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The power of music.                         

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The best part of Friday night. Put a record on – Evan Parker’s “the topography of the lungs” and opened a bottle of Brixton’s Atlantic Pale Ale. Curry delivery is on its way. What else can you ask to finish the week?

Listening session on Andy Evans’ tomorrow. I will take the jFET buffer as well as the latest UV-201a preamp for some thorough tests.

Enjoy the weekend!

Ale


CX-301a DHT Pre-amp Build from Malaysia

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A fantastic build of the 01a preamp using the gyrator PCB from Cheah:

I have just completed my 01a preamp with STP3NK60ZFP output follower. The preamp sounded great !!

A great looking preamp using Rod Coleman’s regulators, output follower and V-Cap capacitors. The jFET is BF862

Well done Cheah!

Flexible HT Power Supply (Part V)

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I’m now back in business. Building a new 4P1L PSE output stage so will reuse the 300B/4P1L Flexible PSU.  I never managed to post an update on the troubleshooting I had to do to get this HT PSU to its optimal state. 

The output voltage was lower than expected and the 50Hz component extremely high. Something was wrong. So I traced the issue down. I found a bad solder in one of the rectifier’s cathode. The supply was operating in half-wave mode. 

The supply is choke-input with 6AU7 rectifiers (hybrid bridge with FRED rectifiers). The transformer is custom made and has multi-taps for 300-400 and 500V. The tuning capacitor for the choke input is 470μF, then choke is 2.5H into 50uF oil cap. The filtering stage per channel is 20H + 100μF Oil caps;

Here’s a test of the supply at 330V/60mA per channel. It’s very rewarding now to see no 50Hz component and that the ripple at 100Hz is just 4mV (ignore the mA typo on the image):

01a Preamp (Gen2) Universal HT Supply

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Introduction

I received many requests throughout this year of folks building the 01a preamp (Gen2) asking for advice on the HT supply design.  Well, I have my own incarnation which serves multiple purposes as is a shared supply, therefore isn’t useful to anyone. 

My friend Andy Evans came up with a supply using available components. This is exactly what you are looking for the HT supply.  I made some tweaks to Andy’s design, so all credit should go to him.

Design

The HT supply design is very simple. It’s mainly a choke-input valve rectifier supply. It has an additional LC smoothing stage. Here is the high-level circuit, see some notes

The transformer is a 250-0-250V / 50VA with an output current of 60mA. It has two windings for 6.3V AC heaters, but you only need one. Here is a great choice from a recommended seller. 

The valve rectifier is a double-diode damper. Of course you can use some other options, but I like the sound of them. Here are some you might want to consider in your build:

  • 6BY5G: a nice double-diode damper with lower filament requirements than the commonly used.  This is my first choice
  • EZ-80: the famous rectifier. This is what Andy used.
  • EY-91: you will need 2 valves as there is one diode per bottle.
  • 6C4P-EV:  a nice Russian small double kenotron rectifier. Very cheap alternative for European builders 
  • AZ1 / AZ11 mesh valves. Their sound is unique, however they are very expensive these days. You will have to add a pair of voltage dropping resistors to accommodate the lower filament voltage requirement. 

For the indirectly heated diodes, it will be better to connect one end of the filaments to the cathode. 

The chokes are commonly available. These are from Hammond (155J) and have 15H @ 30mA. The downside is the high resistance, over 1kΩ. This isn’t an issue here as the current consumption is low so the voltage drop is minimal.  With a choke input supply you need a minimum current to operate. In this case is about 15-16mA so a bleeder resistor is needed (29kΩ 5W wire wound) . 

C1 serves to equalise the output voltage. It pushes the supply to operate a bit more like cap-input (hybrid) by increasing the output voltage. I use it to tune the output voltage to 200V. 

C2 and C3 are classic motor run capacitors. I personally use ASC Oil ones 450VAC rated.  You can choose what you can get hold. You can use any good quality film capacitors. I like the WIMA DC-LINK ones, they are great.

The output ripple is about 7mV. This isn’t a problem as the gyrator load has a very high supply rejection (PSRR) so no need to go crazy on this. If you 

The design is so simple that anyone should be able to build this easily. 

Hope it works for you.

Merry Christmas!

