KT66 to EL34

[Travis8]

Hi, I have a tubeampdoctor JTM45 build with KT66.

How can I changue it to EL34? Thanks.

[HotBluePlates]

Make sure Pin 1 of the output tube socket is connected to Pin 8.  Re-bias the amp when you install the EL34s.

You probably have some healthy screen resistors, but if not add 1kΩ 5w resistors going into Pin 4 of the output tube sockets.

[Travis8]

Here is the schematic

[Travis8]

Make sure Pin 1 of the output tube socket is connected to Pin 8.  Re-bias the amp when you install the EL34s.

You probably have some healthy screen resistors, but if not add 1kΩ 5w resistors going into Pin 4 of the output tube sockets.

Pin 1 go with pin 8, so I must swap 470ohm resistors with 1k resistor (between 4 and 6) and rebias when I install el34? No OT swap ?

[Willabe]

Here is the schematic

That's not a schematic. That's a layout drawing.

[Willabe]

Pin 1 go with pin 8, so I must swap 470ohm resistors with 1k resistor (between 4 and 6) and rebias when I install el34? No OT swap?

The layout drawing shows pin 1 is connected to pin 8, double check to make sure.

Yes, swap 1K/5w screen grid R's for the 470R.

Yes, re-bias when you swap the tubes.

No need to change the OT.

[pdf64]

… No OT swap ?

What are the electrical characteristics of the OT you’ve got, eg intended primary impedance?
If it's the typical JTM45 8k, then you may wish to try setting the impedance selector a step higher than the total effective load impedance.
That will take some stress off the screen grids.

[Travis8]

… No OT swap ?

What are the electrical characteristics of the OT you’ve got, eg intended primary impedance?
If it's the typical JTM45 8k, then you may wish to try setting the impedance selector a step higher than the total effective load impedance.
That will take some stress off the screen grids.

125c1a is my OT

[jordan86]

That model # looks like a choke?!?

[Travis8]

https://www.tubeampdoctor.com/en/choke-4h-90-ohm-for-fender-boogie?number=125C1A


This one

[Willabe]

No, that's the choke.

That's not the OT.

[Travis8]

Sorry!!

https://www.tubeampdoctor.com/en/output-transf.-for-marshall-jtm45-2xkt66-with-8-16-ohms?number=MJTM45A

This one.

[Travis8]

… No OT swap ?

What are the electrical characteristics of the OT you’ve got, eg intended primary impedance?
If it's the typical JTM45 8k, then you may wish to try setting the impedance selector a step higher than the total effective load impedance.
That will take some stress off the screen grids.

https://www.tubeampdoctor.com/en/output-transf.-for-marshall-jtm45-2xkt66-with-8-16-ohms?number=MJTM45A

[pdf64]

Yes, that’s an 8k primary, so it’s worth trying the ‘alternative loading’ I suggested.

[Travis8]

Yes, that’s an 8k primary, so it’s worth trying the ‘alternative loading’ I suggested.

So I must swap 1K/5w screen grid R's for the 470R.
Then I must use the impedance selector one step higher, for example, If I wanna use it with 8 ohms cab I must select 16 ohm output.
Finally I install EL34 and bias it

That´s correct?

[pdf64]

… I must use the impedance selector one step higher, for example, If I wanna use it with 8 ohms cab I must select 16 ohm output.
Finally I install EL34 and bias it ….

I don’t think the impedance thing should be considered mandatory. The change I suggested may be a bit easier on the EL34.
Consider trying it both ways.

[Travis8]

… I must use the impedance selector one step higher, for example, If I wanna use it with 8 ohms cab I must select 16 ohm output.
Finally I install EL34 and bias it ….

I don’t think the impedance thing should be considered mandatory. The change I suggested may be a bit easier on the EL34.
Consider trying it both ways.

OK, I will try using EL34 with the step higher option. Thanks.

[Travis8]

… I must use the impedance selector one step higher, for example, If I wanna use it with 8 ohms cab I must select 16 ohm output.
Finally I install EL34 and bias it ….

I don’t think the impedance thing should be considered mandatory. The change I suggested may be a bit easier on the EL34.
Consider trying it both ways.

