Vox AC-15 plus Matchless Reverb build - Sluckey schematic

[shaun]

Edited.

Hi All, I cannot find my original post on this topic, and it's been a few months since then, so I thought I'd start again. Hope that's okay.

Mr. Luckey drew me the schematic below, which I've now built, with the addition of a 6-way tone switch as demonstrated in his other drawings for Vox amps. I'm pleased with the build, generally speaking, but I've hit a snag. While doing the initial start-up testing, I put the variac at 65vac and checked voltages. Everything looked good, with the expected readings for a loaded amp - all tubes in place (yes, yes, I tested without tubes in place first, and all was well, mostly). But until I resolve this basic issue, I don't want to run it at 120vac.

So, the only issue I can find is that the plate voltage on pin 1 of the PI (V4) shows only 1.2mV dc.  Meanwhile, the PV on pin 6 of V4 shows 106vdc, which seems about right for 65vac on the variac. When I checked the 100k res going to pin 1, it read fine, although I had to disconnect one end to get it to read properly on my meter. Plus, when I lifted one end of that same 100k res off pin 1, I get a reading on the plate side of the res of 160vdc, which seems okay for an unloaded plate voltage. (This occurs whether there's a tube in the socket or not, so I discount the tube being the problem.)

It seems like the right voltage is getting to the tube, but something is awry that is beyond my ken. Mr. Luckey said this Chieftain reverb hybrid would be an experiment. I've looked at many reverb schematics, and most simply send the wet signal into the same end of the PI as the dry signal, so I cannot find anything comparable to see where I could improve the build.

I'm wondering why most reverb amps run the wet signal back to the same side of the PI as the dry signal, rather than into the second triode of the PI, and I'm also curious about the reverb pot (100kA) and how it's wired with no connection to lug #1 (although I assume this style of wiring is simply to allow the wet signal to go to ground when not wanted in the mix).

In any case, I'd appreciate any guidance. As usual, thanks for your help and expertise.

[sluckey]

Run the amp at 120VAC so we can see some realistic voltages.

So, the only issue I can find is that the plate voltage on pin 1 of the PI (V4) shows only 1.2mV dc.

I suspect a wiring error. Look closely at the wiring and components for pins 1, 2, and 3. Be sure pin 3 is connected to pin 8. Show us some hi rez pics please.

Mr. Luckey said this Chieftain reverb hybrid would be an experiment. I've looked at many reverb schematics, and most simply send the wet signal into the same end of the PI as the dry signal, so I cannot find anything comparable to see where I could improve the build.

The reverb is a proven circuit. The only thing experimental about it is the fact that it has never been used in a VAC-15 circuit.

I'm wondering why most reverb amps run the wet signal back to the same side of the PI as the dry signal, rather than into the second triode of the PI, and I'm also curious about the reverb pot (100kA) and how it's wired with no connection to lug #1 (although I assume this style of wiring is simply to allow the wet signal to go to ground when not wanted in the mix).

The Chieftain is the first amp I've seen that connected the reverb into the second triode. I was intrigued by this circuit and I fully expect it to work in this AC-15 amp. Many reverb amps also use NFB. The NFB is connected to the second triode of the PI, so the second triode is not available to use as an input for the reverb, second preamp channel, or anything else. Therefore the reverb is passively mixed back in with the 'dry' signal and the combined signals are sent to one PI input.

The reverb pot will work just fine. I would have wired it like a standard volume pot if I was designing this circuit. But I was simply copying the original Chieftain circuit.

Let's see some close-up pics of the V4 socket and circuitry. We can probably help find the pin 1 issue.

EDIT... Here's the complete docs for this amp...   http://sluckeyamps.com/misc/AC-15R_Lite.pdf

[shaun]

Thank you, Sluckey. I'll re-check my wiring, and I'll take some pics.

[shaun]

Thanks for posting the extra info. I didn't know there was a wiring diagram available, and I wanted to learn how to create one so made an attempt, but the result is a rat's nest. I imagine it would be difficult to see exactly what is going on in the pics except that it's amateur work. Still, I know so little that I haven't perfected much of what I do yet. I guess that comes with time.

In addition, the schematic you posted originally used one less node than this most current version just made available, so I built my version with only nodes A thru D as per the first schematic you helped me with.

What I can say is this: I've checked and re-checked my wiring, and I can find nothing that accounts for the minuscule vdc on pin1 of V4. Of course, that doesn't mean I haven't done something stoopid:). In fact, if I were a betting man...

