EL34 vs 6CA7: high plate voltages
eme...@elwha.evergreen.edu
I've seen people express concerns about using EL34's in amps designed for
6CA7's due to the much higher plate voltages sometimes encountered.
1)
For no informed reason, I've come to associate "6CA7" with the large
envelope types typical of the US manufacturers, and "EL34" with the
slender european varieties. So when a mullard "EL34" is labeled as a
"6CA7/EL34", I am skeptical as to whether the tube is REALLY designed to
support the same plate voltages as the US 6CA7s,... Is there any reality
behind my unguided intuition?
2)
What would typically be the outcome of using an EL34 in a 6CA7 circuit?
Specifically, I have some british GEs (==mullard?) that I'd like to use in
a musicman guitar amp that uses 700V! Would this degrade the tubes in
extremely short time? I love the sound of these- so much so I don't want
to trash them.
--Ethan
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Gruvmyster
The EL34 was designed by Mullard in the early to mid 1950s. The original
spec calls for them to handle 800 plate volts, though not many circuits
to my knowledge ever approached that. Early Marshall amps with EL34s
put, I think, 525 volts on the plates of these Mullards. Angela
Instruments, in last year's catalog, said 450v was tops.
Music Man designed their 6CA7 amps around the Sylvania version, from
what I've been told. I spoke with a well-known guitar amp manufacturer
who told me these tubes were the guts from Sylvania's 6550, pinned as an
EL34, in a 6L6GC bottle. If that's the case, the 6550 had a max of 660
plate volts.
According to the spec of 800 plate volts on either the 6CA7 or EL34 (the
main difference being that the EL34 is a true pentode and all the 6CA7s
I've seen are beam tetrodes) you theoretically shoud have no problem in
your Music Man. If your Music Man is truly pushing 700v (and I
understand MM did push things pretty hard), it is, IMHO, not leaving
enough margin even at the 800v spec. If it was me, I'd buy some cheaper
and more readily available Siemens, or Svetlanas, to try first before
frying my Mullards.
Doug
--
"Think think think"- Winnie the Pooh
The opinions expressed are not really
opinions at all. They're facts.
Ernst C. Land, Jr.
In article <8504410...@dejanews.com>, eme...@elwha.evergreen.edu
says...
>
>
>What would typically be the outcome of using an EL34 in a 6CA7 circuit?
>Specifically, I have some british GEs (==mullard?) that I'd like to use in
>a musicman guitar amp that uses 700V! Would this degrade the tubes in
>extremely short time? I love the sound of these- so much so I don't want
>to trash them.
Sylvania says the EL34 will max at 800v (design center rating system) plate,
while pentode connected. Max cathode current is 150ma in this mode.
The tube style indicated is the "MT8", which is the familiar slender,
euro-style tube.
-Ernst
Winky1005
I have Sylavania 6CA7s and 6550s. Another posting points out that the
6CA7 is the same plate as the 6550, which certainly seems correct based on
the appearance. The 6550 is rated for 42 Watts plate dissipation at 650
Volts. Since the two plates are identical (BTW, the wafer base of the 6550
improves the dissipation rating), the higher plate voltage rating of 800
Volts for the 6CA7 would derate the dissipation (higher voltage causes the
electrons to strike the plate with greater velocity and greater kinetic
energy = heat). The 6CA7 should handle 700 Volts without problem. Keep
the plate current below 36mA per tube (25W/700V).
I compared the bias voltage for the 6CA7 and 6550 in my amp which is
screen voltage regulated pentode at 400V and about 495-500V B+ with the
following results:
6550
Bias = -45.40V Current = 41.25mA
Bias = -44.60V Current = 45mA
6CA7
Bias = -33.33V Current = 41.25mA
Bias = -32.50V Current = 45mA
The control grids on the 6CA7 are apparently very different (? narrower
spacing) in comparison to the 6550 and give it very similar biasing to the
EL34 (Amperex data sheet shows -36V grid 1 bias with Class B P-P 500V/400V
and 30mA plate current).
Winky1005
>Sylvania says the EL34 will max at 800v (design center rating system)
plate,
>while pentode connected. Max cathode current is 150ma in this mode.
Max cathode current is 150mA, but not at 800V.
