Vox ac4 build question

[green drake]

Hi All,
This is my first amp, I was encouraged to do this after reading the thread “Vox AC4 question(s)” here.
It has been an awful lot of fun so far, I finished it a bit over a week ago and commissioned it last weekend. There were a few items that I caught and fixed, now it runs quietly; too quietly in fact, almost no sound comes out at all.
I would greatly appreciate any help!
I suspect the issue is with the EF86, here are the voltages with 5Y3, EF86 and EL84 installed:

C1 (RC) 324VDC
C2 (B+1) 266VDC
C5 (B+2) 259VDC

EF86
pin1 41.4VDC
pin3 0.4VDC
pin6 204.1VDC

EL84
pin3 8.4VDC
pin7 252VDC
pin9 266VDC

My power transformer is a Hammond 290AX and I’m using the 275-0-275 wires.
My output transformer is a Hammond 1760C and I’m using the 5000 Ohm primary and 16ohm secondary wiring.

I build the amp with a Fender style 2 inputs however I’ve ripped that out now and want to get it running properly with one before I re-install. 

Any help that you could provide would be most appreciated!

Thanks,
Brent

[AlNewman]

You would be dumping some signal with the fender input installed because you have the 2 grid leaks in parallel.  The  1M attached directly to the grid of the ef86 could be removed.

You should have more voltage at the cathode of your ef86.  Maybe you used a 15k resistor rather than a 1.5k?  Is the 5.5M resistor at your screen correct?  I would recheck all values and solder connections around that tube.  Not likely you have another ef86 to plug in and compare?

[AlNewman]

Also, your tone control is wired differently than the vox schematic.  The bottom leg shouldn't be grounded.

[kagliostro]

just to see in an easier way about what we are talking



Franco

[green drake]

Thanks for your reply!
I’ve removed the input shown, now it is a 68k resistor only, and a 1meg from pin 9 to ground.
I confirmed that the resistor is 1.49k ohm but it’s just 1/2 watt. The 5.6meg is R8 on the Vox schematic.
Thanks for catching the ground on the tone, the original schematic has the on/off switch in it, I’m using a regular switch and didn’t know what to do with the 3rd pin on the pot. I’ll fix that.
I’m going to buy another tube, they are kinda hard to find here so I think I’ll order a couple just to be safe.

You would be dumping some signal with the fender input installed because you have the 2 grid leaks in parallel.  The  1M attached directly to the grid of the ef86 could be removed.

You should have more voltage at the cathode of your ef86.  Maybe you used a 15k resistor rather than a 1.5k?  Is the 5.5M resistor at your screen correct?  I would recheck all values and solder connections around that tube.  Not likely you have another ef86 to plug in and compare?

[AlNewman]

I wouldn't order a $50 tube just yet, it could still be something simple.

The tone control as it's wired could dump a lot of signal, so I would fix that 1st.

I would also check heater voltage between pins 4 and 5 to make sure you're getting enough heat.

Your tube's conducting, just not where it should be.  There may be a work around.  There's others who would have better input than me.

Do you have pictures? 

[green drake]

Thanks.  Let me fix the tone control and tidy it up, then I’ll post some pics.

[sluckey]

Also, your tone control is wired differently than the vox schematic.  The bottom leg shouldn't be grounded.

It's perfectly fine to connect the bottom lug ***AND*** the wiper to ground. In fact, many people prefer doing that.

[AlNewman]

It's perfectly fine to connect the bottom lug ***AND*** the wiper to ground. In fact, many people prefer doing that.

I would have thought that it would create a parallel connection, reducing resistance to ground in most settings at the tone control.

So how about the screen resistor?  I'm not sure how the relationship works between screen and anode, but would elevating the screen voltage allow more current through the plate?

[sluckey]

It's perfectly fine to connect the bottom lug ***AND*** the wiper to ground. In fact, many people prefer doing that.

I would have thought that it would create a parallel connection, reducing resistance to ground in most settings at the tone control.

You simply jumper the wiper to the bottom lug. Operation is unchanged. But now if the wiper should fail you will have full pot resistance in circuit rather than an open circuit.

So how about the screen resistor?  I'm not sure how the relationship works between screen and anode, but would elevating the screen voltage allow more current through the plate?

I think replacing the EF86 should be the first step.

[green drake]

Thanks Sluckey, I’ll order one and tidy up my wiring tomorrow. I have to travel for work next week, the new tube should arrive by the time I get home

[sluckey]

Thanks Sluckey, I’ll order one and tidy up my wiring tomorrow.

Meanwhile, replace that 5.6M screen resistor with a 1M. Any joy?

The attached pic is my AC15 preamp and real voltages.

[green drake]

Tidying things up, I’ll replace that resistor tomorrow and post a photo.

[green drake]

Ok, so I re-wired the input as per the Vox AC4 schematic and changed the screen resistor to 1meg. Voltage at pin 3 of the EF86 increased to 1.3V. A bit more sound now (my pots are backwards but I’ll fix that later)
My thought is to drop the size of the screen resistor again, but does this point to a bad tube?
Here’s a pic, please note it’s my first time soldering, feedback is appreciated.
I put the wires into the holes of the turrets, I bent them to create friction, because it’s easier to change them if required. Should I wrap them around the turrets instead?

[AlNewman]

What are your voltages at pin 1 and pin 8 after changing the screen resistor?

You could try increasing the cathode resistor to get closer to spec.  I see Sluckey is using a 2.2k.

