5F2A with a 6L6 prebuild questions

[Gerard]

Hi, I have just about all the parts on hand to build a 5f2a with a 6l6. I have some questions about the bias resistor and O.T. I will be running the 6l6 at about 400 volts with a 5y3 and about 450 volts with a gz34 recto. The P.T will handle the load of the tubes, so thats not an issue. What O.T would be a good choice? I had my eye on a Hammond 125ese 15 watt with 5k primary.Also should the bias resistor be a 5watt or 10watt at around 270R or so depending on the voltage? and should I raise the value of the filter caps , if so, what to values?


Thanks again for your time. This is my third build, my first one was a Mission 5e3 that I kept stock and love the sound. This forum has been a great help to me over the past years and I appreciate all the help.

[MWJB]

"I will be running the 6l6 at about 400 volts with a 5y3 and about 450 volts with a gz34 recto" those figures don't scan...what PT are you using what is the B+ secondary rating for VAC & mA? A single ended 6L6 amp will only draw 60-75mA at idle, not enough current draw to to create the voltage drop that you are anticpating, a handful of volts difference is more typical.

At over 410vdc to ground from the 6L6 plate (pin 3) a 680ohm cathode resistor (or 800ohm depending on voltage) will be good. At 410v and less, 470ohms will be spot on. Bias to the highest voltage with the GZ34 and the amp will still work fine with 5Y3. DO NOT use 270ohms unless you are thinking of running the B+ at well over 100v less than what you are currently anticipating. 10W resistor rating, bypass cap should be rated for 100vdc.

100uf for the main B+ filter, 40uf for the screen supply, 10uf for preamp supply.

Plate current for the 6L6 should be 55 to 65mA-ish so an OT rated at 70mA (125DSE?) will suffice. Try the 5K tap (will also work fine for 6V6 if voltages allow), then try the 2.5K tap (6L6 only) and use the one you like the sound of best.

[Gerard]

Maybe I misinterpited the power transformer (or the math). It is an old stancor 360-0-360 at 90ma. 6v at 3a and 5v at 2a. So I figured 360v x 1.1(5y3) = 396 volts and 360v x 1.3 (gz34) = 460volts, am I missing something? I am still kind of new to this. Thanks for the quick answers to my questions.If I have the wrong P.T. what kind of voltages to I need to have?

[tubeswell]

6L6s will run fine at ~400V on the plates. You probably want to have the bias a little hotter than you would if the plate voltage was ~460V. (Its probably a matter of experimenting with the cathode resistors a bit. In Class A, where the current is flowing for the full signal cycle, a 6L6CG should be biased to 30W, and a 6L6GB/5881 should be biased to 25W)

[Gerard]

Do I need a larger O.T , other than the 15 watt I was looking to get? maybe a 20 watt one? and what primary ohm rating does a 6l6 like to see in and output transformer

[tubeswell]

15W one should be okay for 1 6L6 - IMHO a 20W one would be better for 2 in parallel

Pri Z for 1 x 6L6 is 3k2-6k6 (say 4k on that Hammond 125ESE)

Pri Z for 2 x 6L6 parallel is about half of the above (i.e. 1k7 - 2k5)

[MWJB]

"a 6L6CG should be biased to 30W" 30W is the max dissipation for NOS 6LGC, the JJ, also claimed for the TAD6L6GC STR, however I don't recomend biasing that hot. Bias to 22-24W. Most current production 6L6GC are NOT 30W tubes. Assume 25W unless testimonals (not data sheets) back up the figures. Basically, 55-65mA will do the job. You don't really want an amp that will eat tubes if you try the wrong brand.

Your amp is unlikely to make more than 10W, a 10W OT with 70mA rating will do. No harm in going larger, but if you were really looking for better headroom you wouldn't use a tube rectifier or cathode bias.

360-0-360 will be perfect for your purposes. However whether you get the voltages that the ratios quoted suggest is another matter. Those ratios work better with fixed bias applications & moderate current draw...even then you might only reasonable expect to get within +/- 20vdc. Remember that your power tube will be drawing a relatively high plate current, plus you lose effective plate voltage accross the cathode resistor. The current rating of the PT also has an effect on final voltage, as will wall AC at the time of day you measure it, bias conditions, brand of tube, etc, etc...Concentrate on the plate current, plate voltage will be what it will be.

With a 125ESE your options are 2.5K or 5K with an 8ohm speaker, 5K with a 16ohm speaker, or 2.5K/5K with a 4ohm load. Either 2.5K or 5K will work fine with a 6L6, use the Primary Z that sounds best.

[Gerard]

Thanks MWJB and Tubeswell for your help. I now have enough information to get this project going ! This will be a great learning experence.

[big-daddy-59]

I just finished (finally!) this exact build ..5f2a with 5ar4(aka gz34) rectifier, 6l6gc power tube. Used a Weber W022772 PT(330-0-330) and WSE15 OT. Bumped the values of the filter caps to 20u/10u up from the stock 16u/8u. Use a 470R 5Watt and use at least a 50v rated bypass cap. I've got a bias voltage right at 30v with plates at 390v for 24watts dissapation.

[Gerard]

Congrat's B-D-59 on your build. How does it sound? What kind of speaker did you end up with? Would you mind posting your tube pinout voltages? I am still coming up with my parts list to order. (I hate forgetting something and having to pay for shipping again) Thanks

[big-daddy-59]

hey Gerard, glad to help in any way i can. don't have my notes with the voltages in front of me but I do remember B+ was 430vdc before the OT, plate voltage was 420vdc to ground and 390vdc plate-to-cathode. preamp voltage was 190vdc on the plates.some suggestions, mainly safety related.I'm going to reference the weber 5f2a schematic because the components are numbered to avoid confusion. Use at least 500v parts for C9 and C2.. 600v even better. C5 must be at least a 50v part and a 5watt power resistor is highly recommended for R8. 2watt rating for R14 wouldn't hurt. I changed the value of R11 to 27k and given my rather high preamp voltage it wouldn't hurt to raise it to 33k. Given that you are proposing to use a PT with higher voltages than mine (mine's 330-0-330 and IIRC yours is 360-0-360) I would think that you should start at 33k for R11 and maybe even raise it further. And absolutely use a good OT designed for single ended amps like the hammond 125ese or weber WSE15

[big-daddy-59]

OOPS wrong schematic this is the right one

[Gerard]

Thanks B-D-59 for all the tips. I will look in to those suggestions. I am Still coming up with a material list. How do you like the weber OT?

[big-daddy-59]

the WSE15 just sings. If your goal is increased headroom,and a bit more output with later breakup it is absolutely the way to go. It dwarfs the OT that is normally used in a 5f2a with a 6v6, nearly the size of the power tranny.

oh another detail form my own build; I bumped up the values of the filter caps to 20u/20u/10u/10u up from the stock 16u/16u/8u/8u. supposed to tighten up the bass a little. Oh, and since the first two caps are in parallel, you could just use a single 40u instead. Don't go over 40u with a 5y3gt.

[Gerard]

Thanks for the tips B-D-59. I will have my parts this week. Is there some spec sheet you got your info about " not go over 40uf with a 5y3". I would like to understand more about the power section electrolytic cap's and how the "uf "relates to the circuit and tubes. I understand how caps work in a tone stack and also how by-pass caps work, but filter caps, I don't know. Will hi "uf's" stress the rectifier tube?
Thanks for your time!