A really big project, Vox AC30/4 clone

[pompeiisneaks]

Video 10 is up:

https://www.youtube.com/watch?v=aBPxKUpY1ds&feature=youtu.be

~Phil

[pompeiisneaks]

Video 11:

https://www.youtube.com/watch?v=kB9Me3xyRis

[pompeiisneaks]

and now video 12, I cover adding the wires and such:

https://www.youtube.com/watch?v=Pq0RDzUTGUw

[pompeiisneaks]

Okay I did the first power up, normal channel sounds perfect.  Trem/vib channel sounds fine, but no trem/vib at all.  I've tried variations of things, I jumpered the footswitch out closed to see if it needed to be grounded to work (It didn't help).  I've tried all variations of the pots to no avail.  The switch on the front when switched on seems to lower the volume a tad, but htat's it.  Here are my voltages from the tubes related to the trem/vib channel (with sluckey's expected voltages from his ac15 schematic in parenthesis after):

V1: (input stage)
1: 149.2V (147V)
2: 0V
3: 1.2V (1.2V)
6: 212V (212V)
7: 10.5V
8: 20V (17.5V)

V4 (modulator 12AU7)
1: 86V (71V) a bit high?
2: 0V
3: 3.5V (3.2V)
6:85.9V (71V) again high
7: 0V
8: 3.4V

V6 (1-3 oscillator, 6-8 PI)
1: 138V (144V) a bit low
2: 0V
3: 1.3V (1.2V)
6: 208V (208V)
7: 15.3V
8: 24.9V (23V)

I've also got pictures of the finished build in my google photo album here: https://goo.gl/photos/8pshWArzTRsh25WW8

The first two are the related ones.  I have gone over the jumpers, (Found a missing one, but that was before the tremolo worked at all)  I've gone over every resistor and cap in the amp twice, I just don't see what may be missing.  Anyone spot anything odd? 

Does the footswtich need to be a switched jack to ground to bypass it?  (I'm not sure if grounding that point enables the trem/vib or disables it).  Does the voltage off by about 10V mean something's off with that tube? 

I had a scope on it and could see some oscillation that looked right in one half of the PI at some point, I don't recall exactly, but then nothing later... It seems like it's being swallowed somewhere.  The unaltered signal is coming through fine. 

No serious noise that I can tell, a minor hum in the trem/vib circuit that doesn't exist in the normal channel.

~Phil

[sluckey]

The footswitch must be grounded to ENABLE the oscillator. No ground equals no vib/trem. I used a Switchcraft 12A jack with the switch lug connected to the sleeve lug so my trem/vib works without a footswitch.

V6 pin 1 will be bouncing all over the place when the oscillator is working. This just looks like an erratic reading on a digital meter. Your plate voltage looks like the oscillator is not working.

V6 also needs to be a strong 12AX7.

[pompeiisneaks]

Okay, I've jumpered that to ground for now until I can get a 12A, I'm using the 11 type.  is the oscillator always suposed to bounce around?  I'm getting rock solid 138 v, swapped tubes with the main input tube and get 140 instead, but still not moving. 

One other thing of note is on the regular channel it is very quiet with only a slight buzz as it gets to loud volumes, but the vib/trem channel has a heavy hum almost like transformer hum in it.  even at low volumes.  I just went over all the jumpers and resistors in that part of the circuit again and it's fine.  How can I trace the signal path here?  Not sure where it should go from the first V1 exactly, but I see it doing great after the second half of that tube and then levels drop pretty low by the time they get back to the shift network.  (it's like 25V after the first tube, but then drops down to sub 5 volts I'd say at the shift network.  (100mV inputs sine with this. )

~Phil

[sluckey]

Okay, I've jumpered that to ground for now until I can get a 12A, I'm using the 11 type.  is the oscillator always suposed to bounce around?  I'm getting rock solid 138 v, swapped tubes with the main input tube and get 140 instead, but still not moving.

Rock solid means it ain't working. It will be jumping around as much as 50v or more.

Keep looking for a wiring error or wrong component. Lot of jumpers on top and bottom. Verify they are in the correct place. Use your meter to check for zero ohms resistance between turrets that should have a jumper. I hope it's not an under board jumper. I didn't like the way you did those.

Signal flow is V1 to V4 then on to volume control.

[pompeiisneaks]

I'll give that a go again, I'd checked all the jumpers again in that area after I missed the one.  I didn't find any wrong, I retested the under board ones as well, they're right so far as I can tell.  I'll go over it again.  I did test the input path all the way to the V4, and at the inputs there it's decent still, about 25V with the sine, but I see nothing at the oscillator side. 

I'll report back after I mess with it some more.

[pompeiisneaks]

Signal flow is V1 to V4 then on to volume control.

Oh and that part I know works, because I get clean signal out just fine, very loud etc, just no wobbly bobbles.  :D.

