AB764 Vibro Champ Build - Voltage Questions

[bridgford]

Hi,

I have built a AB764 Vibro Champ/Bronco clone, which seems to work well, is generally quiet and clean, but has a few voltages that don't match the reference voltages on the schematic/layout. I am not sure about where all of the pots are supposed to be at while taking measurements, as it seems that they would affect readings, but the schematic doesn't make any comment on that issue. Volume was always at zero, though the remainder of the pots were not set to any specific value. Also, the RCA jack for the tremolo was grounded. Anyhow, house voltage was 122V, B1=381V, B2=373V, B3=358V.

V1 (12AX7)
Pin 1=233V (255V) Reference voltages from schematic in parentheses
Pin 2=1.71V (1.9V)
Pin 6=325V (260V)
Pin 8=4.23V (1.7V)

V2 (12AX7)
Pin 1=156V (210V)
Pin 3=1.80V (2.0V)
Pin 6=375V (410V)
Pin 8=193V (215V)

V3 (6V6GT)
Pin 3=348V
Pin 4=374V (410V)
Pin 8=21V (24V)

V4 (5Y3GT)

Also, with the tremolo RCA jack shorted, if I turn the Intensity all the up, it reduces the output volume. Should this happen, or have others experienced this. Or have I missed something when I wired it together.

Any insight into the reference voltages, as to whether this is even an issue, is appreciated.

Thanks
Dale

[g1]

V1 pin 8 is the only one jumping out at me.
Double check it, and check the 2 resistors from pin 8 are the correct value (1500R and 47R) and going to the correct places.

[bridgford]

Thank you for your reply. I had concerns with Pins 6 and 8 on V1 since my B1 voltage (383V) is lower than the referenced 420, and all of the other measured voltages are proportionally lower than the reference voltages. Seems they should be lower, also. I will take a look at all of the resistors that connect with Pins 6 and 8 this weekend. That is why I was curious if the pots (volume, bass, and treble) had any bearing on these measurements.

Thanks, I appreciate your time.

Dale

[bridgford]

So I took the chassis out and measured the resistance of the two suspect resistors. They were 1460 and 52, respectively. With much trepidation, I submit a picture of the board. Bear in mind, this is my first build and a proof of concept. It started life as an Estey organ..

[Chuck H]

Thank you for posting the board, but it doesn't help much as it is. Component values can be seen on the resistors but the circuit is impossible to trace.

You definitely have something wired differently from the schematic if those 410V readings are accurate. It should be impossible to obtain that voltage because it exceeds the available voltage. So something is hinky. Did you include a standby switch on this build? If so, did you take some readings with the amp on standby? Perhaps those 410 readings are unloaded voltages.

As to pin 8 on V1 that g1 pointed out, those values you measured are fine. So something else is going on there. The NFB loop is elevated on that circuit so I suspect there may be DC on the NFB lead from the output of the amp. That would be bad. Test for DC on the output jack.

[bridgford]

Thank you for your post. I think the transformer I bought for this project may have been designed for the earlier AA764, which had a lower output secondary. My B1 voltage off of the rectifier tube is 381, while the schematic shows the 410 voltage. The highest measured DC voltage in my build is 381 and there is no stand-by switch, yet.

I agree that something odd is going on, as all of my measured voltages are lower than the AB764 reference voltages, as I would expect since my PT has lower output. Except for Pins 6 & 8 on V1, which are higher than the AB764 reference voltages.

So, if I unsolder the negative feed back lead from the positive lead on the secondary of the OT, could that play into it? The other thing that just dawned on me is that I am using the Estey organ's OT, but I don't see how that would affect the tube that receives the instrument input.

Again, thank you for your advice.

[Chuck H]

I think the transformer I bought for this project may have been designed for the earlier AA764, which had a lower output secondary.

Won't matter

My B1 voltage off of the rectifier tube is 381, while the schematic shows the 410 voltage. The highest measured DC voltage in my build is 381

Ah! Right. Good then. My bad. You explained that.

So, if I unsolder the negative feed back lead from the positive lead on the secondary of the OT, could that play into it?

That wouldn't solve the potential problem of DC on the output jack. So I'm not sure what you mean by "play into it". The wire is SUPPOSED to be there. If it being there doesn't work it's because something else is wrong.

The other thing that just dawned on me is that I am using the Estey organ's OT, but I don't see how that would affect the tube that receives the instrument input.

