'62 JMI VOX AC10 TWIN-- Any experts out there?

[ndross475]

I recently acquired what I believe is a 1962 JMI Vox AC10 Twin.  I am attaching a link to a gallery of images and videos I have taken.... I found a couple of schematics online but there isn't a whole lot out there on this one. Anyone have one? Or have experience with one?  I replaced the e caps as the originals were leaky and suspect and rigged a temp fuse, powered it up on the variac and she does live, there are issues that need to be sorted but at least now I know the trannys seem to be ok... and she breaths...

There was a resistor I couldn't find on the schematic, and I would like to have get confirmation on the OT wiring. I would love to hear from anyone with some insight...  Once I get further down the road I will start the debate on Blue Pups vs Golds ;-) First things First....

https://njdimages.smugmug.com/VOX-AC10/

[ndross475]

https://njdimages.smugmug.com/organize/VOXAC10

Separated the videos from the images... easier to navigate...

[sluckey]

I believe this is the correct schematic...

     https://el34world.com/charts/Schematics/files/Vox/Vox_ac10_2.pdf

Here's another that is probably correct also but it's hard on the eyes...

     https://el34world.com/charts/Schematics/files/Vox/Vox_ac10.pdf

[ndross475]

I have this one also... at first glance I haven't found the differences...

[ndross475]

My first concern is if anyone knows for certain the wiring on the OT.  There is a Brown and a Green coming off, and black... I am thinking the brown is 15ohm and the green is 8ohm... just looking at some AC15s where I could confirm the wiring and speaker impedance.  But a few days on the net and I haven't been able to confirm/ the AC10 Twin, The amp has / had 2 15ohm speakers, but don't know if they were wired series or parallel.. I realize I can apply voltage and measure the transformer, it would be nice if I could avoid that exercise as I have so much else to do. But I want to be very careful w/ the OT, so that may end up being the safest course....  There is a (6.8k)  3W? carbon comp resistor from the 16uf filter cap to pin 9 on V5 EL84, I couldn't spot that on the schematic, did I just miss it or is it a mod and does anyone know its purpose?

[shooter]

to pin 9 on V5 EL84

9 is screen so probably there to mitigate screen current

[sluckey]

You may be able to use an ohmmeter to determine which OT lead is which. Check resistance between chassis and the brown lead then between chassis and the green lead. The higher resistance reading is the 15Ω lead. If the two readings are too close together to tell then you will have to resort to the voltage check.

The voltage check is easier than you might think. Pull the EZ81 tube so no B+ will be present. Then use two gator clip leads to connect the 6.3VAC filament voltage to the plates of the EL84s. Ie, jumper pin 4 to pin 7 of one tube and pin 5 to pin 7 of the other tube. This applies 6.3vac  to the OT primary. Then just measure the voltage between chassis and the two secondary leads. The lead with the higher voltage is the 15Ω lead.

I don't see a 6.8K CC resistor in any of your pics. Does it appear to have been added later? Sounds like someone just wanted to add a shared screen resistor to give the tubes a little protection. If you want the amp to be bone stock, just replace it with a piece of wire.

[66Strat]

I have to say that this is one of the most intriguing Vox amp circuits that I have come across. Eliminate the 100 ohm plate resistors of the EL84 output tubes and you have an AC15 output section. The ECF82 (6U8A) tube is a readily available (cheap) NOS tube. What's not to like? Tweakage is encouraged.

[ndross475]

From the little research I have done, there have been those who said with the right tweaks it is a screamer, I keep seeing people replacing the 220 inputs w/ 68ks...

Tried Sluckey's voltage test and am concerned.... I am reading 7.08v across the heaters...  when hooked to the OT I was seeing .341 and .280 on the 2 output leads to ground... That seems low....

[sluckey]

Tried Sluckey's voltage test and am concerned.... I am reading 7.08v across the heaters...  when hooked to the OT I was seeing .341 and .280 on the 2 output leads to ground... That seems low....

No, that seems very good. Voltage ratio of 7.08 to .341 is 20.76. Now square that to get the turns ratio which is 431 to 1. Connecting a 16Ω speaker to the brown wire will reflect a 6896Ω load back to the primary. Good enough for EL84s.

IOW, brown is your 15Ω tap and green is your 8Ω tap. Easy peasy.

