Vox AC4 Question(s)

[sluckey]

Quit torturing yourself. You have the schematic. Just build it.

[dwinstonwood]

Quit torturing yourself. You have the schematic. Just build it.

Aye aye!

It is now brilliantly clear why people solder their pots in external jigs.

[dwinstonwood]

Making progress. The Hoffman turret board was a pleasure to solder up.
Next session will be connecting everything up (the right way).

[dwinstonwood]

I have one wire left to hook up.

I'm stumped. 

Both of my OPT wires are connected to the EL84 - pins 9 and 7. So... how do you all connect your B+ to the OPT?

I feel like it might be so obvious that I can't see it. Can the wire from my second filter cap go straight to the tube? If so, which of the two pins?

Help and thanks!

[sluckey]

Connect the B+ wire to pin 9 exactly as shown on the schematic.

[dwinstonwood]

Thank you sluckey!

It doesn't look like it, but, I am getting somewhat better at reading schematics. Slowly. Studying an old copy of Weber's desktop reference.

OK, I lied. I have one other wire left. I didn't have any 100K resistors (could swear I ordered some). Always one part short.

[sluckey]

OK, I lied. I have one other wire left.

The schematic can solve that puzzle too. 

[dwinstonwood]

I meant I was going to solder the 100K resistor to the EF86's pin 9, and solder the input wire to that.

One more shot of the wiring. It's not art, but it's not too cluttered, either.

[dwinstonwood]

Dang! I left off one connect! I just noticed it in the photo.

[sluckey]

Dang! I left off one connect! I just noticed it in the photo.

Better look closely. I see at least three things on the board that will prevent this amp from working.

[dwinstonwood]

Thank you. Some wire are under the board:

1.) I connected the el84 cathode resistor to the cap (that's what I noticed earlier).

2.) the el84 cathode cap and resistor are connected to the first two filter caps' ground side by a wire under the board.

3.) those four components are connected to chassis ground by a wire under the board.

4.) the third filter cap b+ (for preamp) is connected to the junction of the two angled resistors by an under-board wire.

5.) there's also an under-board wire connecting the .1u coupling cap to the .001 cap connected to the tone pot.

I have it wired exactly like this revised layout.

David

[sluckey]

Some wire are under the board:

That covers the issues I saw.

[PRR]

>>     Some wire are under the board:
> That covers the issues I saw.

We often see (or don't-see) underboard troubles.

Maybe we want clear board-stuff?

[sluckey]

Maybe we want clear board-stuff?

Totally agree. This board is so simple I would have put every jumper wire on top in full view.

[dwinstonwood]

Thanks PRR and Sluckey!

That's a very easy mod to do. I'll make the changes while I'm waiting on the last of the parts.

Looking at the (almost) completed chassis I'm amazed that this project started by studying an 8" x 5" reprint of a 60 yr-old schematic in a book I dug out of the closet.

I definitely came to the right place to attempt this. DIY communities are incredible.

[dwinstonwood]

Moving the wires didn't take long. As you all said, everything can be seen, and worked on without having to remove the board. The one thing you can't see is the chassis ground which is beneath the upper-right corner of the board.

The only thing I know of that's missing is the 100K resistor on pin 9 of the EF86. It's in the mail.

Thanks

[kagliostro]

My 2c


I'll put the wires  under the board and draw connections on the top of the board as to see what Is connected to what


Franco

[dwinstonwood]

Thanks Franco. I wouldn't have thought of that.

One of many things I'll do differently on the next build will be to orient the tube sockets to make wiring easier. I just lined them all up straight. But, now I realize why rotating them 45 degrees - as I've seen - makes sense.

[sluckey]

The only thing I know of that's missing is the 100K resistor on pin 9 of the EF86. It's in the mail.

100K is not a magical tone component nor is the value critical. I would use anything between 33K and 100K. I used 33K on all my EF86 preamps. Vox used 68K, 100K, and even 220K on the various AC-XX models.

[kagliostro]

I wouldn't have thought of that

Here on the forum someone dit it in a much better way (Using a CNC - I don't remember who was)

On that layout there were no hidden connections, but the principle is the same

draw components like in many PCBs, so, why not the hidden connections ?








