Cathode bias EL34 power amp

[Tone Junkie]

Anyone have any experiance with cathode bias EL34 tubes I see the matchless cheiftan uses 270 ohm 250 uf . I guess what I really want to do is build another cap and resister box .
 Any suggestions for a good starting and ending point on the resisters. the caps are easy start with 10uf and end at 250 uf its the resisters I dont quite understand how to figure that out.
Thanks Bill

[stratele52]

You can use a higner resistor than 270 ohms if not sure and monitoring your bias ( cathode current ) and see how is you bias . Change the value of the resistor to get the proper bias.

[TIMBO]

Hey Tone, This is one of our local amps, you could try this http://www.el34world.com/Forum/index.php?topic=15582.0 

[Madison]

Try this thread.


http://www.el34world.com/Forum/index.php?topic=14652.msg139977#msg139977

(I went with a 270 ohm resistor on each tube)
YMMV

[stratele52]

Try this thread.


http://www.el34world.com/Forum/index.php?topic=14652.msg139977#msg139977

(I went with a 270 ohm resistor on each tube)
YMMV

Resistor should be matched with tubes manufactured and plate voltage.

[Madison]

Try this thread.


http://www.el34world.com/Forum/index.php?topic=14652.msg139977#msg139977

(I went with a 270 ohm resistor on each tube)
YMMV

Resistor should be matched with tubes manufactured and plate voltage.

So true

[labb]

200 ohm works pretty good with a plate voltage of 366. Go take a look at the DD-30 on ddawgamps.com

[PRR]

> suggestions for a good starting and ending point on the resisters

Depends on supply voltage!!

With +395V and a 6.6K loading with several K of common screen resistor, I liked 250 ohms. This put Pd near 20W and output at 17W-23W. It also allows use of 6L6 or 6550, no change.

However with other supply voltages the optimum and practical cathode resistor WILL be different.

Get a pile of 50r 1W and 100r 2W, series them up to 500 ohms (should be safe for any reasonable supply voltage), add a 1r at the bottom for instant mA reading, then short-out 100 or 50 ohms at a time until you come near Pdiss. Then adjust your loading for optimum power output.

[stratele52]

> suggestions for a good starting and ending point on the resisters

Depends on supply voltage!!

With +395V and a 6.6K loading with several K of common screen resistor, I liked 250 ohms. This put Pd near 20W and output at 17W-23W. It also allows use of 6L6 or 6550, no change.

However with other supply voltages the optimum and practical cathode resistor WILL be different.

Get a pile of 50r 1W and 100r 2W, series them up to 500 ohms (should be safe for any reasonable supply voltage), add a 1r at the bottom for instant mA reading, then short-out 100 or 50 ohms at a time until you come near Pdiss. Then adjust your loading for optimum power output.

+1

[Tone Junkie]

Thanks guys there is a lot of really good info here. i will take a little time to absorb this I dont quite understand it all but will read further and built my switch box . PRR I love the little add on of the 1 ohm to measure your MA i would not have thought of that .
How do I figure out what Pdiss would be. I will read everything again to see if I missed anything.
Bill

[Tone Junkie]

In output transformers I have a 4.2K,4K, and a 3.3k I had no idea i would use anything differant than standard marshall style 3.3 k . PRR do you think the 4.2K would work I want to push this one a little bit
but I dont want to replace the tubes every 6 months either. the pre amp is going to be a bit of a high gain type. I was thinking 3 gain stages with a plate driven tone stack.
 But if I follow the matchless design with no negative feedback. and a cathode follower  type tone stack i could probably get into high gain territory with 2 gain stages sadly Im undecided (LOL).
Thanks Bill

[HotBluePlates]

How do I figure out what Pdiss would be.

An EL34 has a plate dissipation rating (Pdiss) of 25w.

To find out what your EL34 is actually dissipating, measure voltage from EL34 plate to cathode. Measure the millivolts across the 1Ω resistor. Multiply the two measurements (being sure to use 0.001 for a measured 1mV across the 1Ω), and you'll find your idle dissipation.

Because the resistor is 1Ω, then 20mA of current will create a 20mV drop across the resistor; 40mA will create a 40mV drop. We care about the current, and use a 1Ω resistor and ohm's law to directly measure milliamps of current by measuring millivolts dropped across the 1Ω resistor.

... PRR do you think the 4.2K would work ...

Depends on supply voltage!!

Best output transformer primary impedance depends on supply voltage, same as with the best cathode resistor value.

Neither of these things are plug-n-play (well, loading is in a way as long as you don't need any particular power or distortion spec). We really need more info on your intended use, to include supply voltage or proposed power transformer.

[Tone Junkie]

Well all of my power transformers were too high, more in line with standard marshall specs so i was going to pick one up that would give me around 400Volts or a little under either a 312.5-0-312.5 or 300-0-300 transformer .
Thanks Bill

[PRR]

> I had no idea i would use anything differant than standard marshall style 3.3 k

Standard Marshall is fixed-bias. The idle current can be small; power consumption will rise with signal.

Cathode-bias, the idle current and power consumption *must* be nearly as high as the full-roar power consumption.

So either you run a lower supply voltage, or a higher impedance.

> pick one up that would give me around 400 Volts

OK. EL34 Pdiss is 25W-30W, 25W is safer. 50W per pair.

Since efficiency will not exceed 50%, we already know power-out will be around 25 Watts. Is that acceptable?

At 400V, 50 Watts is 40W/400V= 0.125 Amps or 125mA.

400V at 0.125A is like a 3,200 ohm resistor.