 

 

 

 

 

01a Gen2 Preamp Build from Barry French

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Barry French has recently build his version of the 01a Preamp Gen2. Here are a couple of pictures: 

And Barry’s impressions:

“The 01a Amplifier is a stunner, personally I feel it leaves the 26 out in the cold, better top & bottom by a Country Mile, this was built using the Russian FT-3 Caps on the Output, Russian PIO Caps on the Boards with Jupiter 0.1 μf Wax/Oil Caps from B+ to Ground, the Power Supplies for both Filaments & B+ are from my original 26.”

4P1L PSE project started

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 A new amplifier build has begun. The 4P1L PSE is coming to life. First top plate developed and arrived this morning. A succesful layout for being the first one. Need to submit for machining the other 2 panels. It will be modular so I can make changes and experiment with different output transformers I have at hand (Monolith Magnetics, Lundahls, etc.). The HT power supply is ready and I’ve been fitting the filament supplies in child-proof boxes over the Christmas holidays. It will take me some time though to get this one up and running. Not much time to work on it am afraid. 

3B7 DHT Preamplifier

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An enthusiast blog reader (Paul Prinz) implemented a nice version of the DHT preamplifier using the gyrator PCB but for the 3B7 triode. The 3B7 triode has a pair of DHT triodes on the same bottle. It has a high mu for a DHT (about 20) but with that it comes the higher anode resistance. This was a drawback when implementing a choke or transformer loaded stage due to the high anode resistance (there is no free lunch am afraid). However, with the mu-stage, this doesn’t become an issue and we can get the most out of this valve using the gyrator load. 

Although I tried the 3B7 in the past, I proceeded to get it out from my valve stash and trace it again. Here is a nice set of a Sylvania military NOS one:

Connecting the two triodes in parallel helps to reduce the anode resistance to about 6KΩ, the transconductance is doubled to about 3.8mS. The μ doesn’t change and should be about 18-20.  

As this valve pair will operate at relatively high current then it can drive any stage. This is a great option when you need more gain compared to the classic 01a/26/4P1L stages. 

Here are the curves for the pair of valves in parallel:

 

The valve is very linear, as all DHTs. 

If we want to implement the gyrator load design with the PCB, here’s a simple one:

 

The design is very close to the 01a preamp Gen2. You can easily adapt the 01a to work with the 3B7. A loctal socket and a probably higher HT supply. I think you will be able to get away with 250V if the drive signal is low. Otherwise 300V is recommended. 

Performance is very good and according to Paul, it sounds even better than the 4P1L. Will have to build and judge it!

Happy New Year!

Ale

4P1L: pump up the current!

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Background

I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought.  We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit. 

The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS. 

For comparison, here is a brief summary of the key characteristics of these three devices:

  BF862 BSH111BK MMBFJ310L 
Ptot  (W) 0.3 0.3 0.225
VDSmax (V) 20 55 25
VGS off (V) -1.2   -4
IDSS (mA) 25 210 60
Gfs (mS) 45 640 18
Ciss (pF) 10 19.1 5
Crss (pF) 1.9 1.5 2.5
Coss (pF)   2.7  

There are 2 advantages as well as a freebie from this device:

  • Pump up the current: It can do up to 210mA. However is unlikely you will use it with more than 40mA with the valves commonly used. BTW: someone asked me if a parafeed amp could be done with a 300B valve and the gyrator board. Yes, it could be done but what a waste of power!
  • Lower the output impedance: with the high transconductance it will provide a lower output impedance, despite being operated at lower VDS which impact the transconductance of any FET. This could be a nice solution when implementing a 4P1L or 10Y preamp stage. 
  • Protection Zeners: this device comes with a pair of back to back zeners between gate and source. This reduce the component count in the board, nice freebie! Don’t get confused with the BSH111, which doesn’t have the diodes!

Free Lunch

There is no free lunch am afraid. Firstly the pinout is different to the BF862. Argh, I hate when the clever engineers play with this when designing devices. The lack of standard pinout is a clear way of generating more business! A workaround to this issue is to solder it at 45 degree. Not easy but can be done with a magnifier glass or microscope for SMD soldering:

I thought that the challenge was worth for, so I set myself to solder a pair of boards to test this promising device. 