[ndross475]

Thanks... "Easy Peasey for some... terrifying for others".  Well now I feel comfortable leaving it on long enough to probe around and assemble a voltage chart...  Thank you!!!.  Seems the Tremolo channel isn't working but the Normal is where I am getting volume... The switch open and closed on test so thats positive.... The switch is the original 62 switch... no bottom rubber pad but I can source or make that...

Progress...

[ndross475]

First Tube Voltages...

[sluckey]

There are no negative voltages in that amp. Maybe those were milliVolts?

[HotBluePlates]

I have this one also... at first glance I haven't found the differences...

R14 (200Ω) between the EZ81 and the first filter cap is what jumps out to me.

My first concern is if anyone knows for certain the wiring on the OT.  There is a Brown and a Green coming off, and black... I am thinking the brown is 15ohm and the green is 8ohm... just looking at some AC15s where I could confirm the wiring and speaker impedance.  ...

Summarizing some stuff ndross475 & I discussed at another forum:

I have a late '64 or maybe early '65 AC10 Twin.  In my amp there are places on the terminal block for all 3 wires from the OT (and I'd highly recommend obtaining one to install in place of the existing 2-position terminal block).

Resistance (lead resistance zeroed out):
Black to Brown: 0.8Ω
Black to Green: 1.2Ω

So the Brown wire is the 8Ω tap if your OT color code matches mine (and if the higher-voltage wire was the Brown one in your amp, it does not match my amp).  Always check with a meter to verify.

...  But a few days on the net and I haven't been able to confirm/ the AC10 Twin, The amp has / had 2 15ohm speakers, but don't know if they were wired series or parallel...

A pair of 15Ω speakers could only be series for 30Ω or parallel for 7.5Ω (call it 8).  Yes, my AC10 Twin has these speakers in parallel for the 8Ω tap.

to pin 9 on V5 EL84

9 is screen so probably there to mitigate screen current

+1.

... I keep seeing people replacing the 220 inputs w/ 68ks...

In my amp, 100kΩ resistors were placed across one of the 220kΩ resistors for each channel, giving a lower-loss input jack.
The actual hot-ticket for this amp is that turning the trem off & adjusting the Amplitude (Intensity) control changes the gain of the EF86.   

[ndross475]

Houston we may have a problem... I switched meters just to confirm... V2 Pin 1 to ground = -3.2v DC.  Different from the other meter but still neg... I shut it down...

[PRR]

> V2 Pin 1 to ground = -3.2v DC

What, the rectifier plate? Which you should be reading with AC Volts setting?

Yes, the DCV is nominally zero and actually leans a hair negative. Why don't you try reading the rectifier on an AC Volt setting?

If I am looking at a wrong V2 line-up, sorry.

[HotBluePlates]

I measured the preamp tubes in my AC10 Twin to help spot issues (though I think Pin 6 of the 6U8 is a problem; probably need to unsolder one side of the 0.01µF cap, C17, soldered to this pin).

Voltages measured with 115vac wall voltage.

V1 EF86

 66.6vdc - Pin 1 (Trem Amplitude Max)
 38.7vdc - Pin 1 (Trem Amplitude Min)
      0vdc - Pin 2
1.805vdc - Pin 3 (Trem Amplitude Max)
1.485vdc - Pin 3 (Trem Amplitude Min)
 6.46vac - Pin 4
   ~0vdc - Pin 5
 69.3vdc - Pin 6 (Trem Amplitude Max)
158.1vdc - Pin 6 (Trem Amplitude Min)
     0vdc - Pin 7
Same as Pin 3 - Pin 8
     0vdc - Pin 9


V3 6U8
26.98vdc - Pin 1
      0vdc - Pin 2
  37.1vdc - Pin 3
  6.44vac - Pin 4
      0vac - Pin 5
  96.0vdc - Pin 6
1.4-1.8vdc - Pin 7  (Same as V1 Pin 3 & 8)
0.687vdc - Pin 8
      0vdc - Pin 9


V4 12AX7
  211vdc - Pin 1
 38.9vdc - Pin 2
 58.7vdc - Pin 3
 6.44vac - Pin 4
      0vac - Pin 5
  210vdc - Pin 6
38.58vdc - Pin 7
 58.7vdc - Pin 8
      0vdc - Pin 9

[HotBluePlates]

Below are the voltages for the power section of the chassis.  Once I went to check the OT winding ratio, I found the wall voltage (and variac output) had crept up to 116.3vac.

