My AC30 build

[pompeiisneaks]

Well I've made some progress on my AC30 build. 

I'm building the Hoffman AC30 as a head unit. 

The chassis is now done, except for mounting the choke, OT and PT and cutting the hole for the OT through the top as well as the screw holes for mounting all three.  I'm linking my google photo album of the build so far, Let me know if anyone sees any major issues so far, I've not yet started soldering the wires between inputs, pots board and the like until I finish transformer install, but then I'll be going wild. 

Let me know if anything screams NO! ;)

https://goo.gl/photos/rFudpcpRy5jQJP3x6

~Phil

[TIMBO]

Lookin good Phil.
If I was to say anything, just watch your link wires. I see that you have used some stranded wire and there are a few loose strand ends that may cause problems.
I'm not saying to pull it apart just make sure that the ends are soldered to the turret, sometimes electricity likes to jump between sharp points.

If you haven't already seen this http://el34world.com/Hoffman/5F1ChampBoard.htm single strand works a treat.

[pompeiisneaks]

Timbo, thanks yeah I saw after I did mine that everyone seems to go for the single strand and I think your explanation makes sense.  I don't think I'll pull it all off, but future builds will have single strand for sure.  If I experience odd behavior though, that may be the first thing I replace.

~Phil

[pompeiisneaks]

This is just for my sanity, I knew I'd done a starter thread on this a while back and found it doing some searches.  So I can find it later, here's my original post

http://el34world.com/Forum/index.php?topic=18973.0

I'm about to do the final stages, I'll be updating this thread with the results! :) (in a few days anyway).

~Phil

[EL34]

Looking good


Holy smokes that chassis is thin!

[pompeiisneaks]

Yeah I didn't know what was the best material, I made it from scratch with the sheet metal used in hvac, so its pretty thin.  heheh learned the hard way this time.  I may end up relocating to a new chassis.

[John]

If you do end up going to a thicker chassis, let me know. I just started experimenting with bending my own. So far the biggest challenge is doing the ends without a finger brake, but I'm getting that figured out too. I'd only charge you for material and shipping, no labor. After all, "I'm just a hobbyist" (Mr. Sluckey) 

[pompeiisneaks]

I saw a technique online, don't recall where, of just using some 2x4's and my vise to bend sheet metal, it worked like a charm on this thin stuff, but I don't know about thicker grade metal.  I used tin snips to precut it, and that was easy too, but I think thicker gauges would require my angle grinder with a cutter blade.  I also have a welder, but this was too thin to weld.  (I tried a tack or two and it just melted the metal :P).  I may experiment with thicker gauge metal, and using my angle grinder, but for now this is still working.  I have the equipment to pull it off somewhat but not the massive manufacturing skills.  My stuff sometimes ends up looking a bit off :)

[shooter]

doing the ends without a finger brake

2 builds ago I *rolled my own*, used a hand-held skill saw, asked around and for $10 found an 'ol guy with a finger break to bend it.  Now I have a *new* neighbor 

[pompeiisneaks]

Next stage done, if you look at my album I posted in my first post, I've added the transformers, choke and pushed them through to the inside.  Next steps are to drill holes for my board, mount it, and start wiring everything up!

[pompeiisneaks]

I guess adding the link again may make it more obvious to some, but basically it is the same google photo's album above:

https://goo.gl/photos/rFudpcpRy5jQJP3x6

I just added 4 more pics to the end with the work I've done. 

Hopefully tonight I'll get some more done on the wiring.

[EL34]

Getting there


I hope your chassis can hold the weight of those AC30 trannys


That power transformer is a monster

[sluckey]

I'm concerned about the location of your OT. It's mounted right in the middle of the most sensitive part of the chassis. I would have put it near the output tubes, ie, about dead center of the chassis.

[pompeiisneaks]

I was a bit worried about it too, but I've got so little space on the thing (I didn't realize how HUGE the Transformers would be until I got them.  I plan on routing the wiring as carefully as possible to run at right angles from the audio, and such, but I may end up needing to move to a new chasiss just because this is sub optimal for that reason.  I just couldn't easily fit it in the middle AND be able to mount my board very well, due to the cramped spacing. 

/sigh  I'll see what happens.

