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Old 12th Mar 2019, 8:08 pm   #1
Russ Prothero
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Default Domino Capacitor identification

Hello there,

I wonder if someone can help me identify the value of the attached domino capacitor. I have my own ideas from researching online but I'd really appreciate some feedback from someone with experience. Someone who can tell whether brown is brown and not gold or faded yellow or faded orange.

This is actually from a vintage guitar amplifier from 1960. Similar to a Fender Champ. I'm thinking that there's common DNA between these type of amplifiers and valve radios. That's where you come in.

Ultimately I probably won't be replacing this exact capacitor at the moment but I'd like to buy a replacement capacitor for future use - a more modern mica capacitor I should think.

Also, in the photograph, I've illustrated more clearly what the capacitor seems to be. So, top row looks like: black, brown, black. And bottom row: no colour, brown, no colour.

Thanks in advance.
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Old 13th Mar 2019, 12:09 pm   #2
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Default Re: Domino Capacitor identification

I cannot guess the value of the ones shown, possibly 100pf? but you may find this link useful. https://www.radioremembered.org/capcode.htm
My experience of this type of capacitor is that assuming that the one in question is mica dielectric, (nearly all are), they are reliable unless the case is fractured, and unlikely to need replacement unless over-stressed by some fault condition. Tony
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Old 13th Mar 2019, 12:26 pm   #3
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Default Re: Domino Capacitor identification

These particular mica caps are not very good, I have a Motorola 7VT2 TV from the USA, which uses these, and I've found 2 duff ones in it, one was completely shorted. they don't have a good reputation on other forums!

That particular cap is a difficult one to ID, I'm not so sure that the 2 brown dots are actually brown, one could be gold and the other silver, or even white that's got dirty over time! best way to ID it would be to get the circuit diagram and look for it on there, or get a capacitance meter and just measure it, if the cap itself is OK and not drifted in value or shorted!

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Old 13th Mar 2019, 12:44 pm   #4
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Default Re: Domino Capacitor identification

I would have thought that black/brown/black was 10pF rather than 100pF. Would the other dot indicate voltage or maybe tolerance? It appears different from the one between the blacks, and might have been silver. I don't recall offhand the colour code for capacitor voltages, but silver for tolerance would indicate 10%.
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Old 13th Mar 2019, 1:29 pm   #5
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Default Re: Domino Capacitor identification

I think this is the information you need.
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Old 13th Mar 2019, 2:21 pm   #6
emeritus
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Default Re: Domino Capacitor identification

Beat me to it! No doubt from the invaluable STC/ITT Reference Data for Radio Engineers.
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Old 13th Mar 2019, 4:09 pm   #7
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Default Re: Domino Capacitor identification

If the tolerance is gold (5%) - by the way I assumed 5% too tight a tolerance for a capacitor (how wrong I was) - and the unfilled spots are brown, then maybe the top middle is either yellow or orange, giving either 400pF or 300pF.
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Old 14th Mar 2019, 8:38 am   #8
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Default Re: Domino Capacitor identification

Quote:
Originally Posted by Russ Prothero View Post
I wonder if someone can help me identify the value of the attached domino capacitor.
As you will by now appreciate, we cannot give a definitive answer...
The code is clear enough, but the colours have probably faded / changed over time. So all guesses.

Might be easier to determine a value if you show part of the circuit diagram and or post a picture so we can see where the cap is in the amp. It looks like there is a 1/4'' socket nearby?

Alan
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Old 16th Mar 2019, 11:06 am   #9
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Default Re: Domino Capacitor identification

I'd guess at 10 pf. based on your info. given.
Please keep in mind I have found at least 3 different color codes for mica capacitors too.
Some have no color for the first dot, some use white and some of the 6 dot types used only 3 of the dots, the remaining dots, had no color. ( The main color codes here in the U.S. were EIA, RMA, Military, and some mfg. just made their own up too.)
Some had the dots all blank, but had the value hot stamped or printed in black ink on the back of the capacitor. ( Yes, us Yanks sort of marked them any old way, depending on the mfg. and the year. Some of the Military NSN markings are a real nightmare to figure out too) (NSN= National Stock Number/ D.O.D. designator- and they changed those numbers for the same part at least twice that I know of to make it even more insane).

Also the paint used to color them can change color with age too. Especially O found the white turning yellowish or even to brown! I try to clean any suspect paint dots with denatured alcohol to see if the color will come back, as dirt & crud can change the color. And the paint dots can fall out too, even witht he waxy coating on them.

I would try testing it for capacitance with one of the "new fangled" digital capacitor testers that puts very little voltage on it.