It seems to me like the modulator is fine, but somehow the oscillator and PI for that separate part isn't doing anything. 

maybe I didn't ask it right, but should the oscillator be oscillating if there's no signal getting to it, or will it only oscillate the signal coming in?

[G._Hoffman]

Signal flow is V1 to V4 then on to volume control.

maybe I didn't ask it right, but should the oscillator be oscillating if there's no signal getting to it, or will it only oscillate the signal coming in?

The oscillator should always be oscillating.  If it's not, you definitely have a problem in the oscillator. 


Gabriel

[sluckey]

maybe I didn't ask it right, but should the oscillator be oscillating if there's no signal getting to it, or will it only oscillate the signal coming in?

The oscillator is a signal generator. It puts out a low frequency sine wave (approx. 3Hz to 10Hz). It does not have a signal coming in. It requires a ground at the footswitch jack to be enabled. This low frequency sine wave is many volts and can be viewed with an o'scope. Or you can measure the AC RMS volts with a good Fluke meter.

As long as V6-1 is a steady voltage THE OSCILLATOR AIN'T WORKING! You must have a ground on the FS jack ***AND*** a strong 12AX7. If it's still not working, there must be a wiring error or wrong value component associated with the oscillator circuit (V6 pins 1,2,3). The problem is in the attached pic...

[pompeiisneaks]

Sluckey,

Excellent, thanks, that makes sense.  (Just never really understood that part of the circuit, now I think I do, or I hope I do).  I have a scope, I fixed the missing jumper issue by tracing the input path from V1 to V4 and found where it 'disappeared' and realized at that point what was wrong.  I'll double/triple check all of those components again and figure out what's what. 

I do have a strong tube, I've been using it in the other AC30, and they're pretty new, only about a year old. 
I did try jumpering the jack to ground, I just clipped an alligator lead from the tip to ground of the jack and ensured I got ground.  It is possible something else is wrong. (does the vib/trem on/off switch do the same thing?  It seem to get a sound volume drop when engaged, but nothing else, but not sure if that's related at all.)

I'll tinker with it tonight when I get home.

~Phil

[sluckey]

It is possible something else is wrong. (does the vib/trem on/off switch do the same thing?

Of course there can always be something else wrong.

But the problem you have described will be fixed by concentrating on the pic I posted.

The vib/trem switch is not an on/off switch. One position gives you a pitch shifting vibrato effect. The other position gives you a simple amplitude modulated tremolo effect. Neither effect will work until you fix the oscillator.

Quit jumping around. Focus on the pic I posted.

[pompeiisneaks]

lol I know, I tend to go on brain escapades when stuck at work and I can't do what I need to yet :)  I'll definitely do that exactly.  I've been looking over the pics I have (the last ones above are kinda crap, but earlier ones I had in the same album) and don't see anything wrong.  I know I tested the resistors, they're all right, and I can see that from the pics.  I can actually see that two of the three caps are .01, but I can't see the third from the angle, but man am I definitely confused as to what could be wrong.  I may try another tube, as well as just try jumpering the 'footswitch' connection turret directly to ground as well.  At any rate, I knw what to tinker with when I get home.  :)

~Phil

[pompeiisneaks]

I think I just found it!  I was not looking closely enough, but I missed a jumper.  See attached pic.  (I'll test when I get home).  I was thinking there was one here, but there isn't that's the wire going off board that's overlapping a bit but it doesn't touch. 

[sluckey]

I was thinking there was one here, but there isn't that's the wire going off board that's overlapping a bit but it doesn't touch.

No jumper there would definitely kill the oscillator. I hope you're right. I had looked at that and decided it was a jumper. Fingers crossed for you!

PS... Ever see this pic before?

[pompeiisneaks]

Yeah in the higher res photos in that album, if you zoom in really close it's still pretty clear it's not.  I'm crossing my fingers too :)

~Phil

[pompeiisneaks]

I was thinking there was one here, but there isn't that's the wire going off board that's overlapping a bit but it doesn't touch.

No jumper there would definitely kill the oscillator. I hope you're right. I had looked at that and decided it was a jumper. Fingers crossed for you!

PS... Ever see this pic before?

Heh yup, I need to be better about that, I do a similar 'exercise' but don't cross them off like that, which is where I failed here.  I keep thinking I"m being fastidious enough, but I've proved myself wrong every time.  I just need to assume the mantra 'something is wrong' and do that.

~Phil

[pompeiisneaks]

BOOM!  That didn't 'solve' the problem, but I checked all around, and there was another jumper in place, but that I'd forgotten to solder, that somehow wasn't connecting at all (Not sure how, I couldn't crimp it tighter, it seemed like it had a good physical connection)  Once I added that jumper, and soldered the other jumper, I got it.  The tremolo/vibrato came in. 

