Back to basics - 5f1

[theprofessor]

I started building amps for fun a few years back with a 5f2a, yielding a thread of epic proportions for such a small amp. I had a lot of questions, and folks were kind enough to pitch in. I hope that, despite its absurd length, that thread has been helpful to others. I then went to a 5e3 tweed Deluxe, a 6g2 brown Tolex Princeton, a 5f4 tweed Super, a few Princeton Reverbs, and then a Vibrolux Reverb.

You know how sometimes you end up with a bunch of vintage power tubes singles and leftover capacitors, resistors, and hardware from previous amp builds and some scavenged odds and ends like Cinch sockets and tag strips from old power amps that a neighbor gave you? And then you decide that you pretty much _have to_ build a single-ended amp so that those things just "sit in a box"? Well that's what I thought!

Yes, a 5f1 is basic, but I am glad that others directed me to build a 5f2a first. There's just more room in a tweed Princeton chassis, and despite being a simple circuit, there are still a lot of decisions to be made and maneuvering to be done in a very small space. Make sure you have a good pair of tweezers handy! A Champ is great, so long as you make all the decisions ahead of time...

With the shuddering of ClassicTone, I decided to use Hammond transformers (PT: 290AX, employing the 275-0-275 secondary; OT: 1760CP). I never use the "Lo" input on a Fender amp, so I decided to put the cathode bypass cap on a switch, just for grins. I used @robrob 's layout for input and switch on his 5F1M Champ Micro as a guide. Beside that, there is his great split-bus layout for a more traditional 5f1. And then I used @King Fan 's thread on an optimized single-bus 5f1 as further inspiration for some "upgrades," such as an "elevated" heater center tap at the cathode (pin 8) of the 6V6GT power tube, a separate power switch instead of the switch-on-the-back-of-the-pot-jobber, the fuse moved to the bottom of the chassis, and so forth. I decided not to use a screen grid stopper on the power tube (at least yet) and to add the backup rectifier diodes as well. I got a stainless chassis from Zach Hunter and Carl in Little Rock, and... voila! - a Champ.

If you like, please feel free to give this a gander and comment on anything wrong or just dumb. Or anything right, too!

 

[jsnwhite619]

I have a question for you - how long is the circuit board? I have a lot of end cuts from my other builds that are 6" - 6-1/4" long and was thinking about doing a Champ or something like it.

 

[theprofessor]

I have a question for you - how long is the circuit board? I have a lot of end cuts from my other builds that are 6" - 6-1/4" long and was thinking about doing a Champ or something like it.

Just measured: 6" exactly. It's a Mojotone board, but I assume that's standard.

 

[jsnwhite619]

Thanks. That's what I thought, but just doing a quick layout on one of the extras looked like I was running out of room. Not sure what I was doing wrong there.

How do you like the stainless chassis? Did you have to do any drilling?

 

[theprofessor]

Thanks. That's what I thought, but just doing a quick layout on one of the extras looked like I was running out of room. Not sure what I was doing wrong there.

How do you like the stainless chassis? Did you have to do any drilling?

The chassis is just fine. Shiny, though! Yes, I had to drill the 1/2" hole in the bottom for the fuse holder, since I moved it from the top panel. And of course I had to drill to mount the fiberboard. And I had to drill for the power ground safety. And for the spot where I ground the high voltage from B+1.

 

[King Fan]

Super purty, sir!!! That may be the tidiest wiring I've seen in a Champ since I don't know when.

Stainless *is* nice except for the drilling. For those planning to drill a stainless chassis; one note: Clamp the chassis with a stinkin' heavy clamp. A big steel propeller rotating near your digits and, um, junk may ruin your day.

Re the shiny issue, Prof, I suspect you mean inside; seeing everything twice? You don't need it now, but for those planning future builds, steel wool prep can get you out of the hall of mirrors.

 

[theprofessor]

Super purty, sir!!! That may be the tidiest wiring I've seen in a Champ since I don't know when.

Stainless *is* nice except for the drilling. For hose planning to drill a stainless chassis; one note: Clamp the chassis with a stinkin' heavy clamp. A big steel propeller rotating near your digits and, um, junk may ruin your day.

Re the shiny issue, Prof, I suspect you mean inside; seeing everything twice? You don't need it now, but for those planning future builds, steel wool prep can get you out of the hall of mirrors.

Yes, inside. It is indeed a hall of mirrors!

 

[King Fan]

The payoff question: How's it sound? I'm especially interested in a comparison to your 5F2a -- like you, many of us have one of those. And hint, hint, your speaker insights / reviews are some of the best I know...

 

[theprofessor]

The payoff question: How's it sound? I'm especially interested in a comparison to your 5F2a -- like you, many of us have one of those. And hint, hint, your speaker insights / reviews are some of the best I know...

