Vox AC100/2 build

[pompeiisneaks]

Here's the latest schematic, the diode and cap area is really ugly, but to the best I could do this late :P

I've also done some testing and measurement. 

1. removed the reverb pan completely, no end of the hum
2. removed the second reverb tube, no end of hum

I've not yet gotten to more, as I ran out of time, I did get measurements though. 

Power Tubes
pins  1/8, 4, 5
tubes
5      184mv, 480V,-45V
6      0mv, 480V, -46V
7      0mv, 480V, -45V
8      0mV, 481V, -45V

Preamp Tubes
pins 1,3,6,8
1      143V,2.0V,477V,4.0V
2      199V,1.6V,318V,198V
3      200V,1.7V,Not,Used
4      411V,3.9V,409V,3.95V

Now I didn't get consistent readings on Power tube pins 1/8 because its really weird, when I try to test it, it seems to have some level of mV for just a second, like say 184 mV and then drops to 0 really quickly, same with any of them, not sure why?  1/8 go to a 1 ohm to ground, so I should be able to measure voltage and get plate dissipation there, but something seems odd

Similarly for the preamp tubes Pins 2/7 I can Measure like .36V or so for a short time and then it dissipates to 0.  Not sure if that is expected or a sign of something wrong.

For the attached schematic you can see that the second half of V1 goes to the reverb and first half of V3 returns from it. 

As for the power rail, here's the measurements:

A: 504V, B: 484V, C: 481V C': 476V, D: 320V, E: 293V

The C' is a spot in the schematic where I'm tying in the cap as required but there's another resistor after it going to C, so I labelled it 'C Prime' or C', not sure if that's normal/copacetic, but I just wanted to remind myself the filter cap went in there, not at C directly. 

I haven't had a chance to take photo's either, I'll try to find time to get that tomorrow, but with this debugging, I've still not found a way to get the hum to stop, I even blew a fuse today as well while I was doing some testing, not sure what I may have touched, but Its been fine again with a new fuse. 

~Phil

[pompeiisneaks]

Oh one other thing I've noticed that was odd that happened again last night.  The half of the power tubes end up not getting their heaters lit up sometimes.  If I turn it back of, jiggle them about a bit, they often come back on, but sometimes that doesn't even do it.  It makes me wonder if it is one of two issues:

1. the pins are a bit too loose on those two, so they're not making good contact?
2. the heater power lines can't quite generate enough amps for the entire circuit?  This one is pretty hot, its taking:
  4x 1.5A for the EL34's
  4x 300mA for the 12AX7's
 = 7.2 Amps.  Is it possible this isn't handling that well?  I would think it should because the same setup was used in the other amp, the only difference is that it used a full wave bridged rectifier to provide DC Voltage to the preamp's only.  That shouldn't increase the amps the heater rail can produce right?  Amp handling is based upon the PT itself and that's pretty much it no?

Also, as noted, instead of using 2 ECC82/12AU7's I'm using all ECC83/12AX7's in the amp to get a feel for what it will sound like.  Per discussions I've heard on a lot of other threads this shouldn't be a major issue, it will modify the tone, and increase the potential preamp gain to get more crunch. 

~Phil

[sluckey]

The heater issue is likely a loose or bad connection. Could be sockets but that would mean two sockets have the same issue. Could also be a bad wire or solder connection.

Still waiting for your layout drawing. I did find an old layout scattered across several drawings but it is confusing how they actually connect together. And there were enough errors on the drawings to make me think they are preliminary. It took about 30 minutes of sifting through all the various topics dealing with this amp. It would be nice if you would pull all that stuff together and post it in this thread. Hi rez pics would be very helpful at this time too.

Take a look at the link in my signature line at the bottom of any of my replies...

[pompeiisneaks]

Oh, sorry, I'll get that in here too, it was late last night when I got it together. 

I've gone over it like that link shows, but it was after I finished before power on, and it doesn't mean I didn't make a mistake.  I'm sorry that the layout is ugly, it was my first try, and its def changed a bit.  I'll also get some pictures.  Its an ugly mess, because I'm still learning this, and my lead dress leaves a lot to be desired, and that may be part of my issues, not sure.  One thing you'll see in the pics is that my heater line coming from the power tube side to the preamp side has to cross the lines to/from the preamp tubes to the board, so I'm thinking I may want to try a different path for that as well, still trying to figure that out.

~Phil

[pompeiisneaks]

Okay here are some images, https://goo.gl/photos/tkdWfVc9nQ43hn339.  I'm also attaching screenshots of the latest visio, also not perfect, but the best I can do with visio and limited time.  I've also been meaning to ask, as I've heard mention of it, how do you clean the flux remaining on the board?  As you can see on mine, its still filthy with it. 

I plan on also filling the top areas of the filter caps with GE Silicone II (The non acid type curing type). to ensure nothing can touch the connectors, but only after I complete the build.  I've not figured out yet the best way to handle the rca jacks for the reverb either, they're just hanging up there, I may get a terminal strip for those, but am open to suggestions. 

Really please be kind, as I'm sure a lot of this looks horrible, but I'm open to suggestions on what I need to improve to make this better in any way. 

Edit: this is only my second build as well, thus the really baaaad stuff :P

~Phil

[drew]

Lots of questionable looking solder joints in those photos.  What kind of iron, tip, and solder are you using, and at what temp if it's adjustable

[sluckey]

Please repost your layout pics, but turn off grids and guides in Visio.

