Single ended EL34 amp thoughts

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labb
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Re: Single ended EL34 amp thoughts

Post by labb »

I believe that the 25 watts for the EL 34 is plate dissipation not output power. Output power nominally is about 60% of plate dissipation. Not sure how they get to the 20 watts output when you normally bias SE amps at 95% of plate dissipation which would put you at 23.75 watts and 60% of that would be 14.25 watts output. But then one would be hard pressed to hear the difference between 15 watts and 20 watts. Maybe one more versed is this will chime in. If you want 20 watts out of a single ended amp I do believe you would have to use a KT88 which would probably give you about 23 watts.
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Re: Single ended EL34 amp thoughts

Post by ChopSauce »

nickfl wrote: Wed Jul 26, 2017 5:49 pm... what I don't understand is how to choose the starting variable. I understand how to take a given power transformer and determine what the B+ should be and from there what primary impedance I need for the tube type, or what B+ I need for a given output transformer and tube type, but in this case I'm working from scratch with neither already selected, so how do I decide whether a plate voltage of say 300 or 375, or 425 is what I want? ...
Maybe "playing" with a "load line calculator" such as http://vapkse.internet-box.ch/?q=el34 might help?
(you seem to be more advanced than I am, though.)
nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

ChopSauce wrote: Wed Jul 26, 2017 11:30 pm
nickfl wrote: Wed Jul 26, 2017 5:49 pm... what I don't understand is how to choose the starting variable. I understand how to take a given power transformer and determine what the B+ should be and from there what primary impedance I need for the tube type, or what B+ I need for a given output transformer and tube type, but in this case I'm working from scratch with neither already selected, so how do I decide whether a plate voltage of say 300 or 375, or 425 is what I want? ...
Maybe "playing" with a "load line calculator" such as http://vapkse.internet-box.ch/?q=el34 might help?
(you seem to be more advanced than I am, though.)
That is a great suggestion. Actually I started doing that today, though I used this one: http://www.bmamps.com/Tech_tds.html I'll definitely take a look at the one you linked to as well. Plotting load lines and checking the theoretical bias point has helped me wrap my head around what different voltages will do, at least to some degree...
labb
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Re: Single ended EL34 amp thoughts

Post by labb »

http://www.bmamps.com/Tech_tds.html you do realize that site is for push pull not single ended amps.
nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

labb wrote: Thu Jul 27, 2017 2:01 am http://www.bmamps.com/Tech_tds.html you do realize that site is for push pull not single ended amps.
There is a box where you can select either single ended resistive load, single ended transformer load, or push pull transformer load. Am I confused about what it means by single ended transformer load?
labb
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Re: Single ended EL34 amp thoughts

Post by labb »

My bad. Did not see that.
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didit
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Re: Single ended EL34 amp thoughts

Post by didit »

nickfl wrote: Thu Jul 27, 2017 4:13 am Am I confused [..]?
You are not. Doing just fine. 400V will work. Keep below 25W dissipation and gentle on the screen. Find a ~10W ~6K OPT and try it. Perhaps with a few primaries and 2-4-8 ohm secondaries, and then experiment a bit. Adjusting supply voltage to stay below 25W.

Best .. Ian
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Re: Single ended EL34 amp thoughts

Post by tubeswell »

pompeiisneaks wrote: Wed Jul 26, 2017 8:21 pm I think for that, you start with the datasheet on the tube and look at the operating conditions for a single valve in Class "A" and see the voltages. They specify on one I see either 250V or 300V. Then you can see the expected output wattage, and go from there. This sheet I"m seeing seems to show a single EL34 should only do 11W not 20 or 25 but maybe I missed somewhere earlier that you wanted parallel dual tubes? At any rate, as I understand it, that's where you decide what voltages you want is based on the datasheets.

~Phil
When I referred to 20W earlier, it was in terms of Lord Valve's recommendation that for modern production EL34s, the plate dissipation rating should be nominally reckoned as 20W (and not 25W as printed on the datasheets). This is due to the decline in reliability of modern production EL34s.

