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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 1:15 pm
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To reach 50 % of power, you have to reduce the negative voltage avalaible at each 6L6's grid ( pin 5 ) . Schematic said -56 volts DC

To reduce this voltage you have to put a resistor across the 70 MFD 100 volts bias capacitor. This resistor will divide negative voltage. And reduce neg volt to 6L6's grid pin 5.

1- Before you should take a mesurement of how much neg volt you have at all pin 5 .

2- Trying resistor ; remove all 6L6 . Mesurement at pins 5 must same as in 1

3- Put a resistor like a 50 k or higher ,( It is trial and error ) across the 70 mfm 100 v capacitor. All 6L6 removed.

4- Take mesuremet at pin 5 ( 6L6 ) If negative voltage is less than -56 and more than - 30 volts DC.

5- Put 6L6s in and check tubes bias if close to 50 % power. Bee careful to not go overpower dissipation. You may have to try many resistor to have the good bias.

Be very carefull ; amp have lethal voltage. Filter capacitors must be discharge before working in the amp. Always have power OFF AND amp unplug from the wall outlet when working in the amp.
If you never work in a amp DON'T DO THIS MOD.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 2:16 pm
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stratele52 wrote:
3- Put a resistor like a 50 k or higher ,( It is trial and error ) across the 70 mfm 100 v capacitor. All 6L6 removed.


+1

I seem to recollect that 68K is about optimum for these UL amps.

Arjay

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 2:44 pm
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Why not just put in a 50K pot with a 25K resistor and make it adjustible? :!:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 3:49 pm
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bluesky636 wrote:
Why not just put in a 50K pot with a 25K resistor and make it adjustible? :!:


+1

This could be the second step after he find the good resistor . Replace it with a pot and another resistor .


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 4:51 pm
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stratele52 wrote:
bluesky636 wrote:
Why not just put in a 50K pot with a 25K resistor and make it adjustible? :!:


+1

This could be the second step after he find the good resistor . Replace it with a pot and another resistor .


Why go through the hassle of finding the right resistor? You suggested starting with 50K ohm, and Arjay said he thought a 68K ohm was optimal. A 50K ohm pot and 25K ohm resistor gives you a range of 50K to 75K ohm. Covers all bases.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 6:29 pm
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bluesky636 wrote:
A 50K ohm pot and 25K ohm resistor gives you a range of 50K to 75K ohm. Covers all bases.


+1

And offers the tweakability for biasing different brands of tubes.

Arjay

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Sep 26, 2012 10:06 pm
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Retroverbial wrote:
bluesky636 wrote:
A 50K ohm pot and 25K ohm resistor gives you a range of 50K to 75K ohm. Covers all bases.


+1

And offers the tweakability for biasing different brands of tubes.

Arjay


Makes more sense than mucking around with individual resistors, trying to find "the one". :lol:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 2:06 am
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I don't have any experience with this amp . I do my best for a short an easy answer . IMO it is more easy for russpaterson to start with one resistor .

But if Arjay know the right pot and resistor and russpaterson can install those , this is better


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 3:11 am
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No offense you guys. But I'd leave the 70 cap alone, the vib would suffer the change. The 33k would be my point of focus. JMO Art

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 6:14 am
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aclempoppi wrote:
No offense you guys. But I'd leave the 70 cap alone, the vib would suffer the change. The 33k would be my point of focus. JMO Art


Please explain. Looking at the schematic, it appears that the 33K ohm resistor is what provides the voltage to the opto-isolator so that changing that resistor will definitely impact the trem circuit.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 6:32 am
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This book covers the surgery required in an easy to understand format
It has a chapter for this exact issue 8)


http://www.amazon.com/How-Hot-Rod-Your- ... 60338477#_

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 6:48 am
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blues bondsman wrote:
This book covers the surgery required in an easy to understand format
It has a chapter for this exact issue 8)


http://www.amazon.com/How-Hot-Rod-Your- ... 60338477#_


I have the same book and it does show how to convert a bias balance pot on a 70 Siverface Super Reverb into a bias pot, but since the OP's amp is a later, ultra linear amp and the schematics are different, I was reluctant to recommend it.

Great book in any case. I refer to it a lot. :D

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 7:17 am
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Ok... I know a dumb reminder... but if you are altering the C- to ground resistor in the bias circuit. Remember, that the HIGHER the resistance the MORE negative voltage onto the grid (and thus LOWER current flow through the output tubes). Gotta think in negative voltages.


You know, I'd keep that balance/hum pot. Then, you won't need to buy closely matched 6L6GC's. You can add a bias pot, if you wish to the present circuit without removing or altering the balanace pot. You'll just need to tap one more hole for the added bias pot.


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

For a Twin reverb. But will work on your Super reverb:

http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm


Note: edit!


Last edited by BMW2002Ti on Thu Sep 27, 2012 7:59 am, edited 1 time in total.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 8:05 am
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BMW2002Ti wrote:
Ok... I know a dumb reminder... but if you are altering the C- to ground resistor in the bias circuit. Remember, that the HIGHER the resistance the MORE negative voltage onto the grid (and thus LOWER current flow through the output tubes). Gotta think in negative voltages.


You know, I'd keep that balance/hum pot. Then, you won't need to buy closely matched 6L6GC's. You can add a bias pot, if you wish to the present circuit without removing or altering the balanace pot. You'll just need to tap one more hole for the added bias pot.


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

For a Twin reverb. But will work on your Super reverb:

http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm


Note: edit!


Glad you made that edit. I have been trying to post a response disagreeing with your original statement but the system would not let me do it for some strange reason. :lol: :lol: :lol:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 8:38 am
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aclempoppi wrote:
No offense you guys. But I'd leave the 70 cap alone, the vib would suffer the change. The 33k would be my point of focus. JMO Art


Sorry but I just see a 3.3 K not 33K . Where is it ?


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