Aims Dual Twelve / Replacing Caps and fixing Tremolo

[Elecmuso]

G'day

New around here. Have an Aims Dual Twelve that I decided was about time I should fix up. I an electronics engineer but not at all handy with valves and circuitry of this era.

First thing is  the 7 x 40uF 450V filter caps. I think they are original from the 70's, with 'Beaver' written on them. I took one out and measured its value and it was only out by 20%. No bulging. But I gather the ESR (which I have no easy way to measure currently) is likely high - i that the case? Does it go without saying these caps should be replaced and if so with what?

Then there is the tremolo which doesn't work well. I saw something about operating it in darkness or blanking out the photo resistor with tape. Is there a good tutorial on how this works etc.? Why does it work in darkness? What are the repair options? I am only at the beginning of researching this.

Cheers from Australia

[PRR]

Welcome.

ESR was not invented when this amp was made. When kids today say "ESR" they are BSing.

If it had high ESR it would motorboat, oscillate at a low rate. You say it plays good.

Not all of us have all Aims schematics memorized, and there's many ways to tremolate. Point to a schematic please?

(Yes, ESR was a known thing from the start, a real pain in 1930s ecaps, which may be why it stopped being a big deal by the 1960s, until computer power supplies.)

EDIT: November 17, 2012

two old codger's with not much to do today.

Schem and PCB in single PDF file.

[dwinstonwood]

Does it go without saying these caps should be replaced and if so with what?

The schematic PRR posted says 620V at the reservoir (first) pair of series caps. So, that's why there are pairs of 40uf/450V caps in series which equals one 20uF cap with a 900V rating. So, if you do replace the electrolytic caps, make sure they exceed the B+ (HT) voltages on the schematic.

[Elecmuso]

Though I didn't actually say it, yes the amp does seem to perform well. If I measure the caps and they are all around 40uF, is it reasonable to continue using them?

[Elecmuso]

Photo of one of the existing caps.

[PRR]

> with 'Beaver' written on them

That was a common brand. Turns out to be one of the many names Cornell Dubilier put on their caps.
https://www.surplusselect.com/collections/capacitors/products/cornell-dubilier-beaver-br30-150-30-mfd-150-wvdc-3-available?variant=10576853059

[sluckey]

Then there is the tremolo which doesn't work well. I saw something about operating it in darkness or blanking out the photo resistor with tape. Is there a good tutorial on how this works etc.? Why does it work in darkness? What are the repair options? I am only at the beginning of researching this.

Your tremolo circuit works like this (see attachment)... The oscillator operates at a frequency of 3Hz to 10Hz. The three .1µF caps and connecting resistors determine the speed. (The caps are known to fail.) The output of the oscillator is sent to the driver via the Intensity control. The driver causes the neon bulb inside the optocoupler to flash at the rate of the oscillator. The flashing neon bulb causes the LDR (light dependant resistor) inside the optocoupler to vary it's resistance. The LDR is connected to the guitar signal path and varies the amplitude of the signal. This amplitude modulation is called tremolo. The optocoupler is also known to fail, ie, the neon bulb flashes dimly or not at all and/or the LDR becomes less sensitive to light changes, resulting in weak or dead tremolo effect.

Some optocouplers are totally sealed in some light proof enclosure, but others are simply enclosed in shrink tubing. Still others have no enclosure. If ambient room light can get to the LDR then the light will affect the LDR just as if the neon does. So, room light can affect some optocouplers but is not an issue with a properly sealed unit.

Now that you know how this circuit works, tell us exactly what your symptoms are. Weak trem effect, poor speed range, totally dead? Or what?

Read my tag line.

[Elecmuso]

What a great and thorough explanation! Nice to see an early version of an opto isolator, did not realise the connection. I'll try to be as thorough with my description of the symptoms.

[Willabe]

What power tubes does this amp use? Must be big, they have 620vdc on the plates.

I don't see them listed on the schematic.

[sluckey]

What power tubes does this amp use?

Since they have a suppressor grid connected to pin 1 and pin 1 is connected to pin 8 (cathode) I would bet they are EL34s.

[Elecmuso]

6550s

[Elecmuso]

Found this.

Virtually the same electronics I think. Rather than using pairs of 40uF / 450V caps to get the voltage rating required they have used some single 20uF sufficiently rated (at least 650V) caps. So a total of 5, not 7 caps.

The repair tech chose to replace them all.

I can see no evidence of leaking and when I measured the values of the caps (in circuit - I was measuring total capacitance) I measured 56 uF which seems about right.

Should I replace the caps? Perhaps I should take the cardboard covers off the caps to have a closer look at them?

[Elecmuso]

Here's a photo of the pcb.

[SoundmasterG]

I would replace all electrolytics in the amp if it was mine. They're from the 70's and while they might still work ok, new ones would work better and last longer. It's also cheap insurance against failure as if the caps do fail, other parts like transformers may be affected. Caps are much cheaper than transformers for an amp that is long out of production and was never super popular or widely used.


I have an AIMS duel 12 combo that I need to restore. I'm missing one speaker but I'll find something that will work to put in it.


