[Boatanchors] 1625/807 question

[Bry Carling]

Cherry Red plate on that 807 at 1200V DC running AM in a Wireless Set 19 MK2... very educational for us young lads... we were just using a different model of dynamotor that produced the 1.2 KV, as verified by our Avo Meter! Boy did it whine loudly! Especially when we keyed that black microphone!

Best regards - Brian Carling, AF4K
Ex-G3XLQ

Tel: 321-262-5471




On Feb 7, 2018, at 3:32 AM, Rob Atkinson <ranchorobbo at gmail.com<mailto:ranchorobbo at gmail.com>> wrote:

Maximum plate voltage is kind of a funny thing.  Let's consider a tube
like the 3-500.  The Eimac spec. sheet will say its maximum plate v.
is 4 KV.  But elsewhere, it gives "typical operation" specifications
for plate modulated AM at 3 KV, which means the modulated plate v.
will swing up to 6 KV on positive audio peaks.  What gives?  The truth
is there's no hard and fast limit but the specified limit is sort of
an "all bets off" limit, as in go over this with continuous voltage
and you're on your own--don't demand money from us if there's an arc
or the final takes off and your costly rig is destroyed.   So yeah,
you might get away with 1 KV on the 1625 but then again, depending on
the frequency, the tube might get squirelly and oscillate etc.
Whatever the screen v. is stated as maximum, you want to pretty
closely adhere to that.  Maybe 20 v. over, but not 100 over.  Also,
factor in transmit time for high B+.  1 KV for a minute okay but maybe
not 10 minutes.

73

Rob
K5UJ

On Mon, Feb 5, 2018 at 7:06 PM, Ron.K3PID <ron.k3pid at sbcglobal.net<mailto:ron.k3pid at sbcglobal.net>> wrote:
I am reading some old surplus manuals and find that they like to use the 1625/807 as a power amplifier ( typical ARC-5 parallel style ) and suggest that one could run up to 1000 volts on the plate. Every listing I can find shows a absolute maximum plate voltage for the 1625 of 750 volts. Yes, the operating plate voltage will depend upon class of service and therefor duty cycle but isn’t absolute maximum absolute maximum? Now I can remember pushing the stated maximums a bit and I even have a friend who admits running a tube in a coffee can full of water to get a few more watts out of it.  But none of it was “published” that I know of. What am I missing?

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