Different tubes in 5f2a based amp

I see some amps online advertised as based on the 5f2a circuit and able to run 6v6, 6l6, kt66 and other tubes. I'm building an amp with switchable cathode bias resistors so I can switch between 6v6 and 6l6. Is there a way to support both tube types without separate resistors and a switch? How do they do this?

From what I understand, a cathode resistor is chosen to be "in the ballpark" for all tubes you mentioned...not exact science for tweed amplifiers to be biased perfectly. The bias is like an idle on a car. Too hot, and the tube will saturate earlier...too cold and the tube will cut-off earlier. That's why cathode biased amps break-up more than fixed bias amps. But, as long as the grid signal has enough "wiggle" room, the amp will work. JMO.

I was hoping that they used something a little trickier than that but I had a feeling that this would be the answer. Thanks.

Pete, my 5f2a works just fine with either 6v6 or 6l6 tubes using a 470 ohm cathode resistor. matching the impedance is more important. You will need that switchable cathode resistor if you want to try el34s or kt66s.

Pete,
What big daddy 59 said is nearly correct. The 6L6 uses a little bit smaller Cr. But with 470 ohm there will be no problem.
Using the EL 34 you will also need a switch for the impedance. SO you can't use the org. Fender OT. Try a Hammond 125 ESE.
You also have a look to the PT. Check if your PT can handle the more current you will need with the 6L6.
I've builded a switchable 6V6/6L6/KT 66 amp with line out and loop a month ago.

http://www.felicienne.com/extra/amp/5f1764.jpg

Karsten

OK. I've already planned on a switchable resistor on the V2 socket. Currently the values are the 470 Ohm for the 6V6 and 360 Ohm for the 6L6. I'm assuming that this will also handle a KT66. Using the Weber WSE15 OT and W022772 PT would this be able to handle an EL34? It looks like the impedance on the EL34 is considerably different.

It's not critical that it can handle more than the 6V6 and 6L6 I just think that it would be cool if it could. I'm sure that I'll be perfectly happy with 6V6 and 6L6.

Hi,
The Wo22772 can handle 100mA. You can't use a 6L6 with this one! The tube overkills (heat) the PT.
You will need a PT with nearly twice the current.

The WSE15 if fine. Using the EL-34 you wil need the 16ohm cable with using a 8ohm speaker.

I'm confused. The Two Stroke amp in Dave Hunter book uses this transformer and he says that his amp will handle a pair of 6V6s, a single 6L6 or even a single EL34. According to Weber (https://taweber.powweb.com/store/022772sch.jpg)the PT provides 4A on the 6.3v taps.

A 12AX7 draws 0.3A, a 6V6 draws 0.45A (so a pair would be (0.9A) and a 6L6 also draws 0.9A. The EL34 is the big one at 1.5A. Still it appears that this PT would handle the load.

Slightly related question...

If you were to wire up two sockets in parallel to use two 6V6s... would it be at all possible to use say one 6V6 and one 6L6 or 5881? or is this a stupid question? :D

I'm hoping to build a two-stroke soonish and would be nice to have that option!

Do you mean to use one 6v6 and one 6l6 Or 5881 at the same time? That's an interesting question. I bet it won't but I can't wait to see the answer.

My Epi BC30 will happily run a pair of JJ-6V6GTS or 5881 or any 6L6 with its single 250R cathode resistor and running some 420V HT (that would be ~500R for a single bottle), and I think pin 1 is already wired for EL34 (not tried). Probably not a perfect match but it does work. To be honest there's little to choose between them. Currently running 6L6WGB. Volume is about the same (gain determined by the bias etc), so just use ears to get your best tone.

Probably not a good idea to mix bottle types across a push-pull pair but SE might work, I've heard of using mixed-pairs across a 4-bottle push-pull, one of each type on either side.

It's not only filament current. Add the bias current of all tubes ! 100mA is not enough.

I don't get it, how does the Two Stroke work? It uses the same PT, the same rectifier, the same preamp tube and the same array of output tubes. The filtering section on the 5F2A is a little different but the rest of the amp is almost an exact match.

I'm not trying to be belligerent I'm honestly trying to understand the difference before I order any more parts.

Pete , don't let iorr confuse you. If Dave hunter specified a transformer rated at 100ma then you are gonna be alright. The Two Stroke and Weber's 5f2H (that's the harp version that uses a KT66) are PROVEN designs that work. I'm using that same weber transformer set and my amps trannies don't get any warmer with a 6L6GC than they do with a 6v6 in the power tube socket.Our friend iorr may be one of those people who come from the hi-fi world and think all guitar amps have under designed power supplies

Thanks again Big Daddy. I'm thinking about adding a voicing switch like on the Two Stroke amp. It's another case of "for a few bucks more I can...". I already have a switch and I picked up the extra cap today. I also found out that the fiber board, wire and balance of the resistors shipped out today. Woo hoo!