EL84 sounds louder than EL34

[Leevi]

I wired an amp based on the following circuit and added there an option for an extra
octal socket power tube.


http://cfile23.uf.tistory.com/image/20669C504E1D248A09B714


If I switch with EL84 and EL34 I noticed that EL84 is much louder than EL34.


Those have been implemented with own bias. Even if I run EL34 very hot it's not
that loud as EL84.


Plate voltage is about 300V


Could the cathode follower cause it?


Leevi
 

[VMS]

I think it might have something to do with the amplification factor of the tubes.

[Leevi]

or tranconductance?
/Leevi

[VMS]

Yes. Here's a quote from wikipedia:

It was developed to eliminate the need for a driver tube in radios, and has rather more gain than is usual in a power pentode, producing full output from a relatively small drive signal (the EL84 requires less than 4.4V_rms for 5.7W output compared with 9V_rms for 5.5W from a 6V6 or 8.7V_rms for 11W from an EL34).

[terminalgs]

Those two tubes are are too dis-similar to get optimal performance out of both with a single circuit (even with a bias adjust).  You'll get good results out of one, or the other, .. or neither!

a few other reasons the tubes can't be interchanged in a circuit without disappointing results from one or the other tube:

• an output transformer impedance will be optimal for an el84 @ 300V won't be optimal for an el34 @ 300V (and vice versa).
• optimal screen voltage for either tube @ 300V won't be the same.

Also, the 300V B+ ought to be mentioned:  if you survey circuits of great sounding amps, you'll find some EL84 amps with 300V B+,  but you probably won't find any 6V6 amps with less than 325V B+ (and you'll find some at 400V+), and you won't find a EL34 amp with less than 400V B+.  Can you take the datasheet for a EL34, do the math, draw load lines and get an optimal configuration for a EL34 to work at 300V B+?  YES,. is it the best use of a el34,  probably not,..  will the same circuit be optimal for a EL84, no.


You'd be better off working with a 6V6 than a EL34. 


side topic: why did you choose that master volume arrangement?  it seems like it would load down your tone stack unnecessarily (at low settings) compared to the more traditional volume pot arrangement (like you have on "volume"), which wouldn't load down the TS.

[Leevi]

Those two tubes are are too dis-similar to get optimal performance out of both with a single circuit (even with a bias adjust).  You'll get good results out of one, or the other, .. or neither!a few other reasons the tubes can't be interchanged in a circuit without disappointing results from one or the other tube:

• an output transformer impedance will be optimal for an el84 @ 300V won't be optimal for an el34 @ 300V (and vice versa).
• optimal screen voltage for either tube @ 300V won't be the same.

Also, the 300V B+ ought to be mentioned:  if you survey circuits of great soundin

I have taken that into account in my tests

It was developed to eliminate the need for a driver tube in radios, and has rather more gain than is usual in a power pentode, producing full output from a relatively small drive signal (the EL84 requires less than 4.4Vrms for 5.7W output compared with 9Vrms for 5.5W from a 6V6 or 8.7Vrms for 11W from an EL34

That might be the reason

/Leevi

[HotBluePlates]

Those two tubes are are too dis-similar to get optimal performance out of both with a single circuit (even with a bias adjust).  You'll get good results out of one, or the other, .. or neither!a few other reasons the tubes can't be interchanged in a circuit without disappointing results from one or the other tube:

• an output transformer impedance will be optimal for an el84 @ 300V won't be optimal for an el34 @ 300V (and vice versa).
• optimal screen voltage for either tube @ 300V won't be the same.

Also, the 300V B+ ought to be mentioned:  if you survey circuits of great soundin

I have taken that into account in my tests

You haven't told us the complete test conditions; the schematic only shows a single output tube.

Load (primary) impedance of the OT? Bias voltage across the cathode resistors of each tube?

Preliminary guess is you have a load impedance suitable for the EL84, so the EL34 won't make any additional output power over what the EL84 will do. If the EL34 bias voltage is also higher, then it takes a bigger drive signal to do the same-thing the EL84 did with less drive signal.

This is all saying what Terminalgs is saying, but a different way.

I think it might have something to do with the amplification factor of the tubes.

or tranconductance?
/Leevi

We don't have apples-to-apples data for the EL84 and EL34, but Gm for both is reasonably close to 11mA/V for either tube. Catch is your screen-to-plate bypass cap puts the output tube in semi-triode mode. Interestingly, the EL84 shows more Gm in this condition than the EL34. So that's a piece of the puzzle.

Amplification factor is mu from G1 to G2, and has application to triode mode but is mostly about bias of a pentode. The EL84's higher triode amplification factor means for a given screen voltage, it will take less G1 voltage to bias the tube. This also implies the EL84 will require less drive signal to reach its full output power (-12v bias can only accept a 12v peak signal, or less, before significant distortion where -35v bias will require a 35v peak signal to reach the limit of its output capability).

Last piece of the puzzle is OT primary impedance and the load presented to the tube(s) and power supply. A load that is suitable to get ~6w from a 12w tube with 300v B+ will allow a 25w tube to make only ~6w as well. 2x tubes will still only do ~6w total; you have to raise the B+ (impractical) or lower the load impedance (mismatch speaker load to marked tap to lower reflected primary impedance) to allow the bigger tube or pair of tubes to make more output power.

Distortion also makes a tube sound "louder" and "brighter".

You might consider a Master volume control for each tube, if you haven't done so already, to control the individual drive to and contribution from each output tube.

[PRR]

> I have taken that into account in my tests

A book somewhere points out at least 5 *different* "best loads" for a tube. Best power, best gain, best efficiency, and some others.

And of course EL34 CAN deliver more POWer than EL84.