Max cathode curent with 800V on the plates is 32.5mA. (25W / 800V)
David Crittle
I have had some hassle with HIGH plate voltages on EL34 valves too.
Here in Australia, we have in the past imported lots of stuff from the
UK, lots from USA, and who ever else would buy our iron ore and wool
and whatever else we dig up. Right now our biggest trading partner is
Japan.
So we don't really know if we should call them VALVES or TUBES or EL34
or 6CA7 or whatever. The Mullard valves 'skinny bottle' exported to
Australia carried both names, and most books refer to them as the
same.
I work on guitar amps, and there is one local brand of amp that I
hate...horrible 80 watt things, early seventies, that whack 800 volts
on the plate and seem to think it is OK. Well, it probably was OK for
the old Mullard, but a lot of new tubes just down cut it. So I see a
lot of these come in for repair. I put a couple more resistors in and
bias them a bit cooler. Seems to work, but I still hate them.
Yet I can see what the designers did. They wanted a bit of power, so
they looked up the valve handbook. It says this:
Anode 750 v
Screen 375 v
Anode current 84 mA each
90 watts out
Looks ok on paper. Lots of cherry red plates if the bias is wrong....
BTW, I have full specs (15 pages) on type EL34 if anyone wants a copy.
--
Yours faithfully
David Crittle
email: retr...@wagga.net.au
ktone
>What would typically be the outcome of using an EL34 in a 6CA7 circuit?
>Specifically, I have some british GEs (==mullard?) that I'd like to use in
>a musicman guitar amp that uses 700V! Would this degrade the tubes in
>extremely short time? I love the sound of these- so much so I don't want
>to trash them.
Even though your Music Man amp has 700 V plate voltage it doesn't mean
that the tubes have that potential across them. The first couple years MM
used a tube (12AX7) phase inverter/driver circuit but dropped it and went
to a circuit using an op amp as a phase inverter and a high voltage
transistors on each cathode as drivers. These drop about 65 volts, which
raises the cathode by that much with the control grid at ground potential
for bias. These transistors add about 15 watts to the output of the amp.
Later MM dropped the 6CA7 and started using 6L6GCs. Your amp could have
been made in that transition era so if it has no driver tube then the
working plate voltage is more like 635.
____________________________________________________________
"Risk your life on every note" C. Haden
............................................................
David L. King | kt...@chitown.com
Chicago Psyberview-Music and Arts | http://www.chitown.com
............................................................
Rhythm City - Dance/R&B | http://www.mcs.net/~ktone/rc.html
____________________________________________________________
P.P.H. Bauwens
Winky1005 <wink...@aol.com> wrote in article
<19961216072...@ladder01.news.aol.com>...
Well, max cathode current depends on the cathode construction and surface
and should never be exceeded, but is _independent_ of anode dissipation.
Of course it is true that the _average_ cathode current must be on the
order
of 32.5 mA in order not to exceed plate dissipation at 800V but that
is a different ballgame. The peak cathode current certainly exceeds 32.5 mA
when driving the tubes in class AB/B
Regards,
--
Paul Bauwens
Rietkerkweg 364
3066VL Rotterdam
Tel: +31-10-4211858
Email: paul.b...@tip.nl
Ernst C. Land, Jr.
In article <19961216072...@ladder01.news.aol.com>,
wink...@aol.com says...
>
>>Sylvania says the EL34 will max at 800v (design center rating system)
>plate,
>>while pentode connected. Max cathode current is 150ma in this mode.
>
>Max cathode current is 150mA, but not at 800V.
>Max cathode curent with 800V on the plates is 32.5mA. (25W / 800V)
>
Whoops! right you are.
O'Connor
David is mistaken: The Musicman amps that applied 700V to the plates of
the 6CA7s always had a conventional tube splitter. So, yes, at idle there
is 700V across the tubes.
However, the supply was a voltage doubler type that exhibits quite a drop
at full output, so things are not as bad as they seem after all. If you
want to extend the life of your tubes, use the low power setting. This
selects a different primary tap and drops the voltage down around 450V.
Kevin
DougBC2
You can also modify the power supple from a cap input to a choke input
which will lower the final voltage substantially plus give better
regulation.
Doug