Vox AC15 build - startup issues I can't seem to sort

[BarryW]

I really needed a small vox amp for my regular country gigs.  I have built everything from Fenders (tweed, brown, BF) to Marshalls to hybrids (18w/AC15 mashup like the 65amps London) to Matchless (spitfire, lightning/TC15) to Wreck to Dumble.  I've been really fortunate, most worked first time out barring a OT lead swap, or some other small issue that I left out/missed and quickly traced down.  After seeing Sluckey's great work on the AC15, I decided to whip one up based on the available boards.  I finally made time to do it, picked up some Mercury iron (which I usually don't do) and got it together.

Only a few variations: I added the trem mod as outlined by Sluckey, the variable "brilliance" or voicings on the normal channel with a rotary switch, and a switchable master volume (crossline).  I included a sort of boost circuit similar to the London.  I also noted that there didn't appear to be a CT on the 6.3v tap, so I set up an artificial center tap on the heaters.

If I recall, it passed the light bulb test, I got the tubes in, took some voltages, and things looked to be in good order.  I fired it up and it sounded awesome on the normal channel - Vib channel was a little lower in level and the trem didn't work (I figured I'd sort that later).  I switched in the master volume and started to turn the MV down.  When I turned it very low, it smoked and the two 100r resistors on the heater line blew along with the mains fuse.  I took the MV out of the circuit, replaced the two 100r resistors and looked over everything I had done.  I noted that I had missed a cap on the trem channel, but not really much else.  So I plugged it into the bulb limiter - no problem with just the rectifier in, then no problem with the power tubes added.  But when I added the phase inverter the mains light stayed on.  I've tried other fresh tubes, and I've even confirmed that the tubes I used were good in other amps.  Still no go. And that's where I sit today.

I've looked at it and I've looked at it, I just can't figure out what's wrong.  I went thru Keen's list to check the PT, choke and OT as best as I could, but nothing seemed to be out of sorts.  I'm thinking I'm just blind to an obvious problem - I just can't see what I missed.  To me I think it has to be something in the heater line . . . B+ seemed to be fine across the board.  I'm in the process of freeing up the pots and pulling the board to triple check the wiring underneath, but I have continuity between all of the spots I'm supposed to so barring some weird short underneath I'm not expecting to find anything.

Gumption trap!  Any ideas?  I'd appreciate any thoughts very much.

I'm attaching the PT diagram, I'll add a pic and a link to photobucket folder once I can sort my connection issues.
edit - for some reason I can't get to my photobucket account, I'll attach a link to the folder and revise this later this evening (apologies).

Link to pics: http://s13.photobucket.com/user/barrywol/library/amp%20builds/AC15%20build

[kagliostro]

.... ....    ....  ....   


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


Franco

[BarryW]

yep,  I do that with every build -

probably TMI.  I'll probably just delete this, do more research and re-post later.  I'll re-post after I've had a chance to find what's shorting .

[sluckey]

That 330-0-330 PT should put out way more B+ than EL84s will be happy with. I used a 275-0-275 PT and have 350V for B+.

[PRR]

> started to turn the MV down.  When I turned it very low, it smoked and the two 100r resistors on the heater line blew along with the mains fuse.

Does not make sense.

> no problem with the power tubes added.  But when I added the phase inverter the mains light stayed on.

Does not make sense.

I think we need a schematic of what you thought you built.

[BarryW]

Sluckey  - stock ac15 is 325-0-325, similar to an 18 watt.  My voltage at pin 9 was about 314v. My voltages at key points were within reason:
A 324
B 295
C 295
D 217

 And thank you for the comments in the other thread, as well as so many other helpful posts over the years.

> started to turn the MV down.  When I turned it very low, it smoked and the two 100r resistors on the heater line blew along with the mains fuse.

Does not make sense.

> no problem with the power tubes added.  But when I added the phase inverter the mains light stayed on.

Does not make sense.

I think we need a schematic of what you thought you built.

