Vox - V491 Valve Effects Loop Buffer

moltenmetalburn · Post by **moltenmetalburn** » 13 Jan 2009, 17:16

Here is the rare Vox V491 Valve effects loop buffer. It is a tube effects loop buffer, I used it with the passive effects loop of my Soldano SLO 100 and Krank Revolution, many people (i have read) also use it in front of the amp with their pedals or PODs in the loop, no documentation available so the intended use is questionable though Id lean toward my application.

Released in the early 1990's and quickly discontinued, see if you can find any info on this one!

This unit works great! there should be enough in the pics for a competent trace if anyone needs any more info just ask. It be great to see this in a much smaller chassis (stompbox style.)

there is something about he thinness of the wires carrying signal to the send,return board that just bothers me!

http://www.redhillmetal.com/uploads/IMG_3087.JPG
http://www.redhillmetal.com/uploads/IMG_3088.JPG
http://www.redhillmetal.com/uploads/IMG_3089.JPG
http://www.redhillmetal.com/uploads/IMG_3090.JPG
http://www.redhillmetal.com/uploads/IMG_3091.JPG
http://www.redhillmetal.com/uploads/IMG_3092.JPG

Enjoy!

lolbou · Post by **lolbou** » 13 Jan 2009, 17:28

Hey,

Thanks for this interesting post! I'm surprised to see a 12AX7 buffer, as I thought it cannot handle high currents...

A 12AU7 seems more appropriate to me, but I never found a unit to copy and I'm not familiar with tube prototyping...

Do you know what are the voltages inside it (at the transformer leads) ?

Cheers! I'll try to find a schem or, worse, to trace it...

moltenmetalburn · Post by **moltenmetalburn** » 13 Jan 2009, 17:34

I do not know the voltages but if you were to explain how I measure them Id be willing to in the near future. I have worked inside tube amps with lethal voltages before, no worries.

lolbou · Post by **lolbou** » 13 Jan 2009, 18:03

Ok, first, I made some stitches to your pics to get them easier to read for any other helper...

Then, for the voltages, I would suggest between the two yellow ones and between the red and green ones... I would think that the yellow ones are for heaters, and the red/green are for high tension...

Can you specify what are the jacks and controls about?

edit: and I try a transparency picture...

lolbou · Post by **lolbou** » 13 Jan 2009, 19:39

The yellow wires are 6.3V AC.

The green/red feeds a half wave rectifier (voltage?)

I'm still tracing it... Work in progress

Here's the power section and the Tube socket PCB (reference)

I miss some values, especially for resistors since I'm colorblind and I really have problems...

Dirk_Hendrik · Post by **Dirk_Hendrik** » 13 Jan 2009, 20:32

Thanks!!

(that goes for lolbou as well!)

lolbou · Post by **lolbou** » 13 Jan 2009, 20:48

Cheers Dirk!

Well, slow progress for me... It seems that triode 2 is a cathode follower and I'm not sure with the rest yet, but I'm so tired and going to bed...

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 03:02

There are NO markings of value whatsoever on the pots. they have some numbers printed on the casings that are identical. 889933 or something like that, not values.

the in outs and all are here in these pics.
http://www.redhillmetal.com/uploads/IMG_3093.JPG
http://www.redhillmetal.com/uploads/IMG_3094.JPG

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 04:05

Here are updated pics with all the component values i could see.

http://www.redhillmetal.com/uploads/V49 ... ts%20A.jpg
http://www.redhillmetal.com/uploads/V49 ... nt%20B.jpg

left pot is send level right pot is return level.

fuse, silver cap and tropical fish unknown.

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 04:51

ugh, need sleep now... right now...

here is my first transparency trace!

http://www.redhillmetal.com/uploads/tra ... traced.jpg

damn missed a trace from the bottom diode to large lower cap right side to 20 uf cap right side.

