Amp repair where do I start

[Helmholtz]

I have 370 V on plate of V4B. Is that the issue ? Should be around 160 v

Seems the triode is not conducting. Check cathode and plate resistor.
Could also be a bad socket contact (slightly wiggle the tube in its socket) or even a bad tube.

BTW, by voltage dividing the signal after the the 3.3M resistor will be only 4%.

[LoveChet]

Now its working. All i did was wiggle tube. Go figure. 20 mv in . 1.5 v out. Plate volt 160 dc. Plate and cathode resistors ok. Gain 75 UT of 12AX7

[Helmholtz]

Now its working. All i did was wiggle tube. Go figure. 20 mv in . 1.5 v out. Plate volt 160 dc. Plate and cathode resistors ok. Gain 75 UT of 12AX7

Great!
Nevertheless you should clean the tube pins and socket contacts with DeoxIT D5.
Starting with measuring DC voltages would have saved you some time.

I don't think you can get more than a gain of 60 from that stage.
I guess the probe resistance is somewhat loading down the grid signal and actual input signal is larger.

Now it's time to check output power.

[LoveChet]

V4 mix is still intermittent. I need to get that fixed before I proceed.

BTW what does it mean if you get a good signal that moves up and down a little on the screen. I get it only when I activate the fx loop. Fx loop has a big sky reverb and a 31 band eq in it.

[Greg Robinson]

BTW what does it mean if you get a good signal that moves up and down a little on the screen.

Without seeing it I can't be sure , but it maybe a small amount of ripple on the B+ line. You can increase your timebase to see if it matches up with 120Hz.

I get it only when I activate the fx loop. Fx loop has a big sky reverb and a 31 band eq in it.

You should not be using the FX loop while troubleshooting to minimize variables. You should only try things in the FX loop if you are either injecting a test signal into the FX return, or you are troubleshooting the FX loop itself. Any other reason just confounds issues.

[LoveChet]

I get ya. But why does the ripple only show up in the fx loop but not in the earlier preamp, mstr volume, and mix section. I suspect it's something inherent in the bigsky or the trade eq !

[LoveChet]

As a learning experience #1 yes I want to find why it's not working but #2 I want to learn what each stage does regarding gain, distortion etc. Ultimately I want to get it sounding as great as it was before it fell over. Per your suggestion I'm also going to take the cabinet with speaker to my sons place in Maine and plug it into one of his amps so make sure it's working properly. After it fell over it still worked but didn't sound right. Maybe the spkr issue. Then sitting for a few years the V4 mix tube probably got intermittent due to corrosion. I just ordered some deoxit like you suggested

[LoveChet]

Who pulls in the pretty gal pics that show up when you log in. Give him a slap on the back for me.

[LoveChet]

How do I apply the deoxit ?

[Helmholtz]

I try to avoid flushing.
For tube pins I use the D5 on a cotton swab (Q-Tip).
For socket contacts I use it on a suitable interdental brush.
I might be necessary to tighten/re-tension socket contacts.

[LoveChet]

Do I need a tube tester and if so any recommendations. It seems now that I have a scope and sig gen that knowing the quality of my tubes is a weak point in my repair setup. Maybe a cap bridge also

[LoveChet]

Forget the extra equipment.

I need help.

The mix V4B 12AX7 had the dirty socket I guess but when I play with tubes I can get 200 v on plate (conducting) and 400 sometimes (non conducting ). However even when it's conducting I can no longer get a signal on plate
What's happening ?

[Tom Phillips]

Having a tube tester is not essential. There is a saying "If a tube tester says the tube is bad, then it is bad, If a tube tester says the tube is good, then it may be bad."

For guitar amp work it is best to use the amp as the "tester" by having spare tubes to swap in and then observe the result. Also note that a really good and properly calibrated tube tester is an expensive acquisition. Even the good ones usually do not test the tubes at the full working voltages found in tube amps.

I need to replace relatively few tubes during my amp repairs and restorations. Many of those that I change are due to noise or excessive microphonics that a tube tester would not indicate.

[Tom Phillips]

...The mix V4B 12AX7 had the dirty socket I guess but when I play with tubes I can get 200 v on plate (conducting) and 400 sometimes (non conducting ). However even when it's conducting I can no longer get a signal on plate
What's happening ?

I recently had an amp on the bench recently with a similar problem. Extensive careful cleaning of the tube socket did not solve the problem. However, rocking the tube in the socket did cause the signal to cut out and make a loud popping noise. It turned out to be a bad solder joint that was hard to see because it was obscured under another crossing wire. The bad solder joint turned out to be untouched since the amp left the factory in the 1960's. There was solder on the wire and the tube socket pin but it was not a good complete flow.

Troubleshooting such issues involves careful inspection and methodical gentle probing, taping and wire wiggling so that you can zone in on the bad spot.

[LoveChet]

Than Crazy. I was sort of suspecting the same thing since i changed a tube and no better. The original tube is know worked at one point so it has to be tube socket or wiring. The cold solder joint seems like a likely suspect.