Blackface Pro Reverb Volume Low On Vibrato Channel

[lkrasner]

amp is an AA165. I just replaced all the pots except for the vibrato ones, since the old ones were insanely scratchy. All values for those are triple checked to be correct. I also replaced all the 100k plate load resistors on the preamp tubes and the the 470R screen resistors on the power tubes with exact match carbon comps. This was to get rid of the loud hiss even with no volume, which worked beautifully. One of the screen resistors was also kind of burnt looking though still working, but I just did them while I was in there anyway.

After the fix, the volume on the vibrato channel is super low (I can turn it up to 10 and it is just kind of medium loud, not blowing my ears like it should be). The reverb also stopped working, and there is no hiss or noise from turning up the reverb pot like there was before. The tremolo also dosn't work, but it was dead before too.

 I also touched up all the joints on the sockets since they looked pretty cold, so I guess I could have made a bad one myself... I swapped tubes around with no changes, so I think they are all good. Anyone have some words of wisdom?

I'd also welcome any advice on the Tremelo. the light is flashing, and the caps in it's circuit are good.

[sluckey]

If it worked before you fixed it then retrace all your steps. You most likely caused this unknowingly. Next time don't do so much at one time. And check that it works before proceeding to the next step.

[lkrasner]

If it worked before you fixed it then retrace all your steps. You most likely caused this unknowingly. Next time don't do so much at one time. And check that it works before proceeding to the next step.

Yeah, I suppose that was probably a mistake to do so much at once. Everything seemed so simple though...

[sluckey]

It was simple stuff but it involved a lot of soldering. And that increases the chances of accidently doing some you did not intend to do.

[AZJimC]

>I just replaced all the pots except for the vibrato ones, since the old ones were insanely scratchy.

That is a clue to me that the coupling and tone caps could be leaky. I had an amp like that, and checking DC voltages and replacing the caps that showed leakage, stopped the scratchy pots without replacement. A quick clean and all was well, plus the sound improved and tightened up as expected.


The trem sounds like a bad LDR. As I recently learned, the resistor component in that roach becomes non-light sensitive. I'm pondering whether to replace the roach or modify the tremolo to bias vary.

[lkrasner]

I had a resistor wired to the wrong place, once I fixed that, reverb was back, but I still have the low volume problem. I can't find anything else out of the ordinary. I'll check voltages when I get a chance, but does anyone have more ideas?

[shooter]

get your volts, keep looking for "that other thing not right".  what's vol low?  5 on vibrato = 2 on normal?  Does volume change when you have rev n trem off compared to on?

[lkrasner]

get your volts, keep looking for "that other thing not right".  what's vol low?  5 on vibrato = 2 on normal?  Does volume change when you have rev n trem off compared to on?

I can turn the vibrato volume all the way up and it's about as loud as 2 or 3 on normal. It sounds fine, no bad distortion or anything.

Tremelo doesn't work, but it didn't before this problem either. I think the optocoupler is bad. I also removed it all together with no diferece. No difference whether I plug something in to turn on/ off Tremelo or reverb. Adjusting reverb has no affect other than adjustign the reverb...


What kinds of things could cause a low voltage on a preamp tube plate? I have some recollection of this being the case, but I wrote it off to my wiring error that broke reverb as well. (the reverb tube voltages were obviously way off, but I think another one was too).

[sluckey]

What kinds of things could cause a low voltage on a preamp tube plate?

Just measure your tube voltages and post here. Then we can be a bit more specific.

[lkrasner]

Here're the voltages, definitely some weirdness. I made up my own numbering since there isn't anything in the layout or schematic. I ignored the rectifier, then started with the leftmost 6L6 and worked my way right (as seen in the layout and how I have the amp sitting). For the dual triodes, "A" is the leftmost plate pin (pin 6) and "B" is the right (pin 1). All voltages read to the same (good) ground with a fluke 117 True RMS Multimeter.

I will not give my input yet, as I don't want to bias (no pun intended) anyones ideas. Thanks for all the help!

Mains: 122.5 VAC
Rectifier pin 8: 458.0 VDC
V1: 455.0
V2: 454.8
V3A: 246.9
V3B: 251.7
V4A: 393.6
V4B: 434.8
V5A: 106.2
V5B: 342.7
V6A 438.5
V6B: 439.0
V7A: 274.7
V7B: 275.5
V8A: 278.0
V8B: 268.0

[lkrasner]

Here is the layout and schematic for reference (pdfs):

http://vintagefenderamprepair.com/wp-content/uploads/2011/04/Pro-Reverb-AA165-schematic.pdf
http://vintagefenderamprepair.com/wp-content/uploads/2011/04/Pro-Reverb-AA165-layout.pdf

[sluckey]

There are more than just plate voltages in an amp. We need grid, cathode, plate, and screen for all tubes, even if the voltage is zero. Start with the small tube at the end of the chassis. That's V1. Number the tubes sequentially until you finish up with the rectifier.

