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Post subject: 1979 Vibrolux Reverb volume boost hum
Posted: Thu May 13, 2010 9:29 pm
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I've gone through quite a bit of possibilities but can't seem to eliminate this loud 120hz hum when the volume boost is engaged.

The hum is coming somewhere from the reverb circuit as it uses this circuit to boost the volume.

New power filter caps didn't make any change. It is not the tubes or the reverb transformer. The hum becomes louder if I unplug the reverb tank from the rear rca jacks.

Changed the 220k and 1k resistors at the junction from pin 7 V4.

Anyone have experience with a silverface with volume boost. Does it hum so loud with the boost engaged?

Regards.


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Posted: Fri May 14, 2010 3:52 pm
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Hi Gene, and welcome to the forum. My first suspect would be a faulty25/25 bypass cap on V4. I would also check all the ground connections, and also the 100ohm resistors off the heater supply. Art

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Posted: Fri May 14, 2010 4:27 pm
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aclempoppi wrote:
Hi Gene, and welcome to the forum. My first suspect would be a faulty25/25 bypass cap on V4. I would also check all the ground connections, and also the 100ohm resistors off the heater supply. Art


Thank you for your reply.

The bypass cap have been replaced for V4. No change. It is across a 820ohm resistor. Ground connections have been throughly checked both visually and with a continuity test. It is good. And the 2 100ohm off the heater supply are new as well. The amp hummed with the original power filter caps and I replaced all 5 of them with F&T 16uf/475v and still hums with boost. Otherwise, the amp sounds quite good.

I can tell you it is not the reverb pan or the cable either. It is also not the reverb transformer, any of the tubes, and I eliminated the pull switch as well.

This hum appears only when the boost is pulled. None of the pots make any change in the hum. If you disable the boost, the hum stops. Very perplexing.

Gurus?


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Posted: Fri May 14, 2010 5:04 pm
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Yes, very interesting. Will the amp hum with no input ? Art

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Posted: Fri May 14, 2010 7:05 pm
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aclempoppi wrote:
Yes, very interesting. Will the amp hum with no input ? Art


What do you mean no input? I am not plugging any guitar to the amp.


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Posted: Fri May 14, 2010 8:39 pm
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Right ! So the power supply is generating the noise, on it's own. OK, how does that 3.3m resistor, 10pf cap, and .02 cap on the output side of that circuit test out ? If they're good, I'd start a voltage check at the power trans and work my way back to the input jacks. BTW I wouldn't run the amp without the reverb tank hooked up. It would present a"no load" condition on the driver section and transformer. I'm guessing that you have already tried the tube pulling method to isolate the noise. Art

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Posted: Fri May 14, 2010 9:45 pm
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aclempoppi wrote:
Right ! So the power supply is generating the noise, on it's own. OK, how does that 3.3m resistor, 10pf cap, and .02 cap on the output side of that circuit test out ? If they're good, I'd start a voltage check at the power trans and work my way back to the input jacks. BTW I wouldn't run the amp without the reverb tank hooked up. It would present a"no load" condition on the driver section and transformer. I'm guessing that you have already tried the tube pulling method to isolate the noise. Art


The 3.3m, 10pf, and .02 is on my list to check. I replaced the 1k, 220k, and the 470k resistors. If I short the junction where the 3.3m and the rest of the resistors meet, the hum stops but that disables the boost as well.

I don't think I have any 3.3m 1/2 watts around, can I sub a 3.3m 1/4 watt just to check quickly?

I just ran the amp for a second with out the reverb just to confirm that the hum increases when I unplug the reverb input jack.


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Posted: Sat May 15, 2010 3:20 am
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aclempoppi ask:" I'm guessing that you have already tried the tube pulling method to isolate the noise. Art "

You did that jazzgene ?

What about unmatched output tubes ?

Or a resistor in the Phase Inverter causing a "unbalanced" PI ?


