Peavey 6505 driving me crazy!

[depodo]

Hi.

I'm repairnig my 6505 (same amp as 5150) and can´t find the soluition.

There is no sound in the amp, and`if I insert some in return jack, it works, so I suppose there is a problem in pre-amp section.

(I´ve checked send and return jacks and circuit. The signal is not present in preamp and send jacks.)

I can follw the signal with a oscilloscope to V3B (cathode follower). The signal doesn't go out, so I suppose the tube is broken.

I changed the tube to a new, checked one, and the amp is equal. No soud at all.

I measured voltages and I have: 285v at Anode 240 at grid and 2015 at cathode.

¿Is this measures rigth? I'm not a Cathode follower expert, I don´t know if these voltages are wrong...

Help please! Thanks.



Peavey-5150EVH-Schematic.pdf

[Dave H]

I can follw the signal with a oscilloscope to V3B (cathode follower). The signal doesn't go out, so I suppose the tube is broken.

I changed the tube to a new, checked one, and the amp is equal. No soud at all.

I measured voltages and I have: 285v at Anode 240 at grid and 2015 at cathode.

¿Is this measures rigth?

2015 is obviously a typo. What's the real cathode voltage? 240V at the CF grid looks too high. I'd expect about 100V to 120V at the grid and cathode. Connecting meter probes sometimes causes it to oscillate and you don't get a true reading.

[The Dude]

The schematic link doesn't work for me? At any rate, are all of the filaments lit?

[Leooooh]

Hey man,

2015V is definitely not right. I would check again this value.

I would also measure the voltages V1, V2 and V3 in the power supply.

Also, try to check if there is indeed a signal coming out of V5, wich is the tube before V3b.

And at last, please, check the solder in the send jack. O normal issue in the 5150/6505 family, is dust or bad solder in those connectors, causing the sound to fade or even no sound at all. Spray some contact cleaner then get a guitar cable, put it in and out a few times or even make a new solder in these connector.

Try that and lets us know what happened.

[depodo]

2015 is obviously a typo. What's the real cathode voltage? 240V at the CF grid looks too high. I'd expect about 100V to 120V at the grid and cathode. Connecting meter probes sometimes causes it to oscillate and you don't get a true reading.

Yep, 2015 is error.

215v

How I can measure correctly? Don´t know about oscilating meter probes...

[g1]

Try with black probe on cathode and red probe on grid. Post result.

[Enzo]

On the schematic, just to the right of the post controls and relay K2B, find R81 and note the CLAMP signal just above. With the amp running, measure resistance of R81. If it measures large, my idea is not the case, but if you measure a very low resistance, like 100 ohms, then I suspect your CLAMP.

CLAMP is mid right on page 2. Transistor Q7. Try removing it.

[g1]

And here is a re-post of the schematic. (There is a forum glitch that does not allow the original to be downloaded)

[depodo]

First, thanks very much everybody.

second, sorry, for my bad english.

I've benn checking the amp recently, and I can send some new data.

If I bypass V3B, sending signal from CLAMP point (Where I can see signal in oscilloscope) to R103 (Just before the Effects Send loop) the amp sounds, so there is the problem, the effects loop and mute transistors are out of the equation.

I checked all the Caps and resistors around, and they are all seem OK.

Measures in V·B:

WITH 12AX7 EH (new) or JJ (used) TUBE:
Plate (Anode) (Pin 6) 289v (V2+)
Grid (Pin 7) 248v (A LOT, is'n it?)
Cathode 250v (A LOT, is'n it?)
Voltage

WITHOUT TUBE.

Plate (Anode) (Pin 6) 373v
Grid (Pin 7) 305v (A LOT, is'n it?)
Cathode 329v (A LOT, is'n it?)

"Try with black probe on cathode and red probe on grid. Post result. "
Voltage cathode-grid: -2.27v with tube
Voltage cathode-grid: -3v without tube

Power supply:
V1: 433v
V2: 287v
V3: 250v

Looking randomly for some short, I've found one between J90 and J29 (CATHODE AND V1=433V) I can't find component between this points but, when I dissconect the preamp board, it still there for a minutes and then dissapear. I power up againg, amp no working, same voltages, power down, short is there again.

Any help?

Please help! I promise some virtual beer to everybody!

THANKS!

[Dave H]

WITHOUT TUBE.

Plate (Anode) (Pin 6) 373v
Grid (Pin 7) 305v (A LOT, is'n it?)
Cathode 329v (A LOT, is'n it?)

You're right. It is a lot. Without a tube in there the grid and cathode should measure 0V. There must be a short somewhere.

[depodo]

DONE!

(A tiny tiny tiny piece of metal doing short in preamp board!!!)

Hard to see even with mag glasses!

THANKS EVERYBODY!