How can I make this amp brighter?

[Cory Nealon]

Hello everyone,
I just bought an unusual 18-watt tube amp.
The speakers (2x12 Rolas from '64) sound great. But the head - a 1962 Hammond chassis modded for guitar - is a little muddy.
It has two inputs (one preamp tube is a 5879, the other is a 12AU7). The low to high tube - I can't think of its name, it's positioned third - is a 12AX7. The power tubes are EL84s and the rectifier is somewhere in the 5 series - not sure which.
These are all old American-made tubes, I'm told.
My question is: are there any tubes and/or other modifications that might brighten up the tone? Also, is there a way to reduce the volume to more easily reach the natural overdrive?
Any suggestions are greatly appeciated. I bought this thing and I'm starting to worry I made a mistake.
Thanks,
Cory

I play a '55 National Chicagoan, a no-name steel from the early 40s, a Gretsch 5120 Electromatic, and an Epiphone Nighthawk.

[John Dahms]

The quality of the components is the main thing. There is probably good stuff in there to work with. The voicing can be changed to suit your taste I'm sure but before modding the design it's important to be sure each component is at the spec it was meant to be. Overhaul the unit first then make changes to resistors and caps to alter the tone after you know what it's capable of.
Quick fixes don't go very deep and probably don't give lasting satisfaction.

[Brad Bechtel]

Moved to Electronics from Steel Without Pedals.

[John Billings]

I'm assuming this is a Hammond Reverb chassis that's been modded? I can't help you out with mods, but,, an interesting side note;
Well-known boutique amp builder Mike Zaite, aka Dr.Z, made the first 50 or so of his Carmen Ghia amps from old Hammond reverb chassis. One nice thing about the old Hammond stuff is that they used great transformers! Re-biasing the amp might get you into the smooth distortion earlier.

[Bill Creller]

If you go to el34world.com and get on the "tweeks and repair" section with your question, you will likely get some ideas for your questions. The folks on there are always building & fixing tube amps.

[Rich Hlaves]

Cory,

Most likely the amp is functioning just as it should. Hammond designed their amps to roll off the highs at 6-7K htz to help remove key click from the sound. Hammonds competitors considered it a defect.

If I knew what amp it was originally I most likely have a schematic and can help you with the sound. It had a Hammond part # stamped on it originally.

Tubes and caps are most likely just fine. We just need to figure which caps need a value change or taken out of the circuit.

RH

[Cory Nealon]

Thanks for the input. I'm trying to reach the seller to get the make of the chassis. It had a reverb tank in it, but that was removed.
Rich, I will do some research, but I'm not sure what you mean by changing the value of the caps, or taking them out of the circuit.
The innards of amps is a brave new world for me.

[Cory Nealon]

The chassis model is an AO-43

[Rich Hlaves]

The AO-43 was a reverb amp from an early Hammond A100 series organ. It may have been used in early M100 series Hammonds as well.

I'm out of town till Monday. I'll have a look through my Hammond manuals and see what I can find for you then.

RH

[John Billings]

As I mentioned above, that's the Hammond amp that Dr Z modded to make his first series of Carmen Ghia amps. Try a web search to see if you can find a schematic for the Ghia.

[Jason Hull]

Painting it white will make it brighter.

[Rich Hlaves]

I managed to get a look at the scematic online. It looks like if C1 a .1uf capacitor is still in the circuit it would result in your loss of brightness.

It is wired by Hammond across the input to ground before the preamp tube in a stock OA-43 or AO-44.

Hope this helps,

Rich