[Willabe]

It's not just the lead dress, it's the magnetic field from the OT also.

[pompeiisneaks]

ahh damn, well then maybe I'll be changing it to the other side after all, I can put a metal plate over the hole I made and try again.

Separate question.  I'm really confused on the wiring of both the transformers, the wiring diagrams don't seem to match the schematics and drawings.

For the PT, the input side has two separate pairs listed as 0/120.  Do I wire them in series, or just use one set and put aside the other set?

for the OT, the diagrams/schematic shows 3 wires, two ends, and a center tap.  The actual one I have has 4 wires two separate sets of 1850 ohm loops.  Do I just wire the outside two on the associated board parts, and then wire the central two together and connect them to the Board A point?  There is a 3 point connection in the schematic/drawings, not 4, and I don't know how to create/emulate a 'center tap'

Edit:  Here are the links to the schematics that confuse me:

PT: http://el34world.com/Transformers/files/MT-VOX-AC30-PT.pdf
OT: http://el34world.com/Transformers/files/MT-VOX-AC30-OT.pdf

[pompeiisneaks]

Getting there


I hope your chassis can hold the weight of those AC30 trannys


That power transformer is a monster

Yeah it is brutal.  I hope it makes this beast scream though ;)

[sluckey]

I see a couple easy options for moving that OT...

1. Put some 1" or longer mounting standoffs on the board to provide adequate clearance between the board and the OT. If your chassis is 2.5" deep this should work. Then move the OT next to the output tubes.

2. I prefer this option. Put some long standoffs on the OT bolts so the OT sits above the chassis and doesn't even project into the bottom of the chassis. Then you don't even have to cut that huge hole for the OT. Just a couple 1/2" grommet holes for wires. This will allow the chassis to be a bit stronger too.

[pompeiisneaks]

Thanks Sluckey, I do have some standoffs for the board, was just trying to avoid overlap at all, but due to the sizing of both transformers it became hard unless I did it this way. 

Any suggestions on my PT/OT wiring questions?

~Phil

[sluckey]

Any suggestions on my PT/OT wiring questions?

The PT primaries need to be connected in parallel for 120VAC operation. Connect BLACK and WHITE together. Connect BLK/WHT and BLK/RED together.

The OT primaries need to be connected in series. Connect the two REDs together and to point A on the schematic. Connect the WHITE to one pair of plates. Connect the YELLOW to the other pair of plates.

[pompeiisneaks]

Any suggestions on my PT/OT wiring questions?

The PT primaries need to be connected in parallel for 120VAC operation. Connect BLACK and WHITE together. Connect BLK/WHT and BLK/RED together.

The OT primaries need to be connected in series. Connect the two REDs together and to point A on the schematic. Connect the WHITE to one pair of plates. Connect the YELLOW to the other pair of plates.

You rock Thanks!!!

[pompeiisneaks]

Two lessons learned last night:

1. don't use a drill gun to tighten down nylon spacers, they become 2 dimensional (had to run to home depot and get some replacements, just used aluminum instead :P)
2. Pay really close attention to your layout.  Since I cut the chassis from scratch, I got everything in, and noticed I'd inverted my positioning vs the board.  So now my tubes are on the bottom and controls on the top.  I'm going to go for it anyway, and just have to run some ugly wiring top to bottom and bottom to top, but I'm sure it will work, I just don't know if I'm introducing noise in doing so... Guess I'll find out.  If so, I may be building a new chassis from scratch :(

At any rate, its progressing, I got about half way through wiring my heaters last night before I called it a night.  (Oh and I did relocate the OT back to the side with the PT)

[sluckey]

OUCH! This is the point where I'd throw that thin chassis in the garbage and buy a blank chassis that's stout enough to tote those transformers. I fear you will regret running all those interconnecting wires criss-crossed across the board. Layout is important. Hoffman designed that logical layout for a reason.

[Willabe]

I agree with Sluckey.

It's not to late to start over with a strong chassis and running the grid wires (back and forth) is probably looking for trouble.


Edit; Added, (back and forth) Willabe

[pompeiisneaks]

Sigh,

I was worried about that.  The only issue with that is there is no "Vox AC30 chassis" here, and I've not been able to find one, or I would have bought one.  So I'll still have to make one from scratch, I guess I'll go buy some thicker gauge steel/aluminum and give that a go.  Starting back at 0 /sigh.