V2 EZ81

299.4vac - Pin 1
      N/A  - Pin 2
334.4vdc - Pin 3
             - Pin 4
             - Pin 5
       N/A - Pin 6
299.5vac - Pin 7
       N/A - Pin 8
       N/A - Pin 9

Series R from Rectifier to 1st Filter:  196.8Ω
1st Filter:  319vdc
2nd Filter: 310.5vdc

V4 EL84

       N/A  - Pin 1
      0vdc - Pin 2
10.28vdc - Pin 3
              - Pin 4
              - Pin 5
       N/A  - Pin 6
296.2vdc - Pin 7    Plate R: 121Ω; 300.4vdc at OT wire
       N/A  - Pin 8
   309vdc - Pin 9


V5 EL84

       N/A  - Pin 1
      0vdc - Pin 2
10.28vdc - Pin 3
              - Pin 4
              - Pin 5
       N/A  - Pin 6
296.2vdc - Pin 7    Plate R: 112.7Ω; 301vdc at OT wire
       N/A  - Pin 8
   309vdc - Pin 9


Ratio Test

6.83vac applied to OT primary wires.
Black to Green:  293mV    ->  23.31:1  ->  8150.7Ω : 15Ω
Black to Brown:  213.5mV ->  31.99:1  ->  8187.2Ω :  8Ω

Voltage ratio check aligns with my resistance measurements earlier.  When I connected the test a.c. voltage directly to the plate pins (including the 100Ω resistors), I got 290.6mV at the Green wire and 211.8mV at the Brown wire.

[66Strat]

HBP, how would you characterize the Normal channel sonics. On paper, the 6U8 triode gain is similar to a 12AY7. However, the plate curves are not nearly as linear.

[HotBluePlates]

HBP, how would you characterize the Normal channel sonics. On paper, the 6U8 triode gain is similar to a 12AY7. However, the plate curves are not nearly as linear.

Clean & wimpy.  If gain is similar to a 12AY7, imagine just a single 12AY7 stage feeding a long-tail & that's what you get here.

The EF86 channel has much more harmonic content, especially if you turn the trem intensity to max (which nearly doubles the screen voltage, drops plate voltage by more than half, and applies a bit more G1 bias).

[66Strat]

HBP, how would you characterize the Normal channel sonics. On paper, the 6U8 triode gain is similar to a 12AY7. However, the plate curves are not nearly as linear.

Clean & wimpy.  If gain is similar to a 12AY7, imagine just a single 12AY7 stage feeding a long-tail & that's what you get here.

The EF86 channel has much more harmonic content, especially if you turn the trem intensity to max (which nearly doubles the screen voltage, drops plate voltage by more than half, and applies a bit more G1 bias).

Sounds like a fun amp.

[Rustybike]

Hi, I'm new to the forum but reckon I'll be checking in frequently. Got given a 60's Vox AC10 "piggyback" by friend. Been in her dry loft 20+ yrs. I couldn't believe it when she said "old valve amp". I was thinking hifi! She doesn't know if was working when it was put away. It appears to be original. I'm scared to power it up but I've checked out the 2x16mF power supply cap. and it seems good. There's nothing protecting the input other than the fuse in the plugtop which is 13A! I've played guitar through valve amps as a teenager and been electrical/electronic engineer for 39yrs and I thought I'd have a crack at this lady. I mean, how difficult could it be? That was until I read this post! I never knew there was so much to tubes. Any advice welcome. Don't mind waiting. Have a load of humble pie to get through.

[acheld]

I'm no expert, but would love to see photos of the circuit . . .

[shooter]

That was until I read this post! I never knew there was so much to tubes.

I maintained Medical systems for 32yrs, the 1st tube amp I put on the bench I was horrified at the crazy distorted waveform I was seeing.
had a guitarist plug in and play, "man that's a great sounding amp........"  down the tube guitar amp rabbit hole I went. 

[HotBluePlates]

... 60's Vox AC10 "piggyback" by friend. ...

