'65 Fender Deluxe - One 6V6 cherrying and I'm stumped

[cjlectronics]

Hi Guys,

I am working on a 1965 Fender Deluxe (schematic is here ) and one 6V6 cherries out after I enable the standby switch. Here is what I've done so far:

1. All tubes are new.
2. Replaced all 4 filter caps with 22uf/500v caps
3. Replaced the 25uf/50v cap in the bias pot circuit.
4. Replaced the vibrato channel caps with 22uf/25v caps.
5. Voltage before the standby switch is engaged is 457vdc
6. Voltage after the standby switch is enabled is 340vdc once it settles and then I disengage the standby switch b/c one 6V6 is getting very red
7. Voltage at the last filter cap is 189vdc (schemo says it should be 230vdc)

I'm stumped, usually when I replace these things everything comes back to life. The output is distorted and power is low... obviously.

Not sure what to check next. Can you give me some pointers, please!!

[Jazz P Bass]

Check that you have a bias voltage at each tube.

If yes, then swap the tubes from socket to socket.

If the trouble follows the tube, then it's bad.

[pontiacpete]

how about the screen resistors?

[Tom Phillips]

how about the screen resistors?

That's the problem. You are just guessing. Follow Jazz P Bass' advice and let's focus on things that can cause the symptoms.
Also tell us if the problem started after you worked on the amp or is the redplating the reason you are working on the amp? New tubes is no guarantee of good tubes.

Note that you can trace through the bias supply and measure the voltages at various points with the 6V6s pulled out of the amp. If the bias at the tube socket isn't correct (around -35V at pin 5) without the tube installed then there is no sense to have the power tubes installed until you have located and repaired the problem.

We can discuss next steps, if required, after you report back.

Cheers,
Tom

[Jazz P Bass]

how about the screen resistors?

If the screen resistors are open circuit, the tube will not conduct.

[pontiacpete]

thanks JP! duh on my part )

[soundguruman]

The screen resistor is burned, the tube may be bad. Check for negative voltage on pin 5 of both tubes.
One 6V6 red: It's not getting bias voltage to pin 5. The wire may be broken, or the track on the circuit board may be damaged.
I prefer to move the screen resistors to the tube socket. I mount them just like the old style, on the socket.
The screen resistors tend to burn the circuit board, so I move them off the board, onto the socket, between pin 6 and 4, and use 2 watt instead.

It's hard to understand why all those caps need replacing....
It's a 65 RE-issue? Right? Not really made in 65???

[cjlectronics]

Tom,
You're right I was in a hurry and guessing. Thanks for being candid and to the point. It made me stop, breathe, and walk away to collect my thoughts. It helped!!

The problem was no -35vdc bias voltage. One of the bias voltage wires had broken under an eyelet. Once I fixed this no more red-platting... Whew-who!!

HOWEVER, there is yet another problem. The normal channel sounds great. Lots of volume and the punch I expect. The vibrato channel is just the exact opposite. Very little volume at 10. It sounds like it's set at 2 when the volume is cranked all the way up. From the image, you can see I replaced the two 25uf/25 volt caps. The voltage at the 100k resistor is 230vdc then 150Vdc on the plate.

I'm going to pull the 12AX7 in the vibrato circuit to see if this affects the vibrator channel. Then try pulling the wire off the intensity pot to totally isolate the vibrato from the input signal path. Any other suggestions?

BTW, this is a real 1965 Fender Deluxe. It still had the original capacitors. Pots date to 1964. My friend found it in a flea market and bought it for $35.00 Yes, I typed $35.00.

The original problem was a weak distorted signal with a slight hum in the output. The new tubes fixed the weak output. Then the red-platting started. Also, I removed the death cap and replaced the original 2 prong power cord with a 3 prong power cord.

[Tom Phillips]

Glad to hear you successfully troubleshot the redplating problem.
Cool amp. Excellent deal!
What test equipment do you have available to troubleshoot the vibrato channel issue?
Tom

[cjlectronics]

Glad to hear you successfully troubleshot the redplating problem.
Cool amp. Excellent deal!
What test equipment do you have available to troubleshoot the vibrato channel issue?
Tom

O-scope; DVM, power supply, Waveform generator.

[Tom Phillips]

O-scope; DVM, power supply, Waveform generator.

Cool. Some ideas:
First thing - swap the two pre-amp tubes with each other and determine if the problem follows the tube.
2) Temporarily unsolder the wire on the center pin of the intensity pot. This takes the tremolo light bug out of the circuit. If the vib channel volume returns to normal then we will suspect a bad light bug. Note that this step will always make the vib channel gain increase a little.
3) Measure all the plate and cathode voltages in the preamp. You have two channels so you can compare the results and look for major differences.
4) If that doesn't turn up a problem then you can inject a sine wave into both channels and trace it through the circuit looking for the spot where the vib channel has excessively low gain. The volume and tone controls will need to be the set the same on each channel. If you don't have a dummy load to substitute for the speaker then the injected signal level will need to be kept low. You can also just pull the power tubes for this test. That will increase the B+ supply level but it shouldn't hurt anything. Especially since you have new power supply caps installed.

Let us know what you find and we will go from there.
Tom

[cjlectronics]

I found the problem. A wire from the treble wiper to the volume pot was broken right at the solder joint. I was hard to see and I only found it by poking it with my O-scope probe. When I soldered that wire back on, the amp came alive. The vibrato circuit worked great as well.

Thanks to everyone who replied and especially to Tom Phillips. You were spot on with every reply. Hats off to you.

Merry Christmas and Happy Holidays!

CJ

[Tom Phillips]

Thanks reporting the outcome CJ. I'm glad the Deluxe was fixed without enduring shotgun parts replacement.
All-in-all not bad for an almost 50 year old amp.
Merry Christmas,
Tom