My least wanted/desired part of this hobby would be manufacturing a chassis from scratch.  Now I get to do it a second time. 

~Phil

[sluckey]

I'm not trying to poopoo your project. But I see difficulties ahead if you stay with a flimsy chassis that's punched backwards. There are plenty of sources for blank chassis that will be very suitable for that amp and Doug's layout. They ain't free, but they are worth the price. I've bought several blanks from this site...

     http://tubeamplifierparts.com/guitar-amplifier-chassis-enclosure-project-box.html#!/Project-boxes/c/11283715/offset=0&sort=normal

[pompeiisneaks]

No I have read enough on the forums to know you're worth listening to :)  I'm just bummed that I got this far and made such a noob mistake.  On top of it, ordering a chassis would still require I get a blank one.  I'd still have to cut all the holes etc.  Shaping it into the chassis I have took like 30 minutes at best, the bigger issue is cutting the holes for tubes, PT/OT, front panel etc.

Also I have a bad case of "I want to get it done this weekend" and waiting for a new chassis will take days (mister impatient here)
I'll give it a go again with better metal and see what I get.

Thanks for all your help/input.

~Phil

[EL34]

I see where you are at and I hate to start over, but It's worth the effort to have a rigid chassis

I sell a nice blank aluminum chassis that would work for your project
http://hoffmanamps.com/MyStore/catalog/ChassisMobile.htm

[sluckey]

Oops! Forgot you still had that blank. I think that's a perfect size.

[pompeiisneaks]

Well due to my inability to wait :D I made a new chassis tonight from much stronger gauge steel.  From the same link above, the last two pics added show the new chassis, I've built it, and added the tube sockets.  I've got the transformers inside upside down and on top on the second pic to show how it will look.  Thanks for the input and help all.  I'll be working on it some more tomorrow.  Hopefully it will go much better this time. 

[pompeiisneaks]

Okay, so for today here's my new chassis with a lot more done (Since I'd cut my heater wires to match the old chassis, I had to splice them with some wire to make them reach the bulb, but that's in place now too.  I'll have some shots of it fully wired hopefully later tonight, but look at the album the last three photos from today:

https://goo.gl/photos/rFudpcpRy5jQJP3x6

[pompeiisneaks]

Okay it is finished, but there are some oddities. 

There's a minor hum, I can probably find some specific area that needs better shielding, but its very low level.

I think I just figured out that I wired the pots in reverse order, because I had the tops facing me and looking at the chart they seem reversed.  I see a few oddities though, so maybe this is just wiring those right:

1. volume pots don't seem to do volume, just adjust something weird like the high end or something
2. cut seems to be a master volume for the bright channel but not the other, seems to ahve no impact on the normal channel is that expected? 
3. the resistor (10k/3w) that goes between D and the spot to the right seems like it is getting REALLY hot, what would cause that (See: http://el34world.com/Hoffman/files/Hoffman_AC30.pdf)

It does work and sound great though, but I don't want to keep it running with that resistor seeming like it coudl burn up. 

~Phil

[Willabe]

Here's a great way double check a build, Doug's handy work  ;

http://el34world.com/Forum/index.php?topic=17701.0

And you need a lamp limiter for 1st start ups to check for shorts or until you find/fix any shorts;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

1. volume pots don't seem to do volume, just adjust something weird like the high end or something

2. cut seems to be a master volume for the bright channel but not the other, seems to have no impact on the normal channel is that expected? 

3. the resistor (10k/3w) that goes between D and the spot to the right seems like it is getting REALLY hot, what would cause that (See: http://el34world.com/Hoffman/files/Hoffman_AC30.pdf)

1. Check the ground connection and then the rest of the pots wiring. No ground, no change in volume as you turn it.

2. No, it's after the PI so it should affect both channels. Somethings wired wrong. 

3. Somethings wired wrong. Sounds like a partial short, drawing too much current.

2 and 3 might be related.
 

