interesting low-power beam power tube- 5686

[imaradiostar]

The other week I stopped by a shop that used to sell a lot of tubes back in the day. Most of the stock is pretty picked over but I did find something kindof interesting. I bought 5 5686 miniature beam power tetrodes. They're the same pinout as an EL84 but with extra pins used to double some connections for RF use.

These little suckers are about the size and fil current of a 12ax7 but they're rated at 8 watts max dissipation and are designed to produce 2.7 watts SE into a 9k load. I couldn't find any info on running them push-pull but I did have my plexi-jr on the bench and a lower voltage PT laying around so I hooked it up and biased them at a few different settings and played a bit. I don't know what plate load to use in push-pull but it can't be too far off from commonly available parts.

I think this could make a really interesting low volume push-pull tube. Using roughly 6k plate load in push pull it was darn quiet; probably not even as loud as a valve junior. I'm going to try some different output transformers and see what they'll do but I bet this would be an excellent solution for someone wanting to do that low volume thing but with a really low power pentode rather than a 6sn7 or 12au7 or similar. They require a substantial amount of input to produce a decent output, similar to a 6v6.

How did they sound? Well, I'm used to hearing that amp with EL84's, fixed bias and plate voltages around 375. Usually it's very marshally. With the 5686's and smaller PT I had 250-260 volts on the plates. I tried bias between 15 and 30 ma per tube. These things have pretty low transconductance so it took a lot of voltage at the grids to get them to bias correctly. Colder bias sounded cleaner and slightly brighter- better to my ears but your mileage may vary. It didn't sound like a 6v6 which is a welcome change to my ears! I'm hoping for a "baby kt66" sound after some tweaking.

More testing to follow if anyone is interested...let me know.

jamie

[Wakculloch]

They could make a very nice p-p, class A, four valve, low power output stage. The low heater current and low HT voltage could be useful for those "hard to find a use for" transformers.
Reverb driver also springs to mind.
Let us know how you get on in your application.

[woodyc]

I recently fixed a no-name single-ended 60's Japanese amp with a similar low power beam tube. This one was a 6AR5. Maybe its another number for the same tube? I had to tweak the amp a little but ended up getting a real nice sound out of it. At 288V/33mA with a 7K transformer it put out 1.5W clean.

[bulldogguitars]

Ive definetly been interested in those smaller tubes for sometime. Just havent picked any up yet. I was curious about possibly using something that small in some pedals. Just a thought. Keep us posted Danny

[imaradiostar]

Does anyone out there in internet land want to wager a guess at the plate load in push-pull? I don't know of any published specs. It seems to be a little happier with a higher plate impedance but I don't want to go too high.

jamie

[woodyc]

Theres no right answer, its a choice you have to make between distortion, dissipation, and output power... but typically with beam tubes you want the loadline to head toward the knee. I looked up the data and for that tube I think at 30mA/250V in push pull you'd want a total primary Z around 10K-12K. If you use a cooler bias, you can go to a lower Z.

[Klaatuboradanikto]

Jamie, Just wondering what ever came of your 5686 amp? How'd she turn out and what did she sound like?
Klaatu

[imaradiostar]

Funny you should mention it...I'm taking my last final tonight then I can get back to amp building.

I have a "tiny super lead" in the works. Chassis is made but I'm not happy with it because it's 7" deep. I may try to make a smaller one to reflect the "smallness" of the rest of the amp. It'll have four inputs like a real super lead, split cathodes on V1 so i can blend high treble and normal inputs. The power transformer is good for around 275 volts DC on the plates. I'm going to use a Deluxe Reverb sized transformer from an old tube hifi- it's got lots of inductance and I've previously proven it has nice flat bass response down to around 40 hz at 25 watts. I'll be using 4x5686's in the output, two per side for a reflected impedance of 13k to each pair of outputs. I'll probably use pre and post phase inverter master volumes so I can tweak which part of the amp is distorting. Depending on how it sounds I may add an Airbrake style attenuator in the head cabinet. It'll be fixed bias in an attempt to get that big sound with small bottles.

I'm going for that Hendrix/Clapton/Eric Johnson super lead tone. I don't really want it to ever be super clean but more in line with the sounds of the aforementioned artists. I'm hoping this thing will allow me to dial in a wide variety of those sounds. I may make some of the critical values (bright caps, etc) rear panel switchable so that it's more versatile.

As for how the tubes sounded- it's been a while but I remember them having a neat midrange rattle to them. They reminded me a bit of KT66's. Last I checked they're cheap on ebay so it's worth getting a few and trying them!

jamie

[Klaatuboradanikto]

Cool stuff! Please up-date your thread when complete. I'll have to check these little sucka's out! Thanks responding given that it's been awhile since this thread was active.
Cheers!
Klaatu