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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 1:47 pm
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So is there a schematic available?

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 2:29 pm
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Not the best copy but it's all I have.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 2:37 pm
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Yeah, I can't really read it. Maybe you can e-mail it to me as an attachement?

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 3:11 pm
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This copy is a skosh more legible......

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Though it may still require an Optovisor to discern all of the component values and specs.

:mrgreen:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 3:54 pm
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That is better.

Am I correct in assuming that the amp uses power tube bias modulation for the vibrato? If so, is that why the bias is nonadjustible except for balance?

Since the amp doesn't have a bias adjustment, perhaps this procedure makes more sense:

1. Hook up bias probe and DVM per bias probe instructions.
2. Turn on main power.
3. After a few minutes, take amp out of standby mode.
4. Let amp warm up for about 15 minutes.
5. Measure cathode current of each tube.
6. Measure plate voltage of each tube.
7. Multiply cathode current (in milliamps) by plate voltage (in volts) for each tube. Product equals plate dissipation (in watts) at idle.
8. Adjust bias balance pot so that the cathode current for each tube is as close to each other as possible (2 to 3 ma is probably good) and remeasure plate voltage (plate voltage will change as cathode current changes). After each adjustment, let amp stabilize for about 15 minutes. Final cathode current and plate voltage measurements are not important as long as the product of the two is within the 50% to 70% range of maximum plate dissipation (max for a 6L6GC is 30 watts, so you want to see anywhere between 15 and 21 watts as the product of your measurements).

Note that for bias measurements it is actually the product of PLATE current and plate voltage that you are calculating, but cathode current is close enough for government work and gives you a little safety margin.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 4:24 pm
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No, it uses the standard trem roach (opto-isolator) system.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 4:38 pm
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bluesky636 wrote:
Yeah, I can't really read it. Maybe you can e-mail it to me as an attachement?



Here there are Bill;

http://www.webphix.com/schematic%20heav ... tr_rev.pdf

All schematics on Shematic heaven;

http://www.webphix.com/schematic%20heav ... ender.html


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 5:06 pm
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Retroverbial wrote:
No, it uses the standard trem roach (opto-isolator) system.

Arjay


Then why does the bias balance pot seem to be tied into the vibrato circuit?

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 5:07 pm
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stratele52 wrote:
bluesky636 wrote:
Yeah, I can't really read it. Maybe you can e-mail it to me as an attachement?



Here there are Bill;

http://www.webphix.com/schematic%20heav ... tr_rev.pdf

All schematics on Shematic heaven;

http://www.webphix.com/schematic%20heav ... ender.html


Thanks. :D

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Tue Sep 18, 2012 1:29 am
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You are welcome ; )


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Tue Sep 18, 2012 4:21 pm
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" Then why does the bias balance pot seem to be tied into the vibrato circuit? "

Bluesky, this circuit seems to wiggle the output of the 12AT7 PI. rather than the bias points. Just guessing, but I bet the wave form maybe a bit more squared off, than a tube oscillator or even an opto-bug altering the PI or gain stage grid or bias.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Tue Sep 18, 2012 4:51 pm
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BMW2002Ti wrote:
" Then why does the bias balance pot seem to be tied into the vibrato circuit? "

Bluesky, this circuit seems to wiggle the output of the 12AT7 PI. rather than the bias points. Just guessing, but I bet the wave form maybe a bit more squared off, than a tube oscillator or even an opto-bug altering the PI or gain stage grid or bias.


I think you are right.

Looking at the circuit again, I see that "B" from the Intensity pot is feeding the PI. The center lug from the bias balance pot is feeding the opto-isolator. Any DC voltage from the bias balance pot is blocked from V5 by the two 0.1 mfd caps and the 0.022 mfd cap.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 11:46 am
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Thanks for all the info on this. I got the tube test rig from Amphead.

I measured the old Sovteks I had in there and the new JJ's I bought to install after the rebuild. I could only locate a tube balance potentiometer "OUTPUT TUBES MATCHING". I set the JJ's so they are reading as close as possible. (see attached chart of readings)

The question I have is that this looks like idle is about 10 watts, which is far from the 50% idle power of the 6L6, more like 30%. Should I be concerned with this or add a couple potentiometers somewhere so I can adjust the current per tube instead of just match them?

Thanks!
Image
Image

Having trouble with the image tag.


Last edited by russpatterson on Wed Sep 26, 2012 12:48 pm, edited 3 times in total.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 12:32 pm
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russpatterson wrote:

The question I have is that this looks like idle is about 10 watts, which is far from the 50% idle power of the 6L6, more like 30%. Should I be concerned with this or add a couple potentiometers somewhere so I can adjust the current per tube instead of just match them?

Thanks!
Image
Image


Adress won,t work.

I understand that you need to do the bias, but there is no pot for that ? An easy mod is avalaible , I try to find a sketch to show you.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 1:00 pm
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Location: Natural Bridge, Virginia
russpatterson wrote:
Thanks for all the info on this. I got the tube test rig from Amphead.

I measured the old Sovteks I had in there and the new JJ's I bought to install after the rebuild. I could only locate a tube balance potentiometer "OUTPUT TUBES MATCHING". I set the JJ's so they are reading as close as possible. (see attached chart of readings)

The question I have is that this looks like idle is about 10 watts, which is far from the 50% idle power of the 6L6, more like 30%. Should I be concerned with this or add a couple potentiometers somewhere so I can adjust the current per tube instead of just match them?

Thanks!
Image
Image

Having trouble with the image tag.


Something is definitely amiss. I would suggest you measure all your power supply voltages, particulalry the bias supply. According to the schematic, you should have -58 VDC provided by the bias power supply applied to the balance pot.

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