IMHO, Just replace any paper mica capacitor of that vintage. The acid in the paper degrades the paper insulation. They go bad even when they are NOS. Especially El Menco brand. MicaMold is not much better. I have found NOS ones that test perfect under designated voltage and capacitance, but fail to work in the circuit. Replacement with new silver mica caps solves the problem. (That cost me a weeks troubleshooting to figure out the NOS paper micas were bad in the horiz. circuit of a '54 Magnavox TV set.).
I have gone to new, silver micas to eliminate the problem.
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Old 18th Mar 2019, 11:23 pm   #10
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Default Re: Domino Capacitor identification

10pf would always be silver mica, and values above a few nf or so, are as far as I know always paper. The only mica would be in the brand name, or just in the shared looks.
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Old 9th Jun 2019, 2:53 pm   #11
Russ Prothero
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Default Re: Domino Capacitor identification

Thanks everyone for your responses. I only just got around to taking the Domino capacitor out of circuit today. It tested as 887pF. From the code, and what I was able to work out, I was guessing that it was a 1nF cap.

I've since snapped off one of the legs trying to put it back in - as you do. So I'll have to get a replacement now anyway.

For those who were curious here's a snippet of the wiring layout diagram (see picture) that shows it is indeed near to the 1/4" input jacks. The figure that I've indicated isn't what the domino capacitor is, rather the figures from the original schematic.

So, where the original schematic says 50nF at 400V the domino capacitor was coming in at 1nF and 600V.

Thanks very much for your interest and your help and sorry for the delayed response.
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Old 10th Jun 2019, 12:53 pm   #12
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Default Re: Domino Capacitor identification

Hi,
Can you tell us what the guitar amplifier in question actually is? I might well have a circuit diagram (schematic) for it. The arrangement you have drawn is a bit odd to my eyes, but maybe I'm missing something.
Colin.
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Old 10th Jun 2019, 2:17 pm   #13
Russ Prothero
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Default Re: Domino Capacitor identification

OK Colin,

The amp is an old American amp from 1960 called an Alamo Amp Model 3. I've attached a diagram of the original schematic and also my own layout diagram of the amplifier - apologies for some of the extraneous things that I've marked up - I was trying to follow the direction of power.

This layout was copied from looking inside my own Alamo Amp Model 3. I realise that there are some deviations from the original schematic. I don't know why. It could have been repairs along the way or maybe just slight modifications carried out in the factory. Some of the variations could be accounted for by just the components that they had to hand.
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Old 10th Jun 2019, 3:22 pm   #14
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Default Re: Domino Capacitor identification

Thanks Russ,

Yes, this does look similar to the Fender Champ 5E1, but there are significant differences, too. The Champ uses a 12AX7, a 6V6GT and a 5Y3GT rectifier. There are component differences, too. I suspect that "Alamo" fiddled around with component values to avoid Leo Fender getting twitchy about copyright. Marshall got away with copying the Bassman and giving it another name because they were this side of the Atlantic.

Your circuit diagram makes sense, but your layout has some peculiarities. The domino capacitor that you are wondering about shouldn't, IMHO, be where you show it, but where the circuit diagram shows it. Where you have it, it is bleeding treble to ground, especially if the capacitance is more than a few pF. There is also a resistor missing from one of the input jacks. What I am assuming is a screened cable going between a tag-strip and the 6J7 has both ends of the screen connected to chassis ground - this is bad practice, as it creates an earth-loop, which can cause hum. If I were you, I would assume that the circuit-diagram is correct and that someone has futzed around with the layout for one reason or another. In that case, correct the layout to be what is shown on the circuit diagram. If it all goes wrong, you can put it back to the layout that you have drawn. Originality is fairly important with vintage guitar amplifiers, but this may be less significant for lesser-known makes (or possibly more important if the amplifier is rare).

Colin.

Last edited by ColinTheAmpMan1; 10th Jun 2019 at 3:42 pm. Reason: Correct to 6J7 preamp from 6V6 power tubes.
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Old 11th Jun 2019, 10:39 am   #15
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Default Re: Domino Capacitor identification

I have tried analysing your layout and, to me, there are some significant errors. There are only two 39k (50K?) resistors for the three inputs and there is a 50nF capacitor connected between the mixed signals and ground; this can't be right, surely? It also seems that a 0.005 capacitor is going to pin 1 on the 6SJ7. This pin is the metal envelope and should surely be earthed. The volume control looks to have all three of its tags connected together and to earth and so is completely non-functional. At this point, I have given up on trying to reverse-engineer a circuit diagram, as I can't make any sense even of the input circuitry.

Does this amplifier work as it should? If it does, then your layout is either confusing me or it is wrong.

Regards, Colin.
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Old 11th Jun 2019, 1:09 pm   #16
Russ Prothero
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Default Re: Domino Capacitor identification

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Thanks Russ,

Yes, this does look similar to the Fender Champ 5E1, but there are significant differences, too. The Champ uses a 12AX7, a 6V6GT and a 5Y3GT rectifier. There are component differences, too. I suspect that "Alamo" fiddled around with component values to avoid Leo Fender getting twitchy about copyright. Marshall got away with copying the Bassman and giving it another name because they were this side of the Atlantic.

