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!

[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.

[tubeswell]

Originally Posted by Gerard

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.

Originally Posted by Gerard

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.

[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.

[Gerard]

All great points Tubeswell and Tryu007. I thank you for your comments. What will adding a grid resistor to the power tube do? and the difference sound wise from a 1.5k to a 5.6k? grid resistor would be?

[MWJB]

"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." - That isn't typical, I normally find that the envelope changes with each different rectifier. On a regular champ with 1x6V6 you might not get a huge change in idle voltage from one recto to the next because current draw through it is not that great.

[tyru007]

Originally Posted by Gerard

All great points Tubeswell and Tryu007. I thank you for your comments. What will adding a grid resistor to the power tube do? and the difference sound wise from a 1.5k to a 5.6k? grid resistor would be?

Used to prevent parasitic oscillation and blocking distortion. It will not effect tone. Fenders typically use the 1.5K on 6L6s while marshalls use the 5.6K with El34s. I'd go with the higher 5.6K.

For more info go to:
http://www.aikenamps.com/InputRes.htm
http://www.freewebs.com/valvewizard/gridstopper.html

MWJB - I compared a rectifier of 4 UF4007 diodes and a 220R 25W inductive wirewound resistor to a GZ34 rectifier so that they both gave about the same B+. Granted that the GZ34 does not produce much sag and the 220R resistor does add resistance to change in voltage. I did not detect much diffence in these two methods of rectification for the SE amp.

[MWJB]

"I compared a rectifier of 4 UF4007 diodes and a 220R 25W inductive wirewound resistor to a GZ34 rectifier so that they both gave about the same B+. Granted that the GZ34 does not produce much sag and the 220R resistor does add resistance to change in voltage. I did not detect much diffence the the method of rectification for the map." By trying to emulate the voltage drop of the tube, with the diodes & resistor, you have muddied the water somewhat regarding a comparison between tube and SS rectification. Diodes alone don't drop the voltage, it would have been a better comparison if you had A/B'd the diodes vs the GZ34 without the dropping resistor. What you did was similar to Weber's copper cap recto & some folks feel that these sound just like tubes, so I'm not surprised that there wasn't much difference.

[tyru007]

MWJB - I may try subbing differnet rectifier some time if I can get my hands on a few types just to see. I just wanted to know if a diode + "sag" resistor = GZ34 rectifier, which it appears it does for an SE amp. I have left mine with the solid state rectifier.

Gerard - One thing I found out worked well for reducing the hum from the filements was to raise the ground voltage reference for the 6.3V power supply. This along with good power supply filter made my amp almost silent at full volume.

If your 6.3V supply has no center tap, create a artificial center tap using two 100 ohm resistor then tie the center tap to either the cathode of the power tube (at about 20-25V) or create a 30-40V supply using a voltage divider with 270K then 22K resistor to ground off the B+ (450V) supply (also acts as your capacitor bleed resistor. The voltage at the connection of the 270K and 22K should be about 35V.

I have mine tied to the cathode of the power tube. For some reason I get a slight hum at the speaker when the amp is in standby, which goes away when the standby switch is turned to play.

[Gerard]

All great points!! Thanks. Now it is off to my build.

[Gerard]

Hi, I just finished my 5f2a with a 6l6gc. When I first started it up, it sounded like garbage! rattling/buzzing distortion. So I disconnected the negitive feed back wire. That made it sound alot better. (do I need the change the value of the negitive feedback resistor? or is something else wrong?)
2nd, When the volume is turned up past 9, the sound cuts out (oscillation?). What pins of the 6l6 do I need to install the grid stopper resistor on?
I am running this amp with a gz34 , 6l6gc and 12ax7 . hammond 123ese O.T. It has 440v on the B+ . 510 ohm cathode resistor
How do I make this amp less muddy with H/B pickups?
I have not adjusted any voltages yet, I could use some help! here's the voltages

6L6GC
#3-436v
#4-395v
#8-31v

12AX7
#1-212v
#3-1.70v
#6-215v
#8-1.66v

[MWJB]

Go bigger on the cathode resistor, 510ohms is borderline at 436vdc. 680-800ohms.

