Crazy idea? Guitar Quickie

[Dr. Toobz]

Ok, here's a thought I had today - tell me if it's too crazy or just impossible. I've been wanting to build something small and cheap for guitar, like a mono tube headphone guitar amp. I don't know all that much about guitar amps (and not much more about hi-fi - thank goodness for the people at BH who do). But it seems to me that the Quickie could be turned into something interesting for a guitar, no? Would it be possible to have one of the 3S4 tubes handle the amplification (perhaps even wired as a proper pentode for a little more juice) and the other one act as some sort of input stage, where one could either use it as a tone control or drive it to distortion for the classic tube amp sound? I could see using the output tube as a pentode in that case, seeing as how maintaining the clean tone of a SET wouldn't be important in this case, but high gain would. Since it would be for headphones, maybe an ultralinear type of arrangement would work?

Would something like this work?

 

[Paul Birkeland]

I think you might be better off starting with a SEX amp.  You could use a 12AX7 or 6AC10 in one socket to kick the gain up on your guitar pickup.  A 6EM7 in the second position would provide even more gain and give you speaker/headphone level output. 

You might even have enough gain in this configuration to put some tone controls in. 

I would gander that PJ may be able to come up with a way to get enough out of a Quickie for it to be somehow useful, we shall see!

 

[Dr. Toobz]

I think you might be better off starting with a SEX amp.  You could use a 12AX7 or 6AC10 in one socket to kick the gain up on your guitar pickup.  A 6EM7 in the second position would provide even more gain and give you speaker/headphone level output.  

You might even have enough gain in this configuration to put some tone controls in.  

I would gander that PJ may be able to come up with a way to get enough out of a Quickie for it to be somehow useful, we shall see!

My S.E.X. amp has a hands-off policy - I've disallowed myself to do anything more to it! It's quite happy as a living room amp. OTOH, Quickies are cheap and I have plenty of spare parts (chokes, caps, etc) and an extra base laying around, so I'll be interested if there's any way to hack a guitar amp from one. Plus, batteries make it portable. PJ? If not, I guess I'll have to spring for the real thing! (It's much more fun to try to rig something up, though).

I do remember Doc rigging up some sort of guitar circuit from a S.E.X. amp on the old forum.....

 

[Paul Joppa]

I'm thinking a 1U5 in pentode mode for the driver - lots of gain, and designed originally to amplify microphones i.e. low microphonics. "Standard" Speco output to high impedance headphones. (I'm working on a draft paper on output modifications for a bit more power and/or gain, but it's not ready for distribution yet.) I'll give it some thought. Ping me in 2-3 weeks if you don't see a post, I'm buried in the task of moving my mother to a new assisted living apartment right now.

 

[NotBen]

Ok, here's a thought I had today - tell me if it's too crazy or just impossible. I've been wanting to build something small and cheap for guitar, like a mono tube headphone guitar amp.

I have been thinking about this exact thing. The Quickie didn't come to mind first, but I was wondering about maybe a mono version of the SEX, but with a more typical 12ax7 input. Ken Fischer of Trainwreck amps has written about adding a single 12ax7 stage for gain. Basically using 100K plate load, 1.5K cathode with 22uf cap across it, a .022uf coupling cap from the plate into a voltage divider then into the next stage. Maybe somehow you could run this into the 6DN7 output stage. Or maybe something like using a single 12dw7/7247, use the 12ax7 as the input and use the 12au7 triode to drive the 6DN7.
But a modified Quickie would be much cheaper.
Not a real expert on how to design amps or do all of the math, but I am trying to learn.

 

[Dr. Toobz]

I've been recovering from having my wisdom teeth all yanked out well after the normal age one has that done (i.e., teens or early twenties), so I've been passing my time with my soldering station and breadboards. I actually have started on a guitar Quickie - but this is proving a bit challenging.

As for the basics, I basically bridged the pair of inputs to mono off the potentiometer. I popped out one of the RCA outputs and put a mono headphone jack in there, and the other RCA out is simply paralleled to it. If I want to use phones, I have a Speco wired to the phone jack, and a switch to cut it out of the circuit if I'm using the RCA output into an effects stack (to avoid loading things down). Then, I've played around with different arrangements to see how much distortion I can get. Pentode mode just makes things loud. I've paralleled the 3S4 tubes, which is louder and a bit more distorted, but still not great. My idea has been to use one tube for distortion and the other to amplify the output. I'm thinking that I would have to maybe have two B+ supplies: one 9V, one 27 or 36V. I could starve the plate of the first tube to get a ton of distortion (B+: 9V, with a 500R or 1K cathode bias resistor), then take the plate of that tube into the grid of the next. Question is, would this really work? An earlier experiment to use the two tubes as separate gain stages didn't work well - I basically used a .1uF cap to couple the stages, but didn't get much sound. Any ideas here? Or should I maybe look into a 1T4 sharp-cutoff pentode as my first stage and the 3S4 as a second? Another option would be to wire up a phase splitter and use the 3S4 tubes in a push-pull arrangement to get lots of odd-order harmonics.....but I have no idea where I would stick a third tube for a concertina or similar.

This has been a fun experiment with the Quickie. If I can get this to work, the Quickie will end up on my desk as a guitar headphone amp/preamp.