Vox AC100/2 build

[sluckey]

Yes sir, sorry was going from memory, the anode pins, someone needs to slap me silly.  Lol

OK. I just had to be sure we were communicating properly. We've spent a lot of time talking about this little detail, which BTW, is a BIG DEAL.

I looked closely at the pics you posted and it appears that you had the actual circuit wired correctly to begin with. So, the error was really just on the layout. There is a much simpler way to wire those pins. Move the 100K from the board and connect it directly between pins 1 and 6. Then put a short jumper between pin 1 and pin 7. Finally, connect pin 6 directly to point "D". (See attached V2 pic)

And this brings up a point I wanted to mention. Look at the attached danger! pic. You have 3 turret lugs laced together and connected to ground. And just below those 3 turret lugs are 2 blue resistors that connect to high voltage. The grounded lacing is dangerously close to the turrets that the blue resistors are connected to. I would remove that giant lacing wire and redo using 22AWG (about the size of the resistor leads). Keep the lacing well away from those turrets that have high voltage on them.

[pompeiisneaks]

Oh yeah, never noted how close that was, thanks!  I'll get that fixed asap.

[pompeiisneaks]

Oh and another note of something I'd realized that many times is used like that change to the pinouts, I forget that pin 6 is not used and can be used basically as a terminal strip, so that's what's happening there in the picture you have, you set them both to that pin 6 and then to D, its being used like that.  Very smart, something I've seen on many others but need to remember more often. 

~Phil

[sluckey]

Oh and another note of something I'd realized that many times is used like that change to the pinouts, I forget that pin 6 is not used and can be used basically as a terminal strip, so that's what's happening there in the picture you have, you set them both to that pin 6 and then to D, its being used like that.  Very smart, something I've seen on many others but need to remember more often. 

~Phil

I don't know where you got that bit of mis-information but please try to forget it. Pin 6 is the plate in that tube and is absolutely being used in that circuit.

[Willabe]

Maybe he's thinking of 6L6's and 6V6's?

[sluckey]

I know. And it hasn't been too long ago that I mentioned using pin 6 on his EL34s to mount those dangling screen resistors in this thread. But we're talking about a 12AX7 cathode follower now. Just trying to stay on track. 

[pompeiisneaks]

Sorry my head is still in the El34 pin 6 not connected mode, ignore my insanity :P

I've been thinking about the power tube side too much and my brain is stuck there. 

And now that my brain is on the right page, I see what you mean, connecting the resistor between 1 and 6 means that the anode in the first side gets the resistor but shares the route from there to D with 6, I've already got 1 to 7 jumpered per the schematic and layout, (there's a light yellow line between the red line and pin 7 there).

Forgive me for being addle brained lol.

~Phil

[sluckey]

my head is still in the El34 pin 6 not connected mode

Speaking of which, did you take my advice about fixing those flying 5 watt resistors?

Take a look at a V2 socket that I wired. Notice that the jumper between pin 1 and pin 7 is actually the resistor lead just bent back around. Looks kinda neat, don't you think? The Fender 5F6A Bassman was the earliest amp I've seen that done. Marshall, Vox, and everybody else has been copying that ever since 1959.

[pompeiisneaks]

Very nice! 

Is that somethign I could pin from 4 to 6 and then from 6 out to B so they're over the tube socket and the wires are less 'open' or flying?  Is it also generally better to do a single run for each tube to the B connector or is it fine to bring them all into one line like I have it now?   

Do you mean flying just because they don't have  strong anchor point for both connecting sides of the resistor?

~Phil

[sluckey]

Do you mean flying just because they don't have  strong anchor point for both connecting sides of the resistor?

Yes. One end of those resistors is totally unsupported. That's dangerous! There's very high dc voltage on those resistors. What's to keep them from flopping around and touching chassis, or something worse. This pic shows a very safe method for mounting those resistors...
 

[pompeiisneaks]

Outstanding, thanks.  I've learned so much with this build it's insane.  I can't say again how much I appreciate your help sluckey, also everyone elses.  Willabe as well I do know your guidance pulled me back into the right space so I could be more methodical about doing what sluckey asked.  You all rock :)

[pompeiisneaks]

Ok, I made all those changes, and its now working again.  I put the 1k5W resistors through to pin6 and then out to the next tube and then off to the B point.  Then I changed V2 to  use the method you showed by removing R8 and putting it right on the tube with its one lead bent over to pin 7 and the resistor between 1 and 6 off to D.  I also removed the ugly wiring around the power tube for the heaters, and went straight into it and then took off of the tail end of the last power tube down to the first preamp tube and ran them that way.  I also then put back the two 100ohm resistors to ground on the heaters at the last preamp tube.  Now I just need to get a few more tubes, (two power tubes and 2 ECC82/12AU7's for the original design).  I should be in good shape at that point I think.