But the obvious thing here is what HBP says: '84 is liable to be sitting with less bias, and need less drive, for the same power (or be louder on the same drive). VMS's Wiki-quote suggests 2:1, which sounds right to me.

[Leevi]

You haven't told us the complete test conditions; the schematic only shows a single output tube.Load (primary) impedance of the OT? Bias voltage across the cathode resistors of each tube?

That's true but I made several tests by changing the bias and screen voltage.
I even ran EL34 with 70 mA. I have impedance selector for 4, 8 and 16 load that made it possible
to adapt right load for the particular tube.


Check the following data sheet. In my case all the conditions except Vin (450mV) were in place.


I strongly believe in that:

'84 is liable to be sitting with less bias, and need less drive, for the same power

http://frank.pocnet.net/sheets/129/e/EL34.pdf


/Leevi

[HotBluePlates]

You haven't told us the complete test conditions; the schematic only shows a single output tube.Load (primary) impedance of the OT? Bias voltage across the cathode resistors of each tube?

That's true but I made several tests by changing the bias and screen voltage.

I believe you were thorough and that your impression were accurate.  However, if you don't provide the data I described, I can't do any more than guess why you heard what you heard.

With data, I could at least make an educated guess.

I think you, PRR and I all lean towards the EL84's small required bias voltage leading to smaller required signal to drive to full output power, in this particular case.

Separately, why did you add the smallish plate to screen bypass cap?

[Leevi]

With data, I could at least make an educated guess.

I agree.


I have almost similar case based on Fender Princeton Narrow panel where
I can use two different power tubes like EL84 or 6V6, 6L6 and EL34.
In this setup the EL34 sounds much louder than for instance EL84 or 6V6,


The only difference between these two cases is the signal level going to the power amp.
The power tube of Fender gets the signal from plate which is the reason for higher signal
and louder output.

Separately, why did you add the smallish plate to screen bypass cap?

That is not my idea, it comes from the Bad cat minicat II circuit


/Leevi

[HotBluePlates]

I have almost similar case based on Fender Princeton Narrow panel where
I can use two different power tubes like EL84 or 6V6, 6L6 and EL34.
In this setup the EL34 sounds much louder than for instance EL84 or 6V6,
...
The power tube of Fender gets the signal from plate which is the reason for higher signal
and louder output.

I assume you're plugging the 2 different tube types in the same amp (or between the Bad Cat-type and the Fender-type, comparing the results of plugging 2 different tube types in each of 2 different circuits).

Forget the plate-driven signal vs the cathode driven signal; G1 of the EL84 or the EL34 or the 6V6 all behave roughly the same. Measure the actual bias voltage across the cathode resistor for each tube when inserted in each of the amps. What do you get?

Separately, why did you add the smallish plate to screen bypass cap?

That is not my idea, it comes from the Bad cat minicat II circuit

Okay, then I'll guess the smallish cap value results in triode-operation at some high frequency (calculation says 1n2 against 100Ω is -3dB at 1.3MHz, but...) which reduces gain above the turnover frequency to triode-mode levels. Probably intended to squelch oscillation. If the OT primary impedance is in the circuit (it may be, but isn't clear to me) it could roll off high audio frequencies to tame the EL84's brightness.

[Leevi]

Forget the plate-driven signal vs the cathode driven signal; G1 of the EL84 or the EL34 or the 6V6 all behave roughly the same. Measure the actual bias voltage across the cathode resistor for each tube when inserted in each of the amps. What do you get?

Unfortunately the Bad Cat based amp is no longer here.


I measured the values from Fender based 5F2-A where 6V6, EL34, 6L6... or EL84 can be used


EL84: cathode 11.2V, Plate 325V, Screen 323.5V, Cathode resistor 250R
EL34: cathode 19.7V, Plate 319V, Screen 274V, Cathode resistor 470R


Try to remember the values in Bad Cat:



EL84: cathode ~10V, Plate 300V, Screen 300V, Cathode resistor 250R
EL34: cathode 17.5V, Plate 300V, Screen a bit less than 300V, Cathode resistor 470R




/Leevi

[HotBluePlates]

... Measure the actual bias voltage across the cathode resistor for each tube when inserted in each of the amps. What do you get?

I measured the values from Fender based 5F2-A where 6V6, EL34, 6L6... or EL84 can be used

EL84: cathode 11.2V, Plate 325V, Screen 323.5V, Cathode resistor 250R
EL34: cathode 19.7V, Plate 319V, Screen 274V, Cathode resistor 470R

With same-load from the OT (sized for the EL84), the EL34 will output the same power as the EL84; it also needs 8.5v peak more drive signal to run to distortion as compared to the EL84.

Staying at the same output-power level, you could drop the screen voltage of the EL34 more. Bias voltage for 100% plate dissipation should go down, though you'll have to experiment to see if a different cathode resistor helps land on that voltage. If you get it to idle at the same ~11v bias as the EL84, the apparent volume difference will probably even-out.

For power tubes, you should remember we're not really looking at voltage gain from grid to plate. We're much more concerned with Gm and how a gird-voltage change results in a plate current change. That changing plate current is then what pulls the plate voltage down due to voltage drop across the OT primary impedance. The plate voltage can shoot up above the idle value on the other half-cycle of the signal input due to flyback in the OT. Therefore, because the EL84 and EL34 have fairly-similar Gm, they should have similar power output with a given signal if the load impedance and idle bias are the same.

Or... don't worry about the volume difference as you have it now. When you use the EL34, cut the OT primary impedance by about half and get a bit more actual power output from the tube. Though now the preamp needs to be able to deliver a peak signal to the EL34's G1 that's at least as-big as the EL34's bias voltage. Can the preamp manage that? You'll have to measure a test signal & see...

[Leevi]

Thanks for tips HotBluePaltes
/Leevi