Franco

[dwinstonwood]

Cool Franco, that's like a high-end kit. What happens when you laser cut turret board material? Would it just burn or melt it? I have access to a laser cutter/engraver at work.

Thanks sluckey. The way I understand it, the grid stopper functions with the tube capacitance to create a low-pass filter. I guess what's important is cutting off the higher frequencies that can cause oscillation?

I tried to look up the capacitance of the EF86, but found numbers like 3.8pf and 5.1pf. I thought tubes had a capacitance of around 100-ish. Otherwise, I was going to use one of those online filter calculators to find out my frequency cutoff point.

[kagliostro]

... I have access to a laser cutter/engraver at work. ...

That job was done with a milling CNC router, I've no idea what will happen using a laser engraver on Boards materials

--

... I guess what's important is cutting off the higher frequencies that can cause oscillation? ...

To avoid oscillation and to avoid to receive Radio Frequency signals like if your amp is a radio receiver

Franco

[dwinstonwood]

It's time for our history lesson. More than you care to know about the JMI AC4:

[dwinstonwood]

sluckey, so this is what I ended up with, which is what you suggested from the outset!

[dwinstonwood]

*First Power-Up - No Tubes*

Pin 1 of EZ81 socket - 266.2V AC
Pin 7 of EZ81 socket - 266.4V AC

Pilot light glows.

Pin 4 of EF86 (far end of the filament wiring) - 3.376V AC
Pin 5 of EF86 (far end of the filament wiring) - 3.377V AC

Am I ready to install the rectifier and charge the filter caps? I mean, those voltages look good to me, but what do I know?

I do not own a variac. I've read that I should use a low-wattage bulb in my dim-bulb-tester for the initial charging of the filter caps. Is that wise?

Thanks.

[Williamblake]

Do you have an alternative cabinet yet?

[dwinstonwood]

Do you have an alternative cabinet yet?

I'm still going to use the Frontman Reverb cabinet for now. I did buy a new speaker and some grill cloth. I'm making a lauan baffle. If this amp ends up sounding good I will most likely add the vibrato circuit and invest in a worthy cabinet. This all started as a "low-budget-stay-at-home" kind of project.

[Williamblake]

If you want to check again read resistance to ground in the high voltage path and get several hundred kOhms anywhere and fire her up.

[dwinstonwood]

Than ks Williamblake, but can you elaborate a bit? I should take an Ohm reading from pin 1 and from pin 7 to ground and look for extremely high resistance? That's what I want to see?
Thanks.

[sluckey]

Drain the filter caps and then measure the resistance across each filter cap. Do not want any low resistance readings.

[dwinstonwood]

I have not powered up with the rectifier installed yet. It's too late to get into it tonight. I'll test in the morning.

However, when I try to measure resistance across the filter caps my meter goes to O.F, overflow, so the resistance is higher that what my meter can read; when switched to continuity they read "Open." That must mean they are functioning like capacitors, yes?

[dwinstonwood]

*Second Power-Up - No Tubes*

Installed the EZ81:

329.1V DC at first filter cap.
327.8V DC at second filter cap.

Upon power up the dim bulb lit up and then immediately dimmed.

[sluckey]

Plug it straight into the wall and get your guitar. Time to play.

[dwinstonwood]

*Third Power-Up - All Tubes and Speaker - Weird*

OK, first the good. No smoke, no smells, no humming or noises.

The weird (bad?). Pins 7 and 9 of the EL84 initially rise up to about 258V and then gradually fall and settle at around 135V.

Pin 6 of the EF86 initially rises up to about 241V and then gradually falls and settles at about 112V.

But... I suspect that I destroyed my EF86. Being nervous, I accidentally installed it in the rectifier socket for the first power up. Nothing smoked or popped, but do you think I destroyed it? Maybe something shorted and its not passing current. 

[sluckey]

Did you plug the amp straight into the wall?

[66Strat]

Pins one and seven of the ez81 tube socket are the anode connections. Pins one and seven of the ef86 tube socket are respectively the screen grid and internal screen connections. Pins two and seven of the ef86 socket are connected to ground. By putting the ef86 in the ez81 socket, AC from the power transformer was shunted to ground through the pin seven connection.

