# Bandmaster Reverb Vibrato



## dcole (Oct 8, 2008)

Hello,

I am troubleshooting a Bandmaster Reverb vibrato circuit shown here: http://www.thevintagesound.com/ffg/schem/bandmaster_reverb_aa1069_schem.gif

The light in the roach oscillates for a bit and then just turns on solid. The voltage on the cathode of the oscillator is around 4.5V and the grid voltage on the LDR tubes grid is -4.5 V all relative to ground. I would have thought that the DC voltage of the grids for both halfs of the vibrato tube would show around 0 volts and that there would be an oscillating signal on the grid for the tube feeding the LDR. Is this correct?


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## nonreverb (Sep 19, 2006)

You have replaced the tube already?


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## dcole (Oct 8, 2008)

Yes I have. I have gone throught the whole amp tube wise and replaced everything with known working tubes.


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## Wild Bill (May 3, 2006)

dcole said:


> Yes I have. I have gone throught the whole amp tube wise and replaced everything with known working tubes.


You do know of course that the vibrato pedal line must be shorted to ground to engage the tremolo in a Fender circuit?

Just asking to make sure.

Wild Bill/Busen Amps


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## dcole (Oct 8, 2008)

Yes I am aware of that and that is happening. Now that the easy things are covered, should there be a negative voltage with respect to ground on the grids?


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## dcole (Oct 8, 2008)

I removed the 3 feedback capacitors, cleaned off the wax that was bridged across one of the caps and put the circuit back together as the caps tested good. All of a sudden I was seeing the roach flash nicely. It looks like the photocell might be shot though because there is nothing coming out of the Intensity pot, nor into it.

The negative voltage I was seeing on the grids was due to "Terrible Teching". I never scoped the point. Once I did I got a crappy looking square wave with a very short duration pulse so my multimeter read that as a negative voltage as the square wave was centered around ground.


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## Wild Bill (May 3, 2006)

dcole said:


> I removed the 3 feedback capacitors, cleaned off the wax that was bridged across one of the caps and put the circuit back together as the caps tested good. All of a sudden I was seeing the roach flash nicely. It looks like the photocell might be shot though because there is nothing coming out of the Intensity pot, nor into it.
> 
> The negative voltage I was seeing on the grids was due to "Terrible Teching". I never scoped the point. Once I did I got a crappy looking square wave with a very short duration pulse so my multimeter read that as a negative voltage as the square wave was centered around ground.


What do you mean b "nothing coming out of the intensity pot"? Nothing is supposed to come out of that pot!

The photocell is a light dependent resistor. When the neon bulb flashes the resistance of the LDR nosedives. This pulls the signal input to the PI down to ground, like someone turning the master volume down in sync with the flashes.

If you put your meter on a high ohms scale and measure with that you should see the resistance diving.

Actually, if you turn the intensity all the way up you should hear the trem acting upon the background hum.

As for those caps, I have found over the years that sometimes when those caps get old they don't do their job. Likely they have drifted too far in value for the trem circuit to fire properly. Replacing them perks things back up! Old Gibson amps seem particularly bad for this.

Wild Bill/Busen Amps


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## nonreverb (Sep 19, 2006)

Personally, I've never seen a problem with the trem caps going south on a LDR type Fender amp....I have however, seen the LDR go south...enough times that I now have a stock of replacement LDR assemblies.


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## dcole (Oct 8, 2008)

I replaced all the feedback caps and the bypass capacitors in the vibrato circuit as I had them on hand and the light now flashes steady.

Wild Bill, that makes more sense now that I look at the schematic again. This is the first vibrato circuit I have worked on and I appreciate all your guys help. Now I have a better understanding of how these work and will probably not make so many mistakes in the future.


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## Wild Bill (May 3, 2006)

nonreverb said:


> Personally, I've never seen a problem with the trem caps going south on a LDR type Fender amp....I have however, seen the LDR go south...enough times that I now have a stock of replacement LDR assemblies.


