Loss of power (volume really weak)

[goldtop5]

Hello,

I've been working on a Soldano SL-60 amp head, trying to restore power to it. It still sounds great but the volume is really weak. The pre-amp section is fine when I go out from the effects/send jack to another amp. I've replaced a few resistors and diodes in the power supply section, but it didn't make a difference. I noticed voltage drops on two filter caps in the power supply section. They both connect to the same voltage rail. There is a wire that connects that rail to one of the 6L6 pin sockets. I think it's pin 7. But there are 3 diodes that connect pin 3 and 7 together (on both 6L6 sockets). Could those diodes be bad?

[MWJB]

Recheck the 6L6 pin no's. Pin 7 is one side of the heaters, I wouldn't expect to see a wire from the B+ running to this pin, nor diodes.

There should be a wire that feeds pin 4 from the B+ rail (screen supply), via 470ohm resistors (yellow/violet/brown, 1W rating or better), one for each tube. I'm not familiar with this model so can't say whether the 470ohm resistors are mounted on the tube socket or circuit board. When you find them, make sure that they measure 470ohms and neither are blown (a blown screen grid resistor can cause weak output). If one of these resistors is blown, make sure that you also replace & rebias the power tubes as it's most likely that one of these casued the resistor to blow in the first place. In fact, I would assume that you are using good known tubes?

A voltage check at 6L6 pins 3, 4 & 5 and plate current would be useful too (then just out of curiosity check voltages at pins 1 & 6 of the PI tube, if a PI plate resistor burns up you get low volume & filthy tone).

The diodes you mention sound like spike protection (don't appear on the schematic) and would more likely run from pin 3 to pin 8 (ground). If so, you could temporarily clip them out (it's not essential but a good idea to have them) to eliminate them from your enquires, replace with the same type & polarity when you're sure all is OK.

Can you be more specific about the voltage drop on the filter caps? The schematic shows that the screen supply has 2 filter caps in series at the screen supply node. Therefore, I would expect one cap to have twice the voltage on its positive end (connects directly to B+ rail) than the "+" of the other cap at that same node (this point connects to the "-" end of the first cap and the junction of 2 smoothing resistors, each resistor bypasses a filter cap).

You say volume is weak? Is it weak & clean or weak & dirty?

Confirm those pin no's before going any further.

[goldtop5]

You're right those diodes do connect both pin 3 and 8 on the 6L6 tube sockets. All the resistors checked fine. I even swapped out some resistors that are a little over their value, but not exceeding their tolerance. About the filter caps. The reason why I asked is that they re high voltage caps and I'm only reading 1.5VDC on the "+" of two of them that go to pin 7. I know that can't be normal.

I replaced the 470 ohm resistors. They are mounted to the socket, by-the-way. I disconnected the diodes that connected pins 3 and 8 and rechecked the voltage on those caps and still no change at all. The volume is a weak but clean sound. No muddiness. I went from the send on the effects loop to the input of another amp and it was good and loud. I will check the PI tube and see if the plate resistor is bad.

The whole problem started when I installed a used set of tubes that I use for testing here in my shop. I know those tubes are good because I've seen them work before. I just don't know how old they are or maybe they just started going bad?

[MWJB]

Are you absolutely sure that these "filter" caps connect to pin 7??? I can't see any reference to such caps on the schematic.

[Pedro Vecino]

Pins 2 and 7 corresponds to the filaments. The only components that it should find connected to this line are two 100 ohms resistors to ground somewhere.
If everything is nice, you find approx. 480 volts in the pins 3 and 4, and it if is not a question of an ultralow bias adjustment, check the cathode resistor prior to eq circuit (100K) and the plate resistors (82K/100K) in the driver. The first one is capable of generating a volume loss without altering of evident form the sound.
Sorry for my bad english. Regards

[goldtop5]

Are you absolutely sure that these "filter" caps connect to pin 7??? I can't see any reference to such caps on the schematic.

They connect to pin 8.

[MWJB]

Pin 8 is ground. If this is the case you won't measure any voltage (you shouldn't even get 1.5v, check resistance from pin 8 to ground). The only caps with the "+" terminals connected to ground would be the bias supply caps (even then I'd be surprised if they connected directly to pin 8, there are better places to ground them). Please confirm cap microfarad value and voltage rating.

[goldtop5]

Pin 8 is ground. If this is the case you won't measure any voltage (you shouldn't even get 1.5v, check resistance from pin 8 to ground). The only caps with the "+" terminals connected to ground would be the bias supply caps (even then I'd be surprised if they connected directly to pin 8, there are better places to ground them). Please confirm cap microfarad value and voltage rating.

The caps in the power supply section are
2 - 270Uf @ 315v
2 - 100Uf @ 315v
1 - 47Uf @ 450v

[Amp Kat]

Try patching a cable from send to return and see if it comes back

[goldtop5]

Try patching a cable from send to return and see if it comes back

Me and my boss looked through the circuit with the schematic. Our signal generator was broke, so we used a cd player to inject signal into the signal path of the preamp circuit.

We found that v1 and v2 was only getting signal and a coupling resistor that connects signal between v2 and v3 opened (100k). I replaced it, turned it on, and the amp fired right back up to full power.

What started the problem was replacing the tubes and trying to set the bias. I know now that using the current method wasn't applicable. Only voltage method. I accidentally touched the wrong 6L6 pin socket with the vom and a spark rang out like a shotgun. This might've caused the drop in signal. The amp is fine now.

I've only been doing amp repair for a year and still in the training phase. I went to electronics school (10 years ago), but never worked on electronics since finishing school. I worked on computers, but the future in that soured out so I walked away from the industry. I've learned so much since working on amps, but still have a long ways to go. Hopefully, I'll get better at this as time goes on. I would appreciate any advice if necessary. I don't expect to make alot of money doing this (playing music is the B&B), but it's something that I enjoy because bringing a 1960's fender back to life does something for me. Fixing someone's 5-year old piece of crap computer doesn't.

I'll be keeping in touch. Thanks guys!