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]

MORE Startup issues..AB763

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]

Tremolo not working

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?