Vox AC-10 repair/mods

Hi All,

Thought i'd loose the Egnater bit of this and concentrate on the Vox!!

First what don't you like?

Second speakers are one the biggest parts for a guitar amp. A good experiment is to take a small 10Watt 8inch solid state practice amp and connect it to a good speaker cab

Interesting it uses a JFET MOSFET cascode input

If you want to adjust it find the relay settings you like and redraw it.

Spencerleehorton said:

Its sounding on the muddy side

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the "Tight" switch is supposed to take care of this. Does it or does it not?

What speaker(s) and cab are you using?

yes you can run el34's, but monitor your pwr xfmr temp, as there will be more heater current drawn,  if pwr xmr cooks, buy 6.3 aux xfmr.

get rid of the fet and put in a 5751

strap the two sections together.

does that Eggbeater have a torroidal pwr xfmr? lots of problems with those, offshore quality control can be lacking.

nightmare to work on those eggbeaters, too many bells and whistles, everything crammed together like a blaupunct car radio, i feel your pain,  :

Spencerleehorton said:

I've put an eq pedal into the fx loop and boosted 3k area and it sounds better but would like to not have to use a processing pedal really if there is any changes I could make?

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With 96 different colours available from all the tone switches, one would think there would be a usable position!  8)

why not try the Bluesbreaker? get some GEC KT66, $$$

or a Dr Z  MAZ 38?

i believe there is a print for the tweed champ OPT in magnetics dept., uses small EI 50 core instead of 625 EI used in BF.

If the tweaker 40 is anything like the 15, I cannot think of a more annoying amp to work On! I've owned a few egnater amps and always sold them because they were such a pain to work on, and like you said the tone in lacking. Most of the egnater amps to me seem to be "okay" at everything but not really have any standout sounds.

Spencerleehorton said:

Yeah that's my point exactly, lacking, it's ok but I am going build a jtm 45 and a BE-50 esk amp.
Just wondered whether I could simplify the egnator to get a better sound?
But it sounds like it's a mess of lots of other stuff in there!!
And maybe it's not worth the time?

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I had pretty much the same thoughts with the 15 and couldn't figure out a way to simplify it easily. I'm pretty certain it would have taken the same amount of time as just building a p2p amp, but still being hard to work on

iampoor1 said:

I had pretty much the same thoughts with the 15 and couldn't figure out a way to simplify it easily. I'm pretty certain it would have taken the same amount of time as just building a p2p amp, but still being hard to work on

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That is a frustrating situation.
I have an italian amp that has everything to sound killer.
Big xfmrs, a pair of EL84, a good schemo (AC15-like), a cabinet built like a tank, a good looking speaker.
Even with a separate loudspeaker, it sounds desperately lifeless.

I've got a very old vox ac-10 which sounds amazing, the cabinet perished as it was left in a garage and it just rotted away, I think its dated 1962.
The rectifier tube blew but I replace with diodes and it's very hissy so needs caps replacing as it's so old, I think my time is better suited to working on this and getting it sounding better.
Vibrato channel doesnt work at moment and jack sockets are very scratchy, defo needs some love.

the vox ac-10 actually only the vibrato channel works but no vibrato? and no normal channel?
All valves look heated, will have to have a prod about in there and see whats going on!!

the fat that both the normal channel and the vibrato dont work point to the ECF82 not working, dont know what else i have here to change it for though? i doubt a ECC83 will work!!??

Spencerleehorton said:

the vox ac-10 actually only the vibrato channel works but no vibrato? and no normal channel?
All valves look heated, will have to have a prod about in there and see whats going on!!

the fat that both the normal channel and the vibrato dont work point to the ECF82 not working, dont know what else i have here to change it for though? i doubt a ECC83 will work!!??

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certainly not! ECF82 is a triode and a pentode in a common bulb. ECC83 is a double triode.

well it seems so far that valve amps are not my thing!! limited experience with fault finding.

attached is the vox ac-10 schematic, i do get a signal through the vibrato input but nothing through the normal input?

could it be a duff ECF82? at somepoint previously both channels did seem to work, but ive never heard the vibrato working?
looking at the schematic of the EF86 didnt work the vibrato channel wouldnt work, so thats fine, both EL84's work as im getting output.
the ECC83 must work also, so that must leave the ECF82 that doesnt work, i have done a continuity test and signal get to ECF82 and sign get to normal volume pot.
could it be C12 0.01uf cap?
is there a good test i can do to make sure the ECF82 works?

Could you give me some pointers on this please anyone?

also i pulled a 25uf/25fv cap out and measured it, it has a red end and a black end so i presume its an electrolytic, it measured 31uf, so its a bit out.
Would this amp beniefit from changing all caps? electrolytics and other caps?

everything needs such a clean as its all pretty dirty in there, ive done my best so far but everything really needs to be unsoldered and cleaned then soldered back in!

here is the Dr Z we had on the bench last week, nice to work on,only problem is the rectifier socket is under the board near the inverter resistors,  :

this is an early serial number, a quad of 84's and a 5AR4, we stuck one of those huge 5U4 Sovtek 's in there and added a tube retainer. the new rect tube gets the voltage up a bit as this thing was running with sub 300 B+ on the pwr tubes,

carbon comps used where they matter and 1% film for the rest,

foot switch on that 0.68 yellow cathode cap for a subtle tone change,

big pwr xfmr, nice 3 H choke, has reverb,

too lazy for a redraw

first tube has both triode sections strapped together, probably for a tad bit low noise,

preamp

cataracts?  me too!

pwr section,

another preamp section and reverb input,  nice move using 12AT7 for preamp sections, saves knocking down gain a bit with tone sucking resistors,

spk cab has a nice pair of alnico's