One question, right now the reverb section is using half of the 12AX7 from V1, is it going to work best with the 12AU7, or should I move it to sharing the same tube as the output stage so that its still in a 12AX7?

The orginal circuit I took for it is here, http://el34world.com/Forum/index.php?topic=7957.0 and its a 12AX7 for input and recovery.  I'm thinking I should modify this so that I use the 12AX7 and leave half of the input tube unused instead (the 12AU7)

Thoughts?

Otherwise, though it is working great, still quite clean sounding other than the not so nice sounding distortion from the 12AX7's when pushed over about 2 on the volume dial. 
~Phil

[sluckey]

I put the 1k5W resistors through to pin6 and then out to the next tube and then off to the B point.

I don't understand what you're saying? I'd like to see an updated pic.

You should have a wire from point "B" to pin 6 of the first EL34. Then another wire from pin 6 of the first EL34 to pin 6 of the second EL34. Then another wire from pin 6 of the second EL34 to pin 6 of the third EL34. And finally, another wire from pin 6 of the third EL34 to pin 6 of the fourth EL34. Then the resistors simply connect between pin 4 and 6 of each EL34. Exactly like the picture I posted.

One question, right now the reverb section is using half of the 12AX7 from V1, is it going to work best with the 12AU7, or should I move it to sharing the same tube as the output stage so that its still in a 12AX7?

Theoretically, a 12AU7 should make a better reverb driver than a 12AX7. I would not change anything yet until you put a 12AU7 in the V1 socket. Then let your ears tell you what to do.

[pompeiisneaks]

Sorry Yes you've described what I did, I did exactly what your picture above shows.  the 1k5W is connected between pins 4 and 6 on each tube.  I take the B line directly to pin 6 of the first tube and run a wire between pins 6 on each of the other tubes to the end. 

Okay sounds good, I'll give that a try for the reverb section. 

One other thing I've read and may want to mess with, is that the reverb works but is pretty mild/soft.  If I want more reverb I either need to change the 1M pot to a 2.2M pot OR add another inline resistor with higher value in the output section before or after the reverb pot.  Why does increasing the resistance increase the reverb?  Does it mean that the amount of time the signal spends there is slowed more due to resisting the current flow and therefore gets more overall color to the tone?  or is it something else?

~Phil

[pompeiisneaks]

I've got the last of the schematic changes, and the visio update.  I hope this is right, I posted them back to the schematics page as well. 

I'm also about to upload the first video of the project, its going to end up as multiple videos, this one is over 10 mins long so I didn't want to go too long.  It covers the planning and schematic setup. 

I've also got pics of the updates to the build, as requested by sluckey.  https://goo.gl/photos/tkdWfVc9nQ43hn339  Look at the last few photo's the first 5 were from before.

~Phil

[pompeiisneaks]

Very frustrated at this point.  Any input suggested. 

I got the new tubes in last week end, and tried them out, and everything blew up.  The 2 100 ohm resistors to ground from the 6.3 volt rails blew up and burned up instantly.  That is at 6.3 volts AC, so it should be at under half a watt and I have 1 watt resistors in it.  I took them back out.  I power on now and I think that first power on with the new  12AU7's either blew them up, or they were bad and blew things up, because the same tube that had the resistors that connected from it to ground (V4) still has glowing heaters and a DMM Test of the pins 9 to 5 work just fine, but the other one seems to have burnt up the heaters.  I can't get continuity between those pins at all on the tube now.    The new power tubes are in with the others and they seem okay, I had to dial back the bias a bit because they were redplating at -39 I think it was, so I took it down to about -42 (I guess the new tubes must have adjusted the bias voltages because they were at -41 or -42 before I swapped in the new ones). 

The 1.5k resistor from the PI blew up and I had to replace it (Also from that first power on with the new tubes). 

Any thoughts here about why an amp that I had working but that just sounded like I needed a few new 12AU7's would fry like that?  Did I get a couple bad 12AU7's? 

