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Old 7th February 2013, 01:45 AM   #241
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Quote:
Originally Posted by Barbieboy View Post
Recently had a Krell amp here, the Krell won*, just, by its articulate upper bass. Then I upgraded the caps in the 10L, made a staggering difference. Now prefer the 10L any day.

*My speakers are an easy load to drive, so the Krell would have come into its own with different speakers

Barbieboy: which/what caps did you upgrade to?
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Old 7th February 2013, 10:26 AM   #242
ilimzn is offline ilimzn  Croatia
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Originally Posted by SY View Post
Wow, there's a lot of work to do. Do they really have 130V cathode to heater for the 6N1? Unbalanced phase splitter? A quasi-SRPP (badly unbalanced) input stage?
Wow.
As someone else pointed out, the unbalanced splitter is actually an attempt to balance the unbalance due to a rather short tail in the 'long tail pair'. I'd give them a positive point for that since most miss this subtlety.
As for the rest, I'm afraid you are right, and it does get worse - look at the bias network. What happens if the pot wiper goes open, and the trimmers used are not of any great quality. Surprisingly this is a typical gross error - bias networks should be wired so that the bias voltage goes into maximum negative should the potentiometer fail.
There are a couple more issues with the mechanics and layout of the amp but all in all it's a great candidate for modifications, especially since the output transformers are sufficiently well made that it's worth the effort. However, there are versions for the EU/US market with 220/110V primaries on the mains transformer which seems to have been wound 'to the edge' and tends to work close to saturation on 230/115V nominal mains when it' on the high side, making the transformer run quite hot.
The input selector and pot could be of better quality, but the problem is they are located on the front which requires the signal wiring to pass around the mains transforemer - and it in turn actually sits on top of part of the driver circuit, which required some components to be bent out of place to accomodate the metalwork. The build is not a solid as it looks. In fact the transformer mounts are rather thin for the size of the transformers and can get bent in transport if the amp is handled a bit rough. The metal work in general is thinner than one would guess from the looks of it, there is a very thick layer of effect paint on the chasis which hider this.
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Old 7th February 2013, 11:26 AM   #243
DF96 is offline DF96  England
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Quote:
Originally Posted by ilimzn
As someone else pointed out, the unbalanced splitter is actually an attempt to balance the unbalance due to a rather short tail in the 'long tail pair'. I'd give them a positive point for that since most miss this subtlety.
No, the unequal anode resistors make the imbalance worse. This may, of course, simply be a drafting error in the circuit diagram and the actual amps may be built with the resistors the right way round.

Quote:
The build is not a solid as it looks. In fact the transformer mounts are rather thin for the size of the transformers and can get bent in transport if the amp is handled a bit rough. The metal work in general is thinner than one would guess from the looks of it, there is a very thick layer of effect paint on the chasis which hider this.
Modern electronics built in China not as robust as it appears? Surely not!
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Old 7th February 2013, 11:40 AM   #244
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I used to own a MC-10T - that I bought on a lark - it was a good amplifier, not great. For the price, however, it is a good introduction to tube newbies.

But...the PCB/wiring layout is a nightmare, making it the least favorite unit I've ever worked on. I had thought of improving the circuitry but gave up after replacing the coupling capacitors. I'm sure someone with a higher pain threshold than my own could put a standard Mullard type circuit in and be much happier with the results.

Of course the way the flimsy RCA jacks are done, the cheapo volume control, the oversized bias resistors, and the general layout - it may be better to just pull the transformers and use them on a different project.
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Old 7th February 2013, 08:53 PM   #245
ilimzn is offline ilimzn  Croatia
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Originally Posted by DF96 View Post
No, the unequal anode resistors make the imbalance worse. This may, of course, simply be a drafting error in the circuit diagram and the actual amps may be built with the resistors the right way round.
A, sorry - didn't look well enough, but yes indeed it's done the right way around.

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Modern electronics built in China not as robust as it appears? Surely not!
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Old 8th February 2013, 11:34 AM   #246
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Originally Posted by audio0412 View Post
Barbieboy: which/what caps did you upgrade to?
ClarityCap ESAs, not an ideal fit, but sonics rock. You can see me at it on a few pages back, page 21 I think
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Old 20th February 2013, 06:41 PM   #247
grg is offline grg
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Yaqin MC 10T not a bad amp, but I recently tried to tube roll replacing the stock Chinese EL34s with some Russian Svetlanas. Bias was double checked and OK at 350mv before the swap. After the swap I left my voltmeter connected to the V1 bias points to monitor the bias immediately at turn on. The meter pegged and the 2.5A power fuse blew. (maybe a bad tube?) I replace the tubes back with the stock originals, replaced the fuse, and at turn on measured V1 bias to be a whopping 25 volts! I immediately cranked the bias pot down to min, but the best I could get was down to 8 volts. The other 3 tubes all were still stable at 350mv. Amazingly, both channels still sound fine, but I shut it down within 30 sec. fearing subsequent damage. Any thoughts before I pop the lid off? I'm hoping to find an obvious burned resistor in the bias measuring circuit, or something closely related.
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Old 11th June 2013, 02:00 AM   #248
jkorten is offline jkorten  United States
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Quote:
Originally Posted by grg View Post
Yaqin MC 10T not a bad amp, but I recently tried to tube roll replacing the stock Chinese EL34s with some Russian Svetlanas. Bias was double checked and OK at 350mv before the swap. After the swap I left my voltmeter connected to the V1 bias points to monitor the bias immediately at turn on. The meter pegged and the 2.5A power fuse blew. (maybe a bad tube?) I replace the tubes back with the stock originals, replaced the fuse, and at turn on measured V1 bias to be a whopping 25 volts! I immediately cranked the bias pot down to min, but the best I could get was down to 8 volts. The other 3 tubes all were still stable at 350mv. Amazingly, both channels still sound fine, but I shut it down within 30 sec. fearing subsequent damage. Any thoughts before I pop the lid off? I'm hoping to find an obvious burned resistor in the bias measuring circuit, or something closely related.
Cathode resistor frittata.
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Old 3rd October 2013, 10:06 PM   #249
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Hi Guy's, Try some E88CC(ECC88) up front on the 10L, better match then the 85, go for vintage Tesla's if you can find some, sounds real good.
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Old 17th October 2013, 12:39 PM   #250
Jo3r1 is offline Jo3r1  Canada
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Hi everyone
Yesterday I bought a Yaqin MC-10L, my first ever tube amp.
There is some hum in it though, at startup it stays for about a minute, later it goes away and it comes back after 30min-1hr of playing.
It's only on one side and the volume control doesn't affect it, it stays on the same level, even with volume turned down all the way. When I switch the powertubes to the other side, the hum also switches to the other speaker.
I still have to check bias, but if that doesn't help, sounds this like an almost certainty that one of the tubes is faulty and that I need to replace them?
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