It would help to see a photo of the amp.  Trapezoid-shaped?  If yes, it's a very different circuit than discussed in the rest of this thread.

[pdf64]

Read up about and build a light bulb limiter, to use for the initial power up, ie after a long ‘rest’ or significant changes or repairs.

[Rustybike]

It would help to see a photo of the amp.  Trapezoid-shaped?  If yes, it's a very different circuit than discussed in the rest of this thread.
[/quote]
Sorry, I don't know how to post pics yet but no, it's not trapezoid shape.

[Willabe]

Sorry, I don't know how to post pics yet but no, it's not trapezoid shape.

To post a picture or an attachment, on the left side, just below the white post box your writing in, click on Attachments and other options.
 

[Rustybike]

This is the speaker and amp cabinets. Screws of speaker cabinet had seized and the threaded inserts all pulled out. Working at this until I pluck up the courage to apply some volts to the amp.

[Rustybike]

Read up about and build a light bulb limiter, to use for the initial power up, ie after a long ‘rest’ or significant changes or repairs.
[/quote]
Thank you, this is most useful. Only thing is, 100w lamps are becoming as scarce as Vox amps:)

[pdf64]

Ask around family, friends, neighbours; someone’s going to have an incandescent bulb or 2 at the back of a dusty shelf.
Halogen types are fine too.

[HotBluePlates]

This is the speaker and amp cabinets. ...

The form-factor looks like an AC30 Super Twin, unless that's a model I didn't know about.

I don't know that Vox ever made a "AC10 Super Twin" with straight sides.  They made an AC10 Super Reverb Twin with trapezoidal sides, and apparently used the same chassis & circuits for the Domino Super Reverb (with straight sides).

[Rustybike]

  "They made an AC10 Super Reverb Twin with trapezoidal sides",
Don't know if I should admit to this but just watched a 90min documentary on the Monkees. No really! When they played live (and they could) there were Vox trapezoidal amps left and right. I agree with you that mine would suggest ac30 and I wish it was but the plate defo says ac10.   

[Rustybike]

I'm no expert, but would love to see photos of the circuit . . .

Could you tell me how to pick out a quote and highlight it?

edit... fixed quote. Click on modify to see the code that was placed in this message.

[shooter]

mest up   

[sluckey]

There are three easy ways to insert a quote...

1. While viewing a message click on the quote link at the top right of the message. The reply window will open with the entire message already quoted. You may edit the text if so desired. BUT DON'T MESS UP THE QUOTE TAGS!!!

2. If you already have the reply window open you can scroll down until you find a post you want to quote. Then simply click the "insert Quote" link. All the text of that message will be inserted into the reply window AT THE POINT OF THE EDIT CURSOR.

3. Highlight any text, right click and choose "copy" to put that text on the Window's clipboard. The text can be anywhere, ie, this forum page, any other web page, any other program (This is a standard Windows operation). Open the reply window if not already open. Click the "Insert Quote" button at the top of the reply window and the quote brackets (tag codes) will be placed in the reply window at the point of the cursor without any text. Now place the cursor between the quote brackets, right click and choose "paste" to insert the text that's on the clipboard.

When quoting (or really anytime you are making a reply) it's a good idea to click the "Preview" button to make sure your reply will appear as you want. Once everything looks right click the "Post" button.

[Willabe]

Could you tell me how to pick out a quote and highlight it?

Click on the quote button, top right in post you want to quote from.

Or when your posting, scroll down the previous post's and click the Insert Quote button, top right in post.

Then delete what you don't want to quote, but, LEAVE the top line that starts with [quote.....  AND LEAVE the last part too [ / quote ]

Edit; Sluckey beat me to it, gave better explanation too.   

[Rustybike]

someone’s going to have an incandescent bulb or 2 at the back of a dusty shelf.

Found one in an old shed and made up the circuit using bits and pieces that my wife said I'd never need. Pah! I rest my case, I said to myself:)
I mest up too!

fixed quote AGAIN

[sluckey]

This ain't hard.

READ, UNDERSTAND, AND FOLLOW INSTRUCTIONS!!! in reply #35. Especially the last part that says PREVIEW BEFORE POST.