I also put back in some 12AX7s and can see them pushing some signal out, but it doesn't make it far through the circuit before it seems dead.   

One other oddity, maybe this is just my measurement that was wrong, but I had my DMM set to VDC and connected to main ground and whne I measured the heaters I get the same voltage as B+, like 510 or whatever it is (I think its 510, didn't write it down).  Is that normal or is that why things are going berserk?  Where could I be leaking B+ into my heaters?  I've checked every pin on the tube sockets and there is a clear gap between the heater pins and the anodes. 

So frustrating that I had it working and now I feel like I'm back to square 1 again! :(

 

~Phil

[sluckey]

I'm betting there is a short between pin 2 and 3 on one or more of the EL34s. Use your ohm meter to check.

[pompeiisneaks]

When you're right, you're right... pins 2 and 3 have continuity on one of the new tubes... I think it may have ended up burning up the heaters on one or both of the new 12AU7's as well, would passing that many dc volts through the heaters end up burning the heaters?  What else should I check that may ahve been damaged?  I would guess I need to go back to the seller and see if I can get replacements?  This is the first time I've bought tubes that weren't from doug.  Guess that's what I get... /sigh.

~Phil

[shooter]

would passing that many dc volts through the heaters end up burning the heaters?

they should be ohmable on the tube.  And ya, look at a tube data sheet, it gives a max rating for heater to cathode volts, typically around 100v

[sluckey]

pins 2 and 3 have continuity on one of the new tubes

Are you measuring that directly across the tube pins with the tube unplugged? If so, the tube is bad. If you are measuring across the pins on the socket, then it could be a bad tube or a bad socket.

Unplug all the tubes and measure between pins 2 and 3 ON THE SOCKET. Don't use the continuity check. Check for actual ohms. If you get any reading other than infinity or open, replace the socket.

I'm suspecting a socket because this is not the first time those 100Ω resistors have popped.

[pompeiisneaks]

Ahh, I tested the tube, I'll have to check the sockets as well.   So then it is posible that the issue the whole time has been a tube socket?  could that have ruined the new power tube as well?

~Phil

[sluckey]

A short between pins 2 and 3 on the socket should not cause a short between pins 2 and 3 on the tube. But, a short between pins 2 and 3 whether actually on the tube or the socket , puts B+ voltage on the entire filament string and that may cause havoc with other tubes, maybe even the EL34s.

[pompeiisneaks]

I'll still definitely check the tube, but it was new to the environment, so it was likely what caused the explosive behavior.  I'm still not sure what heated up the resistors the first time but it wasn't this drastic for sure.   It may have been due to something arcing that shouldn't have before I rewired things the way you showed me. 

~Phil

[pompeiisneaks]

Confirmed, no issues with the sockets between pins 2 and 3.

Phil

[shooter]

puts B+ voltage on the entire filament string

did you repair the smoked parts?  IF NOT, leave them out for a test, ohm (meaning amp unplugged) from pin 4 to 5 of each socket (all tubes out), whatcha get?

Now ohm EITHER 4 or 5 to ground?
Ohm either 4 or 5 to the last B+ tap?

[pompeiisneaks]

Shooter, I'll give that a go.  I did replace the 1.5k resistor that burnt from the last preamp tube, but not the 100ohm to gnd from the heater rails.  (no point if they'll just burn up eh)  I'll do that though, I will see what pins 4 and 5 have with the ohm meter, and report back.

~Phil

[shooter]

they'll just burn up

They are besides reducing noise, a safety *part* if things are bad, so keep power OFF and tubes out til you can sort this out.

[pompeiisneaks]

so from 4-5 on each socket I get 1.38M resistance on all 4

from 4 to gnd I have a really odd behavior, it starts (depending on randomness) either at about 2M and then just starts going down as I watch it, or sometimes it seems to start at about 44k I think and then go down.  It doesn't make sense to me that while I'm watching it, the resistance can just count downwards.  Any idea on that? 

From 5 to gd I get about 1.3M on all 4 sockets

from 4 to last b+ rail I get the same behavior where it starts at a high resistance like 2M or 44k at random depending, (I can remove, and retouch it and get different results on the same pin?) and then slowly it just goes down in resistance. 

From 5 to last B+ rail I get 1.37M on all but one I think it had 1.34M.

As Sluckey showed, I'm pretty sure we found the culprit was that tube.  But then again the random fuse blowing while I'm taking test measurements with all tubes out is definitely odd as well. 