The circuit 

It’s straight forward as per my 4P1L preamp using the gyrator load. The changes are simply to avoid the Gate-Source protection zeners (but leave the one in for Drain-Source). If you want to use this valve as a driver, you can simply avoid the filament current starvation and run it at 650mA (or 325mA in series filament) and increase the HT. Of course you will have to adjust the filament resistor as needed:

As you can see from the datasheet extract below, the VGS will be somewhere between 1 and 1.8V. In this case for 30mA operation is somewhere about 1.5-1.6V. Only drawback is that a gate stopper can’t be easily added without hacking the PCB, so I decided to omit it and see how stable the circuit was going to be. 

Dial it up!

Building the board is really easy and fast. As you can see below, the BSH111BK is soldered rotated 45 degrees. Not easy to solder this way!

The initial tests were very successful. With the scope and quick frequency response the HF pole was around 2.5MHz. When added the output load below is the automated FR analysis. It’s very similar to the BF862 and showing a great HF performance of this stage:

Now looking at the THD response of the 4P1L stage at nearly 10V, you can see how low the distortion is as well as a nicer spectrum with lower odd harmonics. Nice, this is what I love from the 4P1L when you run it above 25mA:

 

 

The proof is in the pudding

Finally, haven’t listened to this gyrator yet. I know, the proof is in the pudding. However, it’s very promising as it measures so far. Hope to report on this more soon.


46 driving 45 – SE Amp

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My favourite valves together

Recently I revisited a beloved amp, the SE 45. This time I will share a more orthodox design without sand in play. Surprised? Well, I love lots of iron as well and here is a design I’ve been playing around for some time as I have all the components at hand. 

Driving the 45

Driving the 45 isn’t an easy task. You can get outstanding performance with the gyrator (hybrid mu-follower stage) and nice triode stages (or triode-strapped pentodes) like 4P1L, 6J52p, D3a, C3g, 6J49P-DR, 6C45p-E, etc. If you want to go full DHT, then you have a challenge. The gain of DHTs is relatively low and unless you want to go crazy and spend money on an Emission Labs’ EML 20 or 30 valves, you will need to resort to a step up transformer to obtain the gain needed if you want to avoid more than 2 stages.

 I looked back into my favourite list of valves and obviously the 46 came up at first thought. I have used it before with great success in my 814 SE Amp. However, implementing filament bias with that valve was crazy. I will not do that again, therefore a more sensible approach would be either fixed bias or cathode bias. 

Let’s look at the 45 driving requirements first:

45 SE Amp with 5K6 loadIn my experience, you can get the best out of the 45 by loading it with +5KΩ and biasing it at 300V/30-34mA. The actual load with a 5K6Ω transformer will be closer to 6KΩ if you factor in the primary and secondary winding resistance. Either way, you’re looking to a 120Vpp driving requirements to get the 2W out of this stage. So you driver has to easily deliver 150Vpp with very low distortion. This rule out many options of course. I’m not going to dwell around this as it has been covered extensively by many people out there.

So if we look at the 46 in triode mode, you will only get a gain of 5. How do we achieve the voltage gain needed? My recommended way is to include a step up input transformer which also provides a galvanic isolation and load the 46 with a 1:1 (or 1:2) interstage transformer. I have used before the LL7903 which can do 1:8 with lower distortion at high signal levels. It sounds really nice and is a fantastic transformer. Recently I reviewed a great input transformer from Dorin Bodea here. This can do 1:6 but I assume that if you ask Dorin, he can get you a  1:8 version.  You can implement this design with 1:6 though.

With a 1:8 input transformer you will get about a gain closer to 40 overall. This means that with just over 1Vrms you can drive the 45 to full tilt. 

A lot of iron

So here is the initial design, you may critique this. That’s great. There is plenty of iron and oil caps for the fanatics:

46 driving 45 – DHT SE Amp

The design is minimalistic and classic. The input transformer drives the 46 directly and has a Zobel network to compensate the HF response. The 46 is cathode bias with a 910Ω resistor and a 45μF oil cap. The choice of the interstage is the Lundahl LL2746. This nice transformer has proper primary inductance for this valve as well as can be wired on 1:2 or 1:1. In my case I prefer the 1:1 stage. It performs really well, I have tried it with the 4P1L to achieve a high gain stage before, see here. The operating point is reflected below:

46 driver

The 46 is biased to 30mA and has enough headroom to deliver the +120Vpp without getting closer to grid current and with very low distortion. 

