Blackfacing a Bandmaster Reverb to AB 763 Vibroverb specs

[tonehoney]

On my 1971 Bandmaster Reverb, while retro-engineering the board to blackface specs (Vibroclone) I rewired the bias pot to AB763 specs per the Hoffman Amps website. I replaced the scratchy volume pot with a new CTS 1M audio taper pot. I installed a 120pf silver mica cap on the bright switch. I installed a 250pf silver mica cap in the tone stack (I only use the vibrato channel). The output is very low compared to normal and produces a very buzzy distortion when the volume is on “10.” I’ve checked and rechecked my work against the schematic and it’s correct. Upon visual inspection, there are no burned components, no arcing in the preamp tube sockets, and I swapped out the rectifier tube with no effect. Where should I go from here?

 

[Swarty]

Did it work right before the mods? What did you bias it at?

 

[tonehoney]

Hi Swarty, The amp worked fine before the mod. I haven't biased it with a meter yet, since I may have other problems to fix first, but turning the bias pot in either direction doesn't make an appreciable difference in the tone, and has no effect on the loss of volume.

BTW, I have used the troubleshooting guide at geofex.com to no avail.

 

[Big Dan]

tonehoney,

You really need to check the bias before you go any further (IMHO). When you change the bias over to BF specs, it puts everything out of whack. Depending on the particular set of tubes, it could be WAY out of range.

I did that to a friends SF Bassman and before I adjusted the bias, it was not even close. Plus, that will at least tell you if you have any issues in the bias circuit.

Dan

 

[TheAmpNerd]

Take a look at the schematic and see how your
voltages are compared to the schematics.

Make sure you didn't reverse the bias
diode & supply by mistake.

We know the problem has to be in the mod related
area.

You said the normal channel works properly?

Double and triple check everything you did.

You could also have fried a cap when soldering
or have a bad one. Could have a cold solder
joint or two as well.

Take a break from it or have a friend come
over to review it, you might be too close
to see it.

Good luck let us know what you find out.

 

[Blue Strat]

You should definitely check the bias and all voltages. Checking the bias can often give clues to something wrong in that section of the circuit.

It's a bit late for this advice, but you should always check an amp after each mod you make to it. That way, you'll know immediately where you've gone wrong instead of having to sift through countless variables as you are now.

Clearly, something happened while you were working on the amp so you've either made a mistake or have changed something that you've forgotten about and are forgetting to mention to us.

Check all your tubes, replacing them with known good tubes. Do both channels behave the same? Start with just V1 and V6 tubes installed and see how the normal channel operates. Then try V2, V4 and V6 and try the reverb channel (there will be no reverb without V3 installed).

Is the speaker plugged into the correct (not the ext) jack?

 

[Swarty]

It could be as simple as it being biased ice cold.

 

[tonehoney]

Thanks to all for your input. I've been working through this over a couple of weeks so that I'm not rushing and missing things. I'll begin to work through these suggestions and I'll post voltages soon. I would appreciate additional posts as ideas come to mind because you never know what it may turn out to be!

 

[Blue Strat]

This would be a last resort, but it may come down to redoing everything you've done, one step at a time.

Holding for bias and voltage information since those can be very illustrative.

 

[tonehoney]

Thanks, Mike. I'll get the voltages and bias current tonight and post them.

 

[kevmin]

Check your grounds on all the cathode bypass caps. I had the same problem and it was a cold solder joint on a replaced bypass cap on the second preamp tube. Those copper grounding plates can sometimes lose connection due to corrosion too.

 

[tonehoney]

Here are the voltages that I've recorded so far.
On standby:

bias voltage set to -55v per schematic

Plate voltage on red wire at circuit board, 549 volts, on yellow wire, -.001?

With power on, off standby:

Plate voltage at circuit board, 482v on red and yellow.

The bias current with power on reads -8.3 ma?????

 

[tonehoney]

I took Mike's advice and removed all preamp tubes except v1 and v6. The problem was the same, no change. I never touched the diode circuit downstream from the bias pot. The power tubes, Phillips 7581a's, seem to have less of the dark coating inside than they used to. In fact, one is now completely clear. Another thought is that I built a bias current tester per the Hoffman website. If some of the wires shorted inside the tester's socket, or if a wire had a cold solder joint, would that have fried the power tubes?

 

[RussB]

Turn the bias up on them tubes, Man!

Try 40mA

 

[tonehoney]

Ok, I installed 1 ohm resistors from pin 8 on the power tubes to ground, measured the bias voltage at pin whil on standby and got readings of -54 and -56. Then I turned on the power and set the bias current to 35ma. I plugged into both channel 1 and channel 2 with two different guitars and have the same low volume with either channel. However, I am able to get a really good "Red House" sound when I crank channel 2, only it's the volume of an acoustic guitar. So, I'm wondering about the PI, OT, and power tubes. What do you think?

 

[RussB]

I worked fine before you worked on it...

And now after you worked on it, it doesn't work, correct?

...I’ve checked and rechecked my work against the schematic and it’s correct...

This is a bad attitude to troubleshoot. You need to to assume it IS something you did, it will help open your mind to what could really be wrong.

OK, so I'm going with the assumption that something you did was wrong. You either have a wire where it doesn't belong, or you used a wrong value component somewhere. There is also the possibilty that heat has damaged an existing component (cap or resistor) I once subbed a 2.2 MEG resistor in place of the 2.2k cathode resistor...took me a while to catch it!

The fact that the "NORMAL" channel work properly tells us that the "blackfacing" of the PI and Bias circuit is good. No need to keep looking there. I would have a real good look at the new pot and tone caps.

You need to focus on the basic stuff at this this point, and quit trying to blame it on transformers an such. They're all fine, because that NORMAL channel is fine!

And where are those voltages?

If you could take a good pic and post it, that might help as well

 

[tonehoney]

The normal channel is NOT fine. While there is output, just as there is on the vibrato channel, the volume is still WAY below normal. I'm only wondering if i may have smoked something (transformer or tubes) while working on the mod. I've posted the voltages I've measured, but would be happy to post any others you require, just let me know what you need.

 

[RussB]

Ok, then you need to go through the PI circuit too!

 

[Blue Strat]

Russ is right. The biggest enemy of an amp troubleshooter is his own ego. To assume that you didn't make a mistake (especially since the amp had been working BEFORE you made changes) is naive and counterproductive.

Without more information and/or seeing the amp, my recommendation is to undo your mods one by one, testing after each one is reversed. Otherwise you and we are taking shots in the dark which seem to be leading nowhere.

If your bias current is truely as low as you say (how are you measuring it?) and you can't get it into the 30s at least, you've got a problem with your bias circuit or something related to the power tubes. -55 on the grids seems on the low side (-48 is more typical) but the idle current is the definitive number.

 

[tonehoney]

I FOUND IT!!!!!!!!!!!!!!!!

I retraced my steps from when I did the BF bias mod and found that instead of 2) 220k ohm resistors, I had installed 2) 220 ohm resistors. So I've installed the correct value caps and all is well!
Thank you to all who offered advice and direction through this learning experience!