EL84 Amp - Baby Huey - Page 81 - 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 27th May 2009, 03:45 AM   #801
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Rhone,
The 100R pot (diff amp balance) can be a single turn, choose something mechanically strong.
The 50K set the bias for the output tubes and so you want to make sure that this is "ultra" reliable, For that reason, and for the additional "fine" adjustment they provide, I use Bourns 10 turn for these.
The 2u2 across the 2 100K bias pots is there to make absolutely sure that no noise (no AC signal) is coupled back into the audio path via the bias pots. A polyester would probably be OK but since I had some polypropylene in my parts bins I used them. Actually you could go to a 3u3 of even 4u7 for this capacitor. It doesn't need to be a 630V rated part, it only has to withstand the bias supply voltage, a 100V capacitor would be fine, even a 63V would do.

Cheers,
Ian
  Reply With Quote
Old 30th May 2009, 12:17 AM   #802
rhone is offline rhone  Australia
diyAudio Member
 
Join Date: Dec 2005
Location: Melbourne
Thankyou, thankyou, thankyou!
I'm designing the power supply and was wondering how the critical is the negative rail voltage value ? Does it need to be spot on -45V or is there an acceptable range?
Rod
  Reply With Quote
Old 31st May 2009, 11:41 PM   #803
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Rhone,

There is ONLY a "guideline". When using a cathode follower (MOSFET Source follower in this case to direct couple to output tube grids then it is recommended that the negative rail is at least 3 times the bias voltage. Bias voltage for EL84 is about -12V so you want a rail of at least -36V.

All,
I had a long play with the original (CCS biased) Baby Huey on the weekend.
The change to run the feedback from the Ultralinear taps rather than the EL84 anodes was something more significant than at first thought. With feedback from the anodes I could ONLY push the feedback level so far before starting to loose stereo imaging and the sound starting to be a bit messy. So we ended up with a fairly "loose" setting.
There has been an "accepted" view that the diffamp was just not coping with the demand for more current swing with higher levels of feedback and that this was the limiting factor. That has turned out to not be the case. With feedback from the Ultralinear Taps it appears that the feedback circuit is now much faster and I was able to push up the feedback levels quite a bit without any loss of imaging or introduction of too much diffamp distortion. Instead of the 10K + 9K1 + 10K, I was messing about with values of 22K, 27K and 33K in place of that 9K1, That is about 3 to 4 times as much feedback. When I got to be using 10K + 33K + 10K I decided to try 6K8 + 22K + 6K8 (identical feedback setting) instead to see if it might be a drive impedance issue. I thought that that did make a very small improvement. At the end of the weekend I ended up running 6K8 + 20K + 6K8 and not really able to make up my mind as to go one stage tighter (22K), one stage looser (19K1) or leave it alone. 15K was too loose and after some extended listening I decided 18K was not quite enough either. This has taken the "Baby" and given it better manners, BH is "growing up".
I haven't tried to apply this to the fixed biased version with the source followers yet - it may not be as applicable to that version.

Cheers,
Ian
  Reply With Quote
Old 1st June 2009, 04:08 AM   #804
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
One major FACT missing from the above. Before pushing up the feedback I made the amp a little kinder to feedback by linearising the speaker load impedance vs frequency. A 10 Ohm + 2u2 Zobel across the 4 Ohm Speaker tap. This make a big difference.
Cheers,
Ian
  Reply With Quote
Old 3rd June 2009, 03:52 AM   #805
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
And another hint from last nights experiments.

At higher feedback levels it was still sounding a little aggressive in its top end frequencies.

It suddenly clicked - the reference (AC 0V) for the shunt feedback (taken from the Ultralinear Taps) is the output transformer centre tap. I checked what I was running as an AC bypass on the B+ supply to this node and found that in this particular amp I had just a 100uF/400V electrolytic. Simply soldering in a 470nF/630V polypropylene cap across that 100uF cleaned up the feedback and got rid of 90% of that aggressive edge. I would guess that the self resonant frequency of that 100uF was well down into the audio band. Improving the AC ground at the Output Tranny centre tap had the secondary effect of cleaning up the feedback since its AC 0V reference was better.

This was a big step forward.

Cheers,
Ian
  Reply With Quote
Old 3rd June 2009, 04:07 AM   #806
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Quote:
Originally posted by gingertube
I checked what I was running as an AC bypass on the B+ supply to this node and found that in this particular amp I had just a 100uF/400V electrolytic. Simply soldering in a 470nF/630V polypropylene cap across that 100uF cleaned up the feedback and got rid of 90% of that aggressive edge. I would guess that the self resonant frequency of that 100uF was well down into the audio band. Improving the AC ground at the Output Tranny centre tap had the secondary effect of cleaning up the feedback since its AC 0V reference was better.
Have you tried an amp with all poly power supply caps?

dave
__________________
community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  Reply With Quote
Old 3rd June 2009, 04:23 AM   #807
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Dave,
A couple of the latter versions of the BH had significant (5 and 10uF) poly bypasses but I haven't built one with ALL poly cap supply. I will be doing just that on a PPP 300B (a quad of 300B per channel) Amp which is in the design phase.

The aim right now is to take the original BH (CCS biased circuit) and make it stunning without recourse to expensive bits. To that end I have fitted "cheap" Hammond 1608 Output Trannies back in this amp. With the last weeks or so of "fiddling" it is one of the best amps (mine or anyone elses, DIY or Commercial) I have ever heard. There is just 1 or 2% left (which I'm trying to bridge) between damn good and absolutely stunning. I'm exploring its limits if you like, and finding that the limits are often not where I originally thought, certainly the last week or 2 of mods/experiments has brought the performance up significantly with just the change to feedback from the UL taps, 3 resistor changes to increase the shunt feedback level and 2 added bypass caps - about a 100% performance boost for $5 worth of components.

Cheers,
Ian
  Reply With Quote
Old 6th June 2009, 06:17 PM   #808
adamus is offline adamus  United Kingdom
diyAudio Member
 
Join Date: Apr 2008
gingertube, i have not done the mods to the feedback resisors yet, but i would just like to say, this amp has impressed so many that have listened. There is something about it that sets it apart. Its not the most neutral or detailed, but so many of my hypercritical mates have gone away very impressed.
  Reply With Quote
Old 7th June 2009, 09:42 AM   #809
adamus is offline adamus  United Kingdom
diyAudio Member
 
Join Date: Apr 2008
If i am going to put a zobel netwrok on the outputs, is is an exact science to spec the components (i know the formula)?

I am using the 8ohm taps by the way. what ratings on the resistors should i be looking at?
  Reply With Quote
Old 8th June 2009, 11:12 PM   #810
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
adamus,
I'm using 10 Ohms plus 2.2uF in series for the zobel across the 4 Ohm tap.That puts the corner on the response due to the zobel at 7200Hz.

For the 8 Ohm tap try 18 or 20 Ohms plus 1uF.

Cheers,
Ian.
  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
Red Light District/Baby Huey (EL84 P-P) Output Iron dsavitsk Tubes / Valves 3 14th February 2009 01:21 AM


New To Site? Need Help?

All times are GMT. The time now is 02:35 PM.


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