Upgrading Vacuum State FVP FVP-5 Tube Line Stage to FVP-6
Please note, the FVP-6A and FVP-6B line stage schematics are (C) Copyright Joe Rasmussen & Vacuum State and only single user license is granted.
The original FVP5 schematics also had shared copyright, but SVP-1 and SVP-2 are Copyright Vacuum State. Their schematics have similarities but will not be shown here as I consider them belong to Vacuum State which will still make them. So understandably, I am not comfortable in posting them.
Here for DIY purposes, we shall show two variations called FVP-6A and FVP-6B:
What makes them new is that they are based on the 12AT7A/ECC81 tube instead of 6DJ8/6922/ECC88 tube. Down here in Australia a number of FVP users were persuaded on my recommendation to change away from the ECC88 tube to the greatly underrated "T7" tube in the line stage (the ECC88 remained the same in the phono stage). Nobody regretted it and reported they liked what they heard. There is something about the "T7" tube, it does not try to be high or low gain or mu or anything special. It seems to hit a sweet spot of its own. Like it is not trying to do anything hard.
OUR PROBLEM? CONTROLLING THE GAIN!
This what the early FVP5 did to control the gain, a resistor was added to draw additional current and it looked like this:
This is a crude way of lowering the gain to something manageable. In effect the 25K and 47K look in parallel and also brings the Anode voltage down to where we need it to be. BUT...we want to avoid using this resistor and there are two ways of doing it as well as being able to set the gain where we want it.
The two methods both involve using a current sources.
The reason for being two variations, FVP-6A and FVP-6B, it has to to with FVP-6A requiring an added negative voltage (-15V), and FVP-6B does not.
Both methods should work well. The "B" variant is simpler as it does not need an added negative power supply.
The schematics as shown uses +250V, but it could be used with as low as +210V, but with lower voltage the circuit adjustments in the circuit become more critical.
Gain is aimed to be around 15dB approx (5-6x) but it can be trimmed a bit lower. For sure these things can be discussed as we go along. We gain discuss what is needed to do to get the gain you want. How that can be done, this we can discuss as we go along.
Here is the FVP-6A schematic:
Some simple modeling:
(100mV in and 560mV out.)
What you use as a current source is up to you. Dial in enough current to get you approximately +130V on the Anode of the input gain tube. This also provides a good bias voltage for the buffer stage "Super Linear Cathode Follower" or SLCF for short. This is the key voltage in the circuit.
How you get the +250V is up to you. You can use a SuperReg for sure, but whatever you use, note that it will have an effect on the result, the sound. Power supplies always does.
The +15V comes from the filament supply. The "T7" filaments are wired in +12.6V (series) and are fed with an inline 15 Ohm 1W resistor with will drop down the voltage by a couple of volts. This also creates a slower start when the filaments are cold and good for long-term life.
The -15V needs to be a dedicated supply and it to needs to be regulated. The R9 resistor value "???" is adjusted, so that +130V appears on the Anode of the "T7" gain stage. The R9 value here should not be copied, the correct value needs to be found.
Some may have noted that the SLCF uses a Darlington instead of a HexFet IRF710 used in the Vacuum State circuit. This is to avoid this 47K resistor:
This resistor should ideally be eliminated. The 47K was required because the HexFet's non-linear capacitance could affect the performance. Allen decided to add enough resistance to prevent it from misbehaving. Of course, the SVP variants used an actual tube and that meant he could get rid of that 47K. But here we don't have that option. Hence the idea was developed to used a Darlington with a gain of 500 or higher. The MPSA29 should do the job nicely, but will need a 33-47V Zener to protect it.
The R4 470K or 560K needs to be adjust so that the voltage drop caused by the Darlington is 15-20V. Hence the Anode of the follower will see around around +230V to +235V.
How you come up with the +250V and -15V respectively, we can have a discussion about that.
Next is the FVP-6B Schematic:
Some simple modeling:
(100mV in and 600mV out, gain = 6)
Now we have done away with the extra negative power supply. That is quite a few components eliminated.
Note: Only four additional components, three resistors and one HexFet, typical an IRF710 or IRF820, per channel.
Note that Anode load value is lower and so it the Cathode resistor value, compared the FVP-6A. In this circuit, it is R11 that is adjusted to give the same +130V.
This current source effectively takes the place of the 47K resistor that we might other wise have used. But a current source presents a hugely larger impedance, indeed the ideal current source is infinite.