Your circuit diagram makes sense, but your layout has some peculiarities. The domino capacitor that you are wondering about shouldn't, IMHO, be where you show it, but where the circuit diagram shows it. Where you have it, it is bleeding treble to ground, especially if the capacitance is more than a few pF. There is also a resistor missing from one of the input jacks. What I am assuming is a screened cable going between a tag-strip and the 6J7 has both ends of the screen connected to chassis ground - this is bad practice, as it creates an earth-loop, which can cause hum. If I were you, I would assume that the circuit-diagram is correct and that someone has futzed around with the layout for one reason or another. In that case, correct the layout to be what is shown on the circuit diagram. If it all goes wrong, you can put it back to the layout that you have drawn. Originality is fairly important with vintage guitar amplifiers, but this may be less significant for lesser-known makes (or possibly more important if the amplifier is rare).

Colin.
Thanks Colin,

That's really helpful. Essentially this amp sounds great as is. So when it comes down to replacing the ancient oil in paper caps - which I'll have to do at some point - I just want to be careful that I don't kill the amp's mojo.

As far as the domino capacitor not being in circuit any more - I assume from what you're saying is that the amp will still work fine although there may be an increase in treble now - is that right?

You're correct when you talk about that being a screened cable going between the tag strip and the 6J7. I actually have a photo of someone else's Alamo Model 3 and it looks the same. What would your modification to this be?

Sorry but my knowledge is too limited to be able to wire an amp direct from the schematic - hence my layout from a real example. Any pointers are much appreciated as to the parts of the circuit that don't make much sense.

As well as being able to replace some of the components in this original I was also going to use this layout to build a duplicate of this amp inside another chassis that I have and see how the sound compares.

Incidentally my amp already has had the death capacitor disconnected at one end which, along with the screened cable, is no doubt something that they used to do consistently but is no longer best practice.

Much appreciated,
Russell
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Old 13th Jul 2019, 12:48 pm   #17
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Default Re: Domino Capacitor identification

Update: Not much progress, as other things have taken priority, but I am trying to reverse-engineer your amp layout - slowly. I pretty much have the input circuitry sorted so far. Your drawing is rather difficult to follow; do you think that you could take a few photos of things, especially the input section? If you could also shift a few wires out of the way, that might make it clearer. What I am making of your layout drawing, there is no way that it would work, not even badly.
Regards, Colin.
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Old 13th Jul 2019, 1:46 pm   #18
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Default Re: Domino Capacitor identification

I tried to edit the previous message, but failed, so here is what I wanted to add.

To address some of your points:-

The capacitors that you refer to as oil-in-paper are "paper in oil". Such capacitors are probably ok, even after several years. I think you were intending to refer to wax-and-paper capacitors (often called "waxies"). Some of these have an obvious wax coating, but some don't. Of those that don't, Hunts (often covered in black plastic) and Wima logos are past their sell-by date and should be replaced with equivalents with the same or higher voltage rating.

If that domino capacitor is located where I think it is, then yes, you would probably get more treble. Of course, it might be associated with the tone control, in which case....?

It is a bad idea to connect both ends of a screened cable used at audio frequencies to ground. This creates what is called an earth-loop and can result in excess mains hum. If the tag-strip is connecting one end of the screening to ground and the other is also connected to ground, then disconnect one end and check that the screen is continuous, using a multimeter on ohms. If it is, don't reconnect that end to ground, but snip it short and insulate it so that it can't connect to anything else. Heat-shrink tubing is fine for this insulation, but please don't use insulating-tape - it looks ugly and will look even worse with time.

I'm glad that your "death capacitor" is out of circuit, but just completely get it out of there - it isn't needed and it just might connect to something else and..... Incidentally, most US-made guitar amps that had a "death capacitor" also had a switch to connect the capacitor to either side of the mains. When US-made amps got imported to the UK, while the mains transformers were changed to cope with the higher European AC mains voltages, the "death capacitor" was usually forgotten and so when a 120-volt rated capacitor was hit by 240-volt mains, BANG and release of "magic smoke", which startled people around, but the "death capacitor" had now blown open-circuit and the amp functioned perfectly, to everyone's amazement.

Colin.
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Old 15th Jul 2019, 10:19 am   #19
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Default Re: Domino Capacitor identification

The OP hasn't visited the forums since 11 June.

You could try emailing him via the forums.
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Old 18th Jul 2019, 7:27 pm   #20
Russ Prothero
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Default Re: Domino Capacitor identification

Thanks for your messages Colin and apologies for my inactivity here. I've been sent a message from the forum with your email address and so I've sent you a message.

Russ
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