Thank you. A voice of reason. If there's no voltage across something (even if it's an evil Capacitor from Planet Mongo) how can there be any distortion? Coupling caps get fetishised for very little reason. Filter caps, like RIAA, another matter.
YOS,
Chris
YOS,
Chris
I find it difficult to believe that moisture could work its way inside without any oil leaking out in the process.
Panasonic "ECQ" metalized polypropylene or polyester caps are quite good, reasonably priced, widely available, and come in a wide variety of values/voltages. Just sayin'.
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An unfortunate event was Matsushita Electric (Panasonic/JVC) discontinuing the ECQ-P(U) line of capacitors.
Those parts were discrete polypropylene film and aluminum foil and they had OFC leads. The ECQ-P(U) caps. offered an absolutely outstanding price/performance ratio. Think 716P series Orange Drops (if not a tiny tad better), at lower cost.Another useful (across regulator IC I/P terminals) series Matsushita discontinued are the ECQ-V parts. Grrr!! 😡
So, I've measured all the resistors. The design is simple, so there were only a couple I couldn't test in circuit. Only one power resistor was out of spec (11,5%), the rest were within 1%. These are all 10% resistors. So I'd say that's pretty darn good.
Everything looks new, it has this shiny gloss as if time stood still. No heat marks. Wire coating is impeccable. As soon as I get some banana plugs, I'll fire this puppy up (yes, something that silly is stopping me atm).
Everything looks new, it has this shiny gloss as if time stood still. No heat marks. Wire coating is impeccable. As soon as I get some banana plugs, I'll fire this puppy up (yes, something that silly is stopping me atm).
Heh, I guess some lessons must be learned the hard way. Or, hopefully, I've just avoided that. Since the guy who sold it has been using the amp the past 5 months, I hooked it up and played some music. Sounded nice, smooth treble but bass was lacking. I assume the reservoir caps are dry as a desert. Anyway, the unit got pretty hot. I haven't really had a tube amp in service at home before, so didn't know what to expect.
Today I had it on for several hours since I'm at home office. After a while, I thought the power transformer was buzzing louder than it used to. Turned off the amp. Later, I turned it on, played some music and checked the tubes. One of the output EL84 was redplating.
So, time to get rid of those coupling caps I guess. I hope I didn't ruin the tube, or anything else. Although I assume the tube had redplated before as well. Temp of the tubes inside the cabinet were about 160C, but internet said that was fine. Must be some toll on the transformers! I have only played at low volume, though.
Is it safe to not change the output tubes, even though one redplated? Or would I risk some kind of runaway, despite changing coupling caps?
Today I had it on for several hours since I'm at home office. After a while, I thought the power transformer was buzzing louder than it used to. Turned off the amp. Later, I turned it on, played some music and checked the tubes. One of the output EL84 was redplating.
So, time to get rid of those coupling caps I guess. I hope I didn't ruin the tube, or anything else. Although I assume the tube had redplated before as well. Temp of the tubes inside the cabinet were about 160C, but internet said that was fine. Must be some toll on the transformers! I have only played at low volume, though.
Is it safe to not change the output tubes, even though one redplated? Or would I risk some kind of runaway, despite changing coupling caps?
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A tube that has been overheated should not be used again. Get a new tube(s). Replace coupling caps ? Maybe, but i would start with fresh tubes.
Does this amp require bias adjustment? If so, check and adjust.
Does this amp require bias adjustment? If so, check and adjust.
@petertub, what could be the consequence of not replacing the tube? Suboptimal or catastrophic? Can I use the old one while waiting for spares to arrive?
Everything in this amp is original, so assume bias in ok in that regard?
Everything in this amp is original, so assume bias in ok in that regard?
The O/P tubes are self (cathode) biased. As long as the cathode resistor (1/channel) is within 10 or (perhaps) even 20 % of nominal there's nothing to do.
Replace all of the Japanese PIO caps. (which are quite notorious for becoming leaky) and install 2 new matched pairs of JJ EL84 O/P tubes, which petertub can provide.
Replace all of the Japanese PIO caps. (which are quite notorious for becoming leaky) and install 2 new matched pairs of JJ EL84 O/P tubes, which petertub can provide.
Do you have a schematic? That will tell you if the amp uses fixed bias or not. It might not have been mentioned but Selenium rectifiers used for bias voltage often fail.
There's one bias pot for each channel. Tube rectified (5AR4). But again, since the coupling caps have a reputation of leaking after these years, I assume that's the culprit? They would need replacement, regardless.
I found a schematic and its fixed bias. The pots are for hum adjustment. Get rid of the caps from the AC power lines to the chassis. Really bad things will happen if those start leaking. Add a 3 prong plug.