I can also use the variac and run it at lower voltages to test if you think it would be good to go there.

~Phil

[sluckey]

but not the 100ohm to gnd from the heater rails.  (no point if they'll just burn up eh)

They won't burn up when you get rid of the problem that puts B+ on the filament string.

[pompeiisneaks]

Agreed absolutely sluckey, and I think we found that.  The tube with pins 2/3 shorted.  500V through 100 Ohm is a lot of watts.  500/100 = 5 Amps... * 500 V = 2500 Watts right? They smoked and popped instantly, which makes sense heh.  I've asked the seller to replace the tube and they agreed to, but I asked them about the damage to the other tubes and they said the tube fault shouldn't cause any other damage to the other tubes, I reiterated that HT voltages in heaters WILL damage the heater in the tube but am waiting for a response.

~Phil

[shooter]

 

Did I get a couple bad 12AU7's

I shoulda stayed home, sorry for the mess-up, didn't read past that statement

[pompeiisneaks]

No worries, heh.  The one thing I've been confused about, though, is if it is normal behavior for resistance to be extremely high like that on pin 4 that slowly seems to recede down to a much lower resistance?  Is that a sign something is failing, or is it just some inline cap charging that acts as a high resistor until fully charged?  And since the DMM has very low voltage/current it takes a while?

[pompeiisneaks]

If you need a reason to never shop at tubedepot.com again, here it is.  I got a reply that they'd replace the tube that had the short on 2/3.  They insisted though that a short from HT to heaters could not damage any other tubes and refused to let me explain why and ended up hanging up on me.  When I tried calling back, they refused to answer the phone afterwards.  I'll be writing up detailed complaints on every public forum and site I can about ensuring people do not use them. 

~Phil

[shooter]

been confused about

My post about pins 4 & 5 were for the preamp tubes, 12AX, not power amp tubes!  I ASSumed

[pompeiisneaks]

So to my disgust, I just went through all my tubes and I think most got fried on the heaters. 

On one power tube the heaters have continuity, but they also have continuity to pin 8...
The tube depot power tubes, one had the 2/3 continuity issue, the other has no continuity on the heaters
one of the tube depot au7's has all 3 heaters no continuity (4,5,9 dead)
The other has no continuity on pin 4 but between 5,9 it does.

two other 12AX7's have pin 4 gone as well

so I think I've got two good 12AX7's and maybe 2 good EL34's.  That means I can't even test the amp until I get more, because I need 3 preamp tubes at a minimum to push signal.  /Sigh

[shooter]

I can't even test the amp

you CAN test, and be 90% sure it's good with NO tubes.
make the filaments produce 6.3vac with NO B+
verify NO shorts, probe EVERY socket pin to it's normal return, say V1A pin 8, it's *normal* path is to ground, and it should be about 1.5K.  Pin 6 has a *normal* path to B+ and it should be about 100K.

with the exception of filament, all that can be done with no power, no tubes, just an ohm meter a schematic and quality time

[pompeiisneaks]

ahh true, shooter, and I had done some of that last weekend.  I'll keep at it until I get new tubes. 

thanks.

~Phil

[sluckey]

Any joy yet?

[pompeiisneaks]

Not yet, tubes are due to arrive on Tuesday.   Once I get them, and test it out, its going in the final video (I have one more where I go over everything in detail, but we have to edit that one still).  and then one where I show it all working and give a demo of the sound/tone etc.

~Phil

[pompeiisneaks]

Tubes all in, all seems moderately good.  If I keep volume down around 11 o'clock, its okay, sounds pretty good (I think the speakers I'm using suck and are not well connected, so they're crackling).  I did a video and will pull some audio for everyone to hear.  I did get a bit of that odd chirping a few times, not sure what that was, but it seems to have only happened when volume exceeds maybe 1 or 2 o'clock. 

I have a working amp, but it seems to need a bit of 'dialing in'  I also have a lot of noise with my P90's if they're too close to it.   Once I moved away (after I stopped the video), the hum went almost totally away. 

I also had some pretty odd come and go volume and distortion issues, but again, that may be a badly connected speaker.  I need to go over the connections there again and make sure that's not related.  I'd be happy for input once I get the audio sample here in a bit (maybe tomorrow evening).

I am pretty stoked that now taht I have a new full set of tubes, its working fine (with all 4 EL34's to boot.)

