Supro 6424 build

[Meteorman]

Inspired by Steve Luckey's amps webpage, I have embarked on a S-6424.

Started by making my own chassis out of 16 gauge steel - I won't do that again, unless I purchase a set of chassis hole punches.
Drilling was tough sledding.
Tacked the corners with my MIG buzzbox.

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Used Steve's layout for turret board and circuitry and a template for the control panels components.
Decided to forgo the channel switch and go with one channel.

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

So this is where I am now. 
Heaters and rectifier wired, powered up.
333v DC on rectifier pins 2 & 8.
331v AC on rectifier pins 4 & 6.
Heater pins all at 2.8v.
Seems like good to move ahead...
Waiting on the 10W ClassicTone 40-18063 OT (mistakenly ordered the 35W OT initially)..

Untitled by Mike Millard, on Flickr

[sluckey]

Hey, that's pretty! Are those water slide decals? Looking forward to your next post.

BTW, there's another forum member building one of these right now.

[Meteorman]

thanks Steve - yeah those are waterslides - I buried them under several coats of clear since that pic was taken.
not totally undetectable, but less noticeable than the pics.

Got the board and leads soldered up.

Untitled by Mike Millard, on Flickr


... was also going to ask PlumMojo the reasoning for the 1M vol pot.
I'm not real keen on messing with the Vol2 pot after all the customization I had to do on the back of it.

Untitled by Mike Millard, on Flickr

[Apexelectric]

Great sounding amp, at least for the time I had mine up and running. Been having an issue with signal dropping out on channel 1 at full volume. Hoping to get it resolved soon. Sluckey is on it. Had it paired with a Jensen neodymium 12" speaker in an open back cab.

[sluckey]

... was also going to ask PlumMojo the reasoning for the 1M vol pot.

I would not worry too much about plummojo. He's just trolling and has been banned under two other names. I suspect he will not last much longer under this name.

I'm considering removing his irrelevant post and my response to him just to keep your thread clean. If you prefer for me to leave it alone just say so.

[Meteorman]

Thanks Steve.
Treat PlumMojo as see fit - I'll be taking my amp building advice from you.
and thanks for the PM !  Cleaned that up.
/mike

[sluckey]

Are your water slide decals for inkjet or laser printer?

[davidwpack]

I used laser with good results. It seems like that's what a lot of people recommended when I was researching it a while back. I haven't used inkjet though so can't compare.

[CraigB]

Thanks Steve.
Treat PlumMojo as see fit - I'll be taking my amp building advice from you.
and thanks for the PM !  Cleaned that up.
/mike

Beautiful build, Sir!

Are your water slide decals for inkjet or laser printer?

Steve, I like the laser ones better if your graphics are black.  If you want color graphics (or just black), the inkjet ones work fine also, but need to be clearcoated before soaking/sliding.  The result is a thicker decal.  With the laser film, you can apply them right after printing, soak for about 30 seconds, apply, then once they're on with no airbubbles underneath or wrinkles, let dry for a bit, then shoot several coats of clear to bury the decal, later wetsanding the surface until the cut edges of the decals are no longer visible.  The inkjet sheets require extra coats of clear to get sufficiently buried before wetsanding.  FWIW, the first couple coats of clear should be misted on so the solvent in the finish doesn't eat into the toner or smear the ink.

[Meteorman]

Yes, mine are laser printer waterslides.
One trick, borrowed from guitar making where we decal the logos on the headstock, is to create your text, or texts, then mirror image the whole page of text.
This can be accomplished in most software packages, usually via a “flip horizontal” or similar command.  Turn your text into an image first in some cases. 
Easier to do than explain.
Then print on laser printer, and apply the waterslide PRINT SIDE DOWN.
This eliminates the issue of the top coating dissolving/blurring your ink, and also lessens the chance of sanding thru the print when you’re smoothing your topcoats.
/mike

[Willabe]

Then print on laser printer, and apply the waterslide PRINT SIDE DOWN.
This eliminates the issue of the top coating dissolving/blurring your ink, and also lessens the chance of sanding thru the print when you’re smoothing your topcoats.