K42s would be perfect because of room contraints.
You can use K40s but they are more costly and in this amp they are harder to work with because they are larger in diameter. The k42y caps are PIO caps and they have the insulated paint on them and they will fit nicely. Once they break in, they are going to sound pretty close in this type amp and they are much cheaper in price being they are more available.
You can use K40s but they are more costly and in this amp they are harder to work with because they are larger in diameter. The k42y caps are PIO caps and they have the insulated paint on them and they will fit nicely. Once they break in, they are going to sound pretty close in this type amp and they are much cheaper in price being they are more available.
@astouffer: Yes, I replaced the old line caps with Y1s. Regarding grounded plug, can I just attach ground to chassis? Or could this cause issues with the amp?
@michaelsamra: Thanks! In the meantime, I'll install MKP caps so that I'll be able to use the amp (and listen) - then, when I get the K32Ys I'll put those in and see if I can hear the difference. As part of this, I'll use the most expensive, objective and straightforward measurement tool I know of to correct any placebo effect (i.e. my wife).
@michaelsamra: Thanks! In the meantime, I'll install MKP caps so that I'll be able to use the amp (and listen) - then, when I get the K32Ys I'll put those in and see if I can hear the difference. As part of this, I'll use the most expensive, objective and straightforward measurement tool I know of to correct any placebo effect (i.e. my wife).
Don't "knock" the female of the species. They tend have excellent ears. AAMOF, their ears hear way too much. 😉
Oh, I know! Which is why, for every change I do, I use her as the control. She doesn't care for the gear, only the sound. And she tells me exactly what she hears, no sugar on top.
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Earth ?
I looked at the images of the inside and it looks like a two-cord mains supply. Am I wrong ?
IF I’m seeing it right, the very first thing that I’d do is replace the mains cord with a modern cord incorporating a MAINS SAFETY EARTH and hard connect it to the metal chassis.
I looked at the images of the inside and it looks like a two-cord mains supply. Am I wrong ?
IF I’m seeing it right, the very first thing that I’d do is replace the mains cord with a modern cord incorporating a MAINS SAFETY EARTH and hard connect it to the metal chassis.
This is why I suggested, back in post #3, that all the gray caps need to be replaced. The output tubes should be replaced at this point but it's very possible that they had red-plated previously, so likely needed to be replaced anyway.
I know it's tempting to just run an old amp amp and listen until the parts order comes in, especially if the previous owner had been using it. I've done it many times. But I would never do this with a Japanese amp that uses those gray caps. They really do need to be replaced first.
Another thing I noticed, when looking at the pic of the guts, is that the primary of the PT is labeled 110v. Assuming you're in the US, your wall voltage will be considerably higher and this results in higher voltages throughout the amp. Measure your wall voltage and see. Keep in mind that it tends to vary depending on the time of day as overall demand fluctuates.
While the buzzing may have been the result of excessive current being drawn by the red-plating tubes, the higher wall voltage alone will make the PT run harder and hotter than it was designed to do. This will be the case even after you replace the (electrically) leaky caps and install a new set of tubes.
You should plan on taking steps to reduce the voltage either by adding a bucking transformer or running the amp on a variac. I don't usually bother doing this with vintage American amps, since their PT primaries are generally rated at 117v, so the over-voltage isn't quite as extreme. But something should be done if the PT is rated for 110v.
As a quick check, after you replace all the caps, measure the heater voltages on the tubes. If they're more than 5% above the nominal 6.3v tube life will be affected.
You're confusing fixed bias with cathode bias. It's confusing because "fixed" actually means it can be adjusted, or set, at whatever level you choose. Cathode bias is sometimes referred to a "auto-bias" and is not adjustable unless you change the value of the cathode resistor.
This amp uses cathode bias. You are correct that the pots are hum pots, not bias pots.
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@FlaCharlie, regarding the voltage, I have thought of making a buck transformer, however that would be a project for another day. It's typically 240-245V here where I live (amp has a 110/220 switch). I'll keep an eye on the filament voltage.
But as far as I understand, I only need to change the coupling caps now? I'll do some of the electrolytics as well, but I guess I can leave filters and cathode cap in for now?
By the way, speaking of cathode caps.. some of them are electrolytics. I have a few Nichicon Muse and Elna Silmic IIs lying around, would it be beneficial to use them there, or just wasteful?
But as far as I understand, I only need to change the coupling caps now? I'll do some of the electrolytics as well, but I guess I can leave filters and cathode cap in for now?
By the way, speaking of cathode caps.. some of them are electrolytics. I have a few Nichicon Muse and Elna Silmic IIs lying around, would it be beneficial to use them there, or just wasteful?