Single ended EL34 amp design

[Stephen1966]

I was recently gifted a Zaerix EL34 - I believe manufactured - made in the GDPR and so - Siemens EL34 from RFT Mühlhausen. Also, a "Belton reverb tank BS2BB2E/T" used by Fender in the Frontman 25 amplifier. This Belton, has an Accutronics stamp on the back which also says: 'Made in the USA'.

1683579825739.jpg

Everything looks good here so I'm thinking of building a single ended amp around them. Something Champ-like with a good 10" speaker, a 12AX7 for the preamp and maybe a single tube reverb.

I was looking at the Beast

Beast-1-1_0.pdf

I'm not bothered about fitting it in a champ sized chassis, I have a slightly bigger one going spare so I'm thinking what would be a suitable tone stack for something a bit higher gain? Transformers would need some figuring out but as an OPT I was thinking the Hammond 125 series, maybe the 125FSE or 125GSE.

125se-1389875.pdf

Designing a single ended amp around a single tube is a gratifying challenge. Parts count will be low, complexity minimised. To all my friends out there, where would you take it? And what kind of sound would you look for?

Look forward to your feedbacking.

[cdemike]

Congrats on the gift!

I'm currently designing a similar amp for my wife, who currently plays through a Fender Champ 600 reissue. I generally play a Les Paul, so between the fixed black panel tone stack having too much bass and a general move to down-size our gear, we're going to replace it and a Marshall SV20C with a small combo that checks similar boxes. I'm currently leaning toward using a 5E3 chassis to make a two channel amp combining either 5F1 and 5F2-A circuits in one box, or a Marshall Super Bass preamp with fixed values for the mid and bass controls (full up and full down, respectively). The Super Bass preamp keeps the phase inverter running into a "dummy" power 12AU7 power amp to keep the PI distortion before running it into the single ended power section. Any updates on your build?

[Stephen1966]

Congrats on the gift!

I'm currently designing a similar amp for my wife, who currently plays through a Fender Champ 600 reissue. I generally play a Les Paul, so between the fixed black panel tone stack having too much bass and a general move to down-size our gear, we're going to replace it and a Marshall SV20C with a small combo that checks similar boxes. I'm currently leaning toward using a 5E3 chassis to make a two channel amp combining either 5F1 and 5F2-A circuits in one box, or a Marshall Super Bass preamp with fixed values for the mid and bass controls (full up and full down, respectively). The Super Bass preamp keeps the phase inverter running into a "dummy" power 12AU7 power amp to keep the PI distortion before running it into the single ended power section. Any updates on your build?

Thank you. None whatsoever I like the Champ design. Elegantly done - eight inch speakers don't do it justice though.

[bepone]

i have built many SE amps.. all power.. the are simple and build is fast and nice..for last one i took complet of transformers from piemme elektra Italy, still not tested but i think will be great,

for reverb amp, it is easy to implement..signal need to be taken after first tube and mix anywhere upstream (in one iteration i have used cathode resistor on one stage upstream ECC83 like an input= ' input on cathode)

[bepone]

mostly i have built champ with volume-treble-middle-bass ctrls, with one ECC83 + 6V6 , reduced voltage at cca 300VDC, very nice sound

[bepone]

provided beast schematic will humm enough to be annoying, with SE power supply must be CLC or CRC before anodes

[bepone]

must read before build, champ with all problems fixed:
https://audioxpress.com/article/Glass-A ... of-a-Champ

[Stephen1966]

must read before build, champ with all problems fixed:
https://audioxpress.com/article/Glass-A ... of-a-Champ

A great read - some great ideas there. Thank you!

I am curious... What OPT would you go for with the EL34?

[Stephen1966]

i have built many SE amps.. all power.. the are simple and build is fast and nice..for last one i took complet of transformers from piemme elektra Italy, still not tested but i think will be great,

for reverb amp, it is easy to implement..signal need to be taken after first tube and mix anywhere upstream (in one iteration i have used cathode resistor on one stage upstream ECC83 like an input= ' input on cathode)

Like a DC cathode follower?

[Stephen1966]

The audio express article on the champ is really interesting.

I've been running some figures for the EL34s power consumption. The RC30 dataheets show for class A, the tube requires a quiescent plate current of 100mA and grid No.2 voltage of 250V (plate voltage = 265V) and that consumes 25W of power but puts out, only 11W and an audible, 10% THD. Adding the huge heater current of 1.5A at 6.3V the power requirement is 9.45W - that's close to 35W of power to run just this tube.

