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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 4:19 am
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Can I simply pull 2 power tubes, to lower the output power.
Can this really be done?


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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 8:33 am
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timmyhatch wrote:
Can I simply pull 2 power tubes, to lower the output power.
Can this really be done?

There's a pattern to it,concerning which ones to pull so that it doesn't mess with the push/pull......I have never owned one with 4 PTs but have read the correct procedure,I just don't remember which ones are recommended to try.


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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 4:22 pm
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Yeah, you can remove the two outside tubes. This changes the impedance the output transformer wants to see. So now the amp would like an 8ohm load. Art

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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 7:58 pm
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The two outside PT's, being V7 and V10... Or V9 and V10?
Is this a save thing to do regarding the amp?


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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 8:17 pm
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Actually, you can pull any two output tubes & prolly not cause harm. You may get some different OD tones---maybe a more single-ended OD with un-paired pull. But, yes to maintain the push-pull symmetry (how symmetric the output stage is totally dependent on having matched pairs of tubes and matched ground-to-grid resistors) --- pull V7 and V10, as a pair. Or V8 and V9 as a set.

May want to rebias the amp... as pulling tubes often causes the plate voltage to rise.

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


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Post subject: Re: Super Six Reverb
Posted: Mon Mar 26, 2012 11:52 pm
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My concern is that this amp is a fixed bias...
Is there a web page you know of that can explain to me (a guitarist not an amp tech) how to rebias a fixed bias circuit?


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Post subject: Re: Super Six Reverb
Posted: Tue Mar 27, 2012 12:02 am
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Yes I have to apologize! What I mean is to remove the two outside tubes, as you look at the four power tubes. This would leave two tubes, close together. This gives you one tube on each side of the push-pull output. Like, I said this changes the output impedance. And like Beemer suggests, the plate voltage will rise a small amount. The hum balance control should take care of any noise. And yes, this is safe for the amp. The output transformer will not reach any where near a saturation point. So the amp should be very clean sounding. Now the speaker cabs, correct me if I'm wrong, Tim. In the amp cab you have 2X8ohm speakers in series, yielding 16ohms. And the extension cab is also, 2X8ohm speakers in series yielding 16ohms. So if you now run the amp speakers into the main spk input, and the extension cab into the ext spk input, we're looking at an 8ohm load. This would be fine for the amp. The sound should be kind of cool, not the same as a Super Reverb, but more independent minded. Let us know if you like the sound. Art

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Post subject: Re: Super Six Reverb
Posted: Tue Mar 27, 2012 2:45 pm
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Tim, the amp does not have a separate pot(s) to adjust the idle bias current. If you have the 100 watt Master Volume Super Six Reverb, the bias looks like it is set by the combination of a 1k-ohm, 3.3k-ohm, and a pair of 68k-ohm resistors. The way it is setup, realistically all you can do is read the voltages on each side of that 3.3 k-ohm resistor and hope it is somewhere near -63VDC on one side and -52VDC on the other.

But, this ONLY measures the applied negative voltage onto the grid. It doesn't measure the current flow through the tubes at idle. You can measure this with a Bias Tool Kit. But you cannot adjust it easily.

Ultimately, you'd like to have around 18-20 watts of idle dissipation per tube. If you plan on keeping this amp and using it as a work horse, I highly recommend you have a good tech install an adjustable pot in the bias supply. Maybe one pot for each push-pull pair, so you'd only need to buy matched pairs of 6L6GC. And this would also give you a bit more flexibility on which of the two pairs to pull and re-adjusting the bias.

Good luck, with all that.



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


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Post subject: Re: Super Six Reverb
Posted: Tue Mar 27, 2012 11:22 pm
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Assuming that the amp is already equipped with a grounded 3-conductor power cord, it might be possible to install a DP/DT short-bat toggle switch rated for 500V/3A where the amp's current "polarity switch" resides. Wire it to interrupt the B+ supply to -- alternately -- V7 and V10 or V8 and V9. Thus, it would not be necessary to physically "pull" any tubes to halve the amp's output power.

Arjay

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Post subject: Re: Super Six Reverb
Posted: Tue Mar 27, 2012 11:42 pm
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That's an interesting approach, Arjay. Wouldn't the output impedance still need to be considered ? Art

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Post subject: Re: Super Six Reverb
Posted: Tue Mar 27, 2012 11:59 pm
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Yes. Two-tube performance would require an 8-ohm load to keep the OT copasetic.

Arjay

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