I use denatured alcohol in a spray bottle and acid brushes to clean flux from a board or chassis. I use one acid brush with full length bristles (approx. 1" long) and another acid brush with bristles trimmed to about 3/8" for a more vigorous scrub.

[pompeiisneaks]

Lots of questionable looking solder joints in those photos.  What kind of iron, tip, and solder are you using, and at what temp if it's adjustable

I've got a hakko 888  http://smile.amazon.com/Hakko-FX888D-23BY-Digital-Soldering-FX-888D/dp/B00ANZRT4M/ref=sr_1_1?s=industrial&ie=UTF8&qid=1451541702&sr=1-1&keywords=hakko+soldering+iron at the default of 750degF.  I have a blade tip i think size 2?  and I've got 5 core really thin solder.  I'd have to double check the exact size though.  I can adjust the Hakko to whatever is usually best, I've just left it at hte default. 

~Phil

[pompeiisneaks]

Please repost your layout pics, but turn off grids and guides in Visio.

I use denatured alcohol in a spray bottle and acid brushes to clean flux from a board or chassis. I use one acid brush with full length bristles (approx. 1" long) and another acid brush with bristles trimmed to about 3/8" for a more vigorous scrub.

Here are the layouts with grids and guides off.

~Phil

[Ugly Distortion]

Hello, I usually hang out on TAG (rp) but I often check the posts here. I finally registered just to ask, why is that 18K resistor even there? Is it just a typical bleeder for safety after the amp is shut off? If so why use something that draws 11W? If not just getting rid of it, wouldn’t a more common 220K 2W be better?

Suggestion to the OP: if I got it right looks like the original AC100 has an unused triode. Most guitar amps using 12AU7s didn’t really leave a lasting impression on history especially in V1, lower gain is a part of the chimey sound but you can also turn your guitar volume down and the amp up for pretty much the same effect. A common mod to the 2 channel AC50 was to swap in a 12AX7 for one channel. If it’s not too late, and an extra pot doesn’t ruin your front panel aesthetic, and you have some room for a little terminal strip, maybe add a second channel and use a 12DW7 which is a 12AU7/12AX7 in one tube, and you can tweak-up the two sides to taste. Also, thinking you could use a stereo pot, though the unused side would go up in tandem and might add some hiss.

Happy 2016

[pompeiisneaks]

Thanks for the input Ugly Distortion, I only have half of a tube left over (and I may end up using that for a second half of a tube in the reverb stack, not yet sure), but I've replaced the 12AU7's with 12AX7's already, and as far as that resistor, I'm not sure if its for more than just a bleeder resistor, but I've got bleeders on the A and B power filter caps already anyway.  If that's the case and others can confirm, I may very well just remove that resistor. 

~Phil

[sluckey]

I'd leave that resistor in the circuit just because the original has one. Once all the bugs are worked out I would measure B+ node voltages and then remove that resistor and recheck B+ node voltages. They will increase, but if they don't increase too much I'd be tempted to leave the resistor out.

[pompeiisneaks]

Thanks,

So does a resistor like that, that goes to ground instead of being inline, still lower voltages?  I thought voltage dropping resistors were inline? 

Is there anything else I should get?  specific pictures you'd like?

I hope to have some more time tonight to monkey around with it.  (Its been chaos with the holidays and inlaws over etc)

~Phil

[drew]

Regarding the solder:  if you were using the really thin stuff intended for working on printed circuit boards, you may not havve been have been feeding enough of it in, fast enough, to get the nice shiny domes on the tops of the turrets.  Try using .032 . 

[sluckey]

So does a resistor like that, that goes to ground instead of being inline, still lower voltages?

Yes, unless you have a perfect zero loss PT.

Is there anything else I should get?

Yes. Get some responses to all the stuff I've asked for already. Your responses will likely determine which direction to go. Heck, they might even solve the issue.

I do suspect that your soldering will be a factor in the problems you're having.

[pompeiisneaks]

Oh yeah sorry that's right I did have a few more things you asked to do that I've yet to do.  I'll get on that. 

I also found one other issue, it seems like my input jack is causing the lion's share of the noise, so I'll work on that first.  I may have to remove the old circuit board from it and just connect it directly to the pins. 

As for the solder size, I think I may order some of the .032 you mentioned drew, I bought this size on a recommendation I saw online on a youtube video but I do think it was for small soldering not things like this, and that may be why its such a chore to use it. 

Is there an optimal temperature for soldering?  Is my 750 too high/low?

Thanks,

~Phil

[John]

I don't have a soldering station, but your tip will tell you what you need to know. It should be nice and shiny; to get it that way I have to wipe it on a damp (not wet) sponge just before hitting the joint. If it gets too hot, it will turn a funky brown color and will not transfer the heat well. I've also taken to using a tiny bit of flux on all the joints, even though I'm using rosin core solder just like you.


For me, to get those nice domes on the turrets I usually have to use .050 gauge solder; the thinner .032 doesn't "fill in" as nice. So, since I like using .032 better, I don't worry about the domes.    But the solder on the joint should not look all frosty, it should still be nice and shiny, looking not-quite wet.


I will say, a 100 watt amp is pretty ambitious for only your second build. Cheers to ya!

