Traynor Bass Mate YBA-2B 120 cycle hum

[misterc57]

Hum.....

I disconnected the dual 40 uF can and clipped in new filter caps. I replaced the third 10uF filter cap. Still have a 120 hz hum with or without all tubes inserted. How does noise get to the speaker with no output tubes?

Also getting a large current draw which trips the circuit breaker inside the amp.

Original PH204 diodes in the rectifier seem to be working properly, could they cause noise? Hum starts with very low V on the variac and gets louder quickly as the variac is slowly dialed up. I am able to test using the variac and keeping the current draw low.

What else can cause 120 hz hum?

Much thanks in advance! MarkO

[Helmholtz]

Could be the OT having a short between primary and core or secondary

[Enzo]

THAT^^^^^

No power tubes leaves darn few routes for energy through the OT. You would be hearing power supply ripple if the primary were shorted to ground through either the secondary or the frame.

And such a short would also draw a lot of current from the power supply and pop fuses.

[loudthud]

Check the tube sockets and resistance on the primary side of the OT. I seen both problems on these amps.

[misterc57]

Took the following readings, no tubes inserted.

OT primary;

346 ohms plate to plate

342 ohms V4 plate to ground
5 ohms V3 plate to ground

170 ohms V3 plate to CT
174 ohms V4 plate to CT

168 ohms CT to ground

OT secondary;

1 ohm across secondary

I am thinking about disconnecting the 3 leads on the OT primary and dial up the variac to check current draw.

Thoughts?

[mozz]

Sounds like 1 side of the primary is shorted to the frame. I'd disconnect it and measure the leads to the frame.

[Helmholtz]

A short between primary and secondary would give the same result - as long as one end of the secondary is connected to ground.

[misterc57]

I disconnected the primary leads and have the same readings to ground at the leads. I also dialed up the variac and there is no longer a current draw.

I have no experience ordering OTs. From what I gather, I need an 8000 ohm primary impedance with a 6AQ5 push-pull. There are some listed at 10 watts.

I happen to have a Hammond 1750H, wondering if I can use that for a quick test, or as the actual replacement. Considering the high plate voltage of 395 V perhaps the 1750H is a good replacement?

[Helmholtz]

You can use the Hammond 1750H for a test.
An 8k/20W OT would be perfect.
Don't mix up 6AQ5s with 6BQ5s.

[misterc57]

Don't mix up 6AQ5s with 6BQ5s.

Woops, I did do that.

The 1750H test was successful.

Thank you

[garytoosweet]

Hammond 1750S is their direct YBA-2B replacement.

https://www.hammfg.com/part/1750S

[misterc57]

Hammond 1750S is their direct YBA-2B replacement.

https://www.hammfg.com/part/1750S

Look at that....

[Helmholtz]

If so - be it.

As a more conservative designer I would have used an 8k OT, especially as the B+ exceeds the plate voltage limit.
A 6.6k OT will squeeze out a little more power at the risk of tube overdissipation with signal and reduced tube life.

[misterc57]

The new Hammond 1750S OT worked out great. Thank you all.

[misterc57]

Update and revisited...

Amp started spiking current again and tripping the breaker. Also there is a noticeable smoke and burn smell.

I disconnected the 3 OT primary wires and all leads are reading good resistance to each other and no shorts to ground.

I noticed a burn spot on the chassis at V3 pin 7. There was also a large blob of solder at that pin. I suspect this blob was arcing to the chassis.

I removed the blob and cleaned off the burn on the chassis. Picture shows a slight remaining burn spot (see the yellow arrow).

Weird thing is that with OT disconnected and no connections at pin 7, I read about 6M resistance to ground at V3 pin 7, but I read open at V4 pin 7. Why the discrepancy? I would expect both to read open. Thoughts?

Thank you! MarkO