No, you shouldn't need to change the NFB loop. Something else is probably wrong.

Grid stopper can go from pin 6 to 5. Connection to 220K grid load connects to pin 6. Though, this shouldn't really be necessary.

Your input jack doesn't look right? 1meg should go from hot to ground, with a jumper from ground to the switch.

You seem to have all your grounds daisy chained, separate the preamp & power amp grouns, run the preamp grounds to the input jack, power amp to a PT bolt. That 0.0047cap should be grounded to the back of the tone pot.

[big-daddy-59]

with a single input the grid resistor should be 33k not 68k. I can't tell from your pictures what the value is. Preamp voltages are a little high still, might want to increase the value of R11.

[MWJB]

Lose the black ground wire that runs from your input jack to the speaker jacks, the speaker jacks ground through their connection to the chassis at the socket nut. If you have insulated them, don't.

[Gerard]

I took your suggestions and did the following:

corrected the input jack 1m resistor wiring

grounded the tone cap at the pot


removed the ground wire from the input to the output jacks

grounded the volume pot to the input jack ground

changed the cathode resistor to 680 ohms, now idling at 51ma

changed the input resistor from 68k to 22k (is that ok?)

the hum has always been very quite even at wide open throttle. no problem with 60hz hum.

the other problem I was having with the buzzing distortion with the NFB circuit hooked up was the O.T wires. I just reversed them from tube to B+. That took care of the buzzing distortion.

When I built this amp, I thought it would have more output (loudness). I have built a mission 5e3 and that to me is quite loud! I know this is a small practice amp , but I thought it would be louder.
I have tried all types of tubes in it , 5u4gb, gz34,5y3gt, 6v6,6l6gc,12ax7,12ay7, 12au7,12at7, I havent got a favorite yet, and yes I will watch the voltage when switching tubes.

The only problem I have now is some sort of parasitic oscillation . When the voulme is past 5, there is a slight vibrato effect, just slightly noticable. The amp is still usable but I need to tace that problem down. Which wires should I look at first .Which wires would I need to shorten or reroute?

Would you recommend using a shielded wire from the volume pot to pin #7 of the 12ax7?

Any thoughts on putting a cap on the second cathode resistor of the 12ax7?

Thanks for all your input !!

[MWJB]

Yes shield the wire from volume pot to 12AX7 pin 7, route it (and any other grid wires) away from plate wires (12AX pins 1 & 6). The orientation of your tube sockets seems to encourage long parallel runs of plate & grid wires? Would it be feasible/easier to rotate the 12AX7 by 180 deg?

Also the wire running from the 220K grid load to the power tube grid is rather long, you could also shield this, or what I would do is remove the coupling cap that connects to 12AX7 pin 6 and see if it will reach from the turret where it meets the 100K plate resistor straight to the power tube? Keep it away from the heater wires, mount a new 220K directly on the power tube pin 5 & ground to PT bolt.

What about the PT centre taps & grounds are these still soldered to the chassis?

A bypass cap at pin 8 of the 12AX7 will shunt a large proportion of the signal there to ground, lessening the effect of the NFB loop (it will still have some effect). It will also increase gain, not something I think you want until your oscillation issues are sorted.

Connect a true RMS AC meter across your speaker terminals, play the amp. Voltage will jump about some but try and determine an average, square the average then divide by speaker load in ohms...I'd expect about 8W?

[Gerard]

I will follow your recommendations and rotate the 12ax7 , move the cap to shorten the grid wire to at 6l6, and install the 220k to ground at the tube. also shield the volume wire.

This might take me some time, but I will get back with you.

Thanks, Gerard

[Gerard]

Thanks for all you suggestions! I rotated the 12ax7 which made me loose about 3" of wire, I installed a shielded wire from the volume pot to pin 7. rotated the .022 uf cap from the board to the 6l6 and installed the 220k to ground at the tube. No hum or oscillation at all, after the repairs!
This amp is way louder than I expected! Almost at loud and my mission 5e3 but with a little less thump! That is what I was looking for. This thing screams with a blues driver in front of it.
Now I need to build a cabinet! Thanks for all your help!