High current low voltage / low current high voltage ?

[Colas LeGrippa]

Hi. I'd like to know what do you guys prefer from a power tube : high I , low E or the opposite ? What tonal characteristic you prefer from both ?
The reason is that I am working on a SE amplifier, design of my own with 2 X EL34 in parallel and I hesitate between 250V/100mA and 350V/70mA. ( the trannies are fine with both ).

Regards

Colas

[Leevi]

I prefer 250V. The higher secondary gives too high B+, more than 450V (after rectifier)
which makes the sound too clean and cold. Of course it's depending on what kind of sound
you are looking for.
/Leevi

[stratele52]

Yes  for 250 volts .

[tubenit]

2 X EL34 in parallel

IF I am understanding your post correctly,  I don't think you will have enough ma current capability for paralleled EL34 tubes if you PT options only allow 70-100ma of current.

Please look at this tube data cheat sheet.
 
With respect, Tubenit

[Colas LeGrippa]

Jeff, my PT can supply .5 A, so no prob about it. From a tone point of you, what do you prefer ?

regards

Colas

[Colas LeGrippa]

what I meant was 250V / 100 mA for each tube, 200mA total.

[jjasilli]

350V is close to the max (375) for self-bias.  Theoretically, the lower 250V will be more in keeping, tone-wise, with the SE design.  But there may not be an objectively true answer.  The tonal result may be purely subjective.  Or you may get earlier / later break-up (headroom) with tone otherwise being the same.  The only way to know is to try.  Hence I suggest you should A/B both voltages.

[Colas LeGrippa]

yeah it is often a question of taste, but objectively, we can hear clearly the difference between a clean tone and a distorted one. I have already tried 350V, but now to get the HV down to 250V, I have to hook up a VVR and I just thought I could save me this mod but now that you have spoken 'bout an eventual better tone at 250V, I gotta try it  , maybe I'll find the right balance at 300V.

Colas

[tubeswell]

2 X EL34 in parallel

IF I am understanding your post correctly,  I don't think you will have enough ma current capability for paralleled EL34 tubes if you PT options only allow 70-100ma of current.

Please look at this tube data cheat sheet.
 
With respect, Tubenit

Hi Jeff

There's a typing error in the heater current draw for the EL34 part of the cheat sheet you loaded.

Pete

[jjasilli]

yeah it is often a question of taste, but objectively, we can hear clearly the difference between a clean tone and a distorted one. I have already tried 350V, but now to get the HV down to 250V, I have to hook up a VVR and I just thought I could save me this mod but now that you have spoken 'bout an eventual better tone at 250V, I gotta try it  , maybe I'll find the right balance at 300V.
Colas

Vintage amps tended to be run at moderate voltages. Modern amps are often run at max or over-spec voltages.  (there are exceptions such as Traynor who ran vintage amps at over-spec voltages).  I think the different voltages treat harmonics differently.  Maybe sag changes also, but your amp is SE anyway.  Nevertheless, it's possible that EL34's might not notice that much of a difference between 250 - 350V. 

The easiest way to A/B test different voltages for design & development is with a variac.  Other alternatives:  bucking transformer; variable bench power supply.

[tubenit]

my PT can supply .5 A, so no prob about it.

A pair of EL34's will require 1.0A heater per the data sheet which is twice as much as what you are saying your PT can supply.

I apologize if I am not understanding how you're reporting the PT's specs? Just trying to help you be on the safe side.

Having said that, everytime I have used a VVR ........ I have preferred lower voltages on the plates of the power tubes (tone wise).

With respect, Tubenit

[tubeswell]

my PT can supply .5 A, so no prob about it.

A pair of EL34's will require 1.0A heater per the data sheet which is twice as much as what you are saying your PT can supply.

That would be 3A per pair

[Colas LeGrippa]

OK, I am sorry if I don't make things clear about my PT specs, but that ain't the subject of my post anyway that concerns the tone of power tubes at a given voltage. For those who cares, the power transformer can handle easily 4 X EL34 @ 100mA each,  the heaters tap can supply around 6A. It was first ordered for a 100 w amplifier from Avel Lindbergh ( toroïdal ).

sorry for the confusion

Colas

[Colas LeGrippa]

Just came back from my shop after modifying the voltage on my 2 X EL34 : with 250V on the plates and 100mA each tube I get a high squeal, like when  the plates lead are reversed on a PP amplifier ( not so loud, though ). Does that mean my OPT can't handle a I of  200mA , or is there another issue ? I am to the point of swapping the OPT but I am running out of time, too many rehearsals and gigs and that damn amp doesn't do what I want to

Thanks for ideas

Colas

[HotBluePlates]

Can you be certain the way you incorporated the modification didn't cause the squeal? That is, wiring, lead dress, etc.

[Colas LeGrippa]

All I did is swapping the 2 X 360 Ohm for 2 X 100 Ohm and lowering the B+ on the VVR to 250V, nothing else. Maybe the OPT can't stand the higher current ?

[Colas LeGrippa]

oh , just remembered, I did not put back the cathode resistor bypass cap...........but this is not the cause, is it ?

[Colas LeGrippa]

problem solved: OPT too weak for 200mA, replaced OPT and everything is just fine at 250V / 110 mA each tube, but I prefer the tone at 350V / 70mA

thanks for your help once again

Colas

[Willabe]

Why/how does it sound better?

Brighter, darker, cleaner, more head room?

Would be good to know what changed.


                
                            Brad        

[jjasilli]

Why/how does it sound better?  Brighter, darker, cleaner, more head room?  Would be good to know what changed.    
                            Brad         

Ditto! Same question.

[Colas LeGrippa]

more defined tone at 350V, a bit of headroom more, more biting highs. At 250V, browner tone , like if AC signal had a hard time surfing on too much dc current !

Colas

[Willabe]

Alrighty then, thanks for the good info Colas. You've done enough builds/repairs to have developed a good set of ears.

Helps all of us get a grip on our builds.


             Brad