Orange Matamp 6 channel PA Head w KT88 schematic

[crisotop]

Hi folks,

I'm desperately looking for the schem of a really old orange/matamp PA head I recently received for repairing. I'm not new to tube-amps, but this thing has been "repaired" before, and it's hard to figure out what's original and what has been fixed to death. Had no luck on the web or at matamp directly. I'm putting all hopes in the schematic collection here

Thanks in advance,
Christoph

 

[crisotop]

In case anyone is searching for this as well - Clare of Orange has helped me out.

Maybe one of the tube gurus wants to have a look at the preamp section - in my understanding R33 is used to drop the voltage from ~700V to roughly ~400V (max. - as spec'd by the ECC82 Datasheet) - isn't this critical, at least during startup when the tubes are still "closed"? I can paint by numbers, but am a bit worried to fire this thing up with such high voltages involved

Thanks!
Christoph

 

[PRR]

> roughly ~400V (max. - as spec'd by the ECC82 Datasheet)

Datasheets say 550V no-current, 300V operating.

> a bit worried to fire this thing up with such high voltages involved

I'm not worried about the little tube. It's cheap, available, and has lots of resistance in series.

I sure can see (have seen) a start-up mistake cause the big bottles to light-up like a welder and then the magic smoke come out of the output transformer.

I'd really want a 2K 100W resistor in series with the power supply choke for first smoke-test. That limits total power to amplifier to well under 100 Watts, and zero Watts in a dead-short (the resistor will smoke instead).

Long-term: yes, banging 700V onto C3 450V is bad practice. (Also violates 550V rating on V1.) R9 will prevent instant explosion, and I have seen amps run this way for decades. I have seen other amps with reduced R3 and a big shunt resistor to ground, so the cold-start (or V1 missing) voltage never goes over 460V.

 

[crisotop]

Many thanks for your insights - would a zener (or 2-3 in series) parallel to C3 be an alternative to clamping the voltage to ~300V in that spot? I quickly put the circuit in LTspice, and voltages on V1 anode are ~400V, which according to datasheets I found is way to high.

EDIT: fired it up with just the ECC82 preamp tubes installed - no smoke, but V1 lit up for a second at startup, C3 shorted a couple of times. 736V before, and 450V after R9.

Thanks again!
Christoph