On the other hand it does seem a bit noisy, and a bit muddy.  I'm not sure if I've got something else amiss, or if there is a specific tube that may need to be better quality etc, but it is working.  (I still hear that hum in that channel only, nothing at all in the normal EF86 channel).

I'll have videos of the last couple phases coming, including playing it.  SOON :)  I have to edit them and put them up etc.  (most of my videos come up a week or so after the work was done due to this.

~Phil

[sluckey]

Now that the oscillator is working, check the plate voltage at V6-1. Should be jumping around. If you have a good dmm such as a Fluke, measure the AC RMS voltage sine wave at the ungrounded end of the depth control. Maybe write that value on the schematic for future reference. I like to set the speed pot at 5Hz for these reference measurments.

The cut control is the only tone control that affects the vib channel.

How is the power amp doing?

[pompeiisneaks]

The tone issues I had were my dumb mistake, I didn't realize I had hte pickup switch in position 4 between mid and bridge out of phase so its muddy on it's own. 

I have a nice fluke with AC and hertz. 
At the anode itself:
Slow: 3.5 -7.1Hz
AC: 19 - 40V
Fast: 4.7 - 9.8Hz
AC: 32 - 41V

and the AC RMS voltage at the depth control at 5Hz
9-12V at one end of the pot
12-15V at the other.

Power amp sounds great, I need to double check the bias, but It was a bit low, as I recall, was aroun 28mA, but I need to get an exact measurement on the resistor, (measured 14.2V at the cathode resistor, 14.2/130 /4 tubes... seems a bit low, I think el84 can go up to what, like 40mA if I recall?  (I'll double check).

Sounds great though right now.

9-15V

[pompeiisneaks]

Just looked, seems 48mA is max so at 70% that's 33.6, so if I'm at 27, I could go up a little, but not sure I need to, I do like the tone now.  If I go higher, I'll get a bit more 'crunch' right?  I seem to recall reading that the JMI ones would often be very hot but chew up tubes, like 40 to 42mA?

~Phil

[sluckey]

I'd leave the bias as is for now and just play it and get familiar with it. When you get tired of playing it, then record all tube pin voltages, power supply nodes and the tremolo voltage. Put them all on your Visio schematic. Never know when you may need them.

4 EL84 tubes with a 50Ω common cathode resistor will be very hot. Short tube life. The original Vox cab compounded the heat problem because of poor air circulation.

You may notice that the trem effect is much stronger than the vib effect. Just the way it is. You'll probably like playing with both channels jumped.

Oh yeah, I have one undocumented mod to mine... I replaced the cathode resistor/cap for V6-3 with one of Doug's LED lamp assemblies mounted on the front panel. Never have to guess if the oscillator is working. It will also significantly increase the amplitude of the oscillator signal.

[pompeiisneaks]

Excellent, I may have to do that too.  This thing sure sounds great.  I played it again for a while tonight, but it's getting late.  I'll definitely let it burn in and give it a whirl.  It also seems like the sound is cleaning up a bit now that it's been running for a while, not sure, (I also think my house has horridly dirty power). 

[pompeiisneaks]

Next vid, going over more of the wiring hookup, shorter one, but getting there (as you know I'm done, but trying to keep the entire build videos together in a semi sequential series)

https://www.youtube.com/watch?v=JSwsRZGcmpY

[pompeiisneaks]

Video 15:

https://www.youtube.com/watch?v=6_lEHQx7PRM

~Phil

[pompeiisneaks]

Video 16:

https://www.youtube.com/watch?v=d22Zf_zZj-o&feature=youtu.be

[pompeiisneaks]

Video 17, a few messes made, but recovery was somewhat manageable :P

 https://www.youtube.com/watch?v=4ve8fmNN_Gc

[pompeiisneaks]

Video 18, In this one, I finish through to the last tube socket and the next should finish the entire chassis. 

https://www.youtube.com/watch?v=bQgIdZEKmjk

[pompeiisneaks]

Video 19, I get all the pots done too. 

https://www.youtube.com/watch?v=XVdiD7JJdw8

~Phil

[pompeiisneaks]

Video 21, I finish up the grounding, and it's pretty much ready to power on, next video!

https://www.youtube.com/watch?v=YTh8vci_coA

~Phil

[roseblood11]

Here you go.  I'm not sure if the layout will be all that much help, because the PDF printed out with very thin lines so it is kinda hard to see the colors, but with the schematic it should get you there.  The big thing you'll notice is I added a bunch of extra sections to the power supply, and I distributed the power supply through out the amp.  If you have any questions, feel free to ask.  And for what it's worth, the little bit I've heard the amp it sounds great, and is the quietest amp I've made.


Gabriel

That looks like a very good version, better than any combination of channels that Vox has ever released...
Maybe it would be even more flexible, if the triode would be used to boost the EF86 instead of waisting it in the TB channel. I found this in another thread:
http://i.imgur.com/7L7s7hX.jpg
(Gridstoppers etc are missing, off course...)