[SoundmasterG]

Nice to see someone else doing an AC100 build. They are a really nice sounding circuit, though on mine I stuck with the 12AU7 setup for the stock channel and just added a higher gain channel to it with switching and LED indicators. I built mine out of a gutted Sovtek MIG 100 U, keeping the power transformer but using a choke and OT sourced from Heyboer. I got it all functioning and had an oscillation issue at the phase inverter due to a layout problem and need to gut it and build a new and revised layout but haven't gotten around to it yet. I used DC heaters on the first tube in each channel and the amp is dead quiet at full blast. It sounded great overall except for the oscillation on my higher gain channel.


Good luck on completing your amp!

Greg

[pompeiisneaks]

I don't have a soldering station, but your tip will tell you what you need to know. It should be nice and shiny; to get it that way I have to wipe it on a damp (not wet) sponge just before hitting the joint. If it gets too hot, it will turn a funky brown color and will not transfer the heat well. I've also taken to using a tiny bit of flux on all the joints, even though I'm using rosin core solder just like you.


For me, to get those nice domes on the turrets I usually have to use .050 gauge solder; the thinner .032 doesn't "fill in" as nice. So, since I like using .032 better, I don't worry about the domes.    But the solder on the joint should not look all frosty, it should still be nice and shiny, looking not-quite wet.


I will say, a 100 watt amp is pretty ambitious for only your second build. Cheers to ya!

Thanks,

I've already ordered the .032, mines a lot smaller, 0.017, so already that's almost double the size :). 

Thanks for that, I'm sure I'll get it :)

[pompeiisneaks]

Nice to see someone else doing an AC100 build. They are a really nice sounding circuit, though on mine I stuck with the 12AU7 setup for the stock channel and just added a higher gain channel to it with switching and LED indicators. I built mine out of a gutted Sovtek MIG 100 U, keeping the power transformer but using a choke and OT sourced from Heyboer. I got it all functioning and had an oscillation issue at the phase inverter due to a layout problem and need to gut it and build a new and revised layout but haven't gotten around to it yet. I used DC heaters on the first tube in each channel and the amp is dead quiet at full blast. It sounded great overall except for the oscillation on my higher gain channel.


Good luck on completing your amp!

Greg

I'll keep the thread updated, let y'all know how its coming, I rewired the input jack today and the massive hum left, but when I was testing that out the two power tubes that were alwasy working were massively redplating, I had to shut it off quickly.  I had made a light bulb tester but thought all was copacetic, and obviously its' not. I tested after that with the tester and it lights up instantly.   More than just the heaters seems to be 'off' on the second half.  I'll have to add solder to all the joints.  I'm getting the thicker solder Sunday.   

~Phil

[John]

If your power tubes suddenly started red plating, and your light bulb tester now lights instantly, you "installed" a short where there wasn't one before. Be sure there's not a solder drip touching the chassis through a turret somewhere.... your ohm meter will be good for that. When I'm looking for something like that I measure ohms instead of just using the beep function.

[pompeiisneaks]

That's what I thought, I just spent a few hours with some new solder getting much better beads atop almost every joint, but some seemed to still be mildly dented inwards.  I tested taht I have connectivity and good readings across the pins on the power tubes, etc, everything still seems fine, but when I power it on, again, still getting redplating etc.  I'll keep playing with it, to see if I can figure out what's going on.   Just wanted to update that I've made some changes but no real progress besides the massive hum being resolved via a fix on the input line grounding. 

~Phil

[pompeiisneaks]

Just doing a little reading on my lunch break today, found this thread:

http://www.thegearpage.net/board/index.php?threads/tubes-red-plating.680379/

that pointed to this thread:

http://forum.metroamp.com/viewtopic.php?f=28&t=30036&start=0

that then linked to something from Aiken amps, but specfically that EL34's at high voltages may need better grid resistors to reduce current, or just reduce overall current to the B+ rail.  Does that jive with what you guys have seen in the past?  I know the original schematic has a bit lower voltages than what I have in my current setup, because the old one seems to have had like 470V input and lowers it down from there, but my current PT puts it in more at the 490-500ish range to start... Would inserting some kind of voltage dropping resistor help here? 

~Phil

[John]

Double check your cathode resistors (if used) or if it's adjustable bias then check all connections there. (Captain Obvious says) something is making those tubes conduct way more than they want to. Measure from each cathode pin to ground, check from your grid pins to ground. You'll find it.


Oh, and I was wrong about my thicker solder, it's actually .062. I have an unopened roll of .050 which I think will be ideal for turret work. I need better eyes!

[sluckey]

The very first thing to do when a fixed bias amp is red plating is to check the negative bias voltage on the grid. That means pin 5 of all EL34s. You can pull the EL34s until you have proper negative voltage on pin 5 of all four sockets.

[pompeiisneaks]

I figured out the redplating issue.  I had put in some 100Ohm resistors to ground on the heaters, but that seemed to be providing some mechanism to ground too quickly from the pins 1/8.  I removed those, as they'd also been a bit browned due to too much current.  Once I did so the redplating stopped. 

I also checked the bias and without the tubes it is at about -29 to -31 on all 4 now, but I don't think that was an issue, if I sort out other issues, I may bias it back up to the higher levels, but for now I want to stay in a more 'cool' area.

