Single Channel AB763 amp build problems PI and Vibrato

[JGk]

Hi been working on a single channel AB763 Deluxe Reverb type amp. I changes some things around in then layouts as I stuffing into Princeton Reverb Chassis. I also changed the PI layout and added a post PI master volume fired it with good tubes and my preamp input and reverb work but the PI seems to way unbalanced and the grid going to the 100K resistor doesn’t have any AC signal as near as I can tell using a homemade circuit tracer. See the attached pic I have checked all the resistors and the caps are new also went through and re-soldered all the connections voltages are
 pin 1 321,
 pin 2 11,
 pin 3 18
,pin 6 388,
 pin 7 15
 pin 8 19.

The hookup and wiring seems in order I have a neg 40 volts coming to the 220k and going to the power tubes.

The bias vibrato circuit isn’t working either voltages there seem wrong to me I have ac going to the feedback wire but not the the pair of .1 uf caps

Voltages there are

pin 1 203
pin 2 0
pin 3 2
pin 6 430
pin 15
pin 8

I appreciate any input on these problems I’m learning more all the time but seem to be stuck here. Thanks JGK

[JGk]

Better photo of the vib circuit.

[JGk]

OK got the PI up and working had a bad cap .001 and a cracked bracket on V5 pin 9 on the heater once those we fixed she fired up. Doesn’t sound too bad I haven’t tweaked any tubes just run of the mill test stock I keep. JJ 6v6s biased around 20 mv of so. My voyages are a bit high 428 on the 6v6 plates and around 225 on pin 1 and 6 of V1. Still can’t get the vibrato to work everything seems wired OK voltages are
pin 1 213
pin 2 .5 mv
pin 3 2.7
pin 6 436
pin 7 212
pin 219

I’ll start back on it in the morning.

[HotBluePlates]

Set your meter for a.c. volts.

Got any a.c. at the wiper of the Intensity pot? Got a.c. at the cathode of V4-B (pin 8 itself or the wire from pin 8 to the junction of two 470kΩ resistors on the board; one then goes to ground, the other goes to a 0.1uF cap)?

[JGk]

Hi Hotbluesplates did a trace and have no ac signal at either place. There is AC at the junction of the 1uf cap and 47k resistor coming off of pin 6 V3.

[HotBluePlates]

Not a.c. signal ... Your trem oscillator, when on, should output a large a.c. voltage, but very low frequency (maybe 3-10Hz). Too low to hear on its own, and too low for some meters to resolve well. But you should see either an a.c. voltage at the oscillator plate (the section with 3 caps connected from plate to grid, and 3 resistors to ground or section cathode).

So you won't be looking for signal from the input jack at the points I mentioned.

Fender did several types of oscillators; for our purposes, the 2 main one are normal-on and normal-off. The blackface Princeton is normal-on; you don't need a footswitch to activate the tremolo, you need one to switch it off (or you turn the Intensity pot down to 0). The blackface Super Reverb is normal-off; you need a footswitch plugged in to give certain resistors a ground to allow the oscillator to work.

The single-channel AB763 from Hoffman is a normal-on setup. You should not need the footswitch plugged in to get the oscillator going. But in the section of the amp where the footswitch jack wire runs back to the board, there are a lot of caps & resistors, and places to make a wrong connection. So I'm trying to see if you have a functioning tremolo oscillator (V4 pins 1,2,3 if you accurately copied the layout) and to see where the tremolo signal path is broken.

See if the first half of V4 is oscillating with power applied by measuring plate voltage (pin 1) with your meter set to a.c., not d.c. There should be an a.c. voltage there. You can follow part-by-part looking for a.c. all the way through to the Intensity pot and output tube grids.

So does the oscillator oscillate?

[JGk]

Hi Hotblueplates and thanks for the insight I’m still tweaking things here finally had some time to work, reverb input and master volume seem OK some noise but not much there, still working the vibrato reheated all the connections and I must have did a cold solder on one as now I have static type noise there, tubes are OK. AC voltages are pretty funky V4 pin 1 is around 3 volts AC, pin 2 and 3 are very low .14 or so. I am also getting vibrato when I turn on the intensity but its low sounding and modulates fast not changing speeds or reacting to any controls except coming on when I turn up the intensity. JGK

[sluckey]

With power off, measure resistance from V4 pin 3 to ground and V4 pin 8 to ground. One probe on chassis, the other probe directly on the tube socket. What do you get?

[JGk]

Hi Sluckey, with the power off I get 4.7K on pin 3 and 471K on Pin 8. JGK

[sluckey]

That's as it should be. I just could not see the connections for either of those black wires on the board.

[JGk]

Hi Sluckey, I ran them out the bottom of they board. I’m going to get into it today again I may have a bad solder joint somewhere and I’ll also check the caps and resistors in the circuit. JGK

[JGk]

Ok I’m getting things to start happening with this build. The Master works fine and the amp is loud with nice reverb. Vibrato is working but is pretty weak tried different tubes and biasing still the same. Additionally I am getting a noisy static sound as soon as the amp warms up bad resistor or a cold solder joint I am thinking, checked the tubes and went through the circuit I thinks its in the PI stage so I will check that out today. JGK

[JGk]

Solving some problems and finding a few others. Got the vibrato working had a damaged .02 cap one leg was broken in side the cap. Very sweet and lush sounding. Overall the amp is quiet except there is a buzz in the reverb circuit that increases as its turned up. Checked all the solder joints and grounds appear good, changed out tubes a few times put the shields on them checked the input and output cables and connected and the reverb unit and still a buzz (sounds like I’m close to neon lights) pretty sure its in the return circuit though.

Additionally the volume pot is scratchy and I’m also getting some scratchy sounding pots on my guitars when I plug in….leaking DC somewhere a bad cap maybe? Otherwise everything is functional the Master works fine and doesn’t seem to change the vibrato much. No problems biasing the power tubes currently running about 20ma each. The voltages are all within the DR range. I have read about allot of reverb buzz in Fender circuits but haven’t found any solutions hoping someone on the forum has some ideas. JGK