I agree, it doesn't make sense - but that is how it presents. Sorry if I wasn't clear, but I built this based on Sluckeys schematic and layout. As I note above, I included the 6 pos switch in place of brilliance and the trem mod. But as to what I "thought" I built ... I followed Sluckeys layout with the boards that were available.

I'm pulling the board to make sure nothing is off or shorting. Hopefully I'll find my silly error and report back with my embarrassing confession.  Or perhaps I'll find that I built something I hadn't thought of!  Now that would be surprising. 

[sluckey]

I originally thought your only problem was the heater CT resistors had poofed. But I see you have other issues too. I don't see how the PI problem, or the crossline MV could be related to the CT resistors. For the MV pot to smoke it would have been wired wrong. But since you have removed it, I wont worry about it.

So now, when you plug in the PI tube your current limiter lamp glows brightly??? Does the lamp glow at all with just the rectifier and power tubes plugged in?

This amp worked at one time. I'd like for you to remove all tubes except the rectifier and power tubes and plug the amp straight into the wall. If the fuse holds, measure the voltages at each filter cap and plates, screens, and cathodes of both power tubes. Also check for any DC voltage on the filament pins. Post your voltages.

I just spent a painful 45 minutes trying to view your pics on photobucket. I was able to get the first seven. I give up. Your pics can be posted directly into your replies on the forum. Please do so.

[BarryW]

 Thanks, I'll do that. My apologies about Photobucket – I hadn't used it in a long time, and it's a huge pain. I was trying to avoid posting photos to the issues I've seen in the forum with them.   I will resize and upload relevant pics as needed

 And yes, when I plugged the PI tube in the limiter lamp close brightly.  My recollection is that with just the rectified her and with just the rectifier in the power tubes it initially would go and then dim down as it normally would when an amp with start up properly.

[sluckey]

I think I see why the vibrato did not work. The footswitch jack appears to be wired incorrectly. That blue wire from the board will have to be grounded to enable the oscillator. And what is that white wire on the FS jack? I know you have bigger issues at this point but just keep this in mind for later.

Do you have the amp opened up? Are you prepared to do some tests and make some checks?

Oh, the 7 pics that I saw were small enough to post directly. I suspect the others are small enough also. Probably no need to resize.

[BarryW]

 Yep, it's open and I shouldn't have a problem testing -  just have to get home and find a few minutes to do it.

FS  is a stereo jack, it controls the trem as well as a simple boost in the other channel.    Thanks – but I'm sure I can sort that once I get the major issue taken care of.

[sluckey]

Tell me more about your boost circuit.

The issue with the FS wiring is that you have connected the wires to the "switched" side of the jack, rather than the tip, ring, sleeve side of the FS. Wire it like this...

[BarryW]

first, thanks all for chiming in.
second, my apologies - I didn't have my facts straight, probably due to building this for short periods at a time in between work and gigs.  No excuse.

Limiter lights with rectifier + newer set of el84s, whether both or either el84 is in.  Doesn't light with el84s pulled and just the rect installed.  This el84 set may have been damaged in previous testing - I haven't tried with the other "known good" el84s yet.  But either way, I clearly have an issue.  I have 3.4v on filaments all the way across, and no DC.   I've removed all variables with MV so that I can concentrate on the pure circuit.

As to the FS - it isn't in the circuit.  I have a switch on the front panel, and I don't have the board connected to it (hence it didn't work).  Stupid me again.  But thanks very much for pointing out my obvious switch wiring issue sluckey - I did it from notes on other projects instead of from my head. Lesson learned - only work on projects when you can spend enough time to complete each area instead of a few minutes here and there.  My worst effort yet.   And the boost - it's really more of an "eliminate the boost" function, simply putting a 10K resistor between the 25uf cap and ground (on a switch). 


Again, I apologize to the group - I really should have done a better job preparing a question and pics.  I'm going to put this on hold until I can dig into this and find my short.  I'll either edit this thread or delete it and start again.