Heathkit A7E Conversion to Guitar

[PRNDL]

I received the HealthKit A7E amp yesterday and it sounds pretty good for guitar, although there are some issues that need to be addressed: It sounds a bit muffled, and there's significant noise. On the good side, it's only 6 Watts and breaks up nicely at low volumes, which is why I purchased it.

I'm considering the following mods and upgrades:

1. Replace mono input 1 with a guitar jack
2. Replace the power cord with a 3 prong and ground the chassis
3. Replace the cathode 10µF/25V cap on the 12SQ7
4. Replace the cathode 20µF/25V cap on the 12A6's
5. Bypass the RC filter circuit after the first gain stage
6. Review and revise the star grounding

The 20µF/25V cap is within the capacitor can with the power supply filter caps.
The entire can might need replacing since it's probably old, but I'll see how it sounds with the initial mods.

The grounds are all connected to the input jack, which you can see in the wiring photo below (I'll take a better picture and post it).

Here's the schematic

An externally hosted image should be here but it was not working when we last tested it.

And the stock wiring

An externally hosted image should be here but it was not working when we last tested it.

Does this sound like a good plan?

Do you have any other recommendations or suggestions?

Thanks!

 

[tubee]

Hi

Use relative small coupling caps to get a bright sound for guitar instead of muffled. But they are relative small allready i see.

 

[Tom Bavis]

Those yellow Astron caps are leaky - trust me. Measure grid voltages (should all be zero) and you'll see.

 

[PRNDL]

I've made a few changes and tested, and it now sounds fantastic! Changing to a three prong ground cleared up all the grounding issues.

I also changed the input resistance to ground to 100K (the schematic says 27K, but there were two 100Ks in parallel). Should I change that to 1M?

I do need to pin the treble all the way up in order to get a reasonable tone.

What do you recommend for removing the filter network between the first two gain stages? Is this some kind of mid boost?

Should I take everything out, except for the .05µF coupling cap?
Or, should I leave in the 470K to increase the input resistance?

Perhaps the problem is the values in the Bandaxal tone circuit, which I assume are meant for stereo instead of guitar.

Thanks!!

 

[PRNDL]

I checked voltages, and all the grids are zero, except for the 12SN7, which shows 4mV.

The power supply runs a little hotter than the specs on the schematic, except for C+, which is lower (specs are in parenthesis):

A+= 403 (390)
B+ = 382 (360)
C+= 145 (180)

The cathode bias (12A6 pin 8) is at 26V

 

[PRNDL]

Arrrgg!!

OK, I took out the tone circuit and the volume dropped to barely hearable, put it back in and the same. Then I noticed I had swapped the 12SN7 and 12SL7 tubes. I traded them, put the circuit back to the way it was before, but still the same.

I checked the power supply voltages and they're the same.

I wonder if I burned one of those input tubes, or if one of those Astron caps gave out. The hard part is these are unusual tubes, so it's difficult to do a swap.

 

[Dave Cigna]

The filter network between the first two stages looks like a RIAA equalization network for phonos. It provides a serious bass boost (or treble cut) which explains your need to dime the treble control. You can remove it by clipping out the .0035 and .01 caps as well as the 22k resistor. You can leave the 470k resistor there. It cuts the gain a little, but it looks like you have plenty of gain already. You can remove it later if you decide to.

Hope you figure out what's gone wrong...

-- Dave

 

[PRNDL]

Thanks Dave, that did seem to work.

It does seem as though one of the gain stages isn't working, which is most likely a bad tube, since all the voltages are fine.

On the bright side, it did sound very nice for a little while