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Old 14th October 2010, 01:41 AM   #1
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Talking heathkit a7???guitar amp

i am new here so please bear with me. i just bought a heathkit a7 amp with the hopes to modify it and make a decent tube guitar amp. i am unshure of how to id the amp to tell what model a7 it is. can anyone help? i am also new to tube amps, i have some knowledge of solid state and a little from an old hallicrafters s-85 shortwave that i repaired. is it possible to ad more to this amp,possibly boosting it to 15 watt and adding more gain and tone controlls? i know that i would definately like to add a standby, ive been hitting the library and am waiting on my tube amp book to show up. any suggestions are usefull and verry welcome
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Old 14th October 2010, 02:32 AM   #2
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Did you find the schematic? I found the A7-E if that helps.
http://www.nostalgickitscentral.com/...schema_a7e.pdf

Here's some simple guitar tone stacks.
Adam's Amplifiers: Tone Stacks

Popular Amplifier and Effects Tone Stacks | Guitar Files(TM)
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Old 14th October 2010, 02:42 AM   #3
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I have one, it works (more or less), I've played bass through it.
Not much power here, you will need a high efficiency speaker.

I've ruined my DVD backup of my last PC and all my Heath PDFs.
But I still have a printed schematic of it.

There is more than 3 versions, and my A7 is not exactly like any.

My high gain pentode (for mike pre-amp?) is real noisy, I have
it pulled out. I notice Heath runs the voltage alarmingly high.

Supposedly A7 output transformers like to blow because there
is no fuse! Upgrade by adding a fuse first...

Last edited by kenpeter; 14th October 2010 at 02:58 AM.
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Old 14th October 2010, 02:46 AM   #4
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A7E mine ain't. I have different preamp tubes and no UL.
But I did buy an unlisted Edcor 10W CXPP for UL purpose.
It almost exactly the same size, but the bolt hole pattern
is are a little different from Heath original.

A7C is closest to what I seem to have.

Last edited by kenpeter; 14th October 2010 at 02:50 AM.
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Old 14th October 2010, 01:02 PM   #5
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A7 B, C, D, E schematics are on my web page: Audiophool
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Old 17th October 2010, 04:02 AM   #6
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what fuse rating is being used for these units?
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Old 17th October 2010, 04:41 AM   #7
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Location: Dallas
There was NO fuse in mine.
I added 250mA to B+ tap of OPT
and 2A slow blow on power main.
Slightly better than nothing...

I upgraded my cord to polarized.
Not quite sure if I should have
safety grounded the chassis too?

Signal ground seems common to
my chassis. Still tingles to touch
it sometimes, but not repeatable
when I try to measure...

One day I'll rip out ancient wiring
and re-do from scratch. With star
ground for signals separate from
chassis/safety...

Probably shouldn't even be messing
with it in its current state without
an isolation transformer I can trust.
Get you an external or something...

I do not understand nor trust the
.02uF from hot to chassis ground.
Probably the source of the tingle.
Why Heath does that anyway?

Last edited by kenpeter; 17th October 2010 at 05:08 AM.
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Old 17th October 2010, 11:21 PM   #8
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Join Date: Oct 2010
Location: ft worth texas
Quote:
Originally Posted by kenpeter View Post
There was NO fuse in mine.
I added 250mA to B+ tap of OPT
and 2A slow blow on power main.
Slightly better than nothing...

I upgraded my cord to polarized.
Not quite sure if I should have
safety grounded the chassis too?

Signal ground seems common to
my chassis. Still tingles to touch
it sometimes, but not repeatable
when I try to measure...

One day I'll rip out ancient wiring
and re-do from scratch. With star
ground for signals separate from
chassis/safety...

Probably shouldn't even be messing
with it in its current state without
an isolation transformer I can trust.
Get you an external or something...

I do not understand nor trust the
.02uF from hot to chassis ground.
Probably the source of the tingle.
Why Heath does that anyway?
where did you get the 250ma fuse and what voltage rating was it? i went to radio shack today and found the mains fuse but the lowest they had was .5a 125v.also why would you question grounding the chassis? i was gonna take the plug and cord from a pc power supply for mine,but im wondering about the grounding now
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Old 19th October 2010, 10:41 PM   #9
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Location: ft worth texas
im installing the new cord and fuses today. does anyone know how i would wire in a standby switch on this thing?
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Old 20th October 2010, 01:28 AM   #10
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Its only a 250V fuse right now on the B+ tap.
Totally spaced that it was for a 500V circuit!
Maybe two in series???

250VAC that would be like 353V peak DC?
Or not safe to assume when rating fuses?

Last edited by kenpeter; 20th October 2010 at 01:31 AM.
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