Sunn Concert Bass woes

[Mr_bibbles]

Hey y'all-

Fixes a Sunn Concert bass for a dude a while back, worked great for about a week until it bit the dust again.

The first time it was on the bench, I replaced all the 2n3055's, a 24ohm resistor (r133 I think??), and set the bias to just above minimum as I don't have a scope.

Worked fine, sent it home with him, comes back a few sessions later with 3 burned out 24 ohm carbon comps and 4 burned out 2n3055 on either side of the rail. Replaced the 2n3055's and the resistors with fat metal oxides, and it's running well again, but I could not find a definite cause of failure so I'm hesitant to give it back yet.

My first thought was that it was biased too hot, and perhaps I had the pot the wrong way. Can anyone give advice on how to bias to safe levels WITHOUT an oscilloscope?

And secondly, is there maybe some other issue I should be looking at that could cause failure over a few practices?? Thanks!

[galaxiex]

I believe it is standard practice to replace the drivers when the outputs have a failure.

Did you?

[J M Fahey]

Modern 2N3055 are weak or straight fake, use MJ15015 or NPN MJ1502X .

Post schematic.

IF this is one of the transformer driven ones, there are no dedicated driver transistors for the power ones, and resistive bias does NOT track temperature, so you either bias them VERY cold and stand crossover distortion or kill distortion but amplifier easily runs away thermally.

Cheesy late 60īs state of the art technology.

[loudthud]

What you are calling a bias adjustment doesn't affect the bias of the output transistors, it has something to do with driver stage symmetry. No schematic shows the adjustment and there is no procedure for adjusting it anywhere I have found.

There is no short circuit protection on these amps so make sure the owner does not connect a too low impedance load or a speaker cable with an intermittent short. Also, there is no DC feedback, DC on the output is set by resistors. Check for DC on the output with the four output transistors removed, then re-check after you install the transistors before you connect a speaker.

Juan's advise on the output transistors is right on.

[Mr_bibbles]

What you are calling a bias adjustment doesn't affect the bias of the output transistors, it has something to do with driver stage symmetry. No schematic shows the adjustment and there is no procedure for adjusting it anywhere I have found.

There is no short circuit protection on these amps so make sure the owner does not connect a too low impedance load or a speaker cable with an intermittent short. Also, there is no DC feedback, DC on the output is set by resistors. Check for DC on the output with the four output transistors removed, then re-check after you install the transistors before you connect a speaker.

Juan's advise on the output transistors is right on.

https://music-electronics-forum.com/...3&d=1405457318


Thanks all for the replies!

Woops, sorry, here's the Schematic!

DC Offset is about 2 MV.

Owner using a 4ohm load.

Driver: The driver was replaced with the rest of the trannies the FIRST time it was on the bench, but I did not replace it the second time. I did remove and test it, It's working fine, performs well in the amp too. I have a new nte3055 from the same batch, but since the driver is new and tests fine, it should be good right? I can always throw a new one in to be safe...

Bias?: I'm reading most of the current through the Variable resistor going through the 33k to ground, so I think it's set as cold as I can make it.


Bad NTEs: If the NTE's failed because of their poor make, would that toast those 3 24 ohm resistors?


Since I've already installed the new NTE's I'm going to run it all day on the bench and see if it tries to runaway,

[J M Fahey]

Thanks

There is some kind of built-in protection, hidden in plain sight :
Driver transformer is driven by a single Class A transistor, which can output only "so much" current, which is its own idle current, think 0.5A or thereabouts.

Since that is the straight power transistor base current, and Hfe, current gain, is low at high currents and to boot even drops more between 5A and 10A , they end up being current limited ... which is what most basic short protection does.
Meaning they start being hard to drive about 5A and usually canīt reach 10A in this kind of circuits.

But itīs quite crude and it depends on uncontrolled variables, so sometimes works, sometimes not so much.

[g1]

Attaching full service manual/schematics. There is reference to 'power amp stability mod kit' on pg.13 & 14

[Mr_bibbles]

Thanks dudes!

The power amp stability mod looks incredibly helpful, I have a different version board so it may take me a bit to create a guide for the other board if it comes to that.

So far it's been up and running on the bench for 3 1/2 hours, no no real issues, 24 ohms are still pretty cool. One weird thing, it like if I leave it for a bit and come back, when i touch the common probe to the chassis to measure something, there's a slight pop like static electricity. No AC or DC on the chassis....