The output stage is fixed bias. I will use Rod Coleman’s fixed bias regulator. He has recently released this and I’m in the process of completing my tests. The secondary of the LL2746 is loaded with a 220K but i suspect that a proper Zobel network may be needed to flat the response. Need to test on the bench and optimise this a bit.

The LL1623/60mA is my favourite 5K6:8 transformer I have used with both 45 and 2A3/6C4C stages.

The HT supply is being reused from my 4P1L PSE amp, but can be adapted as you need.

So if you have high efficient speakers and want the best sound of the first Watt, then you should look into this amp.

I’m interested in hearing different experiences and comments about this amp. I’m sure everyone has a view.

Happy New Year!

Ale

DHT preamp “The Mule”

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The birth of the Mule

The name I guess says it all. This is yet another DHT preamp with the gyrator PCB. So what’s different? Simply, a breadboard DHT preamp module ready to be abused.  I’m planning to mod this to death and try a long list of other DHTs with the gyrator load. 

I will only need to change the valve sockets (or build an adaptor) as well as the filament resistors and Rod Coleman filament regulators. Simple changes which can be done fast, will open the door to quick tests on my system.

In order to make this simple and a rapid build, I opted to use an IKEA chopping board. These are made of a laminated hardwood and are dirt cheap. A couple of hours are required to drill all the board like this:

Job done. You only need to do this once. Here is another look at the half-build Mule:

The initial sockets are NOS short pin UX-4/UV-4. I will play around with the 01a before I move to other DHTs. I still need to add the tag strips for filament resistors, output capacitors and the filament regulators. 

Wiring will take a couple of hours and we should have another DHT amp to play with 🙂

 

4P1L PSE Amp: assembling top plates

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The additional machined top plates arrived yesterday. I assembled them this morning. Just need to assemble the chassis now before soldering!

01a Preamp Gen2: New Build

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Yet, another 01a preamp. This time with a beautiful machined top plate and wooden frame:

Will try the Russian filament resistor which we tested at Andy Evans’ and sounded amazing. Just need to build a new pair of Rod Coleman Boards and do the wiring:

01a Preamp Gen2: Build Complete

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Yesterday I started with the build of a new 01a preamp Gen2. I made some component changes during the build process thanks to Andy Evans who reminded me of the Russian FT-2 teflon capacitors.  I had a pair left of 220nF FT-3 caps in stock!

The circuit is the same as the original preamp but with some component changes:

  1. C1 is 100nF/630V ClarityCap polypropylene 
  2. MOSFET is DN2540 and jFET is BF862
  3. Rmu is 330Ω Kiwame
  4. Filament resistors are NOS Russian wirewound 51Ω/20W. I use a pair of them in parallel. Bias is about 5V. 
  5. Filament bias using Rod Coleman v7 regulators. Set starved to 200mA
  6. The output caps are Russian NOS teflon FT-3 220nF / 600V. You can use a pair of FT-2 100nF alternatively.

The bias point is changed slightly up to 5V so the anode voltage is increased to 115V to get the 3mA of anode current. This time I’m using the BF862 which can be soldered in the gyrator PCB instead of the 2SK170. I preferred the sound and higher bandwidth as well as lower output impedance. The BF862 is a real winner as lower FET. 

Here is a view of the preamp inside:

The heavy FT-3 caps are mounted on top of the gyrator PCB boards. The top anodised plate is 4mm thick and anodised. The teflon UX-4 sockets from Luciano Bandozzi (Jakeband) are mounted with silent blocks and Rod’s regulators are bolted to the top plate. you don’t have to as they dissipate very little power in this case. 

How does it sound? Well, just played it for a couple of hours and I’m amazed with the subtle differences that the Russian wirewound resistors and output cap + BF862 can bring to this preamp. We did some listening tests recently with Andy Evans comparing filament resistors and these ones were real winners for both of us.

I hope it improves with time after breaks in a bit more.

