(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

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


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)

"Keith G" wrote in message
...

(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in
90 degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad
distortion kicks in I'm told. Any immediate ideas where to look as to
cause? I have both amps here and can make comparison measurements -
I'm told all the passives measure OK...??

Here they are *again*

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


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)


Actually, it's even worse than that - I would like to rip all the 'bias
meter' palaver out of 'em and change them to cathode bias.

(No idea how to do that, needless to say!! :-)

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

(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in
90 degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad
distortion kicks in I'm told. Any immediate ideas where to look as to
cause? I have both amps here and can make comparison measurements -
I'm told all the passives measure OK...??

Here they are *again*

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


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)



Actually, it's even worse than that - I would like to rip all the 'bias
meter' palaver out of 'em and change them to cathode bias.

(No idea how to do that, needless to say!! :-)


Again, depends on the circuit, you may not have enough B+ to do that.

--
Nick

"Nick Gorham" wrote

Again, depends on the circuit, you may not have enough B+ to do that.


OK - that's a *secondary objective* atm!

(The meters have been swapped and both seem to be OK in the 'good' amp!)

Keith G wrote:
(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday),

Chance would be a fine thing. I never got your email Keith. Anyway....

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

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


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!


Without knowing the circuit its a bit of a guess, but my first thought
would be to check if the coupling caps have become leaky. (assuming
there are any).

Check the voltage between the 300b grid and ground. I would expect
something like -70v. If the coupling cap is leaking it will be higher
than that.

Have you considered painting the valve bases and sockets so he may be
able to work out what goes where? Or faining that, strike hime on the
head with the nearest outout transformer and tell him to get a clue?

--
Nick

"Nick Gorham" wrote in message
. uk...
Keith G wrote:
(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday),

Chance would be a fine thing. I never got your email Keith. Anyway....


Direct response received and replied to!



If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop
for the odious techie *Andrew* to sort them out and I don't think I
could stand the 'loss of face'...!!


Without knowing the circuit its a bit of a guess, but my first thought
would be to check if the coupling caps have become leaky. (assuming
there are any).


He has replaced the two 100 uF caps which I deem to be the coupling caps
with a single 220 uF cap on the 'bad' amp and it has been fine for a
while...



Check the voltage between the 300b grid and ground. I would expect
something like -70v. If the coupling cap is leaking it will be higher
than that.


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?



Have you considered painting the valve bases and sockets so he may be
able to work out what goes where? Or faining that, strike hime on the
head with the nearest outout transformer and tell him to get a clue?


Oh, he's learning!! - It doesn't help the bloody amps are *handed*!! :-)

They're both still on - are we doing further damage??

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

"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

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

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