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
Did you find the schematic? I found the A7-E if that helps.
http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_a7e.pdf
Here's some simple guitar tone stacks.
Adam's Amplifiers: Tone Stacks
Popular Amplifier and Effects Tone Stacks | Guitar Files(TM)
http://www.nostalgickitscentral.com/heath/schematics/heathkit_schema_a7e.pdf
Here's some simple guitar tone stacks.
Adam's Amplifiers: Tone Stacks
Popular Amplifier and Effects Tone Stacks | Guitar Files(TM)
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...
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:
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?
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:
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
is the a7 e the only model that uses the phono tube? mine does and im trying to figure out if i want to add another preamp tube,or just use the two in combination for a heavy gain. im looking into the idea of changing the outputs to 6v6 for a little more get up and go. i would also like to add a stand by switch. has anybody had any success doing that?
Front end on mine was microphonic, hissy, and useless.
I've not yet managed to dig up a decent replacement.
Since you can switch it out, not been a real problem.
Wait, you said Phono? I thought it was a mic preamp..
I gots to go back and look if there was RIAA? Its not
high on my priorities, so don't hold your breath waitin'
Plate voltage is crazy high for 6V6. But suit yourself...
You mean 12V6 anyway, right? Or you going for series
heaters 6+6=12? I recall 12A something or other was
what Heath had stuffed in the holes last I looked.
Concensus here seems to be: Standby is a myth.
But I'd still flip SB if I was leavin the amp unattended
for a few momets to go get something. No standby
mods planned for my A7 at this time.
Put you a tube screamer clone in front, precisely what
they are made for. Had one for years and never could
figure what was the big deal about that pedal till I got
a Marshall Plexi clone (Bugera(Behringer?) 1960).
Some amps lack a high gain front end, but overdive well.
Screamer itself doesn't have much of an overdive sound.
There's clipping diodes if you push it far enough, but you
are really missing the whole purpose of it then.
I've not yet managed to dig up a decent replacement.
Since you can switch it out, not been a real problem.
Wait, you said Phono? I thought it was a mic preamp..
I gots to go back and look if there was RIAA? Its not
high on my priorities, so don't hold your breath waitin'
Plate voltage is crazy high for 6V6. But suit yourself...
You mean 12V6 anyway, right? Or you going for series
heaters 6+6=12? I recall 12A something or other was
what Heath had stuffed in the holes last I looked.
Concensus here seems to be: Standby is a myth.
But I'd still flip SB if I was leavin the amp unattended
for a few momets to go get something. No standby
mods planned for my A7 at this time.
Put you a tube screamer clone in front, precisely what
they are made for. Had one for years and never could
figure what was the big deal about that pedal till I got
a Marshall Plexi clone (Bugera(Behringer?) 1960).
Some amps lack a high gain front end, but overdive well.
Screamer itself doesn't have much of an overdive sound.
There's clipping diodes if you push it far enough, but you
are really missing the whole purpose of it then.
Last edited:
thanks ken. i thought the extra input was phono, i may need to read up a little more. i read a forum on using the 6v6 tubes with series heaters. supposedly the output is boosted to around 15w. could i use a voltage dropping resistor for the plate current? and yes i would like to add a standby switch for when i walk away for a bit so it can just idle. im not understanding fully of how to make that work. do i use it to break the circuit before the rectifier to leave the heaters on?
This is a very common feature of older amps- it's designed to keep RF and other 'noise' on the power line from getting into the amp. With a non-polarized plug, if the .02uF cap goes short the chassis will be 'live'. Not good.
In the guitar amp world some call these 'death caps'.
If you know what you are doing, upgrading to a 3-wire power cord with ground would be a good idea.
John
Switching 400volts DC is not easily done with the average switch you will find in your parts box.
You don't need a standby on a little amp like this, with a tube rectifier. Just turn down the volume when you take a break.
However, if you want to put in standby and have a 7A-B or 7A-C amp with a center-tapped HV secondary (red/yell to ground) then you can switch the ground connection of the CT for standby.
It will be worthwhile to check into schematic heaven.com and print out a bunch of tweed fender schematics and layout diagrams to see the common ways these things were laid out.
BTW, 'hifi' amps like the A7 usually don't make very good guitar amps, since the output transformers are too good to distort properly! The OT on a similar-power guitar amp would be about 1/3 the size of the OT on that A7. In a guitar amp you want distortion (of the nice kind) so circuits like the UL A7 aren't too common.
BE careful messing around in your A7 if this is your first dip into higher voltage equipment.
Cheers
John
Are you using a guitar amp speaker? 5-10 watts wide open with a decent speaker is lease-breaking territory in most places.
If I were you, I'd think about getting a second A7 and rigging up a stereo tube setup for hifi. (they go for $30-50 on eBay). Grab something like a ValveJr for guitar playing (and modding, if that is your interest.)
John
my idea is i have a marshall mini stack that i want to gut the head and make a modified chassis for the a7-e to go into. i hooked it up stock to one of the cabs,,which hold 10" celestions and it didnt sound to bad. distortion wasnt great but with a stomp box in front it was pretty nice. yes im going 3 prong and fusing it. ive found some tone stack mods to chk out and some info for disabiling riaa. not really looking for another stereo,i just built a pioneer sx1010 that does all i need
I have OEM 12" Celestion around here somewhere, but its 4ohm.
Ripped from Line6 Spider amp. Replaced w. Eminence DeltaPro12.
Did not strike me as high efficency, nor quality in any way.
A "Celestion" in sadly tarnished name only.
I don't think you have to worry A7 output transformer being
too good for guitar. If anything like mine, its nothing fancy.
Have an Edcor CXPP10W to replace it, but not installed yet.
I've not modified anything yet aside from adding fuses and
yanking out the noisy pre-amp tube. All original suicide caps
are there in aging brown glory. Available for side by side
before/after listening test if you decide to mod.
Ripped from Line6 Spider amp. Replaced w. Eminence DeltaPro12.
Did not strike me as high efficency, nor quality in any way.
A "Celestion" in sadly tarnished name only.
I don't think you have to worry A7 output transformer being
too good for guitar. If anything like mine, its nothing fancy.
Have an Edcor CXPP10W to replace it, but not installed yet.
I've not modified anything yet aside from adding fuses and
yanking out the noisy pre-amp tube. All original suicide caps
are there in aging brown glory. Available for side by side
before/after listening test if you decide to mod.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.