Biasing a Marshall JCM 800 2203ZW Zakk Head
Kurt Fennig
information
along with my Groove Tube bias kit manual to measure the cathode
current
using a bias probe through one of the 6550 tubes in my JCM 800 2203ZW
zakk wylde head. The tubes are stock, have had less than 10 hours of
play, and no previous biasing has been done on the amp.
I took the following measurements using the bias probe and multimeter:
Plate voltage (pin 3 to 8): 425V
Current: 24mA (factory set)
Using P=IV we have 10.2 W. I have read that 6550 tubes are rated for
max
power dissipation of 27W (according to LV), and the Groove Tubes
manual states a max of 42W. Seems like the head is running pretty
cold at 10W!
So... I opened up the head and turned the bias potentiometer. The
Groove
Tubes manual recommended a current of 54mA for 100W marshalls fitted
with
6550 tubes (23W dissipation). Upon turning the pot, I noticed that
the
max current I could get was ~35mA (14.9W dissipation) which still
seems
cold.
My question to the newgroup is as follows:
Does this all seem correct? Are the JCM 800 fitted with 6550 tubes
biased for 24-34mA, or should I be getting higher readings? The Groove
Tube bias kit manual recommends biasing 100W marshall fitted with 6550
at 54mA (23W dissipation which is more in line with 27W/42W).
I have biased it temporarily for 30mA and it sounds much better to my
ear.
However, I'm concerned that the pot maxes out at 35mA, which is far
from 54mA
(nominal operating cathode current for 6550 as suggested by Groove
Tubes).
Did Marshall do this intentionally to increase the life of 6550s in
the
amp?
BTW, I understand that running the tubes -hot- (lots of current) may
muddy
things up, and running them too cold (minimal currents) may make the
amp
brittle. I also understand that running them cold increases their
life,
etc.
Thanks in advance,
Kurt
Lord Valve
Kurt Fennig wrote:
Lord Valve Speaketh:
Often, the bias supply will be too *hot* for many
6550s. (In other words, the coldest setting will be
too hot.) When this (or your) condition is encountered,
the easiest way is to decrease (in your case, increase)
the value of the bias supply input resistor, which is in
series with the bias diode. (Resistor value increase =
less AC voltage applied to bias diode = less negative
supply = hotter settings possible. Resistor value
decrease = more AC voltage applied to bias diode =
more negative supply = colder settings possible.)
15W static is indeed on the cold side for 6550s, but
*if it sounds good to YOU* (which is, after all, the crux
of the matter) then by all means, leave it that way. You'll
gain some tube life.
You may also want to consider that the set of tubes
you have may have been pulled from the lower end
(plate-current-wise) of your dealer's stock, and may
simply want more a lower negative voltage than the
supply (as presently configured) can deliver. A simple
resistor change will fix things right up.
Lord Valve
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Steve Cowell
"Kurt Fennig" <kurt_...@telus.net> wrote in message
news:17ae078.03010...@posting.google.com...
...>
> Seems like the head is running pretty
> cold at 10W!
You need to change a single resistor to bring your bias
voltage in line... often you can parallel the resistor
into the circuit to bring the bias voltage down. This
would be the resistor that pulls the bias to ground,
probably... need your schematic to be sure. If it's a
10K, you could probably start by paralleling a 33K
to it (carefully!) on the top side of the board. This
is where a decade box comes in handy (or external
pot jumpered in).
This is a common change, one that techs often
have to do to accomodate different sets of tubes.
__
Steve
.
Gilbert Bates
before you start modding the bias supply.
The Zack Wylde head *comes* with 6550's, you indicate everything is
*factory*, stock condition correct? Marshall seems to run everything
on the hot end so something leads me to an observation.
Plate voltage at 425? *Most* newer Marshalls have around 475-500V on
the plates, except for the some of the reissues like the JTM45, when
operated in the US. Are you in Europe, which could explain the lower
plate voltage? The line voltage hovers around 120-123V at my house
most of the time, what is it at yours?
alann
kurt_...@telus.net says...
> I have read Lord Valve's paper on biasing and have used his
> information
> along with my Groove Tube bias kit manual to measure the cathode
> current
> using a bias probe through one of the 6550 tubes in my JCM 800 2203ZW
> zakk wylde head. The tubes are stock, have had less than 10 hours of
> play, and no previous biasing has been done on the amp.
>
> I took the following measurements using the bias probe and multimeter:
> Plate voltage (pin 3 to 8): 425V
> Current: 24mA (factory set)
>
That plate voltage seems a bit low.. My 2204 ( 80's vintage ) runs about
479 on the plates. I would suspect that your's should be higher, but
since these are reissues, maybe they changed something.
Jerry
Jerry
Kurt Fennig
> The other posts are fine but I'd like to suggest some other items
> before you start modding the bias supply.
>
> The Zack Wylde head *comes* with 6550's, you indicate everything is
> *factory*, stock condition correct? Marshall seems to run everything
> on the hot end so something leads me to an observation.
>
> Plate voltage at 425? *Most* newer Marshalls have around 475-500V on
> the plates, except for the some of the reissues like the JTM45, when
> operated in the US. Are you in Europe, which could explain the lower
> plate voltage? The line voltage hovers around 120-123V at my house
> most of the time, what is it at yours?
>
Everything's stock with 6550s and no previous biasing done. Came
straight from Marshall with the measurements I mentioned. I'm
in Canada... I read a line voltage of 117V from my outlet.
I wouldn't be surprised if Marshall just -eyeballs- the bias pot
in the factory, throws in a set of Svetlanas and ships.
Oh, I should ask the newsgroup... as an aside, I noticed a second
potentiometer that seemed to be connected to some 100 ohm resistors,
just next to the 4 diodes in the power supply rectification circuit.
Any idea what that pot is for? I think it's labelled VR2... I could
not locate it in a 2203 schematic. Perhaps it is involved in setting
the plate voltage?
Vinny
2203? when playing through it, is it doing anything strange (noise, hum,
unusual sound) to make you think something may not be right? Or are you
trying to bring out some special "magic" that could be in there, thereby
changing something? STG i'm not trying to be a smartass, i'm asking an
honest question here. Vin
andrewunix
: I have a question regarding all this: Is there anything wrong with the
Current Marshalls these days ship from the factory biased so cold that
they don't sound as good as they can. It's pretty much a good idea on any
brand new tube amp these days to check the bias and make sure that it's in
a reasonable operating range. The manufacturers do this to cut down on
warranty returns due to tube failure.
--
agreenbu @ nyx . net andrew michael greenburg
http://www.nyx.net/~agreenbu/
Kurt Fennig
Hey Vinny,
There is nothing wrong with the 2203. No strange noise, hum, or anything.
I was just trying out my new GT bias probe that I got for Christmas, to
see how the bias setting was on the 2203. I did notice that increasing
the current from 24mA to 30mA made a noticeable difference in the warmth...
which to my ear sounds better.
Cheers,
Kurt