Ping Don..
 

"Don Pearce" wrote


No idea why that cap is so big. In combination with the 100k grid bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.


Well, I've got the owner's blessing to butcher the amps as I (and
you/Nick) see fit, so tomorrow will be interesting.

(I fully expect him to come round anyway - he works Saturdays at the
local 'Bike shop and has Fridays off!!)

Ping Don..
 

Keith G wrote:
"Don Pearce" wrote



No idea why that cap is so big. In combination with the 100k grid bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.



Well, I've got the owner's blessing to butcher the amps as I (and
you/Nick) see fit, so tomorrow will be interesting.

(I fully expect him to come round anyway - he works Saturdays at the
local 'Bike shop and has Fridays off!!)





Well, the cynic in me would wonder:

Audio Note 1uf 600v (copper) PIO = $69.60
Audio Note 0.1uf 600v (copper) PIO = $41.69

Though without starting a flame war, I would have suggested

Mundorf M-CAP® Supreme Silver & Oil 0.1uf 1200v = $23.45

But you could try

BC Components 2222 368 Series 100nf 400v = 13p

--
Nick

Ping Don..
 

"Don Pearce" wrote


Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.


OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)

Ping Don..
 

"Nick Gorham" wrote in message
...
Keith G wrote:
"Don Pearce" wrote



No idea why that cap is so big. In combination with the 100k grid
bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.


Well, the cynic in me would wonder:

Audio Note 1uf 600v (copper) PIO = $69.60
Audio Note 0.1uf 600v (copper) PIO = $41.69

Though without starting a flame war, I would have suggested

Mundorf M-CAP® Supreme Silver & Oil 0.1uf 1200v = $23.45

But you could try

BC Components 2222 368 Series 100nf 400v = 13p


Thanks for the suggestions but the cap hasn't been proved to be faulty
yey - from what I can see of it?

(My suspicions still hover round all the extra crap between the 300B and
the meter itself..??)

Ping Don..
 

Keith G wrote:
"Don Pearce" wrote



Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.



OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)






Yes, assuming there is still the other connections to the grid pin.

Ok, so is the voltage on the grid still high?

It would help having a clue how the amp was wired, I am not sure just
how having adjustable bias fits with having a cathode resistor (ok it
can be a mix of both). And if it soes have adjustable bias I would not
have expected the grid to be a 0v.

Maybe a close up picture of the amp would allow us to see how the thing
is wired.

--
Nick

Ping Don..
 

"Nick Gorham" wrote in message
. uk...
Keith G wrote:
"Don Pearce" wrote



Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.



OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)






Yes, assuming there is still the other connections to the grid pin.


No, this is what's mystifying me - there's only the cap connected to the
grid pin. I don't see how all the crap connected to the heater pins can
alter/affect the grid voltage? (See pix below...)


Ok, so is the voltage on the grid still high?


No, only 1.8V or thereabouts (not steady).



It would help having a clue how the amp was wired, I am not sure just
how having adjustable bias fits with having a cathode resistor (ok it
can be a mix of both). And if it soes have adjustable bias I would not
have expected the grid to be a 0v.

Maybe a close up picture of the amp would allow us to see how the
thing is wired.


Do these help:

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG

??

Ping Don..
 

Keith G wrote:


Yes, assuming there is still the other connections to the grid pin.



No, this is what's mystifying me - there's only the cap connected to the
grid pin. I don't see how all the crap connected to the heater pins can
alter/affect the grid voltage? (See pix below...)


Ok, so is the voltage on the grid still high?



No, only 1.8V or thereabouts (not steady).



Ok, the grid will be floating, so the voltage you are reading will be
from grid current.


Do these help:


Yes, I guess you have disconnected the 100k resistor thats connected to
the cap. Either connect this resistor directly to the grid, or reconnect
the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.

Looking at that, it is using cathode bias, its just got a meter across
the cathode resistor.

That resistor looks like a vishay bulk film, seems a tad excessive for a
grid resistor. Ho, well, keeps in with using a big cap when a smaller
one would do as well.

--
Nick

Ping Don..
 

"Nick Gorham" wrote


Yes, I guess you have disconnected the 100k resistor thats connected
to the cap.

No, I hadn't...


Either connect this resistor directly to the grid, or reconnect
the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.


But disconnecting the *other* end of the cap has put the bias meter
*exactly right*!!!

:-)

Is that it then - the cap's fuct?



Looking at that, it is using cathode bias, its just got a meter across
the cathode resistor.


I would still like the meters out of the way, but not if they are going
to behave - that's a decision for someone else at another time!!


That resistor looks like a vishay bulk film, seems a tad excessive for
a grid resistor. Ho, well, keeps in with using a big cap when a
smaller one would do as well.


It's suppose to be 'PR's (you know who) best work...??

Ping Don..
 

Keith G wrote:
"Nick Gorham" wrote



Yes, I guess you have disconnected the 100k resistor thats connected
to the cap.


No, I hadn't...


Either connect this resistor directly to the grid, or reconnect

the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.



But disconnecting the *other* end of the cap has put the bias meter
*exactly right*!!!

:-)

Is that it then - the cap's fuct?

My money is on that.

--
Nick

Ping Don..
 

"Nick Gorham" wrote in message
...
Keith G wrote:


Is that it then - the cap's fuct?

My money is on that.


Well, your money is safe - I swapped the cap out of the Good Amp and put
it in the Bad Amp and the Bad Boy is singing like a good 'un as I type
and...

.....rushes to check again....

the meter is (dare I say it?) - *rock steady*!!

:-)

Well done Nick and many thanks (you too, Don) - with your brain and my
soldering iron and we could conquer the world!!

Of course, the Good Amp's pretty silent now - what we want now is a
*pair of caps*. What do you suggest - same rating? (Forget *bling* -
just gimme the specs and I should think Tony'll get summat from
Maplins!! :-)


Checks again - yep - still right on the same mark on the meter!!