The FVP-6B does draw more current from the +250V power supply than the FVP-6A.
Please note re the Buffer or SLCF, it does not have the same output grunt as the ECC88 version. But it is quite adequate for our purposes. We are mainly concerned about the sound.
So there you are. I am sure that there will be lots to discuss, but much of it should be obvious to seasoned DIY'ers and also for them to be helpful to others.
Let the building commence...
Please note, the FVP-6A and FVP-6B line stage schematics are (C) Copyright Joe Rasmussen & Vacuum State and only single user license is granted.
The original FVP5 schematics also had shared copyright, but SVP-1 and SVP-2 are Copyright Vacuum State. Their schematics have similarities but will not be shown here as I consider them belong to Vacuum State which will still make them. So understandably, I am not comfortable in posting them.
Here for DIY purposes, we shall show two variations called FVP-6A and FVP-6B:
What makes them new is that they are based on the 12AT7A/ECC81 tube instead of 6DJ8/6922/ECC88 tube. Down here in Australia a number of FVP users were persuaded on my recommendation to change away from the ECC88 tube to the greatly underrated "T7" tube in the line stage (the ECC88 remained the same in the phono stage). Nobody regretted it and reported they liked what they heard. There is something about the "T7" tube, it does not try to be high or low gain or mu or anything special. It seems to hit a sweet spot of its own. Like it is not trying to do anything hard.
OUR PROBLEM? CONTROLLING THE GAIN!
This what the early FVP5 did to control the gain, a resistor was added to draw additional current and it looked like this:
This is a crude way of lowering the gain to something manageable. In effect the 25K and 47K look in parallel and also brings the Anode voltage down to where we need it to be. BUT...we want to avoid using this resistor and there are two ways of doing it as well as being able to set the gain where we want it.
The two methods both involve using a current sources.
The reason for being two variations, FVP-6A and FVP-6B, it has to to with FVP-6A requiring an added negative voltage (-15V), and FVP-6B does not.
Both methods should work well. The "B" variant is simpler as it does not need an added negative power supply.
The schematics as shown uses +250V, but it could be used with as low as +210V, but with lower voltage the circuit adjustments in the circuit become more critical.
Gain is aimed to be around 15dB approx (5-6x) but it can be trimmed a bit lower. For sure these things can be discussed as we go along. We gain discuss what is needed to do to get the gain you want. How that can be done, this we can discuss as we go along.
Here is the FVP-6A schematic:
Some simple modeling:
(100mV in and 560mV out.)
What you use as a current source is up to you. Dial in enough current to get you approximately +130V on the Anode of the input gain tube. This also provides a good bias voltage for the buffer stage "Super Linear Cathode Follower" or SLCF for short. This is the key voltage in the circuit.
How you get the +250V is up to you. You can use a SuperReg for sure, but whatever you use, note that it will have an effect on the result, the sound. Power supplies always does.
The +15V comes from the filament supply. The "T7" filaments are wired in +12.6V (series) and are fed with an inline 15 Ohm 1W resistor with will drop down the voltage by a couple of volts. This also creates a slower start when the filaments are cold and good for long-term life.
The -15V needs to be a dedicated supply and it to needs to be regulated. The R9 resistor value "???" is adjusted, so that +130V appears on the Anode of the "T7" gain stage. The R9 value here should not be copied, the correct value needs to be found.
Some may have noted that the SLCF uses a Darlington instead of a HexFet IRF710 used in the Vacuum State circuit. This is to avoid this 47K resistor:
This resistor should ideally be eliminated. The 47K was required because the HexFet's non-linear capacitance could affect the performance. Allen decided to add enough resistance to prevent it from misbehaving. Of course, the SVP variants used an actual tube and that meant he could get rid of that 47K. But here we don't have that option. Hence the idea was developed to used a Darlington with a gain of 500 or higher. The MPSA29 should do the job nicely, but will need a 33-47V Zener to protect it.
The R4 470K or 560K needs to be adjust so that the voltage drop caused by the Darlington is 15-20V. Hence the Anode of the follower will see around around +230V to +235V.
How you come up with the +250V and -15V respectively, we can have a discussion about that.
Next is the FVP-6B Schematic:
Some simple modeling:
(100mV in and 600mV out, gain = 6)
Now we have done away with the extra negative power supply. That is quite a few components eliminated.
Note: Only four additional components, three resistors and one HexFet, typical an IRF710 or IRF820, per channel.
