Vox historians: help reading minutiae on 1960 Vox AC15 OA/031 schematic

[dchang0]

Hi, all--

On the old 1960 Vox AC15 schematic, OA/031, there are a few notes in the sidelines that I need help making sense of. (I'm relettering the schematic using the original drawing but brand new digital typesetting so that it's more legible--the results will be posted online, free, open-source when done.)

Here's the first one:

At the top right corner of the drawing, it says:

MODIFICATIONS
C42 ADDED
7 - 2 - 61
C17(?) DELETED
R18(?) (DELETED)

My first question: does the drawing really say C17 and R18? I've looked at three different scans of the original schematic, and none of them are particularly legible in this area.

The second question: IF they really are C17 and R18, where on the drawing were/are they? There is one of the OA/031 scans that has scribbles on it where components were scribbled out. I'm guessing that's where they were/are (see attached schematics).

This one has the scribbles.


These two don't.




The second note:

It says near the bottom right:

* DENOTES CHANGE OF COMPONENT
VALUE FOR "VOX" AC 15 "BASS"
AMPLIFIER

Which component does the asterisk refer to? Probably a capacitor somewhere. What are the guitar and bass values?

Most everything else can be deduced by comparing the three different scans of the original drawing. When I eventually post the relettered schematic, I'll need help double-checking it for errors.

Thanks in advance!

[dchang0]

Sample of relettering of OA/031 schematic

This'll give you an idea of what the relettering looks like.

BTW, if you see an error, please let me know.

[dchang0]

Found the first of the "Bass Model" changes. C3 is 0.05uF* Glen_L mentions "about six" component changes in this thread: Vintage Amps Bulletin Board ? View topic - ac 15 twin bass model cap value He says to use 4.7nF-400V as C3.

I have a new question: can anyone read the voltage marked at pin 6 (plate) on the EF86? It looks like it says 80V, but none of the three schematics I have are clear enough about it. See attachment--it is outlined in red.

[MarkusBass]

I think that the first remark refers to C17 and R15 (and not R18). They are just below the V2 tube.
Most probably R15 is removed and C15 is shorted. Take a look at the schematic.

Mark

[dchang0]

I think that the first remark refers to C17 and R15 (and not R18). They are just below the V2 tube.
Most probably R15 is removed and C15 is shorted. Take a look at the schematic.

Mark

Cool--thanks! I see them and will make the change from R18 to R15 in the note.

[loudthud]

From the deluxe edition of Jim Elyea's book. Jim had someone clean up the scans but I suspect the person didn't know how to actually read a schematic.

Voltage pin 6 of EF86: 50V

Change for Bass model:
C2 ?? 25-25 the * looks a little suspicious.
C3 .01 to .05
C6 .01 to 0.1
C10 .01 to 0.1
C7 .01 to 0.1
C9 .01 to 0.1
C11 .005 to .01

[dchang0]

WOW--thanks! You just saved me a ton of work!

I do have a question, though about the voltage on pin 6 of EF86--is it really 50V? That FEELS kinda low--of course, I don't know squat about what it's supposed to be, and I don't have an early 60s AC15 to actually measure. royaljelly says it's supposed to be 95V (but he does not mention where he got this value) in this thread: http://music-electronics-forum.com/t25383/

Looking at Merlin's EF86 theory website, it SEEMS to me that 50V would be really low too: The Valve Wizard -Small Signal Pentode
The curves in the middle of the page--50V is right at the very edge of where they bend. Again, I have no idea whether 50V is right or wrong--it just FEELS low... It is entirely possible that Dick Denney purposefully chose 50V right at the edge of the nonlinear region.

I'll put 50V as you report. If anyone else can shed light on the matter (especially with real-world measurements, although we know that wall voltages were lower in the 60s than now, and modern transformers are probably wound differently than the originals, etc.), please do.

--
As for C2, I am very sure that Jim Elyea's cleaner person misread the V as a *. It is 25-25V (there are other points where 25-25V are written, such as C37, etc.) Also, that would leave exactly six caps (C3, C6, C10, C7, C9, C11), which matches the number Glen_L said was different for the Bass Model.

THANKS AGAIN FOR THE HELP!

[dchang0]

Two new questions:

See attached screenshot.

It looks like VR3 (250K LOG), aka "Cut," has an asterisk on it, but the asterisk has no value next to it. Any idea whether this is an asterisk or not and if it is, what does it mean? (I am assuming that it means that the 250K LOG pot does not exist in the Bass Model.)

And there appears to be some sort of squiggly mark/letter/number just in front of R16 that was edited out in some scans of the schematic. I assume it is not important, but if anyone knows if it is important, please let me know.

Both marks are circle in red--for reference, the valve shown nearby is V2.



And the relettering is coming along nicely:

[loudthud]

Yes, 50V seems low but that's what the schematic says. It might measure that low if an old 20,000 Ohms/Volt VOM was used to take the measurement.

I'm not seeing anything around R16 or VR3.

[dchang0]

Cool--thanks for confirming. 50V it is--I am trying for historical accuracy more than modern applications.

[dchang0]

Two more hard-to-read labels, this time next to the output transformer.

Please look at the screenshot. I've outlined the hard to read text in red rectangles.



The first one says something like this:

?????? IMPED.
ANODE TO ANODE
8000 OHMS
T1

I think the ?????? may be "REMARK" but I'm not sure.

As for the other one, the label on the ground lead of the output transformer T1, I have no idea what that says. My guess is 10 Henry or something like that, but that's just a wild guess.

Any help is much appreciated.

As for the relettering, I'm pretty much done with the normal channel and output section and have started on the Trem/Vibrato channel.

[The Dude]

The first box, I believe, is "primary" (indicating the primary impedance of the OT) and the second is "com" for common (usually indicating speaker ground or negative).

[dchang0]

You nailed it--thanks!

Here's the updated schematic so far. Looonnnng way to go before done... The final will be high-resolution at least in regards to the lettering.

[dchang0]

Pin voltages on V.6?

Here're some more tiny, illegible voltage notations.

Please look at the V.6 schematic--the unreadable text is boxed in red.



I think the first two boxes on pins 1 and 6 read:

80V
74V?

The bottom box on pins 3 and 8 appears to read:

3?V
4V?

Any idea what these say?

Thanks!

[dchang0]

Pin voltage on V.5?

And here's one hard-to-read voltage on pin 6 of V.5 (12AX7):



I'm pretty sure it says 180V, and that value would make sense since there is a 210V right on the other side of R39 (22K). It MIGHT be 150V.

What do you think it says?