Nice trick. 

[CraigB]

Yes, mine are laser printer waterslides.
One trick, borrowed from guitar making where we decal the logos on the headstock, is to create your text, or texts, then mirror image the whole page of text.
This can be accomplished in most software packages, usually via a “flip horizontal” or similar command.  Turn your text into an image first in some cases. 
Easier to do than explain.
Then print on laser printer, and apply the waterslide PRINT SIDE DOWN.
This eliminates the issue of the top coating dissolving/blurring your ink, and also lessens the chance of sanding thru the print when you’re smoothing your topcoats.
/mike

And the other nice thing about doing it this way is, with a really steady hand you can backfill, as a for instance, block lettering with paint to really make the graphics stand out.
Thanks Mete, do it that way on guitar headstocks, but never thought of doing that on an amp chassis - brilliant! 

[drew]

Heater pins all at 2.8v.

Why so low?

[Meteorman]

Heater pins all at 2.8v.

Why so low?

I can't answer that Drew - that's each wire of the pair of course, but still adds a little low. 
Will check with different meter when all guts are in.

Which they are now, but haven't fired it up yet.
I'll be using a light-bulb current limiter.

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr

[sluckey]

Measure the heater voltage ***BETWEEN*** the two green wires on that terminal strip near the PT. No current limiter. Pull the rectifier tube if you're scared.

What have you?

[Meteorman]

thank you sir.
no tubes.
6.3 VAC between heater wires.
just what the doctor ordered, I believe......

[Meteorman]

Went thru the start-up procedures as prescribed on Rob Robinette's page.
All seemed well except one issue.
One of my preamp tunes is a 12AX7A, the other two are 12AX7. 
Read several places this is of no consequence.
Upon firing them up, I see yellow filament glow from the 12AX7s, but not from the 12AX7A.
Switched positions, and same result, so it's not a socket issue.
Wondering if I got a bad tube, or if the 12AX7A infrastructure is such that, under power, it would present differently than a 12AX7.

s-l1600-001 by Mike Millard, on Flickr

[sluckey]

I have some 12AX7s that are difficult to see the filament glowing. Turn the lights off. See it now?

There are several ways to check the tube. You can pull the tube and check resistance between pins 4, 5, and 9. Should have resistance between all three pins. Or, you can just plug the tube in and play the amp. Got sound? Tremolo work? Or, you can plug the tube in and measure voltages. If the filament is not lit, plate voltages will be high.

I don't think I've ever seen a 12AX7 with an open filament.

[PRR]

It's probably not the "A".

Anyway "all" 12AX7 have been "A" for over 60 years. Since they (formerly) were not used in long series strings much, they were not always marked; but "all" heaters moved to standard sizes to get standard warm-up, and there's no point making different heaters for a few types.

Some 12A_7 have heater extending past cathode and mica, you can see glow.

Some the heater doesn't peek out, you don't see glow except in a very dark room.

If the tube "works" (passes audio, has cathode voltage), obviously the cathode is hot "somehow". Maybe a miracle, but probably hidden glow.

A truly cold heater is a problem but not an emergency in your smoke-test. Go on to plate-grid-cathode DC voltage checks.

[Meteorman]

Thank you, gents.
I will conduct further probing as per your suggestions this afternoon.
Will report back the (hopefully) good news and press on to seeing how it sounds.
cheers! 
/mike

[MFowler]

Very nice looking build.

[bnwitt]

Your amp is coming along nicely and your build chops are great.  Looks really good

[Meteorman]

Some more start-up activity and data, plus a confession that will certainly diminish my alleged build chops !