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Posted: Sat May 15, 2010 9:22 am
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stratele52 wrote:
aclempoppi ask:" I'm guessing that you have already tried the tube pulling method to isolate the noise. Art "

You did that jazzgene ?

What about unmatched output tubes ?

Or a resistor in the Phase Inverter causing a "unbalanced" PI ?


Yes, tubes have been checked and swapped out. Output tubes are matched and it also has the balance control.

Remember, this hum appears only when the boost is turned on. It must be somewhere in the reverb circuit.

It is not the phase inverter. If I pull v3 or v4, the hum stops.


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Posted: Sat May 15, 2010 1:58 pm
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Don't forget those CBS amps have a reputation of bad wiring making parisitic oscillations.

So maybe the "hum" could be the bad wiring too.


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Posted: Sat May 15, 2010 6:29 pm
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stratele52 wrote:
Don't forget those CBS amps have a reputation of bad wiring making parisitic oscillations.

So maybe the "hum" could be the bad wiring too.


I did suspect lead dress. I spent some time moving the wires around but this did not help.

The boost circuit sends the guitar signal through the reverb stage for additional gain. And then it mixes it back with the original signal at V4. So between V3 and V4, I suspect the noise is coming in and been amplified when the boost is used.

I also suspect that with the boost off, the reverb pan would filter out much of the 120hz hum so no noise with the boost off.


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Posted: Sat May 15, 2010 9:26 pm
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Well, at least, the three of us are on the same page. OK, I'm gonna go with a voltage check of the B+ power and the bias circuit, for a clean supply. It's not unheard of to get a faulty cap, although the F&T's are a good bet these days. BTW how did you form the caps? If that doesn't turn up an abnormal AC reading, then I'd start checking at the input jacks of V2 and continue all the way through to the power tube grids. Like you said Gene, the AC hum is probably lurking in the chain and the reverb driver and trans just bump it up, when the switch is closed. Just wondering how this noise first started. How is the reverb working now? Art

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Posted: Sun May 16, 2010 10:21 am
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aclempoppi wrote:
Well, at least, the three of us are on the same page. OK, I'm gonna go with a voltage check of the B+ power and the bias circuit, for a clean supply. It's not unheard of to get a faulty cap, although the F&T's are a good bet these days. BTW how did you form the caps? If that doesn't turn up an abnormal AC reading, then I'd start checking at the input jacks of V2 and continue all the way through to the power tube grids. Like you said Gene, the AC hum is probably lurking in the chain and the reverb driver and trans just bump it up, when the switch is closed. Just wondering how this noise first started. How is the reverb working now? Art


I did not form the caps. I've never had any issue with new caps and having to form them with a variac. I'll check the power filter caps but if they were not good, won't the amp have hum even with the boost off? When I received this amp, it had original 1978 power filter caps. It hummed and I replaced them with the F&Ts. It still hummed and the hunt was on.

The reverb is working fine. I am going to replace the 3.3m, 10pf, and .02 first. Have to order them.


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Posted: Sun May 16, 2010 7:47 pm
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Glad to hear the reverb is in good shape. Yeah, just trying to cover the bases with a good clean power supply. I'd check out all of the components surrounding V2 & V3. It would be nice to know what the primary- secondary ratio is spec'd at for that transformer. Any way I'd check out the other components and voltages before ordering parts. Happy hunting ! Art

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Posted: Sun May 16, 2010 8:47 pm
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aclempoppi wrote:
Glad to hear the reverb is in good shape. Yeah, just trying to cover the bases with a good clean power supply. I'd check out all of the components surrounding V2 & V3. It would be nice to know what the primary- secondary ratio is spec'd at for that transformer. Any way I'd check out the other components and voltages before ordering parts. Happy hunting ! Art


Took some voltages. My B+ is a bit low at 400v but that shouldn't be a problem. The schematic shows 450v. The other voltages seems pretty consistent with what it should be.

Is there something I should look for?


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