Heathkit A7E for guitar - preamp gain - diyAudio
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Old 15th March 2007, 11:56 PM   #1
PRNDL is offline PRNDL  United States
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Default Heathkit A7E for guitar - preamp gain

I'm converting a Heathkit A7E to use for guitar and have some questions about preamp gain. Here's the schematic
http://www.naturdoctor.com/Chapters/...eathkita7e.gif

I modified the switch so that it has a clean and dirty channel.
The clean channel connects to the top of the volume pot.
The dirty channel switches in the first half of the 12SL7.
(The .0035/.01/22K RIAA filter has been removed).

The clean sounds fantastic, although it could use a little more gain.

The lead channel has good distortion, but has a loud hum. I'm not sure where the hum is coming from, since it wasn't there before rewiring the switch. It could be that there's too much gain, or the 470K/680K and 1M volume pot are an improper load for the 12SL7.

I'm wondering what to do next.

I could wire in the second half of the 12SL7 between the switch and master volume pot. This would be to increase the gain in the clean channel. I'd most likely have to drop most of the gain.

I could also try replacing the 470K/680K resistors with a volume pot to adjust the lead channel gain.

I thought I'd ask everyone here, since you've been very helpful in the past!

Thanks!!
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Old 16th March 2007, 07:23 AM   #2
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I am not sure how you rewired the switch, but it sounds like that is the likely hum source.

As far as the gain. You could start by bypassing the cathode resistor with a capacitor somewhere from 1uF to 10uF. And regardless, i would change that 27k input resistor to 500k or 1M.

You could also bump up the value of the clean stage's plate resistor to 180k or 220k, and then raise the cathode resistor to 1.8k or 2.2k
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Old 16th March 2007, 02:09 PM   #3
PRNDL is offline PRNDL  United States
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Here's an updated schematic that shows how I wired the switch.
http://www.naturdoctor.com/Chapters/Amps/Retro.pdf
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Old 16th March 2007, 02:34 PM   #4
Trout is offline Trout  United States
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Quote:
Originally posted by PRNDL
Here's an updated schematic that shows how I wired the switch.
http://www.naturdoctor.com/Chapters/Amps/Retro.pdf

Hey, Just curious, What is the power output like and what speaker are you using? I have one of those sitting on the shelf along with the assemble manual & schematics.

Trout

Edit

Oh Also,

No Filament CT on the Transformer? Might that be part of the hum on the high gain channel?
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Old 16th March 2007, 02:42 PM   #5
PRNDL is offline PRNDL  United States
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It's only a few watts, so it's not very loud, which is why I got it.
The clean channel is very smooth and mellow, which may be the Baxandall tone stack.

I've got it connected to a 16 ohm celestion speaker in a little Marshall cab.

I'm wondering if I can disconnect the feedback resistor to make it louder and more open sounding.

>> No Filament CT on the Transformer?

It's not in the schematic, but there's one in it.
I have to redraw the power and output transformers to show more connections.
The OT has taps for 4,8 and 16 ohms and it's ultralinear.
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Old 16th March 2007, 04:40 PM   #6
PRNDL is offline PRNDL  United States
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I'm wondering if the old radio switch is not able to handle the amplified signal, even though it's probably only about 3VAC. It's a Stackpole DPDT slider mounted on the treble.

Any thoughts?

I could try replacing it with a new push-push DPDT pot combination, but that's pretty expensive for a guess ($12).
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Old 16th March 2007, 06:17 PM   #7
PRNDL is offline PRNDL  United States
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It is the switch.

The next question involves whether to wire in the second half of the preamp tube to give extra gain to both the lead and clean channels.

I'm currently thinking that it's fine as it is, even though the clean channel volume is a little low.

Also, the lead volume is much louder. I might fiddle with the gain resistors (470K/680K), but lowering the lead gain might also lower the total distortion, unless it's preamp distortion.

I'm having fun!
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