Anyone Here Blackface a Silverface Fender?

Paul Marossy · March 21, 2013, 11:46:26 AM

I just did and I'm having two *unexpected* problems with it now. Need a little help...

Johan · March 21, 2013, 11:52:11 AM

Too little info.some silver faces were just born with the wrong colour face others are radically different beasts...what model number did you have and what is the problem?

Paul Marossy · March 21, 2013, 12:04:31 PM

Quote from: Johan on March 21, 2013, 11:52:11 AM
Too little info.some silver faces were just born with the wrong colour face others are radically different beasts...what model number did you have and what is the problem?

It's a '74 Twin Reverb (100 watts). It has new power filter caps in it, and I changed all the "chocolate drop" coupling caps to Sprague Orange Drops. Also replaced the caps in the power tube bias supply. I did those things a long time ago before I decided to blackface it the other day. Was previously working great.

Here is what I did:

I disconnected the master volume control and changed out a few resistors and caps, everything is shown on my layout HERE.

Problem #1: When nothing is plugged into Channel 1, unless I have the volume control all the way down and the tone controls a certain way, it will start oscillating - even when I have something plugged into Channel 2.

Problem #2 -

A: The reverb is working fine with the exception of it I shake the amp even a tiny bit (like even just adjusting a control), it will rumble like you kicked the crap out of it for quite a while before stopping - this is with reverb level at about 3 or 4. Weird because it never did that before.

B: The vibrato switch pops when you turn on the vibrato. Not a big pop, but a pop. Louder as you turn up the intensity control.

Did I miss something in my conversion? I'm a little perplexed. I haven't done the preamp tube swapping dance yet, but I have a feeling it's not due to a premp tube.

drewl · March 21, 2013, 02:24:42 PM

Well the reverb springs will make noise if you move the amp.
Pull the tank out to see if one of the springs is loose and just resting on the tank.

Swap tubes for channel 1 to see if it's just a microphonic preamp tube, check to see if the input jacks are okay and tightly grounded.

Tremolo "thump" is common, search some amp forums for a few ways to quiet it down, I forget off the top of my head.

Paul Marossy · March 21, 2013, 02:36:56 PM

Quote from: drewl on March 21, 2013, 02:24:42 PM
Well the reverb springs will make noise if you move the amp.
Pull the tank out to see if one of the springs is loose and just resting on the tank.

Swap tubes for channel 1 to see if it's just a microphonic preamp tube, check to see if the input jacks are okay and tightly grounded.

Tremolo "thump" is common, search some amp forums for a few ways to quiet it down, I forget off the top of my head.

Yes, reverb makes noise if you shake the amp, but this is not normal - it's WAY over the top. I mean all I have to do is pull out the master volume switch and it will rumble like a S.O.B. for probably five to seven seconds. I have to check this to make sure, but I think since the B+ voltage is a little higher now, the 470 ohm resistor on the cathode of the reverb driver tube is probably way too small now. It was 2.2K in the blackface amps. That's probably the cause of that problem (I hope).

The input jacks on either channel haven't been touched, I know that's not the problem for Channel 1 squealing. Could be the preamp tube on that one, I'll have to check that. It might not have mattered before if it was a little microphonic, but now it does with the higher B+ voltage.

The tremolo is not a ticking problem, that's easily fixed. Might be a resistor value somehwere once again...

Johan · March 21, 2013, 03:09:50 PM

So it all started when you removed the master volume pot? My gut feeling says bad or intermittent ground.did this amp also have the pull switch on the master? If so, did you remove those wires or are they still in there? My limited experience on those particular amps says the layout is very sensitive to changes and wires not used should be removed

Paul Marossy · March 21, 2013, 03:37:59 PM

Quote from: Johan on March 21, 2013, 03:09:50 PM
So it all started when you removed the master volume pot? My gut feeling says bad or intermittent ground.did this amp also have the pull switch on the master? If so, did you remove those wires or are they still in there? My limited experience on those particular amps says the layout is very sensitive to changes and wires not used should be removed

I think it actually started after I made the B+ voltage higher by changing two resistors back to blackface specs in the B+ supply. So until I get home to verify yes or no, I think the reverb problem may be due to the silverface circuit using a smaller resistor value on the cathode of the reverb driver tube - it's like 20% of the original value (470 ohm vs. 2.2K). I think the gain going to the reverb tank is just way too high now.

Everything has been disconnected from the MV pot, switch included. I did also pull all the disconnected wires out of the way so that they can't act like antennas somehow. That was actually one of my first hunches, but turns out it was not the cause.

Channel 1 oscillation may be due to a microphonic preamp tube, will have to also check on that. But it starts to squeal LOUDLY at only like 1-1/2 or 2 on the volume knob, so I'm not convinced it's a preamp tube at this point. I haven't really messed with any grounds, so I don't know if it's that, but I can check on it.

Paul Marossy · March 21, 2013, 11:48:55 PM

OK, so I compared schematics some more and found that there were a few bypass caps on the cathodes of the preamp tubes in the vibrato and reverb sections that were either reduced in size or omitted in the silverface Twin Reverbs. So I added/changed those caps, and changed that 470 cathode resistor on the reverb driver tube to the blackface value of 2.2K. I also reflowed a couple of places where several preamp tube cathodes are grounded just in case. The vibrato pop is very minimal now, I think I can live with it.