I'm still seeming to pull a lot of current though, as I blew another fuse after doing this.  I have checked the pins over and over and I can't see any path to ground other than that one.  I've replaced the 3amp fuse with a 6 amp for now, since the newer one asks for it (the old '65 schematic wants 3, so I'll go back to that if everyone think's thats better, but I wanted it to stay up a bit while I tried to figure out what's up).  I still see and hear significant cycling of some kind of sound, and the output after the very first input tube seems to go from 200mV to about 10V or so, which seems way wrong, but that was also something I'm not 100% sure of, because I kept having to shut it down because I got worried.  I also tried to remove the first preamp tube as you requested sluckey and as I did on the 4th, ended up breaking the tabs on it trying to get the tube out. (the metal keeps flexing while I'm trying to get them out just slowly pulling while doing a circular motion)  I think the breaks are because the stupid amp I have had the holes opened for the exit of the tubes, not for the sockets themselves, and therefore the holes I put into them are a bit too 'close' to the actual hole and it lets the screws tilt a bit.  Basically a bad fit.  I'm going to have to order 4 more preamp sockets and try that all again, after I do something to tighten up the holes in a good way (maybe those metal plates Doug sells for adapting an 8 hole to a 9 hole might do it?)

At any rate the nicer belton sockets seem to have a better quality metal as well, so maybe I can get a more solid seat against the metal with those also.   At any rate, I don't want to keep trying so hard to get the tubes out of the sockets and end up breaking the screw tabs on all of them, its a royal PITA. 

Why are they so hard to get in/out?  Is that because the thinner solder gauge allowed it to flow down into the pin area making them mroe 'tight'?

Anyway, I'm going to defer more troubleshooting until I can get the sockets all replaced.  ugh.

 

I don't see anything yet wrong with the power tubes, maybe somethings in the preamp phase that is sending too much to the power stage?  at any rate, until I can replace the sockets, I don't want to keep fighting it.

~Phil

[Willabe]

Is there anything else I should get?

Yes. Get some responses to all the stuff I've asked for already. Your responses will likely determine which direction to go. Heck, they might even solve the issue.

Oh yeah sorry that's right I did have a few more things you asked to do that I've yet to do.  I'll get on that.

Hmmmmm........     

Sluckey has been asking for these things for, what a few weeks now?
 

[pompeiisneaks]

I'm trying honestly, but I keep making one step forward, and two back.  I'll try and summarize all he has asked for, and what I've done, so I know where I'm at

Phil

[Willabe]

I'm trying honestly, but I keep making one step forward, and two back.

This amp is a BIG amp to completely rebuild.

For the guys and Sluckey to be able to help you what Sluckey asked you for is very much needed.

If your "making one step forward, and two back" maybe it's because your going off in different directions than you should be. There is normally an order of attack in any amp to follow and not just I think I'll work on this now.

What I see you doing is what you think you should do before you take the time to organize the things Sluckey asked you to and post them. You go off on your own out of the normal order of attack and then make a mistake and take 2 steps back. Then you try to fix that and post questions about what the amp is now doing and looking for answers to fix it.

So you are ending up chasing your tail and don't have the time to organize and post what was asked of you because your distracted.

And round and round it goes. 

[sluckey]

I figured out the redplating issue.  I had put in some 100Ohm resistors to ground on the heaters, but that seemed to be providing some mechanism to ground too quickly from the pins 1/8.  I removed those, as they'd also been a bit browned due to too much current.  Once I did so the redplating stopped.

Those 100Ω resistors should have nothing to do with pins 1/8 or redplating. Maybe you had them connected to the wrong socket pins. One should be connected between pin 2 and ground and the other should be connected between pin 7 and ground. Recheck that the green/white filament wires connect to pins 2 and 7 of all four big sockets.

I've looked closely at the pic that shows the socket that you have the burnt 100Ω resistors connect. I can't figure out how the filament wires or those resistors are connected. That may just be the view angle of the pic. I see a green/white twisted pair of wires that are butt spliced to two 'white' wires and I assume those go to the PT. Then there is another green/white twisted pair going to the next EL34 socket. And finally, there is another light green/white twisted pair that goes to the little sockets. All three of those twisted pairs and the two 100Ω resistors should tie together at pins 2 and 7 of that socket (3 whites to pin 7, 3 greens to pin 2). They should not connect to anything else. I cannot tell what they actually connect to. And there is a solder blob at the top side of the socket. Some of the green wires seem to be connect to that mess. And there seems to be a yellow wire connected to that blob. If so that's a big problem and would burn up those 100Ω resistors.

There is also a resistor just to the left of that solder blob. It has a brown blob on the top and a red wire connected to it. What R# is that? There should be a 100Ω resistor (probably 2 or 3 watts) connected to pin 3 of every EL34 socket. This would be R26, R27, R28, and R29. Where are they? R47, R48, R49, and R50 (1K/5W) screen resistors are mounted very dangerously. Those resistors should not have one lead flying freely in the air and connected to those red wires! Most people would connect those screen resistors between pin 4 and pin 6 and the red wire would connect to pin 6 also.

I also checked the bias and without the tubes it is at about -29 to -31 on all 4 now, but I don't think that was an issue, if I sort out other issues, I may bias it back up to the higher levels, but for now I want to stay in a more 'cool' area.

You 'may' misunderstand the relationship between that negative bias voltage and 'cool' running tubes. _29 to -31 is probably pretty hot. -40v would be cooler. I suggest you set the bias pot for maximum negative voltage on pin 5 of all EL34s for now. That will keep the tubes running cool. BTW, what is the maximum negative voltage on pin 5?