[Rustybike]

[quote
This ain't hard.[/quote]


Bet you're wondering how a chap that can't post on a forum is gonna fix a valve amp. I know, I'm wondering too! I deleted a bracket and back slash last time. I'm almost afraid to post again:)

[sluckey]

Bet you're wondering how a chap that can't post on a forum is gonna fix a valve amp. I know, I'm wondering too! I deleted a bracket and back slash last time. I'm almost afraid to post again:)

All you gotta do to fix your last quote is to put a close bracket after the first "quote".

What kind of device are you using to connect to the forum?

READ, UNDERSTAND, AND FOLLOW INSTRUCTIONS!!! is killing you. You're gonna have to demonstrate that you can do that before I'm gonna offer any instructions about amp repair.   

[Willabe]

READ, UNDERSTAND, AND FOLLOW INSTRUCTIONS!!! is killing you. You're gonna have to demonstrate that you can do that before I'm gonna offer any instructions about amp repair.   

[Rustybike]

READ, UNDERSTAND, AND FOLLOW INSTRUCTIONS!!! is killing you. You're gonna have to demonstrate that you can do that before I'm gonna offer any instructions about amp repair. 

Do you think if I sent you my amp that it would be less bother? :)
 I apologise for turning this very informative thread into a tutorial. Even more humble pie now. I'm using a laptop and the preview is displayed off screen, above the dialog box and I missed it.
 Thanks everyone for your patience, your interest and your help.

[sluckey]

 

[Willabe]

I apologise for turning this very informative thread into a tutorial.

No need to apologize, you haven't done anything wrong. No harm done     

Maybe try looking for an amp repair man near where you live?

I'm using a laptop and the preview is displayed off screen, above the dialog box and I missed it.

Well, now you know.        It took me a little while to figure out how post and do quotes and attachments. And guys helped me through it.

[Rustybike]

   

Maybe try looking for an amp repair man near where you live?

Thanks for the vote of confidence there Willabe:) Gonna power up using the afore mentioned lightbulb limiter and take it from there.
 Great to see a Rory fan btw. Regrettably, only saw him once but took the bikes to the Ballyshannon festival in 2010 to see his statue and then down to his grave in Cork.

[sluckey]

Now that you've conquered the mechanics of the forum it's time to fire that thing up on your light bulb current limiter and see what happens. If no smoke of bright bulb you can plug the amp directly into the wall. I'd love to see you get that amp back in shape. Heck, it may even work as is. This should be a fun and interesting project. The amp is fairly simple and straightforward and troubleshooting should be pretty easy. Lot's of friendly help on the forum too.

Having only a 13A fuse in the plug concerns me. Can you simply replace that fuse with a 2A fuse? If not, can you rig up a 2A fuse in series with the power cord?

[pdf64]

Standard UK plug top fuse values are 3A and 13A, but from more specialist suppliers, other values are available.
An AC10 should be fine with the lowest value I’ve seen, a 1A mains fuse https://cpc.farnell.com/eaton-bussmann-series/tdc180-1/fuses-mains-plug-top-1a-x10/dp/FF01691?st=1a%20plug%20domestic

13A is ridiculous, especially so as the mains cable is probably far too skinny to carry that current.

[Rustybike]

13A is ridiculous, especially so as the mains cable is probably far too skinny to carry that current.

Agreed. Cable looks like 0.5mm. Makes me wonder about countries where there are no fuses in their plugs. I have found it so difficult to find the correct size of fuse. There was nothing on the rating plates back in the day. I fitted a 3Amp but have changed to a 2 after reading your post. Thanks for that and the lightbulb limiter idea. Ready to power up today. Ironically, the chap who sold me my '58 Hofner years ago, has just posted an AC10 for sale this week. I asked him what fuse was fitted to his. He sent me a picture of the plugtop pointing out that it says "13A Fuse Fitted" on it.

[zircontweezer]

I'm no expert but I had my hands on a Vox AC10 twin about ten years ago. Got a few pictures of it as well. Mine had a reverb unit added to the amp at the factory which was DOA and the two by ten speaker cab had Goodman? speakers. Bit late in the night here but I can post some pics later. Whats with the 13 amp fuse? I'd guess that was 1.3 amps if anything. Great looking amp (mine) but due to whatever it didn't make me go Wow! when I played it. The amp was a grey faceplate but yours looks a bit blue or is that just the camera? I don't think Vox ever did a blue.