They are basically a Twin Reverb clone with the higher voltages and the use of the 6550's instead of 6L6GC's, and a master volume. One thing the AIMS circuit designers did that I don't agree with is the whole preamp runs off one filter cap section. They got away with it due to the low gain of the circuit, but when I rebuild mine I plan to modify that so instead of 4 stages of gain sharing one cap I reduce that to two or may one stage of gain per filter section but we'll see what there is room for.


Greg

[Elecmuso]

Cheers Greg. Yes I had seen your other posts. Where would you get the caps from?

[SoundmasterG]

My amp looks like this picture though rather than the one in the video.


Greg

[SoundmasterG]

Cheers Greg. Yes I had seen your other posts. Where would you get the caps from?

I'm not sure what the best supplier would be if you are in Australia. For me here in the US I use CE Distribution/Antique Electronics Supply/Amplified Parts if I want F&T or similar brands. If I want to go with radial caps (cheaper, better quality than Axial caps today) then I would likely get them from Mouser or Digikey and get Panasonic caps, and find a good way to retrofit radial caps into the amp safely. As long as you are getting the approximate same value with at least the same voltage rating as what they used originally you would be ok. Make sure you put them in with the same polarity as what was used originally also.

Greg

[Elecmuso]

Mine is exactly like yours. Btw i also had only one speaker working but fixed it last week. Attached are a couple of photos which may help you with yours. The problem I had is much easier to see with the dust caps off.

Hope what I did was ok. Re-soldered incoming wire to coil wire and then secured it with a bit of nail polish. The nail polish also closed up the hole for the wire coming through the speaker material.

[Elecmuso]

Speaking of speaker dust caps I noticed that in the two photos you posted that they have different dust caps. The first has the metallic ones (as did mine once) and the power photo has black felt or cardboard material. I'm guessing these are not your actual amp just some photos you found of the same model. Or have you actually replaced the dust caps already?

[Elecmuso]

Now that you know how this circuit works, tell us exactly what your symptoms are. Weak trem effect, poor speed range, totally dead? Or what?

Based on my findings with the caps I decided to solder the capacitor sample back into the circuit and turn it on. All good, no smoke, and after cleaning up the pots, very quiet. Reverb works.

But the tremolo doesn't work properly. Here's what I found.

1. There is just a hint of tremolo when intensity is turned up to 10.
2. The tremolo is only audible when reverb is turned up, best at 10.
3. Speed control works and the range of speed from 0 to 10 seems appropriate - at a guess I'd say about 10Hz at the 10 position, 1Hz at the 1 position.

In the photo above you can just see the half of the reverb bug and it is wrapped in what looks like black insulation tape.

[sluckey]

The most common suspects for weak tremolo in your circuit would be Tube (replace with 12AX7), three .1µF caps, or the optocoupler bug. A common Fender bug will work in your circuit. Here's what they look like...

     https://www.tubesandmore.com/products/switch-vibrato-ldr-fender-amps

Forum member Timbo is in Australia. Contact him via PM. He can likely tell you where to buy parts.

[PRR]

....They are basically a Twin Reverb clone with the higher voltages...

The preamp's 3.3Meg plate loads and 30V-50V plate idle are very non-Fender. This may only matter for very hot guitars/players.

[SoundmasterG]

Speaking of speaker dust caps I noticed that in the two photos you posted that they have different dust caps. The first has the metallic ones (as did mine once) and the power photo has black felt or cardboard material. I'm guessing these are not your actual amp just some photos you found of the same model. Or have you actually replaced the dust caps already?

You're right, those are pictures I've collected of these amps but not my amp. In the case of my amp, I only have one speaker, and have no idea where the other one is as I got the amp this way. I don't even know what the stock speakers are supposed to be similar to? Vintage JBL's maybe? No idea though until I can get mine going. It was in a barn so there are rat droppings and corrosion that need to be addressed. The whole thing will need to be gutted and cleaned before I can do anything else.


You have a better idea of what these truly sound like than I do since your amp actually works at present.

....They are basically a Twin Reverb clone with the higher voltages...

The preamp's 3.3Meg plate loads and 30V-50V plate idle are very non-Fender. This may only matter for very hot guitars/players.

I wasn't clear enough I guess about what I meant. They aren't exact Twin Reverb clones, as there are differences like the one you noted, but they are similar in overall amp design and circuit to a Twin Reverb. Certainly closer to that than say a Tweed Bassman. I've read anecdotes that they were billed as being similar to a Twin Reverb but better when sold new. I was very young in the early 70's though so I just know what I've read and heard about them.

Greg

[Elecmuso]

SoundmasterG I am a piano and sax player, do play a little guitar, but don't really have a lot to compare with as far as tube amps go. I think it sounds good, certainly is loud! I picked up the AIMS when I was living in Cedar Rapids IA in 1989. It has been sitting under the house for some time.

BTW, I am a Greg too!

Greg

[SoundmasterG]

SoundmasterG I am a piano and sax player, do play a little guitar, but don't really have a lot to compare with as far as tube amps go. I think it sounds good, certainly is loud! I picked up the AIMS when I was living in Cedar Rapids IA in 1989. It has been sitting under the house for some time.

BTW, I am a Greg too!

Greg

I was planning to use mine for guitar and also for a Fender Rhodes. I got mine probably 12 years ago for free from a friend who was getting divorced and had to downsize severely.


Greg