R23, it is grounded, right, hence 0V. Then if you are seeing 23V across it and 23V on the Input Cathode, that means that you are running more than 10mA and hence the tube is turned hard on (23/2200 = 10.45ma). There must be a wiring issue to cause that.
The voltage on the Input Anode needs to be up around 100V or else the SLCF Buffer will not work correctly.
Momentarily remove R5 and measure the Input Anode voltage and report it here. It should be in the 70-100V range I would think.
If it is, then connecting the R5 again, there should be no real change in that voltage. If there is a massive drop, then the problem is not the input stage, but somewhere after that. The problem would then be in the SLCF Buffer. These basic steps in fault-finding. Make sure the input stage is working first.
Can you verify the above and report back here. Depending on what you report, we will determine the next step.
The voltage on the Input Anode needs to be up around 100V or else the SLCF Buffer will not work correctly.
Momentarily remove R5 and measure the Input Anode voltage and report it here. It should be in the 70-100V range I would think.
If it is, then connecting the R5 again, there should be no real change in that voltage. If there is a massive drop, then the problem is not the input stage, but somewhere after that. The problem would then be in the SLCF Buffer. These basic steps in fault-finding. Make sure the input stage is working first.
Can you verify the above and report back here. Depending on what you report, we will determine the next step.
Other than being busy here, I have been thinking along the same lines.
The 12AT7 version is known as the FVP-6 Line Stage. There are already a number of users (close to 10) of the FPV-6 here in Australia and it has been universally received as an improvement when compared with the EC88 when 'upgraded' to 12AT7.
If I start a thread on this, can anybody here suggest what we should call it? It will basically be about the Line Stage and versions FVP-5, FVP-5A (which became the basis for the SVP-1 and SVP-2) and FVP-6. The Phono Stage is largely unchanged and still uses ECC-88.
Hi Joe,
I think Viljo incorrectly reported his measurements first time round - if I understand correctly, the 23V measurement was for the cathode of the bottom valve of the SLCF of the output stage, which is close to what it should be. I think the high voltage on the anode of the first valve, as you say, is the problem, and is consistent with the low voltage of 5.15 V on the first cathode resistor, implying a very low current of 2.3mA. This is also consistent with the initial measurements of my own FVP5, which I remedied by reducing the value of the cathode resistor from 2K2 to about 1K0. This circuit needs about 6mA to get 100V on the anode.
Do you remember why 2K2 was chosen for this resistor?
Alex
There is a problem with the FVP Line Stage that the bal/differential RTP does not have: Problem controlling the gain and to some extent high gain also gives you unnecessaries noise. With 25K Anode load and 1K on the Cathode, are you getting too much gain. Also, listen close to the speakers, what is the background noise like.
So 2K2 is a compromise, and I perfectly understand you going for 1K to get the Anode down to 100V.
The SVP-1 and SVP-2 uses a trick, but the these schematics are not posted on the Vacuum State website. I have the SVP schematic, but I don't feel at liberty to show them. People can still order it and get it built by Thomas Bruentrup, Allen's technician, and will build them for those willing to pay. But Thomas knows that the Line Stage in the FVP was my baby.
In the new FVP-6 Line Stage, using the 12AT7, there is a way to control the gain (and reduce noise) that I will reveal. In fact there are two solutions, the best one needs a negative power supply, nice and clean and about minus 10V will to the job. The other solution is maybe not quite as good, but it requires no negative power supply and uses only a few components (3).
What shall we call the new "FVP" thread?
I don't want to use the Vacuum State name, there are people who have made unkind accusations against me and I don't want to give anybody fuel, know what I mean?
I know for a fact that nobody has built as many FVPs than I have. All the fully working FVP boards that was sent to Passage Lab in the US, they were all built by me. The Passage Lab preamp that got a bad review by Stereophile was not an FVP. Stereophile gave the fake 'FVP' a scathing review and that pleased Allen no end. I won' t mention the person who cancelled the order and went back on his word, except his initials are MB and he originally came from Sydney. I am only stating facts.