The instrument input goes to one triode of that tube. The second triode of that tube drives the power tube and is connected to the NFB lead. The NFB lead is connected to the output jack. Since the NFB circuit floats on the second triode of that tube, if there were voltage on the output jack that voltage would be divided by the NFB circuit and also float on the cathode of that triode. This could only happen if the OT is bad. Since the OT is an old and not necessarily of known quality there is reason to suspect it. And THAT is how the output transformer can affect the instrument input tube.

Check for DC on the output jack.

[g1]

I guess it would help to get the schematic up in here.
After looking at it I see pin8 of V1 also goes to the intensity pot.
Was the intensity turned all the way down when you measured the pin8 voltage?

[bridgford]

Thank you for your suggestions g1 and Chuck H.

Chuck H, What i meant by "play into it" was would the negative feedback affect voltages on Pins 6 or 8 on V1? If I unsoldered (is that what they mean when they say "lift the ..."?) the NFB lead at the speaker jack and remeasured voltages, and at that point, they fell in line, it would also meet your other point about checking for DC voltage on the secondary of the OT, correct?

As for schematics, I am referencing http://schematicheaven.net/fenderamp...hamp_ab764.pdf, which is the cleanest version and matches the AB764 Bronco. On Sheet 1 of 2, lower left, it provides the caveats, but I don't understand the "(except V2)" part of #1; "Voltages read to ground with V.T.V.M with Vibrato-grounded (except V2) - no input signal.

Also, the notes to the schematic referenced above are silent as to what any of the pots should be set at, as I alluded to in my original post. Both Speed and Intensity are reverse audio curved, so if you were to put them at zero for a regular pot, they would be at minimal resistance rather than the max specified resistance, correct?

While composing this post, I also ran across this one (http://music-electronics-forum.com/t17672/), which I will be reading now.

Thank you both for your assistance, I really do appreciate it.

Dale

[g1]

The note means most of the voltages must be read with the vibrato footswitch jack grounded. Except the voltages given for V2 are with the vibrato running. Control pot settings should not matter.
Right now we are concerned with pin8 of V1, so please recheck pin8 voltage with f/s jack grounded and intensity pot set to zero.
If it is still far off from the 1.7V, check for DC at the output as Chuck mentioned.

[bridgford]

With all pots fully ccw, here are my measurements:
House voltage was 122.2V AC
Red leads to 5Y3GT
A=343 VAC
B=344 VAC
V4 (5Y3GT) Pin 8=382VDC

B1=382V, B2=374V, B3=348V.

V1 (12AX7)
Pin 1=225V (255V) Reference voltages from schematic in parentheses
Pin 3=1.66V (1.9V) (I made a mistake in my first post, reference voltage was Pin 3, not Pin 2 as I initially posted)
Pin 6=222V (260V)
Pin 8=1.65V (1.7V)

V2 (12AX7)
Pin 1=154V (210V)
Pin 3=1.78V (2.0V)
Pin 6=367V (410V)
Pin 8=190V (215V)

V3 (6V6GT)
Pin 3=345V
Pin 4=368V (410V)
Pin 8=21.1V (24V)

No DC voltage on the positive lead on the secondary of the OT.

So, apparently all is well. The trick was to have all the pots turned fully counter-clockwise.

Thank you both, g1 and Chuck H for your time and input. I really appreciate it.

Thanks,

Dale

[Chuck H]

g1 got this one

So... How does it sound?

[bridgford]

When my daughter's boyfriend plays it, it sounds very good. Not so much when I "play".

It does have a low hum at zero volume and a buzz as volume is cranked with nothing in the inputs. Once, while I was checking voltages, it was dead silent, I pulled the tremolo shorting plug because it kind of freaked me out. That brought the hum back. Which brings up a question, if the 6.3V heaters are a cause of the hum, wouldn't it be possible to run the green leads through a full wave rectifier?

Thanks,
Dale

[Chuck H]

DC filaments are the long path to hum-less-ness for this model amp. Very low hum AC can be done without the high current DC circuit and it's associated components. Though this is pretty manageable anymore, actually. Still... Using a simple twisted pair arrangement as seen on most of Fenders push/pull amps, and then using a false CT with a pair of 100R resistors and their junction wired to the top of the power tube cathode resistor (rather than ground) is a very satisfactory arrangement and there are no silicooties to heat sink.

[Enzo]

Turning the heaters to DC will ONLY affect the hum caused by heater wiring. There are many other sources of hum, and that will not affect them.