Standel 25L15 Info and Advice?

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Standel 25L15 Info and Advice?

Postby BudP » Thu Aug 14, 2008 8:33 am

Anyone familiar with the Standel 25L15? I'd like to build a similar amp but replacing the 25L15's push-pull 807 finals with something even more "impressive-looking", such as the 75TH. Possible? Feasible?

The circuit should be simplified somewhat, since the 75TH is a triode, and thus wouldn't need the screen supply, but I'm concerned about the higher plate voltage needed and whether a modern OPT like the Hammond (claimed to be Hi-Pot tested at 2000v) or Edcor would stand the 1-1.5kv plate supply.

I guess I also would need to research whether the 12AU7 would be adequate to drive a pair of 75TH's or whether I'd need to add an additional driver stage. (So much for circuit simplification...)

Thoughts or advice would be appreciated. (Or ... arguments to the contrary such as, "Are you NUTS?")

Bud
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Postby Gingertube » Thu Aug 14, 2008 10:17 pm

Bud,
Let me be the first - You are Nuts! Welcome to the Club.

Seriously while 75TH could be used it is not a really good choice.
Coupled with the High Voltage requirement, each 75TH also needs 6.25 Amps of heater current (at 5V) and it requires about 2 to 3 watts of grid drive POWER, you certainly are not going to be able to drive them with a 12AU7.

It would be a lot cheaper and easier in all respects to use a pair of Cathode Biased 300B's.
Of course they dont look quite as impressive but there are 1000% less headaches in implemented a 300B design vs a 75TH design.

Cheers,
Ian
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Postby Gingertube » Thu Aug 14, 2008 10:39 pm

Bud,
Thought I should add some justification for the 300B suggestion.
A pair of Push pull 300B wil give you 20 to 25 Watt, very similar in power to the Standel. Run them at about 420 V Vak and 75mA idle per side. You would generate about 80V across the cathode bias resistor so you are at a B+ of around 500V. They don't draw significant grid current (unlike the 75TH) and so you could drive them with a 12AU7 although I'd probably look at ECC99 if you want to stay with a 9 pin tube or go to a 6SN7 if you want to go Octal.
Output and Power Trannies then become "off the shelf" items. In fact anything which suited push pull 807 would also suit push pull 300B. Required heater (filament) power is 1.2 Amps per 300B so your total heater power is about 12 watts compared to 62 watts for 75TH. And finally, you will be able to get 300B's for a long time to come. Unless you have a stash of 75TH then the amp would have a lifetime limited to the 75TH life.

Cheers,
Ian
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Postby BudP » Fri Aug 15, 2008 6:44 am

Ian, you hit the nail on the head! (About the 75TH stash... and MAYBE about being nuts!) My goal here isn't to build an efficient circuit, but to show off the 75TH's.

I've been collecting Eimac triodes for years, trying to obtain the entire series, and while the 750TH/TL has been eluding me, I've finally got all the rest, having just purchased a few brand-new, in-the-box 75TH's. They're cute little things, and thought they'd be the appropriate candidates for showcasing in a small guitar amp rather than a pair of, say, 450TL's. (Maybe I'll build up a ham radio linear using them...!)

I'm not deterred by the HV requirements (I'll "show off" a pair of 866A's for that) nor the filament current requirement. The light given off by them will just mean I won't have to turn on a room light when I run the amp! And, for that matter, I'll probably be able to turn the furnace thermostat down a bit during the winter, although the A/C will probably have to run a bit harder during the summer...

Anyway, it's not intended to be a "practical" project. As JFK said, "We don't do this because it's easy, we do it because it's hard." Sort of on the order of that 304TL audio amp at www.alumrocktech.com...

...Maybe I can convert the existing circuit to SE, and use the output of the single 807 to drive the 75TH's...? I'd run the 75TH's at the lowest possible plate voltage that would make them work, probably around 1000v or less. Efficiency would fall off, true, but I'm more concerned with having a "conversation piece" here. When I want the best quality amp, I'll just play the Standel!
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Postby Gingertube » Sun Aug 17, 2008 10:03 pm

Bud,
Good to see you are doing this for all the right reasons.

For you BUT mostly for those folks saying WTF is a 75TH - its simply gorgeous. Pretty Picture here.
http://pic3.audiogon.com/i/c/f/1218202546.jpg

And a thread on another forum - no real details to help.
http://www.audioasylum.com/forums/tubed ... 47744.html

Your idea of using an 807 to drive an interstage transformer to in turn drive the 75TH is the way to go.

Cheers,
Ian
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Postby Gingertube » Thu Aug 28, 2008 10:21 pm

Bud,
A few tips on using 75TH might be able to be "gleaned" from this SET Amp.
http://www.edsawyer.com/audiogallery/75 ... ic.gif.php

Cheers,
Ian
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Postby BudP » Sat Aug 30, 2008 6:22 am

That's the strangest schematic I believe I've ever seen! Beautiful construction, and that's what I'm after, but I don't think I want to drive a 36w plate dissipation tube with a 75w one... maybe it's an "attenuator" rather than an amplifier. Could be he runs the speaker output from one of the boxes on the shelves through this thing to reduce the power delivered the speakers. Sorta be like driving a 4-125 with a 4-400! He seems to like heavy iron, what with all the chokes... think I'll stick to resistance-coupled. If that thing actually works with only 400v on the 75TH plates, I think I'll be OK running them at 1000v. Thanks for the tip!
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Peace through superior RF (or AF) firepower

Postby EWBrown » Sat Aug 30, 2008 12:47 pm

The 75THs would be more suitable for a Class B P-P circuit, with an interstage transformer handling the phase splitting duties. These tubes, as mentioned previously, do draw considerable grid curent and need a few watts of drive power to their grids. I'm going on somewhat "fuzzy" memory from reading my 1950's vintage ARRL ham radio handbooks, they cover a lot of this type of tube in both RF and (mostly Class B) modulator circuits.

Now for a bit of health and safety advice:

These tubes, with their exposed anodes, can, and will be a definite shock hazard, unless they are well=protected. Add in the 1000V plate voltage, and there is no such thing as a "little jolt"... THis kind of voltage can be extremely dangerous, and even lethal. The amp in the photo is pretty, but definitely needs to be kept out of reach of casual observers, kids and household pets. ...

Unforch, I can't just grab the books for a quick reference look-up, they are currently about 1050 miles SW of here Yellow_Light_Colorz_PDT_02

Granted, in a proper guitar amp, the tubes will be under cover, and therefore, some air circulation would be a good idea.

Class B typically runs with a zero volt, or positive grid bias, and some amount of grid current, unlik Class e A or AB, which consume no, or very little, grid current.

Class B does inherently have more crossover distortion than Class A , AB1, or AB2. But then for guitar amp use, this can be a good thing...

Good 807 stand-ins are the 1625 (12V filaments, and UX-7 bases)) and the 6BG6G, (octal bases).

I know Ed Sawyer, and he has some amazing designs, and he is a very accomplished builder and woodworker, far beyond my meager capabilities. I had lunch with him last week, (along with a half-dozen other NNETG'ers) and as usual we discussed all things tubular Yellow_Light_Colorz_PDT_03 He uses the 75TH as a driver stage, which is definitely a non-standard application, and at the relatively low B+ and plate current, it should last about forever...

/ed B
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