If I replace resistors and I bias, I could still using the 4 ohms output?

Maybe I prefer to still having this option.

[tubeswell]

Optimal load for an EL34 in centre bias Class A with a plate voltage around (say) 450 = 450/(25/450) = 8,100R which is 4,050R for a pair of EL34. Go slightly lower (say 3k2) to ensure the screens don’t overheat under big signal conditions.


You could run the pair at 8k but you’ll get less power output, and you’ll want good screen resistors to soak up excess screen current under big signal. (Higher load resistance = shallower load line = more Ig2 when plate voltage swings closer to zero).

[Travis8]

Optimal load for an EL34 in centre bias Class A with a plate voltage around (say) 450 = 450/(25/450) = 8,100R which is 4,050R for a pair of EL34. Go slightly lower (say 3k2) to ensure the screens don’t overheat under big signal conditions.


You could run the pair at 8k but you’ll get less power output, and you’ll want good screen resistors to soak up excess screen current under big signal. (Higher load resistance = shallower load line = more Ig2 when plate voltage swings closer to zero).

Maybe the safer way to do it is use 1k/5W and higher step on impedance selector. I always use 8 and 16 ohms cabs

[Travis8]

I change resistor and I rebias the amp, it´s work but sometimes loose power and I get some intermittent sound

Recommendation?

[pdf64]

So did it do that when the KT66 were fitted?
Has it ever worked right?

[Travis8]

So did it do that when the KT66 were fitted?
Has it ever worked right?

No, with KT66 was fine

[pdf64]

What condition is the 1k 5W board mounted resistor (that feeds the screen grid resistors) in, eg looking overheated?

[Travis8]

What condition is the 1k 5W board mounted resistor (that feeds the screen grid resistors) in, eg looking overheated?

They look in good condition.

Maximum bias I can get it´s 30ma, I think is very low, how can I get more bias range?

Could low bias making the amps sound comes and goes?

thanks

[Williamblake]

Could low bias making the amps sound comes and goes?

thanks

Loosing screen voltage would do that. Dind't somebody just change the screen resitors?

[pdf64]

…
Maximum bias I can get it´s 30ma, I think is very low …

Why do you think that?
Bear in mind that the anode of KT66 isn’t as beefy as that of EL34.

how can I get more bias range?

Reduce the bias voltage (ie applied to pin5).

Could low bias making the amps sound comes and goes?

I can’t see how that could feasibly occur, especially as 30mA does not seem a particularly low idle current.

[Travis8]

Could low bias making the amps sound comes and goes?

thanks

Loosing screen voltage would do that. Dind't somebody just change the screen resitors?

Yes, I use 1k 5W on screen resistor, but sound comes and goes :/

[shooter]

sound coming and going is typically blocking distortion in the PI.  A scope comes in handy to know if that's the case.

[sluckey]

Maximum bias I can get it´s 30ma, I think is very low, how can I get more bias range?

See attached pic to provide proper bias for EL34s...

[Travis8]

Maximum bias I can get it´s 30ma, I think is very low, how can I get more bias range?

See attached pic to provide proper bias for EL34s...

I will try 150k...

Blocking PI distorsion means?

[shooter]

this explains it pretty well;


The Long-Tail Pair (aikenamps.com)

[pdf64]

There’s also http://aikenamps.com/index.php/what-is-blocking-distortion
If the issue can occur at non overdriven signal levels, then it won’t be blocking distortion.

[Travis8]

There’s also http://aikenamps.com/index.php/what-is-blocking-distortion
If the issue can occur at non overdriven signal levels, then it won’t be blocking distortion.

The amps sound but the sound comes and goes in all positions.

I will try to fix the bias issue and look if that helps...

[PRR]

> sound comes and goes

Missing or not-really-connected grid resistor. Any stage.

[Travis8]

There’s also http://aikenamps.com/index.php/what-is-blocking-distortion
If the issue can occur at non overdriven signal levels, then it won’t be blocking distortion.

I changue to 150k and now I can bias it better.

Bias now is 43ma and it´s look like the sound goes well :) thanks guys