5f2: rectifiers, tubes, and OT primary impedence

[cminor9]

I have been searching for answers three days now, but I don't think I got my questions answered. Hence, yet another 5F2 thread. I have two things I am wondering about, and I did enough research to make some plans/assumptions. I'd appreciate input on my assumptions.

1) read here that for a single-ended amp like a 5f2, ss vs tube recto makes no difference other than voltage (a 5Y3 drops a good bit of it). I have some 1n4007's, so I think I might use these for the rectifier. I also have some 22v zeners in stock to handle the extra voltage. I was going to just use two diodes and wire em up just like the tube recto only on a little board. 1n4007's seem to be rated at 1a/1000v which should be PLENTY considering I will be using a 1a fuse. All correct? Is there some reason to use a tube recto?

2) intrigued by the idea of being able to drop a 6L6 into this thing. The OT I have is 15w, has a 5k primary and a 6k primary. It has 4, 8, and 16 ohm taps. The speaker I bought is 4 ohms. Using a 6L6 isn't a primary concern for me. But it seems like I could just switch out the zeners to get the higher voltage and then plug the 4 ohm speaker into the 8 ohm tap (to get 3k on the OT primary which would be better for a 6L6 than 6k). Thoughts on this?

My sources:
http://music-electronics-forum.com/t12634/
Single-ended 6L6 amps - AMPAGE Archive
5F2 circuit sounds great in 6L6 format - Telecaster Guitar Forum

[tubeswell]

Nothing you don't already know:

1N4007s are fine for SS rectification for the B+. If you want sag, just put a ~100R really chunky power resistor in series with the B+.

If you use tube rectifiers you have a bit more convenience in adjusting the B+ voltage (i.e.: for running different output tubes)

SE 6L6 will run fine on anything between about 2k5 and 6K6 with 3k2 to 5k being 'optimum'. That OT sounds quite flexible.

[Chuck H]

I was corrected about this on a previous thread about three weeks ago. Because the amp is class A there won't be any sag. Class A has the power tube drawing consistent current. But...

I countered that ANY non linearities would have an effect. Possibly even change the operating class (yes you can run a single tube amp in a different class of operation, or at least like one half of that class of operation). Also, in class A the tube is drawing max current at idle, sometimes less when actually amplifying. So there are some current changes, though smaller than you would see in AB1. One interesting thing to consider is that some of these changes would be going in the opposite direction drawing less current at lower signal input and possibly then drawing more current on a square(ish) clip. This would depend on whether or not the tube is spending more time in cutoff or saturation. So...

ANY current changes are going to be affected by resistance in the power supply and what this does to the feel and tone of a Champ may or may not be significant. For example, if the amp draws a tad less current as it begins to amplify then you get a kind of reverse sag. Dynamics would be accented, not just unfettered. Then as the amp draws more current the dynamics again are softened. And this flip flops depending on the nuances of your playing. Play soft and the amp is articulate, almost animated. Play hard and the amp softens as it clips.

How profound this is I don't know. I've never tested it and I don't have a single ended amp to play with right now. It may be insignificant and barely audible if at all. Perhaps you could "feel" it more than hear it too. I wouldn't ignore that.

Of course you don't need to use a tube to get sag. Any resistance in the power supply will do. I like using rectifier tubes for this type of amp for romantic reasons. It's a classic tube line up. One rectifier, one preamp and one power.

So maybe try it both ways and see for yourself. Build the amp as you describe and then pull the zener and put a big resistor in that drops the same voltage instead. Then you'll know for sure. If it were me I might even design the layout to allow for the addition of a rectifier tube in case I decided to put one in after all.

[Jazz P Bass]

. If it were me I might even design the layout to allow for the addition of a rectifier tube in case I decided to put one in after all.

Or build the amp with both rectifier & ss diodes.
Make it switchable.

[cminor9]

good info, thanks guys.

I don't get sag. I have heard it described, but I don't really know that I have ever heard it enough to notice it. Or maybe I do and it just doesn't stand out as this separate thing. I have built a few 5E3s, a 6G3 (5ar4 recto, which I understand sags very little), and a Matchless Spitfire (5ar4) and that's all for my experience with playing amps with tube rectos. I know that a 5E3 has a sloppier feel when its cranked, but I don't run it that way much. It also has quite a bit less filtering than the other two amps. Maybe what I attribute to less filtering is actually partly sag? I don't know.

As to whether or not single ended amps sag, it would seem to me that since sag can be defined as a temporary lowering of voltage until the rest of the circuit "catches up", why would this NOT affect single ended amps? That never made sense to me, but I read it here so I generally trust what I read here at least moreso than most places. Either way, I don't *think* I care much about sag.

Maybe I'll just build it with the tube recto though. I could make it switchable, and then I don't have to zener if I want to raise/lower the voltage. Not a fan of extra switches on the chassis, but I could bury it on the back. Had a 5E3 build with switches all over it, and funny thing: after all the fiddling and experimentation, I preferred it mostly stock.

[cminor9]

Thanks, Tubeswell for that info about using a 6L6 tube. Sometimes its good to have someone more experienced sanity check theories, so thanks for doing that.

The OT is from triode store, made in the USA by Magnetics Corporation. I like their transformers. Not too expensive, either. Though for you in NZ shipping might be ugly. I'd gladly hand deliver one if someone *cough* were to pay my airfare to NZ. I have always wanted to see NZ.

