• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

stupid question? re-capping

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i have a martel reciever that i need to recap, it uses 6bm8 outputs and my question is......which are the coupling caps? are these the 4x .05uf 400v caps that connect to the output tubes? are these the only caps that i should replace with polypropylene?
i am aware the large caps at the power supply are electrolytic but what about the rest of the caps under the chassis? should they also be polypropylene or electrolytic? and how much is a safe amount to raise the rateing of the caps uf and v ?

thanks, mike
This is one of those questions that scares me. There are numerous places where caps <i>could</i> be, but without seeing the circuit, it's gonna be really hard to tell. You could, for instance, have DC coupled drivers into the output tube grids. It's not that difficult to arrange DC coupling between a pair of tubes.
As a general rule, you keep the same (or greater) voltage rating, but you can play around with the values--particularly if you intend to go larger. The most obvious exception to this is the compensation cap on the NFB loop. That one you're going to want to keep the same value.
Tube amps are extremely forgiving. Much more stable (in nearly all senses of the word) than solid state. My recommendation would be to back-engineer the circuit, drawing a schematic as you go. That way, you'll be able to see what is hooked to what. After you've done this sort of thing a few times, you'll be able to glance at a circuit and tell what's going on, but I'd advise taking it slowly this time.
Polypropylene is an excellent choice for small to moderate value capacitors. Polystyrene and silver mica work well for smaller values. The big ones will still need to be electrolytics unless you want to go and do something silly like I did when I built my amps.
Keep your wits about you, and you'll be fine.
Don't forget that there are voltages in there that will give you a serious tingle if you mess with the wrong thing at the wrong time, but don't get freaked, either. I've been shocked more times than I can count and I'm still here to amaze and annoy people. If you get zapped, just let the cuss words out, then go on.

Grey, thank you for your time and for your reply. the reason i asked about changing values is that i am ordering these from from my local rat shack and they dont list all of the same values that i have. example would be replacing a .05uf 400v with a .10uf 630v cap. there are 3 under the bottom that are 20uf 300v and 1 that is 40uf 150v if i replace these with electrolytics then the rest can be poly or mica? what about metal film?

i bought this reciever to practice and learn on after i decided not to chop up my harman kardon for parts to build a project amp. i also have a sansui 1000a comming and want to get a little practice before i start on it.

thanks again, mike
When you see the term metal film, it generally is in the context of a 'plastic' cap, meaning poly-something. There are two options here, metalized, and metal foil. Metallized is when there is a thin metallic sheath applied directly to the poly. Metal foil is when they literally have aluminum/tin/copper foil as the plates of the capacitor. Foil is the better of the two, but even metallized is better by far than electrolytic or (ugh) tantalum.
There's an article you should read about capacitor selection. I believe you can find a copy online at www.capacitors.com (note the 's'). It was originally published in the '70s, but is still pretty much the straight deal even today.
Consider ordering from digikey or mouser...they'll have a wider, better quality selection, and are possibly cheaper in the deal. They will almost certainly have the exact values that you need.
And draw that schematic, fella!

Check those higher value caps close and make sure they are not electroltics. Most vintage stuff did not have much in caps over a few microfarads except for electrolitics, but one never knows. Capacitors and transformers were the most costly componants going on the chassis and most often scrimped on so don't worry about bumping the values as long as you don't go over board. And by the way Nichicon has come out with audio grade electrolitics but unfortunatly at only 100 volts or less so you can't use them in the power supply, but could be a real upgrade in quality for not a lot more $$ in other spots of the amp circuit.
ok heres the new question, i ordered caps from aes and they came in yesterday. there are 4 .05 400v oil caps at the output tubes and for replacements i ordered .47 4oov poly propylene caps, the problem is the new ones are huge and will not fit, is this normal? should i always expect this?

Joined 2001
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FlMike said:
.05 400v oil caps-> .47 4oov poly propylene caps

Those new caps are 10x the value of the old ones so one would expect them to be a bit bigger.

Also this Svetlana tech article has both an SE & a PP schematic that you might be able to use as an aproximate guide to how things are laid out in your amp.

The $100 each AV8 monoblocs that have been getting raves is a PP 6BM8...

6BM8 amps

A great bargain in a vintage PP 6BM8 integrated is the Calrad AP 15 mono block. They often come up on Ebay and with a bit of tweaking are very good. I just got one out of the closet and put it on the variac and will be using it to run my center channel for movies. The Japanese used to have a 6BM8 club on line and some of those little things were real works of art. I bet they sounded great too.
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Re: 6BM8 amps

Thatch_Ear said:
A great bargain in a vintage PP 6BM8 integrated is the Calrad AP 15 mono block.

I have a Calrad stereo SE 6BM8 down in "the lab" -- it does sound quite good -- i do need to swap something else in for a bit so i can clean up some hum and replace the volume control.

Monarch also made some nice 6BM8 amps...

i found some .05 600v orange drops at work and used them instead. the thing actually works now and doesnt sound to bad.
i have another that i'm building thats goung ti use 6bm8's but i got my sansui 1000a yesterday and am putting the others on hold for a while so i can get it up and running.

mike :)
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