Monoblock blues.

[catch29]

Just finished a refurb' on another pair of Heathkit MA-12's. Completely rebuilt apart from the transformers, there never faulty are they!?. Amp 'a' fine. Amp 'b' oop's. Powered up loaded but without the rectifier valve, all heaters working, pop in the GZ34 and the el84's start getting brighter and brighter and pop/flash blows the resister between the pins to the output transformer, also ruins one el84!. So I replace and double check all wiring and resister and cap values, also swop valves with the good amp. Power up, all good for 20seconds then the new el84 start getting brighter - I turn off quick before I loose any more valves!. What can be wrong?, I check everything again against the good amp and AH! a difference, the good amp shows a resistance of about 80ohm between the two sets of pins on each el84 socket attached to the wires going to the output transformer, the dodgy amp is open circuit!!. I've probably answered my own question here, I have two pairs of wires with a break in each, unlikely, or the transformer has failed internally, probably knowing my luck. Would this fault produce the strange behaviour of the valves?. Your thoughts would be welcome.

[Hartley118]

Defective valveholder? Those ‘top quality’ McMurdo valvelders from the 1960s are nowadays suffering from embrittlement of their brass bifurcated contacts and may have lost contact with some valve pins.

Martin

[Kalee20]

It almost sounds like the anode connection is open-circuit.

That would make all the cathode current go to the screen-grid which would indeed glow bright orange, what you could see of it. It might fuse the grid wires, which could then spring any which way.

[joebog1]

Did you check for continuity and resistances for the windings of the transformers, before you started the upgrade ?.

The FIRST check I make, without even removing valves from their bases, is the quality of the iron.

Joe

[catch29]

Well, problem solved. I have a S-99 waiting re-furb' and it has the same output transformers, Hinchley 51-507's, so I've 'borrowed' one and now the MA-12 sounds sweet. Thanks for your input guy's, now I need to find one for the S-99!.