[pompeiisneaks]

So to explain the heaters, I've got the heater splitting at a point above the first tube, one side going to the preamp, the other down to the first socket of the power amp tubes.  I have tested them for continuity and they're all connected correctly on pins 2/7 on all power amp tubes (and 4/5 and 9 of preamps, and all heaters are heating correctly, reflowing and adding solder to the last two tubes heater pins seems to have solved a dry soldered connection that was being intermittent).

to remove the confusion on the other resistor, that's going off to my LED, its the current limiting resistor for the LED, that's all.  I've tied the LED into the heaters there as well, but down at the pins of the first power tube. 

As for them causing redplating, I was able to figure it out becuase with them in circuit, I was getting continuity between pins 1/8 and 2/7, with them out, that stopped.  I don't know why, but probably because they weren't creating enough resistance to stop looping between 1/8 and 2/7 of the power.  That may also be wrong in the 'why' but I clearly saw continuity between the two stop when I removed those.  I may restore them or link them to a different ground line at some point later, I'm not sure, but for now it removes the redplating.  I suspect they may have burned on an earlier error when I had connected the wrong lines for the heater to them, and instead of 6v they were running at 70 or so volts.  I just didn't realize they were that browned until I figured out the issue.  I think one measured only 45 ohms after the burn, and the other was like 30 ish so they were definitely not in good shape. 

If you think it is wiser to just move the connection of the heater wire down to tube 1, and then connect off to tube 2, led and preamp tubes from those pins, I definitely can.  it is just tight at that tube and I was trying to create a bit of space by sending off the preamp lines a bit earlier.  Is this generally considered bad form?  Or maybe I should even just remove that entirely and send the power lines from the last power amp tube on the left down to the first preamp tube?   

I'm open to anything that way.

Willabe, I don't disagree, I'm sure you're right.  I tend to be bad about that kind of mental flights off on tangents.  It sometimes can be very helpful for me, and others it can be very detrimental.  In this case I think I agree with you that its what is causing the issues.  I need to, as stated in my last comment, sit down, review all of sluckey's suggestions, write them all out and go at them one at a time (I've done many of them but not all, and its been such a silly mess since I don't even recall which I did and didn't do.)  I can then repost the list here, and then I'll go tackle them serially.

I really do appreciate the help,  you guys' input has already been invaluable, and I'm learing tons (mostly due to my own mistakes, but that's sometimes the best/only way to learn I guess heh).

I ordered the sockets from Doug, and some 8 to 9 pin adapters to see if I can get these to fit better.  but I can still keep testing with the sockets as is, and leave the preamp tubes in since I can't seem to get them out.  (any tests that required removing them can wait until I fix the sockets, but I did do some of them for sure). 

~Phil 

[sluckey]

OK, I understand how the filaments are wired. You still have a big mess around that socket. At least the camera angle view makes it look messy. Maybe when you replace the sockets it will look neater.

As for them causing redplating, I was able to figure it out becuase with them in circuit, I was getting continuity between pins 1/8 and 2/7, with them out, that stopped.  I don't know why, but probably because they weren't creating enough resistance to stop looping between 1/8 and 2/7 of the power.  That may also be wrong in the 'why' but I clearly saw continuity between the two stop when I removed those.  I may restore them or link them to a different ground line at some point later, I'm not sure, but for now it removes the redplating.

Of course you will measure continuity between pins 1/8 and 2/7. That's perfectly normal. If you look at resistance rather than continuity beeps, you'll see that continuity is actually about 50Ω.That's because you have a 100Ω from pin 7 to ground. And another 100Ω from pin 2 to ground. Those resistors are practically in parallel and give a total resistance of 50Ω. Now you also have a 1Ω resistor from pin 1/8 to ground. So, pins 1/8 are only about 50Ω away from pins 2/7. If those resistors were causing redplating, they must have been touching something they should not have.

What about all the other issues I mentioned in that same post? Especially the questions about the negative bias??? Those answers may lead to a solution to your red plating and fuse blowing issues.

[pompeiisneaks]

I can't set the bias from work, so I didn't mention that, I'll take a look, but again, the redplating is gone, but I can make the bias colder by pushing it up to whatever is max (If I recall in my testing it got up to about -65V?)  Sorry I was even reading the valve wizards tube stuff and read the same thing this morning on the bus.  That the higher the voltage the colder the bias, so I'm glad I got it from two separate sources on the same day, its a counterintuitive concept but reading it and seeing you remind me of it as well is good.  I'll test that out when I get home, and have had time to write down all the things you've asked.  I'm going to try to ensure I'm going at this methodically this time :)

~Phil

[PRR]

> the higher the voltage the colder the bias,....its a counterintuitive concept

When you want to strangle somebody, you "press harder" to reduce his air-flow.

Zero grid-cathode voltage allows MAXIMUM current. Humans work well this way, less-well when strangled. In tubes, we never want to idle wide-open this way. We "strangle" it to a lower current, half or less of the peak current we want on the loudest signal peaks. In a push-pull amp, we can strangle down (press harder) to a very small current and still get full power: the signal voltage on top of the bias voltage can push current to the max if desired.