Here are voltages, fully loaded, at 120vac going in from the variac (I should also point out that my limiting lamp stays a little too bright for comfort):

Rec plates: both 168vac  (Unloaded, the PT was delivering 340vac per side before the build).
Node A: 164vdc
Node B: 151vdc
Node C: 127vdc
Node D: 126vdc

EL84 plates: 159.8,  160.
PI plates (i.e. the symptom): Pin 1: 4.5mV     Pin 6: 104vdc

V1 (EF86): Plate voltage: 43v
V2 (12AX7): both plates, 149vdc.
V3 (12AX7): pin 1: 102   pin 6: 120.

I've changed out all tubes several times with little difference in result. Generally speaking, these voltages seem lower than what I was expecting, so that may indicate a problem and account for the limiter bulb staying too bright (although it does dim about half).

[sluckey]

Voltages are meaningless since you are plugged into a lamp limiter. Plug the amp straight into the wall.

[shaun]

Doh. Sorry.

Rec plates: 325 & 320

Node A: 348vdc
Node B: 306vdc
Node C: 243vdc
Node D: 240vdc

EL84 plates: 338  &  333.
PI plates (i.e. the symptom): Pin 1: 7.5mV  Pin 6: 161vdc

V1 (EF86): Plate voltage: 50v
V2 (12AX7): both plates connected, 305vdc.
V3 (12AX7): pin 1: 158   pin 6: 214.

[sluckey]

Unplug V4. What is the voltage on pin 1 now?

[shaun]

Pin 1: 9mV
Pin 6: 249vdc

[sluckey]

Check the voltage at these four points and report back.

[shaun]

A: 254.8
B: 254
c: Jumping around from approx. 1.1mV to 45mV
D: Jumping around from approx. 1.1mV to 45mV

If I lift the lead off from point D and read point C, I get 250vdc.
If I take a reading from the other side of the resistor closest to point A, I get 252.

It may be hard to make out, but there is a dark cable carrying Node C that meets the two 100k plate resistors. But you probably figured that out already.

Thank you for your time, btw.

[sluckey]

Looks like a cone board from an old Hammond organ amp. Be sure the cone at point D is not contacting the chassis underneath. Pretty easy to shove a wire all the way through and touch the chassis. Also easy to create a solder blob that touches the chassis. If this is not the case then just replace the resistor.

[shaun]

Okay. Thanks. Yes, I checked for clearance underneath, and I've already changed the resistor. And yep, the board is from a Hammond. I like using old crap for some reason. It's interesting to see how things were once done, and I've learned quite a bit from hand-wiring, with or without a board. Plus, I'm cheap. But I like the idea of making my own boards for upcoming projects.

Hmmm. Now what to try?

[sluckey]

With power off check resistance between point D and chassis. If it reads low ohms then remove the brown wire that is connected between point D cone and socket pin 1. Still read low ohms at point D?

[shaun]

With power off, the reading at point D jumps from OL. and all over the place. With the brown wire disconnected from point D, the meter reads 0.00 ohms at point D.

[sluckey]

The cone at point D appears to be shorting to ground. Remove the screws that fasten the board to chassis and lift the board up as much as you can. Then inspect that cone, looking for anything that might touch the chassis when the board is screwed down.

[shaun]

So, I checked under the board - for about the third time:) - and all was good. On a whim, I gator clipped in a new resistor between C and D, just in case. I mean, what are the odds? But hey presto, suddenly the voltage reads good, so I guess I got a couple of bad resistors in that batch - they both read good before install. A first for me even to get one bad resistor, let alone two.

Here are the new readings:
Rec plates: 325 & 323
Node A: 348vdc
Node B: 312vdc
Node C: 267vdc
Node D: 264vdc
EL84 plates: 340  &  334.
PI plates: Pin 1: 202  Pin 6: 202

V1 (EF86): Plate voltage: 55v (kinda low)
V2 (12AX7): both plates connected, 309vdc. (not sure how to interpret this, but assume 154vdc per plate)
V3 (12AX7): pin 1: 179   pin 6: 234.

There's sound coming from the speaker - but not good sound. There is a lot of hum (I'd say 60 cycle) which disappears when I remove the EF86, so I'll upgrade to shielded cable from pin 6 signal out to the 6-way switch and see if that helps. But I suspect there's more to it than that.

