Sunn Concert Bass question

[pontiacpete]

This amp had 3 blown output transistors(2n3055). Also all but one of the 22R resistors were burnt badly and all 4 diodes 1n934s were shorted.
Replaced the outputs with mj15024s and replaced damaged resistors and diodes. The amp is up and running well so far.
This amp is the one shown in the second schematic down on the attachment. It the power amp shematic that has the driver transformer.
It has trim pot R104. Dialed down it is measuring 1.1vdc on the wiper.
Does anyone know what this should be set at?

Thank you
pete

[nsubulysses]

Shouldnt it just be set for symmetrical clipping at the output?

it seems misleading to call it bias adjust on the schematic. Unless I am misinterpreting it.

[Enzo]

It biases the input stage. It does not bias the outputs. Don't assume that only output stages ever have bias.

And yes, I agree it is just there to balance the drive side for max ssignal room before clipping.

[J M Fahey]

EDIT: be aware that the one in page 3 and with "89" written inside is a *bridged* output amplifier.

While the Q4/Q5 "speaker out" swings up, the other Q2/Q3 one swings down.

To confuse matters more, Q2/Q3 speaker out is grounded.

All of this being possible because all 4 transistors are driven by independent driver transformer secondaries.

This amplifier is a wolf in sheep´s clothes, and supplies unexpectedly high signal voltage to speakers, I expect 150/200 W RMS into 8 ohm which is unexpected given the relatively low +/-34V rail voltage and the humble dated 2N3055 transistors.

.....................................................

The earlier (1975) one on page 2 is more conventional, power transistors in parallel, and I guess must have been designed to drive down to 2 ohm loads.

Not sure which version you have on the bench.

Both versions are almost the same, guess can even be built using same PCBH, just wiring driver transformer secondaries and 2 of the power transistors in a different way.

And bias pot adjusts idle current in the Class A driver transistor, which is a 5W amplifier on its own right, for symmetrical swing both ways .

[drewl]

Tell me about it!

This one band has like three of them that they love to abuse and use odd speaker/impedance combinations, so, they blow them up every few years.


I have rebuilt and beefed them up to handle the abuse.

[pontiacpete]

Thanks for the information guys.
I adjusted the trim pot to the point where the crossover notch was disappeared.

This one is the first one you are speaking of, with the drive transformer.

Thanks again!