New Build Start-Up Problem AB763

[Duck Dodgers]

Correct. It's the 6L6 version. I built it in a single channel, single input configuration. That is; no "normal" channel and a single jack input. Tubes are all Groove Tubes, transformers are Hammond.

[bnwitt]

Did you perform the 6L6 tremolo mods to make the tremolo stronger?

http://www.el34world.com/Forum/index.php?topic=604.0

[Duck Dodgers]

I had not seen the thread you referenced. Some good info. I'm thinking that the trem isn't oscilating.

I do have a 25mfd cap on the osc cathode. I will try the other mods tonight. Also the bias....I'm set at 30ma.

Thanks!

[Duck Dodgers]

OK...tried the bias first. Everything from 15 to 40mv.
Then did the component changes...100K in place of 470, .047mfd in place of .1 and .022mfd in place of..I forgot! (Did this before dinner..)
Anyway still no tremolo.
Suggestions?

[loudthud]

The voltages don't look right on the oscillator tube. Cathode and Plate are too high. Can you measure the cathode resistor to ground at the tube pin?

[Duck Dodgers]

It's Alive!!

OK, I'd checked everything elevendyseven times. Per loudthud's post, checked osc cathode to ground one more time and read 47.3K......should be 4.7K ish.
Lifted the resistor to measure it...oopsie!!
So much for old eyes and the color code. (Bad Boys Ra.....well, you know.)
Put in a 4.7K and.....Tremolo lives...still need to tweak a few values for speed and intensity.
Thanks to bnwitt for that info as well.

Thanks to all.

Best Forum EVER!

[Duck Dodgers]

Tremolo depth and Hissssss

Had some time to work on the amp this weekend.

First issue I'm working on is a background hiss. Pretty much white noise kind of sound. It's not affected by the volume control and it will disappear when I pull the reverb recovery tube. Have a very faint 60Hz over this at full volume.

Second issue is a variation of the Thumping Tremolo thread from tubeswell going now.

I've been experimenting with values per the info sent by bnwitt (see above) to get more depth. I made up some parts on jumpers and tried modifying the cf buffer cathode resistor that goes to the caps and the .1mfd cap that goes to ground. I also monitored the 6L6's bias while doing this.

Anyway, after progressively lowering the values, best results were with lowering the 470K to 82K and leaving out the .1 cap altogether. I set the bias at 18ma which raises to 29-30 with the tremolo cranked full intensity.

I will get a thumping at high intensity and volume. At lower speeds it sounds like someone tapping a microphone with their finger...hiss-pop...hiss-pop.

I'm concerned leaving out the cap could cause other problems. Will this be OK? Any suggestions on the hiss?

Thanks everybody...been a great help so far.

[Duck Dodgers]

Update

OK, progress so far:
Tracked hiss to reverb recovery tube. Swaped second stage 1.5K/25mfg cathode parts for a LED (blue, hi intensity). Hiss is GONE (except...see below). BTW also tried a metal film resistor (original is cc) and higher rated part. No luck with these.

Tremolo thumping: played around with values per bnwitt. Could change speed and depth but thumping persisted. Put a diode across intensity pot. Cut thumping about 95%. Will only slightly thump at full speed and full intensity. If you play at any volume, esp. chords, you can't hear it.

There is a slight hiss that follows the tremolo. Almost sounds like someone breathing thru a stuffed nose. Only noticeable when you're not playing.

OK for now. Ordered lasered panels (Prodigy), Weber speaker (15") and will start to build cab. My son (amp is for him) wants Blonde with Oxblood.

Comments??? Suggestions??

Thanks again guys (and gals?)

[TheTinMan]

For that "hiss following the tremolo", try chopsticking the leads from the board to the Speed and Intensity pots. IME it's best to have those leads go straight up from the board and away from everything else, especially the return from the Reverb pot.

HTH

Chip

[Duck Dodgers]

Chip....thanks for the suggestion. Hiss was pretty well surpressed and when the amp was put in the cab it became 99 44/100% quiet. Much better than most commercial amps.

There's an aluminum liner (about .01 thk sticky back) in the top of the cab. I think being fully shielded did the trick.

I posted pics in the Building Your Amp section.