As for the reverb, it seems to be working but has a great deal of static when turned up - kinda like radio white noise when between channels. There is also hum (60 cycle) when the reverb is turned up with nothing in V1, albeit quite a bit fainter.

But that's progress! Thank you, Sluckey!

[sluckey]

V1 (EF86): Plate voltage: 55v (kinda low)

I had noticed that earlier. Time to figure out why. The voltages listed on my schematic are what I measured on my working AC-15 lite. Measure voltages on all pins of your EF86 and compare with my voltages. Check your wiring and component values around the EF86. Shouldn't be too difficult to find.

Leave the reverb pot set to zero until the EF86 is fixed.

[shaun]

Okay, so here's a newbie question. I've studied some of your schematics for Hammond conversions to AC-15 and Marshall 18watt, plus the current AC-15R. I decided to include a 6-way tone switch in the reverb model. I presumed that the 0.1uF cap, that would normally go between Pin 6 and the tone switch, would no longer be needed because the 6-way switch is full of capacitors to act as couplers. At least, that's what I'm guessing is demonstrated in the A0-39 conversion to AC-15, which has the 6-way switch. So I did not include the 0.1uF coupling cap. Was that a mistake? I believe the 6-way switch does not automatically connect to the chassis through its mounting, but I've been wrong before:).

EF86 Readings are:
Pin 1: 88.2 vdc
Pin 3: 2.19 vdc
Pin 6: 59 vdc

When I move the tone switch through it's positions, it pops, even though it is a MBB switch.

Thanks again.

[sluckey]

When I move the tone switch through it's positions, it pops, even though it is a MBB switch.

The .1µF cap will probably fix that. Put it in.

[shaun]

I added a 0.1uF between pin 6 and the 6-way switch. It did nothing to diminish the hum, nor the popping from the switch. But now that there was a 0.1uF coupler in place, I jumped the 6-way switch to see if I'd screwed up the switch somehow. However, still no change.

The only differences I can see that I made around the EF86, compared to the schematic for the AC-15R, were to replace the brilliance switch with a 6-way, and I haven't added the 180pF bypass cap on the Vol pot yet. The humming increases as the volume is increased, and it is considerably louder than normal (normal for my builds, anyway:).

Here are a couple more pics that demonstrate my wiring in all its hideousness.

[shaun]

Well, I've tried rewiring everything using shielded cable and re-routing to avoid hum. I pulled the board off to check underneath - all good. I realize such a hum is often due to poor grounding, but I cannot find anything that's not according to the schematic.

The 220k plate resistor on V1 had moved up to 265, so replacing that raised the vdc at pin 6 to 85, which is a bit better. But I can't account for it not getting nearer to 100. I guess I could put a smaller resistor on there.

I hate to say it, especially as I'm not 100% sure of what I'm doing, but it seems vaguely possible that I might need to re-route the reverb to return to the usual place, a la the Fender Princeton reverb. Or the Vox Berkeley II Super Reverb - attached. Just to see what happens. They use similar blending styles, a 3.3M and 10ppF cap.

Anywho, I'm at a loss. An amp tech friend suggested that the 22k plate res on V3 was an error and should read 220k. But I doubt such an obvious error would exist for long in this company.

Here again, before I reconfigure the reverb entirely, are the Vs:

Node A: 355
B: 321
C: 273
D: 270

EL84 plates:  340 / 334

V4 Inverter plates:  209 / 205

V1 - pin 1: 91.5     pin 3: 2.3v      pin 6: 85v
V2: (combined) 316
V3 - pin 1: 189    pin 6: 240 (22k plate res)

Thanks, Mr. Luckey, for your help thus far. I appreciate the time you've put in on my behalf. Maybe we should have a Patreon link somewhere with your name on it:).

[sluckey]

Divide and conquer. I suggest you set the reverb pot to zero. Pull V2 and V3 out and lay them aside. Then concentrate on making V1, V4, V5, and V6 work properly. Only then add the reverb circuit back in.

This would be a simple task to troubleshoot and make it work if you had a decent layout. But what you have is one big cluster f#*k. It's impossible to troubleshoot that over the net. I wouldn't even want it on my bench. Maybe your amp tech friend would like to put his hands on it.

That 22K plate resistor on V3 was copied from the Matchless schematic. It should not be 220K. Matchless was purposefully keeping the gain of that first reverb recovery stage low to reduce noise.

