1949 5B3 Fender Deluxe Issues

[gtrplr2]

I seem to have this same problem on these older amps that I can not figure out. I have serviced and modded many blackface and silverface Fender amps with great success, I recently had a chance to work on some of the older tweed models and they are killing me. I have a Very high bias reading on one of the 6V6 tubes, and the other one is in the normal range. I have found several bad coupling caps and replaced all the electrolytic and power supply caps. The tubes all check good on my tester. The transformer readings to the rectifier are good as well as the readings on the pins of both 6V6 sockets. I have the 5B3 layout and schematic and do not understand how and where this amp is failing. I am about to go crazy, and as in the recent past, I thought I better come here for some good tech advice, as I am just learning as I go. Thanks in advance.

[Enzo]

What do you mean by "very high bias reading?" Do you mean the grid voltage on that tube is very high? DO you mean the tube current through the tube is very high?

If the voltage at pin 5 of either output tube is away from zero, then fix it. Pull the tube from the socket. NOW what voltage is there? If it changes, the tube was at fault. If the voltage is positive, then the cap from the phase inverter is leaky.

The tubes share a common cathode resistor, so how did you measure the current through the individual tubes? Just for science, pull the 6SC7 phase inverter tube. Does that affect your bias? If so it is bad.

You have verified that there is B+ voltage on pins 3 and 4 of both power tubes? ANd zero DC on both pins 5. ANd both pins 8 have whatever the bias voltage is from the cathode resistor?

With power removed, what is the resistance from either output tube plate to the OT center tap?

[nico.verduin]

I am assuming you have a tubetester that measures the dynamic conductance (micro-ohmos). This can be measured anywhere on the curves of the tube. What should also be measured is the Idling plate current. The better tubetesters (like AVO MK4) can measure that as well. It would have shown you the big difference. Through time, the plate current will deteriorate and give an imbalance. I don't have a schematic on hand, but I am assuming it is cathode Biassed. This will cause the values of both plate currents to average.
My guess is the power tubes need replacement.

[tuberattler]

that particular phase inverter is a real tricky one and is difficult to get the tubes balanced. I ran into this with mine, I had to go through several PI tubes to get it to balance out.

[gtrplr2]

I am sorry for the slow response to all that have replied, I have been out of town and returned last evening. I will try to go thru Enzo's list of things. To preface this a bit I would like to add that I am not a tech, but by no available techs in my area, I began to work on my own amps about 4 years ago, mostly black face and silver face Fenders for my personal use. By other friends in my area playing my amps at jams and such they asked if I would try to look at there old Fender's too. I was hesitant at first but I did take in a few for the basic service of caps and small repairs. I have accumulated quite a bit of test equipment now and some of it I am still in the process of learning as I go. So if I might sound like I am not very well schooled on most of my questions, you here on the board are correct in that. Well over the years more and more amps started coming my way and with some luck and the Internet have been mostly successful. Now I do repair work for my local guitar shops and again with some luck I have been mostly successful. I have a friend that lives out of state and is a collector-player, so that is how I ended up with this old Fender. First by high bias I was meaning that I checked it using my tube socket bias probe, where I plug in a DMM to the socket. I do not have a quality tester like you spoke of, mine is a hickok 799. I did take the very same 6V6GT tubes down to my 65 deluxe reverb and again checked the bias with the probe, and they test at 22ma-26ma respectively. 1st I will give some readings that you suggested with tubes installed, then pulled. 6V6GT closest to the rectifier tube (V4?) Pins 1 and 2 are connected together with pin 2 going directly to chassis ground. Pin 3 (plate?) reads 330VDC. Pin 4 (grid2?) reads 250VDC. Pin 5 (grid1?) reads 0VDC. Pin 6 (NC). Pin 7 (heater?) reads 6.5 VAC. Pin 8 (Cathode?) reads 30VDC. The second 6V6GT (V3?) Pins 1 and 2 are connected together with pin 2 going directly to chassis ground. Pin 3 reads 290VDC. Pin 4 reads 260VDC. Pin 5 reads 30VDC. (As you suggested I will now pull the phase inverter tube and read pin 5 again) Pin 5 PI pulled 0VDC. Pin 6 (NC) Pin 7 (PI back in) 6.4 VDC. Pin 8 30VDC. ALL tubes pulled except rectifier. V4 Pin 3 515VDC. Pin 4 517VDC. Pin 5 0VDC. Pin 6 NC. Pin 7 7VAC. Pin 8 0VDC. V3 Pin 3 518VDC. Pin 4 517VDC. Pin 5 0VDC. Pin 6 NC. Pin 7 7VAC, Pin 8 0VDC. The OT has a brown wire (pin 3 on V4 6V6GT) another brownish wire (pin 3 on V3 6V6GT) another red wire that goes to pin 8 on the rectifier, and a white wire that goes to chassis ground and the other end of the white wire goes to the mounting tab on the OT speaker mount. With power removed the resistance from the white wire chassis ground to both 6V6GT tubes has no reading on my DMM. On the rectifier pin 8 to pin 3 on both 6V6GT pin 3 is 0VDC. Sorry this is so long, but I wanted to cover all the questions that you had asked Enzo. I would also like to thank nico.verduin and tuberattler for the response.

