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Ebay Tube Amps - Page 4

post #46 of 89
1/29/07 at 8:25pm
Well the full complement of tubes (in my amp anyway) is 4 x 6P1 and 2 x 6N1. valves 1-4 are the 6P1, 5 and 6 (smaller at the front) are the 6N1.

If you want to try it out, you should also be able to get the 6P1P offa ebay too.

Heres a link to the 6P1P-EV tubes on ebay. If you buy them, I'll take 4 of them off you! Just confirm with the seller they are 9 pin 6P1P and not 7 pin 6AQ5.

And heres one to the 6N1P also on ebay.

Both from the same seller and all EV rated tubes.

That'd be about $50 incl postage. I'd be willing to cough up $25 for a full set of the tubes!

Fran
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post #47 of 89
1/30/07 at 5:26am
As an update for anybody who has been following this saga. Myself and fran will be trying some of these Russian tubes, and will keep this thread posted when they arrive.
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post #48 of 89
1/30/07 at 10:15am
If someone just could find / draw a schematic on this amp...it would be an easy / simple job to modify it!

Arne K
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post #49 of 89
1/30/07 at 2:38pm
I have 90% of a schematic drawn, it just needs a few more details yet......
Its PCB based so its not that easy to modify and I don't know how clear my schematic will be!!! I ain't no expert thats for sure!

When I get the last bits done, I'll post it here.

Fran

Fran
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post #50 of 89
2/7/07 at 5:48pm
Thread Starter 
I recieved a new pair of matched tubes from the seller. There is still a crackling when the amp is warmed up now, but only with the AMT headphones. When the amp is connected to my speakers, or when somethign is connected to the headphone socket, it isnt there.

Most frustrating given I bought the amp for use with the AMT.
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post #51 of 89
2/7/07 at 6:21pm
Comiserations... could you leave a dummy load on the headphone socket as a fudge?

That reminds me, as this thread resurfaces from the depths. As myself and fran are getting these ebay NOS tubes, will we have to do anything about bias with them, or should they be plug and play?

Worried that they may end up either under-fed or burning out too quickly...
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post #52 of 89
2/7/07 at 8:28pm
Another update:

I replaced the centre tap trim pot (<50R) on the 6N1 tube sockets with a pair of 92R resistors. Definitely made it a bit quieter, now all I have a smooth hum, it measures at 1.2mV one side, about 2mV the other (for the headphones). On the loudspeaker terminals its more, about 4mV and 7mV or so. No noise at all through the speakers (not very efficient but glorious Quad ESL57s) but the hum through the headphones is still there. I know one guy on another forum that bought a different amp and had noise too, bought svetlana 6N1P and ended up with lots of noise! Hope that doesn't happen to us!

I ended up ordering copies of the 2 Morgan Jones books on valve amplifiers, cost me €75 but I suppose its an investment.

Don't know about the bias Q but I'll ask around
Fran
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post #53 of 89
2/7/07 at 8:36pm
Duggeh, if connecting a pair of headphones makes the hum through the loudspeaker terminals/amt "energizer" box (if these are the amt's you have) then it should be fairly easy to solder in something to mimic the headphones.. maybe a low value resistor or something.

Fran
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post #54 of 89
2/11/07 at 7:22pm
OK, some good news for us!

I put in some additional caps into the power supply - I added in some 470uF/200V caps in at the voltage doubler circuit - added to the 100uF ones that were already there. My hum has just about halved, now down to 1mV on headphones and 3mV at speaker outputs. Myself and stewtheking are still waiting on the milspec tubes, but its looking like the amp might well be alright but some of the components are a bit low. Anyway, I have more caps ordered and enough stuff to change teh heater supply from AC to DC so we'll see how things go....

Fran
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post #55 of 89
2/12/07 at 2:52am

What does it mean...

Quote:
I just pulled the trigger

Regards

John
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post #56 of 89
2/12/07 at 8:02am
It means he bought one.

Oh, any news on that schematic yet fran?
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post #57 of 89
2/12/07 at 5:44pm
heres a link to a high res file click here

and a smaller one to display



This is my first attempt at tracing the circuit so if you stick your fingers somewhere and die, it ain't may fault OK?

shagging pictures probably won't display anyway I really need to host these elsewhere!

Fran
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post #58 of 89
2/13/07 at 5:29am
Good job...If you want to make it cleaner/easier, just draw one channel.

Do you have a chance to measure the output-trafo?

Arne K
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post #59 of 89
2/13/07 at 10:26am
Well, I have a decent DMM and the amp sitting there....

if you can give me instructions on what measurements to take, I can certainly do them (very much learning yet!)

Fran
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post #60 of 89
2/14/07 at 5:31am
I'm "new" with tubes too...
But to measure output transformer; de-solder from PCB, measure resistance of the primary,(half value=center-tap/B+), then measure from speaker-end.
To measure transfer-ratio, you need a signal-generator, or something that can give a volt or two sinewave at ~ 100Hz, in to speaker-terminals, measure voltage on input, and output on the de-soldered wire-ends.
Diffrence will give the transfer-ratio.

Maybe some other members will correct me...or give better advice?

(With a signal-generator and a scoope, it could be checked for 10-30+kHz response)

Arne K
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