Uneventful
Guess what? My amplifier works. Sort of.
There are some problems. My brilliant idea to use quick connectors to connect all the tubes and pots to the main board turned out to be a not-so-brilliant idea. It certainly made assembly cleaner, but the connectors are not very reliable. I’ll be replacing them all with proper permanent soldered connections next. Still, the new layout is a good one (using perf board instead of point-to-point wiring was a GREAT decision), and the inside is actually quite clean and, I daresay, rather pretty. I’ll post pictures on Thursday.
Also, I need more gain after the tone stack. I used a unity-gain phase inverter before the power tubes. This, coupled with the large attenuation factor in the Baxandall/James tone stack preceding the inverter, means that I can’t get the full possible volume out of my power amp, even if I drive the tone stack buffer into saturation with the preamp. I plan to fix this by using a single anode-follower to drive the tone stack (instead of a direct-coupled two-stage cathode follower), and use the extra triode stage to form a long-tail pair phase inverter, which itself has substantial gain. This should allow me to get the full volume from the amp.
But the sound. The SOUND – it is pretty breathtaking (when it works right). Very high gain, but very tight and clear. I can play full chords at full overdrive levels and they still ring out pretty clear. Couple that with a healthy dose of sag from the series resistor in my power supply, and I’ve got myself a tight little high-gain rhythm guitar amp.
There are a few other issues- something is wrong with the grounding on my input jack, my power tubes are biased a little hot, I’ve decided to redo my power supply to use the full B+ available from my transformer. There are a lot of little fiddly things to fix here. And it should be a blast! Man, I feel like Rachel right now.
Alright, enough amp stuff for now. I’ll see you on Thursday.
Schematics, please? Do you *want* the output B+ to sag under full load? Sounds like the filter caps, esp the output cap, need more capacitance. You might go for a choke, too, instead of the simple series RC filter.
sounds like a great project, though. brings back many memories from a long time ago.
pure hack idea to play with: add a little adjustable positive feedback around the tone stack and it’s gain stages. That let’s you tune the Q and shape of the tone stack and that can be pretty interesting – just make sure it isn’t driven into full-blown oscillation or things get very nasty and magic smoke escapes from the speakers.
good hacking!
I should upload some schematics. I’ll draw them up on something other than a legal pad this weekend and post them.
The sag IS intentional actually. I’ve got a 10W 1k resistor in series with my rectifier diodes, so the entire power supply B+ is reduced. It sounds pretty good (I get a ton of punch-sag-swell when playing percussive rhythm power chords), but once I get full volume it will probably be too much. I think I’m going to make my “sag resistor” switchable so I can have clean power if I want it.
The positive feedback sounds like fun, though I’ll have to be careful, given how much feedback I already had to deal with in my circuit before. I REALLY need to pick up a secondhand oscilloscope next time I’m at the electronics flea market. Doing it by ear is tiresome (thank god for cheap speakers).
I actually have a design for a fully-active, fully-tube Baxandall tone stack that I originally planned to use on this amp (before I got intimidated by the complexity and simplified it from a 12-triode circuit to a 6-triode circuit). You use the standard active (op-amp) Baxandall circuit with a tube op-amp formed by 2 dual triodes (one is a diff amp, the other is a direct-coupled output buffer. I based this off a tube op-amp circuit from the 50s that I found, but changed the output buffer). The open loop gain isn’t huge, but it is enough to get some pretty good results (in simulation, anyway). I’ll have to play around with that idea too. And my balanced input idea. Soooo many things to try!
Hmm… How do you reboot your boss?
In a manner that doesn’t get you fired afterwards, I mean?
Wave the money under his nose–that always works on mine.
I think I foresee a plea for her to use her influence for tickets and/or a backstage pass in her future. 🙂