Would you mind to share a bit more info about your amp, including some pictures? Esp. some explanation of the mods in the power supply (filtering...) would be helpful. And would you share your original schematic- and layout files? I'm planning a very similar build, but maybe with some changes...

regards from germany, Immo

[pompeiisneaks]

Are you asking Gabriel or Me?  mine are all at the first post of the thread if you want them.

~Phil

[roseblood11]

Gabriel!

But I really like your amp as well, I watched all the videos via the Vox facebook group... Maybe I'll use that layout to rebuild my chinese  AC15H1TV.

[pompeiisneaks]

Well, here's video 21, with first power up and some of the troubleshooting shown here:
https://www.youtube.com/watch?v=TQUUurAKBOA

[G._Hoffman]

Here you go.  I'm not sure if the layout will be all that much help, because the PDF printed out with very thin lines so it is kinda hard to see the colors, but with the schematic it should get you there.  The big thing you'll notice is I added a bunch of extra sections to the power supply, and I distributed the power supply through out the amp.  If you have any questions, feel free to ask.  And for what it's worth, the little bit I've heard the amp it sounds great, and is the quietest amp I've made.


Gabriel

That looks like a very good version, better than any combination of channels that Vox has ever released...
Maybe it would be even more flexible, if the triode would be used to boost the EF86 instead of waisting it in the TB channel. I found this in another thread:
http://i.imgur.com/7L7s7hX.jpg
(Gridstoppers etc are missing, off course...)

Would you mind to share a bit more info about your amp, including some pictures? Esp. some explanation of the mods in the power supply (filtering...) would be helpful. And would you share your original schematic- and layout files? I'm planning a very similar build, but maybe with some changes...

regards from germany, Immo

I split up some of the of the preamp power sections.  It's pretty easy, in the AC30, since the power sections are in parallel, instead of in series.  I think I just added one extra section, so the bright and normal inputs aren't sharing the same power resistor and cap.  Double the value of the resistor, and all's good.  (You could use a smaller cap, but why?)  Then, instead of putting them all in the power amp chassis, I put them right next to the rest of the circuitry they are driving.  It's just about trying to decouple things a bit more.


Gabriel

[hesamadman]

These are great videos. I was curious what you were using to capture such clear audio?

[pompeiisneaks]

I have a azden mic that fits on a camera hot shoe that's on my camera that I've used for a long time because it works so well.  I believe it is this one:

https://smile.amazon.com/Azden-SGM-990-Shotgun-Microphone-Black/dp/B00CRMB79C/ref=sr_1_1?ie=UTF8&qid=1487140295&sr=8-1&keywords=azden+dslr+microphone

[hesamadman]

Thanks. I'll check that out.

[roseblood11]

Phil, we're all waiting for video 22! ;-)

@Gabriel: Thanks for explanation! As I'm planning a normal channel with an ef86 driven by a triode, decoupling of the power supply doesn't work for me anyway. I think I'll use the weber layout and try to put in the ef86. Which might be difficult, because I have a pre-drilled chassis...

[G._Hoffman]

@Gabriel: Thanks for explanation! As I'm planning a normal channel with an ef86 driven by a triode, decoupling of the power supply doesn't work for me anyway. I think I'll use the weber layout and try to put in the ef86.

I can't think of any reason you couldn't add a couple extra PS sections.   It's one extra resistor, and one extra cap per section, so pretty easy to shoehorn in.

Which might be difficult, because I have a pre-drilled chassis...

Greenlee punches are wonderful things.  Expensive, but wonderful.


Gabriel

[roseblood11]

I can't think of any reason you couldn't add a couple extra PS sections.   It's one extra resistor, and one extra cap per section, so pretty easy to shoehorn in.

Sure, but it wouldn't make much sense, because I'll use one triode of the first tube for the top boost and the other for the normal channel anyway. We'll see...

Greenlee punches are wonderful things.  Expensive, but wonderful.

The hole is not the problem... I'll just have to find a good place for the EF86. There's no space to put it in a row with the other preamp tubes.

[pompeiisneaks]

Here's the final video, the demo! Let me know what you guys think.  (I always post here for the gang first :D)

https://www.youtube.com/watch?v=G8E-oJm_8vc

~Phil

[roseblood11]

Several of the members have built this 1960 AC-15...

     http://sluckeyamps.com/VAC15/ac15.htm

There's a very legible schematic at the bottom of the page.

Sluckey, which software do you use to draw your layouts?

[pompeiisneaks]

I know this one, because I've asked the same question, Visio.  I ended up buying a version so I can too.  I live in Redmond WA though and have friends that work at the evil empire (Microsoft that is) so it was quite cheap for me, vs normal price :)

~Phil