Champion GM 8
Posted: Sat Jan 07, 2017 6:49 pm
It´s more rare these days but this was a pleasant surprise! A Swedish made Champion PA amp from the early/mid -40´s.
Surprise? Why´s that? Well see..Champion GM 8 is a rather low powered and small push/pull amp that uses a pair of ECL-11´s for power. ECL-11´s being a "mix" tube. Triode and tetrode in one bottle.
A while ago i did a GM 20 which kind of etched. A twin EL-12 amp that certainly put the goods out. This tho...a smaller jobbie.
A complete half small PA. Haven´t measured the diameter of that speaker but it isn´t overly small at least. Now..take a note of that piece of plywood...
...which seals off a compartment where the speaker wire is stored. Jack for it tho...a no-no as it´s of the same type used as mains jack back in the day here in Sweden.
Yep. Adjustable primarys.
As i first saw the pictures of the amp i thought it was an EL-12 amp. Not so. Didn´t know that it was a GM 8,which traditionally means 8 watts,and thus.. The GM 20 looks basically the same just this one carries fewer pots. That´s the difference them between,and now with amp in hand..this one´s a tad smaller too. ECL-11´s then. A tube released in 1939 and it must have been all the rage as a mix tube and super avant garde at the time. Powerwise...unsure,or was,as i laid eyes on them. Rather large,about the size of EL-12´s approx. So. Checked e-bay and the going rate for them is between 10 and 50 Euros a pop. IOW not all THAT bad,IF they turned out to be da shiznitz. Then again..not exactly mundane glass either is it...
Onboard we also found a 6J5. A single triode octal based jobbie.
...and in turn the very fabled EF-12. A tube of guts and glory for various reason. Famous as it powered a number of real high end microphones and mic preamps of the time. A tube i´ve worked with previously...and that one stays no matter.
...in turn an AZ-4 as rectifier. Overkill you won´t believe. A qualified guess is that the amp sported an AZ-1,which looks the same,and got replaced by this AZ-4 as that was what was at hand. Well. The 4 carries 200mA easy..and with this amp we´ve got no use for that what so ever,and the 1 in turn a 100mA.
Having measured DC resistence of the speaker i´d say we´ve got a 4-8 Ohm load setup here. Good enough. There is yet another set of taps that were just soldered to a solder support and i guess that´s for either a negative feedback system or some sort of line system. 100V or whatever..
Speaking of which..this is a little different. The HT centertap does NOT reach ground instead is does so via this big ass resistor which in effect brings the entire amp to be ground lift. Different to say the least. This resistor in turn doubles as cathode resistor for the pair of ECL-11s too. As usual with the old Champion amps it looks like a friggin grenade went off under the hood. The stock "tower",can,e-lyte was dead as a doornail and had been left in place while being replaced within by an axial. This job done ages ago too...
Kind of says it all doesn´t it?
So what´s the idea here? Phase 1 is to get the thing running. At this stage i had the AZ out of there and just jumpered mains voltage in there and flicked the switch. The PT is alive at least. So idea 1 here is to replace anything capacitor and reference measure the resistors at hand to get the POS running again. Reason for that is that i want to hear them ECL-11´s tell me what they´re made of. If they come up short i´ll just turn this into a dual 6V6 amp,something the transformers will handle with easy.
Have checked datasheet for the ECL-11´s and they are on par Ra-a wise with 6V6´s so..need be..