I agree with your comment about the high line stage gain of the FVP5! A few years ago I swapped the 6922 in mine for 5687 and reduced the cathode resistor to 820R, which gave a useful reduction of a few dB, though I don't remember the exact figure I measured.
I saw with interest your suggestion a while back to replace the resistor with a CCS in parallel with a resistor, but haven't got round to trying that yet.
Alex
There is some scathing text in the TPCB where Allen complains about a certain Matthew Bond - might that possibly be him?
Alex
I was there!
In the famous words of Sergeant Schultz:
"I know NOTHING!"
That should answer your question. I must be on safe grounds as MB did not sue Allen.
I need to post two FVP6 schematics on the new thread. Perhaps best to wait until then and look at the two options.
"Upgrading Vacuum State FVP FVP-5 Tube Line Stage to FVP-6"
Does that sound OK? Some have unkindly accused me of 'cashing in' on Allen's name and legacy. In fact I actually owned the "vacuumstate.com" domain name, but I gave it to Hanni his widow, after he passed away. I am still in regular touch with Thomas Bruentrup his German/Swiss technician, and also Hanni from time to time. I am not trying to take advantage of anybody.
So will calling the new thread above get anybody's nose out of joint?
What I'm curious about too, and forgive me if this is not to be discussed too much on forums, is who was responsible for the absolute dead ringer copies of the factory SVP-2?
Back in 2015 such a thing would never be on my radar. I bought with relative confidence a pre I thought was an SVP-2. All the casework appeared identical. The faceplate had the machined Vacuum State name (except light blue not black filled paint). It was only later confirmed suspect when comparing the main transformer. This thing even had all the back panel Schaffhausen etching.
I was truly impressed because all the internal circuit boards were custom single layer boards with attendant "copyright and revision" numbers.
I guess my question is to what extent do you know of the details from such period in time when Vacuum State may have been trying to combat an active cloning market.
How is your FVP of different iterations different from what Thomas may build under a current VSE spec?
Back in 2015 such a thing would never be on my radar. I bought with relative confidence a pre I thought was an SVP-2. All the casework appeared identical. The faceplate had the machined Vacuum State name (except light blue not black filled paint). It was only later confirmed suspect when comparing the main transformer. This thing even had all the back panel Schaffhausen etching.
I was truly impressed because all the internal circuit boards were custom single layer boards with attendant "copyright and revision" numbers.
I guess my question is to what extent do you know of the details from such period in time when Vacuum State may have been trying to combat an active cloning market.
How is your FVP of different iterations different from what Thomas may build under a current VSE spec?
Yes, I have heard of the fake SVP-2, but I have never actually seen one. I don't know the price of a now ordered genuine SVP-2 built by Thomas Bruentrup, but it would not be cheap.
I can't say exactly, other than I do not want to intrude on their turf. I have a schematic of the real SVP-2 and have never shown it publicly. It would not be right for me to do that. But if others have posted schematics, then it was not from myself or the original Vacuum State guys. There is TT Yap in KL Malaysia, myself here in Sydney, Brian in Auckland New Zealand, Warren in San Francisco and Bill Thalmann on the East Coast of the US. Other than those guys, whom I trust, there could be others who have done it without permission.
The original FVP5 Line Stage was my idea and I convinced Allen that it was better than the SRPP Line Stage he favoured then. I have been present at every stage of FVP and RTP development since the late 70's and Allen considered my ideas for improvements and said so in his TPCB book.
Besides, the SVP2 will always be a complete phono preamp, whereas we shall be dealing with the Line Stage only as that is where the interest seems to at. I have a stand-alone JLTi Phono that goes really well with the FVP6, but that is a commercial item, just as the SVP variants are.
