hot rod deville, low volume

[funkmeblue]

Just got this in from a friend. Figured it was tubes because they are 10 years old. So far voltages seem to be right. Just thought I would ask if there is any common things that go wrong with these suckers. So many relays and op amps......thanks man

[funkmeblue]

pin one on phase inverter 0 volts, pin 6 223 volts. Should be 14.7 volts each side, what could cause this, thanks

[RevD]

Never seen inside of one, but do they use a ribbon cable to connect to the tubes? On some of the smaller Fenders (blues jr) reflowing the tube sockets to the board often fixes some of those type of problems. (on the blues jr it often starts redplatting due to this). Just a thought but as I say I've never been in one.

Regards,

Rev. D.

[azatplayer]

Its the load resistors on the phase inverter. This amp gets really high B+ to the phase inverter, like 440V or so before the load R's. Then drops to about 220-240 from memory. As they are cruddy little 1/2 watters, they fail.
Throw some nice 1watters in there on both sides, will be great forever more.
You have to remove the board to get them out completely, its about a three minute job if you snip the resistor off right at the body, then reattach to the wires that are left standing. See a lot worse done by techs that are regarded, also youre not dicking around with possible damage 0f the pcb.
Dude, pull out those crappy illinois elytics while youre in there, cos they fail too, and leak crud all over the board and sockets below them..
Cheers Don

[funkmeblue]

hey man, thanks alot. This is what I figured after looking at it last night. Have not had a chance to get back at it.

[ampgeek]

Interesting that the "expected" voltages on the PI plates is so low (14.7 V). Could that be correct?

Isn't this a typical long tail PI plan firing into a 6L6'ish power section? I can't easily see how that low a voltage could EVER swing a high enough signal to drive the power tubes.

Maybe the decimal point is off and it should be 147V? Even that feels slightly low but would at least be in the correct "area code".

Dave O.

[jaysg]

There was a college student who had messy but comprehensive website dedicated to problems with these amps. I've only been in one and it was a royal pain. There was a bad IC cap and I reflowed a number of suspicous solder joints. Also, the previous guy had biased the whole amp at 30mA when it should have been > 60mA (both sides, one resistor)

Needless to say, there's a litany of issues with them:

Cold solder joints
JFets
Input and output jacks
MV should be audio taper
Illinios Capacitors -- 6 count
PI: plate resistors undersized
Bias supply
+/-16V power supply -- there are a couple 330ohm/5W that can cause cold solder joints locally
Screen resistors

[funkmeblue]

ya, the voltages I said were from the schematic. They are wrong (444v) or I read it wrong or something went wrong. Replaced 82k resistor on the phase inverter plate and all is well. Thanks everybody for the help. Only took me an hour and I did it the right way

[azatplayer]

Good work! I had one on my bench a few months ago, same issue.
For what its worth, i reckon this is a fantastic sounding amp, at least the 4x10 i did was. Killer tone.

[funkmeblue]

well.....it did it again. Worked fine yesterday when I cranked it up for a few. As soon as I turned it on today, back to quiet. I didn't have a one watt 82k only a half watt which is what was in there oringinally. I've never had this happen, but could the tube be freaking out and eating that resistor. It tests o.k. but it's 10 years old and the owner doesn't want to buy new. Anything else that could cause this......try a 1 watt?

[azatplayer]

Gotta use 1 watt man. And did you change the 100k as well? Do the pair of them with 1 watts. Its a pretty hefty drop they have to make, ordinarily there would be a 3 watt dropper after the screens, but the phase inverter gets screen voltage straight to the load R's sans the dropper. Bit cheap ass of fender.

[funkmeblue]

ok, the resistor is fine. I hear a buzzing sound that I can't quite pinpoint. Problem was "fixed" now is back? I think he needs a new amp!