I'll get back to you in just a bit on that. Gotta do the whole startup process. And then, yes: the speaker. I want to see how it sounds first, before deciding on a direction. I already have an extra Eminence 1028k, so there's that. But also thinking on the possibility of an old AlNiCo 6x9"...

 

[theprofessor]

The payoff question: How's it sound? I'm especially interested in a comparison to your 5F2a -- like you, many of us have one of those. And hint, hint, your speaker insights / reviews are some of the best I know...

This thing works and sounds ~great~! I don't recall my 5f2a having this much cajones. The wall voltage when I was doing measurements was 124.7 ACV. With everything up and running, using a NOS GE 5Y3/6807 and the lower secondary winding of the Hammond PT, the B+1 was 348 DCV, B+2 was 306 DCV, and B+3 was 266 DCV. The heaters were at 6.86 ACV.

I used a JJ 6V6S and a JJ ECC83S for startup, and I played the amp through my Princeton Reverb cabinet, which I have loaded with a Celestion G10 Vintage. This yielded stellar sonic results, which was a bit of a surprise to me. The ability to bypass the cathode bypass capacitor is actually really nice. I find you need the bypass cap at lower volumes, or it sounds too thin. But at higher volumes, you can switch it out, and it tightens everything up nicely. The bias is on the cool-ish side, as is probably predictable with the 470R cathode resistor. My Eurotubes Pro One bias probe read out 305 plate volts, with a current draw of 41 mA. I believe that's 12.5W of dissipation. That JJ 6V6S don't be caring about 12.5W of heat.

These little 5f1's can have a reputation of working better with lower-wind single-coils, and it did certainly sound good with a Strat. But I also plugged in my Les Paul Special with two Sanford Magnetics P-90's, and it sounded fabulous too. Just switch off the bypass cap when necessary.

I'm very, very pleased. Now I'm just going to make a little head cabinet for this thing out of some walnut I've got in the basement.

Viva la Champ!

 

[James Knox]

Looking real good Prof!

 

[chas.wahl]

Very nice job!

I see you've opted for a fairly "stiff" 22 uF 1st filter cap, which, if that's a difference from your 5F2-A, may contribute to the cojones.

As I pointed out in @King Fan's Tremolux build thread, the stainless chassis has about 1/4th the electrical conductivity that a carbon steel one would.

While the heater voltage is within Merlin Blencowe's recommendation for no more than 10% variance from nominal 6.3 V, he also says it would be better to be within 5% (say 6.6 V). So in the interest of saving tubes, you might consider dropping that.

The Valve Wizard

www.valvewizard.co.uk

Despite the small size of your board, it looks to me like you could fly an eyelet strip off the board at the filter cap edge, and put in a pair of dropping resistors there between the PT's heater leads and the pilot light terminals.

 

[theprofessor]

Very nice job!

I see you've opted for a fairly "stiff" 22 uF 1st filter cap, which, if that's a difference from your 5F2-A, may contribute to the cajones.

As I pointed out in @King Fan's Tremolux build thread, the stainless chassis has about 1/4th the electrical conductivity that a carbon steel one would.

While the heater voltage is within Merlin Blencowe's recommendation for no more than 10% variance from nominal 6.3 V, he also says it would be better to be within 5% (say 6.6 V). So in the interest of saving tubes, you might consider dropping that.

The Valve Wizard

www.valvewizard.co.uk

Despite the small size of your board, it looks to me like you could fly an eyelet strip off the board at the filter cap edge, and put in a pair of dropping resistors there between the PT's heater leads and the pilot light terminals.

Thanks very much for these great suggestions! Fortunately for me, I have an AmprX Brown Box, which I use all the time. I can dial down the voltages and make the heaters happier. We'll see if that has any negative effect on the tonality. In my experience, it never does.

Yes, I decided for a 22μF capacitor there, rather than a 16μF. It may well be that is a big part of what I like about the way this amp is responding. And I did not realize that stainless was only about 25% as conductive as carbon steel chassis!

 

[Mongo Park]

I would like to know if the grid stopper has any effect. And what value you used. I guess you would have to disconnect it to see tell if it has a audible difference.
Nice build for “free” parts.

 

[theprofessor]

I would like to know if the grid stopper has any effect. And what value you used. I guess you would have to disconnect it to see tell if it has a audible difference.
Nice build for “free” parts.

Do you mean the grid stopper on the power tube screen? I decided not to use one. I'd have to put one in and audition to see the difference, which I'm not inclined to do at this point. And, well, yes... almost free. I did order a few parts!

 

[King Fan]

Thanks very much for these great suggestions! Fortunately for me, I have an AmprX Brown Box, which I use all the time. I can dial down the voltages and make the heaters happier. We'll see if that has any negative effect on the tonality. In my experience, it never does.

Yes, I decided for a 22μF capacitor there, rather than a 16μF. It may well be that is a big part of what I like about the way this amp is responding. And I did not realize that stainless was only about 25% as conductive as carbon steel chassis!

