1975 Fender Silverface Twin Reverb Vibrato repair issue

[Planobilly]

Hi,

http://ampwares.com/schematics/100W_Mast_Vol_SF_Twin_Rev.pdf

I am a bit lost on this amp. I replaced the filter caps and installed new tubes. The amp plays and sounds good. The Vibrato does not work. The two Vibrato channels themselfs work and the reverb works.

Plate voltages for V1,V2,V3,V4, and V6 are OK

V5 has high plate voltage on pin one, 423V and should be around 300V, pin 6 reads 380V and should be around 440V

Plate voltage on V7,V8,V9, and V10 all read 435V which is more or less normal

I set the bias to 40mv on V10 and checked V9 which read 39mv

Both V7, and V8 read only 10.8mv

I can not find a bad cap or resistor in the vibrato circuit.

One other strange bit. Per the schematic there is a 10K and a 100K resistor in series on the 22uf filter caps. The 100K is actually a 2.2K (red red red silver) but the voltage at the cap is correct. Schematic not correct?? Should I change it to 100K. Part of the problem?

What could cause the low plate current on V7 and V8? Are the two conditions likely related...low plate/vibrato not working Where do I test next?

Thanks,

Billy

Edit: The 100K resistor is one watt but I only have a 120K 2Watt on hand.

[Guitarbro]

The LDR usually goes bad (the thing to the right of V5B).

[voltz]

Vibrato footswitch working ok?

[Planobilly]

Just woke up.

I replaced the optoisolator LDR and I don't have a foot switch for this amp. As I understand it the foot switch only turns the vibrato off.

After I drink another coffee I will get back into this. Need to wake up before I go sticking my finger in the 500v  light socket.....lol

Cheers and Happy New Years

Billy

[voltz]

And happy new year to you too

The footswitch needs to be shorted out for the vibrato to work. Try a shorting link.

[cvanc]

How are you measuring output tube bias?  I don't see cathode resistors or bias test points on the schematic?

[Planobilly]

I shorted the rca connection without results a day or two in the past

I measured the bias current with a bias probe which breaks the connection between the tube socket and the tube and runs the circuit through a DMM set for 200mv or can be set on auto. Some probes use a cathode (1 ohm) resistor but this one is direct. I have read that in some rare cases this type of probe can make a amp unstable although I have not had any issues in the past with either probe I use. I have a 8 pin and a 9 pin probe.
The nominal output for the 6l6 is 25 watts so the plate voltage I measured was 435v. 25/435v X.7 = apx 40ma is how I came up with the bias.

I will close the foot switch circuit again at the RCA jack on the amp just to check. AS I understand this amp the vibrato circuit is always on unless turned off by the foot switch.

Billy

[voltz]

Yep, the reverb footswitch closes to kill the reverb, and the vibrato footswitch closes to turn ON the vibrato. So they work opposite.

[Planobilly]

Thanks guys....

I will get back into this issue later.

Phone is ringing off the hook this morning, Fedx banging on my door, wife asking me a thousand questions. Too many distractions!!!!lol

Just why I like to work at 3 AM....lol

Billy

[gadget73]

Hi,


One other strange bit. Per the schematic there is a 10K and a 100K resistor in series on the 22uf filter caps. The 100K is actually a 2.2K (red red red silver) but the voltage at the cap is correct. Schematic not correct?? Should I change it to 100K. Part of the problem?

What could cause the low plate current on V7 and V8? Are the two conditions likely related...low plate/vibrato not working Where do I test next?

Thanks,

Billy

Edit: The 100K resistor is one watt but I only have a 120K 2Watt on hand.

If the voltage is right, then I'd leave the resistor, especially if it appears to be original.  Schematic errors or production changes without documentation are not uncommon.  If it worked with that resistor once, chances are good that it will work again with it. 

Low current on V7 and V8 would either indicate those tubes are pooched or the balance is not set properly.  Moving the tubes to the other spots would tell you if its the tubes.  I don't actually see a bias level control pot in here, unless it was added.  Normally you'd set the bias on one set of tubes, then adjust the balance so they are equal.  Go back to the bias and adjust as needed, etc.  Repeat until everything is happy.  Without a bias level adjust, you just balance it and call it square.

Not sure where the resistor you mention is.  Not sure who drew this schematic, but they sure did a hell of a job making it hard to follow.  No part numbers on anything either, bah.

[vk6zgo]

If the voltage on V5A anode (Pin 1) is high,that tube may not be conducting.
If this is the case,you will not see any volts across the 2700 ohm cathode resistor attached to Pin 3.
The schematic says that voltage should be 2v.

Check the other cathode voltage on Pin 8------it should be 13v,but don't be surprised if it is greater,aFter all,V5B anode is reading a bit low,so it is probably drawing excessive current.

[Planobilly]

Thanks guys,

Sorry for not getting back sooner. After replacing a few bad caps and a resistor or two plus a new LDR everything went back to normal.

Thanks for all the help. All your post cause me to think and learn new stuff...so thanks a bunch.

Billy