Good thinking, NR. I think we will be seeing more and more of them as those vintage amps are just getting so old!

Neon bulbs slowly age and eventually stop working, like the indicator lights on an Ampeg V4. Or even the power light on an old stove. It's just their nature.

Hell, I don't work so good, some mornings! LOL

Wild Bill/Busen Amps


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## LexxM3 (Oct 12, 2009)

Zombie thread, be reborn ... I am the lucky new owner of StevieMac's Bandmaster-highly-modified-into-a-sort-of-blackface-SuperReverb head. The amp is great, but as fully disclosed by Stevie before the sale, the vibrato doesn't work. More precisely, it kind of works, but definitely not well. I have finally had a few minutes on the long weekend to have a look and could use a little advice on the findings.

First simple things: vibrato enable jack shorted, V5 tube is good, don't see any bad wires and just (carefully) poking around to see if anything is intermittent or loose produces no new information. The vibrato oscillator caps look fine to the naked eye, but I have not replaced or tested them. I can see the neon bulb light up and vary with input in the "roach", at least for a while ...

.. in fact, I can even get some vibrato, but only for a few moments from the time the vibrato enable is shorted; in other words, if speed and intensity are preset at say half way and then I enable vibrato, I get some very passable vibrato that quickly (in a few seconds) decays to no vibrato. I can repeat this indefinitely. Sometimes, but not often, the vibrato will be active for longer than a few seconds.

Luckily, NextGen Guitars has a generic replacement LDR/neon bulb roach and I now have one on order. But in the meantime, would love to hear some thoughts on what might be going on and/or additional diagnostic procedure ideas to narrow it down.


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## nonreverb (Sep 19, 2006)

Does the neon light stop flashing when this happens or does it continue to flash after the effect dissipates? Is it a 12AX7 in the vibrato position? Also the tube emission may not be good. Best to swap it with another 12AX7 just to be sure.


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## LexxM3 (Oct 12, 2009)

The original V5 tube was an RCA Made in USA 7025, which looks like it might very well be original for that 1969 amp (in fact, all preamp tubes are RCA Made in USA, all except that one labelled 12AX7A). I decided to replace it by a new Mesa 12AX7 I had lying around. The results changed (!), vibrato is working "more" now, see below. Note that old and new tubes read pretty much exactly the same on both an emission and a DMC tester, although I am questioning the veracity of my emissions tester right now.

The new behaviour is as follows:

the neon bulb is on at all speed and intensity settings when vibrato is enabled; that was also the case with the old V5 tube
vibrato audibly consistently works for the midrange of speed, but seems to get quenched at both the low and high speed settings
mid to high intensity positions definitely work, but there is little dynamic control range in the lowest 1/3 to perhaps 1/2 of the intensity pot
when moving speed from say low to mid setting, it takes a fair number of seconds (~10) before vibrato ramps up to audible and stabilizes
even when non audible, I believe I can see the neon bulk flicker at all speed settings, although it is most clearly visible in the audible middle speed pot range
Does that provide any new clues?


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## dtsaudio (Apr 15, 2009)

Have you checked voltages at the tubes. Particularly the cathode voltage to see if the tubes are actually operating where they should be.
Barring testing, just start replacing caps, starting with the bypass caps on the cathodes. It sounds like the circuit doesn't have enough gain to start oscillating


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## nonreverb (Sep 19, 2006)

With the new tube installed, what's cathode voltage now? You indicated it was 4.5 volts initially. Has that changed? Good idea to check the cathode bypass caps as dtsaudio suggests. Also, make sure the values are correct. The 12AX7 cathode bypass cap on the side driving the neon bulb is only 5uF whereas the other side is 25uF.
Can you take a pic of the circuit for us to see?


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## LexxM3 (Oct 12, 2009)

Thanks @dtsaudio and @nonreverb. I'll try to do some measurements in the next couple of days and the new roach should arrive soon so that will motivate me to get the soldering iron out. 

