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Re: Several questions...

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9/6/1997 7:40 PM
John Edwards	Re: Several questions... In the last post, I meant to say, add dropping resistors to the center-tap and the screens (there aren't any screen resistors). Also, didn't mean to imply that plate glow indicates cold bias-I DO know better than that! I just meant to suggest that the bias is probably within range. Still haven't had time to check it yet.

9/6/1997 10:38 PM

Report from tonight:

1. Charles B. asked via E-mail how I measured the plate voltage. I simply measured plates to ground--but since this is a cathode bias amp should I have measured to the cathode? I'll do that next.

2. The 6V6's are drawing about 42ma each-OK for cathode bias, right? (BTW, I went back to the Phillips tubes for the time being. They've been in for awhile and have held up, and I don't want to take a chance on smoking the RCA's...)

3. Voltages on the preamp/driver: 128 and 136 on the preamp, 178 and 210 on the driver (pins 1 and 6 respectively).

4. No discernable clues from the scope, either reading the guitar or a sine, square, or sawtooth wave. I don't think the tone generator, even at high level, was triggering the harmonic. I switched to another guitar to make sure I could still hear the overtones (to re-confirm it wasn't something squirrely with the guitar), and they were loud and clear. I played with several different range values on the scope but the guitar signals were jumping around too much to really be able to tell much. There was some slight crossover with the tone generator, but it was real minor and didn't occur until significantly after clipping. It also didn't vary with frequency.

4. Maybe a SPEAKER problem? I brought in my Deluxe Reberb. I had put a Mojo MP12R HD speaker in the DR at the same time I put the MP12R in the tweed. The only difference in the speaker models is the HD has a plastic coil former instead of paper. So, I subbed the HD (still in the DR cabinet) and guess what? Couldn't hear the "dial tone" on the F# and G#, and only a faint dial tone on the G (whereas the G had real intense nasty crossover hide-the-women-and-children distortion on the non-HD speaker).

So, ANYWAY! A combination of factors, maybe? Amp running too hot, and the excess gain drives the lower-power-rated speaker into crossover distortion at a particular resonant frequency???

Man, my head hurts...

Regards,
John E.

9/6/1997 11:10 PM

Boy, is my face red... I can't even bear the thought of posting this on AMPAGE...

I measured the voltages on the plate PINS with the tubes REMOVED, thinking that was OK... voltage is potential, right?, and it didn't occur to me that voltage would vary with load (duh)...

Anyway, with the tubes IN the socket, plate to ground is 364v, plate to cathode is 342v. Back to square one. Speaker??? (see previous posts)...

9/7/1997 9:31 AM

Ah... that's better... the tube are not going to cook at 340V across the tube. That is (342x0.042)=14.3Watt; the 6V6 is rated at 14W, and many Fender amps run them at 18-20W, so you're OK.

If it changes when you change speakers, think really hard about it being either the speaker or a resonance (or loose screw!) in the cabinet. Try removing the speaker from the cabinet and seeing if the annoying cross modulation is still there. If so - speaker. If not, probably cabinet or some thing loose in the cabinet.

You're pretty close to gettting this one nailed.

9/7/1997 8:19 PM

First of all, I apologize for wasting your time and bandwidth on a problem I should have been able to solve for myself... (BTW, the cabinet's tight as a drum, and the speaker can't have more than ten hours on it... but it's bad! I was just determined to have it be a problem in the amp... And I've gotta say I've never heard a speaker resonate in that range before.)

Now, I've gotta deal w/Mojo for a replacement. This oughtta be fun.

Anyway, as always, thanks for your time, patience, and help. BTW, I'll buy your book, too!

Regards,

John E.
scetv2@teleplex.net

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