A single ended output tube operating in centre-biased Class A with an optimum reflected load is capable of delivering output power which is about equivalent to 40% of the maximum rated plate dissipation. (Therefore, assuming setting up the output stage for a nominally rated 20W modern production EL34, this would be about 8W output power.)
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Re: Single ended EL34 amp thoughts

Post by pompeiisneaks »

Yeah and someone else mentioned the same thing, I'm confusing plate dissipation in watts with the actual output wattage, sorry! ;)

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Re: Single ended EL34 amp thoughts

Post by RJ Guitars »

tubeswell wrote: Thu Jul 27, 2017 4:08 pm...Therefore, assuming setting up the output stage for a nominally rated 20W modern production EL34, this would be about 8W output power.)
That's a number that I like better... seems like it would get real tricky to crack 10 watts with the type of stuff we are familiar with.

However, I'll contradict myself a bit because a few years back I did get Heyboer to wind me some 25 watt OT's for single ended use. I have no clue what I though I might build but I was playing the KT90 tubes at the time and was thinking maybe of using KT120's. Ultimately I found that a KT88 was plenty loud and it was challenging to keep the hum down where I liked it as I brought the power levels up.
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Re: Single ended EL34 amp thoughts

Post by martin manning »

It's all right here, isn't it? About 10W output power. Or, Blencowe's example at the link above comes to a very similar conclusion.
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nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

martin manning wrote: Thu Jul 27, 2017 11:20 pm It's all right here, isn't it? About 10W output power. Or, Blencowe's example at the link above comes to a very similar conclusion.
I think it would just depend on where you are comfortable biasing it. If you assume it is a 25W capable tube like the old data sheets, then 10-11watts biased at about 90% is about right, but if you consider a new production tube to be capable of only 20W dissipation you end up with more like 8w output when you bias it to 90% of that.
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Re: Single ended EL34 amp thoughts

Post by dorrisant »

What would be different if you were using fixed bias, with a negative supply?
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nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

Thanks to everyone for all the information, between your responses, some reading, and drawing a lot of load lines the last couple of days I've begun to wrap my head around the possibilities.

I've started to settle in on a plate to cathode voltage of about 310v, which seems to be a pretty ideal number for center biased class A with a 5K transformer (which seems to be the most commonly available value for a single ended 15w transformer).

So, my current thoughts are:

Plate - 330v
Screen - 318v
Cathode - 21v (270ohm resistor)

If I am doing things correctly, this is biased to about 90% and pretty close to right in the center of the load line. The interactive data sheet says this should be around 9.5 watts output, maybe a little less in practice.

I'm going to work on implementing this in my schematic and then start thinking about how I might want to tweak the preamp (I'm probably going to move away from some of the matchless derived elements, since there are some odd values there and I don't really understand why they work...). If anyone sees a major flaw in my plan, please let me know!
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RJ Guitars
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Re: Single ended EL34 amp thoughts

Post by RJ Guitars »

nickfl wrote: Fri Jul 28, 2017 2:46 am... So, my current thoughts are:

Plate - 330v
Screen - 318v
Cathode - 21v (270ohm resistor)... If anyone sees a major flaw in my plan, please let me know!
Lemme know if you've already explained this but I'm curious about your plate and screen voltages and specifically the delta between the two.

I don't know but I've been told...that if you get too much of a voltage drop between the plate and screen voltage you going to generate some compression that you may not want. This is something I picked up off of a HiFi thread here recently so it is hearsay but has me wondering?? In that thread there was a lot of talk that an amp with a SE design using an EL34 output might not have but a couple volts difference between the plate and the screen. It was in fact possible to have them at the same voltage or even the screen a couple volts higher than the plate and the amp performance was still considered to be satisfactory.

Disclaimer - it might not be true but it has my attention. Anybody else heard of this?
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