5F2A with a 6L6 prebuild questions

[tyru007]

I've only built a few amps so far but I spent a lot of time on a BF Champ (AA764) 15 watt SE build with various tubes.

I used the Hammond 270DX power supply (285-0-285 at 120 VAC) with a Hammond 125ESE. I ran it using parallel output tubes, tube recitifed, SS rectified, with and without a choke, etc.

Here's my take.

The circuit does not appear to sag. Except for a voltage drop, I get the same response using a tube or diodes to rectify the voltage.

The SE is sensitive to power supply ripple. At the very least, put a resistor between the first two caps to create a PI filter and increase the size of the caps (as tubeswell describes above). The 10-25W wirewound resistor 100-330R would work well.


I'm currently using 4 UF4007s to a 40 uf cap -- 150 mA, 4-5H choke -- 20 uf cap prior to the plates (only because I have the choke).

I have tried a EH 6V6, TAD 6L6GC and a JJ KT77 with JJ and EH 12AX7. I like the EH 12AX7 with KT77 the best so far.

I would recommend adding 1.5K to 5.6K grid resistor to the power tube.

I lose a lot through the Fender BF tone stack and the 6L6 or KT77 did not seem to break up as early. I added back some gain by changing the plate resistor on the first gain stage to a 220K and I have a bypassed cathode resistor on the second gain stage. You may/may not need extra gain with the Tweed tone stack.

With the B+ at 400 to 450V, you may need another filter stage to drop the preamp plate voltages.

At 450V you may need to use filter caps in series for the first two filter stages so that you have a filter rated for the high B+ voltages. Two 350V 40uf caps in series would give you a filter with 20uf 700V rating.

[tubeswell]

Is there some spec sheet you got your info about " not go over 40uf with a 5y3".

Most 5Y3GT datasheets of yesteryear say 20uF as a maximum capacitance. But the info on those old datasheets is there to ensure that you get a long life out of your tube in your TV, radio or whatever. 40uF is what most geetar amp peeps push a 5Y3Gt to in SE amps in order to get most of the spongy flab out of the supply. 20uF will work fine however, as will 16uF.

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?

F is the standard unit of capacitance. uF is standard shorthand for 'microfarad' and 1 uF is one-millionth of a farad. (although in the old days they used to notate this is 'mF', which is confusing because in shorthand for other units of measurement where a lower-case 'm' is included, people usually understand this to refer to a 'milli' of something, meaning 'one-thousandths' of a unit of measurement. Similarly in the old days they used to denote picofarads as 'mmF' meaning micro-microfarads).

1 nF (nanofarad) is one thousand-millionths of a farad. 1 pF (picofarad) is one million-millionths of a farad. The order of units-of-capacitance from big to small is F, mF (millifarad - 1 thousandths - okay big deal - but where do you see this being used?) uF, nF, pF. The more capacitance, the more stress is on the rectifier because more charge appears on the electrodes inside the rectifier tube.

The first filter (a.k.a. the reservoir cap) cap's job is to 'smooth-out' the 'gaps' between the peaks of AC swing in the rectified power rail (after your typical Full-wave rectifier has done its job of putting all the VAC bumps on the same side of the 0-reference point voltage potential).

The reservoir cap does this by storing charge on the upswingin' part of the cycle and releasing this back into the power supply on the downswing side of the cycle. This effectively removes the 'bumps' in the rectified AC power supply - leaving a sort of 'smaller-sinewave' of power at a higher potential than 0, (but looking nowhere near as bumpy as the pre-filtered supply does).

The resistor or choke (on a typical CRC or CLC filter) then further 'reduces' the 'waviness' of the 'rectified-and-capacified' power signal (each in its own respective way).

Then the output filter cap on your typical CRC or CLC filter takes care of the little bit of ripple that the other bits can't deal with, leaving you with a pretty smooth DC voltage several hundred volts above 0 (ground) potential.

There's plenty of sites on the net where you can looking up AC power supplies and rectification and filtering types with pretty pictures that better explain all of this BTW.

[big-daddy-59]

I;m just passing on info from this forum and others on the size of the first cap when using a glass rectifier. I'm sure there is a spec for it but I'm still pretty much a noob at this as well. I don't understand all the abreviations on the spec sheets for instance. I can just pass on what's worked for me and some lessons learned the hard way by blowing stuff up.

[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!

[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 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]

OOPS wrong schematic this is the right one

[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

[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]

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]

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

[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.

[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)

[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]

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)