Mesa Boogie Simuclass bias?

[Mark]

I have a Mesa Boogie 295 dual power amp to repair.

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It has an unusual story as the owner was going to throw it away and my brother took it. Oddly enough I never have this sort of luck.

The EL-34’s are shot as you’d imagine, and a couple of the 12AX7’s are no good.

I have never bought into the whole simuclass design. I have thought they ran two sets of valves at different levels of dissipation.

That said, what levels of dissipation are the 6L6’s and EL-34’s run in the simiclass amp?

To tell a little more on the repair, the amp was blowing the mains fuse without valves in the amp. This turned out to be the bridge rectifier going S/C. I thought the 1N4007 diodes were a bit light on for this application and replaced them with 1N5408 diodes.

[Stevem]

I never thought the class A and class A/B thru the same output transformer was worth the hassle sound wise, and indeed Mesa gave up on it.

Fact is once a class A/B amp is idling ( at whatever dissipation that this) it’s in class A since all the output devices are on.

I would rebuild it into pure A/B.

A big 8 output tube amp like that idling in class A/B might be able to put out 15 watts of class A power.

[leadfootdriver]

Great find!
Hoover dam levels of power reserve..

[Stevem]

I have never worked on one of these, but if I recall right the first guitar combo they made with that output stage did not put out any more then 90 watts rms.

[Mark]

Whilst looking up info on how this output stage works I came across this.

https://www.ampbooks.com/mobile/classic ... xtended-a/

It harkens back to Randall’s days of patenting other people’s ideas. I don’t know why he thought that was a good idea.

The real name is extended class A.

I have come to the conclusion that there are a few ways to run with it. One is to have 6L6’s with high transconductance so a high bias voltage is required and the EL-34’s aren’t taking a beating. The other option is to run the EL-34’s at 100% and the quiescent current through the 6L6’s is what it is.

What are your thoughts?

[pdf64]

Do the OTs actually present different impedances to the EL34 and 6L6 anodes, as per the patent and marketing?
I recall reading that the extra OT primary wires were just for show, both anodes each side were fed from the same point.
It might make a difference because if the EL34 really were working into a higher impedance load with a less steep loadline, it might work out that they really could operate in class A, so it would be feasible to idle them up near 25W.

[Mark]

Hi PDF64

Good question, that’s the sort of thing I would be curious about. Given my curiosity I had to find out. I measured the transformers from the 6L6 tapping to the EL-34 tapping.

On the first output transformer both side were 12 ohms. On the other transformer the measurements were 9.8 and 10.6 ohms.

Is there any other measurements that you would like me too take?

[pdf64]

What I was looking for was the blue to brown resistance, and the blue/white to brown/white resistance.

Are you able to check the primary to secondary voltage ratios?
eg if a measured low V AC is applied to the 4ohm secondary, what V AC is produced across blue to brown, and blue/white to brown/white?

An isolated heater winding (5 or 6.3V nominal) could be used to provide the low V AC.

[bepone]

to throw all in scrap and reuse hardware? rebuilt some normal power amp..caps are too old now also

[Mark]

to throw all in scrap and reuse hardware? rebuilt some normal power amp..caps are too old now also

I measured the filter caps with my Peak ESR meter and they measured fine. As there are so many caps, I didn’t want to replace them unless I had to.

[Mark]

What I was looking for was the blue to brown resistance, and the blue/white to brown/white resistance.

Are you able to check the primary to secondary voltage ratios?
eg if a measured low V AC is applied to the 4ohm secondary, what V AC is produced across blue to brown, and blue/white to brown/white?

An isolated heater winding (5 or 6.3V nominal) could be used to provide the low V AC.

I can do the resistance measurements tomorrow and the voltage measurements when I track down where I have put a suitable power transformer.

[Mark]

I measured the resistance of the primary windings of the output transformer.

The channel A transformer measured from blue/white to brown/white 87.2 ohm (EL-34’s class A).
108.6 ohms was measured from blue to brown.

The channel B transformer measured from blue/white to brown/white 89.2 ohms. 111.1 ohms was measured from blue to brown.

My meter for some reason didn’t like measuring the primary and was unable to give a reading, it kept going from nothing it O/C. I found I was able to get a reading by turning it to diode mode and then resistance. The diode mode readings were different of course but led me to a similar conclusion as was shown by the resistance measurements, the meter is a 1980’s Fluke 77.

MB Stereo Simul-Class 295.pdf

These measurements weren’t what I was expecting. I expected the “class A” section to have greater resistance.
I checked gut shots of other amps and they are wired the same.

I hope this is of some assistance.

[pdf64]

…
My meter for some reason didn’t like measuring the primary and was unable to give a reading, it kept going from nothing it O/C. I found I was able to get a reading by turning it to diode mode and then resistance. The diode mode readings were different of course but led me to a similar conclusion as was shown by the resistance measurements, the meter is a 1980’s Fluke 77.

Try selecting manually selecting the resistance range; I suspect that the back emf kick as autoranging homes in on the best range upsets the autoranging function.

These measurements weren’t what I was expecting. I expected the “class A” section to have greater resistance.
I checked gut shots of other amps and they are wired the same.

Yes, I agree.
Maybe Mesa built them incorrectly? Perhaps accidentally and then decided it was better.

[Mark]

I fixed the blowing fuses, but I was getting bugger all out the amp.

I checked the voltages on the plate and the first plate was 7vdc. I checked the plate resistors one was 22K and the other was 42K. I didn’t think that was the source of the fault.
I lifted the coupling caps and the voltage returned.

There were both short circuit. The caps were SBE 0.1uF 400v, so they weren’t poor quality caps. I can’t remember ever coming across a shorted coupling cap.

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Hopefully, this will help someone else who encounters this issue

[Mark]

Here is a bit more information on the circuit diagram of the 295. It includes the presence control which is omitted.

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Measurements will be forthcoming.