One thing I notice with your layout, it looks like your signal wires are in close proximity with your OT wires.  You may have luck moving things around and seeing if it has an effect on your volume level. 

(Correction, not pin 1 and pin 8, but pin 1 and pin 6.  Screen and plate.)

[sluckey]

I finished it a bit over a week ago and commissioned it last weekend. There were a few items that I caught and fixed, now it runs quietly; too quietly in fact, almost no sound comes out at all.

I'm trying to sort the order of what you said. Here's my interpretation. Please correct me if I'm wrong...

1.  You completed the build a bit over a week ago.
2.  Then you commissioned it last weekend. (I'm assuming it played fine.)
3.  Then you noticed a few items and fixed them.
4.  After fixing the items, the amp no longer makes much sound.

Is that the order in which things happened?

BTW, don't make the screen resistor any smaller. That amp should sound fine even with the original 5.6M. I still think you need to replace the EF86.
 

[dwinstonwood]

green drake, may I ask, which AC-4 is your amp based on? Is it the V-1-1 version I uploaded below, or a different one? Am I correct that you built yours without the Vibrato Oscillator?

Thanks,
David

[green drake]

They were within 1-2 volts as my previous measurements

What are your voltages at pin 1 and pin 8 after changing the screen resistor?

You could try increasing the cathode resistor to get closer to spec.  I see Sluckey is using a 2.2k.

One thing I notice with your layout, it looks like your signal wires are in close proximity with your OT wires.  You may have luck moving things around and seeing if it has an effect on your volume level. 

(Correction, not pin 1 and pin 8, but pin 1 and pin 6.  Screen and plate.)

[green drake]

Here's my interpretation. Please correct me if I'm wrong...

1.  You completed the build a bit over a week ago.
2.  Then you commissioned it last weekend. (I'm assuming it played fine.)
3.  Then you noticed a few items and fixed them.
4.  After fixing the items, the amp no longer makes much sound.

Is that the order in which things happened?

BTW, don't make the screen resistor any smaller. That amp should sound fine even with the original 5.6M. I still think you need to replace the EF86.

Not quite.
I finished it mid-week and commissioned it on the weekend. Pretty much no sound. I did some reading and then put it down for the week.
Decided to re-wire the input per the original schematic to remove a variable, re-measured voltages on Saturday and compared with some examples which led me to the EF86.
After tidying up my input wiring and changing the screen resistor to 1meg I got a bit more sound, but it’s still really quiet.

I think I will order the new tube, this one is an Electro-Harmonix, should I try a different brand?

[green drake]

green drake, may I ask, which AC-4 is your amp based on? Is it the V-1-1 version I uploaded below, or a different one? Am I correct that you built yours without the Vibrato Oscillator?

Thanks,
David

That’s the one, and yes without the vibrato. Similar reasons as you, I had been looking at a Champ or 5E3 but wanted something a bit different.

[sluckey]

I finished it mid-week and commissioned it on the weekend. Pretty much no sound.

OK, so this amp has never worked right? The most common reasons for faulty new builds are wiring errors and incorrect component values.

What do you mean by "commissioned"? To me that means a piece of equipment has been thoroughly checked out and found to be working as advertised and is put into service, ie, commissioned into service.

I noticed your speaker jack is located very close to the EF86. That's a terrible idea!

[AlNewman]

They were within 1-2 volts as my previous measurements

Well, that's cool, but initially you were running .25 mA at the cathode, and now you are at .86 mA.  What type of meter are you using?

[green drake]

I finished it mid-week and commissioned it on the weekend. Pretty much no sound.

OK, so this amp has never worked right? The most common reasons for faulty new builds are wiring errors and incorrect component values.

What do you mean by "commissioned"? To me that means a piece of equipment has been thoroughly checked out and found to be working as advertised and is put into service, ie, commissioned into service.

I noticed your speaker jack is located very close to the EF86. That's a terrible idea!

Ah I mis-used the word. I meant I checked the wiring and component values and powered on, voltages to the filaments and rectifier were good, fuses didn’t blow.
I wondered about the speaker jack location… I bought an enclosure that was smaller than it should have been. I mis-calculated the space the power transformer would require. Once I can find all of my mistakes I will re-build into a larger chassis

[green drake]

Well, that's cool, but initially you were running .25 mA at the cathode, and now you are at .86 mA.  What type of meter are you using?

I bought it on Amazon, AstralAI digital multimeter

[green drake]

Hi All,
My apologies for disappearing, work, family and now school have been occupying most of my time.

Following the advice I received, I ordered a pair of EF86’s, alas no luck. I decided that between my output jack over top of my EF86 and the small chassis making it very difficult to troubleshoot this attempt required a do-over.
I bought a 10x16 chassis and revised my layout. I also salvaged some scrap glastic sheets from work to make my turret board. It was much easier this time with the extra space.
I’m happy to report success!!  I have sound!  I made a few tweaks to eliminate some noise and now it’s very quiet, a minor hum starts ~9 o’clock but not bad at all.
I’m going to play it a bit, then I’ll make as-built drawings and record measurements.
Thank-you all for your help, I greatly appreciate it!

[tubeswell]

can you write all your voltages on this schematic (assuming you haven't changed any component values?)?

[green drake]

can you write all your voltages on this schematic (assuming you haven't changed any component values?)?

Mines a bit different as I didn’t include the tremolo. When I complete the as-built schematic I’ll add tables for measurements. But work came up again and my day off tomorrow isn’t :-(