Single Channel AB763 W/no trem

[whoops]

That's the plan. But it is an interesting point you bring up. For most of the members on the forum that is the case. My local supplier is terrible, and shipping from mouser is $20 to Canada, Digikey is decent at only $8, but its not exactly fast. For one off parts, it's actually a fair bit more expensive and time consuming. Sorry for the mini-rant. It is a source of frustration for me.

Where are you located?

You can get a part that works from your local supplier and then in the future when you do an online order from a better supplier you add the "better" part you want.
Just make it work with what you can have now.

[dunner84]

I live in Victoria. I travel a fair bit for work, and I usually pick up parts I need when I'm away. Halifax has a fantastic parts store, so I stocked up on a few bigger parts when I was there. I was also just in Boston for a few weeks, and I was able to stock up on filter caps, turrets, and grill cloth.

I pulled the bypass caps, and I clipped in some spare filter caps. Problem is still there. That was really the only time to troubleshoot I have had the last several days. I still plan to post pictures, layout/schematic, and a few audio clips. I may not be able to do so until Wednesday.

[dunner84]

I have attached the schematic and layout I used for the build. My new voltages are included in the table in the schematic. I took two soundclips as well. One is of the distortion issue. The volume is at 1.5, and you can hear how the amp sounds ok when the guitar is played gently, and the nasty distortion comes in when played moderately. The other is of the noisefloor of the amp with V2 and the reverb tank installed. It is much noisier with them.
SoundClips:
https://drive.google.com/open?id=0B2gStVV-5NtrVGdtV2hnTlFUNUE

I will post gut shots shortly.

[PRR]

Put the volts near the pins. Also B+ node volts.

It looks wrong both sides of V3. Too-too-small drop on the plate resistor, yet a not-wrong cathode voltage.

[dunner84]

Voltages with diode rectifier.
B+ Nodes:
A - 481
B - 479
C - 466
D - 432

Measured resistance is 830 Ohms

[PRR]

??? Are you talking a different setup and all new voltages?

LOOK at what I sketched for your V3 voltages. Plate should NOT be 92% or 284/308 (or whatever it is now). Plate should be, for Fendery stuff, 70% of the node B+. Also the currents should add-up.

[dunner84]

The setup hasn't changed. I used the diode rectifier when I measured the voltages for the chart. All of them were taken with respect to ground.

Edit- sorry, I see where the confusion is. I did not notice the node voltages on the schematic when I was updating it. The main voltage chart is correct, but the b+nodes labelled on the schematic are not. I apologise, I did not notice them there until now.

[dunner84]

I dialed in my tone generator to 100mVac and traced it through the circuit today, and had some interesting findings. First of all, the tone came through very loud and clear. Second, the voltages were odd. I had 2.7Vac out of V1A, 11Vac out of V1B, 18Vac out of V3 and over 110Vac out of the phase inverter!! I double checked those voltages, and they are correct. Those are also with the volume control turned all the way up.
This is also with V2 pulled and the reverb tank disconnected.

Is the average amplitude of a guitar much higher than 100mV? If it is, that would explain the volume is so overdriven, but I am not sure how that would relate to low output volume.

[whoops]

Is the average amplitude of a guitar much higher than 100mV? If it is, that would explain the volume is so overdriven, but I am not sure how that would relate to low output volume.

[shooter]

Is the average amplitude

"my" standard is typically 50mVrms.

make a cheater n plug your guitar into the scope, hammer 1 note, softly brush your tips over a string, divide by 2

[PRR]

These amps full-up should clip with 20mV input.

I've lost track of your tubes, but if 110V at PI becomes 22V, that's pretty-near clipping for the grids of most power bottles.

100mV-200mV is more typical playing, but either you turn-down some or you want some clipping.

If you have that much inside the amp but it isn't screaming as it should, you have bad contact or bad parts in the final stage from big bottles to loudspeaker.

[dunner84]

I injected my 100mV tone, and adjusted the volume on the amp until there was 10Vac coming out of the PI. With 10V at the output of the PI, I get 69Vac and 79Vac at the plates of the 6v6s and 135Vac between them.

There is something odd though. My 100mV signal comes through loud and clear. It's crystal clear and annoyingly loud when set with 10V leaving the PI.
I plug in my guitar, and low distorted output. I measured the output of the guitar at the input jack and it varied between 40mV and 300mV. Regardless of the amplitude, the amp sounded the same.

[tubeswell]

Have you checked to see whether any of the coupling caps are leaking DC? That can result in the symptoms you describe.


Also check that the ground side of each filter cap is electrically connected to the ground return. If the pre-amp cap ground is not connecting, it can give you funny voltages and cause unwanted coupling between the pre-amp gain stages through the power rail, which results in mushy distortion

[dunner84]

I have checked for leaking DC and there is none that I can find. All of the filter caps are connected to the same ground point at the power transformer, and the connections are all sound. This is a weird one. I will definitely keep at though and report back any new findings, and hopefully a solution!

[Willabe]

I have checked for leaking DC and there is none that I can find.

How did you check?

[sluckey]

I didn't see anything in his voltage readings that would indicate leaky coupling caps.

[tubeswell]

I didn't see anything in his voltage readings that would indicate leaky coupling caps.

He hasn't reported any DC voltages on the AC-side of the coupling caps. Even 1V will throw off the bias at the grid of the following pre-amp stage, and reduce gain.

[tubeswell]

I have checked for leaking DC and there is none that I can find. All of the filter caps are connected to the same ground point at the power transformer, and the connections are all sound. This is a weird one. I will definitely keep at though and report back any new findings, and hopefully a solution!

