[TenTec] Centaur Tubes odd ones in parallel

Wed Apr 1 16:38:45 EDT 2015

Why not equalize the grid current for each tube. Small resister to ground should do the trick. Some folks turn it into a grid fuse by using a very small wattage resister. If the grid current goes too high, they pop open. 

I've played with that concept, but I'm not a fan. 

73,

Mike
W2AJI 

Sent from my iPhone

> On Apr 1, 2015, at 2:06 PM, Rick - DJ0IP / NJ0IP <Rick at DJ0IP.de> wrote:
> 
> Mike, that's a good procedure.
> Unfortunately I haven't figured out a way to do that when you have multiple
> triodes.
> 
> With Sweep Tube amps it was very simple.
> We biased the grid directly and using diode strings, each tube got its own
> dedicated tap.
> 
> But what can we do to match multiple triodes in grounded grid?
> 
> 73 - Rick, DJ0IP
> (Nr. Frankfurt am Main)
> 
> 
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Mike
> Schatzberg
> Sent: Wednesday, April 01, 2015 7:23 PM
> To: Discussion of Ten-Tec Equipment
> Subject: Re: [TenTec] Centaur Tubes odd ones in parallel
> 
> 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
>>> 
>>> 
>>> ------------------------------
>>> 
>>> Subject: Digest Footer
>>> 
>>> _______________________________________________
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>>> 
>>> 
>>> ------------------------------
>>> 
>>> End of TenTec Digest, Vol 147, Issue 34
>>> ***************************************
>>> 
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