Vox Ac-10 problems

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Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
4,035
Location
Felixstowe, Suffolk, UK
Hi all,

As usual I'm having issue with equipment!!

Ac-10 made in 1962, vibrato channels works, have changed all electrolytics apart from c10a/b 8uf dual cap.
Have changed the input resistors 220k.

Normal channel doesnt work and also vibrato effect doesnt either.
Have changed ecf82 for another one and no changed?

Could one of the el84s be the problem?
 
Dead short valve - Unlikely
Open circuit 220K resistor - Possible
No 255 Volts - Possible
No Ht at all - Possible
 
Well I tested voltage at 16uf and 16uf, and I'm getting 287v and 235v a fair bit lower than I'm meant to?
I'll test it at the diodes which are in place of the rectifier.
On both ecf82 valves a lot seems to light up on them? Is this a sign both my  ecf82 are faulty? I just bought the new one!!!
I'll see if I get the 255v behind the 220k and see what's in front of it.
Not the most confident on valve amps!!
 
Suggest you also check/change the 220k resistor going to pin 6 of the ECF82.
For that matter, check all the high value resistors.
 
You dont have to replace them, just leave the amp off for about an hour to make sure all the caps have completely discharged, and then measure them with your meter. They should be within 10% of their value, but anything that is wildly wrong needs one leg lifting to check it out of circuit.
 
Been a bit busy with our dog having puppies but got a minute to test the other dual 8uf cap and it only measures 8
10.84uf on one side!!
So I've changed this cap but need to look at what damage not having 8uf would do?
Going to carefully check all plate voltages and check all the resistors in the vibrato oscillator.
Hopefully then I'll be able to find out why the vibrato doesnt work.
The amp so far sounds so much better, much quieter but there is still about 70%of resistors that are way out of range.
 
Grid resistors usually wont matter much even if there 20% high. `
The one place it will make a difference is an oscillator  circuit like used to create Trem , leaky caps and out of spec resistors will often prevent the oscillations from starting up , most likely whats going on here .
 
I do feel the power amp valves are running a bit hot as the heat that comes off them has me a bit worried.
That's why I havent really run the amp for that long yet.
On my next days off I'll hopefully be able to get all resistors changed and record all voltages to give a proper indication of where this amp is at.
 
Most of the Vox class a amps run hot enough to cook eggs on.
Do check the value of the bias resistor though and make sure the bypass cap is good too .
If your still unhappy about it you can also make a small increase in the bias resistor value without any drastic effect on the sound , generally speaking as long as your plates arent glowing red your probably ok , ocassionally in an AC30 badly mismatched valves will show up as red plates ,reduced tube life and high hum levels.
 
use the screen voltage to adjust tube heat if running cathode bias, raising cathode resistance will not do much, everybody runs those tubes hot, wish i had a dollar for every Fender Blues Jr i had to tweak for less plate disp.  not easy, you have to tack a pot onto the circuit board resistor, dial it in, remove pot, measure pot, solder fixed resistor across resistor on board, otherwise you have to gut the amp and remove the board.

put those 30 percent off resistors back in lest you lose your vintage tone.
 
Hi CJ,
I may well put the off resistors back in once I've found the problem with the vibrato circuit, I may also build up another turret board with modern in spec parts and compare the two.
With what ive changed so far it hasnt changed the tone and feel of the amp, which is good.
As soon as I hear a change I will make note.
It's only the vibrato channel which needs looking at now and all overall plate / cathode voltages
 
Well I've changed a few more resistors, r24 330k r26 330k r40 220k and r43 330k.
I found also there was no connection from  r40 220k to pin 6 on ecf82!!
Have put in a patch lead now.
Now I need to check all plate voltages as when I checked voltage drop on el84 pin 3 to gnd it measured 12.11v.
12.11v / 130R = 93.1ma
305v x 0.0931 = 28.39 watts!!! This is only an ment to be a 10 watt amp!!????
 
If R40 wasn't actually connected to pin 6, then I would say you have found your Vibrato problem and can stop changing components and give it a whirl!  :)
 
Well something else didnt look right so I checked supply voltages and they are quite off, +305v is at 320v and +255 is at 196v.
R6 is showing at 605k meant to be 470k
R10 is 1.020meg meant to be 750k

When turning up the vibrato channel on either channel brings up a big hum now and the speed and amplitude do seem to bring in a sort of oscillation noise, it's not very nice at the moment but definitely signs I'm getting there.

After every turn off I'm discharging the caps with 1k 7watt resistor to gnd.

I've got a dedicated MM on the HV all the time so I can see what's going on.
I'm only putting on hand inside at a time.
Generally trying to be as safe as possible.
I will make the bulb current limiter later in the week.

Need to look at R17 22k and see where it's at along with all the resistors to the plates.

Also check from the dual caps exactly what voltage they have.
 

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