Amazed and Disappointed

[Tim Hall]

Well I just finished my first from-scratch build, which is also my second build ever. I'm totally amazed that it generally does what it's supposed. BUT I have a few problems with it. I really hope someone can offer some help...this was a lot of work, and I don't want to just write it off.

First problem, BIG problem: BAD hum. I'm guessing it's 60Hz hum. When you have it dimed and the tone completely cut it's minimal, but still completely unplayable. The hum gets much worse as you turn the tone up...it's almost deafening with the tone at max. I suspect this may just be because I have such a small chassis and tight layout (see pics). I would really appreciate ANY help resolving this.

Second problem: Moderately harsh sounding with the tone completely cut. Sounds like shattering glass with tone turned up. I paralleled a .047u with the .0047u between the last gain stage and PI, and that brought up the bass a fair amount, and subdued the harshness somewhat.

Third problem: I love the quality of breakup and distortion (that you can barely hear over the hum), but it comes on a little too soon. Could I gain a little more clean headroom if I drop the value of my first couple of grid leak resistors?

Schematic is attached.





Thanks for any help.

Tim

[Amp Kat]

Nice little build there Tim. Really sturdy and neat and nice components (mustards and good e-caps). Onlyt two things I see that may contribute to hum is twisting the heater wires going to the tube sockets. Try moving wires around with a stick to see if it kills some hum. Those transformers are very close together but not much you can do about that and they are turned the right way but there may be a sweet spot among the two.

[Euthymia]

Ground the heater supply

From the schematic, it looks like your 6.3V heater supply is just floating. This will cause LOTS of hum, about as you describe, if what you describe is indeed 60Hz and not 120Hz.

If there's a center tap for the heater winding, ground it, if there's not, put a 100 ohm resistor from each side to ground.

Sweet lookin' lil' build, BTW.

[Don Moose]

One common hum-reducing practice I didn't see that you followed is to refer the heater winding to GND in some way. The most common method is to add a 100R0 1W resistor from each leg of that winding to GND.

OR you could GND the center tap of that winding if there is one - DON'T do both.

I also don't see a cathode bypass cap on the output tube's Rk (at least, not in the schematic) - this may have something to do with the early breakup. Look at a couple of other low-power amps with cathode-biased outputs for some starting values/voltages.

Hope this helps!

[Tim Hall]

Thanks A Bunch!

Thanks for all the replies...you guys nailed it! I jumped one side of the heaters to the chassis and the hum dropped about 70-80%. Hopefully a proper ground connection will eliminate all of it. I had originally drawn in a heater CT to ground on my schematic because I had seen this PT in a schematic before with a CT. But when the tranny arrived from Angela...no CT.

I'll have to source some resistors to do it right...I don't have anything below 300R in the 1-5W range. Should I ground the heaters to the main star ground on my chassis?

Any ideas on settling the harshness? When I turn the tone (treble I guess) up the attack sounds like a box of razor blades exploding. With the tone completely cut it's still somewhat harsh but sounds a little like your playing down in a well. The design currently sports a "brownface" Princeton tone control - originally I had designed it with a tweed-style tone control. Would a tweed tone control help?

[MWJB]

The brown (and early black) 6G2 Princeton has the same tone control as 5F2A, 5E3 (inst channel) and most tweeds (inst channels), so I'm a bit confused by your question.

Can't read your schem, what voltages are you running on preamp & output plates?

[Rob Mercure]

Hey Tim,

As I reviewed your schemo - before reading the other responses - the gain of the third stage jumped out at me and "harsh" it be. I'm a lazy tweaker and tend to simply clip in a cap sub box to get me close to values I like (putting up with the temporary noise that a clipped in box causes) so my suggestions may not be that great but I'd reduce the second stage cathode bypass cap to about 5 uf as a start and then drop the 0.022 uf caps to 0.01 and see what the results are. You may also consider grid blocking resistors for the 12AX7 stages and then, if you need to restore gain after these changes, bypassing the third stage cathode with a 1 uf or so cap. But everything I just stated is "seat of the pants" guesstimates. I'm not familiar with the triode outputs you're using - what p-to-p drive signal level are you looking for there?

Of course if you've got a scope you can step through it pretty quickly and see where your distortion gets funky.

Oh, nice neat layout by the way - shoulda mentioned that first.

Rob

[MWJB]

Had a look at the schem now - as well as Rob's suggestions, are you sure you need a second preamp triode (5751b) at all? I'd maybe think about dropping 50-70v before the preamp node as well.

[Tim Hall]

The brown (and early black) 6G2 Princeton has the same tone control as 5F2A, 5E3 (inst channel) and most tweeds (inst channels), so I'm a bit confused by your question.

I'm referring to the 5F2 vs the 5F2A.

Can't read your schem, what voltages are you running on preamp & output plates?

Uploaded a different format. I recall measuring about 280...290...310. I had designed for 290...300...320

Oh, nice neat layout by the way - shoulda mentioned that first.

Makes me feel good that so many of you more experienced builders seem impressed (at least somewhat) with the quality of the build. I really didn't think the guts turned out so clean compared to what I had drawn up in CAD.

BUT it seems I was a little optimistic last night with the hum issue. I think I may have two sources of hum, and both are bad. One is definitely the heaters, but the other I still can't figure. Can't find any loose connections and all my grounds read good.

Hmmm...maybe I need to just plug into my other amps for a while and sleep on it. I don't know that I'm up to pulling parts right now because getting everything soldered to those sockets was like brain surgery. I'm starting to have the sinking feeling that the whole thing may just have to go into a bigger chassis.

