rp wrote:BTW posted this cool site long ago but it might have some relevance here. Some of the amps have schematics, maybe there are some pp triodes in here. I think I remember there being a 2x2A3 amp. Granted RJ's going balls out for an Express triode!
I did a little more work on the triode amp tonight, but sorta hung up until some parts arrive. I ran the heater wiring with #20 teflon covered wire. I figured there was no sense in trying to stay with the Wreck standard with a build that is this far off the beaten path... it sure was nice to work with wire that didn't melt before the solder did.
I straightened up some of the parts on the preamp board and then pulled it off to run the power wires on the back side. Then I added the front panel components... we'll see if I get parts in the mail tomorrow.
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I had a couple parts orders show up yesterday including the heater power supply caps.
I reconfigured the original size Wreck PS board to make room for the Heater power supply. I have the board all finished and ready to install it as soon as I get the output tranny in place...
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That is an interesting tube... somebody mentioned them earlier as well. Its not a tube that most folks have in their stash, but i would say absolutely it will work. The 1626 is sorta considered to be a similar tube in a smaller size... if this amp does anything interesting, the 2A3 would likely do likewise only a bit bigger. The one issue with a very big triode is going to be the amount of space that is not there in a Trainwreck package, especially the crowded distance between the power tubes on an Express. I'd be inclined to go with a larger chassis like the tejas twister (Z-Weck or ODS). This would also get rid of some transformer hum issues.
The HiFi community might not be thrilled to know that we are abusing big power triodes, but they would make very cool looking amps.
RJ Guitars wrote:The HiFi community might not be thrilled to know that we are abusing big power triodes, but they would make very cool looking amps.
They probably wouldn't. The whole point of the triode power tube revival is the purity of the sound achieved. Real 2A3s are at least $250/pair used.
if I had pair I'd try one of the 2A3 amps in the prewar amp link, but I'm not going to spend $250 just for the power tubes to find out. Go for it andresound, at the moment I get my thrills vicariously through other people's amps here, and I'm getting kind of bored with all the Dumbles!
RJ Guitars wrote:...For a output tranny i studied several of the Darling audio amp designs and found they often used a 5K OT in a single ended build. For this project i ordered a 2.5K PP tranny from Edcor... still waiting on that to arrive....
Why would I have chosen that primary impedance value? I figured out after the fact that I had very little experience or expertise in calculating primary impedance for a push pull output transformer. I typically am copying an existing formula of tubes and transformers that work well together.
Does anybody want to give a tutorial on how this can be done... the readers digest version first please if it can be done?
Plate resistance is around 5K and I looked at lots of other tubes with similar plate resistance and I still haven't really nailed it down but now I'm thinking the primary impedance should be something between 5K and 8K.
I've struggled with the same thing you are...Fortunately, from what I understand, triodes are more forgiving with impedance matching. Often, people will get a theoretical value, and then swing one way or another to see what matches best.
Bob, Merlin's discourse is a distillation of what you'll find in Radiotron Designer's Handbook on the topic. That's the right way to do it.
The other way, which is a bit less work but requires a "leap of faith" here and there, is to consult the tube datasheet and interpret from the operating point(s) shown to where you would like to go.
For example, I'm working on a cathode biased PP pair of EL37. The datasheet shows 4K primary impedance at 325Va and 325Vs for 35W output, using a 130 ohm shared cathode resistor. This is pretty much old school type conditions with high plate and screen dissipations, although it is actually a high AB by the look of it.
I don't want to go there because I don't want to flame out those costly EL37s, but I would like to go to about 375 Va and a bit lower Vs. So I looked at another page in the datasheet which compares conditions at 300, 325 and 350V. Very handy, I wish other sheets did the same kind of thing.
Anyway, at 350Va they move the primary impedance up to 5K, and the cathode resistor Rk goes up to 165 ohms. Output power goes up very slightly to about 36W.
So here's the leap of faith, or SWAG might be a better description. If I go up to 375 Va, up the Rk to 180 ohms and keep the Vs at about 360V or so, I should get about the same output power without overdissipating the plate+screen allowance.
Checking Vsquared/R for the OT primary, 375*375/5000=28.125W. Vs. 31W from datasheet. Also the Ik will drop a bit as Vk gets a bit larger, so that would act against the dissipation calculation a little. And of course lowering the Vs a bit below Va will help too.
Thanks guys... it will take me a bit of study before this all soaks in. I did find a little bit of help from an antique radio web site. http://www.radioremembered.org/outimp.htm
Using my best guess so far and considering what I already had available - I've decided to use a 15 watt Fender Pro Junior transformer to finish assembling the amp and start tweaking. I really like the sound of these transformers with a pair of EL-84's... I'll let you know how well it works with this pair of octals.
Do you know what the primary impedance is of that OT? The reason I ask is that 6sn7 tubes liked to drive a load of 22k. Not many OTs have a primary impedance that high. So, I used an Edcor with 10k and doubled the seconday load (16ohm on an 8 ohm tap) to give me 20k (close enough).
Of course you are using different tubes. But I kinda think (and could be wrong) that triodes like to see a larger load than pentodes.
Mike's got a point there. I haven't noticed what your plate voltage is going to be, which will change things some. You might at least try doubling the speaker impedance. I'm going to guess this might like something in the 16k plate-to-plate range, based on the thread I linked.
I am basing most of my guess work off of the work done (within the HiFi community) with the other big triodes... you'll notice that the ones one that chart from the old radio site are similar to our common pentodes... but more directly, most of the single ended Darling amps are using a 5K primary. This was sorta what had me thinking along the lines of 5K to 8K. I think the Fender OT I used is ~8K.
I've actually got the amp finished... but behind on my posts. I will definitely try my 16 ohm cab and see what my ears can tell. More to come...
