6SL7 + EL34 SE Audio Amp

[dr._sleep]

I have all the parts to begin the following:



I plan on staying fairly close to the schematic except for using Lundah LL1664 output transformers and perhaps connect the cathode circuit to the secondaries ala:



Any suggestions, criticisms, warnings, poo-poos welcome and appreciated. The amp will power a pair of LaScala's owed by my brother, CD as source.

Dr._sleep

[Bas Horneman]

Not much to add..it's a proven design I believe and should perform beautifully!

[Tubes4e4]

Hi,

I have build such an amp, quite closely to the schematic shown above, also using the 125ESE OPT.

I strongly suggest to use a reflected Ra of 10k, somewhat higher B+ and a small bit of gNFB, you won´t regret it I am sure.

Have a look at my interpretation of this circuit (scroll down to the link table to get the moded schematics).

Tom

[dr._sleep]

Tube4e4:
Great site with plenty useful information.

I am now financially committed to using the Lundhal LL1664 OPT's, primary inductance 3K @ 8 ohms. One variation I may try is

Here the screen grid is connected to a 50% tap on the primary.


If the primary is configured as taps 1 to Plate/Anode,5 strapped to 11, and 7 to B+, then what/where is the 50% tap on the LL1664? Taps 3 and 9 are not tied, is it one of these.

help, dr._sleep

[dougigs]

Could you explain a bit more about what you're proposing for the cathode-bias circuit?

I can't see clearly enough -- you say you're connecting the cathode resistor (and its bypass capacitors) to the secondary of the OPT -- does that mean it's connected to the *ground* side of the secondary? This would be fairly conventional.

But your schematic seems to show it connected to the "hot" side of the secondary (i.e. the one connected to the + of the speaker). If that's the case, I don't understand at all, and I'd enjoy some explanation.

[Griz]

Originally posted by dr._sleep
Tube4e4:
Great site with plenty useful information.

I am now financially committed to using the Lundhal LL1664 OPT's, primary inductance 3K @ 8 ohms. One variation I may try is

Here the screen grid is connected to a 50% tap on the primary.


If the primary is configured as taps 1 to Plate/Anode,5 strapped to 11, and 7 to B+, then what/where is the 50% tap on the LL1664? Taps 3 and 9 are not tied, is it one of these.

help, dr._sleep

Hi,

The 50% tap is at 5 / 11. It's all pretty well explained in Mr. Byrith's article:

--- from the article ---

Connections to the output transformer are shown on the next page. The primary is divided into
two sections each with a tapping point. You have two options:
1. Supply voltage connected to 1, 5 strapped to 11 and anode connected to 7. You have tapping
points at 16%, 50% and 83/%.
2. Supply voltage connected to 5, 6 (ed. SHOULD BE "7") strapped to 1 and anode connected to 11. Tapping points
are now at 33%, 50% and 67%.
I have chosen my tap at 50% so both configurations will do. It is essential that polarity is right
when the secondary is included in the cathode circuit of the output valve. Make sure that output
signal drops when you make these connections. Otherwise feedback becomes positive and output
resistance will increase.

----------------------------

Remember that the cathode feedback is fed back as voltage feedback to the grid of the driver tube in the Byrith circuit, and not as current feedback to the cathode as is more common.

You really should give Mr. Byrith's article a read, if you haven't already done so.

Griz