All Triode Express - Standard Wreck Sized Chassis

[David Root]

Bob, can you advise your OT primary impedance? Also are you using 135V screen or some other number? Trying to get a handle on where on the loadline your operating point is. I assume you are using fixed bias.

I too am intrigued with this tube. Thinking about 4x6Y6G in cathode bias, not a Wreck circuit. If I used three 6SL7 as preamp I would need 6A heater current! There are not a lot of choices in PT at the low HT required--maybe 230-0-230--also with 6.3V @ 6A and 5V @3A for a 5931/5U4WG rectifier. (I have four of these and they are pretty tough looking tubes, I have seen some ratings as high as 300mA DC). Edcor has a 2K5 25W OT that would be perfect.

[RJ Guitars]

I am using a Fender Pro Junior OT... I believe it is 8K on the primary and 8 ohms on the secondary. It seems like the amp really likes it although I spose a 5K Ot would still be in the zone.

I have it setup for fixed bias with an independent adjustment for each tube. I think I have it biased kinda hot at 48mA measured on the cathode, so it's that minus whatever the screen current is.

I'm not sure I have it figured out if you are asking about the measured voltage on the screens or the calculations I used on the load line... or maybe I am lost altogether. At any rate I have 245 VDC measured on the screens and about 253 on the plates.

Lemme know if this helps and/or if you think I need to wear safety glasses each time I turn it on.

...almost forgot to mention, in one of my previous not so bright ideas, I bought a bunch of Edcor 120-0-120 @125mA power transformers with a heater tap at 3 amps that I will plan to use for the next iteration. If somebody wants one I can sell them for a decent price.

[David Root]

You're very close to max dissipation but still within limits. I guess the 8K primary is keeping things under control at that high screen voltage. At least I now know one operating point on the 8K loadline!

I can't graphically follow it because I don't know where the 8K load line fits in a plate curve at ~250V screen, can't estimate it. Don't have any way of knowing where the 250V screen curve is relative to the 135V curve. I'm sure this can all be done with the math but that would be uncharted waters for me; it's all in RDH4 somewhere I'm sure.

OTOH this theoretically means I could run 4 x 6Y6G into a 4K ohm OT !

[johnnyreece]

Hmmm...that PT sounds interesting...you think it can handle the filament current? My math says this would take 3.4A of filament current. If you think it will work, count me in.

[RJ Guitars]

Yeah, I continue to play the amp a few times a day... it has inspired me to learn a few new jazz chords. I did pick up more tubes but I'd sure like to see this amp go with the current set for reasonable amount of time. Since none of the manufacturers predicted that we would flagrantly over-voltage their tubes, they didn't give us load line graphs with operating points where we are working. Those guys making tubes back in "the day" had no idea what was coming!

As far as the PT's go, if you know a little bit about how Edcor goes about rating their transformers it might convince you that they will work. I typically call them up and give them a list of specifications and they tell me what size core they will build it on. It turns out they have a volt-amp power rating based on the ability of the core size to stay cool. In this case this particular tranny also has a 2 amp 5V tap. If the entire power capacity is maxed out, and I were to ask for the same tranny with a 3.5 amp 6.3V tap, they would tell me I have to give up some power from one of the other taps and all would be well...

I know this because I actually went through this scenario with them before. I also recognized that they put the ratings you ask for on the label, not the max power that the core can handle... so there can be more power available but if you only asked for 3 amps, that's what the label says. Now we know they probably aren't going to wind that 6.3 tap with a different wire size between a 3 amp capacity and a 3.5 amp capacity so all that really changes is the label. I think fudging small amounts can be done safely especially if you pay attention to the temperature of the tranny.

When I was trying to get a little more heater tap power rating in the past, the owner and chief engineer guy there named Larry also told me that they included a healthy over-rating factor... but he wouldn't say what it was. I sorta believe it's around 100% based on what I could drag out of them. Thus far I have not had any of the Edcor PT's run hot but I also don't tend to run the amps flat out for long time periods so YMMV

[johnnyreece]

Okay, I'm convinced; gonna give this a whirl. I'll probably e-mail soon about some transformers, etc. Gonna have to pick your brain a bit.

[johnnyreece]

So, I have a feeling the transformer fairy is visiting soon...RJ - Do you have an updated schematic or layout? I'll be using the traditional filament wiring, so that won't be an issue. Are your 6Y6 tubes still holding up? I have quite a few 6W6 as well that I might try, if the 6Y6 can't take the beating. They seem to have slightly more robust screens. The 6F6 is another candidate, possibly. So many tubes; so little time.

[RJ Guitars]

I predict that you'll be looking at transformers within a day or two...

So far the 6Y6 tubes have been solid... I haven't played it long and hard on many occasions so there is still some unknown quantity concern how robust this circuit will be.

I tried 6W6 tubes and they worked well... a little too well if you are trying to keep the overall volume down but decent sounds just the same.

I put 6F6 tubes in there and I gotta say that my greatest disappointment in them is that the metal case hides the warm glow of the heaters... not to mention any redplate activity. Tweak them within specs and I think you'd get a pretty decent amp that way as well.

The 6Y6 may not be the tube for everybody but for something 10-12 watts it's a pretty cool option

[johnnyreece]

Awesome. Versatility is always a bonus, so it's good to be able to try many different types. I'm assuming that 6V6 would do just fine, as well, if one wanted?

I suppose I'm most curious about your bias circuit values. Are they standard Express values, or did you have to tweak? Of course, since I'm using a different PT, I suppose that will all go out the window, anyhow...

Either way, I'll report back in a new thread when all is said and done. May take a while, as I've got some other projects/personal commitments right now, but I hope to really tackle it by the end of July.

[10thTx]

IF it has NOT already been done ...................

I'd love to see someone build a Wreck 6BM8 version and report on it.

I've built a few 6BM8 amps and like that tube a lot. Good bass response, smoother than an EL84 to my ears, they are a current manufactured tube and come with triodes for the LTPI.

7-9 watts in push/pull.

With respect, 10thtx

[RJ Guitars]

...I'd love to see someone build a Wreck 6BM8 version and report on it.

Yeah that might make a tidy little package... could use the same basic layout for a Rocket version as well. I'm overcommitted until retirement but would be interested in contributing to the project.

[johnnyreece]

Interesting. Cathode biased output section makes this seem more Liverpool-esque. In case it isn't obvious by now, I'm really into these designs that use non-traditional output tubes. Also, most of the people in my area play in smaller clubs that are suited to lower power amps. My 6J5 triode output version was a hit, but it's a little too low powered. Short of running it through a 4x12, it just doesn't put out quite enough for the folks gigging here (although it's perfect for me playing at home with a wife and child at the other side of the house). It seems like 2 6V6 are just a touch too much, but this is another that could be in the Goldilocks zone.

[10thTx]

FWIW, I gigged with a band with keyboards, loud drummer, typically 2-3 vocalists, bass & acoustic with a HoSo56 using 6BM8's with the speaker mic'd. I used the amp off and on for several yrs with the band and it was fine for me.

AND the amp was a much better volume at home for me. It had VVR instead of the PPIMV which also would have worked just fine for me.

With respect, 10thtx

[johnnyreece]

Dragging this back to page 1...I've been working on a layout for this, and have a first version up. I've not got the filaments or OT secondary in, but this should be enough for now. I think I've implemented all the changes RJ mentioned. If you see something wrong, feel free to point it out!