[EL34]

As Willabe mentioned above, do this first
http://el34world.com/Forum/index.php?topic=17701.0

Let me know when you have finished it

[pompeiisneaks]

Thanks,

I actually didn't wire them wrong, I'm a total noob, I'm used to looking at it from the underside, so I was reversing all the pots.  They're fine now.  Remember how I mentioned the cut seemed like a volume.  That's because I was using the volume :P

I did check on that resistor, and it doesn't seem grouned out, and I think I may have had something else with my input jacks that I sorted that may have caused the ground.  I don't know if it is dead, but in series when I test it, it seems to be 0 resistance,but the amp is playing great now.  (after I moved some wires a bit the hum is almost non existent. 

I'll tinker with it a bit more but man it sounds amazing.

[pompeiisneaks]

So I'm mostly done with this, I need to take some more pics and let y'all see the finished chassis.  I do want to take a minute and thank everyone for the help.  Doug, you have an amazing website, forum and product. Thanks so much for the detailed notes everywhere on how to make this work. 

Thanks to the forum members for being so helpful and patient with a neophyte like myself. 

This was a blast and I look forward to many more amp tweaks, builds and modding. 

Cheers!

[EL34]

Glad you got it working

[Willabe]

 

[pompeiisneaks]

Yeah ultimately my issue came down to the fact that I messed up the shielded cable on the inputs, I didn't isolate the shielding well at the tube input and so it was grounding out.  I forgot I'd read somewhere here about cleaning it back a bit and putting a heatshrink on it.  I did that and the issues just disappeared.  I think that was why that resistor was getting so hot, but everything is working great now. 

Like I mentioned, in a day or two I'll take some high res photo's of the completed project and get people's input on what I could have done better.   But overall its a really silent amazing sounding amp.

[Willabe]

Yeah ultimately my issue came down to the fact that I messed up the shielded cable on the inputs, I didn't isolate the shielding well at the tube input and so it was grounding out.

Ahh, dog gone shield.         But you found it. 

Like I mentioned, in a day or two I'll take some high res photo's of the completed project and get people's input on what I could have done better.

Great, the guys love to see amp pics. 

But overall its a really silent amazing sounding amp.

 

[Ritchie200]

I've been following this with interest as this will be my next endeavor!  Great job!  All these little "noob mistakes" you keep talking about have bit all of us at one time or will again!  That's the learning process - heck your an old hat now!


Along with the pics, I want to hear some soundclips!


Jim

[pompeiisneaks]

I've not yet had time to record audio samples but here are some high rez photos I took with my DSLR:

https://goo.gl/photos/b9EhN94RQvUUfvgC6

I'll get some audio samples hopefully this weekend.

My extremely lacking skills at metal fabrication are visible by the massively misaligned pots and inputs etc :)  I hope you get a giggle out of that.  I never wanted to be doing that stuff heheh. 

The wiring is pretty sloppy I'm sure as well, but I am pretty amazed at how quiet it is,  I only get some noise at full volume really.  Minor hum at mid way, that goes away when I turn down the treble and bass or remove the 2 and 3 preamp tubes.  I'll have to look over the tone stack for potential noise issues, but that is only at really high volume.  up to mid or more is really quiet.

[pompeiisneaks]

So I was going to record some audio last night, but the hum in the amp seems to have gotten significantly worse.  I've taken the steps to trace out the entire circuit with a highlighter as Doug suggested before, but I'm still getting a pretty nasty hum.  One other thing that seems weird as well, is the plate voltage on the EL84's is running at 316 on two and 320 on the other two, and it gets really hot.  The PT is only supposed to supply 280v per the schematic, so why are mine so hot?  And the resistor is a 50 ohm 10watt one, do I need to adjust that then to reduce the plate voltage?  Would excess voltage do that?  It seems like no, it would just distort sooner and be a bit louder than expected right? 

Edit:  I also read the EL84 data sheet and it is supposed to run at most 300V, so I'm over voltage as well... What other measurements should I take and where?

What other measurements should I get? 

I took this for the Power tubes(EL84):

Pin 1: .022 V,
Pin 3: 12V,
Pin 7: 316/320,
Pin 9: 314 on all.