 

Gyrator PCB board updated (Rev06)

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After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:

  1. Protection Zener (D3) between drain and source (through-hole)
  2. Back to back protection Zeners (D1 and D2) between gate and source to ensure positive gate bias for higher currents on jFETs and use of enhancement MOSFET

Layout was carefully adapted to ensure track separation due to HV in place. Result is that the new gyrator board provides all protection needed on the lower device and simplifies the build process

 

 

Here is an example of a completed board tested:

Gyrator Board Rev06

 

4P1L PSE Amp Finished!

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I’ve been travelling a lot lately so haven’t had the chance to update on this project. A couple of weeks ago I finished the 4P1L PSE Amp:

4P1L PSE Amp

The amp is outstanding, just like previous incarnations and tests I have conducted over the years.  The level of detail and tone is unique. This is what I always loved from the 4P1L. I’m running it very hot (70mA per pair) and the output transformer is Amorphous Core 3K2 (more detail to be shared soon). It’s a simple stage with filament bias, so no cathode capacitor. The filaments are wired in series to reduce the heat dissipation. Despite this adds a bit more on the output impedance, the bass is powerful. I’m very surprised with the bass, but the level of treble is amazing. It goes over 40-50kHz, I will still need to undertake the classic measurements but so far is great!

It’s absolutely dead quiet. No traces of hum. 

Some more pictures below:

Glowing 4P1L and filament resistor stack!

And the full system below:


Gyrator FET options (More!)

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Someone had to invest and sacrifice some gyrator boards to test various lower FETs (either depletion or enhancement devices as well as TO-92 or SMD options). That was me. 

Why? Because I want to push this circuit further and find the best options as well as provide to the builders out there some other device alternatives when they can’t solder SMD components. 

So let me present you the abused test mule and the various boards under the mercy of my tests:

Boards with different FETs under test

So What FETs where analysed? Here is the list:

  1. BSSH111BK: N-Channel, Enhancement, 55V, 335mA, SMD device (needs to be mounted in 45 degrees or with an adaptor)
  2. 2N7002: N-Channel, Enhancement, 60V, 115mA, SMD device (needs to be mounted in 45 degrees or with an adaptor)
  3. 2N7000: N-Channel, Enhancement, 60V, 200mA, TO-92 – fits perfectly on the PCB board.
  4. BS107A: N-Channel, Enhancement, 250V, 250mA, TO-92 – fits perfectly on the PCB board.
  5. BSS123: N-Channel, Depletion, 100V, 150mA, SMD device (needs to be mounted in 45 degrees or with an adaptor) 

The test circuit

The test circuit is very simple. The classic DHT preamp stage with a 4P1L in filament bias. The HT supply is regulated 300V with a decoupling 100μF ASC Oil cap with a 100nF film cap in parallel. To separate FR impact from the load, the load is the test set in parallel with the 510kΩ R8.  The following components differ between boards as am planning to use them in different circuit from the 4P1L but shouldn’t affect the results of the below tests:

  1. R4: Either 300kΩ or 390kΩ (preferred for 4P1L)
  2. C1:100nF and R6 10MΩ
  3. R7: 300Ω or 470Ω
  4. Rf: 15Ω

All tests were conducted with If=650mA and the 4P1L biased to Ia=30mA

Below are the tests conducted.

BSSH111BK test
BS107a test
BSS123 test
2N7002 test
2N7000 test

So in summary, here’s my doggy bag take:

  • BSSH111BK will give you the best performance. However, there’s isn’t any free lunch. You will have to solder the SMD device on 45 degrees. Which is tricky, see my previous post here
  • If you want to avoid the hassle of SMD soldering and need +30mA current in your stage, then go for either 2N7000 or BS107A. Both TO-92 devices fit on the board and perform really well up to 1MHz. 

 

 

 

C-299/CX-299 DHT Preamplifier

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The start of a different DHT experience with the Mule

I built the “Mule” to provide enough flexibility to test other DHTs as pre-amplifier / line stage. Using the gyrator board, the flexibility is fantastic. Can share same HT and dial the right anode voltage. The LT supply can also be shared amongst many DHTs and Rod Coleman provided me with a set of different resistors to test the list of 9 or 10 DHTs I have in mind which haven’t listed carefully on this design. 

