Dr. Jekyll & Mr. Hyde: a Dumble with a GU-50 in Single Ended

[roberto]

Ever tried to make a guitar amp more controlled without killing what makes it sound like a guitar amp?

That was the rabbit hole.

We ended up building a GU50 single-ended amp with two feedback paths: anode-to-anode voltage feedback from the output tube to the driver, and variable negative-null-positive current feedback from the OT secondary to the driver cathode.

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The project started on the Italian DIY forum DIYItalia, when Kagliostro proposed the idea of a Dumble-inspired GU50 SE.
http://www.diyitalia.eu/viewtopic.php?f=22&t=11331
From there it became a real team effort.

I worked on the output stage, power supply and effects loop, bringing some ideas from my hi-fi work into the design, but adapting them to guitar rather than trying to make a guitar amp behave like hi-fi. Dom made a masterpiece work building the amplifier and developing the FET preamp.

The anode-to-anode feedback cleans up the GU50 considerably while keeping the character of the pentode.
It also allows the OT to be driven from a lower impedance, improving the low end without taking feedback from the secondary.

The interesting part is the positive current feedback from the OT secondary to the driver cathode.
It gives more control over the speaker, but the result doesn't feel more restrained. If anything, it gives the amp more movement and life.

That's where the Dr. Jekyll & Mr. Hyde name came from.

Then there's g3 raised by around 15V compared to the cathode, to give pentode curves a sharper and higher knee.
To make them more petodeish.

TRUMBLE - PRE + AMP - COMPLETO - DEFINITIVO.pdf

The driver/recovery stage has an LED on its cathode, directly connected to the secondary current-feedback path.
The PSU is regulated for low noise, with G2 selected according to the OT impedance and G3 optimized for the GU50 pentode curves.
The effects loop was also part of the design from the beginning and can be used either as a conventional unbalanced loop or with a balanced send.

TRUMBLE - PSU - DEFINITIVO.pdf

Then we plugged a guitar into it.
Luca Mongia played his Danelectro, Telecaster and Stratocaster through a Laney 2×12 with Eminence V12 Legends.
No re-amping, no studio processing. Just guitar, amp and 2×12.

VIDEO-2026-09-25-19-31-30.mp3

I'll put the schematics and build details below.
Always interested in different approaches, so feel free to pick it apart.

[erwin_ve]

I haven't gone through the tech details, but the amps sounds fantastic in the mp3!
Well done!
My interest is mainly the positve fb at the driver tube. Looking into that soon.
Thnx for posting!

[roberto]

Thanks Erwin!

It is born from the experience of all three letting more “space” where each of us has more competences, and we are very proud of how it, indeed.

We already started discussing a PP applying same concepts (plus a few more), to have a more powerful companion for the Single Ended.

Kagliostro and Dom will intervene here too to add some more information and most probably a new dress.

[Guy77]

Very cool sounding for just a single EL84 and it looks great.

Congrats Roberto!

Ciao
Guy (Gaetano)

[Kagliostro]

May be there is a bit of misunderstand, the power tube is a GU-50 and with only one in SE it is a 15W amp

Many people call it a ruggedized KT88

https://en.wikipedia.org/wiki/GU-50

Franco

[roberto]

Ciao Gaetano,

thank you for your kind words!
It has been a team work, and we all are proud of it!

As Kagliostro said, the tube is a GU-50, a tube that was developed in Germany one century ago mainly for radio transmissions under the original name LS-50, then built in Russia after the war until few decades ago.
It has 40W plate dissipation, g2 limited to 300V, g3 with a dedicated pin. A great tube for HiFi too, but also bass amps (you can get around 400-450W from a sextet if you push the anodes to 950V, 300W if you push it to 650V).

[Guy77]

Ciao Gaetano,

thank you for your kind words!
It has been a team work, and we all are proud of it!

As Kagliostro said, the tube is a GU-50, a tube that was developed in Germany one century ago mainly for radio transmissions under the original name LS-50, then built in Russia after the war until few decades ago.
It has 40W plate dissipation, g2 limited to 300V, g3 with a dedicated pin. A great tube for HiFi too, but also bass amps (you can get around 400-450W from a sextet if you push the anodes to 950V, 300W if you push it to 650V).

Thank you correcting me Roberto & Kagliostro . This probably explains why it sounds nice and full like a 6L6 or KT88 or similar Octal power tube ( I realize this is a much different pin out) and why I enjoyed the sound.

You have lots of great sounding tubes in Europe that we don't see much here in the West!

I visited an amp builder in Italy last summer by the name of Ernesto. He lives near the town of Bojano. He owns several 400 yr old homes with rooms filled with vacuum tubes with names I have never heard of!

Ciao
Guy

[roberto]

Thanks Guy!

I'm not aware of any famous amp builder named Ernesto, and was not even aware of the small town called Bojano in Molise.
Old TV and radio always had many hidden treasures, and russian tubes have other gems too.
You most probably can find them too in USA if you go to radio amateur meetings.

There are also copies of most known tubes with different names, just because heaters are supplied at different voltages (16, 25V) or because the anode is connected to a cap on top of the valve.

[domx]

Hello guys.

Thank you all for your appreciation!

...Perhaps some of you might like to see some porno pics????

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[domx]

...a few photos without the cabinet:

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[Colossal]

Very nice work!

[domx]

For now, there are only these recordings made in a hurry with my phone:

phone rec 1.mp3

phone rec 2.mp3

phone rec 3.mp3

phone rec 4.mp3

I should be recording some better quality samples soon.
...I hope soon!