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Clubman type amp

8/19/1999 2:37 PM
Steve K
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
J Fletcher
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
Steve K

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 - 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

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
J Fletcher
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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