Fender Super 60 Voltage Questions?

[p0pNfr3sh]

Hi, I have a Super 60 that I have been working on. I originally found that there was a plate resistor bad on V103, I replaced that and the 22uf capacitors due to some bulging. the amp works better than before the output is noticeably higher, but I feel like I haven't found all the problems in the amp.. most of the voltages on the test points in the power section are almost on average +20 volts higher than on the schematic. where the preamp's are -10v to -20v lower on average, there are two points of interest to me.

The reading from the grid, pins 7, and 2 on V103 A and B read +22v where they should be +40v. I have checked all of the resistors in the amp at least twice now, and I haven't found anymore open ones. There is a nice ground hum coming from the amp, this stops when I test pins 7, and 2. I would also like to note that when testing the voltage from pin 2 on V102A as it does read the 0v it should the hum becomes worse.

The second thing that I came across was when testing the voltage on pin 3 for the plate of V105. when testing over the diode D102 the screen on my voltmeter goes nuts and there is a very high frequency pitch that is emitted through the speaker; almost inaudible. this doesn't happen when I test the pin from a different angle, nor from V104 when tested in the same manner as mention on V105. I have also swapped the position of the tube, the result of the testing stays on the socket of V105.

Any thoughts?

[Enzo]

First, what is your mains voltage as you take these readings? When your B+ is supposed to be about 475v, that is just about four times the mains, and for every volt the mains moves, expect your B+ to move the same way 4 volts. That is one thing. Also, all the parts have tolerances, and those toleraqnces allow voltages to vary amp to amp. Look at note #4 on the schematic. Yes, always read those notes. It says that all voltages 0n the schematic are reference only and might vary 20%. COnsider how far off from the printed voltage 20% would be. It just is not critical, this is a guitar amp, not a NASA project. next, look at the preamp, B+ node D mostly. From node C, the voltage is to drop from 417 to 223 across that 120k resistor R167. 194v. I get 1.6ma current from that. That 1.6ma is the total of the three triodes on that node, the first three triodes in the amp. Due to tube variations, if the sum total current of those three tubes rises a half a milliamp, that would increase the voltage drop across that resistor, and thus lower the B+ on node D, by over 50 volts.

SO I don;t see a problem with your B+ voltages.

V103 is your phase inverter. You cannot read the voltage on those grids with a meter to ground. You can measure the voltage at the cathode, pin 8, to ground and not it down. Now connect your meter to pin 8 and measure voltage to pins 2 and 7 from there. You can calculate the voltage on pins 2 and 7 from that difference voltage from whatever pin 8 has. I suspect pin 8 is close to 50v. And my guess is about 3v between cathode and grid. Your meter has internal impedance that reacts with the circuit to give erronoeus resdings on those grids. Your really low grid volts are probably reeally just fine. If your two plates both have B+, on that tube, it is probably working.


When you go to measure the plate voltage on the output tubes, your meter leads can act as antennas. What you are hearing is high frequency feedback oscillation. I often find that one tube lets me read it and the othe won;t in your fashion. SOmetimes moving the probe wires makes it OK, and sometimes I can reverse my meter leads. The two plates are opposite in phase from one another, so one can cause positive feedback and thus oscillation, while the other side makes negative feedback which does not upset the stability of the amp. Once again, you have discovered an artifact of the testing procedure, and not a problem with the amp.

[p0pNfr3sh]

Thanks for the help, I setup the guts of this amp on a board for testing and repair due to the way its mounted in the chassis. I want to make sure that I'm happy with the results, and that there isn't going to be anything I missed.. I should have though about note #4, of course there would be tolerance in the voltage if there is tolerance in the components. the B+ voltage is at 493V, and the amp is quite louder after replacing the open resistor that I found at R145 feeding the plate to V103A. I will have to try the reading of the grid using pin 8 as you suggested.

While writing this reply and looking at the schematic, studying I think that I have found the answer to my problem. I find that there is quite a difference in the output of channel one and channel two. I see that channel two the Gain pot R5A,R5B and the volume pot R16 are rated at 1M, where the volume pot of channel one R8 is rated at 250K. I do believe that would account for the vast difference in volume and drive between the two channels.

[Billy R aka DynaFreak]

I just repaired a Super 60 and my notes show the plate voltage as 492V. L ike yours, also had a bad plate resistor on the PI tube. I would be a little concerned about low preamp voltages. I would check the voltages on each pin of the preamp tubes. This can help find a leaky coupling cap or bad cathode bypass cap.