Questions about tubes in Marshall Power amp
j.h. jansen
I was wondering if someone could tell me more about my Marshall 50 + 50
stereo valve power amp model 9005.
I wanted to change the tubes on this amp, so I opened it and found four
ECC83m and four 6L6GB tubes. However I'm not familiar with the inside of a
tube amp, especially with the bias adjusting, so i found someone to do the
job for me. This person asks for a diagram and the bias-adjusting values
drawn from the wall. From the Marshall importer I've got a diagram send
over. However in this diagram I found no 6L6GB tubes but four EL34. In the
Series 9000 Handbook I found the following note concerning the 9005.
"When valve replacement becomes necessary use only high quality Marshall
EL34 and ECC83/12AX7 valves."
Another note if found was:
"Never mix valve (tubes) types. So if your amplifier has EL34 power valves
(tubes) you must always replace them with EL34s. If it has 5881 or 6L6
valves you must use those".
So I'm confused, my questions are:
-Which tubes do I have to take for an exchange, EL34 or 6L6s?
-Why did they send me a diagram with EL34 drawn, while I have 6L6 in it. Is
this amp an earlier or later model?
-Do the 6L6 tubes belong to my amp?
-Can the guy who's going do the job for me use this diagram?
-Can I use 5881 or do I have to stay with the 6L6?
-What are the correct settings for the bias? They are not in the diagram
Many Thanks
M.Jansen
CERREM
>I was wondering if someone could tell me more about my Marshall 50 + 50
>stereo valve power amp model 9005.
>-Can I use 5881 or do I have to stay with the 6L6?
>-What are the correct settings for the bias? They are not in the diagram
>
>Many Thanks
>M.Jansen
This is a sticky subject...because there are folks that think that whatever
Marshall says or does is the law.....according to my recollection this output
transformer made for them by Drake U.K. has always used EL34 valves from day
one except for 6550 valves sent to the US for a few years in late 70's to mid
80's.... This output was first introduced in 67 with a 3.4K plate load....
If you plot this load line based on the 3.4K plate load on 6L6 family of curves
with the screen voltage at 450 and plate voltage of 450v.... You will see a
PERFECT fit....the load line will cross the 0-bias curve just a hair above the
knee.... So it is my opinion that the 6L6/5881 valve will have no electrical
problems working in this amp....It is then just a matter of personal taste with
the tone...
When this load-line is plotted against EL34 valves in the same operating
condition...the load line does not intersect the 0-bias line BUT it slams into
the linear region of the pentode curves prior to the saturation
region....obviuosly not a nice fit BUT will limit power output ..ie AC voltage
swing.... and most of all this will produce lots of even order distortion....
Looks like they just grabbed whatever was cheapest and worked...
The biggest transfromer goof that sounded great was the JTM 45 which used KT66
valves.... The plate load was 8K...which is a gross mis-match of load line to
curves....The load line does not even make half the distance to the 0-bias
curve when it intersects in the current axis..... This makes for some serious
even order harmonics.... Whats funny is that the 3.4K plate load fits this amp
perfectly.... If you get the plate curves of a KT66 /6L6 valve with 250 volts
on the screen...you will notice the 8K load fits perfectly....Someone goofed
when they got the curve sheets when trying to fit this transformer to pentode
operation....This Drake output was originally intended for KT66/ 6L6 valves in
triode wired configuration for Williamson style amp and is intended for 30
watts Max..
I will not even get into core distortion or reactances of the transformer
models....
The botom line here is that Marshall doesn't have a clue..... But I am
glad...since the amps wouldn't have accidentaly turned out sounding the way
they do...CRUNCHY..
So you can use 6550, 6L6/5881, EL34 valves...
In fact Marshall use to put 6550 valves in the USA models...
You just need to bias the different valve type accordingly....
If you adjust each valve type to sink 40mA per valve in the amp you should be
fine....
CHEERS
CM
TROOBKA
>glad...since the amps wouldn't have accidentaly turned out sounding the way
>they do...CRUNCHY..
>So you can use 6550, 6L6/5881, EL34 valves...
>In fact Marshall use to put 6550 valves in the USA models...
>You just need to bias the different valve type accordingly....
