Single tube practice amp?

[choosebronze]

While I wait for Mojotone to send my new Dual Lite PT I've got some time on my hands to think and that usually doesn't end well...  Long story short, I've got a 13-year-old cousin who recently started playing guitar. I thought a little combo amp would be a cool present for her, and she could help me build it. Sound quality would not be the #1 objective. Low cost would be the biggest thing (Aluminum hammond chassis, toroidal PT maybe?). #2 would be simplicity. She lives 5 hours away and the more she can troubleshoot without me being there, the better. If she can make electric guitar sounds she'll be happy.

The dream in my head is a one tube amp. Troubleshooting via tube swaps becomes pretty simple, costs would be low, and there shouldn't be many components to fail. Low wattage is fine, I'm sure she'll only play in her bedroom. That led me to the ECL82/84/86 and ss rectification. I came across this simple Magnatone 411 schematic. They're using the ECL86's triode for the tremolo, but is there any reason I can't cut the tremolo out entirely and be left with a triode and pentode that can all be handled by 1 tube (82/84/86 with appropriate pinout). That seems like it would be a really simple, cheap point to point build.

I'd just take the pentode's 1M grid resistor to ground and cut out the rest of the tremolo, right? Is it that easy? Is this a terrible idea?

[DummyLoad]

if you've been shopping for 6GW8 recently then you're probably aware they are getting expensive and harder to track down. 6BM8 is still in production and there are a LOT more of them in NOS circulation.

can't go wrong with a single 12AX7 and single 6V6 - 2 different sized bottles shouldn't be all that difficult to shoot trouble on. yeah, the build another SS rectifier champ mantra. :)

stiffen up the filtering, use a heftier OT and you'll be very happy with the tone through a 12" speaker. spice it up with a bypass cap on the second preamp stage. 



--pete

[DummyLoad]

I'd just take the pentode's 1M grid resistor to ground and cut out the rest of the tremolo, right? Is it that easy? Is this a terrible idea?

yes, ground 1M and no, not a terrible idea, but you probably won't be happy with the overall gain.

no speed control on that trem anyway - easy enough to add though.

--pete

[sluckey]

That'll work

[choosebronze]

if you've been shopping for 6GW8 recently then you're probably aware they are getting expensive and harder to track down.

Confession: I've had a 6DX8 (ECL84) sitting around for a while. Part of the motivation is to finally use it for something, I guess. Plenty of cheap NOS.

can't go wrong with a single 12AX7 and single 6V6

you probably won't be happy with the overall gain.

That's interesting. I've built a couple champs and was hoping to try something a little different. If I were going to be stubborn and stick with the dual purpose tube, but was okay with adding another tube, a 12ax7 could get me an extra gain stage and the tremolo. I mean I guess at that point I'm really just talking about a Gibson GA-5T. Maybe that's the way to go.

[Gnobuddy]

The dream in my head is a one tube amp.

I've never seen that dream really work out for anyone (myself included.) With a single tube, there's invariably never enough gain for anything more than clean tones so clean that they sound like they come from a solid-state amp. If you so manage to get enough gain for some overdrive (say with a small-signal pentode in the preamp wired for maximum possible voltage gain), then the overdrive is buzzy and sounds like a crude solid-state fuzz-box rather than a good tube amp.

My most recent attempt at this used a 6JW8 triode/pentode in an SE design that put out a whopping 200 milliwatts or so of output power. A sort of micro-Champ, if you wish. Wired to the stock speaker in my '65 PRRI, 200 mW was actually plenty loud enough for use in my apartment, in fact, too loud to overdrive at night without worrying about annoying the neighbours.

I used the 6JW8 pentode as the output tube, with a small Hammond OT with a primary set to around 22 kilo ohms, and B+ around 135 volts. The triode in the 6JW8 was the input stage. One tube, but gain was too low, and all I got was low-volume, too-clean tone.

Then I added a 6AQ6 in front (ignored the two diodes, just used the triode.) With this, I had enough gain to put a two-knob tone stack between triode and pentode stages of the 6JW8, and still go from clean tones to classic-rock levels of overdrive.

I kinda liked it - I think it was a pretty decent all-tube mini amp - but I didn't like it enough to take it beyond the prototype stage.

Each of the two valves cost a whole buck (USD) each, so two bucks total. 

I think this little amp has some potential to be turned into a kinda-sorta Herzog, feeding the output to a solid-state power amp or P.A. system. I haven't explored that, though.

Back to one-tube amps, I think the guitar signal has to go through a number of tubes, each adding just a tiny bit of distortion and non-linearity, for a tube amp to sound "tubey". Even classic Fender amps designed only for clean tones (like my '65 PRRI) have the signal go through five tubes before reaching the speaker. A one-tube amp is just not going to sound tubey, and if it doesn't sound tubey, what's the point? Might as well go solid-state at that point.

-Gnobuddy

[shooter]

Gibson GA-5T. Maybe that's the way to go.

 

don't use Gibson's original wire layout 
It is one of my favorite small amps

[Adrien]

Have a look at these:

http://www.solorb.com/elect/musiccirc/squirrelmonkey1/
https://zeppelindesignlabs.com/product/percolator-2w-amp-kit/

Both are single-tube designs using a 6AF11 compactron.  Pretty cool!