Bias caps in '68 Traynor BassMate; one missing?

[jdw3]

Upon reading the schematic for my BassMate, I have discovered a 8uf/250V cap is missing. It is C11 on the schem. The amp was running for quite awhile.

Would the cap missing cause the amp to become unstable?

[rdjones]

Upon reading the schematic for my BassMate, I have discovered a 8uf/250V cap is missing. It is C11 on the schem. The amp was running for quite awhile.

Would the cap missing cause the amp to become unstable?

Yes, a missing cap can cause an amp to be unstable.

Which version do you have ?
It is a YBA2(x) we are talking about, correct ?

There are at least 3 major variations of these amps with a few more minor revisions.
I made a .pdf with the versions I could find, 5 pages.

rd

[jdw3]

Wow, never seen the second one.

Yes, it's the first, larger print. It is C11.

I'm amazed to find it missing it the 2nd schem. I called a guy who described it and said there was no such cap; he was looking at the wrong schematic.

I have a cap sitting here, ready to install. Should I record any voltages? The amp cranked was burning tubes up and popping fuses. I took it to mean it needed screen resistors, which I am also going to install.

Do you think the cap missing has caused the problems? I have used it a lot without it.

[jdw3]

So, I installed the cap and screen resistors. Amp sounds great.

Just wondering what the cap or lack of it would do to the amp.

[rdjones]

So, I installed the cap and screen resistors. Amp sounds great.

Just wondering what the cap or lack of it would do to the amp.

In a general sense, the bias circuit can be compared to the fixed bias Princetons which take its AC supply from the HT winding.
These only have one cap and a lower total capacitance.
As long as there's enough filtering to suppress audible hum I don't think there'd be any other difference in sound caused by the cap.

reddog