This issue of back emfs in unloaded tube amps was never worried about
in the thousands of old radios with 1 x 6V6 used without any NFB and
connected to OPT and a speaker with no Zobel damping network.
The reactance of speaker inductance rises so that by 3kHz there is
virtually no load on the 6V6 and the shunt C is low and leakage L also
high. RDH4 does not mention that arcing in the tube would be a
problem, and I've seen no evidence of it. Stored magnetic energy in
bean couter designed radio OPTs is low; it uses a tiny core which is
air gapped. B+ voltages in radios tend to be less than +280V, and even
if anode goes to +560V, the insulation will survive.
But in PP amps the B+ can be +500V and as we all know the anode swing
can be to +/-1,500V, and thus threaten to arc. There's a simple answer
which is well know and been around ever since Si diodes began to get
good reverse voltage ratimgs, and that's to connect diodes from each
end of the OPT to 0V with diode anode connected to tube anodes. I like
to use 3 x 1N7007 in series with 1M strapped across each diode to
equalise reverese voltages, and when one end of the OPT tries to rise
to more than 2 x +B, the other end tries to go below 0V so diodes
conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping
with diodes has been around for ages.
There is a schematic example at
http://www.turneraudio.com.au/100w-monobloc2-2004.htm
The amp also has Zobel networks across each 1/2 primary of the OPT to
load the OPT at HF.
The book I have with 17 designs of amps from 5W to 1,100 W written in
anout 1960 has no provisions for dealing with excessive unloaded OPT
voltages except on the high power types where an adjustable spark gap
is used with a 10k0 load resistance in series between each end of the
OPT.
But arcing in OPTs is not confined to amps which have B+ of over 1kV
or more. I once rescued a Geloso PA amp made in 1950s moments before a
trundling bulldozer went over it after someone dumped it when people
used to be able to take stuff to a tip and then raid each other's tip
off's. This kind of amp was used at Italian weddings for an acordian
player and singer. 2 x EL84 for about 15W max. The amp arced just
sitting there doing nothing while turned on; there was a fault in
insulation where B+ wires went into the winding and it arced between B
+ and the core. Some careful observation showed a blackened area and
it had been doing is for awhile and probably because it had got DAMP
at some time. I scraped out the carbon and wax in the area and dressed
wire positions, heated the tranny to dry it, re-waxed it, and it never
ever arced again. I then rewired it totally to give better guitar amp
usability and sold it for $350, about 12 years ago, to a guy here who
calls himself Dr Zot, and who dabbles in vintage electric guitars and
amps.
Patrick Turner.[/quote]
Before 1960 the Selenium Thyrector Diode by GE & others was available to circuit designers. These were meant for power applications & came in both single ended & back to back stacks in several voltage ratings. Reverse leakage current specs were low enough so as not to adversley effect the questionable fidelity of those Class AB & B PA amplifiers. In a guitar amp they might add something.
Cheers, John
If the designer chose to he could have selected a pair of back to back devices attached across the OPT primary. But caps & resistors were less money.