Bias adjustment??
 

OK, another one for all you who haven't managed to get a box at the opera
tonight:

My recent Chinky EL34 amp acquisition comes with fixed bias and trimpots and
no damn instructions. I don't know - is the *bias* fixed by the valve type,
the circuit (which I suspect it does) or by this figure which all four
valves are (rather conveniently) labelled with:

'1a: 31 mA' and 'S: [nothing]' - (wossat one?)

as per:

http://www.apah69.dsl.pipex.com/keit...pychinky08.jpg

.....and any idea what the voltage might be and would it be + or - ???


I'm not yet au fait with the thermionic ping-pong games that actually go on
*inside* valves - only the broadest principles. (The only other amps I've
got that require bias adjustment rather conveniently have the voltage
printed on the chassis!!)

TIA

(out now, back later)

Bias adjustment??
 

The labels are just test measurement labels for selecting matched or quad
pairs out of their valve bins. Email the supplier for correct readings for
your amp as it all depends on the resistance value of the output valves
cathode resistors as to what your actually measuring in relation to the
current through the valve.

If the bias is read from that point you can always see what the value of
cathode resistance are and use ohms law to calculate the anode current from
the reading you get on your multimeter at the test points... EL34's aren't
they usually arround 50mA? Its so long since I've used '34's ;-)

Mike

"Keith G" wrote in message
...
OK, another one for all you who haven't managed to get a box at the opera
tonight:

My recent Chinky EL34 amp acquisition comes with fixed bias and trimpots
and no damn instructions. I don't know - is the *bias* fixed by the valve
type, the circuit (which I suspect it does) or by this figure which all
four valves are (rather conveniently) labelled with:

'1a: 31 mA' and 'S: [nothing]' - (wossat one?)

as per:

http://www.apah69.dsl.pipex.com/keit...pychinky08.jpg

....and any idea what the voltage might be and would it be + or - ???


I'm not yet au fait with the thermionic ping-pong games that actually go
on *inside* valves - only the broadest principles. (The only other amps
I've got that require bias adjustment rather conveniently have the voltage
printed on the chassis!!)

TIA

(out now, back later)

Bias adjustment??
 

Mike Gilmour wrote:
The labels are just test measurement labels for selecting matched or quad
pairs out of their valve bins. Email the supplier for correct readings for
your amp as it all depends on the resistance value of the output valves
cathode resistors as to what your actually measuring in relation to the
current through the valve.

If the bias is read from that point you can always see what the value of
cathode resistance are and use ohms law to calculate the anode current from
the reading you get on your multimeter at the test points... EL34's aren't
they usually arround 50mA? Its so long since I've used '34's ;-)

Looking at these curves, I would have expected a bit more than 50ma for
HiFi use, maybe 50ma for AB1.

http://www.mclink.it/com/audiomatica/tubes/el34.htm

but as you say, it all depends on the resistor you are measuring the
voltage across.

Keith, what you could try, if you can't get the maker to give the info,
is measure the resistance between the test point and earth, that should
give the resistance you are measuring, so from that you can find what a
particular voltage means in terms of current (do this with the amp off).

--
Nick

Bias adjustment??
 

Keith G wrote:

My recent Chinky EL34 amp acquisition comes with fixed bias and
trimpots and no damn instructions. I don't know - is the *bias* fixed
by the valve type, the circuit (which I suspect it does)

A bit of both, as I understand it.

Specs on the web say that the bias resistor (between cathode and ground)
should be around 470R, giving a bias voltage of -28V (at the signal grid).
Put a voltmeter between ground and pin 8 of each octal valve base (valves in
place), and see if you get something like +28V.


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment??
 

"Nick Gorham" wrote in message
...
Mike Gilmour wrote:
The labels are just test measurement labels for selecting matched or quad
pairs out of their valve bins. Email the supplier for correct readings
for your amp as it all depends on the resistance value of the output
valves cathode resistors as to what your actually measuring in relation
to the current through the valve.

If the bias is read from that point you can always see what the value of
cathode resistance are and use ohms law to calculate the anode current
from the reading you get on your multimeter at the test points... EL34's
aren't they usually arround 50mA? Its so long since I've used '34's ;-)

Looking at these curves, I would have expected a bit more than 50ma for
HiFi use, maybe 50ma for AB1.

http://www.mclink.it/com/audiomatica/tubes/el34.htm

but as you say, it all depends on the resistor you are measuring the
voltage across.

Keith, what you could try, if you can't get the maker to give the info, is
measure the resistance between the test point and earth, that should give
the resistance you are measuring, so from that you can find what a
particular voltage means in terms of current (do this with the amp off).

--
Nick

Keith, As a matter of interest what DC meter readings do you get from each
of the testpoints with your meter on the mV range? (with no signal input
going through the amp i.e. no music) Its usually the negative meter probe to
the chassis and the positive meter probe to the amp testpoints in turn
noting down each reading to each valve.

Bias adjustment??
 

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

My recent Chinky EL34 amp acquisition comes with fixed bias and
trimpots and no damn instructions. I don't know - is the *bias* fixed
by the valve type, the circuit (which I suspect it does)

A bit of both, as I understand it.

Specs on the web say that the bias resistor (between cathode and ground)
should be around 470R, giving a bias voltage of -28V (at the signal grid).
Put a voltmeter between ground and pin 8 of each octal valve base (valves
in
place), and see if you get something like +28V.


--
Wally
www.artbywally.com
www.wally.myby.co.uk



Fixed bias? with bias adjust pots I would expect the cathode resistor to be
lower at around 10 to 50 ohms. But I stand to be corrected :-)

Bias adjustment??
 

Mike Gilmour wrote:

Fixed bias? with bias adjust pots I would expect the cathode resistor
to be lower at around 10 to 50 ohms. But I stand to be corrected :-)

I ain't no expert, but I believe it's the voltage that counts. :-)

I'm assuming his outputs are push-pull, and not too dissimilar to my
nominally-20W AB1 amp, which uses 470R (no trimmers). My 50W guitar amp has
a bias supply (cathodes to ground), and the schematic specifies -32V at the
signal grids. If Keith's amp is higher o/p than mine (40W, Keith?), maybe he
should be looking for something like 30V?


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment??
 

http://tdsl.duncanamps.com/show.php?des=EL34

Have a look at the above. In fixed bias, you should be seeing something like
-30 to -35 volts on grid one, which is pin 5. Pin numbers are allocated from
the underside of the valve base. If you think of the little indent that the
valve bases' guide goes into as zero, then go clockwise around the underside of
the base and count 1,2,3,4,5,6,7,8 and then you're back at the indent.
If it's fixed bias of the usual kind, it'll either have no cathode resistor or
something small like 10 ohms for measurement purposes. (the 470 ohm cathode
resistor would be for self-bias)
Now, I don't know how your amp is constructed so the above is a 'guesstimate'.


=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment??
 

Andy Evans wrote:

If it's fixed bias of the usual kind, it'll either have no cathode
resistor or something small like 10 ohms for measurement purposes.
(the 470 ohm cathode resistor would be for self-bias)

Ah, self-bias - *that's* what I was talking about! ;-)


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment??
 

"Mike Gilmour" wrote

Keith, As a matter of interest what DC meter readings do you get from each
of the testpoints with your meter on the mV range?


Damn good question! We've either been *very* selectively burgled or the
bloody thing's left home without leaving a forwarding address!!

I've only just got in and I've an appointment with a curry in aboot 5
minutes, so I'll try and find it and get back here later. (The amp's warming
up.)

(Needless to say, we *don't* have a box at the opera tonight!! ;-)