[lkrasner]

There are more than just plate voltages in an amp. We need grid, cathode, plate, and screen for all tubes, even if the voltage is zero. Start with the small tube at the end of the chassis. That's V1. Number the tubes sequentially until you finish up with the rectifier.

Sorry, I was hoping Plate voltages would be sufficient.

Here they are again, numbered from right to left (starting with the first preamp tube, ending with the rectifier). Everything is by pin number for simplicity. My mains dropped a bit due to a heater (in my house) kicking on around V8, so all voltages there are a bit lower than they should be. Starred values are tied together on the socket. I still measured on each one though to be sure.

Code: [Select]

V1:
1: 266.5
2: 0.00
3: 1.96

6: 277.1
7: 0.00
8: 2.07


V2:
1: 268.9
2: 0.00
3: 2.08

6: 275.0
7: 0.00
8: 2.07


V3:
1: 439.1
2: 0.63 **
3: 8.75 *

6: 4.37
7: 0.63 **
8: 8.75 *


V4:
1: 341.5
2: 0.00
3: 3.52 ***

6: 107.9
7: 3.61
8: 3.52 ***


V5:
1: 442.6
2: -48.78
3: 0.00

6: 392.5
7: -48.81
8: 0.02


V6:
1: 250.2
2: 62.8
3: 100.3 ****

6: 248.1
7: 63.9
8: 100.3 ****


V7 (6L6):
3: 453.5
4: 453.4
5: -45.6
8: 0.00


V8 (6L6):
3: 449.3
4: 449.4
5: -46.9
8: 0.00

[eleventeen]

V3, a 12AT7 is your reverb driver, which gets BIG volts on pins 1 and 6, its plates. Pins 1 & 6 are common, they should be jumpered together and have the same volts on them. It's hard to see how they could be disconnected. They are fed by the blue wire from the reverb tranny. Maybe you made a scale error [439 vs 4.37] here; they should have ~~430 volts on them. Appears you have good volts on pin 1 but a 12A_7 isn't going to work at all with 4.x volts on its plate. Does that third tube get VERY hot? (It should...very noticeably hotter than any other tube for given period of being on) At the same time, the voltage readings seem fine on the FIRST list you supplied. If the volts on your first list are correct (suspect they are) then the reverb driver tube V3 is OK. Move on.

V4 does not seem right. Section 1-2-3 seems OK. Section 6-7-8 your plate voltage is low (107 vs 280) and you probably should not have "the same" volts on the grid and cathode (pins 7 & 8) of that section. Pin 7, grid, comes (through a resistor) from the wiper of the reverb control. Did you make the ground connection on the cold side of that pot? (leftmost terminal as viewed from the rear) And/or...if you have a leaky cap in the reverb coupling cap (a .003) that is putting DC on that grid (via the reverb control) that will mess with the bias of that tube and disrupt its mojo. Possible wiring error or leaky cap.

V5 is your tremolo tube. Pin 1 should have about 280 volts on it. It is much higher in your readout, plus, BOTH cathodes of the two triodes within your V5, are effectively at zero. As for the 1-2-3 section, it is clearly not conducting. If it was conducting, we would see a voltage drop across its 220K plate resistor (we don't; that plate sits at the supplied B+ of 440 v) and its cathode would lifted off zero by about 2 volts. It's not (from strictly your voltage readings) possible to derive what's going on with your V5 but it's very not right. Both those cathodes should be at about 1.5 - 2 volts. If they are not---the tube (either half) is not conducting.


Otherwise, things seem pretty normal.


Do you get reverb crash if you shake the amp cabinet (with the reverb can in it) ?


Are you sure your reverb cables are sound?

[sluckey]

Focus on V4 pin 7. That voltage should be zero. Make pin 7 zero and the plate and cathode voltages will probably be happy too.

The reverb recovery tube .003 coupling cap could be leaky causing the pin 7 voltage to be positive. Disconnect that cap. If V4-7 now reads zero volts then that .003 cap is the cause. Replace it.

However, there is another way for V4-78 to be positive. The .02 coupling cap from V2-6 plate could be leaky (Look for point Z on the layout). Disconnect that cap. If the voltage at V4-7 now reads zero, replace the .02 cap.

Just ignore the voltages on V5. They are perfectly normal for a trem oscillator that is biased into cutoff by the -48v on the grids. That is the norm for trem turned off by footswitch or no footswitch plugged in.

