Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Stability question for the gurus
Stability question for the gurus
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 11th January 2018, 12:32 AM   #1
EL34Dave is offline EL34Dave  United States
diyAudio Member
 
Join Date: Apr 2010
Default Stability question for the gurus

This is in regard to my custom home-built ST-70. After reading a lot about this, I am considering removing the Zobel stability network (series RC shunting to ground) between the input stage and the phase splitter. And in addition removing the cap in the GNFB loop. As I understand it, these are meant to reduce the possibility of HF oscillation, but may not really be necessary. My GNFB is approximately 15db (which I believe is fairly moderate). The OPT's are Hammond 1650K.

So I really have two questions, 1) do you think this is a wise mod (ie. will it really improve the sound?), 2) If I do this, how can I tell if it has introduced any adverse behavior? Please keep in mind that I don't have any test equipment, other than my old ears.

Thanks,
Dave
  Reply With Quote
Old 11th January 2018, 01:43 AM   #2
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by EL34Dave View Post
1) do you think this is a wise mod (ie. will it really improve the sound?),
2) If I do this, how can I tell if it has introduced any adverse behavior?
Depends on the circuit and output transformer. In the original ST70 circuit,
removing the RC on the input pentode's plate will cause strong ringing.
The sound will have a hard, pinging quality. A scope is necessary to adjust
the capacitor value to be smaller but still not cause ringing, but don't change
the resistor's value. The speaker load can affect the tuning as well.

Last edited by rayma; 11th January 2018 at 02:00 AM.
  Reply With Quote
Old 11th January 2018, 04:30 AM   #3
petertub is offline petertub  Sweden
diyAudio Member
 
Join Date: Jul 2015
Location: gothenburg,sweden
If you don't have any test equip then the suggested changes ( or any changes) are detrimental
for the amp.
__________________
My home is at www.tubular-well.se
  Reply With Quote
Old 11th January 2018, 04:47 AM   #4
leadbelly is offline leadbelly  Canada
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Stability question for the gurus
Good point about not attempting without test equipment. However, this is an ST-70. Just about every permutation and combination of input stage mod is already worked out and published. Just google it. If you have books like Morgan Jones, there's one in therr as well.
__________________
There are still unbridgeable divisions between cultures. How can two peoples ever come to understand each other when one of them is ignorant of TotÚ? Umberto Eco
  Reply With Quote
Old 11th January 2018, 05:34 PM   #5
richwalters is offline richwalters
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Schwartzwald
Quote:
Originally Posted by petertub View Post
If you don't have any test equip then the suggested changes ( or any changes) are detrimental
for the amp.
spot on.....Don't do it. If you tinker with the values, one could end up destroying a precious tweeter without knowing one has done it....and a mystery orange glowing anode from an output tube. One of the tell-tale signs of amp instability in reactive loudspeaker loads is momentary squealing when the amp is turned on/off. It may even pick up an AM station too.

rich
  Reply With Quote
Old 13th January 2018, 08:38 PM   #6
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
It is hard to tell how much different the phase response versus frequency, and the frequency response of the Hammond 1650K transformers are, versus the
original Dyna A470 output transformers.

It may turn out that you need to find a person in your area (and highly suggested for you to do so) who has the following:
An Oscilloscope
A square wave generator
A non inductive 8 Ohm power resistor load
The knowledge to know how to adjust the feedback loop(s).

There is a dominant RC pole in the pentode plate circuit.
There is a feedback capacitor from the (correct) ultra linear taps back to the pentode circuit.
There is the global feedback network.
All of these, plus the A470 Output transformer are what makes the Dyna Stereo 70 as stable as it is with different loudspeakers.

If I remember, there was about 26 dB of global feedback in the ST70.

If done and adjusted properly, it should be possible to use 10 dB or so less global feedback. That actually should be easier to optimize than 26dB. But the frequency response, distortion, and damping factor will likely be slightly worse.
However, it might make the amp better with some types of loudspeakers that have more reactive loads.

In some cases, it may be better to use one tap lower, i.e. 8 ohm speaker on the 4 Ohm tap.
  Reply With Quote
Old 13th January 2018, 08:59 PM   #7
Wavebourn is offline Wavebourn  United States
Designer & Technologist
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Stability question for the gurus
One "ST70 Guru" on facebook Vacuum Tube Amp forum insisted that dominant pole compensation in ST70 is not needed. May be he was right.
__________________
Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
  Reply With Quote
Old 13th January 2018, 09:01 PM   #8
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Try any feedback scheme you want. Just not on my speakers.

I will have to use the scope, generator, and resistive load first.
Only after that, when I connect to loudspeakers I still watch the scope (my hearing does not go to as high of a frequency as my loudspeakers, but I do not want to burn out the tweeters).

Last edited by 6A3sUMMER; 13th January 2018 at 09:22 PM.
  Reply With Quote
Old 14th January 2018, 08:41 PM   #9
Globulator is offline Globulator  United Kingdom
diyAudio Member
 
Globulator's Avatar
 
Join Date: Aug 2004
Location: Cambridge, England.
The problem with the ST-70 circuit is one shared with many circuits: the designer has decided to include a laggy band limited piece of iron inside a GNFB loop - which needs to avoid and laggy band limited device in order to function correctly. This is a hangover from the faulty 'Williamson' method of GNFB.

The phase lag can turn your ST-70 into a nice oscillator to blow out your tweeters so that's what the RC is for - to damp down HF in the loop.

While it's not sabotaging your GNFB loop the OPT is band limiting it so that the bass and treble don't get as much feedback as the midrange, which is unfortunate because they are the areas that generally need it most.

Then if you increase the feedback to compensate the lag turns your ST-70 into an oscillator again. All of these problems are caused by the OPT being in the loop.

Therefore the only way to avoid these issues is to remove the laggy, band limited device from your feedback loop in the same manner that a SET amplifier does. However you'll still be wanting the same power as the original ST-70 had originally, so just put the GNFB loop around the driver and power tube - or all the tubes - instead. Do this to turn your multiple tubes into a 'superTriode' - i.e. a collection of tubes with GNFB around them to create a single linear, low drive impedance unit - like a perfect triode in fact.

This will then drive the OPT correctly with no lag or bandwidth issues so oodles of NFB can be used, and finally the OPT can be driven properly and will sound the best it can with full stability.
__________________
Battling the Loudness War with SeeDeClip4, multiuser decompressing, declipping music server.
Currently running on a lenovo through some Grundig Box 550's
  Reply With Quote
Old 23rd February 2018, 10:10 PM   #10
EL34Dave is offline EL34Dave  United States
diyAudio Member
 
Join Date: Apr 2010
Is this why the original Dynaco amps connected the feedback capacitor to the input side of the OPT (specifically the UL tap), whilst connecting the feedback resistor to the output side (16 ohm tap)?
  Reply With Quote

Reply


Stability question for the gurusHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Question for the amp gurus thejessman Class D 4 21st September 2016 08:43 PM
Need help in finding stability margins/loop stability in LTspice for my amp maouna Solid State 32 1st February 2014 06:21 PM
WinXP question for gurus. This is a tough one! mpmarino The Lounge 0 5th March 2006 03:15 AM
Quick crossover schematic question for you gurus :-) Florian Parts 2 1st November 2005 09:24 AM
Question for SE Transformer Gurus frankmalz Tubes / Valves 5 14th January 2004 07:36 PM


New To Site? Need Help?

All times are GMT. The time now is 02:43 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki