Question of EL34

[Alan0354]

Hi, I am about to put the EL34 into my Bassman. This is the first time I use EL34 and just reading the data sheet the first time.

file:///C:/Users/Bo/Desktop/Music%20...hilips1969.pdf

I have a few questions.

1) I notice the data sheet specified Rg2 ( Screen resistor ) of 1K or 470 ohm. What is so sacred about this? I 6L6 or 6V6, you use different resistors depends on situations.

2) The max power is 25W only which is surprisingly low to me. So if I run 450V, say 60% idle current, or (25W/450V)X0.6=33mA!!! Is that correct?

3) When looking at page 5 for 33mA, the grid 1 is about -33V. This is in the ball park of 6V6 bias. The reason I said this is because My bassman is a Bassman 100 platform with 4 power tube socket. I am using the middle two for 6V6. I am planning to use the outer two for EL34. Looks like I can use the same -ve bias and can get to the ball park. If so, I can add a little switching circuit and can switch between 6V6 and EL34 to compare the sound difference.

4) I am too lazy to change the OT, I am going to use the existing Vibrolux OT with 4K primary. That is close enough to 3.4K, I just loose some power. BUT my concern is the Vibrolux OT is 40W. Pair of EL34 can give 50W. Should I be worry? From first pass, I think it's going to be ok. I am not going to gig with the amp, just use it as prototype and then build into the Marshall Dual reverb combo with the Classic Tone Marshall Plexi 50W OT.

I know I have to ground the suppressor grid, other than that, anything else I have to be concern?

Thanks

[frus]

1.
compare the transfer characteristics of el34:

http://www.triodeel.com/6ca7ap8.gif

with, say, 6L6

http://www.triodeel.com/6l6gc_p8.gif

the curves for el34 are much more curved, so there's no definite "knee" you would point your loadline to.
and since the sum of plate and screen current is virtually constant, el34 WILL draw more screen current when you push the amp enough so you reach the low plate voltage condition (left side of the graph). and your loadline will have even more gradual slope (since your OT primary has a bit higher plate impedance, but it's nothing to worry about). so it's wiser to put a bit higher screen resistors

[frus]

..

[frus]

damn

[teemuk]

1. To cut a long story short we are essentially discussing pentode tubes (e.g. EL34) vs. Beam power tetrode tubes (e.g. 6L6, 6V6).

Unlike in a true pentode, in a beam power tetrode the grid and screen are aligned interweavingly and have the same winding pitch. This type of construction results into less electrons hitting the screen than in a true pentode. As a result the beam power tetrode tube has higher effiency, lower third harmonic distortion AND lower screen current draw than a true pentode tube.

Since screen current draws are different with pentodes and beam power tetrodes designs naturally call for different screen resistor values.

[Alan0354]

Thanks guys, that is very helpful. Yes, now that I pay attention, the curves are a lot rounder. Is that the reason EL34 sound different from 6L6 and 6V6 that gives the "Marshall" sound?

How about question 2 and 3?

I am thinking about the load line. The link of the EL34 data sheet I provided in the OP has the curve of the 25W power limit. Instead of drawing the load line to the distinct bending point of the 0 grid curve, I should:

1) Determine the idle current and draw a point of the idle current for the given voltage. In my case, I use 60% to give 33mA. The PT gives me say 450V. So I draw the point of 33mA and 450V.

2) Draw the line from the idle point touching the max power curve to intersect the 0 grid curve as shown:



But with that you see it gives (450-10)/(0.18-0.03)=2.933K. So the primary is 4X3K=12K. That's too high!!!!

How do you get the load line to use a 3.4K primary. That will violate the power dissipation limit!!! Even if you go to Class B with 0 idle current, (440)/(0.18)=2.44K. This implies primary =4X2.44=9.76K!!! So how did people come up with 3.4K primary?

EDIT:

Plate going down to 10V does not seems real. Lets say it saturates at 100V. So RL=(450-100)/(0.18-0.03)=1.94K. Still, Primary has to be 8K!!!

[jazbo8]

IIRC, one of the first posts you ever made on this forum was on screen voltage scaling, so perhaps a quick refresher is in order. In any case, Eg2=360V (from the datasheet) is a still fairly low, what is the actual screen voltage you plan to use? The tonal difference between the 6L6s and EL34s are well documented, I think even on this forum, so I'll leave you to look them up. Do a search on Marshall 100W load line (try AX84), and you will see just how badly the Pda are violated, Aiken plotted them out quite a few years ago.

