5F2-A SS rectifier and other ?'s

[UniPlank84's]

Hi All,

As I work my way through my beginner attempt to learn and repair the small Mesa I am mentally tinkering on a build for a small 5-8watt Blues Blues/Rock amp I can maintain and mod as desired.
I have poured through the usual suspects and arrive at the ax84 P1 or eX, and the 5F2a at Weber-the Weber has the updated supply wiring that seems sensable to me.

So I am wondering:

1) Is it a straight exchange between using tube rect'tn and doides on the board similar to the ax84,SmokinJoe etal? Or are there other considerations-like sag being desired in a blues amp?? Or is sag a non-issue in small wattage like this?

2) If diodes are OK, are 2 adequate in this case, or 4 better? I also assume 4 diodes = 'FullWave' rectification with no leak back possibilities(?) or better at smoothing the a/c to d/c in the first stage, or something. As the 6v6's use a higher pt v'tage I was wondering if that is a determiner in diode count, vs. the ax84 P1 w/ 190/0/190v pt secondary and 2 diodes.

3) Is a 5F2a an easier/simpler/cheaper first scratch build project then the ax84? ...My first amp in the late '70's was a SF PrincetonReverb which was cheap then. I have never crossed paths with a true tweed Princeton, but from what I gather it seems what I'd like today in the small Fender circuits for a simple Vol Tone first build, and the board has fewer and lower value components, 2 pots, etc etc.

But....The ax84 is attractive since I love the 84's and I have a stock of them at hand, and it uses a smaller/cheaper transformer set, no rect'y tube. But maybe the 5F2 is a better simple first build choice.

Thanks, and Good New Year to you all,
John

[tubeswell]

1) If you're using diode rectification on a SE amp of about 5-8 watts with a 6V6Gt where you're looking for a plate voltage of about 340VDC and you have two pre-amp stages, then you should aim for a Pt with an Ht secondary of 220-0-200VAC at 60mA min, and a Centre-tapped 6.3VAC heater winding of 2A. SE amps don't sag, only Push-pull amps sag. Also be prepared to use a choke in the Pii filter if you want a quiet amp with little or no hum. Or you can set up a CLC filter before the plate supply using a small 40-50mA 4H choke and then a resistor at the screen node if you want. That will keep the character of a 5F2A but be really quiet. I'd run 240-0-240 HT secondary on the Pt if you're going to do that.

2) 2 x 1N4007 are fine for a FW diode rectifier. If you built a diode bridge rectifier you need 4 diodes (but then you wouldn't use a CT on that HT secondary winding). If you are just using a FW rectfier with 2 diodes in series on each side, you can just run them in series for something at the voltages described in 1). If you are using higher voltages you might want to put a 1kV (or higher) 10nF to 47nF cap in parallel with each diode in order to spread any Peak Inverse Voltage spikes evenly across all the diodes.

3) No real diff in simplicity or cost for the simpler AX84 vs tweedy SE amps - the sounds are a wee bit different between an EL84 and a 6V6. I built a 5F2A as my first amp - it is heaps of fun to play and takes pedals really well, as well as being loud enough to jam with (or even to mic up at gigs). Of course if you do a Tweed Princetony thing you've got the flexibility of a tone control. They breathe a bit better with a low wattage 12" speaker (if you make the cab big enough)

[Bruce / Mission Amps]

If you don't want to use a tube rectifier, and choose to use a SS rectifier (it makes very little difference in tone with respect to a single ended class A amp).. take a look at my 5F2H amp design over at Weber VST...

https://taweber.powweb.com/store/5f2h_schem.jpg

In the process of developing this amp, one of my builds was with a non center tapped 360vac@70ma PT and a SS full wave bridge rectifier feeding a 5Hy@150ma choke input filter to a 16uF 450v filter cap... then the actual amp started.
This, with an over the counter Hammond 125SE output transformer, gave me about 320vdc at the power tube plate, with a very solid idle current level and a terrific sounding +7 to 10 watt, SE KT66, 6L6 or JJ6V6 amp driving a tweed Harvard amp cabinet with a cheap 10" alnico speaker.
I later refined it of course but it would give you an easier start then designing your own.