In comparison the 6L6GC (used in the champ article) running at a grid No.2 voltage of 250V (plate voltage =350V) in class A, requires 54mA = 250 x 0.054 = 13.5W, plus 5.67W for heater current when it pulls 0.9A at 6.3V. This tube used in the champ article is clearly "greener" putting out around the same output power, 10.8W (15% THD) but consuming much less at 19W.

As much as I like this EL34 tube it just doesn't add up, a marginally lower THD and that might be of significance as it relates to odd order harmonics but very inefficient in a SE, class A design.

What do you think?

[Stephen1966]

I also followed up on the Kirk Elliot article linked above with this one, 'Revisiting The Remaking of a Champ': https://audioxpress.com/article/you-can ... of-a-champ Driedger's article takes it a step further with a look at other different kinds of output tubes and reflects on the idea to replace Elliot's DC regulated supply with a more traditional AC string.

I opted to stick with simpler AC filaments and, in hindsight, wished I had followed his advice. There is a slight AC hum present in the amplifier that I could never completely eliminate.

His pentode/triode switch on the output beam tubes is a new feature in this design - but does anyone like triode mode? Even so, the variety of output tubes he tested is interesting and will require a bit more number crunching for power consumption and output power. It's the latter I am interested in first...

6GK6 5.7W
6L6 10.8W
6F6 5W
6V6 5.5W

9 PIN SUBMINIATURE (dual triode)
12BH7A 7W (class A both plates)

Thinking about this long list, the 6L6 and 6V6 are still relatively easy to come by and of these, the 6L6 appears to offer the greatest headroom.

[Stephen1966]

Good article on the SE design based around an EL34 here: http://www.valvewizard.co.uk/se.html

cdemike, have you seen this? https://el34world.com/Forum/index.php?topic=21867.0

[Andy Le Blanc]

El34/el84 are power pentodes, verses 6l6/6v6 which are beam power tubes. The "knee" that optimizes power output curves with the beam tubes has a tone character that is different than the older power pentodes. In general terms, if you mind the limitations of your available parts, like the max heater draw of the PT and max dc ma. of the OPT, you can run them fine in a variety of designs. Its the soft knee of the device, distortion totals are up to 10% class A, 6l6/6v6 have greater distortion totals, and the hard knee, perceptively less musical. Old school debate. It's like a marshal with 6550, you might have the power but you ain't got the tone. Go for it,...

[LOUDthud]

In comparison the 6L6GC (used in the champ article) running at a grid No.2 voltage of 250V (plate voltage =350V) in class A, requires 54mA = 250 x 0.054 = 13.5W, plus 5.67W for heater current when it pulls 0.9A at 6.3V. This tube used in the champ article is clearly "greener" putting out around the same output power, 10.8W (15% THD) but consuming much less at 19W.

Your math is wrong. Plate: 350 x 0.054 = 18.9W Screen: 250 x 0.0025 = .625W Heater 6.3 x 0.9 = 5.67W Total: 25.195W

You can't really trust the power output. His OT was 5.1K, he didn't use a scope and signal generator to measure the output, just a guitar and a Volt Meter. I did better than that when I was in High school and that was 50 years ago.

[Stephen1966]

In comparison the 6L6GC (used in the champ article) running at a grid No.2 voltage of 250V (plate voltage =350V) in class A, requires 54mA = 250 x 0.054 = 13.5W, plus 5.67W for heater current when it pulls 0.9A at 6.3V. This tube used in the champ article is clearly "greener" putting out around the same output power, 10.8W (15% THD) but consuming much less at 19W.

Your math is wrong. Plate: 350 x 0.054 = 18.9W Screen: 250 x 0.0025 = .625W Heater 6.3 x 0.9 = 5.67W Total: 25.195W

You can't really trust the power output. His OT was 5.1K, he didn't use a scope and signal generator to measure the output, just a guitar and a Volt Meter. I did better than that when I was in High school and that was 50 years ago.

My misunderstanding, thank you for your feedback. I was assuming the screens drew the lion's share of the current [hence the screen voltage in the calculation]. Not giving up on the EL34 just yet though... Andy made a good point with tone.