Building an amp around the QB5/1750 transmitter tube - diyAudio
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Old 23rd December 2011, 03:56 PM   #1
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Location: Belgium-Europe
Default Building an amp around the QB5/1750 transmitter tube

Hi,

I'm playing with the the idea of building an amp around the QB5/1750 transmitter tube. Mostly because of the looks, but it should sound good as well of course.

Apparently it does sound good according to the reviews of the NAT transmitter amp:
NAT - Products - Transmitter - Single Ended Mono Block Power Amplifier

Did someone come up with a publicly available design/schematic?

I don't need that much power (driving Klipsch La Scala II's), so an another tube with similar looks but less power would be ok as well.

I'm comfortable with building electronic equipment but I don't have the electronics skills to design one myself.

Thanks,
Henk

Last edited by brain1970; 23rd December 2011 at 04:04 PM.
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Old 23rd December 2011, 09:52 PM   #2
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Join Date: Jan 2008
Location: Salt Lake Silly, Yewtaw
If you have LaScallas then consider the Chineese FU-29 amplifier instead. It is based on a pair of 829B 40 watt transmitter finals. It's cheaper at $420 USD delivered to you via UPS 2 day saver. Sound is incredible from this amp that would cost you at least several thousand if it were made in the USA. if you refit the amplifier with RCA 829B toobes and russian 6N1 input toobes then bias it properly after a burn in period it is going to be almost impossible to beat... I am running one on my Dynaudios and even though it is just 30 wpc it can drive them to a very nice listening level...

I have worked on transmitters in my past life quite a bit (1980's) and the QB5/1750 should at least have a glass chimney around it and forced air cooling. It is designed to be operated this way or in a slightly pressurized sealerd air plenum. This way there are no rediclous temperatures to have to deal with. The amplifier shown is a very poor thermal design!!

EDIT: Also note that the QB5/1750 will require a plate voltage of 4000+ volts at pretty hefty current. You need to posess specialized knowledge to safely build amplifiers that operate on this sort of voltage if you are not to kill your self in the process... The Output Transformer alone might cost you 5+ thousand USD per channel to have custom wound.

Mark

Last edited by Mark Allen; 23rd December 2011 at 09:59 PM.
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Old 23rd December 2011, 10:05 PM   #3
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Thanks for the tip. Tube esthetics are similar and probably a much better value proposition.
I did worry about safety as well, better having somebody experienced building this amp or me.
Is there no way around this expensive output transformer?

Henk

Last edited by brain1970; 23rd December 2011 at 10:21 PM.
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Old 23rd December 2011, 11:35 PM   #4
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Join Date: Jun 2011
I have played with the idea of using transmitter tubes like this one. I wouldn't be surprised if they set this thing up like a 211 or an 807 (10K plate transformer @800V). or even sticking the transformer in the cathode circuit.(I would do that if I used 4KV on the plate but it would require lots of drive signal) Those transformers don't look like high voltage transformers, they look more like medium voltage type

Last edited by DavesNotHere; 23rd December 2011 at 11:37 PM.
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Old 24th December 2011, 02:03 AM   #5
db! is offline db!  Canada
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The way that transmitter tube is being run, it'll probably die early. By running it conservatively(at 55-60%), the anode will not get hot enough to getter properly.
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Old 24th December 2011, 02:04 AM   #6
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They may very well be in the cathode but I don't think it would be a very efficient way to go about it. No matter where it may be if the tube shorted from say a catastrpohic internal self destruction or went gassy they would still have to be rated for at least three times the B+ and max current the amplifier runs on for a decent safety factor. Also, I doubt the B+ would be that low since the toobe is not capable of all that much current (~450 ma) you would have to keep the B+ high in order to be able to generate that kind of output power. To me those are fairly largish transformers when compared to the QB tube which is itself 4.65 inches in diameter... I would guess them to be about 6 to 7 inches square.
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