[lkrasner]

Thanks Guys, I'll take a look at those caps tonight and see what I can find. What kind of cap should be used as replacement? I might have access to some of the original kind, but there are equally as old, so should I just replace it with some orange drops? or is there another type you recommend for coupling?

[sluckey]

Do the checks first. If in fact you find a leaky cap I'd use whatever I had on hand unless you're concerned with preserving an original look. If I had to buy some I'd use Mallory 150s or Sprague ODs from Doug...

http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Capacitors&ORDER_ID=!ORDERID!

[lkrasner]

Do the checks first. If in fact you find a leaky cap I'd use whatever I had on hand unless you're concerned with preserving an original look. If I had to buy some I'd use Mallory 150s or Sprague ODs from Doug...

http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Capacitors&ORDER_ID=!ORDERID!

Alright. I'll see what I can find. I'm about to make a huge mouser order for my next build, so I can probably add a few things into that. or I might get everything from Doug, that seems like it may actually be more practical...

[lkrasner]

Focus on V4 pin 7. That voltage should be zero. Make pin 7 zero and the plate and cathode voltages will probably be happy too.

The reverb recovery tube .003 coupling cap could be leaky causing the pin 7 voltage to be positive. Disconnect that cap. If V4-7 now reads zero volts then that .003 cap is the cause. Replace it.

However, there is another way for V4-78 to be positive. The .02 coupling cap from V2-6 plate could be leaky (Look for point Z on the layout). Disconnect that cap. If the voltage at V4-7 now reads zero, replace the .02 cap.

Just ignore the voltages on V5. They are perfectly normal for a trem oscillator that is biased into cutoff by the -48v on the grids. That is the norm for trem turned off by footswitch or no footswitch plugged in.

Pulling the .02 cap from V2 brought V4-7 to 0 volts. However, I don't have a replacement. would a .022 have any adverse effects?

[sluckey]

Pulling the .02 cap from V2 brought V4-7 to 0 volts. However, I don't have a replacement. would a .022 have any adverse effects?

A .022µF would be perfect.

Anything you have in the range of .01 to .1 would do for a temporary fix. Gotta be rated for 400V or more.

EDIT... If I didn't have some spare caps, I would even borrow that .047 from V1-6 to test to see if just the leaky cap will fix it.

[lkrasner]

Pulling the .02 cap from V2 brought V4-7 to 0 volts. However, I don't have a replacement. would a .022 have any adverse effects?

A .022µF would be perfect.

Anything you have in the range of .01 to .1 would do for a temporary fix. Gotta be rated for 400V or more.

EDIT... If I didn't have some spare caps, I would even borrow that .047 from V1-6 to test to see if just the leaky cap will fix it.

Yeah, since the trem doesn't work anyway, I put the .02 from that circuit (which actually was new, in an early attempt to fix the tremelo) and it worked beautifully. I'll order the right part, and replacement optocoupler for the tremelo when I place my next order.

Only remaining problem is that the reverb has a lot of hum when turned up. Not terribly worried about it, and it is likely just bad wires or something, still sounds great when playing.

This thing totally sounds awesome, and I'm loving the new Eminence Red White and Blues speakers I put in! I can't thank you enough for the help getting it up and running again.

[sluckey]

Yeah, since the trem doesn't work anyway,

Do you know for sure that the trem does not work? I ask because your voltages indicate that there is not a footswitch plugged in. This trem circuit REQUIRES a footswitch plugged in and the trem switch contacts must be closed. You can simulate this by putting a ground wire on the hot lug of the trem footswitch RCA phono jack.

Short that jack. You may be pleasantly surprised. You will need to put that .02 cap back in the trem circuit.

[lkrasner]

Yeah, since the trem doesn't work anyway,

Do you know for sure that the trem does not work? I ask because your voltages indicate that there is not a footswitch plugged in. This trem circuit REQUIRES a footswitch plugged in and the trem switch contacts must be closed. You can simulate this by putting a ground wire on the hot lug of the trem footswitch RCA phono jack.

Short that jack. You may be pleasantly surprised. You will need to put that .02 cap back in the trem circuit.

I made a "footswitch" by stripping an old rca cable, and shorting the tip and sleeve wires together. Plugging that in gives me a ticking on the trem (if I ever get it working, I should probably add the mod to stop that, but it's kind of nice for debugging ;)) that varies with the speed and intensity. It also makes the light blink, but still does not produce sound. I tried replacing the .02 cap and 2 .01 caps in that circuit with no luck. This leads me to believe that the resistor side of the optocoupler is dead.

[sluckey]

This leads me to believe that the resistor side of the optocoupler is dead.

I agree.