Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 92Volts on December 03, 2017, 11:51:43 am
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This project started life as an 80w PA amp with ultralinear 6CA7s. The chassis was a bit awkward to build in, and after conversion to SS rectifier, I was getting over 600v B+. 5AR4 was used originally so it wouldn't have been much lower. This would eat modern EL34s alive... I added the transformer at the top-right which gets me 340v or so for the screens:
(https://i.imgur.com/EzPkN1i.jpg)
The small transformer is for heaters because the PT has just enough 6.3v for the power tubes-- it ran all preamp heaters in series off the bias supply!
I got it running and used it for hifi for a while, sounded good, but an 80w mono amp was impractical. Got my hands on 2x UBT-2 OPTs (thanks DummyLoad!) so it's now a stereo single-ended amp. Lots of empty spaces for knobs and tubes in this configuration:
(https://i.imgur.com/2OIcD34.jpg)
Running KT-90s at the moment because I got a good deal on them. This is nice for single-ended use as they should be capable of at least 15w, significantly more than EL34s.
Immediate next step is better power supply filtering. Long-term, I am thinking of moving the power supply off-chassis (as well as switching to another transformer for another 100v B+) and putting the original OPT where the big power transformer is now. The result could be a 3-channel amp, with 15w of single-ended stereo plus 100w for a subwoofer or something. Impractical? Of course! But I do want to find a use for the original OPT... 7.5k impedance at roughly the size of a Hammond 1650R. EL34 or KT88 at high voltage are really the best way to take advantage of it.
I guess I should also try to move some of the wiring inside the chassis. Here's my current audiophile-grade speaker hookups :icon_biggrin:
(https://i.imgur.com/pm4U8PQ.jpg)
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i recognize those output trans!! i forgot to include my usual filler parts with your order. 😜
sorry!
--pete
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hope you're havin fun!
This was my best audio build, pretty flat to 15K, ran it through a pair of audio Nirvana 8" I still have it in my do again file :think1: Next time instead of a mono Quad el84's I think i'll do 2 PSE stereo
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This thing is ugly as ever but simpler and lighter-- now in triode mode with better PS filtering and no separate screen transformer. Attached schematic shows one channel of audio, there's a second channel but they share one power supply.
Diodes are some part number I don't remember with a 2500v PIV rating. They were $0.75 each from Mouser which is a lot for rectifier diodes, but not insane.
About 560v goes to the OPTs, 520v to the preamp. Preamp plates are "safe" at 200-250v (thanks to the plate resistor/current) but this is high of course.
I'm considering raising the 6SN7s to 4ma through a 68k resistor (up from 2.5ma) which "should" decrease distortion-- they'll have no problem with this. Also considering replacing the 12AX7 with a couple 6SN7s for durability.
Has anyone run a 12AX7 over 500v (less at the actual plate of course) and found out if it survives for long?
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:thumbsup: Ugly is beautiful!
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6SN7s to 4ma
I took 'em to around 8mA, can't say how it effected sound and it was the days before better notes :think1:
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6SN7s to 4ma
I took 'em to around 8mA, can't say how it effected sound and it was the days before better notes :think1:
I'm sure the 6SN7s would take 8ma! But I don't have the voltage for it. I need about 100v of negative voltage swing out of the 6SN7 to cut the KT90s off and reach max power.
So whatever idle plate current/voltage is, the 6SN7 needs to be able to draw +1.5ma with -100v on the plate before it runs out of steam. The solution to this is a fairly large plate resistor and low current.
I'm starting to think there are reasons people don't try to get big power out of triode output tubes! :laugh:
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> don't try to get big power out of triode output tubes!
They did.
http://www.mif.pg.gda.pl/homepages/frank/sheets/188/f/F862A.pdf
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So you only need two Majors and a Plexi to drive that thing?
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The MRI tubes I believe were triodes, pulsed to 25KW, avg more like 500W.
here's some good reading; well, it's too big, the care and feeding of a triode, Eimac, should get you close :laugh:
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> don't try to get big power out of triode output tubes!
They did.
http://www.mif.pg.gda.pl/homepages/frank/sheets/188/f/F862A.pdf
I need a pair of these! Do you think I can get them with free shipping? And do you think Edcor can wind me an 18kW output transformer?
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Would they also wind you the 33V 207amp filament transformer? What a beast!
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Don't forget the pair needs individual insulated water jackets, and more water flow than I can get in my house. Several feet of water-hose to stand >12,000V. I would not try my well-water at 12KV; these usually ran distilled water (constantly tested for low conductivity) in the jacket and a heat-exchanger to some other cooling (a pond).
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for a slightly tamer triode :icon_biggrin:
https://frank.pocnet.net/sheets/140/3/3CX800A7.pdf
we used these to drive the PA, I had a dozen laying around the barn, most have been passed out as novelty items :laugh:
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for a slightly tamer triode :icon_biggrin:
https://frank.pocnet.net/sheets/140/3/3CX800A7.pdf
we used these to drive the PA, I had a dozen laying around the barn, most have been passed out as novelty items :laugh:
Hey, those look almost usable in a residential setting! Actually I'm familiar with the tube, they're used a lot in ham radios, right?
PRR, the cooling is something I hadn't thought about. I did wonder if a typical household could get enough electricity to run one of those heaters, let alone use the tube anywhere near its potential!
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they're used a lot in ham
I believe so, MRI's that I worked on using that tube ranged from ~~32Mhz - 127Mhz
Not sure but I think we were paying about $800ea new, lasted about 10K hrs
in the early 2000's you could pick up a scrap .5T mri for about $8,000, scraped out around $10K so you could keep the RF deck and still make money :icon_biggrin:
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Unless ham-rules have changed, I think you are thinking of the 500W little-brother to the 3CX800.
The 3CX800 is easily house-trained. Heater is like a couple EL34. Plate power similar to an electric room heater. Main problem is that the small heater and big plate leads to high voltage low(ish) current, and it is hard to wind a audio transformer for that.
F862A needs more power than my clothes-dryer just to heat. The plate power may be 4X what my whole-house power line is rated. It could pull steady more power than the average of 50 or 100 houses on this road (but there's not 50 houses here). There's no 50KW AM transmitters in my state. Two TV transmitters at Portland run 100KW; the others have lawn-tractor power (3KW-15KW). (But F862A won't work that high.)