Gotta say, Prof, it's nice to have you back in the asylum here -- you do great reports on how mods sound. For just one example, your note about the switchable bypass is very helpful.

I'm all for testing reduced input voltages. Please report back. But although putting a soldering iron in that tiny crowded space would scare me, I'm also interested in the Merlin heater-specific voltage droppers @chas.wahl helpfully links. IME my 'brown box' at reasonable drops doesn't hurt the tone; in fact it often warms it but it does *change* it. It'd be fun to give your heaters a break but leave B+ in the big cajones zone.

In fact, (actually, pure speculation) is the extra-ballsy tone here partly the result of the 'Goldilocks -- just right' B+ you're getting on your low HT taps on high wall voltage? And maybe even avoiding the temptation to add the output tube control-grid stopper keeps the OD raw and vintage?

 

[theprofessor]

Gotta say, Prof, it's nice to have you back in the asylum here -- you do great reports on how mods sound. For just one example, your note about the switchable bypass is very helpful.

I'm all for testing reduced input voltages. Please report back. But although putting a soldering iron in that tiny crowded space would scare me, I'm also interested in the Merlin heater-specific voltage droppers @chas.wahl helpfully links. IME my 'brown box' at reasonable drops doesn't hurt the tone; in fact it often warms it but it does *change* it. It'd be fun to give your heaters a break but leave B+ in the big cajones zone.

In fact, (actually, pure speculation) is the extra-ballsy tone here partly the result of the 'Goldilocks -- just right' B+ you're getting on your low HT taps on high wall voltage? And maybe even avoiding the temptation to add the output tube control-grid stopper keeps the OD raw and vintage?

Great to be here! Yes, lower heater voltages would be nice indeed. But I just can't see how to cram in a tag strip or to add board eyelets without taking a bunch of things apart (that's a 5f1 for you!). If I had thought of it ahead of time... I will probably just leave it alone at this point and call it good.

In just a bit, I think I'll dial down the voltage and re-check the bias and also see how things sound. I like the OD character so much that I'm now very hesitant to add a screen grid stopper to the 6V6. And regarding the heaters again: aside from the rectifier, which is mil-spec, the tubes are new-production JJ's, anyway. If they live a shorter life, then I'm not out a bundle.

 

[King Fan]

With apologies to Edna St. Vincent Millay: "I burn my JJs at both ends, they may not last the night, but oh my foes and oh my friends, the tone I get is right..."

 

[theprofessor]

Gotta say, Prof, it's nice to have you back in the asylum here -- you do great reports on how mods sound. For just one example, your note about the switchable bypass is very helpful.

I'm all for testing reduced input voltages. Please report back. But although putting a soldering iron in that tiny crowded space would scare me, I'm also interested in the Merlin heater-specific voltage droppers @chas.wahl helpfully links. IME my 'brown box' at reasonable drops doesn't hurt the tone; in fact it often warms it but it does *change* it. It'd be fun to give your heaters a break but leave B+ in the big cajones zone.

In fact, (actually, pure speculation) is the extra-ballsy tone here partly the result of the 'Goldilocks -- just right' B+ you're getting on your low HT taps on high wall voltage? And maybe even avoiding the temptation to add the output tube control-grid stopper keeps the OD raw and vintage?

I just went back and looked carefully at the chassis, and there's just no way for me to shoehorn something in there that's dedicated to reducing heater voltage -- at least nothing I'm willing to do at this point.

I played on the same guitar for a while, using my Brown Box, and switching back and forth between 122 ACV under load and 119 ACV under load (3% reduction). I did not measure anything; I just played and listened. I liked both, but they were different. At the higher voltage (wall voltage is still ~124 ACV today), the sound is tighter, and there is more lower midrange. The tightness coincides with more midrange congestion.

At the 3% reduction (119 ACV under load), the sound is more open and rootsy. Some of the midrange is there, but the sound is not as tight, nor is it as congested. Honestly, it reminded me a bit of the comparison done a while back between a 5f1 and a Gibson GA-5. The Gibson sounded more "rootsy" to me, whereas the 5f1 sounded more mid-focused.

 

[jsnwhite619]

I would like to know if the grid stopper has any effect. And what value you used. I guess you would have to disconnect it to see tell if it has a audible difference.
Nice build for “free” parts.

I can't find my referencing it in the build threads, but I added a 1.5k grid stopper and 470R screen resistor to both of those 5881 5F2 amps I built. I think I started with the 1.5k just because I use it on everything, then added the screen resistor in the final tweaking to smooth it out.

I think you can end up making amps almost too clean with all the mods to smooth things out, but I think I like them better that way in the end. Take a 5e3 or other 2x6v6 tweed amp - by the time you add the phase inverter grid stopper, power tube grid stoppers, screen resistors, and reduce the phase inverter input capacitor you can end up with an extremely smooth & clean amp and nice, tight overdrive. But, I'll admit that it loses some of the vintage vibe in the end as well.