DVD-Software.info

[lord_glum]

I just got a Heathkit AA-13 off ebay and against my better judgment, I
plugged it in. From what I have read, its not a bright idea, but oh
well it worked in the end after a good once over. That and the
wiring/caps/resistors are all in near flawless shape. But now the
questions...
1. I want to rebuild it just as a power amp and bypass all the tone
controls. Is it as simple as bypassing the selector switch? I've got
the original owners manual for reference.
2. It came with mullard 6bq5's in it and darn do they get hot! Like
not touchable hot. I know in the EA-1's they got hot, but I didn't
here much about heat issues in the AA-13's.
Thanks for the help,
Jeremy

[Carroll Conklin]

Jeremy,

I just replaced the coupling caps in my EA and the cherry red plates problem
stopped. You should have about 13 Volts of self-bias when measuring between
the cathode and grid of either output tube. If you have less, your coupling
caps are probably leaking.

Carroll
"lord_glum" <lord_glum@yahoo.com> wrote in message
news:1105405957.660226.70890@c13g2000cwb.googlegroups.com...

[Rod Keys]

Hi Jeremy,

NO! Don't "rebuild it as a power amp". You'll destroy resale value and
make performance worse.. And no, bypassing the selector switch will not
defeat the tone controls. In fact taking the tone controls out of the
circuit would take some doing .. and rserious edesigning. The tone controls
are driven from a relativly low impedence stage and are followed by a high
impedance voltage amp. You could tap in at that point but it's not really
what the doctor orderd for a normal amp input (lacks volume control, you'll
have to look at grid bias questions and likely add an input cap, there is
potential for noise and hum issues .. etc. etc. etc.).

Instead, set tone controls in thier mid position, (this actually is pretty
flat on those old Heaths) and use the "aux" input. Actually, I don't
remeber if a AA 13 has an "aux" input so if not use "tuner".

6BQ5's can get pretty hot .. they might be okay. Look at the plates. (Not
the little thing in the middle, that's the "heater" or "filement", look at
the bigger metal "box" that's around all the tube's guts). Are they glowing
red? (Some output tube plates glow just a little red normally .. it's a
judgement call how much is normal .. but for 6BQ5's there should be very
little glow, certainly none that can be seen with room lights on.) If the
plates are glowing, you do have a problem. If not, you're probebly okay. I
don't remeber if the AA 13 had a bias adjustment pot. If it's there, it'll
be a screwdriver setting poking through the chassis somewhere near the
6BQ5's. If it has one, start there and tweek till the tube's plates no
longer glow. But I think the AA 13 lacked one. In that case glowing plates
most likley means leaking coupling caps.

Don't go crazy replacing and changeing things. Only change out stuff that
you're sure is actually bad. Those old Heathkits were actually pretty good
gear as they were originally designed and made from decent parts. Changes
almost always make them worse.

Rod

"lord_glum" <lord_glum@yahoo.com> wrote in message
news:1105405957.660226.70890@c13g2000cwb.googlegroups.com...

[EC]

I tried a tone-bypass when rebuilding my Heathkit aa-151, but
honestly couldn't tell much difference. I ended up rebuilding
the tone stack with new parts (pots, resistors, caps). I was
lucky enough to find the schematic for the Heathkit SA2 that
layed out the P.E.C. that was there. The stock 1M pots
had each drifted up to about 1.6M!

After the rebuild with 1M pots, I found my CD-player in the
AUX input was overdriving the pre-amp, so I added a AUX-trim
circuit; two 50k ohm pots that bleed some signal to gnd.


"Rod Keys" <rkeys@comcast.net> wrote in
news:xcydnT-l5JMmZH7cRVn-jg@comcast.com:

[lord_glum]

I started looking at rebuilding it as a power amp and i am starting to
not go that way. It looks like alot of work that i don't think i want
to do after reading what i see here. I think i am going to start small
and replace the caps, but my main concern is that there isn't any hum
and all the caps look fine even though looks can be wrong. How can i
tell if a cap is "broken?" The components are in pristine shape and
the person that put it together did a bang up job.

The volume pot needs to go as well its either off on full bore. I may
replace all the pots in it, they all are touchy. I am going to add
on/off switch as well. Right now the treble turns on the amp.

It does have the bias adjustment, i noticed that after i unplugged it
and inspected it again.

I tried all 3 inputs (crystal phone, mag phono and tuner) and got the
best sound out of crystal phono. The tuner input was so loud it
distored the speaker.

Other than that is just replacing the input jacks, speaker terminal
strip, and power cord.

Thanks for the feeback. I'm sure there will be much more to come.
Jeremy