ST-70 mono makeover - Page 3 - 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 December 2012, 12:08 PM   #21
diyAudio Member
 
Join Date: Aug 2008
If I wanted to have another go at trying feedback with the LED bias, should I just add 100 ohms between the LEDs and the ground and use the point between the LEDs and the resistor to inject the FB?

Thanks!
  Reply With Quote
Old 13th December 2012, 01:29 PM   #22
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
What your measurements appear to show is a highish output impedance so voltage rises at speaker impedance peaks (16Hz (bass resonance?) and 2000Hz (crossover or tweeter resonance?)). Was this with or without feedback? My guess is without - as this lumpy frequency response into a real speaker is exactly what I would expect to see without feedback.

If you are using LED bias for the first stage then inject feedback below the LED, although above probably won't do any harm either.
  Reply With Quote
Old 13th December 2012, 01:38 PM   #23
diyAudio Member
 
Join Date: Aug 2008
Hi,

Yes, this is without feedback. Yep, it is all easy to see looking at the impedance profile where these things occur.

If I inject below the LED, do I need to put a 100 ohm resistor between the LES and ground?

What value would you start with for a feedback resistor?

Thanks everyone for all your help!
  Reply With Quote
Old 13th December 2012, 01:44 PM   #24
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by deicide67 View Post
Hi,


What value would you start with for a feedback resistor?

Thanks everyone for all your help!
Blair,

Have you determined what the gain of the amp is?
  Reply With Quote
Old 13th December 2012, 01:58 PM   #25
diyAudio Member
 
Join Date: Aug 2008
Hi 20,

At 1K with .101V input, I get 11.42VAC output. So, 113.07 is the total gain factor right?

11.42vac/.101vac= 113.07
  Reply With Quote
Old 13th December 2012, 02:05 PM   #26
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes. What gain do you want? Or, what feedback factor do you want? What values of feedback and cathode resistors gave you the bass problem?
  Reply With Quote
Old 13th December 2012, 02:10 PM   #27
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by deicide67 View Post
Hi 20,

At 1K with .101V input, I get 11.42VAC output. So, 113.07 is the total gain factor right?

11.42vac/.101vac= 113.07
Yep, just don't mix your scope against VOM readings. (peak vs. RMS)

I did a quicky calc from your table and came up with 225, ( 4.5/.02 ) so not sure why those numbers are so much higher.
  Reply With Quote
Old 13th December 2012, 03:29 PM   #28
diyAudio Member
 
Join Date: Aug 2008
Here are the results from a few FB resistor values. Looks like 33K is the most linear to me.

I had to inject the FB between the LEDs to get it to work. Is that a no no?

Click the image to open in full size.
  Reply With Quote
Old 13th December 2012, 03:42 PM   #29
20to20 is offline 20to20  United States
diyAudio Member
 
20to20's Avatar
 
Join Date: Jun 2010
Location: W-S, NC
Quote:
Originally Posted by deicide67 View Post
Here are the results from a few FB resistor values. Looks like 33K is the most linear to me.

I had to inject the FB between the LEDs to get it to work. Is that a no no?
The table shows your gain increases with FB. That cannot be allowed as that would be positive FB.

LED's are a gimmick. Get your amp working with conventional means first.

If you use a pot for the FB resistor you can dial down the gain to make your input sensitivity realistic and give you the output power max you want. Then set it with a fixed resistor. But remember as you change the FB resistor which is in parallel with the driver cathode resistor you will shift the biasing. So start with a slightly higher K resistor value and then add in the FB signal through the pot. If you are just stabbing different FB resistors onto the cathode you are shifting the bias point without realizing it.

Last edited by 20to20; 13th December 2012 at 03:47 PM.
  Reply With Quote
Old 13th December 2012, 04:02 PM   #30
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Often, amps are set up so that about 1v input drives them to full output... this is NOT a requirement by any means, just what is approximately the usual situation.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  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 10:44 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