Rob, I've taken note of your suggestions, and I'll probably tinker with them when I work up the courage Unfortunately I don't have a scope, and wouldn't know what to do with it if I did.

[Tim Hall]

...are you sure you need a second preamp triode (5751b) at all?

In case it isn't apparent here's the concept (and it's all been done before): Take a TWreck Express gain-stage topology, sub in a simplified tone/volume stack, sub in a simple concertina for PI, and use a low output triode for the PA (so the wife doesn't plot ways of eliminating your amp, your guitar or you).

Here's my overall assessment, and where I'd like to go with this: I've decided I can be happy with the amount of clean headroom I get. The one thing that works exactly as anticipated is this amp just SUSTAINS when you have it cranked, and standing in the right spot(s) is swells into feedback mayhem very easily. On this point I get butterflies in my tummy and am as giddy as little school girl . Aside from the hum, the main problem is it seems to be too bright(?) brittle(?) harsh(?), and turning up the tone knob only makes it (and the hum) worse. Bringing up the bass by using a larger coupling cap between 12Ax7(a) and PI seemed to help the harshness a fair amount. But too much of this makes it flub out at higher volume/distortion. Ultimately I want to be able to get decent cleans dimed, blusey grit midway up the volume, and smooth lead sustain at full tilt.

Of course if I can't eliminate the hum, it's pointless I think this amp has some serious potential...just needs some work.

[Amp Kat]

Even though you did cure some of the hum grounding the heater also make sure they are twisted in between tube sockets as it cancels the hum much like a humbucker pick-up. The 100 ohm resistors isolate the hum or balance the tubes but will not buck hum. Also try moving those trannies listening to the signal and moving wires around may get rid of all of it. Sometimes shielding a certain wire will do it so you may just have to tweak it some.

[Euthymia]

I'll have to source some resistors to do it right...I don't have anything below 300R in the 1-5W range.

Radio Shack part 271-135 is a pair of 10W 100R. Overkill, but under $2 and local.

For .99, Radio Shack 271-1108 is 5pcs. of 1/2W 100R, which is what Fender used.

[Euthymia]

Shield those tubes from the power transformer

Of course if I can't eliminate the hum, it's pointless I think this amp has some serious potential...just needs some work.

In the photos, I see the power transformer pretty close to the first triode tube. Magnetic radiation from the PT can tickle the tube elements.

Something to try would be to get a piece of sheet metal (steel), clip a ground wire to it, and stick it between the PT and the tubes. If the hum drops, there's a problem with the proximity. Might have to permanently attach a shield in that location.

A simple preamp tube shield might help, too, although that would interfere with the beauty of the exposed tubes.

ZVex' Nano Amp (which your amp reminds me of) puts the power supply in a separate box.

[Tim Hall]

Something to try would be to get a piece of sheet metal (steel), clip a ground wire to it, and stick it between the PT and the tubes. If the hum drops, there's a problem with the proximity. Might have to permanently attach a shield in that location.

I had originally thought I would need shields on either side of the tubes...had considered big rectangular aluminum heat sinks running lengthwise with tubes in lieu of the the black handles I have on there now. Anyway, I played with some copper sheets (which should be the best material for magnetic flux shielding), and it doesn't have a bit of effect on the hum. Practically wrapping both transformers didn't even make a difference.

I also tried dropping a tube shield down over the preamp tubes, and pressed to the chassis...no effect either. I thinks it's all internal, probably my heater wires too close to other components.

ZVex' Nano Amp (which your amp reminds me of) puts the power supply in a separate box.

Hmmm...that's an idea. But it would disrupt the symmetry

[Rob Mercure]

Tim,

To "steal" one of Bruce's tricks, disconnect the tranny's heater leads and move then outside the chassis and then run the heaters off a 6V "lantern" battery - a QAD switch to DC heaters to see if this is the hum source. As far as shielding remember that there is a difference between electrostatic fields and electromagnetic ones - ferrous/magnetic materials will shield from electromagnetic fields but copper and aluminum are practically useless unless you make a "winding" of copper that looks like a shorted tranny turn. You need iron or steel or nickel or mu metal - if you've got an old tube type scope with a CRT shield in your junk box the CRT shield is probably one of the mu metals and will provide a "lifetime" supply of magnetic shielding. Frankly the copper makes a good electrostatic shield but has little effect on magnetic fields. Of course the classic "test" if you think the tranny is to blame is to unbolt it and turn it 45 degrees, or so, with the amp powered up and see if the hum characteristics change.

You already know the trick of biasing the heaters to a positive voltage to lift them 20-30 volts above the noise floor. I've also just run the heater leads inside shielded mic cable - 34 ga. wire will handle 0.33 amps within a chassis - or where I just wanted to use the wire I already had in place I've unsoldered one end and slid the wire inside a piece of brass "hobby" tubing or copper tubing. Yeah, ferrous/magnetic would be better but the brass/copper was handier, solders well, and looks neat polished up with a coat of shellac - or heated in a propane torch and quickly quenched in water which creates some interesting "rainbow-ish" patterns. And if you find that you only have a hum issue with one tube there's no "shame" <grin> in doing "spot" DC heaters - a bridge recto made from 1n4001s, a 47-100 uf 10V electro, and an appropriate dropping resistor stuck in a good spot "upstream" of the problem has worked for me when I've done one of my "famous cram jobs" of the past (and my "innards" weren't anywhere near as neat as your amp's are!).

Rob

Rob