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larger tube & output in Single-Ended amps?

8/11/1999 7:42 PM
larger tube & output in Single-Ended amps?
I was wondering about how big (in terms of power output) can one make in a single-ended amp design? (in practical terms)  
I'm still gearing up to basically make a Fender Princeton clone (single-ended with an 6V6GT power tube), and reading about the advantages & disadvantage of single-ended vs push-pull (such as the point that in a push-pull design you have "crossover distortion") and I rather like the idea of the SE style... anyway, at some later stage, I'll want to think about building a higher power amp for live use (for when I play well enough ,grin>). So, would it be possible to build an SE amp that puts out say 30 to 40 watts? or more? What output tube would I need to use?  
Is this another rediculous idea?
8/11/1999 8:50 PM
John Martin

Try a Hammond 1640SE output transformer with a parallel pair of 6550s. Maybe even EL34s or KT90s.  
8/11/1999 9:20 PM
Reid Kneeland

First off, understand that "single-ended"  
doesn't necessarily mean "single-tube".  
You can build a single-ended amp that  
uses two or more tubes in parallel in  
the output stage. In fact if I recall  
correctly, there's a 50W SE amp design  
that uses 4 6550s in Kevin O'Connor's  
Principles of Power. You could certainly  
start by looking at that. And if you  
want to get crazy, you can either use even  
more tubes or big transmitting tubes, and  
get over 100W. The biggest problem will  
be finding an output transformer that is  
designed for single-ended operation at that  
power level. You'd probably have to have  
it custom made, it would be expensive (several  
hundred dollars) and it would weigh a lot.  
(Kevin's design uses a stock Hammond, which  
is about as big as they get, and costs around  
As far as crossover distortion, it does  
not exist in push-pull class A amps, either  
(it's eliminated by class A operation, not  
by being single-ended). What you get a lot  
of from an SE amp (especially a guitar amp)  
is even-order harmonic distortion. In a  
push-pull amp, the even harmonics tend to  
cancel. I think that's really where most of  
the tonal differences lie.  
Also keep in mind that SE amps are more prone  
to hum that PP types, because there's no  
common mode cancellation happening. In other  
words, you'll want a lot of supply filtering  
and maybe a DC filament supply if you want a  
really quiet amp.  
If you want your small-amp sound at higher  
volumes, mike your Princeton clone and run  
it into the PA. Much easier.  
8/11/1999 9:21 PM
Reid Kneeland

Sorry about that... it kept saying  
"timed out".  
8/11/1999 9:37 PM
Ken Gilbert
Jeez Reid, really trying to get your point across, aren't you??  
Kidding around aside, spot on about the post, though. Interestingly enough, I have a Hammond 1640SE that is rated 50W. This is a rare beast, since Hammond, for some reason not completely disclosed, de-rated the tranny soon after I purchased mine to 25W. Perhaps they were saturating to hell with 50W of flux @ 30Hz? That's completely plausible.  
The best thing about that 1640SE is the DC idle current that's allowed--200 mA. You won't find another production SE tranny ANYWHERE with that kind of flux capacity.  
The downside is the very low primary Z (1K25), and low primary L, which is <10H if I remember correctly. This means paralleled tubes are necessary.  
There was a Japanese guy I saw who used a Svetlana 3cx300 "hockey puck" tube to drive a 1640SE. He seemed to like it quite a bit for home HIFI use. He went with the Hammond for its DC current capacity.  
I think you could still get an honest 50W, especially in the guitar range of frequencies, from a 1640SE, which are a little under 100 from Angela Instruments. When I bought mine from them it was 142. Damn. It DOES say 50W though.  
If you want to get even more SE power, you can get a couple of them and stack their secondaries. Or you could get a custom SE traffo wound, for big bucks.  
8/12/1999 6:01 PM
John Stokes
Maybe 3 paralleled KT90s would drive this thing? That should get you close to 50 Watts. For driving the paralleled grid load, maybe a CF would be in order.
8/12/1999 8:07 PM
Ken Gilbert
Completely agree here. But I'd try to steer around those KT90's, since they are NOS now.  
Probably best to go with three KT88's or 6550's. The low impedance of the tranny means high B+ isn't necessary, or desired, so 6550's would probably be best in terms of cost.  
You COULD parallel three SV811-3's or 811-10's for the DHT sound... that might sound really good.  
One of these days I'll get around to doing something with that iron.  
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