Ampeg SVT6PRO question

[Mars Amp Repair]

Hey folks,
Just rebuilt the negative side of this amp & was able to power it up on a load to full power out with no problem.

this amp has a rather weird config where one entire power amp handles the positive half of the waveform & another entire amp section runs the negative half.

you cannot ever ground the speaker negative for obvious reasons. The 2 of these I've rebuilt have the negative side blown so I suspect that's what's happening.

Anyway, I went to adjust the bias & it went into oscillation (or something) for a second, drew major current & blew itself up all over again

I suspected the bias pot & should have replaced it in the first place, but it measures fine & watching the graphic on my meter as I turn it up & down, it seems smooth & never intermittent. Parts on order at this point.

I'll for sure replace this pot along with everything else next time, but has anyone had an experience like this on this amp?


Thanx, glen

[Enzo]

Did you remove the load to make the adjustments?

[Mars Amp Repair]

In retrospect,
No...I actually have never in some 20yrs removed the load to adjust idle current. I don't ever recall seeing any info that suggests that. Are you thinking the lack of a load might cause a solid state amp to oscillate?

Are you suggesting to remove the load to adjust the idle current? I know if you're adjusting the idle current by the crossover distortion method (on solid state only...like the SWR's) the load must be connected to accomplish that.

glen

[Enzo]

I was surmising that if the control was noisy, a momentary spike might have occured that was made worse by having to feed it to the load. Not thinking oscillation. No, I don't normally remove the load for bias setting. I was not suggesting it to be a standard procedure, sorry.

I don't usually use the crossover method anyway. I set it for the onset of mains draw rise and back off a hair.

[Mars Amp Repair]

Ah, I see.
Yes, I can see where having the load on would cause that issue. Good thought as it all happened so fast that it might not have actually oscillated at all. What I saw on the scope might just have been that huge current draw.

The 'SWR' crossover distortion method only seems to work on some amps. Sometimes, I can't even get any crossover indication on the amps they claim you should adjust that way.

Funny even in the crossover method, SWR claims if you adjust even slightly over the correct point, the current will increase exponetially. So why not just do it the way you suggested.

P.s. for those who do Loud warranty or get parts from them, I have been advised that they will no longer supply many individual components, but will be moving to carrying boards & assemblies for warranty repairs & I guess whatever you might want them to be for.

BTW, The P-chnnl MosFet IRFP9240 is on back order until mid-june. I guess we'll have to find a method to rank these things.

(I'll let you know when the stand arrives). Thanx, glen

[Enzo]

And no one will hear a little touch of crossover anyway.