[pompeiisneaks]

Oh groovy, that uses words that make even more sense!  Thanks :)

~Phil

[pompeiisneaks]

First up a question, sluckey, could you clarify number 10 below?  I've put my question there after the request.

TL;DR post coming :)  Here's the complete list of what sluckey's asked for, in 'done first and then those I'll be doing' format,  so if you don't want to read, that's fine, but I wanted it here for my sake :)

1.   Replace 18k 2W with 18k 25W - Done
2.   replace full wave bridged rectifier with full wave rectifier for PT from AC100cph - Done
3.   use different bias circuit for the above rectification - done
4.   What is voltage level of input - Done, Showed that a bit later as well as all input  voltages across the A-B-C-D-E
5.   Use 200mV peak to peak on input for testing the audio circuit - done
6.   Recheck all solder joints - Done - Did this over the weekend, I got thicker solder and re-soldered everything on the boards, and redid the power tubes as well.  I also looked for any issues on the preamp tube sockets but they're all looking good.
7.   Measure and post voltage readings for all filter cap nodes and every tube pin - Done
8.   Post some high rez pics, post layout - Done
9.   please put all pertinent layout and schematic in this thread - done
 
And TO DO:

10.   Swap Tubes - I've swapped tubes around, but I don't have extras, do you mean to try some other power tubes? They were new when I got them, and the issues were occurring, but it is possible that Iv'e done something to toast them?
11.   Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked at each end - will do
12.   Divide and conquer.  If you  have added reverb, disconnect it, any better? - will do, I attempted to do this by pulling the reverb recovery tube, but i guess it may be possible that there's still something bleeding out after it gets in from the input tube side?
13.   Pull V1 and V2, - Will do I've got to fix the tube sockets first but I will
14.   Temporarily connect an input jack to the treble control wiper and plug a guitar in.  will do
15.   Recheck your drawings, make sure schematic agrees with the original (except for the mods used for the PT), make sure the layout agrees with schematic, make sure you amp is wired IAW schematic and layout.  - I've done this, but I think i may do it again just as a sanity check
16.   Plug in speaker and guitar, adjust bias - Done initially, I'l be doing the later recommendation to bias cold until I've resolved the issues (max of about -65V as I recall)
17.   Take a look at my signature - that links to the how to cover that you've done it all right, as mentioned I've done this, but do think I need to do it again to ensure I've not missed something, because something IS still wrong
18.   Use denatured alcohol to clean the flux - Started and got partly done before I re-soldered everything, so I'll repeat this too, is this part of the troubleshooting or just to see clearly that the solder looks good and post pics post clean up?
19.   After everything else is balanced out, it may be worth removing the 18k 25w resistor - Will do (its now 22.5k because I didn't find one locally, but I may replace that with said 18 or remove depending on post fix voltages)
20.   bias the tubes colder until issues are resolved by pushing the pin 5 to the maximum negative voltage possible (-65V I think) - Will do
21.   restore the 100 Ohm resistors as they should not cause red-plating, just ensure they're not near anything that could cause issues by touching other connections etc.  - Will do
22.   Adjust the heater wires to be less ugly at the power amp first socket - Will do, I intend to just remove the preamp split, and go directly to the pins on the power amp, and then pull of the end of the last power socket straight down to the preamp tubes.

So I'll report back with my findings to the above second half and more of the list. 

Thanks again for all the help, I hope to have a fix or more data soon :)

[SoundmasterG]

You may find that now that the tubes have been red plating, that they are permanently altered and will not work correctly anymore. It depends on how badly and for how long they were red plating before you shut the amp off. I was using some JJ KT77's in my AC100 project and had a problem with the screen grid resistors not being big enough and before I noticed that one tube on each side had started to red plate. Once I had fixed the issue, that set of tubes wouldn't work correctly anymore and sounded very weak. A new quad of tubes resolved the issue. Generally speaking, once you notice an issue with the tubes red plating, after you have shut it down, take the tubes out and connect it up through your current limiter with no power tubes in it initially and see if you can spot the issue. If you can't then put the tubes back in and run it on the current limiter with the power tubes in there. The tubes will likely try to red plate but being current limited they can't get to that point and the light bulb on your current limiter will light up brightly. Then perhaps you can spot the issue. Once you have found it, then test it without the current limiter and verify your bias first. Assuming you have adequate bias, then you shouldn't see any more red plating, though again I would be suspicious of those tubes now and use another set. It can get expensive quickly if you are always burning through tubes so make sure to use your current limiter as it is intended so you don't burn stuff up.


Greg

[pompeiisneaks]

SoundmasterG, thanks, I'll keep an eye out for that.  So far when I've powered it up, I don't have lack of sound, the hum coming in right now due to whatever's wrong is so loud at almost no volume, I'm pretty sure the output tubes are doing well right now.  I'm considering getting another set but I don't want to replace them until I'm done troubleshooting (unless people thing the tubes may have been damaged enough to exacerbate the problem).

