Giaime's new toy - aka the UniOctal

[Giaime]

Hello folks!

Well, time to do amp designing

What do you think about it? Schematics attached.

I will finish soon designing the PSU: it will be SS rectification, capacitor input filter and LC filter.

I'm studying the possibility of building an amp that can let me exchange the output tubes: from a rapid look, 6V6 and its cousins would work.

You would ask: why so many coupling caps? Just DC-couple the first and the second stage. Well, that's an hard thing to do with only 300V B+, but if you have ideas please feel free to post them!

Gain is pretty high, I think I will add 3dB of global NFB from the secondary of the output transformer back to the (actually grounded) minus input of the LTP input stage.

[Giaime]

NO!!!!

Wrong coupling caps values

Updated schematic:

[Geek]

Hi Giaime,

Mmmmmm, EL36 in triode

Since you have a negative supply on the input stage, have you thought of DC coupling to the 6CG7?

[Giaime]

Yes Greg, I thought about it: but how can it be done with only 300V max B+?

You already have 200V at the ECC81 plate, you can have only 60V on the plate resistor of the 6CG7: of course cathode should be at some 210V above ground.

1) how could you bias a 6CG7 with those values?

2) where do I put the negative supply? I don't see...

[Tubes4e4]

Hi Giaime,

using EL36 in triode mode, with about 300V across it, and Vg1 = -42V, you will get about 100mA of plate current and a plate Pd = 30 Watts but probably only for a few minutes

Tom

[Giaime]

Hello Tom,

I've downloaded the EL36-as-triode sheets at your site, and to get 50mA quiescent current I need about -50V, not too far from what TubeCAD calculated.

100mA at 300V I need at least -38V...

This surely imposes the interesting topic of the accuracy of TubeCAD calculations, but when the bias is regulated with a simple trimmer, frankly I don't see the point of the question

I'll need some more volts drive, that's not bad the driver can already do 70Vpk.

[Tubes4e4]

Hi Giaime,

but when the bias is regulated with a simple trimmer, frankly I don't see the point of the question

Good argument, indeed!

Sorry for my misconception, but I just thought the values you gave in your schematic were measured / in vivo ones.

Tom

[Giaime]

Oh excuse me for this Tom. No, the prototype hasn't been built yet.

Please don't use that schematic it hasn't been tested.

So, back to the topic: Greg suggested DC coupling. For me, it's not doable.

what do you think?

Are there other points I should improve?

Notice that I tried to take care of the phase shifting associated to the 2 coupling caps: in fact the time constants of the two RC couplings are very far from each other, I could increase them if you find it useful. I plan on having a 0.5Hz cut at the first, 5Hz at the second and 20-30Hz at the output transformer...

[poobah]

Giaime,

There is no reason to make the time constants different.

Choose one time constant to be dominant (the OPT). Select the others at a much lower frequency. The total phase shift in your loop will always be the total of the separate shifts anyway.

[Tubes4e4]

Hi Giaime,

Greg suggested DC coupling. For me, it's not doable.
what do you think?

Probably hard to do with this tube selection (I mean the ECC81 in front, which needs quite some voltage across it and its plate resistors) and when not using a SS CC sink to make some voltage available which is burned in the tail resistor at the moment.

Regarding the time constants, they are not that critical at all, since you don´t show/use a gNFB loop (Poobah might have missed that in his article).

Tom