The push-pull plate-to-plate load should be around twice that, 6,400 ohms.

I can testify that 395V, 200 ohm Rk, large Rs, around 120mA, 6,600 ohms load, gives in excess of 17W clean and happy. (I'd put "24W" on a guitar-amp badge.)

> I have a 4.2K,4K, and a 3.3k

Take 4K as example. So we want a 2K load on the supply, and suck 50 Watts idle power consumption. V^2/R=P, P*R=V^2, 50W*2000r= 100,000, square-root of 100,000 is 316V. Cathode bias drop is likely to be near Vg2/Mu or 300/10 or 30V. Total supply can be 316V+30V or 346V.

But why guess? Back in 1956 people were paid to *sell* these tubes, by making it easy to design-in to equipment. EL34 data page 5 shows cathode-bias "AB" conditions for EL34. I _suspect_ there is a typo on this sheet: "Va+Vrk" should be "Va-Vrk"? So 350-400V, 3k4 to 4k load, cathode resistor near 130 ohms (start above 150 ohms and work down). This heavy load and loose distortion spec allows current consumption to rise 26% from idle to roar, efficiency touches 49%, so power output is more than half the idle Pdiss. That's what hours of experimental work can do, better than cut-and-try or napkin calulations.

[Tone Junkie]

I was working on this so had to bump it up. Thanks for all the help guys.
Bill

[Tone Junkie]

PRR would a 6k6 OT with a 25VA rating be enough for this .
Thanks Bill

[Ed_Chambley]

Bill, I built a 18 watt TMB with a Chieftan power section with very low voltage.  I used a 3k4 OT, Heybour.  Did the resistor string, but I cannot remember the voltages exactly, but have them somewhere. I put the clean copies of the hand drawn schematic in a plastic pouch in the amp.  If you want them, let me know and I will find them.

The amp was great for 1 thing and that is High Gain.  It would not play nice, but I did not give up on the idea of a cathode bias EL34.  I changed it to a 5f4 super preamp.  Long story short, it was not me.  I put it on consignment at a friends music store and it sold in a few weeks, so I cannot say it did not sound good, just not my sound.

Madison made one with sort of a 5E3 preamp if I remember correctly.  I never heard the amp, but those who like 5e3 I would think it would be nice.

[Tone Junkie]

I would love to see it Ed. Im like a schematic whore I have layouts and schematics poster size all over my computer room . (LOL) Now one wall has all the ones Ive built I find I love to compare and mix and match.
Thanks Bill

[HotBluePlates]

PRR would a 6k6 OT with a 25VA rating be enough for this .

I'm not PRR, but here's my take: first guess, yes it will be fine.

OT's usually aren't specified in VA, but maybe that's just showing your preference for that term. Regardless, a true class A EL34 amp won't do more than 25w and will likely do noticeably less. So it should be fine.

PRR alluded that even a cathode-bias amp may be run as a hot class AB amp. So let's guess that you might bias a bit cooler and under heavy drive you get a bit more than 25w. If the OT is a Hammond, it already has more bandwidth than you need and passes the extra power without you noticing.

If it's a transformer which isn't so conservatively rated (or was intended for guitar use from the outset), you may notice less bass, maybe a bit more distortion or a "saturated" sound when you push max power. The transformer will pass the extra power but will tend to do this with less bass, maybe with less treble. Depending on the specifics of the transformer and its core, coloration from "overpowering" the OT may be a noticeable part of the final sound.

So the answer is still yes; with a smaller or less-conservative OT, you still get all the power output but possibly less-bass and more coloration.

[Tone Junkie]

Thanks HBP I just wanted to make sure I wasnt building one that would go down at the wrong time because I used an undersized  OT .
Bill

[PRR]

> OT's usually aren't specified in VA

If they were, the unit has to be VA/F^2, to account for the serious effects of Frequency.

[Tone Junkie]

Had to find this one one. sorry for the bump.

[SILVERGUN]

I guess what I really want to do is build another cap and resister box .
 Any suggestions for a good starting and ending point on the resister.

Don't forget that you can use a 500 ohm (or 1K) potentiometer/rheostat for this purpose....if that's all the range you'll need

Just make sure you get a nice high wattage one....don't use a "regular" 1/2 watt pot

[Tone Junkie]

Yes good thought SG I had all the parts sitting here and had the amp about ready to go. So i thought maybe I should put it together.
 But I had to find this page and read through it so i could remember what my thought process was. Man it sucks getting old (LOL).
I have a 5 watt pot with a 2 watt 1 ohm resister im going to put between it and ground to clip my meter to.
Thanks Bill

[Tone Junkie]

> suggestions for a good starting and ending point on the resisters

Depends on supply voltage!!

With +395V and a 6.6K loading with several K of common screen resistor, I liked 250 ohms. This put Pd near 20W and output at 17W-23W. It also allows use of 6L6 or 6550, no change.

However with other supply voltages the optimum and practical cathode resistor WILL be different.

Get a pile of 50r 1W and 100r 2W, series them up to 500 ohms (should be safe for any reasonable supply voltage), add a 1r at the bottom for instant mA reading, then short-out 100 or 50 ohms at a time until you come near Pdiss. Then adjust your loading for optimum power output.

PRR was that one 250ohm split between the 2 tubes I was just looking to see where I want to have it set before I fire her up.
Thanks Bill

[Tone Junkie]

After looking at the Matchless Chieftan. I might have to redesign my little box to have the option of one cap and resister for 2 tubes, or separate ones for each side.
Bill