Please help me diagnose this Deluxe Reverb

[mlp-mx6]

The amp stutters when you take it off standby - but only for a while, and not always. Please listen to the attachment and tell me what you think it could be.

I can tell you that the power supply voltage jumps up (and jumps around a bit) when it's doing this, then settles to a normal level once it stops.

I have replaced all 5 16uF caps in the power supply.

Please help!

[strelok]

First thought is its an issue with the tremolo section. Does it make a difference if you have the tremolo switched on or off? Or with a change in the controls (speed,intensity)?

You might try pulling the tremolo tube out and see if that makes any difference. If you're not sure which one that is, its the second preamp tube fom the output tubes (second leftmost preamp tube when looking at the back of the amp).

Is this a Re-issue?

[Stevem]

Did the amp do this before the new filters got installed?

Where any new / different tubes installed when this issue kicked in?
Do both channels do that?

Here's some things to check out in no particular order.

Does rocking the rectifier tube around make things cut in and out as in a loose fitting socket?

More than once in my 50 plus years of doing this I had a bad solder connection on a rectifier tube pin ( and output tube ) itself and had to reflow it!

It looked so bad I could not belive it ever made any dam connection at all!

If it was the standby switch or anything that is passing a DC voltage along with audio signal you would be getting big time crakling and popping noises taking place also !!

You need to look at sections where there is just signal passing and no DC , or after the DC has been blocked!

It would not be the first time that the solid core wire used has fractured but still looks connected, I would prove out and tug on all the wires on the secondary side of the OT , and check on that stock crappy Ciramic cap they use to couple to the input of the PI section.

I would look close at the power suplly dropping resistors as sometimes they just crack open but you need to look close to see that and in I would ohm check all the connections on that filter board for high resistance ,and just for kicks I would jump out the choke with a section of wire .

[mlp-mx6]

First thought is its an issue with the tremolo section. Does it make a difference if you have the tremolo switched on or off? Or with a change in the controls (speed,intensity)?

Trem makes no diff.

You might try pulling the tremolo tube out and see if that makes any difference. If you're not sure which one that is, its the second preamp tube fom the output tubes (second leftmost preamp tube when looking at the back of the amp).

I replaced that with a known good tube. No diff.

Is this a Re-issue?

No, Silverface. '72 I think.

[mlp-mx6]

Did the amp do this before the new filters got installed?

Yes. New filters made no diff.

Where any new / different tubes installed when this issue kicked in?
Do both channels do that?

No new tubes. Yes on both channels, which indicates to me that it's a power supply problem. That's why I did the filters first. Fresh filters never hurt, right?

Here's some things to check out in no particular order.

Does rocking the rectifier tube around make things cut in and out as in a loose fitting socket?

Nope. And I installed bear trap clips for the octals, so it doesn't rock now.

More than once in my 50 plus years of doing this I had a bad solder connection on a rectifier tube pin ( and output tube ) itself and had to reflow it!

It looked so bad I could not belive it ever made any dam connection at all!

I'll check that!

If it was the standby switch or anything that is passing a DC voltage along with audio signal you would be getting big time crackling and popping noises taking place also !!

My first attempt was to sub in a new standby switch. No diff.

You need to look at sections where there is just signal passing and no DC , or after the DC has been blocked!

Will do.

It would not be the first time that the solid core wire used has fractured but still looks connected, I would prove out and tug on all the wires on the secondary side of the OT , and check on that stock crappy Ceramic cap they use to couple to the input of the PI section.

Will do.

I would look close at the power supply dropping resistors as sometimes they just crack open but you need to look close to see that and in I would ohm check all the connections on that filter board for high resistance ,and just for kicks I would jump out the choke with a section of wire .

Those resistors looked good last night when I installed the filters, but I'll double-check them too.

Thanks for the guidance.

[Stevem]

I have also had coupling / blocking cap who's lead at one end was making a intermitant connection to the foil rap inside it even when the cap leads looked like they where never manhandled / mistreated .
With the issue that this amp has that could only be at the input to the PI section.
Is the bias voltage on pin 5 of each output tube stable when the amp is acting up?

[mlp-mx6]

Is the bias voltage on pin 5 of each output tube stable when the amp is acting up?

Good question. I'll check it tomorrow. (band has a gig tonight)

Thanks!

[Firestorm]

The fact that the B+ goes UP (and fluctuates) implies that the power supply is losing its load intermittently. Check all B+ connections, including OT leads; check the screen supply (resistors and connections) choke leads and bypass the choke as Stevem said. Also check the bias supply, especially the ground on the load resistor (if that loses its ground, the negative bias voltage will jump up).

