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Old 5th August 2011, 10:33 PM   #1
PJN is offline PJN  United States
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Default Need a little help with Chinese tube Preamp

Hi Guys,

I was looking for a cheap pre for a chipamp that I had recently built, and wanted to try a tube version to see what they were like, so I took a chance on one of the cheap Chinese options on EBay. I got a version that was supposed to be based on the Matesse Atom that uses two 6N3 tubes for about 50 bucks shipped including the transformer. It looks to be put together nicely and the board seems to be of pretty good quality, but the tubes that came with it look really cheap. Well the bottom line is that it doesn't work. When hooked up I get a very faint sound coming out with the volume turned all the way up. I checked out things a bit with my DMM. The transformer is fine, with 115 vac it gives 227 vdc and 7.7 vdc after rectification with no load. The dodgy green led's under the tubes light up and the tubes glow very dimly inside, but don't get even warm. I tested the vdc on the tube pins and the results are in the attached pic. I don't know a thing about tubes being a SS guy. I'd appreciate any advice on how to proceed to get this thing working.

Thanks,

PJN
Attached Images
File Type: jpg Pre1.jpg (62.4 KB, 339 views)
File Type: jpg Pre2.jpg (67.7 KB, 331 views)
File Type: jpg Tube vdc.jpg (54.7 KB, 325 views)
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Old 5th August 2011, 10:38 PM   #2
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Both anodes get 224V, that's OK but what about the rest? Circuit please. Please measure pins 8 & 2 (cathodes).
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Old 5th August 2011, 11:35 PM   #3
PJN is offline PJN  United States
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Hi Horntube,

I measured the sockets the first time by placing the negative lead of the DMM at the negative rectifier dc outlet and touching the positive lead to each pin. I'm guessing to measure the cathode I need to reverse the procedure and put the positive DMM lead on the rectifier + dc out. When I did that every pin except 4 and 6 measured 224 vdc. 6 & 4 were 0.

PJN
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Old 5th August 2011, 11:45 PM   #4
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No, use any ground rail or PSU cap minus side and measure the anode & cathode pins.
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Old 6th August 2011, 12:22 AM   #5
PJN is offline PJN  United States
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OK, I'll do that and report back.
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Old 6th August 2011, 12:49 AM   #6
PJN is offline PJN  United States
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Ok, I measured all of the pins against the ground plane. With the negative lead of the DMM on the ground plane I get 224 vdc on pins 4 and 6 as shown in the diagram above, I get 0 vdc on all of the other pins. Unfortunately as of yet I have been unable to get a schematic, I'll email the seller to see if he has one. I've attached an pic of the underside of the board, the out RCA's are in the upper left, notice the two spots for smd resistors that are empty. On the input RCA there are smd's there.
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Old 6th August 2011, 02:51 AM   #7
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Pic is not very good but if the SMD's R9, 10, 15 & 17 are missing than you have to put some resistors in there! It can't work without cathode resistors, no wonder you measure 0V on the cathode pins.
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Old 6th August 2011, 02:56 AM   #8
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Oh, just forgot to mention: Throw the Chinese 6N3 away and put some Soviet Red Army 6N3P in there and you will be amazed!
Click the image to open in full size.
Well, not that much please!!!
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Old 6th August 2011, 01:50 PM   #9
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Hello,

If the tubes are not hot take a look at the heater voltage. Between pins 1 and 9 there should be ~ 6 volts.

DT
All just for fun!
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Old 6th August 2011, 08:57 PM   #10
PJN is offline PJN  United States
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Hi DualTriode,

I measure 7.6 vdc between pins 1 & 9. Between 1&2, 1&3, 1&5, 1&7, and 1&8 get 0 vdc. Between 1&4, and 1&6 get 224 vdc.

Horntube,

There are smd resistors on R9, 10, 15 & 17.

Thanks for helping.

PJN
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