Sunn Concert Bass power transformer specifications?

[spy]

I'm bringing back in life a Sunn Concert bass amp and when I'm done with the recapping, etc it would be great if I can replace the PT with a 220V version. Does anyone know the specs? Voltages and currents at each secondary so I can order a custom one? I have the version with 3 secondaries and one primary (8 wires).

[Enzo]

You have a working one? I assume you have a step down to provide 120vAC for it? If so, power it up and measure the voltages coming out of the transformer. If you want, do this while running signal into a load for a reailistic draw off teh thing.

Now you will know the secondary voltages to specify.

As to currents, we can infer from the circuit and amp performance what those might be.

There is one that makes the +32v and +25. Judging from the circuit, the 25v branch carries less than 10ma. I doubt the -15v carries any more than that. And the 32v for the driver transistor? I'd doubt over half an amp.

And the main voltage supply? Calculate enough current to provide for full rated power into rated load, with margin for reliability.

[spy]

Enzo, thanks for your help! Didn't answer all these days because I was changing caps and some parts like the bridges, some power resistors etc. Yes I do have a step down and that's how I power up the amp.

After I finished everything I powered up the amp but it doesn't work. Preamp stage is fine but there's a problem with the output stage. Without a load I have a perfect sinewave at the output but with an 8 ohm dummy load it distorts on the positive side and the output is maximum 10W. I have replaced all 22 Ohms, .27 Ohms except 910 Ohms, also R323 22 Ohms and C307 .22u.

I'm following this schematic, sunnconcertbass.pdf

I'll try to do some more measurements and come back with more details.

EDIT: Just replaced 910 ohm resistors with 1K but the problem is still there. Hope it's not the interstage transformer! I took off all the wires from the pcb and I did have a signal at each output with the correct phases. It wasn't a perfect sinewave but it was there.

Voltages at the output transisotrs are,

Q2
C: 0V
E: -22.6V
B: -22.1V

Q3
C: 23V
E: 0V
B: 0.5V

Q4
C: 23V
E: 0.1V
B: 0.6V

Q5
C: 0.1V
E: -22.6V
B: -22.1V

[loudthud]

With the dummy load disconnected, check the voltage at the output and the voltage across the 0.27 ohm 10W resistors.

[spy]

The voltage at the output is 0V and across 0.27 ohm resistors also 0V.

EDIT: The output is around 150mV, there's a voltage drop across 22 ohm - 0.22u ( R323 - C307 ).

[spy]

bump

Any help?

[loudthud]

Your symptoms indicate that one of the four output transistors is not doing it's job. The voltages look ok.

The problem could be:

1) A bad transistor or open connection to the transistor. Check junctions with ohm meter and transistor out of circuit. Check continunity from the board to the socket and to the actual transistor pin where is sticks through the socket.

2) A bad transformer or connection to it. Check with ohm meter. Each secondary should measure 2.3 ohms, the primary should measure 17 ohms.

3) A bad .27 ohm 10W resistor or connection to it. Check with ohm meter. There should be a small voltage drop across these resistors when the amp is operating, perhaps 0.005V . This voltage will increase as the amp warms up. If you measure 0.000V across a known good resistor, there is a problem with the transisor whos emitter is connected to it. Try heating the heatsink up with a hair dryer. The voltage across the .27 ohm resistor should increase.

[spy]

Thanks loudthud! I'll follow your instructions.

Is there a possibility to test good a transistor with the "diode check" mode with your DMM and be faulty?

[loudthud]

It's possible that a transistor could have low beta or low breakdown voltage and still pass the diode test. However, your symptoms are more than likely caused by a transistor that does not conduct or some other fault causing it not to conduct.

[spy]

BTW, all the output transistrors are replaced with PL-909 and the driver with 2SD425. 2N3055 are pretty cheap, I think I'll replace them and I'll clean every contact at the sockets and the whole mess from the thermal paste on the heatsink.

[spy]

Ok, I found the problem and it was my fault! As I was replacing caps, power resistors, etc I replaced R303 330R with a 33R resistor.
I replaced it with a correct value and the amp is working great! Full 50+W output at 8R load.

One last question, how do I set the "bias adj" pot? From this, Setting Bias on a Sunn Concert Bass Head?, thread it seems that it's not a bias pot but a "symmetry" adjustment pot. What if it's not set properly? Does it matter if it's not in the correct position?

EDIT: Found this, How to set bias on the Sunn Concert Bass, thread! Thanks for one more time loudthud!

[spy]

Back to the power transformer windings, I measure 36Vac at RED-BLK, 36Vac at GRN-GRN and 44Vac at BRN-YEL. It's pretty weird that there are 44Vac where only 15Vdc are needed from this winding.

The size of the transformer is pretty large, from the core dimensions (96mm length x 80mm depth x 55mm height), it's around 230W or more depending on the quality of the laminations, so if I'll order a PT with secondaries 36V/1A, 44V/500mA then what left of the power can go to the winding for the power output this means I can have minimum around 36V/5A. By placing a 1 ohm resistor after the bridge CR1 I measure 3A at full power so with 5A secondary I'm just fine.

Let me know if you agree with my calculations!

[SmartAlex]

Does anyone have a different schematic? I have a Solid State Sunn Concert Bass Head without a foot switch jack. The preamp only has three transistors. Does anyone have or know where I can find it?
Thanks,
Sam

[spy]

There are two versions of footswitch outputs, which version do you have? One is with two RCAs at the back panel and the other version is with a DIN.

[SmartAlex]

There are two versions of footswitch outputs, which version do you have? One is with two RCAs at the back panel and the other version is with a DIN.

Neither. Mine is a 1977 Solid State Sunn Concert Bass Head without no foot switch jack. Just a plug to fill the hole. 3 Transistor preamp.

[spy]

Maybe someone took off the RCAs or whatever was there. Can you post a pic of the amp? Especially the inside with the PCBs?

[SmartAlex]

Maybe someone took off the RCAs or whatever was there. Can you post a pic of the amp? Especially the inside with the PCBs?

Here are two picts if I've done this correctly.

[SmartAlex]

[spy]

Ok, it's time to replace the power transformer! Anyone can help?
My last calculations are below,

Back to the power transformer windings, I measure 36Vac at RED-BLK, 36Vac at GRN-GRN and 44Vac at BRN-YEL. It's pretty weird that there are 44Vac where only 15Vdc are needed from this winding.

The size of the transformer is pretty large, from the core dimensions (96mm length x 80mm depth x 55mm height), it's around 230W or more depending on the quality of the laminations, so if I'll order a PT with secondaries 36V/1A, 44V/500mA then what left of the power can go to the winding for the power output this means I can have minimum around 36V/5A. By placing a 1 ohm resistor after the bridge CR1 I measure 3A at full power so with 5A secondary I'm just fine.

Let me know if you agree with my calculations!

Here's the schematic, Sunn Concert Bass.

SmartAlex, any luck on finding the schematic of your version?

[SmartAlex]

No, I never found the schematic for that particular version. The customer was considering selling the amp because it sounded so thin.