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(2)6V6Champ works fine

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10/14/1997 6:57 AM

(2)6V6Champ works fine

Its up and running now, and I think its going to be OK, that is I don't think anything is going to blow up. I used it for at least 6 hours Sunday, no problem. I have a fan in there to keep the PT cool, but there are a couple of things I would like to do yet. Like wire the OTs in series so I can get 8 ohms at the speaker. Maybe then I could put a 10 in there. If someone could confirm that the way to do that is:

OT1 OT2
+ - <-> + -
spk spk

I would greatly appreciate it.

Mike

10/14/1997 8:18 AM

Mike,

If the topology you're describing is one tube per OT then you would want to hook up the secondaries of the OTs in series (each OT having a 4 ohm impedence) to get an 8 ohm impedence. I think the wires are green and black on a Champ OT. For series hook up, connect the black wire of OT#1 to the negative <-> terminal of the speaker (assuming you are using a single 8 ohm speaker), then connect the green wire of OT#1 to the black wire of OT#2. Finally connect the green wire of OT#2 to the positive <+> terminal of the speaker. This will connect the OTs in series and give you an 8 ohm load.

Steve

10/14/1997 10:02 AM

Steve,

Yes, thats the deal, one tube per 4 ohm OT. And your description of how to wire it is what I thought, but I wanted to confirm it because (1) the (2) 6V6s are paralleled (as the OTs are now) with no phase inverter to retain the single ended structure, and (2) when I drew a flowchart on paper of the actual series circuit, I had to step back and look at it as the entire signal passes through both OTs before reaching the speaker.

But I will hook it up like this. Thanks for the confirmation, thats what I was looking for.

Also, since I have the power amp somewhat stable, I've been working on the preamp, with Kevin O'Conner's observation of "Q Point as Reflected in DC Plate Voltage and Output Swing" as a guide to "absolute linearity and purity". Since there is no such thing as crossover distortion in a SE circuit, I'm trying to clean up the preamp as best I can, so the distortion that does come through is not that of clipping from untweaked circuits (from too high a positive or too low a negative half cycle-adjustable by the values of the plate load and cathode resisters on each triode), but pure uncontrollable distortion in its rawest form, once everything that can be optimized is optimized, and there is nowhere else for the signal to go.

Mike

10/14/1997 7:32 PM

It did work fine...I put the OTs in series and smoke came from the 3 1n007s in series between pins 3 and 8 on the new tube. I was fooling with some preamp stuff, but I don't think that is the problem. I shut it off. Checked all the connections and turned it back on. Blow came from the speaker like it was OK. I plugged the guitar in and played about 2 notes when the guitar signal stopped and the amp hummed a little. I turned it off and rewired the OTs in parallel. Turned it back on and the same thing happened. I played 2 or 3 notes and poof...no more guitar. I checked for resistance from the primaries to the secondaries of both OTs and got nothing. The secondaries were disconnected put the primaries were still soldered to the board, I'll try it again after I remove the OTs completely. But I don't think they are blown because I get a guitar signal for about 3 seconds before it goes. I feel real strongly that it is something really obvious and stupid, it usually is, but I'll be damned if I have a clue at this point.

Mike

10/15/1997 10:49 AM
Doc	Mike: If the secondary windings are series connected the circuit may not tolerate a grounded reference, such as is normally seen for single hookups. Is your output winding grounded or floating ? ....just an idea of something to check... Doc

10/15/1997 5:17 PM

Doc,

The secondary on the Champ OTs have 2 leads, I assume one from each end of the winding. There are no other leads except the primary. If the case of the transformer is the ground reference, I suppose I could put a piece of cardboard or something between the OTs and the chassis. But it doesn't seem to me that it is a grounding problem, because they do work for a hot second. Something is loading up when a voltage from the guitar signal is applied.

Mike

10/16/1997 7:59 AM
Steve M.	Mike, Have you tried insulating the output jack on the Champ from the chassis? What was the speaker and its impedence that you used for this project? How are you driving the power tubes? Regards, Steve

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