OLd Fender Twin Sliver FAce Amp

[bsco]

Looking for some advice..I have an old Fender Twin Silverface amp...this one has the four output tubes(6L6) and a switch on the master volume control. Two of the output tubes were red-plating...swapped them with the other two and the problem moved with the tubes, so I replaced them. Replaced two screen resistors..they checked ok but didn't look the best....the other two looked great...installed four 1 ohm 2 watt resistors on the cathodes so I could measure the bias current...going from left to right on the output tubes with the unit upside down and front facing you, I got 15.5mV: 19mV: 17.6mV and 14.9mV with a plate voltage of 427 volts. according to calculations, these readings should be in the 40mV range at least. There is no physical adjustment for the bias...I tried the amp out and it sounds good but it is supposed to be 135watts and there is no way that it is that loud....I am assuming that if the biasing current was higher then the amp would have more volume??? what is the best and safest way to increase the bias on these four output tubes??? I guess I would have to replace grid resistors???Any advice greatly appreciated as I am still a newbe to these types of electronic repairs...
Cheers,
Bernie

[nickb]

Those numbers in the correct range. This amp uses a -55V grid bias and that sets the plate current on the low side. It's quite common to do this. The amp runs more efficiently and so you can get a little more power out. The tubes are not especially well matched but IMHO they are close enough and will add give a richer harmonic spectrum. If you like how it sounds then there no need to do anything.

[bsco]

Thanks for the info....I checked out the bias voltages on pin 5 of all tubes and I varied the hum balance pot and noticed that as one side increased the other side decreased...so I adjusted for as close a reading on both sides as I could. I then checked the voltage drops across the 1 ohm cathode resistors and the readings were pretty much the same as before. So I guess, this is the way this amp is set up....The grid bias is set for -49 volts before I started and I put it back to that level....then I saw your reply.....I am learning as I go....so far I haven't run into anybody that made me feel like an idiot but I guess that will happen sooner or later for sure. Thank you very much for taking the time to reply to my post....I appreciate it very much....By the way I also had a pulsating pop in the speakers with this amp...I started to remove the smaller tubes but decided to remove them all and just leave the outputs....the popping noise stopped...so I started to install the tubes back in one by one...when I installed the 12AT7 which goes right next to the first output tube, the noise returned...installed another 12AT7 and the noise disappeared so that tube was bad as well....hopefully now just a good cleanup and it should be good to go...will have to use a pair of rubber gloves in order to clean the inside of the cabinet though...very, very dirty and dusty...you can easily tell that this amp was not looked after. Once again, thank you very much......
Cheers,
Bernie

[pdf64]

The plate voltage looks rather low for a 135watt model, especially for such low static currents; expect around 500V.
Pete

[bsco]

Thanks Pete. Just took a quick look at the schematic.....Somehow I must have overlooked that.....I guess I was more concerned with the red-plating of a pair of output tubes....I will give the filter caps a check and see if any are getting bad....where this amp is so old they might be.....I don't think this has had much work done on it in the past....thanks again for pointing that out to me.
Cheers,
Bernie

[Gtr_tech]

Speaker efficiency plays a big role on how loud the amp is to your ears. A speaker with a sensitivity rating of 98dB will sound way louder than one rated 94dB.

12" Retro 30 - 75 Watts | Warehouse Guitar Speakers

Dump a pair of those in there and see how loud it is then....

[Gtr_tech]

As mentioned earlier, the plate voltage is suspiciously low. I bet you have leaky filter caps pulling the voltage down. If it still has the old brown Mallory caps in there...get'em out now. Don't overlook the bias filters either (80u 100v x2).

[bsco]

Actually, I just opened up the capacitor cover and it does have those old brown caps in there. A couple of them have signs of bubbles on the side.....so I am going to dis-connect them and check them out and replace them........and I'll check the bias filters as well...Thanks for the tip......much appreciated....
Cheers,
Bernie

[Gtr_tech]

If the vents have bulged and/or ruptured, they're junk. preplace all your pwr supply filters and don't look back.

[bsco]

For the fun of it, I measured the capacitance of those 5 caps and also checked them for excessive ESR...They tested good but I will replace them anyway....The bias cap that you talked about.....is that the 70uf,100V cap??? It's on the schematic but couldn't find it in the chassis unless there is a different one there.....and there is only one, right??? Also found broken wires in the reverb tank as well.....
Cheers,
Bernie

[JoeM]

The bias filter cap isnt with the other filter caps in the filter cap can mounted on the bottom chassis, it's inside the amp chassis on a separate eyelet board with the rectifier diodes.

[bsco]

OK. I know which one it is now...I'll change that one as well.....Thanks...
Cheers,
Bernie

[Gtr_tech]

Often old caps will test OK for value and ESR, but not for leakage. Leakage is what happens when they get dried out and quit doing their job as a capacitor. They start drawing current, build up heat, and finally explode spewing corrosive cap guts everywhere.

[Shawnobi]

Actually, I just opened up the capacitor cover and it does have those old brown caps in there. A couple of them have signs of bubbles on the side.....so I am going to dis-connect them and check them out and replace them........and I'll check the bias filters as well...Thanks for the tip......much appreciated....
Cheers,
Bernie

I rebuilt my girlfriends Silverface 2 years ago. It had those original caps and they were showing definite signs of leakage. I replaced all of them with Spragues and it's been solid ever since. So I'd definetly reccomend you do the same. The UK company I used had a specific set they sold for that amp which made it nice and easy.

[bsco]

I have the caps on order. I thank everybody for their help, time and patience with me...Much appreciated. I have another vintage Fender amp but I'll make a new post for that one....
Cheers,
Bernie