All Triode Express - Standard Wreck Sized Chassis

[RJ Guitars]

I've been thinking about this for years but finally found some inspiration from the similar build by Johnny Reece... https://tubeamparchive.com/viewtopic.php?t=26380

I had stashed a few old 1626 power triodes a few years back... these are cool tubes (Darling Audio Amp) that make just under a watt each as single ended output tubes. I am figuring I'll get maybe 3 or 4 watts output in push-pull. I had an Edcor PT that gives me 180-0-180 and that should be just about right for these tubes. The tubes require 12V on the heaters so I'll be doing some trickery there with plans of ending up with a 12V DC heater supply.

I had a chassis that suffered from a bad calculation in tube placement on my part so I had the guys at the shop weld in a plug sprayed a coat of black hammertone over it. I also found an old vectorbord preamp board that I had made for a project long ago and populated with premium parts. Then decided to go with the can caps because have performed excellent for me historically.

I've started but don't hold your breath for fast progress... I have to go back to work next week.

[Colossal]

Very cool! Subscribed. Love old brown base RCA anything.

[David Root]

Me too! Uber-cool, Bob!

[David Root]

Sorry, double post.

[johnnyreece]

I love the look of that old glass! Anxious to see how this turns out!

[RJ Guitars]

Thanks for the encouragement - feel free to join me if you have the inclination to build one yourself. I saw there was someone selling a lot of 100 of the 1626 tubes on Ebay yesterday.

I find I tend to find amps to build around the various "Cool old tubes" I have acquired... but this one has the potential for being a practical build.

I have a couple questions for the guys who have built an Express circuit using a triode output. Does the Express keep the edge and attitude that is prevalent in the pentode version of these amps? Big triodes are known for their smoothness and I am curious how this plays out in the Express circuit.

I am also curious if anyone has built a voltage doubling DC heater power supply for use in a guitar amp? These tubes require 12.6V for the filaments and I am going to use the Darling style voltage doubler - see attached schematic.

[cbass]

I could be wrong But won't that circuit put out around 17 Vdc ?
Are you planning on using resistors to drop the voltage to 12.6?
A regulator might do the trick. I don't know I've never done DC heaters

[johnnyreece]

I'm a little confused on that schematic there...R25...it looks like both sides are grounded? Or is the label "GND" generic there? If not, maybe R25 and 26 are used to drop a little voltage, then, as cbass mentioned? Either way, I really am looking forward to this!

As far as the triode sound, I (unfortunately) don't have a full-size Express to compare mine to. I'm sure it's a compromise of sorts, but my ear is tin enough to not care. I believe there are some good 4-4-0 clips on youtube that might be helpful for comparison.

[rp]

Always wondered why no triode powered guitar amps. Subscribed.

BTW What you using for an OT?

[RJ Guitars]

Lemme add the 1626 spec sheet to help us talk about some of that other stuff...

[johnnyreece]

I'm guessing you're going to have around -40v for your bias? That might get you into range for a more robust tube I'm wanting to try: 6AH4. It's so hard to find a load resistance for these guys, though, making it hard to determine OT primary impedance. I'll be following this closely!

I am starting to wonder if we'll see a big movement on these low power triode amps. Although, some claim to not care for the triode sound, so I guess we'll see. I'm kind of wanting to mix things up and try a pentode preamp with a triode power, but that's for another thread...

[rp]

Although, some claim to not care for the triode sound, so I guess we'll see.

Yes but it's pretty much always regarding some kind of PP tetrodes or pentodes strapped into "low power" triode mode. Any true PP triode guitar amps out there?

[johnnyreece]

Good point, rp! I think the AX84 4-4-0 Studio is a well-documented P-P version. There are others, but that's the one I've seen the most info about. There's been a fair amount put out using each half of a 6SN7 or 12AU7 in P-P scattered about the interwebs.

[rp]

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!

http://prewaramps.org/

[RJ Guitars]

As far as heater voltage, i recall something about comparing DC and AC with regards to adjusting the comparison for RMS vs peak vs?... I'll have to study up but I'll post what i get in the actual PS build. Several of the iterations of the Darling amp use this same style heater supply... i posted it because it wasn't straight forward to my intuitive thinking.

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.

Edit - just read the 1626 tube spec sheet and it says that with a plate voltage of 250 i'll need a minimum of -35v bias in a fixed bias circuit....

I read somewhere that the audio guys typically put at least 35mA of bias current on their builds although all of them are cathode biased. This will give me a starting point and well see where goes from there.