[tubeswell]

one of my builds was with a non center tapped 360vac@70ma PT and a SS full wave bridge rectifier feeding a 5Hy@150ma choke input filter to a 16uF 450v filter cap... then the actual amp started.
This, with an over the counter Hammond 125SE output transformer, gave me about 320vdc at the power tube plate, with a very solid idle current level and a terrific sounding +7 to 10 watt, SE KT66, 6L6 or JJ6V6 amp driving a tweed Harvard amp cabinet with a cheap 10" alnico speaker.

Hi Bruce

Were you running a 270R cathode resistor (as per that schematic) or was it lesser resistance (to run a KT66 or 6L6 at 320V on the plate)?

(I'm curious because I have a spare 376VAC non-centre tapped PT and a 20-25W SE OT and a 4H 90mA choke lying around waiting for another burst of amp-building energy. I was thinking of maybe a dual SE with 2 x 6V6GT or 1 6L6CG/KT66 with an AX84 SE Super Lead type preamp)

[Sir Cuitous]

Tubeswell, you could do dual 6L6 SE with those trannys. Single Ended amps are not as efficient as Push Pull. You won't get more than 20W from two 6L6s single ended. It's about double what you would get from the single tube.

[Bruce / Mission Amps]

Hi Bruce

Were you running a 270R cathode resistor (as per that schematic) or was it lesser resistance (to run a KT66 or 6L6 at 320V on the plate)?

(I'm curious because I have a spare 376VAC non-centre tapped PT and a 20-25W SE OT and a 4H 90mA choke lying around waiting for another burst of amp-building energy. I was thinking of maybe a dual SE with 2 x 6V6GT or 1 6L6CG/KT66 with an AX84 SE Super Lead type preamp)

I'd have to go back and look at all my notes but I'm pretty sure I always used either a 250ohm or 270 resistor at the lower pate voltages.
Personally I try it but probably not use two 6V6s in parallel with that OT but I would expect good success with a single 6L6GC or KT66 using it, if it is around 4000 to 5000 ohms.

The 4hY@90ma choke is a little to small for this project.
That isn't much different then what a 20-50 watt amp uses for 4-6 preamp tubes and the very small additional screen current developed in the power tubes.
You really need something more substantial, like 4-5Hy@150ma as a choke value.

[UniPlank84's]

Thanks for all the details and insights Bruce and Tubeswell. I did notice the 5F2h at Weber, but did not look at the schematic/layout. Before looking at it I assume it is spec'd for the higher power 6L6/KT66 etc rather than the 6V6's. It said 'Harp' amp so I wasn't sure it was appropriate for guitar, but obviouly that is a non-issue if it swaps to 6V6's etc.-still learning many small details in all this. This sounds like a likely candidate to get a bit more SE'd juice.

I looked close at the Angela Super SE project also, but the single tube, SS rect arrangement if designed for correctly is a good beginner project to keep invested parts to the minimum and yeild still a nice useable amp that won't be shunned away for something better.

Still the Mesa is backburnered, surviving on Roland circuitry-barely.

Thanks again for the recommendations and deep details Tubeswell-that info is very educational-I will come back to it when I have more time to absorb it as I'm in a rush now.

Also, in no way would I try to assemble anything from my own bits and pieces and minimal scraps of understanding. At this point I am asking ?'s to learn how this stuff works. Pretty amazing once you look behind the curtain into electronics design and function beyond the jacks,plugs etc.

Best Wishes All,
John

[ncusack]

Hey guys I may be off topic here but my room mate is an electronics tech and is looking to build one of these 5f2a amps. The thing is he doesnt play guitar or anything so he doesnt know what the best choice would be out all the 5f2a schematics out there. If any of you could suggest a quiet and good clean sounding 5f2a circuit I know he'd have no trouble building it. Please PM me or fire an email to zigmen_zigmen@hotmail.com if you can offer an opinion.

Cheers