[TenTec] Centaur Tubes odd ones in parallel

[Mike Schatzberg]

You can check the match both statically and dynamically by installing one triode at a time, and double checking you don't short the unused plate cap. Follow all safety precautions and all interlocks should be engaged. Always allow time for the capacitor bank to bleed down and then short the HV to ground while the amp is open. 

Key the amp with no drive and observe the idle current for that triode.  Remove that tube and replace with the second triode. 

The results should be very close for well matched tubes. 

If you wish, you can plot input drive vs output power through the useful range of the limits of plate and grid current for each triode. Again, the curves should be similar for a matched pair. 

A good quality dummy load should be employed. 

You will also observe the linearity of the amp and the saturation point when additional drive no longer produces a linear output. 

Obviously, good quality instruments should be used to produce the curves. 

It takes only minutes to do the static test. Why guess if you will have an issue? 

This methods can help determine the needed bias voltage for best linearity also. I usually build a diode string and add or subtract diodes to get the desired idling current. The 3-500 is a zero bias tube and is pretty easy to deal with. The higher the idling current, the hotter the tube runs, and the easier it is to drive to full output. 

73,

Mike
W2AJI 

Sent from my iPhone

> On Apr 1, 2015, at 9:30 AM, Rick - DJ0IP / NJ0IP <Rick at DJ0IP.de> wrote:
> 
> Bob, you have been very lucky if you have never had any problems with
> unmatched tubes.
> I certainly have.
> 
> In fact I finally decided not to own any multi-tube amps anymore.
> That solved that problem. (almost)
> 
> I now only have two amps, each with a single 3-500z, but... 
> I also have an old Heathkit HA-14 mobile amp with a pair of 572B tubes.
> Can't seem to find a replacement for that one.  There are no tiny amps with
> single tubes running that much power.
> 
> I don't care much for most transistor amps.
> Their linearity is the pits!
> Though I wouldn't mind having the Elecraft 500w amp...
> Jim sure raves about his!
> 
> 73 - Rick, DJ0IP
> (Nr. Frankfurt am Main)
> 
> 
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Katz Ajamas
> Sent: Wednesday, April 01, 2015 5:14 PM
> To: Discussion of Ten-Tec Equipment
> Subject: Re: [TenTec] Centaur Tubes odd ones in parallel
> 
> Kris-
> 
> Are these 811 or 572?  I've never owned a Centaur but have not had any
> trouble with non-matched tubes in other amps, provided they were from the
> same OEM.
> 
> A single used 811 or 572 should be pretty cheap, so how about hunting up one
> each of the missing?
> 
> 73, Bob ah7i/w4
> 
> 
> 
> 
> 
> 
>> ------------------------------
>> 
>> Message: 9
>> Date: Tue, 31 Mar 2015 15:39:40 -0400
>> From: <km2km at twc.com>
>> To: tentec at contesting.com
>> Subject: [TenTec] Centaur Tubes odd ones in parallel
>> Message-ID: <20150331193940.EK621.275017.root at dnvrco-web01>
>> Content-Type: text/plain; charset=utf-8
>> 
>> Ok, guys, here is the scoop.
>> 
>> My trusty Centaur seldom needs to be re-tubed, BUT now I have two 
>> pairs of tubes,  one pair Taylor, and the other pair Svetlana because over
> time one
>> tube left the troika for the electronic dump.   Yes, matched tubes are the
>> ideal, but having a couple of pairs sitting around doing nothing for 
>> me and no foreseeable makes me wonder.  Just what would happen if they 
>> were a mixed troika.  That would leave one tube in the bin ready for
> another time.
>> 
>> Any ideas on dire consequences?
>> 
>> Many thanks,
>> 
>> Kris KM2KM
>> MERSCHROD
>> 123 Warren Road
>> Ithaca, NY 14850
>> 
>> 
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>> 
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>> ------------------------------
>> 
>> End of TenTec Digest, Vol 147, Issue 34
>> ***************************************
>> 
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