Note that Anode load value is lower and so it the Cathode resistor value, compared the FVP-6A. In this circuit, it is R11 that is adjusted to give the same +130V.
This current source effectively takes the place of the 47K resistor that we might other wise have used. But a current source presents a hugely larger impedance, indeed the ideal current source is infinite.
The FVP-6B does draw more current from the +250V power supply than the FVP-6A.
Please note re the Buffer or SLCF, it does not have the same output grunt as the ECC88 version. But it is quite adequate for our purposes. We are mainly concerned about the sound.
So there you are. I am sure that there will be lots to discuss, but much of it should be obvious to seasoned DIY'ers and also for them to be helpful to others.
Let the building commence...
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@Joe Rasmussen Some of the schematics are missing 33V zener. Does both versions need to have 33V zeners? Instead of IRF820 can I use IRF710, IXTP08N100D2 or STF3LN80K5 ? What would be the CCS mA values for these schematics?
If I like to use it as unity gain and also maybe use some switch to get gain from 250mV input to 2V output? What resistors or changes I need to make in these schematics?
What brand and year range 12AT7 tube sounds really really well in this schematic?
Thank you!
If I like to use it as unity gain and also maybe use some switch to get gain from 250mV input to 2V output? What resistors or changes I need to make in these schematics?
What brand and year range 12AT7 tube sounds really really well in this schematic?
Thank you!
Actually a schematic was missing, now fixed.
There are plenty of others suitable. Just be sure that the resistor needs to be adjusted for correct current to give that +130V.
Most 12AT7's should work fine. I haven't found one that does not sound good.
So A vs B purely for simplicity and no great difference in performance?
I am finding this current 820 part. Ok for this use?
https://www.mouser.ca/ProductDetail/Vishay-Semiconductors/IRF820PBF?qs=cvaI6ThkwxvWaKnofo/V8w==
And who is a good source for super regulator pcb?
I am finding this current 820 part. Ok for this use?
https://www.mouser.ca/ProductDetail/Vishay-Semiconductors/IRF820PBF?qs=cvaI6ThkwxvWaKnofo/V8w==
And who is a good source for super regulator pcb?
There is a preference for the A version since all the current goes through the tube in the gain stage. The current here in the B version is lower but still OK. I have not auditioned them side by side, but both versioned are offered and people are welcome to try both. They should both be fine.
Yes, that IRF820 is suitable, so is IRF710 (indeed the 710 has lower capacitance, but that is less critical here). There are a number of high voltage Fets than can be used for this purpose.
Re SuperReg, this could be configured to work:
http://www.vacuumstate.com/index.dna?rubrik=24
People can also make up their own power supply and are welcome to post/publish them here. I might even do one myself, one that is based on TL431A reference and should be easy to do.
Yes, that IRF820 is suitable, so is IRF710 (indeed the 710 has lower capacitance, but that is less critical here). There are a number of high voltage Fets than can be used for this purpose.
Re SuperReg, this could be configured to work:
http://www.vacuumstate.com/index.dna?rubrik=24
People can also make up their own power supply and are welcome to post/publish them here. I might even do one myself, one that is based on TL431A reference and should be easy to do.
Thanks, Joe. I think using the CCS to load the anode resistor in the SVP6B is a really neat idea!
Have you ever considered (as an SVP6C, perhaps?) the option of using a negative regulated supply to the voltage amplifier stage, so you can use a larger cathode resistor to reduce the gain?
Alex
Have you ever considered (as an SVP6C, perhaps?) the option of using a negative regulated supply to the voltage amplifier stage, so you can use a larger cathode resistor to reduce the gain?
Alex
Thanks Alex. I thought it was neat too and glad you thought so too. 😊
Be sure we call it FVP and not SVP.
And yes, a negative power supply could do that and I am sure that you would realise that option has occurred to me, even if it is something you don't see often. But since you are taking away the ground reference for the Cathode, the negative voltage would have to be very quiet I would think. But I did model it as below shows and it would work. Indeed -15V would give the result below, it should get us near +130B and the same gain around 5x.
Be sure we call it FVP and not SVP.
And yes, a negative power supply could do that and I am sure that you would realise that option has occurred to me, even if it is something you don't see often. But since you are taking away the ground reference for the Cathode, the negative voltage would have to be very quiet I would think. But I did model it as below shows and it would work. Indeed -15V would give the result below, it should get us near +130B and the same gain around 5x.