[Chuck H]

Your 5E3 definitely has an "envalope" to the attack even running fairly clean. Sag is the effect of the amp drawing more current on attack or during hard playing and consequently lowering operating voltages. Tubes sound different at lower operating voltages. In most cases this also affects the bias temporarily as well and that also changes the tone/feel. The overall effect is that pick attack and dynamics are softened. Notes and chords swell up in volume as they sustain and the circuit recovers voltage. Like built in compression but with some interesting behavior. It's subtle in most cases but since as a player amps just sound like they sound or feel like they feel you might not notice much if you don't push the amp. You would probably notice if it weren't there. If you took your exact 5E3 and put in a hefty PT (of the exact same HV), changed it to fixed bias, a diode rectifier and increased filter values you would notice then for certain. All those changes affect sag and not much else.

So, use the zener in the CT of the PT to drop your volts. Parallel it with a resistor (value chosen to drop the same volts) and use a big switch to choose one or the other. If you hear a difference then pick the one you like and remove the other.

[cminor9]

good info, Chuck. I think you hit the nail right on the head: I would probably notice it if it weren't there.

I decided to just keep it stock and build the 5F2 with a tube rectifier. I was sort of half thinking of building this, breaking it in for a mew months, and giving it to a friend who will be moving away soon, I know he will appreciate it. I'd rather give someone else a simple amp. If I really like it, I'll build another one for myself and make the mods.

Though I think I might just temporarily wire some diodes on the recto tube socket to see how it sounds without a tube there. I thought I saw that done anyhow on the new 5E3 reissue schemo from Fender. THough it seems leaving them would just negate the effect of even having a tube recto.

[tubeswell]

Though I think I might just temporarily wire some diodes on the recto tube socket to see how it sounds without a tube there. I thought I saw that done anyhow on the new 5E3 reissue schemo from Fender. THough it seems leaving them would just negate the effect of even having a tube recto.

R.G.'s protection diodes are a good idea for tube rectified amps. I put them on all my amps nowadays. They don't change the voltage drop by much (only 0.6V), so they make no difference to the effect of the tube rectifier. If the tube rectifier shorts, they act as (temporary) SS rectification. (But more importantly they protect your PT from going up in smoke where the High Tension winding could otherwise short to ground through a shorted rectifier tube - via shorted filter caps)

[ThermionicScott]

The conventional wisdom is that a single-ended amp doesn't "sag" because all of the tubes are conducting for the whole cycle, so the current draw is more-or-less constant. That's true enough for hi-fi and other applications where the user isn't pushing the amp into distortion, but in guitar amps that wisdom goes out the window. The output tube's plate draws a little more current due to nonlinearity in the curves and the screen current shoots way up. (Even in the RCA textbook examples, screen current can go up by a factor of 2 or 3.)

As an example, the B+ in my 5F2A sags about 20 volts when I've got it cranked up and I'm playing the guitar as hard as possible. The screen voltage sags by about 90(!), but much of that can be attributed to the 10K power supply resistor. The net result is that there is a very noticeable compression on the attack, and the amp sounds "overwhelmed" for lack of a better word if I keep pummelling the guitar.

- Scott

[cminor9]

As an example, the B+ in my 5F2A sags about 20 volts when I've got it cranked up and I'm playing the guitar as hard as possible. The screen voltage sags by about 90(!), but much of that can be attributed to the 10K power supply resistor. The net result is that there is a very noticeable compression on the attack, and the amp sounds "overwhelmed" for lack of a better word if I keep pummelling the guitar.

Interesting explanation. That makes sense. So all guitars amps will sag (which makes sense) but the sag manifests itself differently in different types of power sections. Thanks for that info!

[cminor9]

well, got the circuitry done now. Finishing up the cab, so I don't know what it'll really sound like yet, but it sure is quiet! No hum or hiss at all! It's always a great feeling to have a new build work perfectly the first time.

The voltages are a bit higher than I'd like (B+ is 403vdc, 390 on plate, 360 on screen), but I have some 22v zeners. One strategically placed zener will get that a bit nicer.

Finish tolex, mount the chassis, and it'll be rockin by the weekend!

Save for using Solen caps rather than electrolytics, this thing is bone stock. If I really love it, I will build another one and try it with a 6L6. Once I get to know this amp a bit, I'll try SS rectification to see what kind of change that makes for.

Thanks for the guidance on this, everyone.

[Chuck H]

Nice. Looking foreward to your observations. FWIW I rewired a Vibro Champ for a friend (about ten years ago) omitting the "Vibro" and cascaded one extra stage as well as adding a cathode follower ala Vox-ish but with the extra gain stage switchable. I didn't make any voltage adjustments though I was concearned, it had a Vp of 425 or so as I recall. The guy is still using the amp and loves it to death. Might be worth a shot just giving it a listen as is and decide if you like it.

[cminor9]

yeah, I just might do that. I was basing my voltage concern off the fact that a 5E3 build was similarly higher (about 20-30v too high) and I liked it way better with the lower voltage. But the 5f2 is a diff amp, so your point stands. I think I might leave it alone for at least a week or so.

[ThermionicScott]

Nice. Looking foreward to your observations. FWIW I rewired a Vibro Champ for a friend (about ten years ago) omitting the "Vibro" and cascaded one extra stage as well as adding a cathode follower ala Vox-ish but with the extra gain stage switchable. I didn't make any voltage adjustments though I was concearned, it had a Vp of 425 or so as I recall. The guy is still using the amp and loves it to death. Might be worth a shot just giving it a listen as is and decide if you like it.

425, yikes. Hope it had a stout 6V6 in it!

As for high-gain uses, I never liked running my 5F2A with a 12AX7 in front until I added grid-stoppers to the 6V6 (5.6k) and second gain stage (270k) -- really smoothed out the "blatty" overdrive.

- Scott