Ping Don..
 

"Keith G" wrote in message
...

"Keith G" wrote

Additional:

*VDC* across the cathode resistors:

Good amp = 59VDC

Bad amp = 154 VDC

Does this add anything - certainly don't look right???


Additional additional:

Anode voltages (amps are warm now)

Good boy = 438 VDC

Bad boy = 312 VDC

Both steady....

??


Another additional additional:

The 'bad' amp *stinks* - Swim says like a 'Chinese amp' but much worse!!
:-)

(Summat's not happy there, that's for sure!!)

Ping Don..
 

On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them? That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes? Check the resistance of the
secondaries, and make sure it hasn't gone open circuit. That grid
voltage should only move with signal.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them?


No - all or any of the valves/rectifiers can be freely swapped to and
fro with no adverse effect, I'm told. Even the meters were swapped (at
my suggestion) and the bad amp stayed bad and the good amp continues to
work OK. (I will check this for myself a little later.)


That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes?


No. This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif


Check the resistance of the
secondaries, and make sure it hasn't gone open circuit. That grid
voltage should only move with signal.


Hmm....

What bothers me is that on my 2A3 amp there is only a single wire to
each of the heater pins (from the filament trannies, naturally) - on
this amp with the meters, there's a whole bunch of crap connected to
them and I'm lost to see how any adjustment can work which is ultimately
connected to the cathode/heater ???

These are the 300B connections:

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

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

Ping Don..
 

On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:


"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them?


No - all or any of the valves/rectifiers can be freely swapped to and
fro with no adverse effect, I'm told. Even the meters were swapped (at
my suggestion) and the bad amp stayed bad and the good amp continues to
work OK. (I will check this for myself a little later.)


That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes?


No. This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif


OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.

d
--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

Keith G wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?



Looks likely.

220uf seems to be a odd value coupling cap, I would have expected 0.22uf
or so. Looking at the picture, I would have thoulf what looks like a big
PIO would be the chap.

BTW, if its 0v, and you have a cathode resistor, then it will be cahode
bias (I would expect)

Track down the coupling cap and remove/replace it. Any old 0.22uf cap
will do to try, as long as its got a voltage rating of 500v or more.

--
Nick

Ping Don..
 

"Nick Gorham" wrote

Looks likely.


OK, I have transferred Don's response here to cover both bases in one
reply.


220uf seems to be a odd value coupling cap, I would have expected
0.22uf or so. Looking at the picture, I would have thoulf what looks
like a big PIO would be the chap.


OK (see pix).



BTW, if its 0v, and you have a cathode resistor, then it will be
cahode bias (I would expect)


Except there are the trimpots for adjusting the bias...??



Track down the coupling cap and remove/replace it. Any old 0.22uf cap
will do to try, as long as its got a voltage rating of 500v or more.


From Don:

OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.


Did that! :-)

Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

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


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

(Thanks to both of you in the meantime - I really would like *not* to
see this amp go off to the shop!! :-)

Ping Don..
 

On 2007-07-12, Don Pearce wrote:
On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:
... This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif

OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.

Seconded. The anode current in the bad amp looks like it may be
*significantly* more than the 100 mA maximum (if I read correctly
a 300B datasheet I found online).

154V / 825 Ohms = 187 mA (if the cathode resistors are actually as
in the circuit diagram at the link).

--
John Phillips

Ping Don..
 

On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

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


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

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.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

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


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

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.


OK. Sounds like a good ploy. I've had to clear the decks for tea and
have got some errands later, so it might be a while 'til I get back on
it! (The trouble with working on the *kitchen table*!! :-)



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


Don't matter - my dummy loads are **** transducers anyway!! :-)

Mora anon....

Ping Don..
 

On Thu, 12 Jul 2007 18:10:52 +0100, "Keith G"
wrote:


"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

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


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

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.


OK. Sounds like a good ploy. I've had to clear the decks for tea and
have got some errands later, so it might be a while 'til I get back on
it! (The trouble with working on the *kitchen table*!! :-)



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


Don't matter - my dummy loads are **** transducers anyway!! :-)

Mora anon....


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.

d

--
Pearce Consulting
http://www.pearce.uk.com