Kalamazoo Bass 50 Microphonics or PO?

[Jecarroll7]

Kalamazoo Bass 50 is back on the bench.

Can't zero out the volume, in fact I can ground the wiper of the volume pot (grid of V1 section 2/6EU7) and I still have a strong audio signal on the plate of section 2.

Audio is somehow getting passed along from V1 section 1 to v1 section 2 apparently without coming through the grid of section 2.

I have swapped the tube and for grins I looped in a new bypass cap on the cathode of section 1, no change.

Only modification I see is someone installed a three prong power cord, although they left in a death cap (disc, 0.022uf) on the ac primary.

The tube sockets are the old fiber type used in early RCA radios, as are the terminal blocks which has me wondering about conductance although I don't read anything.

I've attached a schematic.

Any thoughts/ideas on a direction for this would be greatly appreciated.

[soundguruman]

Kalamazoo Bass 50 is back on the bench.

Can't zero out the volume, in fact I can ground the wiper of the volume pot (grid of V1 section 2/6EU7) and I still have a strong audio signal on the plate of section 2.

Audio is somehow getting passed along from V1 section 1 to v1 section 2 apparently without coming through the grid of section 2.

I have swapped the tube and for grins I looped in a new bypass cap on the cathode of section 1, no change.

Only modification I see is someone installed a three prong power cord, although they left in a death cap (disc, 0.022uf) on the ac primary.

The tube sockets are the old fiber type used in early RCA radios, as are the terminal blocks which has me wondering about conductance although I don't read anything.

I've attached a schematic.

Any thoughts/ideas on a direction for this would be greatly appreciated.

I am making an educated guess that your socket / plug assemblies have lost continuity.
On this type of amp, there is a habit of grounding one of the heater conductors, at the factory.
If you install a three prong plug, lift those heaters entirely off ground.
And add 2 X 100 ohm balance resistors instead.

[52 Bill]

Try grounding grid of 2nd stage directly at pin 5 instead of at the volume control wiper. Any difference?

[Jecarroll7]

SGM - You're right, heater is tied to ground at Pin 1 of V1. Where do you suggest placing the resistors?

52 Bill - I grounded the grid pin 5 of V1, strong audio still present at the plate.

[soundguruman]

SGM - You're right, heater is tied to ground at Pin 1 of V1. Where do you suggest placing the resistors?

52 Bill - I grounded the grid pin 5 of V1, strong audio still present at the plate.

Anywhere that has space, connect 100 ohm 1 watt resistor between heater wire and ground
do this for both heater wires.
It's better to locate this at the power supply end of the amp and position away from signal path.
If you put the resistors near the signal wires, it will pick up 60 cps.
And then lift the heaters off ground entirely.

Back in the day before grounded chassis, (3 prong), it worked OK, but when it's grounded to heaters and 3 prong it's an issue.
It can cause hum or other problems.

[Chuck H]

The ground on that amp is connected to the circuits via a non soldered connector. Oxide or faults could be the problem. Also, old power supply filters may not be decoupling signal.

[Jecarroll7]

You can still ground pin 5 grid ov V1 and have audio at the plate, so volume still won't zero out.

Ground looks pretty tight, I metered every ground connection. Just finished floating the heater leads with 100ohm resistors and removing the death cap. Tightened the transformers and choke to the chassis, cleaned, scraped and retensioned the V1 socket. Thinking about swaping out the V1 socket (old RCA radio type) but I hate to shotgun parts.

Funny, I was just thinking maybe the filters but the amp is fairly quiet? I'll check them - thanks for the nudge Chuck.

I'm open to any and all ideas at this point, have reached my quota of tail chasing on this project.

[Jecarroll7]

Chuck H you nailed it - Amp is up and running.

Playing like it should and volume is responding like it should.

Appears to be the filter cap off the 2nd stage plate resistor, I noticed a tiny amount of audio there so looped in another filter cap and everything started working like it should.

Appreciate the help for sure.

[Chuck H]

I'm happy for both of us