Output Transformer Selection

[bepone]

Hey group,
I am still having some trouble understanding how to select an output transformer for my build project. I have two 6L6GC output tubes, I will probably grid-bas them and I'd like to get around 30W output.

did you open any 6L6GC datasheet? there is stated output load for various power.

[JJH0906]

Yes, and that's where things got confusing. The data sheet I have list to 360 volt plate voltage columns, and a 450 volt column. The 360 volt column, with very little differences in the rest of the data, shows two different anode to anode resistances.

Somebody needs to do a really good video on how to read two data sheets, but I couldn't figure out what the difference was between the two different columns of the 360 volt.

[bepone]

there are only reccomended guidelines.. you can see that 5k6 with 6L6 is giving 25 watts.. todays standard is 4k, 4k2 which with 450V gived 60Watts of clean!
with varying the voltage on anode you can vary also the output power! the same is for the output load, lowering the impedance (5k6->4k) you can see the power is increasing!

you can find also the graph impedance VS power, so you can select desired power..or you can just follow the standard 4k which is working 50 years and you are ok! lower some voltage and you will have lower power

to draw loadlines and exactly calculate power in PP circuit, you need to know how to draw and calculate output power in SINGLE ENDED , class A design. Then for the PP is almost similar but you need to know that mode of work in PP circuits is very often AB class which is combined with composite of 2 graphs.. graph from A class, and graph from B class..i know is too difficult now but it is better to mention. and ouput power is easy to calculate from the area below the graph.

[JJH0906]

there are only reccomended guidelines.. you can see that 5k6 with 6L6 is giving 25 watts.. todays standard is 4k, 4k2 which with 450V gived 60Watts of clean!
with varying the voltage on anode you can vary also the output power! the same is for the output load, lowering the impedance (5k6->4k) you can see the power is increasing!

you can find also the graph impedance VS power, so you can select desired power..or you can just follow the standard 4k which is working 50 years and you are ok! lower some voltage and you will have lower power

to draw loadlines and exactly calculate power in PP circuit, you need to know how to draw and calculate output power in SINGLE ENDED , class A design. Then for the PP is almost similar but you need to know that mode of work in PP circuits is very often AB class which is combined with composite of 2 graphs.. graph from A class, and graph from B class..i know is too difficult now but it is better to mention. and ouput power is easy to calculate from the area below the graph.

This is very helpful information and I appreciate your advice. I do appreciate learning from other's experiences!

[tschwarz]

...all of the above plus one important variable: Screen grid voltage. With fixed HT the screen grid voltage determines the height of the Ug1=0V curve; determines the height of the knee; determines the inclination of the load line(s), which is the OT impedance across both valves. The steeper it is, the lower the OT primary impedance...

Raa = (Uht / Iu0) * 4 with Uht = HT or B1+ and Iu0 = theor. current at Ua = 0.

[JJH0906]

I think I had attached this before but this is the datasheet I was referencing. I also got the JJ datasheet, as that's the actual tube I have for the output tubes but again, this info differs from the other sheet. Oi!

[tschwarz]

Yes, and that's where things got confusing. The data sheet I have list to 360 volt plate voltage columns, and a 450 volt column. The 360 volt column, with very little differences in the rest of the data, shows two different anode to anode resistances.

Somebody needs to do a really good video on how to read two data sheets, but I couldn't figure out what the difference was between the two different columns of the 360 volt.

Sorry about going back to an older post in this thread but I've just now looked at the Tung-Sol datasheet.

You've got this the wrong way around: they deliberately set HT, Uscreen and Ugrid to the same values in both columns for 360V plate voltage. Then they use two different OT impedances at those values - 6.6k vs. 3.8k. Which is why only the output power and - obviously - the anode current varies between those two columns. 6.6k ist the ideal impedance @ that HT/screen voltage combination, the load line runs right through the 0V curve's knee. You can try the values from the datasheet with the loadline calculator tool and see the effect on the load line - especially where it crosses the 0V grid curve. You will need to divide the "zero-signal plate current" from the datasheet by two as the loadline calculator specifies every parameter except for Raa per tube...

IMHO it is a bit of an esoteric example for a datasheet, though, as in most real-world applications the screen voltage runs off B2+ - i.e. is much closer to HT, usually only off by a couple of volts...

[JJH0906]

Thanks for that in-depth review! I just couldn't figure out where everyone was getting all these impedance values from the data sheet when I was using that one.

I had seen a couple of other videos where they were showing other two data sheets and they looked a little bit more simplistic, meaning easier to read, than what I have in front of me on this one. The JJ data sheet seems too simple, but then again, that's what I'm using so maybe I should just refer to that and toss the other one.

[JJH0906]

Just following up on this a bit more, although the answers provided by the members seem to hash things out for the most part:

I've attached an online datasheet for a common(?) 6L6GC which shows info on Class AB1 and AB2, a description of those two classes from the Tung-Sol website and then a sketch of a fixed-bias Push-Pull circuit from Merlin Blencowe.

Among the other confusion I had, does anyone have any insight as to what Class AB (1 or 2) do tube amplifiers operate in? I'm not familiar enough with the power stage circuits to fully understand what the Tung-Sol website is describing.

Thanks for your thoughts!

[pdf64]

Push pull valve guitar amps operate in AB1. AB2 requires somewhat more complex circuits to drive the output valves, but in the era of output triodes, could achieve greater anode efficiency. This was largely eroded by the introduction of the ‘modern’ output pentodes we use, which could achieve good anode efficiency with simple circuits.
AB2 may sound different, as it should eliminate or at least great reduce, bias shift when overdriven.

[JJH0906]

Excellent input! I think I had read another source which more or less mentioned the same thing but it's always good to hear from people who have been there, done that.

[pdf64]

I’m probably just reiterating the same sources, I’ve not actually tinkered with AB2 myself. Modern high voltage MOSFETs make AB2 a bit simpler.

[JJH0906]

Just out of curiosity, does this match up with the example that Merlin gives at his website:

http://www.valvewizard.co.uk/pp.html

In the example he gave, plus some other sources I came across, it's talking about quartering the impedance to figure out What the valves see in a push pull configuration.

I did do a load line calculator and the 3800 ohm impedance seem to push the tubes way out of the safe range, the 6600 ohm version fits much better but still unsure how that works out with the calculations that Merlin gives in his example.

[pdf64]

…I did do a load line calculator and the 3800 ohm impedance seem to push the tubes way out of the safe range, the 6600 ohm version fits much better but still unsure how that works out with the calculations that Merlin gives in his example.

How about taking and providing screenshots of the calculator with your data, then we can all see the context of your query (and also have a chance to spot any bad data)?

[JJH0906]

Fair enough. I got the calculator from

www.vtadiy.com

Unfortunately, because it's a website, the entire page is filled with ads and other useless information. However, I can try to take some screenshots of the load lines that this calculator puts out and post them shortly for review.