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

To: Carl <>
Subject: Re: [Amps] high drive/low output QRO HF-2500
From: Steve NR4M <>
Date: Wed, 04 Mar 2015 16:26:27 +0000
List-post: <">>
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

On 3/2/2015 21:28, Carl wrote:
What about the input VSWR?

That amp is notorious for frying input circuits as well as that 40/80/160M toroid coil, especially in high duty cycle modes. Bandswitching a toroid at those impedance transformations is not a good idea; they work OK as the L coil in a pi-L.

Once fried it should be replaced.

You can also test one tube at a time by stuffing a rag in the empty hole and using 20M. I run 40W drive to make the A-B comparisons.


----- Original Message ----- From: "Gerald Williamson via Amps" <>
To: <>; <>
Sent: Monday, March 02, 2015 4:08 PM
Subject: Re: [Amps] high drive/low output QRO HF-2500

Steve, do you suppose one of the two tubes is not working?

Gerld K5GW

In a message dated 3/2/2015 12:44:00 P.M. Pacific Standard Time, writes:

Hi Carl,  and others,

My apologies for taking so long to get back you everyone on  this.
'Life' got in the way.

I got the amp moved over to  another position and put the output into a
dummy load.

Driving it  with an Omni 6, I got the same output on 40 as I did into an
antenna  driving it with a K3.
The 4 square on 40 showed a 1.2:1 SWR and the dummy  load a 1:1.

On 20 meters, I tuned it up gradually to several levels of  input, doing
it by the manual. At 100 watts input from the rig, I got 1250 watts out
to the Bird wattmeter..  The input is 2100w +  (2500 volts @.850 ma)
That's 59% efficiency if I don't count the 100  watts drive.
It tunes up easily, but I am not seeing the gain I once did. 40-50 watts input used to give me 1500 easily. It's as if I lost 3-4 db of amp gain.

I used 20 meters as a second tune-up band as I don't  remember using any
toroid inductors for the 20 meter tank circuit.   The 40 meter toroid
part of the tank circuit looks like a 'grilled cheese  sandwich', with
the plastic having heated and drooped down around the 40  meter toroid.
Regarding the 40 meter toroid:  On 40 (and 20) it  tunes up at the
approximate settings, per band, given in the manual, so I am not looking
at the cosmetic issues of the 40 meter toroid.  It  seems to have the
correct inductance to resonate with the described  capacitor settings.


73 de Steve, NR4M

On  2/20/2015 01:53, Carl wrote:
Same problem on all  bands?


-----  Original Message ----- From: "Steve Bookout" <>
 To: <>; <>
Sent:  Thursday, February 19, 2015 7:40 PM
Subject: Re: [Amps] high drive/low  output QRO HF-2500

The K3 that is driving it shows  1.2 or 1.3 to 1 standing wave.


On 2/20/2015 00:31 ,  wrote:
90W drive; how much reflected?
In a message dated 2/19/2015 5:22:23  P.M. Central Standard Time,  writes:

    Hello  all,

    Took a QRO HF-2500 out  of storage and put it on my 40 meter mult

    Tuned by the book  (because I never can remember how to tune this
    without the book...) and  the output is low  at about 1050 watts.
is with about 90 watts of drive. When last used it was used on 15
    but always got plenty of power  out.  Efficiency looks fine at
    about  62%
    (700ma @ 2400  volts)

    As I recall, this  amp used to drive full output with 35, or so,

    Power doesn't drift,  and tunes up easy.  It's just acting more
  like an
    SB-220 than the amp it  is.

    Plate and load cap  settings appear correct per the manual.


    73 de  Steve, NR4M
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