PP EL 34 cathode bias value?

[Madison]

Just want to be sure about this.
Using a single cathode resistor on a push pull amp.
Would 270R be okay?
300R?
330R?
Plates at around 440vdc.

[sluckey]

Matchless uses two each 270Ω resistors, one on the cathode of each EL34 in their Chieftan and Clubman amps. So, if you use that as a guide, you would want a 135Ω if using a single cathode resistor.

[PRR]

I liked 250 ohms under two EL34 at 400V plate ~~360V screen with 6.6K load.

[tubenit]

Madison,

(Ignore posted voltages on the SCH schematic. This was a quick cut and paste I drew for Madison & don't have time to edit it this am).

With respect, Tubenit


EDIT:  I added a SCH layout that can easily be converted to match the schematic. Most of it matches up correctly already.

[darkbluemurder]

Just want to be sure about this.
Using a single cathode resistor on a push pull amp.
Would 270R be okay?
300R?
330R?
Plates at around 440vdc.

Don't use the Matchless values with 440vdc on the plates - you will most certainly fry them. In my Chieftain build I have 410vdc on the plates and use two 390Rs in parallel, i.e. 195R, which leads to a power dissipation of about 26W per tube - which is OK for the JJE34L which are rated at 30W.

270R would be a good start IMHO.

Cheers Stephan

[phsyconoodler]

Sluckey,are you sure about using ONE 135 ohm cathode resistor with a pair of EL34's in push pull?

According to my calculations that would be over 100ma current and about 49 watts disipation or thereabouts depending on cathode voltage measured.

A 270 ohm shared resistor would be closer IMHO. Am I missing something here?

( I seldom question Sluckey cause he is usually right)

[sluckey]

Sluckey,are you sure about using ONE 135 ohm cathode resistor with a pair of EL34's in push pull?

I'm sure of the example I cited. If an amp has two individual 270Ω resistors under each tube then you would combine the two cathodes using a single 135Ω resistor to maintain the same operation.

According to my calculations that would be over 100ma current and about 49 watts disipation or thereabouts depending on cathode voltage measured.

Please give details of those calculations.

[Madison]

Exactly the reason I asked about this.
I searched forever on the net trying to find a clear answer. Resulting in not much.
This is the ONLY thing that is halting the continuation of this "super grinder" build.

I think......I am going to go with a 270R on each tube.
My load will be 3.4K
Hopefully, this is right.

Tubenit, thanks so much.
This will definitely get me on my way!

Peace all.

[phsyconoodler]

Ok if you look up the schematic for a matchless clubman,the specs show 270 ohm cathode resistors for each tube,405v on the plates and 380v on the screens(380v = 405-25 for calculations.
  With math it comes out to 33 watts plate dissipation and 88ma of current on each tube.
That's cooking!a 25 watt tube at 33 watts?

 So you have 440v on the plates and a 135 ohm cathode resistor spells trouble to me.

[darkbluemurder]

Exactly - no need to turn this amp into a tube torture chamber.

Cheers Stephan

[sluckey]

I still don't see your math and I disagree with your numbers. Here's what I see...

The schematic shows 430v for plate (actually OT cT), 380v for screen, 405v for screen supply, 25v for cathode, 270Ω for cathode resistor.

Screen current = (405-380)/1000Ω = 25mA
Cathode current = 25v/270Ω = 93mA
Plate current = cathode current - screen current = 93-25 = 68mA
Plate voltage = 430-25 = 405v

plate idle power = 405 x .068 = 27.54W

Yes, that's hot. Duncan shows 25W for PA max, but look at the Application Data on the same page, especially the line I highlighted. 55W for an AB1 P/P! We know that tube amps typically operate well beyond mfg. suggested max ratings. That Clubman is running hot IMO, but EL34s are often abused in the guitar amp world. That probably adds to the magic for the EL34 sound.

Those two Matchless amps are the only examples of cathode biased EL34 amps that I've seen. Running hot may be part of the magic. And Madison's estimated 440V ain't very different from the Clubman running 430V. I would not be afraid to copy that schematic. 'course, I ain't gonna be changing tubes every 3-6 months either.
 

