63 Pro Amp cap job. I think I broke somthing

[dmartn149]

Okay, now I have new caps in my amp, but the volume is really low, and distorted. I had never done a cap job before, so after changing the 7 smaller ones inside the chassis, I hooked up the speaker, and made sure the amp still worked. Then I changed the first 2 in the filter can (the ones without resistors) and made sure amp still worked. Then I changed the other 5 and the associated resistors, and the 470 resistors on the power tube sockets. When I plugged the amp in again, I neglected to check the power and standby switches. THEY WERE BOTH ON. Now It seems to be running on just one power tube. I can remove the one closest to the preamp tubes and get no change in sound. Pins 3,4& 6 all measure over 500v on my $2.00 meter, which is higher than speced on the schematic.

Any advice? Did I ruin the OT? What do I do now?

[Steve Conner]

I think it's unlikely that you broke it by leaving the switches on.

Much more likely that you made a wiring mistake during the cap job. Go back and check the pinouts, solder joints etc. Especially the resistors you changed on the power tube sockets...

[dmartn149]

Thanks Steve. That is what I'll do.

[Bruce / Mission Amps]

I can remove the one closest to the preamp tubes and get no change in sound. Pins 3,4& 6 all measure over 500v on my $2.00 meter, which is higher than speced on the schematic.

Any advice? Did I ruin the OT? What do I do now?

If you didn't blow up your meter and you don't have a short on the socket from your B+ rail to your bias supply, subsequently blowing a power tube open.... and only one tube is reading high like this, I think you also might have disconnected it's cathode ground wire... lug 8 to ground. Check it.

[dmartn149]

Okay, I did find a loose lead. It was the yellow wire coming from the rectifier board (-55v) I fixed it and didn't hear any improvement in sound. Would having this lead open take out other components?

Thanks a lot for all your help. It must get fairly tedious answering questions for guys like me who didn't bother to get a formal electronics education. I really, really appreciate it.

[Bruce / Mission Amps]

Okay, I did find a loose lead. It was the yellow wire coming from the rectifier board (-55v) I fixed it and didn't hear any improvement in sound. Would having this lead open take out other components?

Thanks a lot for all your help. It must get fairly tedious answering questions for guys like me who didn't bother to get a formal electronics education. I really, really appreciate it.

Yes, if the power tubes loose bias voltage they will try to turn on full blast at idle and draw maximum plate/cathode current.
You'd think when that would happen and the tube runs away, the current through the output tranny would cause a giant surge in power tranny current and open up the mains fuse though... but maybe not.

I still think you might have a completely open tube or a bad ground on the tube socket from lug 8 to ground. Check lugs 8 on those power tube sockets.

[dmartn149]

Thanks bruce. I'll look at that again when I get time. I won't be able to get to it till monday.

[dmartn149]

I had a few minutes this evening, so I checked all the connections at the sockets, all good. I pulled the tubes and checked the resistance on each side of the OT to the center. The one going to the tube that seems to be working measures about 130 ohms. The other, 40 ohms. They should be the same, right? or nearly so? Do you think the loss of bias voltage ruined the OT? If so, what else should I check before replacing it, so I don't ruin the new one too.
Thanks!

[Bruce / Mission Amps]

Watch that you don't shock yourself!!

I had a few minutes this evening, so I checked all the connections at the sockets, all good. I pulled the tubes and checked the resistance on each side of the OT to the center. The one going to the tube that seems to be working measures about 130 ohms. The other, 40 ohms. They should be the same, right? or nearly so? Do you think the loss of bias voltage ruined the OT? If so, what else should I check before replacing it, so I don't ruin the new one too.
Thanks!

Well, forget the OT for minute.
Remember, you said the voltages are all high on lugs 3, 4 and 6.... all B+ levels and that is showing me there is no quiescent current is flowing through the tube.
I'm worried about your $2.00 meter!!

Regardless what the actual DC resistance is (yes 40 ohms seems bad by the way)... you can use the current meter portion of your DMM (set for maybe 1000ma) and shunt across the OT from lug 3 to the center tap with the meter's very very low shunt resistor.
That resistance would probably be in the .1% 1 ohm or less range... whatever, quite a bit less then your 40 ohms.
That bypasses any resistance of the OT and will allow the tube to draw lots of idle current.
If that tube doesn't draw current then; either the tube is bad, the screen resistor is open, the socket is bad, the connection from pin 6 to 4 is bad, the connection from pin 8 to lug 8 or from lug 8 to ground is bad. Any of those would have it acting like a dead tube....

OH, one more thing that can cause this effect is if the actual filament voltage is not being delivered to the tube or socket.... or if the filament element itself is open inside the tube.

[dmartn149]

I'll see if I can borrow a good DMM from the maint. dept. at work and recheck. I guess I should mention that I subbed 10uf 160v caps on the rectifier board for the 8uf 150v originals. The place that I bought the caps didn't have 8uf 150v. Also they are all inexpensive Asian caps. (miec)

When I changed the resistors on the tube sockets, I just stuck them in the large wad of solder that the old ones came out of. Maybe I should remove all the old solder and re-do the connections.

On the sockets, it looks like the solder tab and the part that holds the tube pin is one piece of metal. Is that right?

The tubes seem to be okay. Everything stays the same when I try known good tubes from my other amp.

[Bruce / Mission Amps]

I think it is going to be something very simple at the socket.... keep plugging away at it even if it means sucking all the solder off the socket and starting over.

[VMR]

I like the bad heater wires theory. Maybe you broke one loose when working on the power tube sockets. I have had quite a few of those come loose, as they are fairly challenging to solder to the sockets well. Reflow those and make sure they are secure.

Good luck,

VMR

[dmartn149]

Okay, I got a better DMM (Fluke 112) resoldered all the connections on the sockets, e-caps, and new resistors. and checked continuity between pin sockets and the leads connected to each one.
The voltages I am getting are all over 500v on pins 3,4,&6 on both tubes. 3.4v on 2&7 and -46v pin5.
Still low power with lots of distortion.

[Jazz P Bass]

Fender Pro

What amp do you have?
Fender Pro?
What output tubes are in it?
6L6?
If so, pin six is not connected internally in the tube.
It is an open lug.
The layout diagram indicates that is where the assemblers attached the 470 ohm screen resistor.
Pin 3 (plate), Pin 4 & 6 (screen resistor) should be at B+ voltage.
Of coarse 500Vdc seems a tad high.
Pin 2 & 7 are your heater voltages.
Pin 5 is your -Vdc bias.
Hook your Fluke 112 (red lead) to pin 5,(black lead to chassis) set the DVM to read Vac.
You should see a nice healthy ac voltage when the amp is cranked.(80-90Vac)
If so, your output transformer is bad.(40 ohms on one winding)

[dmartn149]

Thanks Jazz. I'll try that tomorrow. Do I need to have a signal going into the input, or can I just crank the amp without all the noise?