Ampeg V6B Repair Question

[Jayboris3]

Hey diy crew, intermediate tech here and first time poster on the group! I’m banging my head against the wall with the repair of this Ampeg V6B ss bass head. It came in with many a blown output transistor. Replaced all 8 of those along with the driver transistors, bias transistor, and some out of spec resistors. The main filter caps are original and seem fine (?), and someone else before me had done a recap of the power amp section.

Unit powers up fine now but only when unloaded and all of the measurements in the schematic are correct,
but when I attach a 4 or 8 ohm dummy load, the dim bulb ain’t so dim any more. I’ve checked all of the components and can’t find anything thats fried. What am I missing here? Grateful for any ideas. Thanks!

Link to schematic:

https://ampeg.com/support/files/Schematics/V Series/V-6B/V6B Schematics 2.pdf

 

[radardoug]

You running this with a series lamp? Once you get it all fixed, if you try to draw power the lamp will light up! Doh!
As with all transistor amps, do you feel lucky?? Sooner or later you have to take it off the load limiter.

 

[Jayboris3]

Thanks for the reply. I would expect to see a dim glow from the bulb, but this is lighting up like a dead short. I’ll try it today without the limiter.

 

[Khron]

Thanks for the reply. I would expect to see a dim glow from the bulb, but this is lighting up like a dead short. I’ll try it today without the limiter.

Is that with or without any input signal?

And are you seeing any DC on the output, with no load connected? If the thing draws a lot more current only WITH a load connected, that's likely DC on the output.

Does the circuit have no provision for output protection then? And/or that particular part of the protection circuitry has failed, possibly along with something else upstream of the output stage, that's stuck up against one of the supply rails.

 

[Jayboris3]

This is with no input. I’m getting around 70mV at the speaker terminals, which is 20mV higher than the maximum that should be there, but not by much? There is a protection circuit (q206 and q207) which according to the service manual protects against mismatched impedances

 

[CJ]

Plug a speaker in for a few milliseconds and see if it hums real bad or stays quiet,

You might have some power supply problems caused by the failing transistors.

What kind of transistors did you use? Newer ones can have increased bandwidth which can sometimes cause hi freq osc.

 

[Jayboris3]

Plug a speaker in for a few milliseconds and see if it hums real bad or stays quiet,

You might have some power supply problems caused by the failing transistors.

What kind of transistors did you use? Newer ones can have increased bandwidth which can sometimes cause hi freq osc.

Thanks CJ I’ll give that a try. The replacement power transistors are MJ15022, originals were 2n4348, and the driver transistors are complimentary MJE15030/15031 replacing 2n5680/25682.

I am seeing quite a bit higher transition frequency in the replacement power transistors… (.8mhz to 4mhz). It’s about the lowest I can see on the substitution chart, most are 15-50 mhz

 

[radardoug]

CJ is onto it! Good suggestion.

 

[Jayboris3]

That is a good suggestion! Silly rookie mistakes by me- I had been ramping up the line voltage with the variac and line limiter out of an abundance of caution, but of course that will cause the bulb to drop all of the voltage before the amp can run properly. With the amp running unloaded, I quickly plugged in the speaker as CJ suggested and everything seemed fine. Then I removed the limiter as radardoug suggested and was able to pass signal just fine as well. With my dummy load I measured the full 240 watts @ 4ohm. Knock on wood but I think it’s working as it should. Learned an important lesson about being overly cautious, thanks a ton!