Lowering gain for a tube power amp

[armstronguser]

Hi

I have a pair of armstrong A10 amp, and I am pairing it with a modern pre amp that it does not have volume control for the left and right channel.

One of the power amp is giving considerable louder sound compared to the other one.

Can anyone offer any suggestion to lowering the gain or lowering the output from the amp play loud such I can get a better balance when playing?

I note the circuit of the amp has a grid resistor or 27k ohm connected to the grid from input together with a grid leak resistor of 1 M ohm value. I suppose I can make a minor mod to these resistors to lower the gain, any suggestion on the values I should try?

If I change the above resistors, would the quality of sound degrade?

Thanks for help

 

[SemperFi]

Before modifying shouldnt u find out what is wrong with one of the channels? Either the quiet one, or loud one, is having issues. Have u tried swapping L R tube sets to see if volume difference follows?

 

[armstronguser]

Thanks SemperFi for the reply.

That sounds logical, let me try first. Just so happen, the amp we’re back from a full restoration by my friend, The tube swap never came to my mind. Thanks for advice.

 

[petertub]

Hi

I have a pair of armstrong A10 amp, and I am pairing it with a modern pre amp that it does not have volume control for the left and right channel.

One of the power amp is giving considerable louder sound compared to the other one.

Can anyone offer any suggestion to lowering the gain or lowering the output from the amp play loud such I can get a better balance when playing?

I note the circuit of the amp has a grid resistor or 27k ohm connected to the grid from input together with a grid leak resistor of 1 M ohm value. I suppose I can make a minor mod to these resistors to lower the gain, any suggestion on the values I should try?

If I change the above resistors, would the quality of sound degrade?

Thanks for help

If you have a pair of amps that is dissimular i'd start with figuring out

why they are dissimilar. Get schematics, and study the interior , note
all discrepancies for later correction.

If you have the means, inject a signal and note ac among the amps stages.
Changing tubes between them could be done, although it should not result
in large differences.
Have these amps always been dissimilar ? Or have something done with
either ? That could be a clue of where to start.

 

[GoatGuy]

PeterTub and SemperFi are giving good advice, ArmstrongUser. SemperFi's approach has the advantage of being a “good first move”, requiring the least amount of fiddling with internals, schematics, circuit tracing, and all that. Just carefully pop out the tubes in each set, and swap them.

IF the gain issue follows the tubes, well … obviously one (or very unlikely, more than one) of them is out-of-spec. This could be intrinsic … from different manufacturers (tho' unlikely), or just a valve “on its way down” from original spec. This seems most likely, with my life's experience with valve aging under my belt. But who knows … there are a lot of other possibilities.

IF the gain issue does not follow the tube swap, well … then the internals of each amplifier are somehow different. Some resistor or capacitor is quite a bit different between each chassis. Again, is is rather unlikely that 3 or more components would be quite different between chassis … unless the previous owner had engaged someone (or herself!) in doing component upgrade/swaps. Capacitor “rolling” and all that.

So, there's your marching orders, Chief.
→ Swap tubes.
→ → Note results.
→ → → Get back to us.

-= GoatGuy ✓ =-

 

[GoatGuy]

PS: … to answer your original question, do consider purchase of a dual-mono stepped attenuator in a separate enclosure. This is an entirely passive device, hooked between your preamp and the main amp. In “the old days” it might have had a master volume knob, and a 'balance' knob to adjust Left-Right relative balance.

A more modern approach is to have a 3-knob unit … a stepped attenuator “master volume” and one-per-channel 'input pads' to reduce the gain of either left or right channel relative to the other. This has the advantage of being a 'set-and-forget' technology.

But it also perpetuates ignoring the “real problem” cited above. STILL … it would be a good thing to have in your system, given neither preamp nor pair-of-monoblocks seems to have a volume knob. You ought not to spend more than $100 on the thing, either.

There's an endless assortment of parts on EBay … as in ( https://www.ebay.com/i/113248763332...80899&itm=113248763332&pmt=0&noa=1&pg=2386202 )

Just Saying,
-= GoatGuy ✓ =-