Stage amps using DHTs

[andyjevans]

Has anybody tried out directly heated triodes for stage amps? I'm not ruling out 300b outputs (!) but really thinking about small tubes for preamps - 3a5 for instance but there are plenty of possibilities. The smaller DHTs like 3a5 could be battery powered for the filaments. Should be small and portable, and could then use a solid state main amp.

I'm a bass player but also play keyboard. I use DHTs in my hifi amps, but haven't tried them on my instruments. I guess I'd also need some tone controls since I'd use it on double bass for jazz.

Anybody interested in this?

Andy

[Allen Wright]

Why?

Most DH tubes are very 1/ vibration sensitive/micrphonic, and 2/ fragile. As most stage gear get beaten aound a lot, I would much prefer IDH tubes so you have a good chance it will still run when the roadie yanks it out of the truck and drops it on way to the stage. I've repaired so many amps that had the tubes half (or all the way) out of their sockets because of rough handling, I wouldn't even think of using DH tubes.

Keep them for your hifi rig.

Regards, Allen

[Rod Coleman]

Andy, would have thought that the gain and power limitations would make for an expensive build. For double bass, even 300B PP would probably not be enough for a stage amp.

So how about GM70 PP? Apart from the robustness considerations mentioned by Allen, running 600V+ on stage is a hazard, but then Trace Elliotts run 700V. Or go the whole way and connect a PP pair of GK71s at 1.2kV like O.H. Schade does!

You would need to drive speakers with decent tweeters to make any use of the DHT sound.

It would be hard work & high cost to build such an amp, so you would need to be convinced you'd hear something that PP KT88s (say) didn't give.

[Rod Coleman]

BTW, try battery heating the 3A5 filaments, but please compare current driven heat as well. Batteries have low ac-impedance, and this usually stamps the life out of the DHT sound.

[andyjevans]

In response to Allen, I'm really thinking mainly of preamp tubes here. I play jazz, and just do local gigs, so no rough handling problems.

I'm not interested in the treble - I'd use a 12" speaker with no tweeter. My interest in DHTs is purely one of tone. I much prefer the tone of DHTs with audio and I'm wondering if the improvement in tone goes down into the bass register. Would have to try it to find out really.

andy

[Wavebourn]

DHT triodes are different. For example, 6C33C was designed for avionics, to stabilize voltage in jetfighters.

Speaking of stage and avionics, the best variant is still GU-50: a pair of them can give you clean tasty 100W, which it optimal for hi-fi output transformers. 3 db of loudness more means a lot more expensive transformer.

Edit: Right now I am working on my Waybeyond prototype. It is a bass rig, consists of master and power sections: small folded horn, and huge folded horn. Master section, all 3 channels, is made with GU-50 outputs. Everything is tubey, except chorus unit. Power section, all 3 channels, are solid state.

[kevinkr]

You might want to take a look at some of the Vietnam era submini dhts like the 5676 which was designed for use in U.S. military backpack radios, it's very rugged even if a bit microphonic. (Deal with that by shock mounting them) I've used them in simple battery operated line stages. Their linearity is not the ultimate, but it is not too terrible particularly if the filament is running current starved at around 900mV or so.. Their rated service life has not been a problem so far, and they are pretty cheap. They are reasonably well suited to full battery operation or use D cells for filament power with an inverter for B+ which would probably be even better than a string of 9V batteries..

[andyjevans]

I actually have a load of 5676. I tried them out and they sounded quite nice. The only problem I found is that they ran down fairly fast. Maybe I should try them again at a lower filament voltage - that would make sense. Maybe even filament bias, though that might not be for batteries - haven't calculated it yet. Still, a small power supply is no problem.