Heathkit AA-100 phono stage acting weird...

[Patrice B]

Heathkit AA-100 phono stage acting weird... - SOLVED -

Hi,

I've finally completely redone the AA-100 my father kindly donated to me 4 years ago (it was at this time my first tube amp). All resistors, caps, selenium and electrolytics replaced. Resistors matched within 1% on each side.

I installed a lot of wire jumpers on the PCB's as a lot of traces were damaged.

However, it exhibits the same behavior as already discovered back two years ago (after a first visit to a tube tech, back then I was a tube noobie and afraid to do the repairs myself):

- When I plug the "R" lead of a TT in the "R" input and the "L" lead in the "L" input (as it should normally go BTW), the left side is louder and has more bass. The right side is notably weaker and has more treble... The ground of the TT is attached to a screw underneath the amp.

- When I plug in reverse, meaning "R" of TT in "L" of the AA-100 and "L" of TT in "R" of amp: all is fine, equal volume and tone on each channel...!!! All of this behavior verified with mono pressings of The Beatles.

As you could doubt, this situation puzzles me. Why reversing the connections could cause something like that?

It was verified with two different working turntables so the source is out of the equation (the actual Garrard type A and a previous Aurex TT).

I swapped the EF86's and 12AX7's of the phono/tape head stage without any change.

I also swapped all remaining tubes without any change (7591's and the the two 12AX7's in the tone stage).

BTW, the amp is now fine sounding with a quad of EH7591A's biased at around 32-34ma, verified by adding 10 ohms 1/4w resistors to each output tubes cathodes.

So, could someone explain to me what's going on there? Something related to ground? I know it's hard to circumvent without access to my amp but maybe you could point me in the right direction.

It's not as I can't use the phono right now as I've found the reverse connections workaround but I want to understand if possible.

Thanks.

Patrice

 

[Patrice B]

I forgot: I swapped the tone stage 12AX7's for 12AT7's instead as the preamp was overdriven. I've read it here in GordonW threads I think...

 

[Patrice B]

Also forgot to say: the workaround is ok if I put the selector switch in REV position to have the correct channels where they should be...

Just to add that the position of the selector switch is not related to the bug however as it sounds wrong in any position when the leads are correctly plugged (R to R and L to L).

The reverse is also true as it sounds ok in any position when connections are reversed (R to L and L to R).

Patrice

 

[dshoaf]

Are you sure this was wired properly to begin with? Since this was likely a kit, the opportunity for mistakes are a bit higher than if it was factory built.

When I used work as a bench tech back in the 70s, the first thing to suspect with Heathkits was poor solder joints and mis wiring.

Have you got a copy of the assembly manual and the schematic. It will pay to be sure that all is wired correctly before going much further.

Cheers,

David

 

[Patrice B]

Are you sure this was wired properly to begin with? Since this was likely a kit, the opportunity for mistakes are a bit higher than if it was factory built.

When I used work as a bench tech back in the 70s, the first thing to suspect with Heathkits was poor solder joints and mis wiring.

Have you got a copy of the assembly manual and the schematic. It will pay to be sure that all is wired correctly before going much further.

Cheers,

David

My father gave me the assembly manual and I remember I've gone through the input bracket point by point following the instructions. I did found a wiring error, two wires reversed from input bracket through PCB preamp. I didn't go further though (all phono wiring on preamp).

BTW, these errors were related to connections going to ground if my memory serves me well. Unfortunately, it didn't made any difference to the original bug...

Just trying to understand why reversing the leads on the source cure the problem. It makes no sense to me.

Thanks.

Patrice

 

[Patrice B]

SOLVED:

The adjustment pots under chassis aren't even (not exact same resistance value).

So, adjusting one of them (the left) a little bit lower and voilà, same volume on each side!

 

[GordonW]

That was going to be my question- had you adjusted all the trimmer pots? Not only those under the preamp board, but the ones between the preamp and amp as well...

My guess is that the phono cartridge has very slightly higher output level on one side, than the other. That sometimes happens on phono cartridges. It's usually not noticeable- UNLESS you have an aggravating ADDITIONAL gain differential somewhere downstream, that COMPOUNDS the level error. At that point, it CAN become an audible imbalance. That's one nice thing about the AA100- you can ACCOUNT for those errors, without affecting the balance of any other source.

In any case... so, how does it sound now?

Regards,
Gordon.

 

[Patrice B]

I guess I could say it sounds not too bad. Not as good however as the phono stage of my Scott LK-72A. Obviously not in the same league but it could be explained by the fact that the TT on the Heathkit is an old Garrard Type A changer (stripped down from his mechanism) with a Shure SC35C

There is a Lenco B-52 with a Shure M91ED on the Scott...

Be aware that I left the 12AX7's in the phono positions (was it you that recommended to swap them for 12AT7's?). I replaced only the line stage tubes with 12AT7's.

There's also a motorboating effect if I crank up the volume at insane level in phono and tape head position. All caps and resistors replaced in this thing BTW.

 

[Patrice B]

Update:

I've worked on the bias pots adjustments following GordonW thread:

- four 20k pots added;
- the four .1 couplers swapped for .2 and the 470k replaced with 220k...

Wow, biased at 30ma, better bass and no more motorboating in the phono section at high levels.

BTW, the phono finally sounds darn good!

No pics, sorry, this thing is ugly anyway

 

[GordonW]

Have you tried 12AT7s in the phono stage yet? I found them to be better than the 12AX7s in the AA100, in all positions. I wound up with no 12AX7s left in mine at all (all four positions replaced with 12AT7/6201s)...

Regards,
Gordon.

 

[Patrice B]

Have you tried 12AT7s in the phono stage yet? I found them to be better than the 12AX7s in the AA100, in all positions. I wound up with no 12AX7s left in mine at all (all four positions replaced with 12AT7/6201s)...

Regards,
Gordon.

No impact on the RIAA curve?

No, I didn't tried the 12AT7's yet in this position but I will. However, apart the slight imbalance, the phono stage sounds good and not overloaded contrarily to the line stage (before the 12AT7 swap).

Thanks.

Patrice

 

[GordonW]

No impact on the RIAA curve?

No, I didn't tried the 12AT7's yet in this position but I will. However, apart the slight imbalance, the phono stage sounds good and not overloaded contrarily to the line stage (before the 12AT7 swap).

Thanks.

Patrice

If there was a change in the RIAA, it was for the better, IMHO. It's definitely worth trying.

Regards,
Gordon.

 

[Patrice B]

Thanks Gordon,

As soon as I go back to my cottage...

Not for a while as we're in the process of moving here in Montréal.