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Clubman type amp

:

8/19/1999 2:37 PM

Clubman type amp

Hi,, Just finished transplanting my first DIY

AC30 amp into a head version with 2-el34's

instead of the el84's. I used the Clubman 35

design type output section with one channel

like the clubman with a pentode and the other

like the stock top boost channel. I really

like the top boost channels tone but looks

like I'm only getting about 20 watts RMS out

of this amp to my dismay... I have only approx

330v on the plates, am using 250 ohm Rk which

gives me about 16v Ek at about 70ma. This

seems and sounds quite well but I want more

output so am looking for a higher output PT

to put in this amp that can handle almost

200ma of B+ current. Not easy to find. My

first instinct would be the the re-issue

JTM45 PT but I have my doubts as to whether

it can supply enough B+ juice... I was trying

to go alittle lower than the stock Clubman/

Chieftain B+ of about 420v and prefer more

like 360-380v B+. I have switchable fixed/

cathode setup and switchable feedback loop

which also both sound excellent after alittle

tweaking of values. With fixed bias I set

the el34's at about 38ma's which seems to be

a good start.. For output tranny I'm using

the stock AC30 which is 4k primary which

seems like a good match...

Any suggestions on a good open-faced flat-

mounting type power tranny that can output

about 330vAC at at least 160ma or so????

The pentode preamp I'm still tweeking but

the first attempt yields a great tone but

too high a noise level so I need to mess

with the gain abit...

Thanks for any feedback,

Steve K

8/19/1999 5:27 PM

A Hammond 272 JX would do it. 250 ma @ 300-0-300. A pair of 6l6GC's with a shared 135 ohm cathode resistor will deliver 36 watts into a 4 K load. Each tube idles around 30 watts plate dissipation. Hot! With a 270 ohm cathode resistor you get 24 watts and the tubes run cooler. Plate voltage is about 380 volts. This might vary depending on your line voltage, mine's around 110 to 115. I used a SS rectifier, so a tube rectifier would give you less B+. Don't know about EL34's but I'd guess results would be about the same....Jerry

8/19/1999 6:37 PM

Thanks for the info. I've considered trying a

SS rectifier and it probably wouldn't make much

difference in cathode bias mode but for fixed

bias I think I'd loose that familiar sag I want.

The Drake PT is 285-0-285 so I don't think I'd

gain much going to the 272JX. I saw another

Hammond that might work but its vertical closed

type and the chassis I'm using already was open

for the open horizontal type. I could just

change that if I have to but don't want to...

So does anyone have even a guess how much current

the Drake/Marshall JTM45 transformer can handle??

Getting specs on their trannys is like getting a

Clinton rape story on the front page...ie - next

to impossible...

Steve K

8/19/1999 6:52 PM
J Fletcher	I'd guess it would supply about 200 ma which is approx what the JTM45 will draw at full power....Jerry

8/19/1999 7:01 PM
Steve K	I think I'll try it as I'm a fool for living on the edge with these toys... I just have my doubts it can handle a steady 150-170ma of current in cathode bias mode... And if it wasn't a hundred dollars for one I definitely wouldn't hesitate!! Thanks again Jerry,, Steve K

8/19/1999 9:23 PM
Carlo	Just thought I'd mention the Hammond 270HX, 275-0-275, 200mA. I believe it sells for about $60 or $70.

8/20/1999 4:36 PM

Well, I was "guessing". But the AC30 reissue power transformer isn't very large physically, and the circuit draws over 200 ma at idle, and it's a 280-0-280, with a 2A 5 volt winding. Only time will tell how long they'll last though. I was interested in your post because I'd recently worked on a Clubman and was surprised at how hot those EL34's were running. Plus I'd been fooling around with a homebrew amp with a similar output section....Jerry

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