The C-299

First one of the list is the rare 99/199/299. This old DHT has thoriated filaments with low current and slightly less gain that the 01a.  I push it hard to 4mA and beyond 90V. It can work well at 110-120V though. I want to minimise the slew-rate of the stage when driving the 4P1L PSE amplifier. 4mA is ok, if you’re worried you can bias it colder. I have a set of the C-299 as well as CX-299. Given I installed the UV-4 sockets, I added the C-299 with the socket adaptor.

They are slightly microphonic, but not much. I haven’t even starved the filaments. This hasn’t been a problem at all on my system.

The circuit is already popular, here we go:

The component selection is on the same lines as before. I have followed the advice from Barry French and trailing the Takman MF resistors on R5 and R7. The filament resistor is Kiwame. Russian caps, the rest is already known. 

How does it perform?

It measures really well. Great frequency response from 4Hz to 250kHz without loading the preamp:

The distortion is really low (lower than 0.002%) but didn’t save the measurement plot, next time.

How does it sound?

The sound is very interesting. I can say after listening to a few records (including Giant Steps from Coltrane), the detail is very similar to the 01a. The bass is strong as it’s the detail of the treble. I like it, sadly has lower gain than the 01a. 

Will listen to it for a while….

 

 

Pentode driver with gyrator load

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If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:


The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR). 

The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please. 

This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.

Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.

As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.

Cheers

Ale

“Schade” SE Amp Example

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Introduction

On my last post I covered how the gyrator PCB can be used in a pentode driver. The pentode driver is the best candidate in a “plate to plate” / shunt feedback or “Schade” feedback amplifier which is the name typically used in the DIYAudio world. The triode doesn’t work well here as you need high gain and low distortion with a load which can get quite low (due to the feedback effect of the feedback resistor). I’m not going to cover the subject as it has been covered (and discussed) extensively before by many people, so I suggest you do a bit of research yourself if you are interested in the subject and want to learn more. 

A Study example

I chose one of my two favourite valves for this circuit. The output stage is a GU-50. It can deliver good power, whilst having a good transconductance and gain, both are good advantages as we will see. The other valve is the  12HL7 that has been discovered recently and praised by many. I have tested it with great success in triode-strapped mode. 

Here is the circuit for analysis:

Fig1 – GU-50 Schade SE Amp Example

Don’t get scared, it’s not that complex at all. Ok, let’s start with the driver first. In figure 1 we can see that the 12HL7 is biased at about 170V/28mA but has cathode degeneration with R12 being 100Ω which isn’t bypassed on purpose. This helps linearising the stage as the 12HL7 has lots of gain as well as transconductance (I measured μ=290 and gm=22mA/V). The gain of the stage is reduced due to this degeneration which reduces the effective transconductance. The simulated gain is about 38dB (or 79.4 times). The maximum gain that could be achieved on this stage without degeneration would be around gm⋅(RA||rp)or about 170 (we ignore Rf as is much higher than the RA || rp pair). The disadvantage of this topology is that you will burn some current through RA at idle. The higher is the anode quiescent voltage the more current is needed so you get the top FET(M2) and RA dissipating heat. The gate of GU-50 is driven from the low impedance output from the mu-follower (gyrator) circuit. This will provide sufficient current to drive the gate and its capacitance. 

The “plate to plate” feedback is taken from the anode of the GU-50 and to the anode of the driver via Rf. The feedback ratio (B) is given by the ratio of (RA||rp)/(RA||rp +Rf). We typically want to keep this ratio around 10-20%. The more feedback, the lower the output impedance, but the harmonic profile of the stage changes as well. In this case the feedback is close to 9%. The output stage is biased at nearly full-tilt. 70mA and 33W of dissipation on the GU-50 (which has maximum of 40W). You can increase and get a bit more out of juice, but this stage already provides about 11-12W with a maximum input of 1.2V peak.  The gain of the output stage is about 0dB. The GU-50 provides about 26x gain as gm⋅(Zaa||rp) = 7.5mA/V ⋅ 6KΩ//8k5Ω = 7.5 * 3.5 = 26.25. This is because the anode resistance is about 8k5Ω and the transconductance I measured was about 7.5mA/V. The x26 gain reduction is provided by the output transformer which is has a gain of 26.45 or 5k6:1 (without copper losses). The net gain is 1 (0dB) which matches with the simulation. 

