e.a.r 859 clone problems...

[smithie]

hi guys
i have a amp thats built on the ear 859 circuit that was in the hifi news mag many years ago.
i brought it from a forum member and its been exellent,i recently transfered it into a better bigger box and changed over some various parts for some designer gear and thats been fun to and ive had no probs...until now!!
i changed some resistors for different types of the same value and all was fine,or so i thought...the next day the left ouput valve had died,didnt think much on it as i thought the valve was proberly a old one....but now im on my third valve so im guessing i have a problem.
my first thought was i must have screwed it up with the resistor change but ive checked my work,the resistor,the valve base and all seems fine.
ive checked heater and ht voltage and that also is fine.
im now at a loss to what it can be,im fast running out of valves and its getting expensive, a valve will only last aroung 8hrs max if i put one in, and you can see inside the valve at the base that the copper pins turn a light carbon colour when being used so im assuming its cooking or something!!!
any ideas guys as im at a loss!
all the best
smithie

[Richard]

Hi Smithie,

I'd meter lumps of the circuit for expected result then home in if it doesn't read right.

Bias is first suspect, check voltage at op valve pin 9 to ground if you have the figure or compare to good channel.

Then, it's cathode bias and there's no coupling cap, so check the cathode connection to ground. Probe op valve pin 9 to ground (amp off) looking for 244R.

Perhaps also change cathode bypass caps, 2200uF 40v, and use higher voltage rating 63v or 100v in case it's going short or intermittent.

If still no joy work back and make checks of the driver and first stage.

Rich

[smithie]

hi richard
thanks for the speedy reply and some possible solutions.
the trouble i have is i can just about read a circuit diagram and i can even copy when building a amp,where i fall down is not understanding it,and why things are done like so so im next to useless really...but i try not to let that stop me!
i shall try as you surgest regarding bias...i dont have a biasing figure but as you say i can use the other good channel as reference (this is auto bias design,does that make any difference regarding how i do things?)
i shall have a go in earnst to night at all that youve surgested richard and i really appricate your help and input.
all the best
smithie

[smithie]

hi richard
just done a quick messaure of cathode bias asb you surgested with amp off and the good channel is reading 248r the bad channel is reading 378ish so im guessing that isnt good
but what does that mean in simpleton english,ie what would cause that?
all the best
smithie

[Richard]

Hi Smithie,

No problem, thanks for explaining where you're at, hard to know where to pitch it sometimes. Maybe start with the amp off and check the pin 9 to ground resistance first as that would be easiest. You can also do the other channel as a comparison perhaps. Then fire up and check DCv at pin 9 to ground. The amp is cathode bias which is same as auto bias in this case. Again, compare the 2 channel voltages if they're not on the diagram. Pins are numbered clockwise from the gap when viewing from underneath the valve/base, so last pin round.

Rich

[Richard]

Ah, ok, you've been busy while I had a cup of tea... the 248R is fine but the other is wrong for some reason. Actually, reading high, I'd expect it to give more bias so less of a problem for the valve. Whatever, the good channel gives what we expect and you must hunt down why the bad channel doesn't. Meter across each resistor separately, a probe either side of the 250R, 1k and 10k to check they are correct values, again confirm tecnique and reading against other channel. If all ok, resolder all the joints from pin 9 through those R's to ground, and meter again, something's high.

[smithie]

hi richard
you found time for a cuppa? lucky you!
will do as you surgest later,way to busy at the mo,even for a cup of tea,but hopefully i will do that tonight.
once again youve been a great help and given me a good starting place...much appricated!
enjoy that drink
all the best
mr clueless

[smithie]

hi
as promised ive been doing some probeing and found that pin1 on the v1(left channel) tube is around ten volts higher then the right hand channel,it seems to be coming down to the 510k resistor that feeds this line as voltage before this stage is identical...whould this cause my problem,or is this just something else ive found.
unfortantly i havent got a 510k resistor to swop and see.
all the best
smithie

[Richard]

Ah, we're on V1, lost me for a minute, still looking at the op stage , no, 10v there will be nothing, the anode resistor or valve that's all. How did the op stage check out, anything not similar across channels, what about that pin 9 to ground cathode resistance difference?

[smithie]

hi richard
i like to keep you on your toes,my eyes are roaming all over the place in this amp,hence my move to v1...sorry about that,ill try and stay focused!
ok let me throw this at you....i found that the 4k7 wasnt soldered properly( guilty has charged),ive resoldered it properly and now im getting 24 on both channels on pin 9 and earth with power off.
so thats progress but do you doubt it was this that screwed 2 of my valves,scared to put another in as im running out and dont even get me started on the expense of it all
all the best
smithie