Repair job bounce

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Tubes can survive the abuse for a surprising amount of time.

Plate dissipation goes high ( may glow dull red) but that is not a cause of immediate failure.

..... Phil

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The full symmetrical cathode drive circuit you see replaces an earlier tube drive circuit that had exactly that issue.

It was however a disastrous step, helping to put the brand off the market.

..... Phil

Ok, thanks. I am stumped now. Last deperate try: is it something like the grid +30V rising up too quickly at switchon doing something bad before the cathodes are hot?

piglet

ey never built any for sale. If you do some research they built one and onl y one for use in house by one engineer. Only one unit. It wasn't until they became Agilent that they started producing equipment to test semiconductor s on the production line.

d out to be some other mfr they had rebadged, then their evaluation rejecte d. It was afaik a 1 off.

I have a Microdyne C-band signal generator that was never sold to anyone. I t was designed to use for final testing and repair of their CATV and Broadc ast Satellite receivers. It may be the only surviving unit. There was no do cumentation in the Archives when t was given to me. The output attenuator w as a $900 Weinschel variable. That was the price 20years ago. I would hate to need a new one, today.

Do guitarists test the amp is working by touching the plug tip to get a mains hum/buzz from the amp - that invites static discharge?

piglet

It looks like a difficult feedback network to stabilize so I think I understand why they didn't AC couple the feedback network they had trouble getting a response over the required bandwidth that wasn't peaked if they did.

Lots of phase shifts going on including the op amp dominant pole.

The more I look at it the more worried I get about those transistors in the cathode. I'd be happier if they were biased from a power rail rather than self biased because it looks messy starting up as the cathodes heat. What is the Vceo rating of those transistors?

piglet

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** Nope.

See what you can find out about Q3/Q4, SJE1692s - non listed "specials".

..... Phil ..... Phil

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Q3 & Q4 effectively current drive the cathodes of the output tubes.

This significantly linearises the transfer curve so the bias setting can be unusually low and THD is reduced.

Bias current is SET by the two transistors, not the tubes, so matching is non critical and different tube brands and conditions barely matter.

Note the use of split plate and screen voltages, 600V and 300V.

All MusicMan amps have this unusual feature.

EL34 tubes can plug in replace 6L6s.

..... Phil

I mean touching with their *finger*

piglet

Tube has to be conducting appreciable current thru the cathode before the cathode voltage drops to normal operating value, only way appreciable current can flow thru the cathodes is those transistors, only way those transistors can turn on is via the bias resistor from their collector/tube cathode junction. So at startup the transistors CE junction get momentarily walloped with the full B+ which they aren't rated for

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The 2000 odd pF capacitance of the cable cuts any spike voltages right down.

Every similar amp would fail in a week if you were right.

So you are not.

.... Phil

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Then some current starts and the voltage rises on each collector up to the bias value - quicker than most amps cos of the current drive.

There is no standby switch, B+ does not rise instantly.

.... Phil

isn't typical instrument cable something like

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Ok, I was assuming the output tube filaments were already hot, how they're connected doesn't appear to be labeled on the schematic.

If they were with +30 on the grid and 250 on the screens I thought it'd briefly behave more-or-less like a vacuum diode

u.

t down.

uctive plastic over that - then copper braid.

so this list is all outliers?

Usually we test to see if the amp is working by plugging the guitar in, turning the volume up and seeing if it makes sound