5 wire and 3 wire output transformer change ?

[nigelwright7557]

I have a push pull el34 valve/tube amplifier for which the design shows it needs a 5 wire primary.
I have found a cheap 3 wire primary transformer, can I use this ?
I obviously need to connect grid2 to this transformer which currently is done using 270 resistors.
Will this be ok.

 

[DF96]

By '5 wire' I assume you mean UL. With a '3 wire' OPT you only have the choice between triode and pentode mode; presumably the transformer will only have the correct impedance for one of these. The gain, power and distortion will be different for each mode. If there is global feedback this adds further complication, especially with a cheap transformer - you might get instability.

 

[nigelwright7557]

Yes the original circuit is UL.
I want to use pentode mode.
I am using a 270r between grid2 and anode.
I am not using any feedback as I want to get a little distortion (warm sound) for a guitar.

 

[DF96]

270R between g2 and anode will give you triode mode. For pentode mode you are likely to need a slightly different value from g2 to the supply rail and a decouping cap to ground. Alternatively, you might be able to connect g2 straight to the supply and adjust the g1 bias - depends on the supply voltage.

You might get better advice in the Instruments & Amps forum - this one is for hi-fi.

 

[nigelwright7557]

I will go for triode mode then.

Will that affect power out much ?

 

[DF96]

Triode mode is likely to give you less power and less gain. This will mean that the phase splitter will need to produce a bit more voltage.

 

[nigelwright7557]

How low can I go with the anode to grid 2 resistor and stay in triode mode and get the best power.

 

[nigelwright7557]

For pentode mode you are likely to need a slightly different value from g2 to the supply rail and a decouping cap to ground. Alternatively, you might be able to connect g2 straight to the supply and adjust the g1 bias - depends on the supply voltage.

I would prefer pentode mode for more power.

Any idea what resistor I would need to connect g2 to B+ ?

 

[nigelwright7557]

I had a good look around the internet for pentode mode circuits.
Using my circuit as it is I can connect the G2 resistors to B+ and that should work OK.
The output transformer is connected to my pcb through terminal pins so I can short B+ to the G2 resistor tap.
B+ is 340 volts.

 

[nigelwright7557]

My understanding of the pentode is G2 is a bit more positive than the anode to accelerate electrons to the anode.

So, putting a resistor in series with G2 to B+ should take the place of the UL taps on the transformer.
Some current will pass into G2 but the resistor will cause a voltage drop so it G2 doesn't take too much current.

 

[nigelwright7557]

OK finally got my head around UL, triode and pentode.

UL is using a 40% OPT tap.
Triode is connecting G2 through resistor to anode.
Pentode is connecting g2 through resistor to B+.

It looks like the resolution of my original question is to use pentode mode.

 

[Tesla88]

It's a cathode bias or fixed bias ?
Anyway you need to know plate voltage , most guitar amps use EL34 in pentode mode , g2 is connected to B+ by a 1K/1W resistor , for plates voltages beetween 350-400V is ok. In amps where tubes are pushed to 450-500V or more , is better to use a bit lower voltage for g2 ...
Anyway looking at the EL34 datasheet s the way , also lookin at some commercial amps such marshall , engl , peavey , ecc..
Of course you need to measure current to be sure you're not exceeding absolute maximum value per plate and screen dissipation !

 

[nigelwright7557]

It just uses a cathode resistor of 270R and G1 is tied to ground with 100k
I am using 350VDC at the moment but I might need to up it a bit.