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    the safety precautions around high voltages.

Vacuum State FVP5A voltages

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no fets

i had some years ago the rtp with fet on top and bottom of the slcf-its way below the all tube version, and the fets can fail for example cahnging cables etc.






And to continue: You also have experience using fets for the slcf:

My idea is, using a cascoded bjt current source, and a mosfet for the upper part. Should I use 6h30 for the cathode follower (best current?)

I really don't understand, why a valve for the upper follower is a benefit...

Did you try with fet and valve in comparison??? Or did you try the fet for current source and upper follower, an then switch over to 6922 valve arrangement?

I highly appreciate your opinion, because I thing, you gained valuable experience...

Regs, Dirk
 
Given a bias point of -2.7V @ 0.006A (2.7/0.006) gives a cathode resistor of 450R.

You could use the 5687 as the Constant Current souce - but in my opinion it would be better to use each triode for each channel of the front end. Then use a single ECC88 for the bottom and middle of the SLCF, with the top been taken by another ECC88 with one triode for each channel.

The is absolutely no sonic benefit to keeping the two channels in different valve bottles as there will be no noticable cross talk.

Shoog

Hi Shoog, I'm also interested to build the FVP5a with Salas SSHV Regs. I've often heard about the problems of using 6922 in the common cathode stage. So I think I will also use the 5687. You suggested to use the bottom and middle triode of the SLCF in one ECC88! Will there not be a problem with the max. Ukf because the heating of the middle triode should be elevated? Best regards, Falk
 
New FVP-6 with 12AT7's, by Joe Rasmussen from Custom Analogue.
 

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Hi Shoog, I'm also interested to build the FVP5a with Salas SSHV Regs. I've often heard about the problems of using 6922 in the common cathode stage. So I think I will also use the 5687. You suggested to use the bottom and middle triode of the SLCF in one ECC88! Will there not be a problem with the max. Ukf because the heating of the middle triode should be elevated? Best regards, Falk

Elevate the heater to about 60V which will place it at about the right potential for both the cathode follower and the voltage follower on the top. The ECC88 is designed for this type of stacked arrangement, you should check on the datasheet as one half is specified to go on the bottom (cathode follower) and the other is for the top (voltage follower).
Build a stack of resistors from B+ to ground with a total potential of about 100K with one tap at 60V for the bottom of the SLCF and one tap at about 170V for the voltage follower - this stack will also act as you power supply bleed resistor.

Shoog
 
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Thanks Shoog, so middle and upper Triode should be shared in one valve, not middle and lower?! And lower triode shared channels?! Falk
Its not critical which way you do it. If you are going to use the ECC88 for the front end then its sensible to use the other half as the CCS because then you can place the heater at +30V and it works for both. Then the other two can share one valve (the CF and the VF) and can have their heaters biased up at a suitable voltage (+130V) for both of them.

If you are using a different front end then it frees you up to use one ECC88 for both the top voltage followers and you can bias the heaters up at a sensible 180V for them both.

Basically for operation it make little practical difference how you mix the different triodes and this allows you to base your choices on what best respects the heater to cathode limits.

I personally would put one valve in each half of the bottom two triodes in the stack, place a valve on the top VF position and use a single 5687 for the front end triodes.

Shoog
 
My FVP5A has remained stock for the last few years, but these new posts have got me thinking about it again. I have some 5687 but really don't want to re-wire the sockets, so that's out. But I do also have some 6N6P, pin compatible with the 6922. Less gain, more Gm....might be worth an experiment.
 
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