I am happy to share two variants of the FVP-6 using the 12AT7 tube. I feel unencumbered to do that. But there are places where I will draw the line based on what I consider to be the right thing to do.
I can't say exactly, other than I do not want to intrude on their turf. I have a schematic of the real SVP-2 and have never shown it publicly. It would not be right for me to do that. But if others have posted schematics, then it was not from myself or the original Vacuum State guys. There is TT Yap in KL Malaysia, myself here in Sydney, Brian in Auckland New Zealand, Warren in San Francisco and Bill Thalmann on the East Coast of the US. Other than those guys, whom I trust, there could be others who have done it without permission.
The original FVP5 Line Stage was my idea and I convinced Allen that it was better than the SRPP Line Stage he favoured then. I have been present at every stage of FVP and RTP development since the late 70's and Allen considered my ideas for improvements and said so in his TPCB book.
Besides, the SVP2 will always be a complete phono preamp, whereas we shall be dealing with the Line Stage only as that is where the interest seems to at. I have a stand-alone JLTi Phono that goes really well with the FVP6, but that is a commercial item, just as the SVP variants are.
I am happy to share two variants of the FVP-6 using the 12AT7 tube. I feel unencumbered to do that. But there are places where I will draw the line based on what I consider to be the right thing to do.
The SLCF is still based on the other half of the 12AT7. It will not have the same grunt as the ECC88, not quite as low output impedance, but more than adequate.
Ah, not evolution really, just development, and not a revolution neither.
But an RTP using the 12AT7? That is a possibility. But let us see how the FVP-6 fares first, and if well received, I will happily help any to try an RTP-6 Line Stage.
@Alex M @Joe Rasmussen I removed R5 resistor and measurements are following.
U1 - Pin 1 - R5 removed - 188 V, changing R3 to 1K resistor lower the voltage to 147 V R5 included -
U1 - R3 - Pin 3 - R5 removed - 5,4V - changing R3 to 1K resistor then it is - 3,9 V R5 included
U1 - Pin 6 - R5 removed - 71,6 V - with a R3 resistor changed to 1K resistor then voltage is 152 V R5 included
U1 - Pin 7 - R5 removed - 19,6 V - with a R3 resistor changed to 1K resistor then voltage is 20 V R5 included
U1 - R6 - Pin 8 - R5 removed - 20 V - with a R3 resistor changed to 1K resistor then voltage is 22 V R5 included
U2 - Pin 2 - R5 removed - 148 V with a R3 resistor changed to 1K resistor then voltage is 190 V R5 included
U2 - Pin 3 - R5 removed - 162 V with a R3 resistor changed to 1K resistor then voltage is 197 V R5 included
U2 - Pin 6 - R5 removed - 162 V with a R3 resistor changed to 1K resistor then voltage is 197 V R5 included
U2 - Pin 7 - R5 removed - 0,02 V with a R3 resistor changed to 1K resistor then voltage is 142 V R5 included
U2 - Pin 8 - R5 removed - 72V - with a R3 resistor changed to 1K resistor then voltage is 152 V R5 included
To get U1A pin 1 anode to 100 V then R3 have to be I think about 380 ohms. How to lower the gain then? How that affects sound quality if anode has much higher voltage than 100V?
Ideas how to get this to sound superb?