[pompeiisneaks]

Well I've gotten through a few more, I'm going to post what I have, and I'll update after I get the sockets fixed, From the other list, I completed

2.   11.   Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked at each end - will do

it goes from -28V to -52V

11.   20.   bias the tubes colder until issues are resolved by pushing the pin 5 to the maximum negative voltage possible (-65V I think) - Will do

I was working (and spent a LOT of time) trying things out with the reverb return (V3) tube out, and things started seeming 'ok' but there is now absolutely 0 output.  I tested both sides of the OT and that comes within what seems like acceptable ranges.  See this site: http://www.etronic-parts.com/product_info.php/info/p1090_VOX--Output-Transformer-AC100CPH.html they show what they get for impedance readings on that OT, and mine are almost identical (the power output side is a bit mismatched, the above image shows 18 and 18 for a total of 36 ohms, I think I was seeing closer to like 15 and 19 or something similar, I'm not sure if that is within tolerances?   On the other side I saw 0.5 Ohms and 0.2 between the 8 Ohm wire and 0.1 between 16 ohm. 

I'll keep working at it, I've gone completely though the drawings and didn't find anything massively bad except that the reverb pot ground wasn't wired down.  I've completed that.  I started thinking I'd messed up on the connections for the V2 and rewired that but I'm going to have to revert it.  My Visio is wrong, because the schematic shows that the second half of the tube's Anode connects directly to D, but somehow I thought it was supposed to share R8, I rewired it to come in at R8 and the weird cyclical sounds seem gone, but I don't honestly think that's related, because its now 'wrong' instead of right.  At any rate, I'm still just not sure why I'm getting absolutely no output now. 

I've retested the voltage readings on pins 4 of all power tubes and they're still looking right...

Any suggestions on why the OT seems silent now when it was working before?  Would that indicate the OT is fried?  my measurements of impedance should confirm its still good right?  The same number of wirings, its not shorted nor is it significantly smaller or higher resistances?

I'll work on the remaining ones, but I'm getting closer.  I think the quieting down of things may have been because I wired the reverb pot to ground, but not sure?  Would that be it?

~Phil

[SoundmasterG]

OT's rarely fail...I am sure it is fine.


Do you have a signal generator and an oscilloscope? If so they are very useful in tracing the signal through your amp to see where it is going and what the signal looks like from stage to stage. If you don't have one, it makes it a bit harder to troubleshoot when you lose the sound.


If the amp will be stable with those power tubes at an acceptable bias level without the tubes red plating or orange plating, then they will be fine most likely. You aren't at a point yet where you can ascertain that though....try them and see over time how they work....but be suspicious about them until they prove they are fine.

Greg

[sluckey]

Divide and conquer.

[pompeiisneaks]

Sluckey, I have been, I've got the oscilloscope up and am tracing the output all the way to the side just after the phase inverter and it is fine.  This means the signal is getting to the power tubes but then nothing.  (the other work I've done has stopped that oscillation, but maybe that is being caused when the output is going to the speaker?)

 I was in the middle of the divide and conquer process but realized I was getting no output (I was before, and I don't know wht else has happened besides me blowing that fuse, but obviously something went with it). 

I can basically say now for sure that I have output from the input all the way to the tubes, I guess I need to check for output at pin 3 of the Power Tubes?  The other parts of it weren't tested becasue the entire input stage now seems stable after soldering the reverb pot to ground.   So either the lack of output indicates my original issue is tied to the output working OR the grounding fixed the cyclic issue.

I'll see what else may be amiss.  I even tested the speakers by plugging them into a different amp and they're workign fine there, so its not the speakers. 

I'll mess with it more tonight, I'd run out of time last night when I'd gotten to this point.

~Phil

[pompeiisneaks]

I should qualify what I mean by "I was".

I have listed on your "divide and conquer" part was to disconnect reverb, my first step in that, (for the second time), was to remove the reverb recovery tube, that way its not able to feed back back 'in' and things seemed stable, no cyclic issues, but I also had no output, thus I started tracing the signal and found it went all the way through the preamp to the power amp stage. 

I hadn't yet gotten to desoldering the input and output parts of the reverb so that it just went directly from preamp stages bypassing reverb to phase inverter yet, and realized it seemed stable after my soldering of the things 'wrong' with the schematic/layout, and then found no output, so I'd done very little yet.

I should have more time to tinker this weekend, and I'll measure what's coming out of the power tubes as well. Pin 3 to the OT that is.

If I get output working again, and the previous steps have resolved the other issues, then we're golden, if not, I'll resume the divide and conquer plan.

~Phil

[sluckey]

I have listed on your "divide and conquer" part was to disconnect reverb

Go back an read my divide and conquer statement. You left out the most important part of what I tried to tell you.

Now that you have a decent bias voltage range, set the bias to -40V on pin 5 of all EL34s. Make sure there is no red plating. Got sound now?

BTW, pull one pair of EL34s so you are only using one tube on each side of the OT. No need to risk blowing all 4 EL34s at this point.

[pompeiisneaks]

Recheck your drawings. Make sure your schematic agrees with the original (except for the mods you've had to make to use that PT). Make sure your layout drawing agrees with your schematic. Make sure your amp is wired IAW your schematic and layout.

Recheck all solder joints.

Swap tubes.

Measure (and post) voltage readings for all filter cap nodes and every tube pin (even the pins that should be zero volts). What voltage is on pin 5 of each EL34? Pull the B+ fuse and measure bias voltage at pin 5 with the bias pot cranked to each end. What voltages do you get?

Divide and conquer. If you have added reverb, disconnect it. Any better? Pull V1 and V2. Temporarily connect an input jack to the treble control wiper and plug a guitar in. This connects the guitar directly to the input of the power amp. It won't be nearly as loud but should be hum free and very clean sounding. Is it? If so, the problem is in the preamp. If not, the problem is in the power amp.