What is interesting to point out is that (ideally) the output resistance of the “Schaded” pentode is close to 1/gm. The higher the transconductance the better. In reality, the output pentode has finite and small gain (about 65 in this case) so the output resistance is closer to 1kΩ rather than the ideal 133Ω. The more feedback the better as well as the higher gm.  So you probably want to look at other pentodes with high gm (vertical TV sweep valves are ideal here). 

The gain with feedback is reduced down to 22dB. Not too shabby for producing 8W at 1.5% in single ended mode! Well, you need a 500V supply (ouch). There are other options to look into if you don’t want to use the GU-50 which likes the high volts. 

Hope this post generates some thinking and debate. Looking forward to the comments. 

And so Tim was right….. (Update I)

You don’t need Ra, when rp can do the job:

GU-50 Schade SE Amp v03

Update 2

After playing a bit with my favourite Russian pentode (6Z49P-DR), I increased the drive on the GU-50, increased HT to get maximum output from this bottle. The RA has to be included as RF is high 100KΩ and at least Spice didn’t like it. I suspect there will be stability issues given the value of Rp. Safer is to add the RA to stabilise the gyrator. Now distortion is reduced significantly.

GU-50 Schade SE Amp V04 – revised with 6Z49P-DR driver

Here is the distortion profile from the simulation:

 

VT-25 DHT Preamplifer

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VT-25 in action 

Now I’m back from our long trip, I found some time to play with the “Mule“. I wanted to revisit my old VT-25 preamplifier. Many years ago I had my first VT-25/10 preamplifier which was based on a gyrator load. Then it morphed to a transformer coupled (LL1660/40mA) version to drive my TVC before I settled into the 4P1L for some long time. 

The circuit design

The VT-25 has always been on my list of favourite DHTs. It’s gone ridiculously expensive these days and is hard to get. I have a couple of pairs in very good shape luckily. 

VT-25 DHT Preamp Circuit

The VT-25 isn’t easy to implement in filament bias due to the bias point and the high filament current. However, for a line-stage / pre-amplifier you can bias the valve at a lower point where it’s achievable without burning ridiculous amounts of heat (Like I did with the 46 driver in filament bias for the 814 SE Amplifier).  The design of this circuit compromises available supplies. The HT is 230V so the operating point was chosen to be around 200V to ensure headroom for the top MOSFET (M3). The bias point operates the valve in about in a region where anode resistance is about 5kΩ. The high anode resistance is a challenge for a transformer couple stage, but with the gyrator this isn’t a problem at all, as the output impedance is determined by the BF862 and its operating point (Id). The gain is close to 8 (18dB).  R4 needs to be 390kΩ to allow higher bias voltage than 180V. I reused a pair of available gyrator boards using the BF862. For a bias point of about 20mA/200V you need a filament resistor of 5Ω. I used a pair of 10Ω NOS Russian wire-wound resistors in parallel. They do get hot as need to dissipate about 7.2W. In my previous designed I starved the filaments even down to 1060mA. This time I dialled up the current to just 1.2A as had headroom on the supply. The bias point of 6V is healthy and provides good headroom for input signal. I’m using the latest version of Rod Coleman’s regulators and just changed the resistor to trim the current. The actual modification of the “Mule” took me a couple of hours. 

Building and Testing

The filament regulators are using mounted TO-220 small heatsinks as well as the top MOSFET (M3). I have a multi-tapped secondary on my filament supply so I can accommodate multiple DHTs. That was a real bonus, so getting it to work was easy and fast job. 

Here is a test of the distortion profile at 8Vrms output. The second harmonic is predominant as expected. Distortion stays below 0.1% when is driven to full 15Vrms output:

The gyrator circuit provides a hybrid mu-follower with high bandwidth. Here is a snapshot of an early test biased at 15mA instead of 20mA:

The actual response will vary depending the load. However, it’s very good! This is one of the unique things of this circuit topology. 

How does it sound?

The most tricky question indeed. I love it, I always loved the VT-25. It has the clarity of the 01a or most of the thoriated-tungsten DHTs. I think the bass is strong and overall response is fantastic. I need to listen to it more to give further impressions. At this stage, this preamp can stage on my system for long. Wow, it does sound good!

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