5F2A upgrade to to use 6L6 or EL 34 as well as 6V6

[Skip Hagey]

What changes would I have to make to the 5F2A circuit to allow the use of any of the three most used octal output tubes (6L6, EL34, 6V6)?

[PRR]

Be sure the power transformer doesn't get hot with the big tube.

Often fine, because the stock PT for these rigs also covers the 2*6V6 models. Certainly for 6L6. The overload for EL34 is so small the PT will probably be fine unless you gig 24/7 for years at a time.

The big bottles WILL run at the lower 6V6 condition. Power output is unchanged (maybe 5% higher due to less loss in the fat bottle). Re-engineering to get the MOST out of 6L6 or EL34 pretty much means new PT and OT (and now no-good for 6V6). And the "most" out of 6L6/EL34 won't catapult you into the largest stadiums... 5W or 9W is sorta all the same. I advise letting it be a 5W-6W amp.

[labb]

If you want to be able to switch to any of these tubes when ever you want to you will most probably need a version of external biasing. At  plate voltage of around 370 volt the 6V6 would be about 33 ma, the 6L6 72 ma, the EL34 60 ma, a KT88 100 ma. Also you would need to ground pin 1 and I think pin 7 to use the EL34. I built an amp to do this and it was a lot of fun to play around with...When all was said and done most folks that played it ended up staying with the 6V6 tube. There was an AX84 build for this that also had  a version to use a EL84...Think it was one of the older versions of the SEL amp. Been a while> I could get into my files if you really want to do this.

[HotBluePlates]

If you want to be able to switch to any of these tubes when ever you want to you will most probably need a version of external biasing.

Nah, you just tie pin 1 to pin 8.


It's a cathode biased amp; the cathode resistor will let each tube settle into its natural idle current for that plate/screen voltage. You only need to make sure the PT heater winding can support the heater current of the bigger tubes (which is part of what PRR meant when he said, "be sure the power transformer doesn't get hot with the big tube").

[labb]

Well HBP I would have to say that you certainly know more about amps that I do so I will bow to your knowledge. What  I do know is that when I changed tubes in the amp that I built I had to re-bias each time. ( I was biasing at 0.9PaMax.) The amp had external bias test points across a 1 ohm resistor and a 1k, 2 watt pot that was in series with a 220 ohm 5 watt resistor. Also the plate voltage would change according to which tube was installed.  I measured the plate voltage for each tube and the required biasing noted that on a chart and put the chart on the amp. Like I said it was fun to play with. I had it set up for 6V6, 6L6, EL34 and Kt88 tubes. Went back later and install a socket for an El84 in parallel with the other power tube socket so  that I could run the 9 pin tube..Note: I used a Hammond 125 ese transformer that was rated 15 watts. Not a perfect fit for all tubes but it worked.

[HotBluePlates]

Well HBP I would have to say that you certainly know more about amps that I do so I will bow to your knowledge. What  I do know is that when I changed tubes in the amp that I built I had to re-bias each time. ( I was biasing at 0.9PaMax.)

You're not wrong but we're working from different assumptions.

The OP has a 5F2A and wants to plug in any common octal tube. He didn't say anything about swapping his (made for 6V6) PT (for more B+ current availability) and OT (for lower primary Z with a bigger tube), which is why PRR said:

... The big bottles WILL run at the lower 6V6 condition. Power output is unchanged (maybe 5% higher due to less loss in the fat bottle). Re-engineering to get the MOST out of 6L6 or EL34 pretty much means new PT and OT (and now no-good for 6V6). ...

Since I'm assuming the PT and OT remain the same, then I arrive at PRR's conclusion that power output will largely stay the same. That's because the relatively-high 6V6-suited primary impedance won't let additional signal plate current flow for the unchanged B+ voltage (which itself implies a maximum possible signal plate voltage swing). If you want more-power but at same B+ voltage, then along with the bigger bottle you need to reduce OT primary impedance to allow "more current" * "same volts" = "more power".

So my follow-on assumption given that scenario is that it's irrelevant if you bias the new tube for max dissipation at idle. B+²/Primary Z = Same possible power output.

I would be interested in knowing if after you adjusted each tube to 0.9PaMax whether you came up with very-different total cathode resistance and resulting bias voltage, or not.

The reliance on a single, fixed value of cathode resistance is why you can plug any common octal tube into a THD Univalve. No magic. The new tube may settle in at a different idle current with the screen voltage and cathode resistance present. If that results in more idle current for the bigger tube, then more-current flows through the same-value cathode resistor resulting in more-voltage across the cathode resistor, or more bias. That more-bias-voltage condition also means the new, bigger tube would require a larger drive signal before the grid-to-cathode voltage is driven momentarily to 0v and into clipping.

That's part of the reason the bigger tube seems cleaner (maybe a hair louder) than the smaller tube in the exact same, unchanged circuit. Additionally, the bigger tube is almost guaranteed to have much greater Gm than the 6V6, which can help it "feel more powerful" even though measured output power is likely to increase very little if at all.

[labb]

Sorry, I never bothered to actually measure what the total cathode resistance was after a change. Do know that it did change because I had to adjust the pot to get the required current. I don't have the amp right now. When it gets back to town I will see if I can get a hold of it and check. Would be interested myself. One thing that I do remember is that I did not have to do any adjusting when going from the 6V6 to the El84. All others I did. And you are correct about the xformers. I used a PT that was good for 125 ma and a 15 watt OT. Biggest problem I saw was not being able to match tube with OT. Still, it was fun to play around with..