Fender Super Twin Reverb with 6 6l6gc's

[Jay D]

Hello Forum:
I have a silver faced Fender CBS 180W super twin reverb amplifier that gets noisy with time. I have replaced tubes in the pre-amp section, and have checked and replaced the rest of the power tubes and drivers. The tube compliment is unusual to me, as the reverb driver and the parametric equalizer circuit is a 6CX8 (half triode, half pentode) tube.
Also, it uses a triple triode, the 6C10. I have tried to replace the 6C10, but the noise from the new tube is quite high. Replace the old tube, and the noise somewhat is lower. Since the reverb circuits use the 6C10, as I adjust the reverb level, the noise gets amplified. As I lower the reverb level, the noise level goes back to the level I started with. I have replaced the shielded reverb lines from the amp to the reverb pan, and swapped out pans with an equivalent reverb pan assembly. Noise is still there. I have also checked the plate, cathode, grid resistors of the recovery stage, and the bypass capacitors associated.
Noise is still there.
Unfortunately I don't have a scanner to upload and post the schematic, but Schematic Heaven has the CBS 180W, 2nd version using the 6C10, and the 6CX8.
Any suggestions?

[txstrat]

Have you checked the ground connections around that part (or maybe elsewhere too) of the circuit? Bad solder joints etc.?

[in a vacuum]

Clean and retension the tube socket?

[km6xz]

Can you record it and post the file? Hearing it would be a lot better than a description as "noisy".
Does it go away with all gain controls down to 0? Do tone controls effect the tone of the noise?
It is best to attack the problem and fix that before swapping a lot of parts that could make it worse or more ambiguous.

[Prairie Dawg]

Have you eliminated external sources of interference such as ungrounded electrical outlets, noisy guitar cords and fluorescent lamps and CRTs and computers with noisy power supplies? If you've done all that without success then it is time to start looking around inside and poking things with a dowel or a chopstick.

[kramps]

This should be a link to the schematic: http://www.webphix.com/schematic%20h...180w_schem.pdf

moejoe

[Prairie Dawg]

What's up, Kevin?

[kramps]

Working harder than I want and making less than I'd like. The usual.

[oc disorder]

I had one ages ago and had to replace all the o/p tube sockets.
Certainly solved my noise problems.
I guess all that heat rising takes it's toll.
The new ones I bought were slightly larger than the originals so had to make
all 6 holes larger.
The schematic link Kramps provided which is the 180w I'm referring to doesn't
have reverb but has a graphic and vile distortion!

[wagdog]

I have that exact amp... or I should say, had that exact amp.

I ended up completely gutting mine and building an AB763 twin style front-end (sans trem), silverface style PI and 4 tube power section into it - it's like a 1000 times better sounding amp. FWIW, I have a new 6C10, a new 6Cx8 and the original 6c10 and 6cx8 that all worked fine in my amp - I'll sell them to you cheap if you want them as I have no need for those tubes (sorry for the SPAM).

As to what the problem is in yours; have the filter caps been replaced? Also fwiw, the inside of that amp features point-to-point wiring at pretty much the height of fenders sloppiness era - I really didn't help mine when I rewired it either.

[Jay D]

Hello,
I have taken the amp out of the areas that contain fluorscent lamps, computers, etc, with no changes to the amp's output noises.
I now are poking around the circuits with a set of chop sticks, and looking for cracked resisors, bad solder joints, etc.

[Jay D]

Hello, I have checked all tube sockets with a fully insulated, straight, dental pick and re-tensioned the power tube sockets, pre-amp sockets, and all reverb sockets.
No change in the output noises.

[Jay D]

Hello:
I have found something very intresting and like to pass on to the forum. The super twin reverb power supply has two (2) 220uf, 300VDC filter capacitors in series, and one of the filter caps was positioned over a #6 sheet metal screw from the opposite side (inside the chassis) to the insided of the cap box. Now, one would think that a short, blunt screw would be used here, but someone used a #6 sharp pointed screw on in this position. After years of vibration, I noticed that the cap checker I have, showed a low value of capacitance , and I then remove the cap from the circuit, and low and behold, I found a small hole punctured into the middle of the cap can! I have sinced removed all filter caps, and all caps showed low values. Replaced the filter capacitors, and at least 90% of the hum has gone away. Thank You!

[Jay D]

All:
Thank you for all your suggestions and help.
The last problem I am now having with this amp is after a few hours of playing, and the amp heats up good, I hear crackling and poping noises left, as most of the hum is gone from replacing the filter capacitors.
I still have to check plate resistors, bypasss capacitors, and any power supply voltage dropping resistors for cracking, noisy operation, etc.
I am open for any other suggestion on the heat / popping problem(s).

Thank You.

[Prairie Dawg]

I would just go ahead and change all the plate load resistors and use metal film or carbon film. Carbon comps are notorious for noise making.