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)

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)

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

"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.

"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!! ;-)

"Keith G" wrote
Damn good question! We've either been *very* selectively burgled or

In 1997 I got home, and after some time, maybe a day or so, realised
that all my 80s New Romantic and goth CDs had gone. The others were
all still there. Nothing else was missing as far as I could tell.

*Strange*

Then I started using *both* locks on the door instead of just relying
on the Yale one.

One week later to the day, I got home to find my door kicked in,
*ALL* my CDs gone (BTW, I'd already been out and re-bought quite a lot
of the previously-burgled ones), my Spendor SP1s gone, and my CD player.

Not. A. Happy. Bunny.

This is how I learnt about the thing called "Insurance"; the hard way.



Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

Keith G wrote:
"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!! ;-)



As said the normal choices are cathode bias (self bias) where the bias
voltage is developed by tying the grid to ground and rasing the cathode
to a positive voltage by inserting a resistor in the cathode circuit.
The current through this resistor creates the bias by lifing the cathode
to a positive voltage, so the grid tied to 0v is then nagative to the
cathode. This method has the advantage that its self correcting, as the
valve characteristic alters, and (for example) passes more current, the
higher current produces more bias voltage and tends to reduce this
current. The cathode feedback while useful for the DC operating point,
will reduce the ability of the valve to amplify, so the cathode resistor
is bipassed with a cap (normally electrolytic) to mean that the cathode
feedback only happens at DC, and at AC the valve can amplify as
expected. The disabvantage is the prescence of the cap, and that the
power output is reduced for a given B+ as some of the voltage is taken
up by rasing the cathode.

The other method fixed bias (odd name, as the bias is adjustable). The
cathode of the valve is connected directly to 0v, so no bipass cap is
needed, and no B+ is wasted. But the bias is then provided by connecting
a adjustable negative voltage to the grid of the valve. To allow the
bias current to me measured, a small resistor is inserted into the
cathode circuit, so the bias current develops a voltage across the
resistor which can be measured allowing the current to be set by
altering the grid voltage. The disabvantages of this method are that the
grid supply must be well regulated and quite, and as the valve
characteristic alter the grid current will alter without any controlling
feedback, so the current needs checking from time to time.

Its also possible to combine both methods, use a external fixed bias
voltage, and a smaller cathode resistor to avoid having to trim the bias
for each valve. As it happens, this Keith is what the WAD Kit88 you have
uses.

One thing I was impressed with was Keiths Dynaco's that had fixed bias,
and a cathode resistor, such that at the correct bias point, the test
voltage would be exactly the same as a new D cell. meant that the bias
could be set very accuratly without needing to have a accurate meter,
just make sure it gave the same reading as the cell, and it was correct.

Sorry for the bit of a waffle for those that already know this, or dont
care :-)

--
Nick

"Nick Gorham" wrote

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).


Thanks for responding. Did that - see my new post "Bias adjustment II 'The
Reckoning' for details of the measurements.

(Will speak to you later/tomorrow am. ;-)

"Mike Gilmour" wrote in message
...
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.


Thanks Mike - I will email the supplier (Germany) on Monday.

See my new post "Bias adjustment II 'The Reckoning' for details of the
measurements.

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