17JB6... guitar amp?

[ThSpeakerDude88]

Yes, I am crazy.

Not because I want to know if it can be done, but more because I was walking through the woods at school and found an RCA tv buried in the mud.

Pulled a few of the unbroken tubes from it and they test new. Hey, weird things happen. So why not? 17JB6 is the same as a 6JB6 or a 6GJ5 apparently, ableit with a 17v heater. I've also got a 10HF8 which might make a cool amp, 5 watts plate dissipation in the pentode section and an AT7'ish triode as well.


The 6JB6 is used in transmitters, and is a big sweep power pentode with 17 watt plate dissipation and 3 watt screen. I think the only question would really be finding what point to operate it at into what load, with how much drive. I don't expect much power, 5 watts or so probably since the screen voltage is limited. I've got all kinds of 12Ax7's,AT7's, AU7's, 6DJ8's, 6AU6/BE6/BA6's. Weirdish tubes could make a cool conversation peice. So what do you say?

[Colt45]

Sure why not. Dissipation and plate voltage isn't too important, but watch the screen voltage.

Shouldn't need much drive at all, 10v maybe, since it's guitar maybe a few stages of AX7, throw a tonestack in somewhere... maybe have an "overdrive" channel with an additional ax7 at zero-bias or something. i dunno. tired.

[ThSpeakerDude88]

wow I didn't realize the drive needed was around that of a 6v6 or 6bq5. This is more than adequetly provided by an ax7. I'll probably throw in a two stage pre with volume btween it and a cathode follower after the second stage for some marshall-ness.

What load should I try? I've got a hammond 125CSE or whatever, should be good for 10-15 watts if I watch idle current... and whatever load I desire.

[Bandersnatch]

hey-Hey!!!,
The critical thing with these tubes is providing a solid screen votlage. This also happens to be at a fraction of the plate voltage. For reasonable loads, it will probably be around 100V with a 250-300V anode voltage. Figure on a 1k5-2k SE load, or double that for PP. 0B3 at 90V or 0C3 at 105 is where I'd want to start.
cheers,
Douglas

[ThSpeakerDude88]

Well the datasheet says max G2 of 220v, so I would think that 180 or so would be fine. Its gonna be a guitar amp so cheap and dirty is the name of the game. Doesn't need to be very linear. Still, I may try the gas regulator as it might look kinda cool

Oh, heres something I forgot to ask... Fixed bias or will cathode work just fine?

[SY]

Not to throw a curve, but that's a perfect candidate for screen drive. tubelab has posted a lot of stuff about his MI applications for that topology.

[Frank Berry]

Not to throw another curve ... but ... where are you going to get the 16.8 volts for the filament?
It might be less costly to purchase a 6JB6 tube so that you can use a standard filament transformer.
Just a thought.

[Bandersnatch]

Originally posted by ThSpeakerDude88
Well the datasheet says max G2 of 220v, so I would think that 180 or so would be fine. Its gonna be a guitar amp so cheap and dirty is the name of the game. Doesn't need to be very linear. Still, I may try the gas regulator as it might look kinda cool

Oh, heres something I forgot to ask... Fixed bias or will cathode work just fine?

The g2 voltage sets the envelope of curves the tube will operate in( under actually ). High voltage like you suggest will dicatate a low numeric plate load( less than 1k ), which in turn limits B+...and thus power. This is SE so you've got to be Class A. Lowering the load doesn't deliver large increases in power like AB and B PP does.
cheers,
Douglas

[ThSpeakerDude88]

Frank, I will just use a small 13.5v (which puts out more like 16v) transformer that I have. Bridge rectified I'll get around that, and then an LM317 for the heaters of the preamp tubes. I already have the tubes,sockets, OPT, regulator, chassis etc. All I need is a suiatable PT. I was thinking along the lines of a 269JX which puts out 315v after SS rectification.

Another thought: I might get around 17v ( remember +/- 10% for these series heater guys) if double the 6.3v winding, which usually comes out to around 7v anyways. Not too concerned with it anyways, easy enough to change out if I wanted to later.


So explain this grid thing to me... I thought that the higher the vG2 the higher the output power? I do understand that pulling it down to a lower voltage helps linearity, but I am not as well versed in sweep tubes as I am your 'regular' pentodes and triodes.

[Tom Bavis]

True... in push-pull. But in SE class A, you're up against the plate dissipation limit, so you can't use the extra current that the tube could conduct with a higher screen voltage. Also, the screen will be hurtin' if you thrash it... better to keep the screen voltage down. A screen resistor MIGHT sound better than a gas tube - you can try both...

Linearity is desired in audio amps, but not necessarily in guitar amps. Smooth transition into clipping probably will sound best. Especially in a small amp that sometimes has to be turned up to "11".