New Build Start-Up Problem AB763

[Duck Dodgers]

I've completed a single channel AB763 using the Hoffman layout and my own design turret board.

A initial power-up w/o tubes PS and transformer voltages looked good. (I'm running about 80vac from my variac.)

Plugged in the rectifier (GZ34) switched out of standby and after 3-4 seconds had funny squeaks and growls and then the fuse blew.

Lifted the B+ from the board (replaced the fuse) and measured 360vdc. Checked resistance of B+ bus on the board and found it was grounded. To make a long story short...traced it to the choke (Hammond 194A). The input lead is shorted to ground. Thinking the choke may have been a symptom, rather than the cause, I checked the resistance of the bus to ground after the choke was pulled. Measured 16.8Mohms. (No tubes other than the rectifier.)

So, it seems it was the choke but I know they're quite reliable. Am I on the right track? Or, does anyone have a suggestion for any additional checks?

Also, it will take about a week for me to get a replacement choke. Can I put a resistor in its place to continue testing until the choke arrives?

Thanks...

[Don Moose]

Or, does anyone have a suggestion for any additional checks?

Was the board mounted in the chassis when you took that 16M8 reading?

Also, it will take about a week for me to get a replacement choke. Can I put a resistor in its place to continue testing until the choke arrives?

Yes, you can use a resistor - you'll want a several-watt one, say 10W for safety.
Value? The 5E3 deluxe used a 5k0 (nearest standard values are 4k7 and 5k1)

Hope this helps!

[Duck Dodgers]

Yes, board was in the chassis. I measured from the B+ bus where the choke output connects to chassis ground.

As I said above, seems like the choke but considering how reliable they are I'm concerned there's something else down stream. Any thoughts?

Thanks!

[tubeswell]

Hi there

I see that the AB763 only drops 10V from the plate supply node to the screen supply node. When you factor in that you need about 27mA on the B+ rail at that point (22mA in total for the current through all the pre-amp tubes plus say 5mA for the total screen current for both 6V6s) therefore; 10V/.027A = 370R. So maybe a 390R resistor would be about right to achieve that same voltage drop for the same current? In which case a 1W resistor should be fine (10 x .027) should it not?. (Maybe 1k resistor at max?). 5W would be more than enough rating

[Duck Dodgers]

I have some 470 ohm 5 watt resistors.......sound close enough?

I also dug up a 5.1K 10W......probably too much.

I should get a chance to work on the amp tonight. (West Coast US time.)

[tubeswell]

Try it and then measure the voltages and see. If you don't think the 470R drops it enough, try 1k, or 4k7

[Duck Dodgers]

Tried the 470 and 5.1k.....no other tubes but the rectifier....everything behaved.
Not much time tonight...but this is where the fuse blew before so, maybe the choke was bad.....
I'll do some more testing tomorrow with the preamp etc.
Thanks Moose and Tubeswell.......

[Duck Dodgers]

Replaced the choke I had an issue with (in previous post). Checked voltages, loaded up the tubes and..a tremendous hi pitched howl from the speaker as soon as I came out of standby.

Pulled and replaced tubes individually starting at the preamp. Noise stopped when I pulled the phase inverter. Also, for what it's worth, no noise with the phase inverter in and no power tubes.

Rechecked the voltages (plates, grids, cathodes etc.) all seemed within reason. Traced the wiring and didn't find anything but we all know it's tough to check your own work.

This is the Hoffman version with the Vibroverb tremolo. I built it with just a single input vibrato channel (no normal channel).

Any suggestions from the guru's?

Thanks!!

[martin manning]

Try reversing the OT primary leads; you probably have positive feedback going on. The negative feedback (that's what it's supposed to be, anyway) is fed in at the PI, so it makes sense that it quits with the PI tube or the power tubes out.

MPM

[Duck Dodgers]

Martin........right on!!! Reversed the leads and bingo!
A little back round noise (lead dress?) but great sound and reverb Dick Dale would die for (3 spring and added a dwell).
Tremolo not working. I will look into it tomorrow. Newb question...is the trem active with the pedal jack grounded or open?
Thanks guys!

[JoeM]

Tremolo not working. I will look into it tomorrow. Newb question...is the trem active with the pedal jack grounded or open?
Thanks guys!

If you've wired it like a stock Fender, the trem won't work without the pedal jack shorted.

[Duck Dodgers]

This is the Vibroverb 6G16 tremolo on the AB763 Hoffman layout.

Oscillator side voltages look OK...a little high maybe but proportioned about right (339 plate, 4.5 cathode) Voltages from the bias supply to, and through, the intensity pot look OK (-42 ish). The plate of the c.f. follower is 400vdc. The cathode measures 350vdc which seems very high to me. I can't find anything connected wrong or shorted. Is this value OK?

If the voltage is OK any other suggestions for trouble shooting?

Thanks again gang.

[martin manning]

The cathode voltage on the CF should be more like 200V, I think. The Plate of the oscillator should be pretty close to that value too, since it is going to set the grid voltage of the CF, and the CF's cathode will come up close to that value, give or take. Check that you have the right value installed for the plate resistor. The 6G16 circuit shows 470K for Rp on the osc and 470K for Rk on the CF. Rk on the osc is 4K7.

MPM

[Duck Dodgers]

Checked values and circuit again. All looks OK. Also resoldered everything. Still no tremolo.
When checking R values the only thing I noticed was the meter took awhile to settle down on the c.f. cathode resistor. I have no idea if this is a clue.
Short of swapping out parts I'm stuck. I used silver mica caps in this part of the curcuit, if that has any bearing. (I used Mallory 150's for the signal path...amp has nice tone.)

Next steps?

[bnwitt]

Duck,
so you've got a hoffman AB763 with the brown era bias vary tremolo. Is this a 6V6 or 6L6 version?

[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]

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]

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]

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.