[Alan0354]

IIRC, one of the first posts you ever made on this forum was on screen voltage scaling, so perhaps a quick refresher is in order. In any case, Eg2=360V (from the datasheet) is a still fairly low, what is the actual screen voltage you plan to use? The tonal difference between the 6L6s and EL34s are well documented, I think even on this forum, so I'll leave you to look them up. Do a search on Marshall 100W load line (try AX84), and you will see just how badly the Pda are violated, Aiken plotted them out quite a few years ago.

That did not work very well!!!

I understand using 450V screen effective raises all the plate curve up vertically. But the power limit is almost Screen voltage independent . It mainly depend on the idle point and max power curve. So what you are saying is they violates the max power curve big time?

I have been doing search on this forum, no luck. Do you have any link handy?

Pushing things like this can only happen in these tube stuffs. In the SS world, you blow things sky high already. It is dangerous to even push 110% or 120% max max. I used a lot of 1000V MOSFET, I tested some, at 1100V, the MOSFET conduct like a zener diode already. For power transistor, you don't even want to push 80%. Any imperfect cooling will cause it to burn fast. Only tubes with big surface area can get away with going over the limit.

[Alan0354]

Well, I wired up the two outer socket for EL34. I just want to verify:

1) I want to measure idle current. I tie pin 1 and pin 8 together and go through a 1 ohm resistor to ground. I decided not to tie suppressor grid (pin 1) to ground with pin 8 through 1 ohm to ground.

2) I use 1.5K as grid stop resistor because that's the only value in this range I have two!!!

3) I am going to adjust to 30mA per tube idle current.

4) I use 1K 5W as screen grid resistor.

Please comment if you see anything wrong. I am going to power up tomorrow, it's too late to make loud noise.

[jazbo8]

So what you are saying is they violates the max power curve big time?

This plot was done by Aiken, it has since been removed from his site for some reason (only Class B load line shown):


For you test, why are the EL34's biased at 30mA. that is only 13.5W Pda.

[Alan0354]

This plot was done by Aiken, it has since been removed from his site for some reason (only Class B load line shown):
[ATTACH=CONFIG]30401[/ATTACH]

For you test, why are the EL34's biased at 30mA. that is only 13.5W Pda.

What is the Red and Green curve? Looks like max power curve.

The reason for 30mA is the tube max power is 25W. For 450V, max current is 25W/450V=55mA. if I bias at 0.6, idle current is 33mA. So 30mA is a good number.

If the Red curve is the 25W curve, then the Yellow load line for 850 ohm per tube violates the max power curve in the middle. That's the reason I drew the load line in post #6. I tried to stay under the 25W curve by touching it at one point and never pass it. For 850 ohm per side, the total primary is 4X850=3.4K.

How do you get 13.5W Pda? What is Pda?

As for me, I am going to use a 3.4K OT, so the result will be like in the Aiken graph.

[jazbo8]

Yes they are Pda (power dissipation of the andode) curves, green 25W, red 50W. This is the actual load line used for many of the Marshall 100W amps. 30mA is pretty cold bias, do you want safe or rock god tone or perhaps something in between? 450x0.03=13.5.

[Alan0354]

Yes they are Pda (power dissipation of the andode) curves, green 25W, red 50W. This is the actual load line used for many of the Marshall 100W amps. 30mA is pretty cold bias, do you want safe or rock god tone or perhaps something in between? 450x0.03=13.5.

Of cause rock god tone. If looking at 450V, Aiken biased at 50mA!!! No wonder last time when I used the Marshall JCM900 to drive 60Hz to break in the speaker, I blew power tubes!!! I guess Marshall count on guitar is not a continuous signal, there are quieter period between strokes of the pick.

[jazbo8]

Here is the original discussion thread where Aiken posted the load line plot - he did not bias it at 50mA but wanted to know why Marshall did it that way.

[Alan0354]

I read the first two post. So the idea of using 50W load line is because if you bias more to Class B, half the cycle is off, so using 50W power curve is like 25W power dissipation.