The can caps are on the list to be done. I wanted to go with the Hayseed, but they're pricey. I'm in Canada too, so with exchange, shipping, tax, etc, they would cost me almost $300.
I also contacted tubesandmore, but they can only provide the 2x30uF, not the other two. Their rep said they would forward my email to the "new products team" to see if the correct caps could be offered in the future.
So I did some research on restuffing instead and purchased the Nichicon caps to do that, which only cost about 30 bucks. I couldn't find the exact values though... 33uF instead of 30, 22 instead 20, 27 instead of 25, and I'm not sure if that's acceptable or not.
If I do go that route, I will definitely do some more reading about common positive, because I'm not too sure what that means.
I also contacted tubesandmore, but they can only provide the 2x30uF, not the other two. Their rep said they would forward my email to the "new products team" to see if the correct caps could be offered in the future.
So I did some research on restuffing instead and purchased the Nichicon caps to do that, which only cost about 30 bucks. I couldn't find the exact values though... 33uF instead of 30, 22 instead 20, 27 instead of 25, and I'm not sure if that's acceptable or not.
If I do go that route, I will definitely do some more reading about common positive, because I'm not too sure what that means.
"Common positive" means the usual can configuration is reversed. In most can caps, the inner tabs are positive and the can is negative. For a bias supply can, the polarity would be reversed.
Those cap values are acceptable.
Since expense is an issue, you might try these. Not as elegant but they work:
https://grangeramp.com/product/multi-section-capacitor-board/
Those cap values are acceptable.
Since expense is an issue, you might try these. Not as elegant but they work:
https://grangeramp.com/product/multi-section-capacitor-board/
I had some spectacularly bad experiences with the CE caps when they were first introduced, about 25 years ago (literally exploding, black goo all over several friends' carpets, not a good look). Hopefully their current production is corrected, but I'm snake-bit.
Hayseed Hamfest OTOH has always come through for me, and having 105C Nichicon working parts is worth the cost to me. A lower cost alternative is to just leave the old cans in place, unused, and arrange new modern replacements under chassis.
All good fortune,
Chris
Hayseed Hamfest OTOH has always come through for me, and having 105C Nichicon working parts is worth the cost to me. A lower cost alternative is to just leave the old cans in place, unused, and arrange new modern replacements under chassis.
All good fortune,
Chris
Just to confirm, is the bias supply can C203 and is it the only one that's common positive? Looking at the schematic I see that the voltages shown near the other cans are all positive, while C203 shows negative voltages. Is that how you know?
Yes, though most schematics will note the orientation with a + sign at one end of the cap symbol.
Thanks for the link. From the pictures, it looks like they're not inherently touch-safe. Do these need an above-chassis finger shield?
Much thanks, as always,
Chris
Mmm, I don't see a problem. You mean are the capacitor cans elevated? I don't think so. A lot of folks over at Audiokarma use them and I never heard of any safety issues, but I haven't used them myself. I suppose some sort of shield wouldn't be hard to construct. I'm not sure they're any different from what Authenticap or Hayseed Hamfest encloses in a can. ;-)
What worried me was the exposed(?) wire connections visible on the top of the board, C1, C2 etc. Can't tell for sure from the pictures, but look like they could be touched by a finger from the top of the board.
All good fortune,
Chris
All good fortune,
Chris
I'm not explaining it well. The pictures look, to me, as if the through-hole via's made for wire connections to the board (and to hot ends of the capacitors) would allow a finger to touch. If that's not true, great, but that's what it looks like in the pictures.
I'll get a few to see if they could be used in a situation where the can caps are enclosed, like a McIntosh.
All good fortune,
Chris
I'll get a few to see if they could be used in a situation where the can caps are enclosed, like a McIntosh.
All good fortune,
Chris
Yes, I see. I've never used them, but if you get some I'd be interested in what you find out.
I was trying to find the post where I saw that safety cap recommended, and it was in this LK-72 restoration thread over at AudioKarma. I read further through that thread last night and saw this response saying the safety cap is not necessary because "C211 is not a 'death cap,' it's a 10KV ceramic wired across the primary (not to the chassis, as a 'death cap' is) to protect from line voltage spikes. No need to replace if so, and a bit hard to find at 10KV rating."
On my LK-72-B I believe the equivalent to C211 is C206 as seen below. It is also 10KV and also wired across the primary. So maybe it's not necessary to change. The safety cap I ordered is only 400/250V. So it's probably not the right one anyway.
That AK thread also lead me to another LK-72 thread where there was some discussion about changing the 330K grid resistors to 220K, as I did. Someone said in this post "No reason to change the 330K resistors to 220K, as the Grid #1 DC resistance of this design is well within the specification for the tube. In fact, if you do lower the value of that resistor, you'll upset the bias voltage to the output tubes (unlike with the Fisher designs), so you're fine to leave them as is".
Now I'm wondering if I jumped the gun by changing those resistors, but not sure if the 72B is the same as the 72. I did ask over at AK when I was putting my parts order together and someone said "Even if they're still exactly 330K, that's too high. Four 220K resistors are cheap insurance against thermal runaway in these tubes." So now I'm not sure what to believe. Any thoughts?
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I don't see why you can't leave the ceramic cap in place, they rarely, if ever go bad, as far as I know. As for the grid resistors, I trust whatever Dave Gillespie recommends. ;-) You can leave them or change them back. The only effect is that they might reduce the LF response a bit. Up to you.
I'm wishing now that I'd just added a 100K resistor in parallel to the 330K. Seems like that would have been an easier way to reduce the resistance and would have been easier to reverse if need be. I'll leave it as is for now and maybe look at it again when I have everything else done.
I wasn't worried about the ceramic cap being bad. I just thought a safety cap was a good idea to protect the amp, but I can revisit that down the road as well.
Thanks again.
I wasn't worried about the ceramic cap being bad. I just thought a safety cap was a good idea to protect the amp, but I can revisit that down the road as well.
Thanks again.
I highly recommend that you read through the Tube Tech Tips & Tricks pages at the archived HHScott site. It tells a everything you ought to know about how to restore and repair the HH Scott line of tube equipment.
https://www.vacuum-tube.eu/www.hhscott/cc/tube_tech_tips.htm
https://www.vacuum-tube.eu/www.hhscott/cc/tube_tech_tips.htm
Thanks for the link. Just had a quick look and I don't think I've come across that site before. Unfortunate that many of the external links seem to be dead, but still looks like some good info.
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I replaced a few more of the Ceracaps and added the CL-80 Inrush Current Limiter. I read that the CL80 would reduce the voltage a bit, and bias did require a bit of adjustment afterward to bring it back to 60mV per tube.
I'm listening to the amp now and with no music playing I noticed a bit more noise (like static or hum) in the left channel than the right with my ear fairly close the speakers. Not sure if it was there before, but I don't think so. Is this the kind of noise and distortion you're referring to and can this be reduced by adjusting bias?
I think the next step will be the can caps and I will start with the 4x100 for the bias circuit.
Not necessarily. Unbalanced output tubes is pretty straight 120Hz. Try swapping the tubes between channels and see if the noise follows. Start with V2 and V3. It might be one of the 6U8s. Also could be a noisy output tube, or a bad resistor. But the first thing is swappingh tubes.