Yet another Pathfinder 15 to vintage (EF86) Vox AC4 conversion

[ChopSauce]

Nothing that great here ...



... but a pair of well seasonned irons, 3 vintage caps and old tubes (to come).

The chassis was flipped upside-down so there is a fresh (black) face to start with.

Note that the black plastic sheet covering the chassis tends to over-react to the drilling bits.
(hence the large washer around the pilot light... )

The wiring is almost done





but a few components still waiting to be delivered. I am rather happy with how it turned out and pretty proud of the layout.

After having build a closer copy of the vintage one, I realised how much the original layout was to be improved but I didn't wanted to go for a board-based(*) layout, either.

Hopefully I haven't made too much mistakes. I'll (re-)revise the wiring before the first tests, anyway.
_
(*) https://www.tdpri.com/threads/vox-ac4-build.322998/

[ChopSauce]

So, that layout, which conforms better to the original schematic with respect to the grounding scheme, proved to be way more silent than the original.

This conversion also was an opportunity for "debugging" the tremolo speed control : breaking the connection from the potentiometer R15 to ground



(that is wiring it as a variable resistor, "à la" Fender) allows for a wider and more linear response.


Yet another communication problem between the R&D dpt and production dpt head?

[Guy77]

Excited to see how it turns out!

Guy

[ChopSauce]

So far it's slightly missing output power, whatever tubes we plugged into it. The trem. is slightly weak so I suspect the EF86 settings/cathode bypass cap, but I haven't it on my bench to check that, at the moment. Another thing I haven't controlled is the ratio of the ouput transformer.

I have been kind enough to rebuild a messy amp, bought for its vintage look but as bad as a builder's nightmare, & whose transformers seem to be sub-optimal ones (voltages on the lower bound for the PT). At least now it works and it does it silently, but there's room for improvement.

Anyway, frankly, I find the amp is missing the warm woody tone of my own build https://ampgarage.com/forum/viewtopic.p ... 36#p426136 but the owner is planning to plug it into another cab with a greenback, most of the time.
(and he expected to spare some money with me using his own parts, of course!)

[ChopSauce]

Afterwards ...

... did you meant "how it looks"?

A real sleeper. The owner is not that much of a carefull guy. So, so far it looks just like a beaten up pathfinder - but the control panel.
(with even hidden tubes, horizontal inside the cab, as the original)

I managed to convince him to put a brown Vox grillcloth on the whole front, to hide vents for the electronics, so it is reminiscent of
(which I love)

24be530a9690b70418106f2a20d620ed.jpg

but he declined and is looking for some reclaimed stuff. I'm pretty disapointed but it's his own business now...

[Guy77]

Looks beautiful! I love the look of Vox grill in black or brown

G

[ChopSauce]

I especially like the contrast between the brown and the black, be it the tolex or the grillcloth...

While talking about revisions to the circuit : anyone ever tried a 4n7 tone cap, instead of the 1n from the original?

The tone stack is pretty inefficient on the original circuit, so maybe that would add some usefullness to the tone control...

[ChopSauce]

A few more things about the Vox AC4.

The two clones show voltages around 190V (instead of 215) on V1 (EF86) plate...

In addition this one shows cathode voltages (approx. 20-30%) too high for V1. I googled that but found no link to explain why cathode voltages would be higher than expected.

I'm too tired to think about that tonight, but would appreciate any pointers/links/etc (to read tomorrow), nevertheless.

[TattooTone68]

Hi, stumbled across this thread & was wondering if you had any other pictures of the p2p layout with the tag strips, looks very tidy from the couple of pics at the start of the thread.
I’ve built a few fender style small amps & fancied doing a vox style build next…

[ChopSauce]

Thank you very much for your interest. I am confused I have not seen your reply earlier.
I think I have the layout but how tidy it looks it is not so silent. Maybe I should have routed the "grounds" to less points?
That might be made more compact too.

[ChopSauce]

So here it is subject to improvements.

ac4-fw_layout-orig.jpg

Afterwards, I think what would be best would be a mix between the original layout from the 60s and the tagboards & shielded cable based design above.

Please, let us know about any experiment you could have in that area...