[phsyconoodler]

I've been using Weber's Bias Calculator and when you plug in the numbers from the Clubman schematic it shows those figures.

 plate voltage 430v-cathode voltage = 405v
 270 ohm cathode resistor on each tube
 25v across cathode

 According to his calculator it gets 36 watts disipation and 88ma of current per tube.So I guess it's not accurate.Maybe SAFE is built into his calculator.
  It doesn't seem to subtract the sceen current from the cathode current.
Either way those tubes are going to be changed often!

[jojokeo]

A very smart old geezer friend helped me with this several years ago. Why not start with 150r or even higher and work your way down while the amp is actually running and in real world conditions under operation???
This was using 6BM8 tubes, change the tube type and data but follow the same steps:

Plate dissipation calculations as follows:

1) Measure the bias resistor (with the amp off & the resistor "cold") for ACTUAL resistance, write that value down.

2) Turn the amp "on" & allow to warm up ~1 minute. NOTE: If this is the 1st startup, watch the plates of the tubes to be sure no red-plating occurs.

3) Measure the DC voltage from the plate to the cathode (one DMM lead on the plate, & the other on the cathode). Write the voltage down.

4) Measure the voltage from the cathode to ground, write that # down.

5) Now, let’s say you measured & wrote down the following: (just examples)
     Bias resistor = 406Ω
     Plate to Cathode = 326v
     Cathode to ground = 23v

6) Devide the cathode volts by the bias resistor value….23v/406Ω = 0.0566

7) Divide the result by the # of tubes (2)……  0.0566/2 = 0.028

8) The result is the cathode current per tube, or 28mA cathode current.

9) Multiply the cathode current by the plate to cathode volts…… .028 * 326v = 9.21

That is the plate dissipation, 9.21 watts.

The max dissipation for a 6BM8 is 7 watts, so you may want to raise the value of the cathode resistor to 460R or even higher, then re-measure all the above again & recalculate the dissipation.
Repeat until you have an acceptable result (closer to 7W max).

[sluckey]

I've been using Weber's Bias Calculator

Ah, now I see. But Weber's calculator states "5% of the cathode current is assumed to be screen current". But in reality, the Clubman numbers show that screen current is 27% of cathode current. Huge difference.

Jojokeo, your method assumes plate current equals cathode current. That totally ignores screen current. We all should know that screen current is significant for an EL34.

[Ed_Chambley]

I built a Cathode el34 with 430 on the plates and used a 18 watt tmb pre.  I copied the Chiefton power section exactly. 2 wirewound 270 resistors.  I used NOS seimens.   Not a single tube problem, yet.  I will say the reason you do not see too many built this way.  Had lots of problems with squealing and humming.  Finally got all the issues worked out and it is a over the top high gain amp.  I have to admit is does sound great with humbuckers and it doesn't like low volume.

[jojokeo]

Jojokeo, your method assumes plate current equals cathode current. That totally ignores screen current. We all should know that screen current is significant for an EL34.

That was directly from geezer the gate keeper of old knowledge. How would you modify what he's listed to include the screen? I've used his method for my SE EL34 and a few others and so far the tubes haven't melted yet!?

[sluckey]

Read reply #10. Your tubes have not melted because by ignoring screen current, your calculations err on the side of caution. And that's fine for many tubes. But ignoring screen current can cause a big error when talking about an EL34.

Try it yourself. If you have a screen resistor in your SE EL34 amp, measure the voltage drop across that resistor. Then divide that voltage by the Ω value of that resistor. This is the actual screen current. Now, subtract that screen current from your calculated cathode current and you will have your real plate current.

[HotBluePlates]

OR... use the PRR method, if you don't trust his cathode resistor choice will work in your amp.

Get 10 100Ω resistors, wire them in series from the cathode to ground. Get a jumper with alligator clips on either end. Comment one alligator clip to the cathode (or ground), and use it to bypass 1, then 2, then 3, etc, of the 100Ω resistors until you dial in the idle current you want.

