E130L PP Power Amplifier

[frentec]

I like the sound of that!

[Kay Pirinha]

Really? I can be shown easily that the balance of both output signals improves with increasing tail impedance. With a high impedance CCS tail you'll get well balanced signals even with dissimilar triodes.
Best regards!

[Ketje]

Originally Posted by v4lve lover

For the bias i was thinking of using the plug in modules ive made a PCB for to set the bias, see schematic and board attached, this will need a time delay to synchronize the application of B+ and bias to the tubes.

How about like this ?
Mona

[gerrittube]

v4lve lover,

Can it be I saw a quite similar auto bias system on zelfbouwaudio.nl? Do you have gerbers available for these boards? I made one with 4 opamps with a PCB I got from a user there. Works fine!

Regards, Gerrit

[artosalo]

Actually a LTP works fine even with a few kilo ohms resistor as a tail.

Really?....With a high impedance CCS tail you'll get well balanced signals even with dissimilar triodes.

Attached is the normal LTP with a resistor at the tail. To get balanced output signals, the plate resistor of non inverting halve must be adjusted to higher resistance.

Usually fully balanced/equal signals from the phase inverter are not essential. It is essential that the level of output signals can be adjusted.
Output tubes may have a bit dissimilar gm and therefore to have balanced signals at the plates of the output tubes (which is essential) the drive signal of the output tubes should be adjusted individually.

The attached LTP produce required 40 Vpp drive with some 0.1...0.2 % THD.
Is that not OK ?

[Kay Pirinha]

Maybe. I prefer equal plate resistors, though, equal signal levels and don't really want to adjust.
Best regards!

[artosalo]

I prefer equal plate resistors, though, equal signal levels and don't really want to adjust.

Why ?
By doing so the output stage generates even harmonics if output tubes are not completely matched which they not always be.

I would first adjust the DC-balance (bias) of output tubes and then AC-balance from phase inverter to minimum overall THD.

I would not be satisfied if I knew that the output of LTP is fully balanced, but the output stage is not.

[Kay Pirinha]

Unequal plate resistors in LTP PI's resemble musical instruments amplifiers, which are FX devices and where it is ok, rather than HiFi ones.
Best regards!

[artosalo]

Unequal plate resistors in properly designed LTP is only a visual disturbance.
Output signals are equal in all respect.

[artosalo]

As a tubes-only version, I would build this: