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Old 18th February 2008, 02:30 AM   #1
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Default EL34 and KT88??? 6550?

A lot of schematics say either el34 or kt88 can be used. is this true for all amps using either tube? also what about the 6550?

For example, my guitar amp uses 2 el34's. Can I plop 2 6550's in there place with no problems or does it totally depend on the circuit. (Thats not really what I want to do, but I am very curious about it).

The real reason im asking is Im looking for a fairly simple amp project to use 2 6550's for power and hopefully 2 6dj8's for pre. This will be my first high voltage amp as I've already built a few lower voltage "hybrid" amps.
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Old 18th February 2008, 02:52 AM   #2
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Quote:
A lot of schematics say either el34 or kt88 can be used. is this true for all amps using either tube? also what about the 6550?
NO! The KT88 and 6550 draw a little more filament current than the EL34. Some of the current production versions draw almost 2 amps. The filament winding must be capable of the additional current.

These tubes bias up differently also. In many amp designs one tube may draw quite a bit more plate current than another without readjusting the bias. The optimum load impedance will also be different for each tube. It is possible that swapping tubes in an amplifier not designed for the swap could cause instability and possible damage.

It is possible to design an amplifier that works with all three tubes, but this is not always the case. I routinely swap between a 6L6GC, an EL34, a KT88 and the 6550 tubes in my SimpleSE amp, but the amp was designed with a higher than "normal" load impedance and the operating point chosen to work with all 4 tubes.
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Old 18th February 2008, 03:58 AM   #3
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Quote:
Originally posted by tubelab.com


NO! The KT88 and 6550 draw a little more filament current than the EL34. Some of the current production versions draw almost 2 amps. The filament winding must be capable of the additional current.

EEKKKKKK!
Are they reasonably consistent from a single manufacturer though? (i.e. series connected filaments).
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Old 18th February 2008, 07:24 AM   #4
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Another reason why it can be wrong to use beam tetrodes (e.g. 6550, KT88) in an amp designed for pentodes (e.g. EL34) is that pentodes tolerate a much higher grid-ground resistance than beam tubes. A high value of grid-ground resistance makes pentodes easier to drive but with beam tubes it can cause the bias to be unstable, leading to plate current runaway and damage.
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Old 18th February 2008, 04:10 PM   #5
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Quote:
Originally posted by tubelab.com


Quote; It is possible to design an amplifier that works with all three tubes, but this is not always the case. I routinely swap between a 6L6GC, an EL34, a KT88 and the 6550 tubes in my SimpleSE amp, but the amp was designed with a higher than "normal" load impedance and the operating point chosen to work with all 4 tubes.
I tried this in an emergency and will work for most KT88's and 6550's, but not really suitable for EL34's. The screen drive on EL34's likes 1K in UL mode.
When using Svet 6550C's into KT88 sockets these will show roughly 10-15% less quies in fixed bias. However there are large variables within this tube group. By and large, poke for poke will be similiar.-A caveat is the grid leak resistor, modern 6550c's can use same values as for KT88 but the earlier NOS 6550A suggests 50K max.That will load the driver stage into producing more thd.
EL34's driving into a o/p load fit for KT88's will give poor upper audio b/w power (10Khz square wave shape suggests non optimum Z).
Bear in mind.. pin 1 on EL34's is g3 and others is shield grnd. In addition, the 6550 group in UL mode is tailored for 470V B+ max working whereas the 88's can rip over 500V.
For lower voltages, the 6550 can provide the same performance as the KT88 at signifigantly lower cost.

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