Cathode bias and being able to use 6l6, el34, 6v6 in a 5f2a

[vmonster]

I bought a webber 5f2a kit and before i get started i wanted to post this to get some questions answered by some of the more knowledgeable people on this board.

1. Where can i find a schematic to show me how what i need to add to circuit to ensure i have a cathode bias setup?

2. What can I add to the circuit to be able to run this amp with either a 6v6, 6l6 or el34 as some of the victoria or the swart amps do?

3. Has anyone had experience with the transformers included in this webber kit? do i need to upgrade to a mercury or allen etc?

thank you

[ChrisM]

I bought a webber 5f2a kit and before i get started i wanted to post this to get some questions answered by some of the more knowledgeable people on this board.

1. Where can i find a schematic to show me how what i need to add to circuit to ensure i have a cathode bias setup?

Straight to the stock schematic. The 5F2A is cathode bias. Look at the output tube's cathode (pin eight) .

2. What can I add to the circuit to be able to run this amp with either a 6v6, 6l6 or el34 as some of the victoria or the swart amps do?

Your gonig to need to adjust the bias for the different tubes. This can be done by adjusting the cathode resistor's value. I'll let you search load lines and biasing.
(EL34s will want half the stock primary impedance btw)

3. Has anyone had experience with the transformers included in this webber kit? do i need to upgrade to a mercury or allen etc?

Weber are complete crap, will never deal with them anymore. Save yourself a lot of hassle and get some new iron. While your at it you might as well replace about 90% of the other parts in the kit.

[Structo]

Some commercial amps claim with their cathode bias circuit that you can run just about any octal power tube in them.
Such as 6V6, 6L6, KT66, etc.

They must bias the tubes pretty cool to allow such a variance of tubes.
Unless they have a trick in there.

The 5F2A is the tweed Princeton amp. Single ended 6V6.

I think you would need a different output transformer to run the big tubes in it as well as change the bias resistor value, down to 250R or so.

Weber amp kits have gotten bad marks over the years for using cheaper components such as jacks, pots, pilot lights, transformers.
While I have never built a Weber or seen one in person, they reportedly have changed a lot of those parts to higher quality.
I believe it was Ted's (the founder) idea to keep the costs of the kits down yet provide a decent amp for not much money.

I don't know anything about their transformers but haven't heard that much bad stuff about them.

[Cliff Schecht]

A common trick is to design the amp to take either a 6L6 and a EL34. When you want to use a 6V6, you just move the speaker jack down one impedance setting (i.e. 16 Ohms goes to 8, 8 goes to 4) to double the primary impedance. This won't work with a single 4 Ohm speaker using a 6V6 (unless you happen to have a 2 Ohm tap) but this is the quick and easy way to get a "universal" power tube socket.

[Phil_S]

Opinions will differ on the quality from a particular supplier. The one discussed above makes no bones about keeping costs low and you do get what you pay for. I have ordered and used many caps and resistors from that particular place because of the low cost and I found the results to be acceptable. Keep in mind, I am a "hobby level" amateur builder and am not picky about this or that. I think, if I were building professionally (for resale), I would get my parts elsewhere and probably go for name brand items.

I'd say that the most critical parts in your amp (tubes aside) are the output transformer and the speaker. That's where I'd put a few extra dollars.

[tubeswell]

2. What can I add to the circuit to be able to run this amp with either a 6v6, 6l6 or el34 as some of the victoria or the swart amps do?

Further to what the others have said about load resistance changes for 6V6 compared to 6L6/EL34:

The pin-outs for 6V6/6L6 tubes differ from EL34s (notice that I have paired the tubes the other way around now) in that in the eL34 Pin 1 (suppressor grid) is not internally connected to Pin 8 (cathode), whereas is the case with the other two tubes.

Also IMHO you will probably want to have a switchable cathode resistor/bypass cap combination for switching from 6V6/6L6 mode to EL34 mode. I think 5W 470R/25uF 63V will work fine for either a single 6V6 or a 6L6, but you will want probably around 5-10W 220R/100uF for the EL34. The EL34 will also want 5W min (or even a 10W) screen grid resistor IMHO.

[jjman]

Single ended has no concerns with crossover distortion so a wider range of tubes can be used w/o needing to change the PS voltage, as well as the bias voltage/resistor. With PP one could employ a VVR to resolve the PS voltage on the outputs. Switch or adjust the bias, adjust the PS and switch the impedance selector and all those tube types can be used. Probably should go with ~high value screen resistors too to cover the bases.