Vox AC30

[RightLurker]

I have an AC30 that my wife gave me for Christmas in 1983. (She was my fiance at the time.) The amp was a floor model at Zapf's Music in Philadelphia, and had been in the store for some time. I'm not sure what year or period it's from, but I think it was made in 1980 or 1981 when Vox was owned by Rose Morris. The amp functions, but it has always made an intermittent hum that grows in volume and then stops with a percussive thump. It also sometimes motorboats, hisses, and produces static. But underneath and behind all that it sounds pretty good.

The last line on the pots reads "3680," which I take to mean the 36th year of 1980. I've attached some photos. I can replace the tubes and filter caps. The itty-bitty electrolytic caps on the circuit boards look a bit daunting - is there even a source for such small caps anymore?

I guess my big questions are (1) what year is the amp, (2) do I have a shot at replacing the little electrolytic caps, and (3) is it worth the money, time and effort?

[Structo]

Most likely needs a recap.

Could have had a bad filter from when you first got it causing the hum/ motorboating.

There are a couple guys here that know their Vox amps and hopefully they will help you.

Strange to see the older style CC resistors and tropical fish caps in a 80's amp.

[chief mushroom cloud]

that amp in such NICE condition is well worth recapping....the little radial electrolytics can be found at Mouser or Digikey...but they are prob ok (I would leave them be).
It's the larger power supply capcans and electrolytics you should replace.

[crbowman]

I would say 80 or 81 is correct. I had one very similar to this. Same vintage but mine had the reverb circuit. It is in fact a Rose Morris AC30. Have you tried re-tubing it yet? It's a pricey, but simple matter as there is no adjustable bias. You just buy your replacements and plug 'em in. AC30's are hard on tubes so I would start with that. If that doesn't fix it, then try replacing the cap cans as has already been mentioned.
The circuit is not exactly true to the original AC30 but pretty close and they sound really great when working properly. They're a bitch to service though. The PCB's are not of the highest quality and the traces lift easily. Also original AC30s used a GZ34 rectifier where this one has the diode FWB rectifier.
Still all in all, a very nice amp that I would be proud to own, particularly in the condition yours is in. At one point in time I had a friends 63 JMI AC30 top boost side by side with mine and the tone was not dissimilar.

EDIT: You also might want to try replacing the blue cap and large gray resistor on the far left side of the power supply board. These take a lot of stress over the years.

[RightLurker]

Thanks for the replies, all. I love this place. Structo - what are "older style CC resistors"? I also wondered about the blue rectangular bits - they have "1K400" printed on the top, so I figured they're 1,400 ohm resistors(???)

The circuit boards appear to be less-than-top-quality, but they also appear to be in "as new" condition. I've looked underneath them with an inspection mirror and everything appears shiny and intact - not a dust bunny in sight.

It's a crime, I know, but that amp has been sitting in our (very dry) basement on an elevated surface with a cover on it for 22 years. Every year or so I take the cover off and wipe it down with vinyl cleaner/moisturizer. The exterior is in as good condition as are the "guts" - no sags or tears in the grill cloth, even.

Thanks again for the suggestions.

[chief mushroom cloud]

Thanks for the replies, all. I love this place. Structo - what are "older style CC resistors"? I also wondered about the blue rectangular bits - they have "1K400" printed on the top, so I figured they're 1,400 ohm resistors(???)

most likely .1 400V film capacitors

[martin manning]

Those blue boxes are film caps- the marking is .1K400 for 0.1uF, K is 10% tolerance, 400V. Nice amp!

[alvarezh]

Strange to see the older style CC resistors and tropical fish caps in a 80's amp.

True Tom, could be left overs (overstock) from the '70s.

All the best.

[Structo]

CC = Carbon Composition resistors.

These can get noisy as they age and absorb moisture.

[Kagliostro]

This is the first AC30 of this version I see

Your amp is really in good shape

Many thanks for sharing High Resolution photos I highly appreciate that

I think that you MUST change all electrolytic cap and the resistor on the B+ rail

you can find also now CC resistor if you want to maintain it, but metal film resistor will do the job in a better way

if after this you encounter noise try to change the other resistors

film cap usually survive for a long long time and is not necessary to change with new caps

VERY NICE AMP !

K

[billyz]

Replace the blue Electrolytic capacitor next to the ceramic bias resistor and move it further away. The Cathode bias resistor gets very hot and destroys the Capacitor, making your bias unstable. very common in the AC30.

For good measure replace the two Red Can Capacitors, dual 50/50uf at 450vdc or possibly 32/32 uf , go to 500vdc rating. these are common enough, the F&T brand seem to work well.

Leave all the other components , you have some nice sounding Resistors and signal caps. After the Cap job then move onto the tubes, check the bias on the el84's , the rest of the preamp tubes are probably fine. I do clean the pins with Deoxit, and use fader lube for the pots. retighten everything, deoxit on the input jacks.

be careful of the wiring, it is probably solid core and very fragile.

I just noticed you have some super sweet tubes in that baby, those Telefunkens can last a very long time and sound superb.

[Reeltarded]

Excellent! Now all you need is a little gravel in there to get my super-secret sound.

(inside yoke)

Good amp!

[RightLurker]

Thanks for the tips one and all. All I need is the time to do the work. There is a Telefunken 12AX7 lurking in there as BillyZ noticed, but it's a "pull" from another amp. I'm winding up with a number of tubes, and I'd like to get some sort of an inexpensive tube tester - but I doubt such a thing exists. Any suggestions?

After the AC30 I'm going to work on a Gibson 79RVT that has a slew of tubes, some of which are just a little odd (e.g., 7199) and I'd like to avoid replacing them if it's unnecessary. Thanks again for the help.

[tubeswell]

Wow! I can't get over the fact that your wife gave to a toob amp for xmas, when she was still just your girlfriend? Sounds like the ideal girlfriend/wife! Any time I want something musical, I have to justify it to my wife, and then get it myself.

[RightLurker]

Some years later she gave me a Rickenbacker 325V59 to go with it. And she gave me a Vox T-25 from Northcoast to go with a Hofner. She's really great about it. Although she is astute enough to wonder why I need "so many Gibsons with the same humbucking pickups in them, especially when you still can't get the rhythm to Louie Louie down." I don't have a snappy come back for that one.