# Peavey Classic 60/60 power amp circuit question



## JivRey (Jul 2, 2016)

Hello!

I have a Peavey Classic 60/60 stereo tubes power amp. It has 4x 6L6GC, two per input. I plan on using only one side of the power amp.

Looking at this schematic the two inputs look completely separated..

Question: Since technically two of the power tubes are not in use, is it possible to completely remove the pair of power tubes of the Input 2 circuit and only use Input 1 safely without compromising the power amp and the transformer(s)?

I don't think it's necessary but for $10, I have hooked a 8 ohms 100w dummy load in the second output even if I don't use it.

Thanks for your help!


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## nonreverb (Sep 19, 2006)

Technically, if there's no signal going to the unused power amp, you don't need a load at all whether the tubes are there or not. There's no power (signal) transfer taking place therefore no load considerations...they're just idling through the primary winding of the output transformer. If you're concerned about bleed to that channel or long term idle plate dissipation, yes, you can remove the power tubes it shouldn't affect the 6.3V heater supply.


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## Granny Gremlin (Jun 3, 2016)

nonreverb said:


> Technically, if there's no signal going to the unused power amp, you don't need a load at all whether the tubes are there or not. There's no power (signal) transfer taking place therefore no load considerations...they're just idling through the primary winding of the output transformer. If you're concerned about bleed to that channel or long term idle plate dissipation, yes, you can remove the power tubes it shouldn't affect the 6.3V heater supply.



Not arguing, just trying to fill out knowledge: how do you know that the heater supply won't spike with less load? Had that happen (I think - so a tech told me) in an old Ampex mic amp - removed the playback repro tubes and it kept blowing the nuvistor inputs. Something about series vs parallel wiring on the heater rail IIRC but I'd like to understand this better.


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## Paul Running (Apr 12, 2020)

The power supply will have less B+ load when only one channel is drawing power, therefore more reserve energy...less voltage sag in the high tension line, a reduction in distortion. If you are only using one channel regularly, maybe consider a monoblock amp.


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## nonreverb (Sep 19, 2006)

Granny Gremlin said:


> Not arguing, just trying to fill out knowledge: how do you know that the heater supply won't spike with less load? Had that happen (I think - so a tech told me) in an old Ampex mic amp - removed the playback repro tubes and it kept blowing the nuvistor inputs. Something about series vs parallel wiring on the heater rail IIRC but I'd like to understand this better.


Not really a huge consideration IMO. It's a turns ratio. Test any power transformer for a guitar amp. It should be roughly 6.3VAC with no tubes in. Therefor, it's a current handling question. The above schematic indicates parallel tube configuration so it's not voltage drop dependent....besides if it was, removing any tube in the series chain would disconnect the heater to all tubes in it.
At the very worst, you might shorten the tube life it it were to rise a significant amount but we're talking 10 or even 15% rise and that is assuming the transformer has an unloaded voltage of over 6.3VAC.
If one were really concerned, meter testing the 6.3VAC with the two tubes removed will tell you.


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## Hammerhands (Dec 19, 2016)

Can that run in bridged mode?

No mention of it.



https://peavey.com/manuals/80301119.pdf


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## dtsaudio (Apr 15, 2009)

Filament shouldn't be affected too much other than your power transformer will probably run a lot cooler.
What I would be more concerned about is the high voltage rise. I would check that before running this too long without the extra pair of tubes. Your voltage may increase to the point you could damage some capacitors.
Peavey is not known for using the best parts, and the caps may already be run to their limits.
If all voltages are under the part ratings then go for it.


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## nonreverb (Sep 19, 2006)

dtsaudio said:


> Filament shouldn't be affected too much other than your power transformer will probably run a lot cooler.
> What I would be more concerned about is the high voltage rise. I would check that before running this too long without the extra pair of tubes. Your voltage may increase to the point you could damage some capacitors.
> Peavey is not known for using the best parts, and the caps may already be run to their limits.
> If all voltages are under the part ratings then go for it.


I wouldn't think so since they're using series capacitors in the B+ supply. @700V the rise wouldn't be nearly enough to surpass that. It might shift the bias point a bit but not to the degree where it would be problematic IMHO.


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## JivRey (Jul 2, 2016)

Thanks everyone for your input. I guess I'll start by measuring voltages at a few spots with all the tubes then try with only two and see what's going on. Your answers have been really helpful!


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## Robhotdad (Oct 27, 2016)

Granny Gremlin said:


> Not arguing, just trying to fill out knowledge: how do you know that the heater supply won't spike with less load? Had that happen (I think - so a tech told me) in an old Ampex mic amp - removed the playback repro tubes and it kept blowing the nuvistor inputs. Something about series vs parallel wiring on the heater rail IIRC but I'd like to understand this better.


Granny is on to something. I have a prototype amp on the design bench in which I use a Marshall 100W transformer with 6L6GC power tubes. The 6.3 Vac secondary is rated @ 7 amps. If you don't load that secondary properly your 6.3 V rises to 6.5 V or higher. I had some power tubes react badly to that higher heater voltage. Marshalls use 6CA7s and their heaters take 1.5 amps. 6L6s heaters take 900mA. I needed to dummy load the heater secondary to make the 6L6s happy. In your case, I suggest you keep a set of tubes in on the unused side or install a load resistor on the heater lines to take up about 1.8 A. I had a power amp like that and I kept tubes on the unused side, no problem.


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## nonreverb (Sep 19, 2006)

Robhotdad said:


> Granny is on to something. I have a prototype amp on the design bench in which I use a Marshall 100W transformer with 6L6GC power tubes. The 6.3 Vac secondary is rated @ 7 amps. If you don't load that secondary properly your 6.3 V rises to 6.5 V or higher. I had some power tubes react badly to that higher heater voltage. Marshalls use 6CA7s and their heaters take 1.5 amps. 6L6s heaters take 900mA. I needed to dummy load the heater secondary to make the 6L6s happy. In your case, I suggest you keep a set of tubes in on the unused side or install a load resistor on the heater lines to take up about 1.8 A. I had a power amp like that and I kept tubes on the unused side, no problem.


If one is worried, take all the tubes out and measure the heater winding unloaded. If it's within 5 -10% of 6.3V you're safe.


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## friendshappy (6 mo ago)

nonreverb said:


> If one is worried, take all the tubes out and measure the heater winding unloaded. If it's within 5 -10% of 6.3V you're safe.


will also try this 


Sent from my iPhone using Tapatalk


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## Paul Running (Apr 12, 2020)

Current regulation of the filaments would prevent excess power draw from the filaments and also eliminate current surge during cold start-ups, probably considered too elaborate for this application unless you were dealing with valuable tubes.


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## nonreverb (Sep 19, 2006)

Paul Running said:


> Current regulation of the filaments would prevent excess power draw from the filaments and also eliminate current surge during cold start-ups, probably considered too elaborate for this application unless you were dealing with valuable tubes.


They're all valuable now.


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