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Uncertainty re 6BQ5's coupling caps in Trio KW55 receiver

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This receiver branded AGS Trio came into my possession for nearly nothing and immediately afterwards I saw that every tube was missing. I received and accepted an offer to provide an assortment of miscellaneous tubes purportedly checked and deemed usable.

I am not so familiar with schematics that understanding comes easy at all so had to slowly review this one to gain just a little understanding.

What I've read about Japanese tube receivers from this era indicate that factory installed capacitors are short lived. Collating the diagram with the underside wiring was time consuming and perhaps not accurate. What my visual inspection revealed was that some electrolytic capacitors were already new and unless I'm mistaken, so were most coupling capacitors in the power output section 12AX7's & 6BQ5's.

I was somewhat sure there were no shorts via variac & DBT. Shortly after feeding in an audio signal, two (that I noticed) 6BQ5's started to red plate and audio on one side started to sputter.

After disengaging an end then checking with DMM of what I believe to be the coupling capacitors, the reading was what they were labeled as.

Reconnecting them, inserting the tubes again and listening to a signal source, only one 6BQ5 appeared to red plate but all 4 seemed to me excessively hot and sputtering resumed.

If I am reading the schematic correctly, and that may not be so, the coupling caps should be a different value than what's in there.

What I see are 0.1mf square brown film capacitors of some kind except for one. That one is a composition unknown white tubular 0.22 capacitor branded Audio Spec. Whether this one is original or not I can't say but the DMM does measure .22 mf.

It is not possible for me to access the schematic at this time, I'm limited to an old i-phone so I cannot confirm but I believe the specified capacitors are neither .1 or .22.

As far as KW55 coupling caps go, what value should be in place?
 
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Yes, that one I can open.

And the capacitors marked C122 & C122 are two of the four I was looking at. The diagram states .05mF but voltage on diagram says 10V? That puzzled me.

The capacitors presently in circuit are three newish looking square brown film of some kind marked 104K - .05mF. The fourth is that white tubular "Audio Spec" branded capacitor marked 600V .22mF.

Could this disparity be because precise capacitance is not critical or could it be contributing to or response for 6BQ5's going red plate?

If time allows when my shift is over tonight (they're 12 hours), I'll attempt determining where those test points are and check fore those voltages you indicated.
 
Japanese gear from that period is filled with grey capacitors (typically paper in oil types) which are known for going bad. They all need to be replaced. You should also replace the all the electrolytic caps in the power supply and elsewhere.

Red plating is usually caused by bad coupling caps that allow DC voltage to leak through to the grid of the output tubes. The coupling caps are C120, C121, C220 and C221. They are 0.05uf. You can replace them with 0.047 630v parts. The 10v you mentioned has an arrow next to it pointing elsewhere.

Receivers are complex and crowded with parts. Not an easy project, even if you have experience working on tube gear.
 
For availability and cost's sake, I can obtain 630V 0.047mF caps locally. They are also those rectangular brown film caps which I now know to be polypropylene.

My only other venture into a tube amplifier was in 2012 when I gutted and re-built the Heathkit integrated that I first assembled in high school. It contained Mylar capacitors and I replaced those with Orange Drops. I may be totally unaware of it but at the moment there seems little reason not to use those 0.047mF Polyprop ones. They're under $2 CAD each and I can pick them up on my next day off.

At this moment, the smaller electrolytic's have been replaced. Either by myself or an earlier owner. I have maybe three large axial's to source new ones for that might require online ordering.

And there's the multi section can cap. With currency difference and shipping I estimate replacing that will cost me $70. Should this receiver function after I replace what I can afford, the original can cap will stay. If something dictates a new one is needed, I'll cross that bridge when I get to it.

But something I'd be interested in learning is what effect do those substitute 0.1 and 0.22 coupling caps have when used in place of 0.05mF identified in the schematic? Might that cause red plating?
 
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For availability and cost's sake, I can obtain 630V 0.047mF caps locally.

At this moment, the smaller electrolytic's have been replaced. Either by myself or an earlier owner. I have maybe three large axial's to source new ones for that might require online ordering.

And there's the multi section can cap. With currency difference and shipping I estimate replacing that will cost me $70. Should this receiver function after I replace what I can afford, the original can cap will stay. If something dictates a new one is needed, I'll cross that bridge when I get to it.

But something I'd be interested in learning is what effect do those substitute 0.1 and 0.22 coupling caps have when used in place of 0.05mF identified in the schematic? Might that cause red plating?

The 0.047 caps you found locally will do fine in those coupling positions. The fact that they were replaced with a higher value will affect frequency response (add more bass) as Eli mentioned. The altered values should not cause red-plating, however. If they leak DC to the grids of the output tubes, red-plating would result, but that's true no matter what value they are.

And it's good that some of the smaller electrolytics have been replaced already. The grey caps I mentioned are not electrolytics, they are mostly PIO (paper in oil) and there are usually a LOT of them in Japanese gear. ALL of them need to be replaced. You can use film caps as replacements. Hopefully you have a parts list so you can work your way through and identify them.

Replacement cans are very expensive. If you can find enough space under the chassis you can just disconnect the can, leave it in place for looks, and add individual caps underneath. The other advantage to using individual caps, besides the MUCH lower cost, is that you can buy ones with better specs. I try to buy ones rated at 105 degrees for 10,000 hours. Nichicon, Panasonic, Cornell-Dublier are all good.