Preamp Tubes(12AX7):

Pin 1:  V1 138, V2 162, V3 222
Pin 3:  V1 1v , V2 1.3V, V3 5.9V
Pin 6:  V1 127V, V2 229V, V3 229V
Pin 7:  V1 0V, V2 161V, V3 42V
Pin 8:  V1 1V, V2 162V, V3 59V

This is with no input, and the volume pot up mostly to 10 to maximize the hum so I can see it on the scope. 

So really two questions:

1. How do I reduce plate voltages
2. Where to I fix the hum?

For the hum side, I did look around for where the hum comes from with the scope, and it seems to come from only one side on the pin 1 inputs coming from the caps to the cut pot.  If you look at the AC30 schematic: http://el34world.com/Hoffman/files/Hoffman_AC30.pdf I think it  was V4 and V5 outputs that seemed to be where hte noise comes from.  Is it just not grounding well there?  Or is it grounding too early somehow?

if I turn the cut to the extreme end it almost completely goes away. 

~Phil

[pompeiisneaks]

OK I've still got the extremely hot output tubes, but the hum is gone.  I read up on ground hum and found aiken talking about taking the A cap and the mains ground to a separate, closer to main ground to separate them.  It reduced it a bit, but then I went in with my trusty wood stick and pushed the new ground around a bit and as soon as it hit the bottom it went super clean, almost gone.  So I just soldered it heavily there in that spot and boom, no hum. 

I'm still concerned I'm going to burn through tubes fast at this rate, with the plate voltage so high.  Any tips?  Or do I just need to try a higher pot in the 50 Ohm 10W to ground part of the power tubes?

~Phil 

[sluckey]

I'm still concerned I'm going to burn through tubes fast at this rate, with the plate voltage so high.  Any tips?  Or do I just need to try a higher pot in the 50 Ohm 10W to ground part of the power tubes?

Nothing wrong with your plate voltage. Tube current is a big problem.

You have 12V across 50Ω. That means 240mA is flowing thru those four tubes! And with plate voltage at 300V, that means the amp is idling at 72 watts. The tubes won't last long.

You really can't just put a pot in line with that 50Ω/10W resistor unless you have an expensive high wattage pot. I would get a 100Ω, 150Ω, 200Ω, and 250Ω, all rated 10 watts. Replace that 50Ω with each one of those resistors and measure cathode and plate voltages. Then recalculate dissipation until you are about 36 watts. I'd start with the 100Ω first.

[pompeiisneaks]

Thanks I'll give that a try.

~Phil

[EL34]

Try the 130 ohm 10 watt I stock
http://hoffmanamps.com/MyStore/catalog/Resistors.htm

[pompeiisneaks]

Oh and sluckey, thanks for noting that I typed the wrong thing hehe, I meant resistor, not potentiometer doh! 

[pompeiisneaks]

Okay so I've tried two of them, the first was a 125 ohm 10W, and the new voltage read about 14 volts and that gives 112 mA which means at 300 its around 33.6Watts (but I wasn't sure that I was supposed to use the 300 or what I measured on the plate which was about 350V, which would be about 39.2 so that means it is a bit hot still). 

When I tried the second a 175 watt, I got 15.3 volts 360 plate current which is 86mA and 30.96 Watts (unless it should be 300? which comes to 25.8)

Not sure what is the right answer.  Or am I doing the math wrong? 

Let me know what seems like the right one to use 125 or 175 for max power output without overheating the tubes?  (I get more crunch that way right?  Whereas if I set the current lower, I get more headroom, and less crunch, but a cleaner tone? )

[sluckey]

You're doing the math right. Everytime you change that resistor you are changing the tube bias and that causes the current to change, cathode voltage changes and plate voltage changes. You always have to remeasure cathode voltage and plate voltage then do the math based on the new numbers.

Did you notice that as you increased the cathode resistor value that the plate voltage increases? That's because the tube is biased cooler, less current flows and causes the plate voltage to rise. Make a sticky note! 

The tube manual says the maximum plate dissipation for an EL84 is 12 watts. Four tubes would be 48 watts. That 125Ω or 175Ω puts you safely below the max power rating. I'd also want to see what a 150Ω would do.

How does the amp sound with those higher value resistors? Need a low level check and a loud check. Let your ears pick the right resistor as long as you keep it under 48 watts! The cooler you run the tubes the longer they will last. I think the old AC-30 was always run hot.