re-capped fender twin silverface and modded to blackface

[Curranproducer]

OK-

had a buddy drop off a 1975 MASTER VOLUME fender silverface twin reverb, needed tubes, all the electrolytics replaced, and I also redid all the old blue caps with orange drops, and did all of the blackface mods to the PI, power section, and turned the Bias Balance into a Bias Adjust! before I did this to the amp it was working "fine": crunchy noisy pots, and standard flat sounding silverface tone. NOW I have a weird noise in the amp. it is heavy on "rushing, popping, thunder rumble" noise. it only come through on the MV of the amp, the controls for both channels do not increase or decrease this noise. The bias adjust only goes from 47mV to 48mV during the full swing of the bias adjust. BUT the noise does increase as I up the bias to 48mV. Now all this being said I can tell the mods are working!!! the tone of the amp underneath is amazing. I just need to get this noise figured out, I do have a scope but I am still new at using it. could this scope help me find the noisy spot with a sine wave runningn through the amp?

[Stevem]

Well you need to back track as its something related to what you did.

Does rocking the PI tube and or output tubes around make the noise change?

If you pull the 12at7 PI tube is the rumble still there?if so install one of the old preamp tubes even if it's a 12ax7.

If the rumble goes away when the PI tube is pulled you might want to replace the two plate load resistors if the socket seems good.

If so pull out one output tube at a time, is the noise still there?
Is the negative bias voltage on the output tubes pin 5 stable?
Doing this by the way will leave you with a 4 watt class A output stage!

Look close at the 1500 and 470 ohm resistors on each output tube socket, as many times they are burnt and or cracked.

Double check and reflow all of your solder connections on all of the work that you did.
Another bug that these amps may have is that the ground for the output tubes screen filter is landed with the preamp section grounds when it should be landed near the pt with the red yellow and green yellow wire,as this will cut down on 120'hz hum levels.

[Blindog]

Well you need to back track as its something related to what you did.

Does rocking the PI tube and or output tubes around make the noise change?

If you pull the 12at7 PI tube is the rumble still there?if so install one of the old preamp tubes even if it's a 12ax7.

If the rumble goes away when the PI tube is pulled you might want to replace the two plate load resistors if the socket seems good.

If so pull out one output tube at a time, is the noise still there?
Is the negative bias voltage on the output tubes pin 5 stable?
Doing this by the way will leave you with a 4 watt class A output stage!

Look close at the 1500 and 470 ohm resistors on each output tube socket, as many times they are burnt and or cracked.

Double check and reflow all of your solder connections on all of the work that you did.
Another bug that these amps may have is that the ground for the output tubes screen filter is landed with the preamp section grounds when it should be landed near the pt with the red yellow and green yellow wire,as this will cut down on 120'hz hum levels.

Yep, try what Steve suggests here...you've basically rebuilt the whole amp so there's likely a poor connection somewhere. I had a similar situation as yours and it ended up being a poor solder joint on the B+ rail.

Also, I would think you should be getting more than 1mV of range on your bias pot....I would check your component values and connections.

Good luck!!

Mark

[Blindog]

Well you need to back track as its something related to what you did.

Does rocking the PI tube and or output tubes around make the noise change?

If you pull the 12at7 PI tube is the rumble still there?if so install one of the old preamp tubes even if it's a 12ax7.

If the rumble goes away when the PI tube is pulled you might want to replace the two plate load resistors if the socket seems good.

If so pull out one output tube at a time, is the noise still there?
Is the negative bias voltage on the output tubes pin 5 stable?
Doing this by the way will leave you with a 4 watt class A output stage!

Look close at the 1500 and 470 ohm resistors on each output tube socket, as many times they are burnt and or cracked.

Double check and reflow all of your solder connections on all of the work that you did.
Another bug that these amps may have is that the ground for the output tubes screen filter is landed with the preamp section grounds when it should be landed near the pt with the red yellow and green yellow wire,as this will cut down on 120'hz hum levels.

Yep, try what Steve suggests here...you've basically rebuilt the whole amp so there's likely a poor connection somewhere. I had a similar situation as yours and it ended up being a poor solder joint on the B+ rail.

Also, I would think you should be getting more than 1mV of range on your bias pot....I would check your component values and connections.

Good luck!!

Mark

[Curranproducer]

So for whatever reason this morning the noise has gone away it was there when I first fired up the, crackling, the rumbling and then it suddenly disappeared I've been playing on it for 20 minutes and it isn't coming back I haven't done anything to the amp

[Stevem]

It could be new gassy tubes, or a tube socket issue, but I would still reflow all the solder connections you touched!

[M Fowler]

Expansion and contraction when amp warms up? Solder joint.

[Curranproducer]

popped up some pix on facebook as well and some one chimes in that one of the caps was backwards in the power section?!?!?!?!? I feel like a dolt. I flipped it around and it sounds more twin-ish. I am concerned that the cap is just waiting to fail now that i ran it backwards for about an hour.

[Stevem]

If that flipped cap was getting hit with more than 50% of its rated voltage, than yes I would replace it pronto!

[Curranproducer]

oh yeah I already ordered the replacement cap yesterday.I am going to use the vibrato tube for a merlin FX loop. and I am wondering the best way to do that. i have the layout for the merlin and want to know if I need to lift all of the tremolo components from the board or can I just pull the wires off the tube and re-route to the tag board I am using for the loop? and then I literally just splice this right before the PI

[Stevem]

Just remove all the flying leads from that socket and you can leave all the stock tremolo stuff there if you like.

have you ever played a amp with a effects loop, as to me running any time delay type effects in the signal chain like reverb and or echo sound crappy if you are driving the amp enough to clip the output stage after the effects are in, as distortion added in after these time delay effects do not sound good to me, but to each his own!

[rp]

You might want to hold on to the old blue gumdrop caps. There's nothing wrong with them and to me they sound warmer than new orange drops. If after you're all done you have ice-pick I'd put them back as a first thing. Yes they aren't terrific like the old astron or ajax caps but people automatically change the blue or brown drops when these were never the big problem with SF amps. Not picking on you here, but an other pet-peeve, some SFs sound real good and people will insist on a rebuild without even a good test drive

[vibratoking]

You might want to hold on to the old blue gumdrop caps. There's nothing wrong with them and to me they sound warmer than new orange drops. If after you're all done you have ice-pick I'd put them back as a first thing. Yes they aren't terrific like the old astron or ajax caps but people automatically change the blue or brown drops when these were never the big problem with SF amps. Not picking on you here, but an other pet-peeve, some SFs sound real good and people will insist on a rebuild without even a good test drive

+1000

Orange drops in place of the molded blues is a bad idea IME.

[Reeltarded]

Shhhhh I was going to ask if I could have them.



I've only seen one bad one and it came in a box of junk I bought so who knows...

[Curranproducer]

I still got em BUT this amp sat in a moldy wet basement for about 20-25 years. All the blue drops are pretty rough, blisters and mold on them. I replaced them on principal. I even went through and swapped some resistors that looked like crumbeld messes