Here is a Weird one ! Vox combo

[Valvehead]

Amp is a AC15CC1 combo. I have a low freq hum and the amp loses signal / no signal on start up . Narrowed it down to V1. Probing around it seemed to come to life when i moved the pin 6 connection at the socket. Checked everything , re -soldered , Still no signal .
So this is the ceramic socket that you cant re-tension. Ordered/ installed new socket. Checked all joints... probed everywhere / moved wires ...Verified with new 12ax7 also. Heres the weirdness...

After the amp sits for a while, i can turn it on , and no signal..just a low freq hum. If i hook up my voltmeter to read that plate , as soon as i touch the positive probe to pin 6 with the amp running... CLIK! the amp comes to life and passes signal !!!

So.. what is happening when i put my meter in line with that plate to ground ??? Is it "energizing " something for a lack of a better term ?? New socket / solder / tube ...

Went back and probed and prodded and i dont find any bad joints / connections. Voltage normal at pin 6 at 169vdc when amp is passing signal .


D:\Guitar\Mods\AC15CC1_Schematic.pdf

[Jon Snell]

Check the grid leak resistor on pin 6.

[Pedro Vecino]

It may be related to a bad ground contact of the input jacks on the chassis. I think they have a toothed washer for this.

[Valvehead]

R1- R10 check good. Cleaned up the input jack and chassis ground . Same issue

[g1]

Did you try resoldering the socket anyway? If you probe the cathode or grid instead of the plate, does it wake it up the same way?

[The Dude]

He replaced the socket, g1.

So, it starts making sound when you touch the tube plate pin with your probe. Does it do the same thing if you touch the corresponding wiring point coming off the circuit board?

[Helmholtz]

Check the grid leak resistor on pin 6.

I guess you mean the plate resistor?

[Helmholtz]

What are the cathode voltages of V1?

[Valvehead]

Just fired up the amp and oddly it worked right away / passed signal (still has a low hum) . I went in and probed The plate resistor R6 where it eventually goes to pin 6.....and Clik ! now amp went silent ! Ive cycled it on/off several times and its stuck in no signal mode . Ill let it cool down and try later .

Voltage is 8vdc lower when amp is stuck on silence at pin 6 V1. Cathodes for v1 = pin 8, 1.3vdc , pin 3 1.2vdc

I can hear noise when probing plates of V2

Ran a jumper from R6 on the pch directly to pin 6 = no change

[The Dude]

I'd get a scope in there right away while it's stuck in non-working mode and see where you lose signal.

[Valvehead]

Ok.. I do have a cheapo scope and a 1x/10x probe. Im new to these so bear with me. Ive watched a bunch of videos but they never answer my questions. Ill post a video of scoping the Volume pot.

Heres my main questions....
1. when moving thru the circuit, I do see the sine wave drop. For example= big sign wave before the 33k grid stopper= small wave after. Normal yes ? So-- do i make adjustments on my scope as i probe AFTER each cap/resistor in line ?? If so what EXACTLY do i change??? and to what setting?? Or do i leave it alone?
Im sure i need to go to the 10x probe at the plates or any higher voltage...yes?


anyway..the biggest drop i see (i think) is at the Volume pot. Video shows me testing the end pin of that , then going to the wiper...big drop. Normal ?? When i probe the wiper i see no change when rotating the pot. Normal? adjust the scope ?? i dont know (see above) . The volume pot does function fine when the amp is working.

scope

[The Dude]

How much signal appears at the volume pot wiper depends on its position. If the wiper is all the way to the ground side of the pot, there won't be signal there. If it's all the way to the "input" side, it should be the same as the input. And of course, some mid position would lower signal level to somewhere in between. It sounds like you may have a bad or intermittent pot judging by your description. The wiper could be open, or there could be a chunk of solder or something lodged in the pot. I'd first try blowing it out with some compressed air and then clean it. If that doesn't work, try substituting the pot.

[Valvehead]

Well you might have solved it dude. Powered on and amp had no signal, shut off then cleaned and lubed that pot. Full volume now ! Ill cycle it more tonight . So, why would probing that plate intermittently cause they signal to come and go ??

[The Dude]

I honestly can't say for sure why probing the plate did what it did, but your scope readings showed signal to the volume pot and nothing coming out, so......

[Mick Bailey]

I've experienced this situation a few times, the last being due to a faulty plate coupling capacitor. The amp powered on with no sound, but as soon as the plate was touched with a probe it came to life until the next time. It didn't matter if it was a DMM, scope or insulated screwdriver. It also didn't matter where it was touched - track or solder pads.

With the amp left off overnight and discharged, I injected a 100mv signal at the plate (using my signal generator) and scoped the circuit - no signal the other side of the coupling cap.

Here's my theory for this behaviour: There's perhaps a very borderline high resistance or open circuit that is on a knife-edge. I think that probing the plate creates a disruptive voltage increase/transient that overcomes or flips the high resistance and enables the circuit to continue working temporarily. I've also seen this behaviour with plate resistors.