Selectable EL34/6L6 Amp OT

[Valvomaniac]

I'm planning my first hifi stereo amp.

For the matter of tube availability-in Egypt-I want to build an amp
that can work with both EL34 or 6L6 (the tubes that I managed to collect-hunting hunting hunting!)

For biasing I'll use either autobias or a bias selector switch whichever is better.

My real problem is with the OT. Can I find or wind an OT that'll work with both tubes?

I've seen a Messa Boogie guitar amp which uses either but some body told me that it is going to be a problem (at this point please dont laugh!!!) he said somthing about one of the tubes -may be the 6L6-having some value of 6.6 & the EL34 has 3.4.

So Is it possible to realize my dream: An amp that uses EL34 & 6L6!?? How???

[ErikdeBest]

Hi valvomaniac

I just looked up at application data for both the 6L6 and the EL34, available at duncanamps.com.

the 6L6
http://tdsl.duncanamps.com/show.php?des=6L6

the EL34
http://tdsl.duncanamps.com/show.php?des=EL34

Seems there is a good overlap of characteristics when operating the valves in AB1:
- This is a good class I think: efficient and easier to make a driver stage.
- Both valves uperate at a B+ of 450V, although at different currents.
- the voltage required at grid 2 are close. As you will have to regulate this one for best characteristics, you may choose a MAIDA regulator, which can be easily made to have two selectable output voltages.
- Bias point is approximately the same (-36 and -37 volts on grid 1) . This does not mean that, when using fixed bias, you just may change the valves and forget about bias. At each change off valves you will have to check that the valves are correctly biased.
- The output Z for the EL34 should be 5k, while for the 6L6 5k6. This difference of 600R can be disregarded, as the real reflected impedance will vary more than that (in most cases).
- socket wiring is the same. Just check what you do with the third grid of the EL34.

Hope this helps

Erik

[Yvesm]

Hi,

The simplest way is to use a 6K OT, 400V B+ and 250V G2 wich are correct for a pair of 6L6 and should give you near 30W output power.

Plug two EL34, rebias and you'll got the same power. (beware of the G3 connection and the higher heather current).

Of course, you could obtain more power from a pair of EL34 at the expense of a beefier power supply and switchable load.

Yves.

<edit>
not so different that erik suggested while typing this answer !

[Valvomaniac]

Hi Eric, Hi Yves
First of all thanx for your help-

The configuration I think will work for me is:

6K OT
400V B+
G2 with a selectable MAIDA regulator
Switch selectable bias-as in the Messa Boogie Roadking-
Switchable load
G3/cathode connection can be switched with small switches may be ganged

How about that??? I really want hight fidelity I dont care too much about reasonable power loss.

Well every thing is switchable & selectable!!! May be buiding 2 Amps and "switching between them!!" would be more realistic dont you think so????

Any I have some questions to ask, so I hope U'll be patient with me.

1-

- The output Z for the EL34 should be 5k, while for the 6L6 5k6

What is the output Z? Is it different from the "load" Yves mentioned?

Of course, you could obtain more power from a pair of EL34 at the expense of a beefier power supply and switchable load.

2- What is a MAIDA regulator?