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Old 19th September 2012, 04:52 PM   #11
M Gregg is offline M Gregg  England
Account disabled at member's request
Join Date: Jun 2010
Location: UK
Just a guess,

Either HT (B+) open, OP Tx open or bad connection. Speaker lead or voice coil open..possible stand by switch fault or broken connection. The mains is OK because the heaters are working!

Do you have a multi meter?
If you don't know what you are doing then be guided by the guys here! Don't play with a live chassis it can kill you easily.
There are some tests you can do with it switched OFF!
High voltage..Take care..

M. Gregg

Last edited by M Gregg; 19th September 2012 at 04:56 PM.
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Old 19th September 2012, 05:08 PM   #12
TheGimp is offline TheGimp  United States
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Join Date: Aug 2009
Location: Johnson City, TN
Several things could be wrong with the amp.

First you need to be competent and comfortable with working around high voltage. According to the schematic, your B+ is 410V. It will be more likely 415-425V as line voltage is higher today than when the amp was designed.

Given that you can follow the appropriate safety procedures, you need to remove the chassis from the cabinet, and support it upside down on your bench.

I would check the continuity in the output of the output transformer first. If that looks good I'd go on to the voltage checks.

Since the tube heaters are on, the fuse and ac switch can be ruled out.

(By the way, you need to change the power cord to a three prong safety cord while you have the amp out of the cab.)

Your problem is most most likely in the HV section. You need to measure the voltage at the standby switch with it off and then on, and at the input and output of TR2. If TR2 is open (not likely) you will lose all high voltage to the preamp driver and screen voltage to the output tubes. No sound will ensue.

If you have appropriate voltages on both sides of TR2 (schematic values up to 1.05X schematic values) then the problem lies elsewhere than the B+.

Also check the B+ on the output tube plates (open OPT).

I doubt the bias voltage is sufficient to shut the tubes off completely, but it is also a possibility. Check the voltage at the two ends of the 10K balance pot. This is the same circuit as the Bassman 50 I just finished working on.

Bias voltage could be as high as -45V. I had to increase it in my case to -45V to get the zero signal tube current where it should be.

If it is greater than -45V I would look at a possible bad connection to the 15K or an open 15K resistor.
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Old 19th September 2012, 11:27 PM   #13
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
In post #2, Hollow asked the OP to check the speaker, but the OP merely restated the tubes were lit.

The speaker wiring may LOOK intact, but that tells us nothing about its integrity or whether the speaker itself works. Before ALL that other stuff, verify that the speaker and its wiring will function.

Your problem is zero sound, not poor sound or weak sound. So your trouble is not from some wrong voltage. EVen grossly wrong voltages will result in some sort of sound. COnsidering the general lack of internal fuses in Fenders, a loss of B+ is possible, but not at all common.

Unplug the speaker wire from the amp, get your ohm meter, and measure resistance from plug tip to plug sleeve. Open? Or some low resistance?
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