An extra couple of 100Ω resistors are handy to parallel one or more of the series 100Ω resistors to get values between 100Ω increments. Adjust on test, find what works, replace the messy arrangement with a single resistor or parallel pair that gives the value you determined to be best.

[PRR]

> load will be 3.4K

At over 400V?? With 25-30W tubes?

That's over-loaded.

At FULL power, the plate-to-cathode path acts as a "resistance" of nearly OTZ/2, where OTZ is the transformer primary plate-plate impedance.

So 1700 ohms.

At 425V, 1700 ohms will flow 0.25 Amps.

If you run cathode-bias, your idle current must be very-nearly equal to your full-roar current. Otherwise bias must shift between idle and full, i.e. on the pluck, and that's distressing. Harsh blurpy attack.

So 425V 3.4K load must idle at 425V*0.25A= 106 watts dissipation per pair. Maybe 90W/pair allowing for bias-shift. That's 45W idle dissipation in a 30W tube!!

Yes, my 400V in 6.6K is conservative. Barely 20W per tube at idle. I wasn't getting paid extra for re-repairs, so I didn't want to ever see it again except happy-on-stage. You sure can work it harder. But not 45W/bottle, not for long.

3.4K load in self-bias with EL34 works out to about 300V of safe B+ plate-to-cathode, say 340V from OT to ground. Alternatively, 440V self-bias suggests 7K (or 6.6K) load.

{Ha! I've just re-proved the _three_ SELF-BIAS conditions on Steve's chart: 430V 6.6K 470r cathode resistor, and 355V (315V is typo) 3.4K 130r cathode resistor.)

Or, as every Marshall shows, 440V 3.4K in FIXED-bias can be an OK amplifier. And a LOT more power output than the 25-35W possible from EL34 in self-bias.

[Madison]

Oh man, my head is spinning.
Too much info.
Let the confusion prevail for me noggin.

How about this.
Since the trouble seems to be the plate voltage is so high, my PT is 350/320/290/70/0/290/320/350.
Will be diode driven.

I have built adjustable bias amps on 290 taps and they were cool sounding.
But, I have built adjustable bias Plexis with 340 taps and they just hurt!(in a good way).

No, I do not want to put adjustable bias in this amp.

I was.....hoping to use the 320 taps for kicks.
Will do the 290s.....if I must :(

My logic.
I have always found that swapping out 6L6s and EL34 in an adjustable bias amp basically no adjustment so.....

Simply.
What I am wanting to do is just stick a couple of EL34s in this,while changing the preamp to my taste,with the highest output voltage possible without having to go out and buy 100W resistors and such.

What's your take?

[sluckey]

Go for it. But, you will need to connect pin 1 to pin 8 and I highly recommend 1K screen resistors on each tube.

[Madison]

Go for it. But, you will need to connect pin 1 to pin 8 and I highly recommend 1K screen resistors on each tube.

Noted.
Will give it a go.
Thanks you Sir.

[PRR]

> with the highest output voltage possible

Why?

Raising the supply voltage on a cathode-biased amp does not raise the power output. If it were already optimized, raising B+ forces you to lower idle current, *less* power output.

[Madison]

> with the highest output voltage possible

Why?

Raising the supply voltage on a cathode-biased amp does not raise the power output. If it were already optimized, raising B+ forces you to lower idle current, *less* power output.

Good question.
Good point.

Going to drop the board in this thing hopefully tomorrow and try both 290 and 320 taps and let my ears answer this.

Unfortunately, my neighbors just gave me a truckload of wood I have to spilt.

I'll report findings later on.
Thanks for everything.

[HotBluePlates]

But Weber's calculator states "5% of the cathode current is assumed to be screen current".

But ignoring screen current can cause a big error when talking about an EL34.

This is an important point that separates EL84/EL34/EL37 from beam power tubes we're used to, like the 6V6/6L6. The screen grid is not aligned with the control grid on (at least old versions of) these tubes. See this deconstruction of an EL37.

So, Mullard's EL34 data sheet seems to show G2 current is about 13-14% of cathode current for pretty much every condition they list.