[dwinstonwood]

Yes sluckey

And it's AWESOME! 

The part you won't believe: with the volume dimed and my guitar wide open all I can hear is a very, very low level hum; the transformers, I guess. I have to put my ear next to the amp to hear it.

Volume and tone knobs work great. With the amp cranked all the way I get nice, smooth distortion. At half volume it is still clear and clean. With tone at noon it is warm and full sounding. It has a very usable range, and is not thin or too bright. I get good bass considering the 8" speaker. All frequencies seem to be coming through.

I honestly think it sounds great. I need some time to come back down! My heart rate was pretty high.

I'll post again to personally thank everyone here who contributed. But, a big thanks to you sluckey.

[dwinstonwood]

Of course, all is not perfect. The tone pot works backwards, and it's a bit scratchy. I noticed it makes a rubbing sound when turned (with the amp off). I hope that's not a symptom of a bigger problem, elsewhere.

[sluckey]

The tone pot works backwards

Easy fix. Refer to your most recent layout drawing. Remove all wires from the pot, including jumper between lugs 1 and 2. Now reconnect ground wire to lug 2 only (no jumper needed). And reconnect yellow wire to lug 1. That's all.

[dwinstonwood]

Fixed. Thanks!

[shooter]

it makes a rubbing sound when turned (with the amp off).

probably knob binding on chassis

[dwinstonwood]

it makes a rubbing sound when turned (with the amp off).

probably knob binding on chassis

Thanks shooter, it actually does this with the knob out of the chassis. It makes a squeaking sound. I wonder if some liquid flux ran down onto the wiper. I can give DeoxIT a try.

[dwinstonwood]

I've been playing the amp for about an hour and it seems stable, nothing has changed in the way it operates. In fact, the transformers are at like room temperature to the touch. The tubes have a nice orange glow that looks normal.

What's still bugging me is what I was doing wrong when I took voltage readings this morning. Why would the B+ voltages rise up and then fall back down? Maybe I was measuring at the wrong points in the amp. I don't know.  But, the amp is clearly operating fine. It's plenty loud, and it doesn't overdrive until I crank it.

Anyway, I would like to take cathode bias measurements and calculate plate dissipation, etc.

I'll try measuring again in the morning, because this is going to bug me. Any ideas are welcome.

[sluckey]

Let the amp warm up for 5 minutes.

Measure voltage on the positive end of each of the 3 filter caps on board. Black probe on chassis, red probe on the caps. What have you?

Now measure voltage on V2 pins 7 and 3. Black probe on chassis, red probe directly on socket pins. What have you?

[dwinstonwood]

thanks again sluckey (for your patience). I didn't let the amp warm up this morning. I'll report back tomorrow.

[dwinstonwood]

OK, I took some measurements after the amp warmed up.

Assuming my math is correct, dissipation is a bit high for the EL84 at 12.231 watts. The tube is rated for 12 watts.

First, the filter cap voltages:
1 - 304.5V
2 - 252.2V
3 - 233.6V

Now, the EL84:

EL84 cathode pin 3 - 7.77V
EL84 plate pin 7 - 243.9V
Current draw - 51.8MA
Dissipation - 12.231 watts

If I change the 150 Ohm cathode resistor for a 180 Ohm, that will give me 10.19 watts. That's about 85% or so.
A 220 Ohm would drop me all the way down to 8.34 watts.

What do you suggest? Leave it running over 100% or bring it down some?

Thanks,
David

[sluckey]

I'd leave it alone. 84s are cheap and plentiful.

[dwinstonwood]

Except mine's a NOS Mullard...

Haha, just kidding. It's the new Russian Mullard.

Staying with the budget mentality, I've installed a $30 Celestion. Still waiting on the grill cloth.

[PRR]

> EL84 cathode pin 3 - 7.77V
> EL84 plate pin 7 - 243.9V
> Current draw - 51.8MA
> Dissipation - 12.231 watts

At least 10% of EL84 cathode current goes to screen, not plate. I get 11.1 watts of plate dissipation.

[dwinstonwood]

Thanks PRR, you just made my amp "perfect."