>If you adjust each valve type to sink 40mA per valve in the amp you should be
>fine....
>CHEERS
>CM
>
>
>
I would not just dump a set of EL34s in without a few considerations. First is
the issue of the increased heater current draw. It may or may not be able to
handle it safely. Second if you decide to do this anyway the screen grid
resistors need to be changed to 5 watt 1K models to prevent thermal runaway. I
would bias them closer to 30ma and use a good quality tube. The easiest, but
most dangerous to the inexperienced tech is to connect the plate of each tube
(one at a time) to their respective center tap on the output transformer. Do
not adjust it by a negative number on the grid because all tubes do not pull
the same current. Make sure the tubes are properly matched as well.
The unit referred by you is an earlier model. Marshall had to use the 5881
because it was, at the time the only reliable thing available at the time.
Good luck and all the best,
Mark S.
James W. Rowell
j.h. jansen <j.h.j...@hccnet.nl> wrote in article
<7hsf58$e9q$1...@news.hccnet.nl>...
> Hello
> I was wondering if someone could tell me more about my Marshall 50 + 50
> stereo valve power amp model 9005.
>
> Many Thanks
> M.Jansen
>
>
>
>
>Mr. M Jansen
In regard to your questions about the Marshall 50 + 50, I am not familer
with that paticular amp but all Marshalls have a family resembalance.
During the early to middle 90's Marshall tube amp production switched over
to the American 6L6 usually Sovtek brands, from the traditional Marshall
tube the EL 34, when it appeared that the EL 34 would no longer be
avalible. This sitiuation has been recified, with several new EL 34 types
currently avalible. If your amp was manufactured during this period, it
came with 6L6's from the factory.
The 6L6 and EL 34 have the same pin basic pin configuration but need
several circuit changes inside the amp to function correctly. The bais
adjustment range is also different.
Therefore you need to continue to use 6L6's in your amp. It is very likely
that the 6L6 is original to your amp.
Your amp tech should be able to replace and perform a bais job with the
6L6's without a schematic, if he looks at you cross-eyed find a new tech,
although any info is potentially useful. The bias voltages are not printed
on the schematic, probably because they are different for every amp and
every set of tubes. The procedure is too detailed to discribe here, and
arguments as to the correct method can form to the left! Tubes are not
baised to a specific voltage reading at the grid of the tube in almost all
cases.
Any quality 6L6 type should work in your amp, this includes the 5881 which
is a military 6L6. I belive your amp came stock with Sovtek 5881's so they
might be a good choice, even though I personally don't really like them,
other types include the Sovtek 6L6 WXT, which I haven't tried yet. You
should really purchase matched sets, I am assuming a 50 + 50 would use a
pair of 6L6's on either side for a total of 4 tubes. I would buy a quartet
to ensure power levels are equal between each side of the amp.
Now to open a real can of worms, The 6L6 is originally an American
designed tube, popularized by the Fender line of tube guitar amps such as
the Super and Twin. The very earliest Marshalls also used 6L6's but soon
switched to the European designed EL 34. In my opinon the classic Marshal
rock sound is a good set of EL 34's, the 6L6's sound different. I don't
know if Marshall changed the circuit in terms of transformers and the like,
which would help to minimze the difference, but the tubes themselfs just
sound different. I tested a factory 6L6 Marshall several years ago and
remember being quite unimpressed (I have a European 1976 50 watt master
volume head with EL 34's as a reference) If you are interested in the
change talk to your tech, he should be able to preform this change as well.
Always leave service to qualified personel and If you poke around you are
taking your own risk. Information provided is not intended to be a repair
guide or a license to mess around with the insides of your gear. I am not
responsible for yours or anyone elses safety or or that of your amp. Use at
your own risk. Enough legal type stuff, good luck and I hope I was helpful,
please keep in touch.
Alan Thompson
not a guitar head. These never had 6550's in them. They originally had EL34's,
Marshall changed to 5881's (6L6 type) when there was a shortage of good EL34's, now
that there are an abundance of EL34's they have changed back to them. As has been
noted, you can use either tube with certain adjustments.
A.T.
CERREM wrote:
> >Hello
> >I was wondering if someone could tell me more about my Marshall 50 + 50
> >stereo valve power amp model 9005.
>