So after all that, it seems to be working more or less normally now. The reverb still seems to make a little more noise than it should when you move the amp but it's a MAJOR improvement over how it was before. Might tweak stuff a little more but for the most part I think it's just about there, finally. I've pulled this chassis so many times I lost count.

alexradium · March 22, 2013, 03:14:02 AM

The biggest problem with oscillations in the silver comes from the wiring layout,try to use shielded cable from inputs and volume pots to the tube grids.
Also,there were 2000pf disc caps on power tubes grids to avoid this.once you perform those changes it should be OK

Paul Marossy · March 22, 2013, 09:19:52 AM

Quote from: alexradium on March 22, 2013, 03:14:02 AM
The biggest problem with oscillations in the silver comes from the wiring layout,try to use shielded cable from inputs and volume pots to the tube grids.
Also,there were 2000pf disc caps on power tubes grids to avoid this.once you perform those changes it should be OK

There already was shielded cable on all the inputs, done at the factory. There were no caps on the power tubes. I'm guessing that they were removed before I got the amp. Mine is a '74, the lead dress isn't too bad on it.

drewl · March 22, 2013, 01:38:42 PM

Did you pull the reverb tank and look inside?
It sounds like one of the springs is loose and banging around in the tank while still connected at one end.
I've seen it alot.

Paul Marossy · March 22, 2013, 03:36:55 PM

Quote from: drewl on March 22, 2013, 01:38:42 PM
Did you pull the reverb tank and look inside?
It sounds like one of the springs is loose and banging around in the tank while still connected at one end.
I've seen it alot.

I think my problem is pretty much cleared up now, but I still ought to check that out....

Paul Marossy · March 22, 2013, 05:41:19 PM

Pulled the reverb pan out. No springs that fell off, but one spring is very loose compared to the other. Not sure if it was made that way intentionally but at least that explains a few things. With the reverb driver tube having too much gain it will pick up that and amplify the heck out of it. Maybe I'll stick some 1/16" thick foam rubber on there so it can't klank as much.

kodiakklub · March 22, 2013, 06:47:12 PM

my somewhat educated (from experience in dealing with a problematic twin reverb) guess is that the B+ voltage hitting V3 is still too high. even on a blackface stock its slightly too hot i feel. try swapping in a 12AU7 for V3 for a quick test if you have one on hand. then try and get that voltage down some.

Paul Marossy · March 22, 2013, 07:06:37 PM

Quote from: kodiakklub on March 22, 2013, 06:47:12 PM
my somewhat educated (from experience in dealing with a problematic twin reverb) guess is that the B+ voltage hitting V3 is still too high. even on a blackface stock its slightly too hot i feel. try swapping in a 12AU7 for V3 for a quick test if you have one on hand. then try and get that voltage down some.

Could be. If I find that it's still a problem I will take a second look at it. I changed the cathode resistor on V3 to a 2.2K. It was a 470 ohm. That made a huge difference. I probably could up it a little more, maybe to a 2.7K. Mainly the problem is that one spring in the reverb tank is very loose and floppy, so it bangs around pretty easily.

EDIT: I did a little surgery on the reverb pan. Just shortened the really floppy spring by 1/8". That was enough to make it where I have to shake the amp pretty good to get it to thunder and rumble. I'm good with that. But I still have a little bit of noise when turning the reverb and vibrato on with the footswitch. The reverb does it when turning it off and the vibrato does it when turning it on. It's not a big noise. Am I expecting too much to think it should be silent when switching? I don't remember what it was like before as I don't really use the footswitch, but I don't remember it making any noise.

kodiakklub · March 23, 2013, 02:33:50 PM

have you tried operating the reverb WITHOUT the footswitch plugged up as the reverb is "NORMALLY ON" and the footswitch is used to take it out of the circuit? if the reverb is not noisy without the footswitch plugged up, i would say you have an issue with the FSW. im sure CBS put the cheapest cable possible on that footswitch

Paul Marossy · March 23, 2013, 03:06:14 PM

The reverb makes no noise when the foot switch is unconnected. I know you don't need it to get reverb. The only thing I would ever use the foot switch for is for the vibrato (more properly the tremolo), which I really don't use. I want everything to be working properly though. The foot switch is a replacement. When I got the amp in 2001 it didn't have one.

kodiakklub · March 23, 2013, 08:13:57 PM

its definitely the footswitch then. maybe you could borrow someone else's to triple check. i would crack that sucker open.

Paul Marossy · March 23, 2013, 10:06:38 PM

Quote from: kodiakklub on March 23, 2013, 08:13:57 PM
its definitely the footswitch then. maybe you could borrow someone else's to triple check. i would crack that sucker open.

I looked at it, I don't see anything wrong with it. It uses a pair of x-wing DPST switches. I'm guessing it's a MIM Fender replacement switch. I got it from Hoffman Amps a long time ago.

PRR · March 24, 2013, 08:52:20 PM

On the simple amps, the remote Reverb switch *has signal on the wire*. It *must* use shielded wire, or that yard of cable will suck-up all the buzz and pops in the room.

Later amps tend to DC-controlled switching and generally do not need shielding on the switch cable.

While we are here: the TREM switch on simple amps generally does not have to be shielded. It is either a simple form of DC switching (debiasing the LFO) or acts as dead-short when Trem is running.

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Anyone Here Blackface a Silverface Fender?