Wednesday afternoon: 
Progressed to installing all tubes for a final look-see & listen.
Brought my 5E3 down to shop, unplugged the speaker from the 5E3 circuit, and alligator clipped the speaker to a 1/4" speaker jack to provide impedance for the initial full-tube Supro fire-up.
Light bulb limiter in line.
Speaker alligator clips in place on speaker.
Flipped amp power switch to on.  Flipped light-bulb limiter switch to on.  Nothing.....
After about 8 seconds, no more than 10 seconds, I noticed I neglected to plug the dang jack-end of patch cord into back of the amp.
Turned off all power immediately, issued a few curses, and hung my head.
Bottom line: I had the amp fully tubed & powered up for 8-10 seconds with no speaker hooked up.  Sigh. A current limiter was in line if that matters.
Bummed, but pressing on, I plugged the speaker patch cord into the amp, and powered up again.
Fairly loud, seemingly 60-cycle hum, from speaker.  Completely independent of any volume knobs position.
Turned everything off, depressed, and left shop to live and fight another day.
First thought: did I fry a cap or transformer with my 8-10 second oversight? 

Thursday
Decided to start again from no tubes, progress thru preamp tubes,  and take careful voltage readings.
Here is what I saw:

NO TUBES:
PT HT leads to rectifier pins 4 & 6:   655 V AC
PT heater leads to rectifier pins 2 & 8:  4.7V DC
filaments: 6.7V AC

RECTIFIER ONLY:
Rectifier Pin 8: 420 V AC

B+1:  421 V
B+2: 421 V
B+3: 420 V

v1-1: 405V
v1-6: 405V

v2-1: 409V
v2-6: 408V

v3-1: 420V
v3-6: 408V

v4-9: 422V

v5-9: 422V

Rectifier and 3 Pre-amps tubes only - NO POWER TUBES:

v1-1: 226V
v1-3: 2,1V
v1-6:123V
v1-8:  0.9V
v1-2 & V1-7: 0V

v2-1:91V
v2-3:0V
v2-6:180V
v2-8: 1.4V
v2-2 & V2-7: 0V

v3-1: 282V
v3-3: 2.1V
v3-6: 109V (ground jumpered to remove oscillation)
v3-8: 0.6V (ground jumpered to remove oscillation)
v3-2 & V3-7: 0V (ground jumpered to remove oscillation).

B+1: 337V
B+2: 334V
B+3: 284V

v4-1, v4-3, v4-6, and v4-7: 0V
v4-8:  333V

v5-1, v5-3, v5-6, and v5-7: 0V
v5-8:  333V

So this is where I'm stopped.
I did find an Uncle Doug video talking about 60-cycle hum that may be relevant: 

So, my questions for you pros are, looking at the above voltages:
(1) any evidence that I damaged my circuitry with the full on mode for 10 seconds with no speaker hooked up ?
(2) any sign of a bad 12AX7, possibly causing that loud hum I heard, a la Uncle Doug ?

I'm not going to expose my good 5E3 Jensen to any more experimentation with this amp.
Have ordered a speaker to test with, and will post a video/audio when it comes in.
Have also ordered another 12AX7 to roll thru.

Thanks for any help - couldn't do this without you folks and this forum.
/mike

[octal]

IMHO, very unlikely that briefly running it w/o speaker load would cause any damage. The scenarios where this is a problem is when someone runs an amp dimed with a signal going in to it, and also on amps with global NFB which can be unstable and oscillate.


I would connect a speaker, install your output tubes, pull your preamp tubes and check the voltage on the cathode resistor for the output tubes. If "reasonable" (tubes are not over dissipating) you can proceed - are you getting hum? If so, power supply issue or wiring error in the power amp. If not, plug in your phase inverter tube and see if the hum starts. You can work your way back to the preamp, plugging one tube in at a time.

[sluckey]

First off, get rid of the light bulb limiter. It has served it's purpose already. Plug the amp straight into the wall.

Fairly loud, seemingly 60-cycle hum, from speaker.

About that hum. Was it no louder than U. Doug's bad 12AX7 example? Or was it so loud you were afraid it would blow the cone out of your speaker? Could you talk at a normal level and be heard?

