Need a little help with Chinese tube Preamp

PJN · 5th August 2011, 10:33 PM

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

HornTube · 5th August 2011, 10:38 PM

Both anodes get 224V, that's OK but what about the rest? Circuit please. Please measure pins 8 & 2 (cathodes).

PJN · 5th August 2011, 11:35 PM

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

HornTube · 5th August 2011, 11:45 PM

No, use any ground rail or PSU cap minus side and measure the anode & cathode pins.

PJN · 6th August 2011, 12:22 AM

OK, I'll do that and report back.

PJN · 6th August 2011, 12:49 AM

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.

HornTube · 6th August 2011, 02:51 AM

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.

HornTube · 6th August 2011, 02:56 AM

Oh, just forgot to mention: Throw the Chinese 6N3 away and put some Soviet Red Army 6N3P in there and you will be amazed!

Well, not that much please!!!

DualTriode · 6th August 2011, 01:50 PM

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!

PJN · 6th August 2011, 08:57 PM

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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5th August 2011, 10:38 PM	#2
HornTube diyAudio Member Join Date: Nov 2009	Both anodes get 224V, that's OK but what about the rest? Circuit please. Please measure pins 8 & 2 (cathodes).

5th August 2011, 11:35 PM	#3
PJN diyAudio Member Join Date: Dec 2004 Location: Pa, USA	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

5th August 2011, 11:45 PM	#4
HornTube diyAudio Member Join Date: Nov 2009	No, use any ground rail or PSU cap minus side and measure the anode & cathode pins.

6th August 2011, 12:22 AM	#5
PJN diyAudio Member Join Date: Dec 2004 Location: Pa, USA	OK, I'll do that and report back.

6th August 2011, 02:51 AM	#7
HornTube diyAudio Member Join Date: Nov 2009	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.

6th August 2011, 02:56 AM	#8
HornTube diyAudio Member Join Date: Nov 2009	Oh, just forgot to mention: Throw the Chinese 6N3 away and put some Soviet Red Army 6N3P in there and you will be amazed! Well, not that much please!!!

6th August 2011, 01:50 PM	#9
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	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!

6th August 2011, 08:57 PM	#10
PJN diyAudio Member Join Date: Dec 2004 Location: Pa, USA	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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