U1 - Pin 1 - R5 removed - 188 V, changing R3 to 1K resistor lower the voltage to 147 V R5 included -
U1 - R3 - Pin 3 - R5 removed - 5,4V - changing R3 to 1K resistor then it is - 3,9 V R5 included
U1 - Pin 6 - R5 removed - 71,6 V - with a R3 resistor changed to 1K resistor then voltage is 152 V R5 included
U1 - Pin 7 - R5 removed - 19,6 V - with a R3 resistor changed to 1K resistor then voltage is 20 V R5 included
U1 - R6 - Pin 8 - R5 removed - 20 V - with a R3 resistor changed to 1K resistor then voltage is 22 V R5 included
U2 - Pin 2 - R5 removed - 148 V with a R3 resistor changed to 1K resistor then voltage is 190 V R5 included
U2 - Pin 3 - R5 removed - 162 V with a R3 resistor changed to 1K resistor then voltage is 197 V R5 included
U2 - Pin 6 - R5 removed - 162 V with a R3 resistor changed to 1K resistor then voltage is 197 V R5 included
U2 - Pin 7 - R5 removed - 0,02 V with a R3 resistor changed to 1K resistor then voltage is 142 V R5 included
U2 - Pin 8 - R5 removed - 72V - with a R3 resistor changed to 1K resistor then voltage is 152 V R5 included
To get U1A pin 1 anode to 100 V then R3 have to be I think about 380 ohms. How to lower the gain then? How that affects sound quality if anode has much higher voltage than 100V?
Ideas how to get this to sound superb?
Hi Joe I have just picked up on this Thread and live in Queensland. I am looking to build a really good valve preamp. I have read the Allens Wrights book in the nineties and did build the Super Reg on an already existing Valve Preamp with good results. I would be interested in building a full kit ? or at least circuit boards if possible. I will watch this thread with interest.
This is the problem with the FVP5/SVP that Joe alluded to. You have to reduce the cathode resistor on U1A to a small enough value to get the anode to 100V to allow the SLCF to work properly, but this gives you a line stage gain that is too high for most systems. My solution was to replace the 6922 with a 5687, which has a lower transconductance. This did reduce the gain somewhat, though I don't remember by how much. To start with I used the other half of the 5687 for the lower triode of the SLCF (the current sink), but since its transconductance is not really high enough for this function I replaced it with an IRF730 as in the "hybrid FVP5", and repurposed it as a tape output buffer with another IRF710 on its cathode, which works well.
Alex
Alex, very well summed up and yes, we need to control both the gain and the voltages seen by the SLCF as well. A voltage that would work well is 100V. I can suggest two ways of doing this and that is why there will be two variations and using the 12AT7 tube.
These two basic variations can then had additional customisations in the SLCF and I will have some suggestions to make there as well. For example, the SLCF current source FET can be an IRF820 as there is no voltage swing on the Gate and hence it rather high capacitance is less of a problem. But the FET on to makes the tube follower in 'constant voltage' mode, if using a FET there, then it is much better to use an IRF710 as it has lower non-linear Gate capacitance. That IRF710 has voltage swing on its Gate. Below is shown Allen's compromise/solution that could be improved upon.
The IRF710 or even IRF820 on the bottom is just fine. But the IRF710 is better up top. But note the 47K is a compromise because of that Gate capacitance, this is not used in the SVP as it becomes a tube. My FVP-6 12AT7 version solves the problem by using a high voltage Darlington there and that gets rid of the 47K resistor.
So there is stuff like that we can discuss, but the gain and noise is top of the list, but also sorting out the best but most simple SLCF is also part of it.
These two basic variations can then had additional customisations in the SLCF and I will have some suggestions to make there as well. For example, the SLCF current source FET can be an IRF820 as there is no voltage swing on the Gate and hence it rather high capacitance is less of a problem. But the FET on to makes the tube follower in 'constant voltage' mode, if using a FET there, then it is much better to use an IRF710 as it has lower non-linear Gate capacitance. That IRF710 has voltage swing on its Gate. Below is shown Allen's compromise/solution that could be improved upon.
The IRF710 or even IRF820 on the bottom is just fine. But the IRF710 is better up top. But note the 47K is a compromise because of that Gate capacitance, this is not used in the SVP as it becomes a tube. My FVP-6 12AT7 version solves the problem by using a high voltage Darlington there and that gets rid of the 47K resistor.
So there is stuff like that we can discuss, but the gain and noise is top of the list, but also sorting out the best but most simple SLCF is also part of it.