All Triode Express - Standard Wreck Sized Chassis

[johnnyreece]

Awesome! Can't wait to see it!

[David Root]

In my description of getting to the operating point I forgot to allow for the cathode voltage in the Vsquared/R check on dissipation. So it should be
(Va-Vk)squared/Zaa. Vk would be around 25V+/- so that further reduces the total dissipation at idle.

[RJ Guitars]

I took some photos of it that I can now share. It turns out that the output tubes were not closely matched so the dual bias controls worked out well. I did find two tubes that I was able to tweak in opposite directions to get them matched. However, the longer they burn in the more they seem to be moving toward each other??

The DC heater voltage has settled in right at 12.8 volts... that worked out really nice!

Lemme know if anybody spots a few extra parts and wires and then I'll explain them if you don't figure it out before I tell you what's up.

More info to come...

[johnnyreece]

I'll bite...the two green resistors...what's the deal there? I can't see what they're feeding, but those two pins are unconnected on the tube, right?

[RJ Guitars]

Yes, you made quick work of that mystery. If you look at the pinout for a 12V6gt tube, it is the same the 1626, except it has screens. I just happen to have a few of those tubes as well. On my first test, i found they do not bias anywhere close to the same point ... but they do work.

That also sheds some light on my OT choice... just in case this doesn't work out i have the 16 watt option using the 12V6gt tubes.

[romberg]

Nice! Bet it is louder than one would guess .

Mike

[johnnyreece]

Nice! Versatility is always a plus. Once I finish my honey-do list, I may get after my 6AH4 version. I'm with Mike; I bet it's loud!

[RJ Guitars]

The first play out of it was thin and ratty sounding... I had to work to get the bias right but that wasn't enough. I had initially used a .001 coupling cap for C8 but swapped that out to a .0047... that helped to make it full but still ratty sounding. I then replaced the V1 12AX7 with a vintage 12AY7 and that helped but not enough. Finally I exchanged the R7 150K resistor with an 82K and that cleaned things up nicely.

It has a lot of smoothness and a distinct triode...ish distortion but lacks the bite of a full blown Express... lots of mellow and not so much chime or edge. This is typical of my experience with triode outputs.

It's decently loud, certainly enough for church or small gigs. The single ended HiFi amps with the 1626 output tubes are rated a 0.7 watts. I figure this amp must produce something around 3 watts when it's really working hard... if it mattered I could do some measurements and the math but I'm not inspired yet...

It does have some decent headroom and a nice transition from clean to mean starting at about 5 on the dial of my Stratocaster. The best sounds are probably with the guitar and amp both at 75% on the dials.

I'm going to call it done although I'm going to study more on this output impedance issue. I may also invest the time to try and tweak the 12V6gt tubes and see how the same amp sounds with pentode output tubes. In the meantime I need to get caught up on everything else then look for the next project...

[johnnyreece]

That's too bad. I was kinda hoping you'd fall in love with the triode tube sound! It's so dang convenient to have an amp that you can plug into a single 8" speaker for practice, then a 4x12 for the show. I'm thinking about maybe trying a 6k6 version at some point for more stage-level volume. So many amps, so little time...

[RJ Guitars]

Yeah, I've built a handful of triode powered amps at this point and they just don't have the bite i like. I was hoping a bigger triode and fixed bias was the answer. Its got several sweet spots and does the smooth distortion very well plus a great clean tone. But thus far i am liking the small pentodes best for the low wattage high gain Wrecks. However those little 7 pin pentodes have a bunch more loud than this amp has. I also have lots of the sub-mini pencil tubes but they bring a bit of pain to the design and fabrication as well as other issues that i dont enjoy...

The quest goes on for big sound in small quantities....

[johnnyreece]

Voltage requirements would be dicey, but have you thought about the 6G6? It's an octal that's similar to the 6AK6, from what I've read, but with lower voltage handling.

[cbass]

The DC heater voltage has settled in right at 12.8 volts... that worked out really nice!
...

Glad it worked out. Shows what I know.

[RJ Guitars]

Even a blind squirrel finds a nut occasionally... I didn't really do anything but follow some innovative pioneers. Actually one of the coolest parts of this build was to create a 12V DC heater supply, especially because of that voltage doubling circuit... who would guessed that would work?

I brought the amp into my studio/office and played it through an open back cabinet with a pair of 10" speakers. It's a 16 ohm cabinet and the amp seemed to really favor that setup. It never got to the point of sparkle but sounded quite nice and was really pretty loud with lots of clean headroom. I even put the 12AX7 back into the V1 spot.

I discovered a couple things...

1) I had wired the bright switch upside down so that I could put this build into a combo cabinet ( or maybe I just messed up)... but that switch was so ineffective even with a 250pF cap that I couldn't tell if it was on or off.

2) The presence control seems to be null... I reduced the feed resistor and it made almost no impact.

Has me wondering about how a triode works in this circuit... I can't get the amp to really brighten up and the normal things to do didn't work??

Finally, I swapped in the 12V6gt tubes and changed the bias resistor until I could bias them at 33mA. That's the correct value according to the weber bias calculator when you have a plate voltage of 250 volts. The amp sound OK but sould use some tweaking. However, when you hit the bright switch it blasts you with highs and the presence control also works... I had to reverse the leads to make the squealing stop though. This experiment told me a little more about the triodes... still figuring out exactly what.

It's a better sounding amp as is with the triodes, just marginal on the high frequency side but really nice cleans and a bunch of headroom. To tweak it for the 12V6gt's I think it maybe would take a better OT.

Look for it in the "For Sale" section soon. It was fun but it isn't a contender for a permanent residence in my herd...

[rp]

To tweak it for the 12V6gt's I think it maybe would take a better OT. Look for it in the "For Sale" section soon. It was fun but it isn't a contender for a permanent residence in my herd...

RJ before you move on or convert to 12V6gts, I'm wondering how it would sound with an over-spec'd high quality OT like a 10-15W Edcor. In my limited experience low power amps seem to benefit from high quality M6 OTs more than bigger amps which can clean up too much and get a buzzy grind (except for such amps as Hiwatts and Oranges which actually succeed at loud-clean-grind). Anyway, maybe you could loose some fuzz and gain some clang.

[RJ Guitars]

Voltage requirements would be dicey, but have you thought about the 6G6? It's an octal that's similar to the 6AK6, from what I've read, but with lower voltage handling.

I hadn't thought about those tubes for a long time... but you might be on to something. I just happen to have a pair of those!

I think I have the triodes working as well as they are going to work. Better than good enough but I was hoping that they would sound more like small pentodes. Once I tried the 12V6GT tubes and heard the tone brighten up I sorta knew that it was the pentode sound I wanted, but I don't think there is anything real special about a 12V6gt Express over a 6V6gt Express, so I want to keep experimenting with output tubes unless someone decides they need this amp for themselves. I did list it on the for sale pages but so far it's not going anywhere.

A slight heater rewire job in order to run the heaters in series on a pair of 6G6 tubes and I'll have another interesting data point... maybe two since I will be pushing them a bit over voltage.

RJ before you move on or convert to 12V6gts, I'm wondering how it would sound with an over-spec'd high quality OT like a 10-15W Edcor. In my limited experience low power amps seem to benefit from high quality M6 OTs more than bigger amps which can clean up too much and get a buzzy grind (except for such amps as Hiwatts and Oranges which actually succeed at loud-clean-grind). Anyway, maybe you could loose some fuzz and gain some clang.

I tend to agree... big iron does good things for small tubes. In this case I'm gonna have to hear more high end sparkle before I can become a triode lover though.