Hi dunner. Did you test the caps like Fig4-4 (P57) of the attached chapter shows?

[PRR]

> He hasn't reported any DC voltages on the AC-side of the coupling caps. Even 1V will throw off the bias at the grid of the following pre-amp stage, and reduce gain.

The thrown-off bias will usually be seen in the plate voltage of the next stage. We would expect say 170V and find maybe 100V. I have not looked, but I trust Sluckey would have hollered if he saw that.

[dunner84]

I had more time today, and I made a big discovery. My power tube screen resistors are not 470ohms...... They are 470k. I am not sure if that is what is causing my issue, but it could be, and I want to swap then out. I don't have any larger wattage 470r on hand, but I do have a bunch of 1K 1/2 watt that I can parallel. Would 1 watt suffice for testing purposes? I think the current resistors are 3 watters.

[sluckey]

You'll be fine using two 1/2 watt 1Ks paralleled for testing. Fender used 470Ω/1W on most all those blackface amps.

[dunner84]

I swapped in some 820s in parallel and IT WORKS!!! The nasty distortion is gone, and the amp sounds as great as it can through the test speaker on the bench. I don't want to know how many times I checked those resistors. On the bright side, I learned a tremendous amount, and it is mostly thanks to all of the help I received on these forums, so thank you!!! It is amazing how much more I learn after each build.

The voltages all look really good now. The jj6v6s are sitting with 448 on the plates, -39 at the grids, and 30mA flowing at the cathode which is as cold as I can bias them. I will be tweaking the bias circuit a little more, but I have some NOS tungsol 5881s that I am thinking of throwing in. The voltages will also drop a bit once I replace the diodes with the 5ar4.

The last issue to sort out is the reverb. It comes through very loud and clear when I tap the tank, but is very weak when coming through the signal path. It also kills the volume of the amp when I turn the reverb passed 5, and causes the power tubes to glow blue.. I think I will remove the dwell pot, go to the stock configuration, and proceed from there. In the meantime, I have a functional amp that I can finally enjoy, and start to build a cab for.

Thank you again for all the help. This was a weird project for me, and I really appreciate everyone putting up with how scatter brained the whole thing was.

I will post pictures when it is all finished, and will probably be back before then if I can't figure out the reverb.

[tubeswell]

The last issue to sort out is the reverb. It comes through very loud and clear when I tap the tank, but is very weak when coming through the signal path. It also kills the volume of the amp when I turn the reverb passed 5, and causes the power tubes to glow blue.. I think I will remove the dwell pot, go to the stock configuration, and proceed from there. In the meantime, I have a functional amp that I can finally enjoy, and start to build a cab for...

What plate and cathode voltages have you got on the reverb driver tube? (And did you test the coupling cap going to the reverb driver for DC leakiness?)

[dunner84]

V2 Voltages

1 - 435
2 - 40mV
3 - 7.9
4 -
5 -
6 - 447
7 - 40mV
8 - 8

It looks like there is about 40mV leaking on the grids. I have heard/read conflicting info on the allowable voltage on the grids. Also, it doesn't look like the reverb driver has a coupling cap. It is tied directly to the reverb transformer.
It is possible I damaged the reverb transformer when I first started my testing. This was my first reverb build, and I did not know the tank and reverb driver needed to be disconnected. When I checked the winding resistance, I remember the values looking right, but there is always a chance it is not good. In my experience, the Hammond transformers are pretty robust though.

[tubeswell]

... Also, it doesn't look like the reverb driver has a coupling cap. ...

There will be something like a 500pF coupling cap in front of the reverb driver grid ('C22' on the Weber 6A40 schematic)

[dunner84]

I have not checked the 500p cap for leaking DC yet, but I am starting to suspect it, so I will pick up a new cap on Monday when I hit the local shop.

I had a chance to work on things tonight. I tweaked the bias circuit to get the 6v6s around 70%, and I also removed the dwell pot, and replaced it with the 1m resistor.

The reverb is now coming through ok until about 3.5 on the dial. After that, the overall sound of the amp slowly dies off, and the bias of the power tubes drifts from -40v and 22mA to -70v and about 40mA and start to glow blue. I am going through the schematic right now to see the correlation, but it seems like it could be DC throwing things off down the line.

[Raybob]

You'll be fine using two 1/2 watt 1Ks paralleled for testing. Fender used 470Ω/1W on most all those blackface amps.

Fender did always use 1W screen resistors, but I'm wondering if that wasn't because maybe Leo got a shitload of 1W 470R at a good price.  There seems to be very minimal current used on the screens.  Seems to me 1/4W would be sufficient.

[PRR]

Screen current can be quite large in OVERdrive. Not easy to predict from normal amp experience. I bet he thought 1/2W was plenty, and they kept coming back blown, so he went 1W.

[tubeswell]

Hi dunner84 - did you ever post a schematic of your amp? (In case I missed it)

[dunner84]

I did post a schematic. Disregard the node voltages, and bias voltage printed on the schem. Only the tube voltages are accurate. I have also replaced the dwell control with a 1m resistor.

I have replaced the 500pF cap, and it did not correct the issue. With the exception of some pre-amp noise, which I suspect is a noisy tube, the reverb is the last thing to fix.
With the tank and driver removed, the amp squeals if I turn the reverb up past 7.

With the tank and driver installed, the reverb works and sounds nice up to around 5, after that it drops the output of the amp, and cranks up the bias on the power tubes.