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Re: [Amps] high drive/low output QRO HF-2500

To: Jim Thomson <>
Subject: Re: [Amps] high drive/low output QRO HF-2500
From: Steve NR4M <>
Date: Wed, 04 Mar 2015 17:42:34 +0000
List-post: <">>
Thanks for you cautionary input, Jim,

I have the two originals, here, on my desk.
Two of the non-pedigreed spares are in the amp now, with no HV, cooking.

I plan on also using my 'never-before-used' hi-pot tester on the new ones. I figure I need be good at 6 kv plate to gnd.


On 3/4/2015 17:25, Jim Thomson wrote:
Date: Wed, 04 Mar 2015 16:26:27 +0000
From: Steve NR4M<>
To: Carl<>
Subject: Re: [Amps] high drive/low output QRO HF-2500

Happy 'Hump day' to all,

I did a little investigation based on suggestions from members of the group.

In preparation for running the amp with only one tube, and then the
other tube, figured I would do some simple resistance checks of the
tube.  I was looking for an open filament.  I pulled both tubes out to
check one against the other.  (I really do hate putting these tubes back
in their sockets...)

Trying to understand the Svetlana pin out info for this tube. /Assuming
/in their 'bottom view' that the gap in the pin symmetry is at the six
o'clock position.  That is, pin 1 (G1) is at 7 o'clock and pin 7 (H) is
at 5 o'clock.  I also located the 3 connected pins for the cathode (K)
at pins 2, 4 and 6.  This, then, seems to make sense to me.

First tube I pulled measured an open across pins 7 and 3 (H)  I am
supposing 'H' is the designation of the filament.
The second tube showed 1.4 ohms across the same two pins.  Therefore, I
believe the first tube has an open filament.  This would explain the
'not working correctly' vs catastrophic event of a short somewhere.

I have a couple of spare tube I got several years ago.  I do not know
their pedigree, but supposed to be NOS.

Any issues with freely mixing tubes, or is the performance of this tube
such that a pair of tubes MUST be matched, or at least recommended, to
be matched for getting the operating bias/quietens current of each tube
the same?

Comments welcomed.

73 de Steve, NR4M

###  DON’T  MIX THEM UP.   IE:  keep track of which is bad, which is
the oem good original.     and the 2 spares, since u don’t know if they will
work , not work, both good, both bad,  1 bad, 1 good.   mark em somehow
like with a jiffy marker.  1-2-3-4

##  dunno if they have to be matched or not.  Unless  u can read individual 
+ screen currents, u would never know anyway.

Jim  VE7RF


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73 de Steve, NR4M

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