Single-Ended Class A - EL34

James Freeman

Rock Star
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3,181
I plugged my Hi-Fi single-ended EL34 amp to my guitar cab running Helix into one channel, got a very impressive juicy round sound that gradually compresses to a smooth growl as I turn the volume higher.
Aside from lack of a Tone knob it is identical in topology to an early Class A Fender Princeton but with slightly more preamp headroom and power, both are basically a textbook implementation of a 40's radio amp.

I got a good taste of Single-Ended Class A and I have to say I understand now, it sounds rounder, smoother, and compresses more gracefully to crunch, very different than Class AB guitar amps.
If I needed a wider range of edge-of-breakup or clean-to-crunch sound controllable with the guitar volume I would definitely go Class A like Princeton.
It also works very nicely with full range speakers for some cork sniffing hifi music listening. :p

Some technical info:
Topology is as simple as it gets, clean 6SN7 preamp tube that doesn't distort (7v cathode bias voltage) into a cathode biased EL34 (25v cathode bias voltage), no negative feedback, no tonestack, flat frequency response (measured with a resistive load) when operating in the linear range.
6SN7 preamp swing is 45Vp with 3Vp (2Vrms 0dBFS standard) on RCA inputs, so the power tubes grids get full swing and output full power or can be pushed to clip.

Single Ended.jpg



Inside 1.jpg


Closeup.jpg
 
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Cool amp. A setup like this can't be beat for home practice. I think the beefy OT's make a difference. Usually 5W OT's are kind of whimpy.

Is it tube rectified?
 
Is it tube rectified?
Yeah, the big tube in the middle, but I also have a solid state rectifier in a tube base socket, you can see it near the tube rectifier in the first image.
It's also Choke filtered, it's inside the chassis.

I think the beefy OT's make a difference. Usually 5W OT's are kind of whimpy.
I've measured around 7W maximum per channel, bigger OT might give a slightly cleaner low end when cranked but the amp is already distorted at that point I don't think it will make it any more Hi-Fi, that topology is all about nice distortion at sane volumes.
 
I plugged my Hi-Fi single-ended EL34 amp to my guitar cab running Helix into one channel, got a very impressive juicy round sound that gradually compresses to a smooth growl as I turn the volume higher.
Aside from lack of a Tone knob it is identical in topology to an early Class A Fender Princeton but with slightly more preamp headroom and power, both are basically a textbook implementation of a 40's radio amp.

I got a good taste of Single-Ended Class A and I have to say I understand now, it sounds rounder, smoother, and compresses more gracefully to crunch, very different than Class AB guitar amps.
If I needed a wider range of edge-of-breakup or clean-to-crunch sound controllable with the guitar volume I would definitely go Class A like Princeton.
It also works very nicely with full range speakers for some cork sniffing hifi music listening. :p

Some technical info:
Topology is as simple as it gets, clean 6SN7 preamp tube that doesn't distort (7v cathode bias voltage) into a cathode biased EL34 (25v cathode bias voltage), no negative feedback, no tonestack, flat frequency response (measured with a resistive load) when operating in the linear range.
6SN7 preamp swing is 45Vp with 3Vp (2Vrms 0dBFS standard) on RCA inputs, so the power tubes grids get full swing and output full power or can be pushed to clip.

View attachment 3857


View attachment 3858

View attachment 3859
I have no idea what's going on here, but I approve
 
I plugged my Hi-Fi single-ended EL34 amp to my guitar cab running Helix into one channel, got a very impressive juicy round sound that gradually compresses to a smooth growl as I turn the volume higher.
Aside from lack of a Tone knob it is identical in topology to an early Class A Fender Princeton but with slightly more preamp headroom and power, both are basically a textbook implementation of a 40's radio amp.

I got a good taste of Single-Ended Class A and I have to say I understand now, it sounds rounder, smoother, and compresses more gracefully to crunch, very different than Class AB guitar amps.
If I needed a wider range of edge-of-breakup or clean-to-crunch sound controllable with the guitar volume I would definitely go Class A like Princeton.
It also works very nicely with full range speakers for some cork sniffing hifi music listening. :p

Some technical info:
Topology is as simple as it gets, clean 6SN7 preamp tube that doesn't distort (7v cathode bias voltage) into a cathode biased EL34 (25v cathode bias voltage), no negative feedback, no tonestack, flat frequency response (measured with a resistive load) when operating in the linear range.
6SN7 preamp swing is 45Vp with 3Vp (2Vrms 0dBFS standard) on RCA inputs, so the power tubes grids get full swing and output full power or can be pushed to clip.

View attachment 3857


View attachment 3858

View attachment 3859
Nice! Did you build this yourself, or purchased it?
 
Nice! Did you build this yourself, or purchased it?

I bought it prebuilt about 6 years ago. I think they go four around 400-500$ today and plentiful on amazon, ebay and aliexpress.



I can’t shake a stick at that! Is that what this build is?

It's a Class A stereo tube amplifier for music, each side has one preamp tube and one power tube that amplify the entire waveform as opposed to Class AB where you need two tubes to amplify each half of the waveform.

Audiophiles love these Class A amps, they are the earliest and purest form of analog amplification even though they have high distortion figures and terribly hot and inefficient, each EL34 is running full on 100% (25W) at all time even when no music is playing.

Early Fender Champ/Princeton amps are basically that, minimal change from what was a Radio/Audio amplifiers of the time.


Mikael-Abdellah-SE-KT88-Amplifier.png




Fender 5C1 Champ (1953).

champ5c1.gif



Fender 5B2 Princeton (1946).

fender-princeton-5b2-schematic-1.jpg
 
I bought it prebuilt about 6 years ago. I think they go four around 400-500$ today and plentiful on amazon, ebay and aliexpress.





It's a Class A stereo tube amplifier for music, each side has one preamp tube and one power tube that amplify the entire waveform as opposed to Class AB where you need two tubes to amplify each half of the waveform.

Audiophiles love these Class A amps, they are the earliest and purest form of analog amplification even though they have high distortion figures and terribly hot and inefficient, each EL34 is running full on 100% (25W) at all time even when no music is playing.

Early Fender Champ/Princeton amps are basically that, minimal change from what was a Radio/Audio amplifiers of the time.


View attachment 3874



Fender 5C1 Champ (1953).

champ5c1.gif



Fender 5B2 Princeton (1946).

fender-princeton-5b2-schematic-1.jpg

Snap, thanks for the info! That's pretty cool!
 
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