Simple tri-triode 360k amp

[roberto]

Hello,

I report here a project that I have sketched these evenings in the hotel, to be developed in the coming months.
A simple amp for clean, pedal-eating sounds, clearly to be refined in detail once built.

It has only two power supply capacitors, one for the preamp and one for the PI and power amp.

Tri-triode because the preamp is a triode, the PI is a triode and ... the power amplifiers thus configured have the curves of a triode.
360k on the other hand is due to the fact that the 12AX7s all have a load of 360k.

The preamp is inspired by the Songwriter, but the values have been changed to suit the EL34s.
The frequency shape is instead between a Vox and a Dumble (controllable with the bass control).

I've used a CCS on the LTP not to waste voltages on the bottom side (the high load "eats" the upper part), and still have a 88k impedance to ground to keep the PI balanced while keeping the anode resistors the same.

I will keep you updated.

Tri-triode 360k.jpg

[_Steve]

Thanks for posting - interesting design!

How does the PI work without the 2nd grid being AC grounded with a cap? Is that an omission, or do I need some learnin'?

[bepone]

Hello,
I report here a project that I have sketched these evenings in the hotel, to be developed in the coming months.
A simple amp for clean, pedal-eating sounds, clearly to be refined in detail once built.

i'm also spending life in hotels and asking myself questions until when will be like this=?
1. to go only with few electrolytics usually lead to instability and oscillations via power supply
2. shade + UL for output stage? too much linearisation IMO, maybe you can convert PI to classic supply and see.. triode output leads to "dull" guitar sound without chime, it is in oposite from guitar sound requirements("controlled" presence but without shrill highs to be able to cut through)

try to build your idea and give some feedbacks.. i have quit with triode outputs on gutar amps 10 years ago due to luck highs (read damping factor, different speaker freq characteristic, pentode nonlinearities)..

[roberto]

How does the PI work without the 2nd grid being AC grounded with a cap? Is that an omission, or do I need some learnin'?

The latter.
Both grids are referred to gnd because in this design, as explained, the ccs helps to recover the missing swing on the high voltages (because of the high value plate resistors) on the low voltages.
This also eliminates the need of the coupoling cap before the PI.

[roberto]

i'm also spending life in hotels and asking myself questions until when will be like this=?

In my case, until work and know-how wont be in the same place. Then...

1. to go only with few electrolytics usually lead to instability and oscillations via power supply

The final part of the amp is similar to what I use for some of my Hi-Fi amps (less UF, more Shade, driver dc coupled to output tubes' grids) and it works good.
The gain stage is decoupled with 220k so it should not bring oscillations. I'll see.

2. shade + UL for output stage? too much linearisation IMO, maybe you can convert PI to classic supply and see.. triode output leads to "dull" guitar sound without chime, it is in oposite from guitar sound requirements("controlled" presence but without shrill highs to be able to cut through)

This is one point I'll look into, but I have two set of trafos with low primary inductance (around 60 H) and 40% UL and I wanted to find them an application.

try to build your idea and give some feedbacks.. i have quit with triode outputs on gutar amps 10 years ago due to luck highs (read damping factor, different speaker freq characteristic, pentode nonlinearities)..

I've heard some designs with UL only as feedback, and they aren't bad. I then added a small amount of Shade to try it, and the Rout is not that low being 6 Ohm. I'll see, but I agree with you that could be too linear, that's why I wrote that it will need some refinements when built.

[pdf64]

So ‘shade’ is referring to the coupling between LTP and output anodes?

[roberto]

So ‘shade’ is referring to the coupling between LTP and output anodes?

It is usually called Shade, but the "inventor" is Otto H. Schade (see attached 1941 patent), and is basically shunt applied feedback from anode to g1, so that means you need more current to drive the tube with the same swing.
The benefit is that the tube is linearized and its internal impedance is reduced, so it becomes a triode but with the capability to swing down the anode of the output tube to less than 50V.

It is mostly used for SE amps, but I like it for PP as well.
The benefit of this specific design, "invented" by Yves Monmagnon in 2004 ( http://www.dissident-audio.com/PP_ECL86/Page.html ), is that with one single cap you can supply PI, screen and plates.