Post some hi rez pics of your amp. Post your layout drawing.

PS... Please put all pertinent layout and schematic drawing in this thread so we don't have to go chasing them down in other threads.

Reposting your divide and conquer post, because I want to understand what I'm missing. 

First off, thanks for the help, it is really making a difference, I don't want the tone of this post to come off wrong, I'm not complaining, just honestly trying to understand what I've missed.  If there's one thing I've learned in my life, its that email/texts/forum posts can be misread for tone/intention, and this is completely "I'm ecstatic you're helping, please know your help is appreciated, I feel like a dumbass for missing something" and nothing else.  So to continue:

In your above post/list, I've done

:recheck drawings and schematics
:recheck all solder joints
:swap tubes (I asked for clarification on this in a post a few posts ago, but I have swapped tubes around quite a bit, from each half of the push pull setup, not sure if that's what you meant or if you meant to swap in different tubes, but I don't have extra's right now)
:Measure and post voltage readings
:pull B+ fuse and measure pin5
:post hi rez photos
:post latest schematics and layouts

all above are done, (with that one caveat of the swap tubes)

that being said the rest was what I considered the "divide and conquer" part of what you asked but maybe you meant all (but all BUT this is done, and is what I was working on).  From above:

Divide and conquer. If you have added reverb, disconnect it. Any better? Pull V1 and V2. Temporarily connect an input jack to the treble control wiper and plug a guitar in. This connects the guitar directly to the input of the power amp. It won't be nearly as loud but should be hum free and very clean sounding. Is it? If so, the problem is in the preamp. If not, the problem is in the power amp.

In that I had only done the first part, reverb, so to be sure I understand what I'd missed, are you meaning the fact that I seem to have good sound right up to the power amp that the problem is in the power amp? 

If so, I guess I just didn't read that far, as I was trying the first part of that paragraph (I've reread it in the past too, but sometimes forest trees, you get the idea)

thanks so much for the help, if I gather what you're saying now, I may have the tubes biased too cold so that they're in that state where they can't produce output?  I've read something about that if you push the bias too far, then it can't produce output, but it didn't click until now, (and even now I may not be right lol)

~Phil

[pompeiisneaks]

Ok so I'm working from home so I figured I'd go down and try removing two power tubes, and bias to -40V and guess what, its working great, I think one of the two tubes was bad... (I biased to -40V first and it still was oscillating, but I realized I'd forgotten to remove the two tubes).  I was getting clean tone w/ no issues.

I then plugged in my guitar instead of the oscilloscope, and bam, its sounding great.  At low volumes it is clean with a bit of crunch when pushed, but it goes into distortion pretty early on for what I'm used to, is that due to using the 12AX7s instead of the 12AU7's for the V1 and V4?  I don't know which of the tubes I've pulled is bad, but I guess I can put the amp to low volume, and swap one tube at a time until I find the one oscillating and then replace that one right?  Or should I get a balanced pair?

Thanks so much!   

I'll have to do some more general checks, I have to desolder/restore that one output from the V2 that I put up to the resistor in error, but it doesn't seem to have caused major issues,  I also need to restore the 2 100ohm resistors to ground on the heaters and then I think I'm pretty good.  (I will get it all good, and then check if I can remove the 22.5k 25watt resistor after all is good and done and tested etc.)

One thing of note, the amp is SUPER quiet, I'm only hearing minor hum at high volumes with P90 pickups pretty close to the speakers.

Edit: clarification, I meant removed the sine input not the oscilloscope, its still connected so I could monitor, but basically all is looking good other than distortion at about I think 11 to 12 o'clock which sounds a bit nice but with some harmonics that seem 'off' a tad, I'll have to get all 4 power tubes in, and what not and see where it starts distorting in the chain (I have a 4 input scope, so i can set it in 4 locations and see what's what.)

~Phil

[sluckey]

At low volumes it is clean with a bit of crunch when pushed, but it goes into distortion pretty early on for what I'm used to, is that due to using the 12AX7s instead of the 12AU7's for the V1 and V4?

Correct. 12AX7 has a gain of 100. 12AU7 has a gain of approx. 20. I would put 12AU7s in the sockets that call for 12AU7s if you want it to sound more like an AC50.

I have to desolder/restore that one output from the V2 that I put up to the resistor in error, but it doesn't seem to have caused major issues

I still don't know if you know how that should be connected. The schematic is right. Pin 6 must connect directly to power supply point "D". Your layout is wrong. You show pin 6 connected to pin 1. Fix that. And fix your layout too.

[pompeiisneaks]

sounds good, will do.  I thought the 12AX7's might just make it overdrive a bit better, instead its a lot earlier and maybe too much.

For the second note, yeah, I basically somehow did the layout wrong, and then when I first did it I think I was looking at the schematic and had it right, but on second review of the layout, doing the 'check it all' all over again, I found that error and duplicated it.  It didn't cause too much of an issue, but it may be causing some of the odd sounding distortion because now both stages have the wrong setup, pin 3 is supposed to have one resistor and 6 none, now they share it, so I'm sure that changes things.

Thanks again for the help!

~Phil

[sluckey]

pin 3 is supposed to have one resistor and 6 none, now they share it, so I'm sure that changes things.

We ain't talking about pin 3! We're talking about pins 1 and 6.

[pompeiisneaks]

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