~Phil

[pompeiisneaks]

Decided to upload it now:

https://soundcloud.com/pompeiisneaks/my-ac-1002-first-play-after-first-power-up-and-new-tubes

I've got that popping squeal noise at about :45 to :50 seconds in.  Let me know if you hear anything else really 'off'  I'm kind of wondering if the awful sounds aren't just a bad connection in my amp, but I don't think the squeal/pop noise is...

Anyone heard that before?  I've also noted that after I stopped recording, I was pretty close to the speakers with my P90's and they were causing most of the hum, when I stepped back maybe 3-5 feet the hum stopped.

~Phil

[sluckey]

I've got that popping squeal noise at about :45 to :50 seconds in.

Is that snap/pop coming through the speaker? Or from inside the amp? Sounds like a high voltage arc to me. I also hear a lot of hum. Does the amp hum if no guitar is plugged in?

Hopefully some of the guys that can still hear good will chime in.

[pompeiisneaks]

Its coming through the speaker.  When I had the dummy load up, you'd still see the oscilloscope bounce like something 'popped' but there's no audible sound that I can think of.  The audio doesn't catch part of the sound it has a bit of a squeal like sound right as it pops like a cricket chirp or something.  I think there's no hum with no guitar plugged in, but I'll check.  Like I mentioned I realized I had some single coil pickups really near the amp and the speakers, when I walked about 3 feet away the hum almost disappeared.

Edit:  Change hum with 'no' guitar plugged in hehe.

~Phil

[Willabe]

There's something, maybe a few things wrong, way too much distortion too early and not a good sounding distortion, it's also muddy sounding.

And when you turn on the reverb somethings really wrong.   

[Willabe]

That popping worries me a LOT.   

[pompeiisneaks]

I think its muddy sounding because I've got the tone controls all mucked up, I was playing with them during the audio and at a few points you hear I somewhat had it dialed in pretty well, then I went and turned it back to muddy.  I think I was paying more attention to the noise and distortion.

So if that popping is arcing, what would be arcing and why? 

~Phil

[sluckey]

Turn the lights off and watch for a spark inside your amp when you hear the pop. Sometimes you can see the culprit, sometimes not.

[pompeiisneaks]

I'll give that a go.  What usually can be done to resolve arcing?  I'm guessing it means something is a bit too close to a grounding bus right?

~Phil

[Willabe]

What usually can be done to resolve arcing?

It depends on what exactly it is that's arcing.

You have to find it first (if something is arcing).

[pompeiisneaks]

Ok I did more messing around today.  I turned out all the lights near it, it was pretty dark, and I played around and could get the popping to happen off and on but never once saw a visible arc of any kind.  I thought that maybe its distorting because it was biased a bit too cold so it was distorting early so I adjusted that a bit, looking at different bias levels between maybe  -50v and about -45V and I think I even went shortly to -38 but then it seemed to stop outputting almost any sound ( was i hitting the quiescent point where the tube no longer can conduct?)  During different points of the biasing, I was getting between 8 and 11mV off each tube depending.  I believe I got about the best sound at about 8.1mV on all tubes, but I didn't write it down as much as just tinker.  If it was 8.1 mV

Which, of course, is way low for that tube.

Then... I also accidentally was monkeying with trying to get voltages to calculate plate dissipation and accidentally shorted through one of the 1 ohm 1w resistors and blew it up.  /sigh.  I knew there was a reason they tell you to connect, power up, read, power off, repeat etc,  I'll have to get another 1 Ohm so I can get that tube working again so that one isn't taking half the load.  It still plays well.  I think part of the hum has been found in two locations. 

1. I realized that the reverb transformer is the same direction as the output transformer and touching it.  I removed one screw, and rotated it away from the OT and a decent amount of hum went away. 

2. I also noted when I was probing the heaters that when I did so, and created a good path to ground, a lot of the hum went away. (I'd removed the 100 ohms that blew earlier due to the 2/3 pin burnout of heaters debacle).

I'll restore those, and hope that helps a lot with hum.  (I'll have to redrill the second hole for the reverb transformer in a slightly different location and tighten it up in that new spot.  That way it has an alternate rotation off 90deg and is also no longer touching against it )

/sigh, tomorrow I'll run to my local shop for the resistors and get those in, and then give er a whirl again.

Side note, I don't see a big change in hum with or without the guitar cable now that I tested that either.

Edit: clarity and missed my '2' even though I wrote it out :)

~Phil