By now you've seen examples of good workmanship. And there are several people on this forum that have built some nice amps in a Hammond AO-43 chassis. It takes some electronics knowledge, time, and effort to do that. If you should decide to rebuild this using a decent layout and put some thought into the project I'll be glad to help you get it going. But I'm through looking at this spaghetti. I don't have that much time left.   

[shaun]

I completely understand. I'm surprised and delighted at the generosity of spirit and knowledge that people share here and elsewhere on the internet.
And I know my work is dogsballs. I can barely stand to look at it myself. But I will follow your advise and take it one step at a time. And next time, I think I'll improve my style - I can't work like this any more.
Cheers.

[shaun]

Success! Boy, that's a great feeling when the amp works as hoped. Or at all:).

The ground hum was due to me grounding the reverb transformer at the same point as the V1&2 signals - doh. So I reworked my grounding scheme and shielded the signal cables, which also helped. It ain't perfect, but it's alive!

Thanks for your patience, Sluckey. I managed to produce something largely due to your help, and I couldn't be happier.

Here's a link to a test bench vid of the amp's first real try-out - reverb up the waazoo, but not demonstrated until about 2:25, if you want to jump over my noodling:

[sluckey]

Wow! You've made a lot of progress. Glad to see you have it sorted. Your brilliance selector switch pops a bit more than I remember, but that may simply be due to the volume setting. I would not worry about it. The reverb sounds fantastic. Def can do Dick Dale. I think I like it better than the Fender style reverb. Are you pleased with the reverb? Any issues?

What speaker are you using? I play mine through a Celestion Blue and that seems to work very well with an AC-15 amp.

I would love to see a full set of voltages for this amp since this is the very first. And also tell what PT and OT you are using (I'm assuming AO-43 iron?). I'll annotate the schematic for future reference.

Perseverance really pays off. Glad you did not become discouraged. Thanks for sharing this project.      

[shaun]

Thanks Sluckey. That's high praise indeed! I'll happily post a final set of voltages, and list all the technical details I can.
I'm very happy with the reverb, although it's the first one I've built, so my experience is limited.
For quick reference, I was running through my workbench speaker, a 16ohm alnico. In the final cabinet, it'll be a Heppner from the old Hammond - there were two speakers in it, one for bassier response with the usual corrugation and one that is more of a horn shape, probably for the mid to higher range. I thought I'd experiment to see what sounds best but will probably end up with the mid-range. And yes, the PT is from the A0-43. The OT is an 18watt from tubesandmore.

Thanks again!

[shaun]

Here are the voltages. And unfortunately there's a problem. The amp is behaving differently today, but the voltages may pinpoint the problem, especially the inputs on the EL84s; they read 20mV on pin 2 of V6, 59mV on pin2 of V5. With my meter connected to pin 2 of V6, I turned the reverb up and the mV reading jumped all over the place. Plus, I get bad static through the speaker. So there seems to be an issue with DC voltage getting to the inputs of V5&6.

I've checked and re-checked the wiring a million times. The signal from the plates of the PI run through .01uF caps, blocking any DC from that direction, and the Hi-cut doesn't have the opportunity to pick up any DC, so that leaves me with the reverb. But that runs through the .02 cap before it hits the PI. That's where my knowledge runs out. I suppose I may have a bad component somewhere.

Many thanks, as always.

Rec plates: 326 & 328
Node A: 352vdc
Node B: 315vdc
Node C: 259vdc
Node D: 255vdc  - I used a 10k 3w between nodes B&C - it was all I had.

EL84 plates: 340  &  340.    V5 pin2: 59mV    V6 pin 2: 20mV    pins 3 connection: 11.4   pins 9 connection: 314

PI plates: Pin 1: 174  Pin 6: 165     connection of pins 3&8: 86vdc

V1 (EF86): Plate voltage: 77v
V2 (12AX7): both plates connected, 315vdc.
V3 (12AX7): pin 1: 223   pin 6: 219.

[sluckey]

Verify that you have a good ground on the reverb pot. Pull the reverb recovery tube. Still got static when turning the reverb pot?

[shaun]

The reverb pot ground is good. When the recovery tube is removed, static disappears.

With the recovery tube in, when the reverb is turned up about half way, as well as static, it sort of flips into that typical oscillation sound, if such a thing exists:).

[sluckey]

With the recovery tube in, when the reverb is turned up about half way, as well as static, it sort of flips into that typical oscillation sound, if such a thing exists:).

Is this a recent development? Used to be good but now it's not?