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Thank you Joe! 🙂
But what could be done with the all tube version of the FV5A?
best regards,
falk
But what could be done with the all tube version of the FV5A?
best regards,
falk
For some reason in #1 some of the schematics are showing up. in my Chrome browser. Can others see these *.png files? I think the problem is local on my computer.
Edit: I checked Edge browser and the *.png files are working there, so that is good.
The same two current sources used in FVP-6A and FVP-B can be adapted to the FVP-5A, but you need to sort the details yourself. But the "T7" tube just sounds nicer and better IMO. Others have agreed with me.
Edit: I checked Edge browser and the *.png files are working there, so that is good.
The same two current sources used in FVP-6A and FVP-B can be adapted to the FVP-5A, but you need to sort the details yourself. But the "T7" tube just sounds nicer and better IMO. Others have agreed with me.
In your first post I can see 6 inline images, and one other attached which is a different file but it's a copy of the sixth one.
I've started to plan a component layout to wire up on a turret board. Have been using images I've found of other FVP online including some Aussie Stereonet listings for old preamp sales....
First of my many questions hopefully some can help with:
Apart fron the 2W anode resistor on first tube, would 0.5W be suitable for all other locations? Carbon comp on grids and metal film for most other stuff.
How much power dissipated in the IRF820 and is a heatsink required?
I may pursue this build as a complete preamp with phono stage. Is the revision of phono in FVP5A the best iteration of this design? No tweaks that would be commensurate with the changes/subjective performance increase seen in FVP6?
First of my many questions hopefully some can help with:
Apart fron the 2W anode resistor on first tube, would 0.5W be suitable for all other locations? Carbon comp on grids and metal film for most other stuff.
How much power dissipated in the IRF820 and is a heatsink required?
I may pursue this build as a complete preamp with phono stage. Is the revision of phono in FVP5A the best iteration of this design? No tweaks that would be commensurate with the changes/subjective performance increase seen in FVP6?
Yes. that should do nicely.
The IRF820 would benefit from a small TO-220 heatshrink.
The phono is still the same and based on ECC88/6922 tube.
Joe, what tier of subjective sound quality would you judge the re-designed circuit?
What about heater to cathode voltages? To elevate it by 60V will be ok?
And a lot of useful information about clean -15V power supply is possible to find here:
https://www.diyaudio.com/community/threads/super-regulator.247281/page-175#post-7559580
And a lot of useful information about clean -15V power supply is possible to find here:
https://www.diyaudio.com/community/threads/super-regulator.247281/page-175#post-7559580
Hi Ken, it's been some time, hope you are doing well and surviving. Last month I got Covid after all and that was after four vaccinations/boosters (as I regard myself as a science person, I am definitely a believer). Life here has been the usual roller coaster.
The sound of ECC88/6922 vs ECC81/12AT7 is curious, all I can say is that when I have replaced a 6922 with a "T7" in any circuit where possible, I have preferred the T7 sound. When I have done it for others, they did not ask me to undo it. This included the input tube on power amps. Do I know the reason? Nope! But I have a theory, the 12AT7 is a regular down the middle signal tube and just does the job with ease, a word that describes the sound well.
The FVP-6 is right up there sound wise. You can still get in touch with Vacuum State and get a quote for the 6922 based SVP-2 and they will build it for you and it will take some months. The price will make your eyes water and it is good. Even then, I will prefer the FVP-6 - but this means getting the power supply right.
You could, but now you would need a new +15V supply or similar, to create bias for the current sources. These would now need only be low current. But basically the answer is yes, if the added complexity is not an issue for you.
Could do that. I just present the fundamental schematic, which is really the important part. The variations in implementations can be infinite. Go for it!
Some time ago in vacuum-state-rtp3c thread #906 post:
https://www.diyaudio.com/community/threads/vacuum-state-rtp3c.151733/page-46
I asked about square wave response test. I performed some tests with LTSpice and real circuit but did not get correct waveforms at the output at 100Hz, 1kHz and 10kHz. I understand that this modification is for line stage and phono part is the same as in FVP5 or FVP5A but I still want to ask again the same question. Any comments are greatly appreciated.
https://www.diyaudio.com/community/threads/vacuum-state-rtp3c.151733/page-46
I asked about square wave response test. I performed some tests with LTSpice and real circuit but did not get correct waveforms at the output at 100Hz, 1kHz and 10kHz. I understand that this modification is for line stage and phono part is the same as in FVP5 or FVP5A but I still want to ask again the same question. Any comments are greatly appreciated.