8 to 10 seconds with no speaker connected... I seriously doubt you hurt anything even if it was a roaring 60Hz sound. Your tubes may not have even been warmed up by then. And you do have a jumper between the ground lug and the switch lug on the speaker jack, right?

Now follow octal's advice. Let us know if you have loud hum with just the power tubes or if the hum appears when you plug in the PI, etc.

When you have eliminated the hum, get a new set of voltage readings. The readings you have while using the light bulb limiter are useless.

[Meteorman]

Thanks for the guidance !
Will do.
The hum was in no danger of blowing the speaker.
Seemed a little louder than uncle doug’s, but his speaker was also layin’ cone down and recorded at some unknown distance. So I’m guessing similar.
I’ll walk thru the guidance and post a vid if hum is still there.
Gotta get my speaker first.
Am I correct in interpreting that v2 is my PI in this circuit?

[sluckey]

Gotta get my speaker first.
Am I correct in interpreting that v2 is my PI in this circuit?

You ain't gonna hurt that Jensen speaker. Use it.

V2 is the PI.

[Meteorman]

You ain't gonna hurt that Jensen speaker. Use it.
V2 is the PI.

OK, you, Jensen ! Get in the game !  Coach says you aren't hurt !  haha - thanks Steve - ur right of course.

Made some progress.
Put in the power tubes and rotated the 12AX7s thru the v2 socket and did indeed have a bad one.
Tubes a,b, and d were dead quiet in v2 socket. 
Tube c made the hum and static crackle.  Twice.  eBay seller guy will hear about that.

Stuck in the 3 good preamp tubes and plugged in the guitar.
It's alive !
Fairly annoying 120cycle hum when volume is turned up.
It goes up and down with the both volumes, but most noticeably with Vol 2.
Hum is kinda' normal in my shop, tho, which is covered with banks of plug-in fluorescent lights, numerous powerstrips, and old wiring in general.
I'll bring it upstairs, which is normally more quiet for amps and see how it sounds and chopstick it.
Trem seemed to work.  (tremmed the hum too).

Many thanks for the guidance !!! 
Hopefully can work off the hum issue and begin on the combo cabinet.
/mike

Voltages with full tube array; anything look amiss here ?

Untitled by Mike Millard, on Flickr

Untitled by Mike Millard, on Flickr



 

[sluckey]

Well done! It's been fun following your project.

[Meteorman]

Working on the cab now.
Modding off plans for a standard 5E3 tweed cabinet.
Reclaimed American chestnut, with a canary wood/maple header board on the face.
Roughed together now - much milling left to do.
The chestnut will get a nice amber glow after dyeing and finishing.
Received some vintage speaker cloth - the baffle will mount from the front.
Aluminum perf plate on the back. 
Does perf plate (instead of solid wood) on the back dramatically affect the resonance of the cabinet, as a whole ?  Seems like it might.....


IMG_2431 by , on Flickr

IMG_2432 by , on Flickr

[Meteorman]

Finished up the project.
Many thanks Steve Luckey !

Untitled by , on Flickr

Untitled by , on Flickr

Untitled by , on Flickr

Untitled by , on Flickr

[davidwpack]

Very nice!

[bnwitt]

Quality work Meteorman.  It looks really nice.  Did you figure out the cause of the 120Hz hum?
Barry

[sluckey]

That really turned out to be a classy amp, inside and out. Thanks for sharing your project.

[kagliostro]

Nice Amp 


Franco

[tubenit]

Bravo, well done!  I think it's cool that you found some chestnut to work with.  The amp looks fantastic.

Thanks for sharing!  With respect, Tubenit

[Meteorman]

Thanks friends.
Couldn’t have pulled it off without the helpful folks here.
The 120hz hum mostly disappeared once I got it up outta my shop and on a cleaner house circuit. Can’t say it’s “dead quiet”, but it’s well within acceptable range.
I have more chestnut - I’m gonna re-cab my Champ build into something similar to this.