ST-70 mono makeover - 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 12th December 2012, 04:21 PM   #11
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The feedback may be introducing LF instability, or at least an LF peak. You have 3 LF rolloffs within the loop: two coupling caps and the OPT. You need to ensure that these do not coincide, or anywhere near, otherwise you will get at the very least an LF/subsonic peak in the same region.
  Reply With Quote
Old 12th December 2012, 04:30 PM   #12
diyAudio Member
 
Join Date: Aug 2008
Thank you,

Makes sense, and the last time I used this topology or a variant, the feedback was injected at the grid of the lower triode of the phase inverter which eliminates one coupling stage. I tried both ways, and it was about the same.

The next question is, how important is Feeback? Do all amplifiers need it?

What/how would the best way to add feedback to this amplifier be? It sounds quite nice, I just want to make sure I am getting "everything" out of it.

Thanks again!

Blair
  Reply With Quote
Old 12th December 2012, 05:08 PM   #13
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
A little over my head but I'm watching. On a stock 1961 dynakit ST70 with new B+ cap and 5AR4 (stock B+ voltage) I got about 8 years out of a quad of 1970 build GE 6CA7's. Maybe 8000-10000 hours on the output tubes? I was checking output voltage into 8 ohm resistors after B+ voltage was okay each time the sound got flabby, that is how I decided to change 6CA7. The 5AR4 was changed when sound got flabby and B+ was low after new HV caps were installed. I've got a quad of matched JJ 6CA7 now on new e-caps and old 5AR4, don't know how they will last at the high voltage ultralinear, which is reputed to be hard on them.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 12th December 2012 at 05:13 PM.
  Reply With Quote
Old 12th December 2012, 06:41 PM   #14
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The method of applying feedback is fine. It is the LF rolloffs you need to address, also the amount of feedback. Measure/calculate the 3 rolloffs. The quick and easy way to get stability is to ensure that one dominates. This may mean reducing the value of one of the coupling caps to make that rolloff dominant, or increasing them to make the OPT the dominant one. Not that this sort of issue is one reason why it is not possible to simply add or remove feedback from an amp; the right cap values for feedback may be different from those for no feedback.
  Reply With Quote
Old 12th December 2012, 08:06 PM   #15
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
The other thing besides the report of distortion is that the damping factor will get higher (lower output Z), so the nice bass might reduced by feedback depending on the Q of the speaker as it rolls off.

And no, feedback is not a requirement at all.

Check your frequency response without feedback. IF you have good iron it will be pretty flat out to 20kHz, and the iron looks big enough to handle the power, assuming enough turns on the primary then there is ample inductance for good LF response.

If you test the response up through 100kHz you will find a bump up somewhere above 10kHz, usually between ~40kHz. and 100kHz. The higher that bump, the better the HF response of the iron.

I'd bypass that LM317T with a cap... don't trust it at HF etc...

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 12th December 2012, 08:21 PM   #16
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Most speakers need a highish damping factor for good bass. Low damping factor with these means lots of bass, not good bass. If you want flattish frequency response with most speakers then this usually needs a lowish output Z; not enough to merely measure a flat response into a resistor.

In the second design (post 10) the 317 is the CCS in the LTP tail, so it must not be bypassed.
  Reply With Quote
Old 12th December 2012, 08:50 PM   #17
diyAudio Member
 
Join Date: Aug 2008
correct, it is the CCS for the LTPI.

As much as it hurts my ears, I did some measurements into my speakers today. I will need aspirin, but compared to the same output voltage on my Emotiva and moving up in one octave steps, the tube amp definitely has a few rolloffs.

The iron is from the ST-70, so it should have a pretty good FR.

It would seem that testing into a dummy bank of resistors would be the most accurate for FR? The speakers have impedance profiles, and the load is reflective to the primary right? So, if my speakers are 2ohms at 20hz, but 6 ohms at 20K, then the amount of power generated is different right? Sorry for the terminology

Blair
  Reply With Quote
Old 12th December 2012, 09:57 PM   #18
diyAudio Member
 
Join Date: Aug 2008
Here is what I'm talking about. Look at the insanity into my speakers vs. a dummy load.

Click the image to open in full size.
  Reply With Quote
Old 13th December 2012, 12:23 AM   #19
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Why are your speakers 2 ohms anywhere?

Anyhow, you have to convert your AC to RMS and then into dB to see what is going on.

But, that looks very flat indeed.

I'm not sure what an "Emotiva" is??

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 13th December 2012, 12:37 AM   #20
diyAudio Member
 
Join Date: Aug 2008
Hi Bear,

Emotiva is my SS amplifier I used into the same load just as a benchmark of sorts.

My meter is an RMS meter. Is that what you mean?

As for the load of my speakers, they dip to about 2.5-3 ohms around 25hz.other than that, they look like an almost perfect 4 ohm load. I have the impedance profile, so that's how I know.

If you look, my source also drops output voltage where the upper end rolloff is occurring, so it is actually fairly decent like you suggest.

Blair
  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
What are your opinions of the ST-70 and St-120 ElCid79 Tubes / Valves 44 21st June 2014 03:24 PM
St 35 or St 70? scetoaux Tubes / Valves 13 19th February 2011 05:18 PM
College-proofing an ST-70 (aka autobias+ST-70) antiquekid3 Tubes / Valves 15 13th June 2010 03:37 AM
Which ST-70 ? percy Tubes / Valves 21 4th September 2004 06:11 PM
St-70 Jack Thomas Tubes / Valves 1 13th January 2003 07:24 PM


New To Site? Need Help?

All times are GMT. The time now is 07:44 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