Re: Question on JCM 800 2204 conversion...
- From: "RichL" <rpleavitt@xxxxxxxxx>
- Date: Tue, 20 Dec 2011 19:32:48 -0500
"Jim" <inse@ttle> wrote in message news:deSdncNaYZm0SnXTnZ2dnUVZ_jednZ2d@xxxxxxxxxxxxxxxxxxxxx
I was able to "piggy back" a single resistor parallel to one in the bias circuit to get enough range to bias the JJ E34L -or- GE 6550A that I had. (Note: JJ E34L are different than JJ EL34, and bias up a bit different from what I understand. I don't know that it will bias up for other EL34 because I've never attempted it.) I did not move the NFB to the other OT tap because I was unsure that I was going to stay with EL34.
To do it "right," you'd change the couple of resistors in the bias circuit (which would eliminate the ability to go back to 6550) and move the NFB to the correct OT output tap. But I'd rather have the range to dial in both tube types, if possible! It might be quicker to "do it right" than experiment with a value that with work with two tube types. But it didn't take me long at all.
Please report the end B+ along with date of production.
Follow up. Today was the big day. Brought the amp in after lunch, together with a schematic that I had marked up. I didn't expect to hear from the amp tech until late today, if not days later.
Anyway I'm on my way home from work and I get a call from the shop on my cell. This guy who's working on my amp starts asking ME (as if I'm an "expert") what the deal is on moving the NFB wire to the 8-ohm tap. So I have to explain to him first why that's done, then tell him that the right wire is probably located on the speaker selection switch. "AHA"! Then he says he doesn't need to change ANY of the resistors, he can get between around 30 and 47 mA without changing a thing. I'm battling traffic, I don't like talking on the cell while I'm driving in the first place, so I tell him OK, figuring I'll call him when I get home (which was five minutes later).
Anyway I call back when I get home and he says "all buttoned up, ready to go!". I ask him what the plate voltage was, after all I had written clearly on the schemo that I dropped off, "check the plate voltage". He says, "I dunno, I didn't check it, it's usually around 500 V, right"?
Hey, he's the expert, right?
I was exhausted after a long day at work, and I didn't feel like going out tonight to pick it up. If he had called 20 minutes earlier, I could have grabbed it on my way home. In any event, for all I know it may sound great, I'll pick it up tomorrow and put it through some paces.
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