MULLARD 5-20 build with mods - 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 14th September 2013, 11:26 AM   #11
diyAudio Member
 
Join Date: Feb 2002
Location: Denmark
The schematic suggested in the first post is very similar to the Michaelson and Austin TVA1 power amp. The TVA1 is well liked by many people. But SY has got a point.
Torben
  Reply With Quote
Old 14th September 2013, 06:20 PM   #12
kward is offline kward  United States
diyAudio Member
 
Join Date: Sep 2013
Location: Western USA
Quote:
Originally Posted by multi View Post
I am thinking of using the Mullard circuit too;
On another thread I found the following.
Best to keep the EF86 and get rid of the 12AX7 and use a 6CG7/6SN7 from what i have read.
Phil
I'd agree with this. The 12AX7 in the position of Long Tail Pair phase splitter is not the best tube to use for that purpose. It's got quite high plate impedance and not a lot of current drive. The 6GC7 or 6SN7 works extremely well in this position.
  Reply With Quote
Old 14th September 2013, 10:06 PM   #13
diyAudio Member
 
Join Date: Feb 2011
Location: South Wales, UK
Quote:
Originally Posted by esltransformer View Post
what output transformer will you use?
I have a set of transformers from VVT on the Isle of Wight.
  Reply With Quote
Old 14th September 2013, 10:13 PM   #14
diyAudio Member
 
Join Date: Feb 2011
Location: South Wales, UK
My amps are based on the article by Claus Byrith.
  Reply With Quote
Old 15th September 2013, 03:27 AM   #15
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by SY View Post
It's not really a 5-20 since you have the input stage set up as a diff amp. Your issue is likely to be in the second stage, since the first stage plates are unlikely to have the same DC voltage on them.
I don't understand. I've read in a number of places that cascading diff pairs will improve the balance at the output of the second diff pair's plates, as compared to the balance achievable by a single long-tailed pair fed from a single ended source.

I know that's in "Valve Amplifiers" but I can't remember exactly where. Probably somewhere in the discussion of the Crystal Palace amp, which uses cascaded diff amp pairs.

Also, if the first stage LTP has a constant current sink in the tail and matched plate load resistors, that will enforce balance in the first stage, correct? Then that pretty good balance should be made a little better by its connection to the second diff (push-pull driver) stage, no?

That all seems to contradict the idea that imbalance in the first stage will "upset the apple cart" and make good balance difficult to achieve from the 2nd stage's plates. However, I'm sure I'm missing something.

--
  Reply With Quote
Old 15th September 2013, 04:04 AM   #16
diyAudio Member
 
scottpetersen's Avatar
 
Join Date: Jul 2011
Here is the 5-20 I used.

Conn organ conversion to amp
  Reply With Quote
Old 15th September 2013, 07:38 AM   #17
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by payitforwardeddie View Post
I have a set of transformers from VVT on the Isle of Wight.

Try to find out what bandwidth your transformer is or measure it, because your amplifier uses overall feedback. Also pay attention on the bandwidth of all individual tubestage.
  Reply With Quote
Old 15th September 2013, 10:59 AM   #18
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by rongon View Post
I don't understand. I've read in a number of places that cascading diff pairs will improve the balance at the output of the second diff pair's plates, as compared to the balance achievable by a single long-tailed pair fed from a single ended source.
You're confusing AC and DC balance. Look at the first stage- the cathode voltages and grid voltages are forced to be equal for the two sections. So if the sections aren't perfectly (and I mean PERFECTLY) matched, the DC plate voltages will have to be different, even if the long tail is a CCS. You can verify this on the bench with a quick test circuit. But all the long tail (or CCS) does is insure that the sum of the two sections' plate currents is constant- the two currents do not have to be equal. So even with a DC imbalance, the AC balance is maintained, since a change in one side must have an equal and opposite change on the other side.

I posted a demo a while back showing the results of a real mismatch, with two different tubes used in the LTP (I think it was a 12AT7 and ECC88). The AC balance was, as advertised, perfect. But the DC plate voltages were severely different, sort of a reductio ad absurdum of this point.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 15th September 2013, 05:11 PM   #19
diyAudio Member
 
Join Date: Feb 2011
Location: South Wales, UK
Quote:
Originally Posted by SY View Post
You're confusing AC and DC balance. Look at the first stage- the cathode voltages and grid voltages are forced to be equal for the two sections. So if the sections aren't perfectly (and I mean PERFECTLY) matched, the DC plate voltages will have to be different, even if the long tail is a CCS. You can verify this on the bench with a quick test circuit. But all the long tail (or CCS) does is insure that the sum of the two sections' plate currents is constant- the two currents do not have to be equal. So even with a DC imbalance, the AC balance is maintained, since a change in one side must have an equal and opposite change on the other side.

I posted a demo a while back showing the results of a real mismatch, with two different tubes used in the LTP (I think it was a 12AT7 and ECC88). The AC balance was, as advertised, perfect. But the DC plate voltages were severely different, sort of a reductio ad absurdum of this point.
I understand that.
The Michaelson and Austin TVA1 power amp has extra resistors and capacitors between V1 & V2. Also, V2 has been changed too. I am trying to keep it simple. Maybe fitting a 50k pot between the 330k resistors and the HT - and the same for the 150k resistors. I don't mind re-adjusting these from time to time, likewise the bias pots.
  Reply With Quote
Old 15th September 2013, 07:03 PM   #20
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
How critical is it for the DC plate voltages to be matched? Can the plate voltages be off by 5% or 10% from each other and the circuit of two cascaded diffs will still work well?
--
  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
Mullard 5-20 JDeV Tubes / Valves 137 17th February 2010 04:53 PM
Mullard 5-20/Maplin 4-20 modification to NON-Ultra Linear mode???? vinyvamos Tubes / Valves 1 28th January 2006 04:07 PM
Another Mullard 5-20. GeirW Tubes / Valves 3 30th November 2004 12:45 PM
My Mullard 5/20 billinchile Tubes / Valves 25 14th October 2004 09:16 PM


New To Site? Need Help?

All times are GMT. The time now is 11:47 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