Put a ground clip on the reverb return jack. Still got static?

[66Strat]

Please excuse my kibitzing, if it is unwanted. I had a similar situation in a 6G3 build where there was positive DC on the output tube grids. The amp was unstable and exhibited the same sorts of symptoms that you describe. It turned out to be a bad solder joint on the output tube grid resistors bias voltage connection. My suggestion would be to re-flow the solder joints and wick away any excess solder relative to the EL84 pin 2 connections.

[shaun]

Thanks for the input, 66strat. I think I found the problem, and as usually seems to be the case, it's a matter of me being a novice.

The problem was simply that I didn't have the reverb tank plugged in! Can you believe it? I've been waiting for a shielded cable to turn up in the post, and without it....well, I assumed the reverb would stay quiet even without the tank connected. I mean, now that I realize why the tank must be plugged in, it makes sense - those tubes have gotta push against something.

But all is well, and it sounds great. So from here on, upward and onward! Thank you Sluckey. Sorry for my slow learning curve.

[PRR]

> I didn't have the reverb tank plugged in!

Plug a cable into your guitar amp instrument input, but not a guitar. Except don't use a shielded guitar-cord, use an un-shielded speaker cable. It will pick-up all the buzz in the room.

A shorter unshielded cable inside the chassis may pick-up less buzz. But the reverb return is often more sensitive than the guitar input. So one way or another, it can be a lot of buzz.

And you may get lower buzz, even usable buzz, with a short shielded cable *with something plugged to it*. Guitar, tank. The idle source will suck-up some of the buzz than an un-shielded open cable picks up.

[shaun]

Still having problems, unfortunately. This issue has been there all along, but I'm only just figuring out what it is.

Everything seems to work fine, but as the reverb is turned up, a serious vibrating develops in the OT. I thought hum was poor grounding until I actually felt the OT and realized it was not a happy camper. Sometimes when I'm testing an unknown transformer with a variac for ratios, if I wire it up the wrong way then bring up the voltage, it'll vibrate, basically telling me to "back off, buddy!" And that's what this feels like.

As usual, I've checked the reverb choke and all transformer wiring, and it's all by the book. Same with all grounding runs. The amp sounds beautiful until the reverb is turned up. So it seems like the OT is fighting with the reverb choke. Or something. Maybe they're out of phase?

Any help appreciated.  Thanks.

[PRR]

> vibrating develops in the OT

What is the speaker cone doing?

[shaun]

It behaves normally. There's no noticeable woofer distortion going on, but I can hear some slight nasty distortion when the reverb is on, a sort of slight clipping on the bass end, so I was thinking the tubes might be overdriving. So I changed all sorts of values there, but then I realized it must something more fundamental for the OT to be vibrating so much.

[shaun]

Hi all. Through trial and error - mostly error:) - I seem to have most of the bugs worked out of this AC-15R. It's a great amp, so many thanks to Sluckey once again.

But there's a tweak or two needed that have me stumped. First, when the reverb is increased (surfin' safari to the max!), distortion occurs from the reverb unit itself, and then once it gets to about 8 on the dial from 1 to 10, it becomes a sort of white noise. Up until about 5 or 6, it runs okay. It almost seems like the return signal is too high. Funnily enough, when the foot switch is plugged in, it seems to ameliorate this issue about 70%.

2nd, there is a hum that increases with reverb increase. I have tried a wide variety of grounding schemes, adhering to best practices, but I have been unable to shake the hum when the reverb unit is turned up. The amp itself is fairly quiet, and the hum occurs even with the amp volume down. I've primarily used the footswitch jack as a ground point for much of the reverb tubes' grounding, and the footswitch ground is connected to the tank cable grounds, which are lifted off the chassis by isolated sockets. I ran the black cable from the reverb trannie to the amp's main ground point near the PT, as this seemed slightly quieter than anywhere else I tried it.

Anywho, I'm stumped once again. Many thanks in advance for your help.

[tubenit]

With the "normal" amount of reverb that you typically play with,  is hum a problem?

With respect, Tubenit

[shaun]

Actually, that's a very good point, tubenit!  . I woke up this morning thinking I'd simply put a 12at7 or ay7 in the recovery section and be happy with it.

I took a look at your one-tube reverb thread, and I really like what you have to say about getting all the reverb you need from it. I think I'm in that ballpark, too. I was just trying to, you know, figure out what I was actually doing .

Thanks!