Re: Sylvania El34's made in Great Britain? A question (6CA7 bias current).



On Sun, 27 Nov 2005 04:17:50 +0000, Patrick Turner wrote:

>
>
> bart wrote:
>
>> On Sat, 26 Nov 2005 15:42:04 +0000, Patrick Turner wrote:
>>
>> >
>> >
>> > Andy Cowley wrote:
>> >
>> >> pfjw@xxxxxxx wrote:
>> >>
>> >> > *Gleep*
>> >> >
>> >> > 6CA7/EL34/KT77 are all "Beam Power Tubes". Whether American, British,
>> >> > German, Chinese, Martian or Pomeranian.
>> >> >
>> >> > As are KT88/6550s, EL84/6BQ5s, KT66/5881/6L6s and a whole range of
>> >> > other similar-application tubes.
>> >> >
>> >> > It is what they are, pretty much that simple.
>> >> >
>> >>
>> >> *Plonk*
>> >> Nonsense. The Philips/Mullard EL34 and 6CA7 are true pentodes.
>> >> They didn't have to pay the RCA royalty for the beam tetrode
>> >> patent, it's pretty much that simple.
>> >
>> > There was such a thing called a beam power pentode,
>> > which acted like a pentode but which focused the electron stream into beams
>> > anyhow.
>> >
>> > But one thing for sure is that 6BQ5 and EL34 were REAL power pentodes.
>> >
>> > The 6CA7 I have in front of me marked made in the USA
>> > is a beam tube with beam forming plates even though the data in the
>> > RCA tube manual shows G3 drawn as any other grid.
>> > Even 6V6 which *are* beam tubes have the same G3 drawn on the pin out
>> > sketch.
>> >
>> > The 6CA7 made by Sovtek and sold now as EH6CA7 is a copy
>> > of the american tubes I have, and perhaps made by Sylvania; I have a pair
>> > in an old amp in my shed for testing speakers.
>> >
>> > The 6CA7 has in fact a higher dissipation limit before cherry red glows,
>> > and is really a poor man's 6550, although it won't pull the same max Ik.
>> > However, it might be able to be pushed into A2 or AB2 like a 6L6
>> > or 807 lsince it is a beam tube; this cannot be tried with EL34.
>> > The 6CA7 can be unstable in circuits meant for EL34.
>> >
>> > Patrick Turner.
>> >
>> >
>> >
>> >>
>> >>
>> >> Andy
>>
>> Howdy!
>>
>> I've recently aquired a quad of Syvania 6CA7's and notice that the grid
>> current ramps up while stabilizing after power up. The grid current draw
>> per pair goes as high as 300ma and then settles down after about 1 minute.
>> Is this normal for this tube or are they showing signs of age?
>
> I assume the grid current you are measuring is with no signal, ie, idle
> conditions.
>
> Its impossible to get 300mA of g1 current.
>
> How are you measuring the grid current?
>
> If the grid bias resistance was say 100kohms, 300mA means there would be
> an enormous voltage across the 100k R.
>
> Ig = dcV across Rg / Rg in ohms.

The Dynaco has -55v supplied to a series resistance chain with it's bias
pots in the center.

IE: -55v --> 10k --> bias pots (parallel 10K) --> 10K --> ground.

from the center tap on the bias pots to a divider (two 270K) and to the
grids on the tubes. The 2 tubes' anodes are in parallel with a 10 ohm
resistor to ground. The voltage at this point with 100ma bias is 1 volt. I
see the voltage go upwards of 300ma.

For full details I reference pages 8 and 9 (of 16):

http://www.curcioaudio.com/st7_mnl.pdf

The original circuit used a 15.6 ohm resistor to achieve 1.56v @ 100ma at
the anode resistor. This was to allow a "fresh" D-cell battery to be used
as a reference for calibration of the builder's VTVM. DMM's are a bit
more accurate and 15.6 ohm resistors are not as common as 10 ohm ones.

>
>
>
>
>> My set up is a Dynaco St-70 with the bias set at 100ma for each pair.
>>
>> My EL-34's (Svetlana, Electro Harmonix), when used in the same amp
>> don't exhibit this behavior.
>
> Pehaps the 6CA7 are oscillating at some radio frequency but it still
> doesn't explain what your symtoms are.
>
> But the tubes can be stabilised if you have to.
>
> Patrick Turner.


When the Svetlana EL-34's are used in the same circuit, the voltage
measured at the anode resistor steadily rises to 1 volt.

When the Sylvania 6CA7's are used in the same circuit, the voltage
measured at the anode resistor steadily rises to 3 volts and then drops
back down to 1 volt.

My apologies for not being clear about how I'm deriving my readings.

-Bart
.



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