Any feedback on prospective amp design?

BoomTexan

Tele-Meister
Joined
Sep 20, 2021
Posts
357
Age
19
Location
Houston
In the break between exams at Georgia Tech, I've been working on a schematic for an amp of my own design: the "Yellowjacket." I'm going for a lo-fi 50s sound with some weird tubes and some switchable features that I've been wanting to try.

SE 6W6 power tube outputting 10W, 6SK7 preamp tube, and 6AT6 preamp tube. I really don't want to make just another 5F1, so I'm taking inspiration from the 5C1, 5F10, and a couple weird Dano and Alamo amps. My final goal is making an amp that you can switch the preamp tubes on or off with two DPDT switches. Basically, if you wanted a 5C1 sound, you switch the 6AT6 off in front, and just run the 6SK7. If you want a low volume sound, take out the 6SK7 and just run the 6AT6 by itself. If you want to really push the amp, run the 6AT6 into the 6SK7.

I do have a couple questions for more experienced amp techs than I. First off, would the preamp tube switch circuit be feasible? I'm expecting a pop when switched on or off, what would be a good way to mitigate that? Secondly, would the gain from a single 6AT6 preamp stage be super underwhelming? I'm not absolutely set in stone on that, it could be subject to change. I think that a 12AU7 would be pretty good in that position, and I have several NOS 12AU7 tubes that would look cool in the amp, but I also think a 6AT6 would be cooler to use.

Also, before anyone mentions this, I am expecting the 6SK7 tubes to have a degree of microphonics, and I know that they're higher gain than the 6SJ7 which is usually used on the 5C1, but I have like 10 of them that I need to use somehow and I think it could work well.

I might also try making a single ended 6SK7 power amp and running a Tweed preamp pedal into there, because they can output around 4W and I could use that as a pocket rig for open mics, but that's a project for another day.

Finally, does anyone know of somewhere that I could get dirt cheap transformers from? I've tried Weber before, and I'll probably get some from there, but if there's anywhere cheaper (including Aliexpress or other chinese sites), I'd really like to know. I'm trying to make these amps for under $150 apiece, and transformers are the vast majority of the costs that go into there. Given the lo-fi sound I'm going for, getting a noise-free OT or PT is not top priority for me, in fact, the sketchier, the better.
 

moonlighter

TDPRI Member
Joined
Sep 28, 2022
Posts
94
Location
New Jersey, USA
A month late here, but better than never...

To mitigate the pops you'd just have to put some 5Meg resistors in parallel with the switch contacts.
The 6AT6 gain would be low, but if you give it a high enough value plate resistor (to get as much gain as possible), the tones could be interesting.
As to the transformers, you'd probably do well to crawl eBay and look for some crusty ones that could be in spec. For your power transformer, you're going to have to eat the price a bit, they're all expensive now. I'd probably also scroll the 'Bay for this, and look for a small vintage industrial transformer.
Are you going to use a tube diode for rectification? The older and lower spec the better in this case if you want sag.

Looking forward to hearing how this went if it kicked off
 

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
6,042
Location
california
Build it and work out the bugs. Some of us build Swiss army knife, do everything amps. Some of us prefer the one trick pony. With the simple Champ-ish builds, I like the simple one trick thing. OMMV.

Antek has toroidal transformers. IDK if they have any small ones with 5v secondaries. 6X4, EZ80, and EZ81 rectifiers can use 6.3v secondaries if tube rectification is wanted. The 50VA models will be more than adequate for the builds you are considering.

From my internet searches I found:
AS-05T280 280 tap with SS rectifier gives V Jr B+, ~354VDC
AS-05T320 320 tap in 18Watt* circuit gives B+ ~354VDC
AS-05T320 300 tap in 18Watt* circuit gives B+ ~337VDC

*18Watt uses a tube rectifier so there is a voltage drop. (IDK how they have wired the rectifier. It may be a hybrid EZ80/SS Bridge Rectifier?) 18 Watt also uses more power than the nominal 50mA PT rating. There will be less voltage delivered when running at 90mA.
 

chas.wahl

Tele-Holic
Joined
Dec 8, 2012
Posts
600
Location
NYC
If you're concerned about microphonics, then one thing to avoid is a combo amp, where the tubes are in close proximity to the speaker.
 




Top