[stokes]

If I am reading this right,and you didnt make a mistake you have 0vdc on pin 5 of one power tube and 30vdc on pin 5 of the other.Is that right?If so you have a leaky coupling cap to the pin5 with 30vdc on it.This will cause the bias mismatch.

[gtrplr2]

If I am reading this right,and you didnt make a mistake you have CDC on pin 5 of one power tube and 30vdc on pin 5 of the other.Is that right?If so you have a leaky coupling cap to the pin5 with 30vdc on it.This will cause the bias mismatch.

Thanks stokes,

I have eliminated the original coupling caps for the purpose of trial, and I have the same readings.

[stokes]

Still getting 30vdc on pin 5 of one of the power tubes?If so,you need to find out where it is coming from,looking at your post again,I notice you have 30vdc on the cathode as well,could it be the tube has an internal short from cathode to grid?That 30v's on pin 5 is a problem and is likely the cause of the bias mismatch.

[gtrplr2]

Well I finally found the problem. It is probably unusual, but I am not sure with this amp by now! The under-board wire that connects from the 6SC7 PI screen grid to the 6SC7 preamp suppressor grid, and control grid, had disconnected from the tag board. I thought I would never find the problem, and was feeling very sick about the whole thing. The bias with my probe is still a bit high. (38ma) Is it possible to adjust the bias on this amp without any modifications? Would I adjust it by changing the 250 Ohm resistor that is in parallel with the electrolytic 25uf/25V from pin 8 on the 6V6? I appreciate all the help that has been offered. It sounds really good with it just hooked up to my out of cabinet test speaker on the bench and I am looking forward to getting it all back in the cabinet and listening to it with the stock P12R.

[stokes]

To adjust the bias you would alter the 250ohm cathode resistor on the power tubes.You can raise the value of this resistor to bring the current draw down,but if your tubes are not red-plating,it is not critical.Check the resistor to see if its value has drifted before making any changes.

[bob p]

while you're in there looking at the cathode (bias) resistor, you might want to double check the position of the 25/25 cap to verify that its not up against the resistor, as heat will dramatically shorten its life. on the subject of cap life, i've never understood why fender chose to use a 25VDC cap in that position, as the working voltage is close to the cap's limit. i prefer to replace the 25/25 cap with a 25/50.

just my $0.02.

[Bruce / Mission Amps]

Well I finally found the problem. It is probably unusual, but I am not sure with this amp by now! The under-board wire that connects from the 6SC7 PI screen grid to the 6SC7 preamp suppressor grid, and control grid, had disconnected from the tag board.

There is no screen or suppressor grid on 6SC7s.. did you mean the 6V6s?