Digi-Key, located in Minnesota, has a free shipping option although it might only apply to orders shipped to the U.S. Details are under Terms and Conditions on their website.

I would not wait to replace all the can caps and large axials unless they can be tested at operating voltage with a cap checker to confirm that they are still in spec and not electrically (or physically) leaky and tested for ESR.

That schematic is very difficult to read since it's drawn in such a crowded manner. It looks like it uses a voltage doubler PS, which may be the axial caps you mentioned. Make sure you connect all the electrolytics correctly.
 
Yes, I prepared a list by recording values of all the old caps still in place.

This chassis is profusely populated by those 600V 0.05's. Including the 4 coupling caps already discussed, about 16 of them. But there are 21 tubes in this thing and it is a receiver.

It looks like I can get caps of similar values locally for those large electrolytics with the exception of that can cap. It's a 350V 3X40mF. I'll leave that until last and with any luck, not need to replace it. If it does, I will ponder the chances of fitting in 3 single caps instead. Not very optimistic about that on the underside, there's not a lot of unoccupied space.

Might need to look at stuffing the old one or fabricating a facsimile can cap cylinder out of something.

I have a feeling that when I leave the part's store later this week it'll be "Whoops, there goes another hundred bucks".

As far as the red plating goes, if brand new coupling caps don't cure it, I suppose it's the output transformer(s) or the 6BQ5's themselves?

The last two times I switched the unit on it on it was with all 12AX7's and 6BQ5's in place. During the first go, only one 6BQ5 visibly red plated. The second time no visible red glow but after a minute or two, the audio signal did start sputtering/distorting again on the right channel as it had before.

But even though I saw no red plating, those 6BQ5's were throwing off a lot more heat than I think they should.

In simple terms, where/how do I check the OPT's? The only device at my disposal is a DMM.

The 6BQ5's might be NOS. They're Russian and by the looks of the boxes, intended for that domestic market who knows when - the staples are rusty.
 
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The Russian 6Π14Π (6p14p) is a quite mediocre 6BQ5/EL84 equivalent. OTOH, the 6Π14Π-EB (6p14p-ev), AKA EL84M, is a nice 7189 equivalent that is tougher than old rubber boots. Check the markings on the tubes in your possession.

Re-examination of the schematic shows "Cheap Charlie" DC heaters as part of a single bias network that all 4 "finals" share. You need a closely matched quad of O/P tubes. A random set of 4 tubes could be part (all?) of the red plating problem. IMO, it will be well worth your while to acquire a properly matched set of the 7189 equivalents from a reputable dealer, like Jim McShane.
 
The Russian 6Π14Π (6p14p) is a quite mediocre 6BQ5/EL84 equivalent. OTOH, the 6Π14Π-EB (6p14p-ev), AKA EL84M, is a nice 7189 equivalent that is tougher than old rubber boots. Check the markings on the tubes in your possession.

Re-examination of the schematic shows "Cheap Charlie" DC heaters as part of a single bias network that all 4 "finals" share. You need a closely matched quad of O/P tubes. A random set of 4 tubes could be part (all?) of the red plating problem. IMO, it will be well worth your while to acquire a properly matched set of the 7189 equivalents from a reputable dealer, like Jim McShane.

Interesting. All four of the boxes those tubes came in have the "EB" suffix but the tubes themselves don't. And just now having examined them, two of the tubes are labeled with red script and the other two with grey.

No tubes, but from having bought some bits and pieces from Jim McShane when re-building that previously mentioned Heathkit, I have a feeling four 6BQ5's from him will be on the pricey side. That local shop I mentioned has those tubes on their website:

Search

It is stated "matched dual/quad". I haven't checked yet but I'll see if Jim McShane has a website with pricing. If not I could ask him by e-mail.

The local source would be in Canadian $ and no shipping, no duty.

I'm reasonable sure the tube glass was clear when I first got them which in my mind, made me understand the vendor's remark that they might possibly be NOS. At present, two of them, the ones with grey script, have silver/black "scorch marks" on opposite sides. I do not believe this is indicative of a healthy tube.
 

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Jim McShane's price for JJ tubes ($49.50 US per quad) would be about the same in CAN dollars, minus any imposed duty & tax, as the local place's new tubes.

His Tungsol reissues are $64.50 and Sovetek - $79.50.

Presuming that either of these choices is within my means and that this receiver drives them at their limit (as seems to be the case) are the Sovetek's more suitable or would it be JJ's and forget the Tungsol's?

Calculating US to CDN exchange the Sovetek's would be a bit of a stretch at about $110. Landed cost might be higher.

Re filament voltage, I spot checked one a week or so ago and it was 6.5V so within acceptance I assume.

Before I'd go dropping additional funds on more tubes, I feel it would be in my interest to determine if the output transformers are OK but I'm not exactly sure how to do that.

Pin 7 of one of the 6BQ5 pairs to pin 7 of the other?

If so, is it a resistance number I'm looking for or just continuity? What I've read so far about doing this was not simple enough for me to comprehend.

Don't want to appear a tightwad but I'm on the OA pension dole now yet still need to work 48 hours a week.
 
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I will test for those pin 7 voltages, it could not hurt to know. But I can't grasp how knowing that would indicate health of the output transformers.

But having thought about it maybe I need not be concerned for the output transformers? An audio signal sounds fine for the minute or two before the sputtering/distortion and red plating begin. Maybe I'm over simplifying but if an output transformer were faulty, would that not mean poor sound quality at all times?
 
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