Vacuum State FVP5A voltages - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th October 2012, 07:57 AM   #11
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
For anyone who might find it useful - I tried the first stage with self bias using an LED worked out fine, but I think I prefer just a plain old cathode resistor. LED bias tends to sound a bit cleaner/leaner which may or may not be to your taste.

Shoog
  Reply With Quote
Old 7th December 2012, 12:49 PM   #12
diyAudio Member
 
Join Date: Feb 2008
Hi Shoog,

a real stupid question:

Allen uses 25k wirewound, and he wants us to change the 2,2k that way, that we reach 100V, this corresponds to a specific current through the anode resistor. If I change this anode resister, would this no completely alter the harmonic spec of the preamp (In my opinion, the choice of the anode load is vital for the distortion...)

Regs, Dirk

P.S.: Another point: Lowering the cathode resistors from 2,2k to around 500 ohm meant 75% less local feedback, in the end: 4 times the amplification and 4 times distortion!!!
  Reply With Quote
Old 7th December 2012, 01:34 PM   #13
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
What I tended to do with mine was to place a wirewound pot on the last stage of the power supply, before the final cap, before feeding the input stage B+. This allowed me to tune the anode voltage into just the right voltage to give me the tone I wanted. A 1K resistor was all that I needed. This allowed a surprising amount of fine tuning to the overall tone - from fat to lean.

Shoog
  Reply With Quote
Old 7th December 2012, 01:42 PM   #14
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Originally Posted by rhone View Post
Thanks for your input, Shoog. I have recently been going back thru' the archives and enjoyed reading about your build of this excellent pre-amp. The way Allen has drawn the circuit we get 6 mA through that gain stage, do you think this is optimum?
I know you experimented with various alternatives to the "plain vanilla" cathode resistor on that stage. What do you think worked out best?
6mA would be about optimum in this situation. Anything more is going to shorten valve life quite a bit with almost no sonic gain. I have since built a new version using ECF80's which passes 10mA in both input and output stages - I think it sounds better than the ECC88 version - but that could be attributable to many other factors.

This is a very versatile circuit which can be made to work with any number of valves. I have always fancied building a version using a DHT in the front end. The 5687 seems like a much better fit for the input valve - lower gain and fuller sound.

Shoog
  Reply With Quote
Old 7th December 2012, 01:55 PM   #15
diyAudio Member
 
Join Date: Feb 2008
6mA is 150V anode resistor drop, is 100V at the anode. What is the cathode resistor value for this? (6922 valve)

Regs, Dirk

P.S.: You recommend 5687. Can I use the second part for the current source of the SLCF? Which current is best for the SLCF?
  Reply With Quote
Old 7th December 2012, 02:13 PM   #16
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
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
  Reply With Quote
Old 10th December 2012, 08:40 AM   #17
diyAudio Member
 
Join Date: Feb 2008
Hi Shoog,

so from your experience you strongly recommend the 5687. Which current (cathode resistor...) you recommend for this valve? (25k anode load!)

Regs, Dirk
  Reply With Quote
Old 10th December 2012, 08:46 AM   #18
diyAudio Member
 
Join Date: Feb 2008
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
  Reply With Quote
Old 10th December 2012, 10:26 AM   #19
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
I like the 5687 a lot. Assuming a plate voltage of 100v and a desired plate current of 10mA gives a bias of -4V. This requires a cathode resistor of (4/0.010) 400R.

I never tried a transistor cathode CCSink in this design so can't comment on how it would sound. I did however use a pentode CCS in my ECF80 version and that worked out fine. I used a very simple screen regulator with a pot to vary the screen voltage and hence the current - this made it possible to set the current to exactly what I wanted using a simple adjustment.
I tried both a FET and a triode as the top element. I can't say as I heard a significant difference between the two - though I never did do any serious side by side listening. Allen always maintained that his thinking matured to go all triode with definate sonic benefits from doing so, and I am inclinded to say that he is probably right about that. The problem with FET's is always going to be large gate capacitance, which if not driven hard is going to effect the signal and introduce a "FET sound". Driving the gate capacitance as well as the output load is going to place more stress on the middle cathode follower element. There is also the small issue of reliability - FET's are not as robust as a valve. If you are going to use a FET consider a small signal JFET rather than a MOSFET.

On balance I would stick to an all valve version - but consider using a Pentode as the bottom element CCSink.

Shoog
  Reply With Quote
Old 10th December 2012, 11:25 AM   #20
diyAudio Member
 
Join Date: Feb 2008
Thanks a lot for your input!!!

Dirk
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Vacuum State RTP3C Binaural Tubes / Valves 479 20th April 2014 11:07 PM
Vacuum State SuperREGS tubesguy Swap Meet 5 1st March 2009 03:05 PM
Vacuum State RTP5. InSides Tubes / Valves 4 15th March 2008 04:40 PM
Vacuum State Electronics FVP5A runeight Tubes / Valves 28 5th December 2003 10:46 AM


New To Site? Need Help?

All times are GMT. The time now is 01:42 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2