But in the meantime, I should mention that I am not the original poster of this thread and I don't think this is the same amp (I suppose it could be, but I don't think so). So that 4.5v discussion is probably a red herring. I just hijacked this old thread because of the clear similarities.


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## LexxM3 (Oct 12, 2009)

Well, it sure took a long time to get back to this topic. Ok, here is the update relative to above:

I've replaced the LDR ("roach") with a newly purchased one; in the photos, you see the new one installed; it did not help, behaviour is the same as described in previous posts, namely that I do get some very limited vibrato, but in a very limited intensity and speed range and it takes many seconds for the effect to turn on when I manage to get into that narrow operating range
I took some oscillator area gut photo at this link: Dropbox - 20171126_Vibrato_Issue_on_Bandmaster_turned_SuperReverb there is also a copy of the Bandmaster schematic
All comments would be appreciated.

I'll specifically ask about something that seems strange: the 3 oscillator caps are supposed to be 0.02uF, 0.01uF, and 0.01uF according to the schematic and I've seen other Fender vibrato circuits from the same era with the same values and topology. 0.02uF is 0.022uF which is fine. One of the Suntan caps is 0.01uF per schematic, but the second Suntan cap is *0.001*uF!!! Could this be the culprit? Those Suntan caps are not original, are they? Thoughts?


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## jb welder (Sep 14, 2010)

First off change that .001 to the proper .01 and see what you get. That's an error, not a mod.
The only speed mod I know of is changing the .01's in that circuit to .02's, to get a slower speed at the low setting.


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## jb welder (Sep 14, 2010)

Also, what are the values listed on the 2 caps I have circled in red?
Am I correct they both connect to the same pin on the tube socket?


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## LexxM3 (Oct 12, 2009)

0.01uF 630V caps on order, should be here by end of the week. I am going to replace both 0.01 caps. Thanks for the confirmation!



jb welder said:


> Also, what are the values listed on the 2 caps I have circled in red?
> Am I correct they both connect to the same pin on the tube socket?



Both of those circled caps are 25uF 25V. And you have a rather amazing eye because, yes, they are both connected to the same socket pin. What are you thinking?
Thanks very much, JB!


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## jb welder (Sep 14, 2010)

Remove the one on the rear panel, the 2 are wired in parallel and result in a 50uF capacitance, which is wrong and may cause some of your issues. Probably someone forgot the other was already there.
Then change that .001 to .01uF.
Hopefully after those two issues are dealt with it will tremolate.


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## LexxM3 (Oct 12, 2009)

Drum roll, please ... SUCCESS!

Replaced both SunTan caps with correct value 0.01uF 630V caps, removed the extra electrolytic, and added a 0.022uF 630V cap across the 10M res for good measure (to reduce future probability of the classic vibrato ticking) and ... the vibrato works awesomely. Full speed control, full intensity control, awesome.



















And I haven’t played this amp for a while because it was apart for months and it turns out that it is an absolutely awesome sounding amp. Super quiet, no noises, crackles, or hums. The reverb is, well, super . 4 or 8ohm. Looks killer. Awesome.




























Thank you very much to everyone that commented on this thread, particularly JB Welder, NonReverb, and DTSAudio — couldn’t have done it without your help and sharp eyes. And thanks to StevieMac for selling me the amp in the first place, and Vokey Design for delivering it. What an amazing community!


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## greco (Jul 15, 2007)

Congratulations! Very impressive!



jb welder said:


> Hopefully after those two issues are dealt with it will tremolate.


Nothing equals the pride in making an amp tremolateable again!


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## jb welder (Sep 14, 2010)

Glad to hear it was a success.
Sorry to be a pain, but for best results one end of that anti-tick cap should go to the other 100K, and it should be a .01 
What you have there is what they refer to as the "old modification". It usually works, but the "new" way is better.


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