122 Rebuild Gone Wrong

[Guest]

Hello to all forum members. This is my first post so please bear with me.

I recently acquired a 122 amp which was a project undertaken by the owner who eventually lost interest. The owner connected it to his C3 to show me that it works, except 1) the volume is very weak and 2) the relay is stuck on fast.

I checked the underside and noted that the four .0047 capacitors were missing. They were actually removed and never put back when the amp was "rebuilt."

When I got home, I checked the amp further and saw that the jumper plug was wired wrong. Instead of connecting points 2 and 4, they were connecting points 1 and 3. I think this means 117vac was connected to the filament wires.

All the tubes are new, but are they now damaged by this wiring mistake? Can this also damage the transformer? Is this the reason why the volume is very low?

Are the missing capacitors also contributing to the low volume?

Are the factors above also causing the problem with the relay? Or is the relay issue a totally separate problem?

By the way, I connected it to a B3 with a working 122 connector so I am sure the switching problem in the amp.

Thanks in advance for any info you can share.

K

[glensound]

K,

PM me if you would like the name of the tech who just checked out / restored my 122 amp...

[David Anderson]

The situation you describe raises many questions. What it sounds like you're dealing with is the previous owner's unsuccessful attempt to fix a problem. Or maybe he acquired it from someone else in that condition. Sometimes these things change hands several times.

Tube filaments would typically be burnt up by having 120VAC applied. If they light up normally, then they may be OK. A tube tester would be useful here to check for heater-cathode shorts.

The missing capacitors need to be replaced, but they should not contribute to the low volume problem. The relay is probably a separate problem, but with something like this . . . who knows?

I rebuild these professionally, and I've had to tear out a fair number of unsuccessful repair attempts and start over. Unlike a rebuild of a factory-stock amp, you can't take anything for granted.

[Guest]

Someone tried to 147ize a 122 more likely....

[jpmas555]

Thanks for all the replies.

@David, the tubes light up normally, so hopefully, they are ok. I'll also try to have them tested in due course. Thanks for confirming that the missing capacitors are not causing the low volume. I will replace them but out if curiosity, what are these capacitors for and how do they affect the signal chain?

Thanks again.

K

[David Anderson]

Thanks for all the replies.

@David, the tubes light up normally, so hopefully, they are ok. I'll also try to have them tested in due course. Thanks for confirming that the missing capacitors are not causing the low volume. I will replace them but out if curiosity, what are these capacitors for and how do they affect the signal chain.

Two of these capacitors provide negative feedback from the 6550 plates to their respective grids through 390k resistors. Because they are capacitors, there is more feedback at higher frequencies. The purpose of the negative feedback is to reduce distortion and prevent parasitic oscillation. The other pair of capacitors connected from the 6550 plates to ground is there to limit the amp's high frequency response, partly because the Leslie output transformer was not really wound as a Hi-Fi output transformer. It has limited higher-frequency bandwidth. (I have the original specs from Triad.) You don't want the output tubes producing large signal swings at high frequencies into an OPT that can't handle them because it risks damaging the OPT. In one of the Leslie manuals, users are warned against removing these. They also help prevent parasitic oscillation.

Replacements for these capacitors need to be rated at least 1500V DC.

[Guest]

The stock capacitors C12,13,14,15 were rated 1600VDC already, weren't day?!? I wondered what was the 122 manual suggesting not the remove C14/15, knowing that C7(0,001 mf mica) grid to grid of the ballanced input stage is being THE high frequency output control in a "put smaller cap get more high end, bigger-less high end" proportion, suggested in 22 H/R manual?!?
Indeed that is the 22H/R manual suggesting: "Do not change the OT primary shunting condensers".

[Hamman]

My 2 cents.....I would point to point from input to output check everything! After that is done I would then have the tubes checked. Then I would fire her back up and check voltages. Tedious but also a great learning experience as well as peace of mind
Benton electronics web page has the voltage check list posted on their site