Bernardduur · Post by **Bernardduur** » 14 Jan 2009, 09:30

tropical fish cap = 470n

lolbou · Post by **lolbou** » 14 Jan 2009, 12:01

great for the values!! and what about the tin can capacitor? and the two big elecrolytics (68 uF?)

how the hell are these switches wired inside???!!!

lolbou · Post by **lolbou** » 14 Jan 2009, 12:25

Ok, here's my sketch for the cathode follower which stands between the amp send and the loop send.

Some questions remain concerning some values (56 or 560 ohms, pot value, and 470n coupling capacitor).

Again, I'm surprised to see the 12AX7 used here, but it might fine for guitar applications... But what's the point of such a device? On the amp, loops are solid state buffered aren't they? So why buffering them again?

To be honest, I'm quite interested by the project to build a tube buffer for my guitar to drive effects and cables...

ps: my previous schems for power section can be uptated with the correct values...

lolbou · Post by **lolbou** » 14 Jan 2009, 12:56

And here's the loop return amplifier...

I did not mention the fuse on the power section... What is the value (amps/slo blo?)?

Do we need to trace completely? to have it in a smaller enclosure, it would be nice to make an other pcb design...

Thanks to moltenmetalburn & Bernardduur (and Dirk for cheering)...

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 15:51

opened it up again this morning.

Two largest caps are 80 uf.

Silver cap 25 MF 25 V.

fuse is buss mdl 3/8 250V.

pot markings are rr9933 r13/8841.

tropical cap 470n (thanks Bernardduur) 10% 100V.

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 16:08

to have it in a smaller enclosure, it would be nice to make an other pcb design...

sure would be nice, maybe dirk will be up for it, ive seen some incredible tube pedal layouts hes done.

But what's the point of such a device? On the amp, loops are solid state buffered aren't they? So why buffering them again?

In fact the amps ive used this with have NO buffering whatsoever in their loops. Totally passive, which a few amps are in fact. The effects loop is just a pair of jacks wired between preamp and PI.

Bernardduur · Post by **Bernardduur** » 14 Jan 2009, 16:12

I have something sililar in my Dumble amp called a Dumbleator and man, that really makes a difference. I never put anything in the loop just turn it on and even out the volumes (Dulbleator also has a double bright switch for both the volume pots) and it really adds something to the signal!

lolbou · Post by **lolbou** » 14 Jan 2009, 17:02

moltenmetalburn wrote:opened it up again this morning.

Two largest caps are 80 uf.

Silver cap 25 MF 25 V.

fuse is buss mdl 3/8 250V.

pot markings are rr9933 r13/8841.

tropical cap 470n (thanks Bernardduur) 10% 100V.

Good! And what about my confusion : is it a 56 or 560 ohms resistor? edit : no problem with blue, green, and brown as for my colorblindness = it's a 560)

25MF??? Is it mF or uF? I'd rather go for the second choice... It's polarized, isn't it? The positive lead going to the cathode 1?

The schematics are being updated, but still I'm lost in all the lugs from the two switches

... Could you test for shorts?

And one last thing, for the PCB being made, I wish you could measure the boards (jacks:tube:main) so that I can scale properly and just add stuff above the actual stitches I've made...

Here's a link to some cathode follower stuff, which might inspire... http://www.freewebs.com/valvewizard/accf.html

moltenmetalburn · Post by **moltenmetalburn** » 14 Jan 2009, 17:41

560 ohm resistor.

the cap is marked 25mf not uf but who knows?

lolbou · Post by **lolbou** » 14 Jan 2009, 17:52

moltenmetalburn wrote:560 ohm resistor.

the cap is marked 25mf not uf but who knows?

Cheers! Anyway, appart those little details, it's a great value sharing it here since it's an informationless device on the net, and again, thank you so much...

Still no voltages for green/red wires?

Well, I don't think I will draw the whole schematic unless i have more time or find an easy way to make it, so if anyone is interested...

Vox - V491 Valve Effects Loop Buffer [schematic]