[AMRadio] Push-Pull Parallel Tubes In Modulator

[Jim candela]

	Don,

	I have a couple of TZ-20's NIB somewhere...think trade for curve tracer...

	Anyway I have a modulator built up with 4 type 808's in PPP driven with a
WRL GK 500 interstage transformer, and then by a pair of 6B4's in PP. I ran
the 808's class B with about -35 volts bias (from memory), and 2250 on the
plates. They happily put out 600 watts of real clean audio. I did nothing
special, and the tubes seemed to match based on the amount of red glow from
the 4 plates when driving to maximum output with a sine wave audio source.
In a dark room, the plates were just starting to glow, and the 4 tubes
seemed to be sharing the load well based on visual measurement of plate
glow. I used no parasitic resistors either. For the output transformer I
used a Freed 125 watt transformer with DC through three 8 Henry chokes in
series, and a 4 uf DC blocking capacitor. The Freed transformer with no net
DC took everything those 808's could dish out, and all the way down to 200
hertz at full output. Someday I'll get that rig back on the air....

Regards,
Jim


-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of Merz Donald S
Sent: Wednesday, December 03, 2003 12:17 PM
To: Old Tube Radios; 'Amradio (E-mail)
Subject: [AMRadio] Push-Pull Parallel Tubes In Modulator


I have had this modulator chassis sitting around here that I was fiddling
with over the weekend. It came to me with no tubes. It uses 4 tubes in
push-pull parallel. The mod transformer is a UTC VM-3 rated at 125 watts.
The VM or "Varimatch" series of UTC mod transformers was sold pre-war and
evolved into the later "CVM" series that is more commonly seen today. This
chassis may have come from a commercial amateur rig because the construction
standard is strictly professional.

Through some artful guesswork using only the finest assumptions, I have
concluded that the modulator tubes should be Taylor TZ-20's. The TZ-20 is
(quoting Taylor) "a high-mu triode designed for zero bias class B audio
operation". The Taylor catalog even suggests this modulator design under the
TZ-20 listing, saying "4 of them in push-pull parallel operation will form a
most economical 140 watt modulator", though no actual design for such a unit
is given.

Fortunately I have some TZ-20's (though I can use a few spares if anyone
wants to part with a couple). But I know nothing about running tubes in
PP-parallel. Are there any special considerations? Should the tubes be
matched in any sense? And lastly, I suspect this should be obvious but I'll
ask anyway, how critical is the resistor between the 2 plate caps of the
tubes in parallel?

Comments?

73, Don Merz, N3RHT

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