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Post subject: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 1:19 pm
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Hello all. This is my first post here although I have been watching for awhile. My question is, "Should I mod my 1973 super reverb?" It is the aa270 circuit and what I'm thinking about doing is adding a bias adj pot along with the balance pot that is there now. I checked it with a bias probe and it read 24ma with 460 plate volts. Power tubes are sed winged c' and I felt like they should be running quite a bit hotter? I took the amp to a local shop and they used a scope and told me it was fine. I am no expert on this. I think the amp sounds good but I can't help thinking it could sound better. bluestrucker.


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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 1:49 pm
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this should work

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


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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 2:03 pm
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joncarr1979 wrote:

Thanks joncarr, looks like a good idea.


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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 3:01 pm
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24 mA is mighty cool IMO. I run all of my 6L6 platforms at 33 to 35.

Methinks you oughta be lookin' for a new amp tech.

A bias adjust pot in conjunction with the extant balance control would be beneficial.

As for improving the amp sonically, you must remember that this amp is loaded with tone-sucking coupling and isolation caps that FMI's einsteins added to increase the clean headroom. There's a bit of trickery involved if you wish them removed in order to emulate the AB763 circuit.

Arjay

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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 4:24 pm
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I spoke with a amp builder about doing a blackface mod and he wanted $300 for the job. Its more than I can afford right now. I was hoping to do a little at a time. The bias adj looked like a easy first step but I'm not sure I could hear the difference. The place where I purchased the tubes said they sound best at 50 ma. That seems like a big jump. I have been told that since I play at low volume ( 4 ) most of the time It wouldn't make a big difference. Thanks for all the help. JD


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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 5:13 pm
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I think it is the opposite ! If the current bias is very high, at low amplitude you can stay in A class amplifier and very clean sound, on the contrairy, if the current bias is low, at low amplitude you swap from A to B and may have more distortion.
Anyway, the =C= are becoming collector ! keep 'em !

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Post subject: Re: ? on 1973 super reverb
Posted: Sun Jan 09, 2011 5:17 pm
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bluestrucker wrote:
The place where I purchased the tubes said they sound best at 50 ma.


BWAHAHAHAHAHAHAHAHA!!!

The place where you purchased those tubes makes a handsome living selling still-more tubes to the saps who believe what they say.

:roll:

Arjay

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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 3:44 am
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(24 mA is mighty cool IMO. I run all of my 6L6 platforms at 33 to 35.) Right now I'm thinking that the amp tech. that checked it out with the scope is right. (24ma to 35ma sound like a very small change). He has been working on amps for a long time. What I would realy like to do is set down and a-b it with one thats been modified.
I,m running out of time this morning but I'll be back! JD


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 5:18 am
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There 2 kind of 6L6's

6L6 or 6L6 GB are max 19 watts

6L6 GC are 30 watts, the most used

Yes 24 ma at 460 volts is very very cold : amp sound is thin.

average bias should be around 40 to 42 ma for 6L6 GC


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Post subject: Super Reverb
Posted: Mon Jan 10, 2011 8:49 am
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I glance over the AA270 and AB763 circuits of the Super Reverb & there doesn't seem to be too big a difference. The main one being that the earlier AB763 used a GZ34 tube. The AA270 called for a 5U4GB. There is also changes around the circuitry of the phase inverter. Trannies all seem to be the same part number. You may want to experiment with 5U4GB and GZ34 rectifiers, first, to see if that changes the tone more to your liking.

Just a suggestion. Good luck with that GREAT amp!




http://www.ampwares.com/amp.asp?id=98


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 10:55 am
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The schematic says the plate voltage should be about 437 so the bias could be from 34mA to 48mA. but it depends on what tubes you are using and the actual plate voltage.But 24mA is way to cold of a sound, you should have at least be minimum be 35mA.that's what i have my vibrolux at and i9 have similar plate voltages


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 11:51 am
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BMW2002Ti wrote:
You may want to experiment with 5U4GB and GZ34 rectifiers, first, to see if that changes the tone more to your liking.


Not advisable.

If you examine the power supplies of each circuit, you'll note a 100-ohm resistor tying both legs of the 6.3 volt secondary to ground on the AA270 revision. That's because the 5U4GB requires substantially more heater current than a GZ34. Substituting a GZ34 into this chassis will likely result in premature rectifier failure.

Arjay

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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 2:22 pm
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The two 100 resistors are there to remove the filament hum. This is use ( and a must if you want to remove some hum) if the transformer filament winding have not center tap to ground. There nothing to do with different voltage or current filament of rectifier tubes.

And yes you can use GZ34 tube in place of 5U4 with no problem. GZ34 is a better tube.


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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 2:29 pm
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Stratele:
Quote:
The two 100 resistors are there to remove the filament hum.
. Do you know exactly why ?

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Post subject: Re: ? on 1973 super reverb
Posted: Mon Jan 10, 2011 2:51 pm
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I'm going to check around to find another amp tech. The shop I took it to to have the =c= biased used a scope and gave me no numbers. He just said it was working fine. What would be a fair price to add the bias pot and change what is needed for the phase inverter? Thanks to everyone here for there help. The last estimate I got to have the amp "blackfaced" was $300. It seems very high and he realy didn't say what was going to be done. Money is tight for an old trucker like me but I love my music. JD


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