[mlp-mx6]

The fact that the B+ goes UP (and fluctuates) implies that the power supply is losing its load intermittently..

I had the same thought. Some intermittent connection is removing the load. I'll check those things as soon as I can. Thank you.

[mlp-mx6]

OK, I may be about to declare victory.

Taking you folks' advice, I started by checking ground connections. It looked like one of the brass plate grounds was supposed to have been soldered to the chassis also, but this did not look secure to me. So I fired up my big iron and re-flowed it. Now it works fine. I can't make the amp misbehave.

Most of the amps I've built have followed the Dumble long ground wire star-point style. I'm not familiar enough with these brass plate grounded amps - is the brass plate supposed to be soldered to the chassis, or is it supposed to be just a friction connection? Regardless, this one has chassis continuity now, for sure.

I'll test it a few more times over the weekend, but just maybe...

Thanks again for all your suggestions.
Michael

[Stevem]

Congrats, it sounds like you nailed it!

The black ground wire that goes to that Brass plate from the output tube screen filter in the filter house is a mistake that Fender made in a lot of there amps that makes for more 120 HZ hum then there should be.

Take this wire , extend it , crimp a ring on it and bolt it down to one of the PT mounting bolts and make sure that bolt is tight , of course you could solder it down near the PT with a big Ass iron , or maybe even easyer , solder it to the bias pot with the resistor that is on that, once again just make darn sure that the bias pot is bolted down good .

You should also check that the pots themselves are bolted on good as that is how that Brass section gets its ground in the first place.

You will also find that if you listen close with these two channel fenders with reverb that since the two channels are out of phase that turning up the controls on the unsed normal channel will knock some hum out also !

Just make sure that the shorting switch section of each input Jack is working otherwise you will get add hum and gain hiss!

[Charlie Wilson]

Hello Michael, the brass plate functions just the same as the Dumble buss bar. Although with the Dumble, the Pot cases are referenced to ground by contact with the actual chassis rather than the brass plate. I have only seen those plates soldered to the chassis in silverface Fenders, never in blackface or before. My guess regarding your issue would have been the 47 ohm resistor that runs parallel to the circuit board in the PI area. When the board warps from heat that resistor tends to break free from it's eyelet. You may want to still check that. Stevem I am still curious about the screen grid grounding in a Fender as you reverenced that in my Twin build post also. All of the Blackface and Silverface Fenders I have seen has the reverb and tremolo sharing that node on the filterboard with the Screens(reverb and tremolo), PI, and preamp all sharing a ground via a jumper under the filter cap board. If the ground wire that is soldered to the brass plate is moved to the power ground then the entire preamp section would then share the ground with the power section. Did they separate the screens from the rest of the preamp at some point?
CW

[mlp-mx6]

Pots are tight. Input jacks are good too.

I did extend that wire, but there was already a solder nut on the PT screw, so I ran the wire there. Good to go. Thanks again.

Take this wire , extend it , crimp a ring on it and bolt it down to one of the PT mounting bolts and make sure that bolt is tight , of course you could solder it down near the PT with a big Ass iron , or maybe even easyer , solder it to the bias pot with the resistor that is on that, once again just make darn sure that the bias pot is bolted down good .

You should also check that the pots themselves are bolted on good as that is how that Brass section gets its ground in the first place.

[mlp-mx6]

The board is a bit warped, but that 47ohm resistor is still solid. This *is* a Silverface. Pot codes show 1968. The tube chart says AB763, but it also says 5U4 rectifier, and it had the nasty ceramic caps on the power tubes (I just clipped those), and it has the chocolate caps in the preamp - clearly this is a transition model.

Hello Michael, the brass plate functions just the same as the Dumble buss bar. Although with the Dumble, the Pot cases are referenced to ground by contact with the actual chassis rather than the brass plate. I have only seen those plates soldered to the chassis in silverface Fenders, never in blackface or before. My guess regarding your issue would have been the 47 ohm resistor that runs parallel to the circuit board in the PI area. When the board warps from heat that resistor tends to break free from it's eyelet. You may want to still check that. Stevem I am still curious about the screen grid grounding in a Fender as you reverenced that in my Twin build post also. All of the Blackface and Silverface Fenders I have seen has the reverb and tremolo sharing that node on the filterboard with the Screens(reverb and tremolo), PI, and preamp all sharing a ground via a jumper under the filter cap board. If the ground wire that is soldered to the brass plate is moved to the power ground then the entire preamp section would then share the ground with the power section. Did they separate the screens from the rest of the preamp at some point?
CW