low power SE with 813

[SemperFi]

I have a couple of 813 tubes, laying there just looking good...I also have a powersupply which will give me around 500V. I have lots of parts and other tubes. What I dont have is lots of money to buy new stuff such as power tranny with higher output voltage.
So I plan to use those bad looking toobs in a SE geetar amp with what I got. If I run the 813 with 0V bias, I can get just under 200mA idle. It can maybe swing +-400V and +-150mA, which is 60Wpeak. That's a lot for geetar sound!
I figure driving it transformer coupled with a smaller power tube, such as 6L6, or parallel smallsignal triodes like 5687 or equivalent.
Any comments on this venture? I know I'll have a lot of wasted heat, which I don't worry about, but perhaps I've missed something and will spend a lot of time for nothing. My main concern being running 813s at this low voltage.

 

[Miles Prower]

I wouldn't do it. Those 813s are really meant for a max legal ham rig, and don't really work well with less than 1000V. You're like lt to be running them in a rather non-linear region, and furthermore, those transmitter tubes could be damaged by running just hot enough to allow outgassing to occur, but not hot enough for good getter action. The thoriated tungsten cathodes are rather more delicate than oxide cathodes, and contamination will ruin them.

I'd either hold up until you can build something that can fully utilize them, or trade them off for the essential goodies to build something of more reasonable power. Unless you're building an amp for an arena, you don't need that much "overkill"

 

[SemperFi]

Thanks for reply. That was excactly my concern, ruining these bad ***** due to low voltages. What are the lowest voltages I can use these tubes at without worrying about shortening filament life?...edit: I just remembered you said 1000V...
In regards to linearity I don't worry too much since I think it may give a very cool and unusual sound to a guitar. It would just be so awesome to have that thing glowing big time and providing tons of single ended tone thru a couple of 15" speakers.

 

[Miles Prower]

According to the ARRL Handbook, the 813 can run as little as 1250V(DC) as a single-ended class C RF amp. For class AB(1) and AB(2) amps, they suggest from 2000 -- 2500V(DC). I suppose that this applies to AF as well as RF. Even here, the figures look conservative. 813's have figured in "extra-legal" ham rigs.

It would just be so awesome to have that thing glowing big time and providing tons of single ended tone thru a couple of 15" speakers.

I don't think those speakers would last very long.

In regards to linearity I don't worry too much since I think it may give a very cool and unusual sound to a guitar.

In this case, you might want to consider 845's instead. These have the same funky thoriated tungsten direct heated cathodes, and would look pretty similar. Since these have carbon anodes, they operate at a vague red heat. In a darkened room, they'd look even more intimidating than the 813. Being triodes, they'd probably sound better in a single-ended, no feedback, topology than a pentode like the 813. (Mainly 2nd order harmonics as opposed to the more unpleasent sounding higher order harmonics.) You just might be able to get away with running an 845 from a 500V(DC) supply.

Or you could always under volt a more reasonable power pentode. The 6KD6 might just work in that, seeing that it is a true pentode, like the 813. These have the more robust oxide cathode, and since it was common as a horizontal power deflection amp in color TV's, probably cheaply available. So if you blow one, it won't be quite the loss as an 813 would be.

 

[Brett]

Ignore the ARRL Handbook. I've built a couple of 813 amps, mainly PP class A triode though I've also experiemnted with them in pentode, and they're fine at lower voltages. Very inefficient though. They seem to come to life at 700-800V and are as tough as old nails.

Been thinking along similar lines for a guitar amp using the 813 in triode. Just too busy with other projects and work at the moment.

Do you really need 60W? It's a lot for a guitar amp unless you are running without PA support in a large hall. For home it's stupid.

Drive it with a pentode, like an EL84 (in pentode) if running it as a triode. It's pretty easy to drive the 813 I'd think in pentode with a range of small signal tubes. Remember you're trying to create part of the sound with the amp and hi-fi clean it might sound like rubbish in that application. Try everything to hand; experiment.

 

[bluerooster]

How 'bout running it GG, and drive it with a pair of 6L6 PP, and a 6SL7 pre amp.
With about 2.5kv on the plate of the 813, it should give about 300 watts out.
It will be linear, but the power supply will be kinda large to provide 2.5kv at ~250ma, just for the 813.

 

[Wavebourn]

I have one SE prototype with 813, 750V B+, pentode with nested feedbacks, A2. 20W output.

 

[multi]

What about an 805 you can get 45 to 50 watts with 750 to 800 volts single ended??

 

[knockbill]

Or an 811SE,, mine (avatar) is driven by 6BG6 and 6SL7 preamp...

 

[PRR]

What about an 805 you can get 45 to 50 watts with 750 to 800 volts single ended??

Need a big hammer.

750V and 45W out suggests 113W dissipation or 0.15A current. The idle point is well up in the positive grid region. The grid impedance is 800 Ohms around the idle point, *varying* from 333r to infinity as you approach full output. Peak grid power is near 70V and 90mA or 6.3 Watts. For low distortion, driver output impedance should be <300r. The high and varying grid current means R-C coupling is no good.

A 6L6 (maybe 2A3) with ~~5K:600 interstage transformer may do OK.

Cathode follower, SE class A, is another option; needs 6L6 6550 8417 or maybe 2A3.

 

[Miles Prower]

Need a big hammer.
A 6L6 (maybe 2A3) with ~~5K:600 interstage transformer may do OK.

Cathode follower, SE class A, is another option; needs 6L6 6550 8417 or maybe 2A3.

A power MOSFET siurce follower would be even better: much lower Z_o and r_d(on) that can slap those grids dilly.

 

[DAK808]

I am thinking of building a SE 813 amp. I got all kinds of parts and tubes for the amp except the output transformer. What would the specs for such an output trans be? I can supply a B+ of at least 1000v, depending on the rectifier scheme. Thanks for the suggestions.

 

[audiowize]

Cathode follower, SE class A, is another option; needs 6L6 6550 8417 or maybe 2A3.

The EL34 is pretty well suited to this duty (trans-conductance is king!). The real fun comes when you need to swing enough voltage that a negative rail is needed for your cathode follower.

 

[SemperFi]

Whoa cool thread rebirth. I saw the title and thought to myself wow I have just that same idea... then I see I am the OP and that was in 2005, 13 years ago!
Well I still have those 813s NIB but now I have 500va transformer with 690vac secondary. So should get >1kV. Also have some nice interstage transformers that are pp-se with a little step up ratio. Plan to use 6AS7 as driver with those ITs.
Lets see in 13 years if Ive actually built something like this.