VOX AC4TV Layout - I'm drawing one - Can you give your contribute ?

[kagliostro]

As I was not able to find a VOX AC4TV Layout (the ecc83 pre version of the amp) I'm trying to draw one

as it is a long time I don't use Visio and don't draw layouts, please give me help

this is the schematic


this is what I've draw till now


attached, in a .zip file, the .vsd file (the .vsd file was too big to be posted)

Many Thanks for any contribute

Franco

[sluckey]

Can you use this? Check for errors...

[kagliostro]

Mant Many Thanks Steve

Franco

[kagliostro]

Ciao Steve

I cheked the layout (for what is in my ability) and your job is perfect

I've a bit of questions to clarify the situation in my mind

I think I'll use a 6N2P-EV (6Н2ПEВ) in place of the 12AX7 tube, so pin #9 connected to ground and filaments heaters only to pin #4 & #5

You rotated the 12AX7 & EL84 sockets as to have the best wire path

the way resulted put the pin #4 & #5 of the EL84 facing the chassis corner and the pin #4 & #5 of the 12ax7 facing the Board

Which will be your preferred path for the heaters wires ?

One other thing

I remember (may be wrongly) that some versions of the EL84 tube can have (internal) connections to pins that oficially are unused, do you see any problem connecting the 5K6 resistor to pin #2 and then to the wire in the way that is connected the 27K resistor (pin # 7 on the 12AX7) to the shielded cable ?


---


I took the liberty of making some small changes, soon post the .vsd file to submit it to your opinion

Many Thanks

Franco

[kagliostro]

Follows the previous post

Here is the .vsd file I modified

if you remember my other thread I want to build a small stereo with AC4 (with ef86) and AC4TV (with 12ax7) channels, and the PT will be only one for both circuits (I'm using components recovered from a Graetz Stereo Radio)

Do you think there are problems with this configuration ?

Can I go further and move the 33uF 450V capacitor at the left of the catode resistor + bypass cap ?

Do you think this version will be prone to have problems ?



Many Thanks

Franco

[sluckey]

Your suggested changes should work just fine. I like to tuck the filament string in the rear corner of the chassis, like this...

[tubenit]

That is some super neat layout and wiring!  Wow!   


With respect, Tubenit

[kagliostro]

Many Thanks Steve

I rotated the socket and swapped  the triodes with the purpose to follow your path near the chassis corner with the heaters wires

Thanks Again

Franco

[JPK]

Wow super clean wiring and dig the chassis holding jig with adjustable tilt angles. Saving this pic for my next build (Mojotone Studio One).   

Your suggested changes should work just fine. I like to tuck the filament string in the rear corner of the chassis, like this...

[sluckey]

Here's some more info on my amp cradle in case you want to build one...

     http://sluckeyamps.com/cradle/cradle.htm

[kagliostro]

...my amp cradle in case you want to build one...

Simple and very effective

Franco

[sluckey]

Tell me about the attenuator. What wattage rating do you suppose those attenuator resistors are?

[kagliostro]

Ciao Steve

About the attenuator .... 

No, no idea about resistor wattage

but I've a pair of photos, those seems 3 - 5W resistors, more 5W (to me)





One consideration

The attenuator is planned for the speaker load of 16ohm, but if OT and Speaker are 8ohm or 4ohm of impedance, resistors value must be adeguated to the load  (and may be capacitors value too ?), however no problems drawing the original circuit

Franco

[DummyLoad]

Ciao Steve

About the attenuator .... 

No, no idea about resistor wattage

but I've a pair of photos, those seems 3 - 5W resistors, more 5W (to me)





One consideration

The attenuator is planned for the speaker load of 16ohm, but if OT and Speaker are 8ohm or 4ohm of impedance, resistors value must be adeguated to the load  (and may be capacitors value too ?), however no problems drawing the original circuit

Franco

Those look like 3W parts. Bourns brand?


--Pete

[kagliostro]

I was intrigued by the proposal to build an AC4 (ef86 with vibrato) & AC4TV (ecc83 no vibrato) as the two channels of a small Stereo

in my archive I found a layout for the AC4 (ef86), this one



As I didn't find a layout for the AC4TV I started to draw one following the same style



But our friend Sluckey kindly did the job for me and draw a beautiful layout for the AC4TV

I remembered that the layout drawn for the AC4 + vibrato by our friend Dwinstonwood was near  in measures to the layout drawn by Steve

So I redraw the AC4 with vibrato Layout by Dwinstonwood in Visio (with very little changes to accomodate components) and modded furtherly the AC4TV layout changing a bit the board dimensions as to have the two circuits on identical (in dimensions) boards
The thread by Dwinstonwood is here https://el34world.com/Forum/index.php?topic=27253.0

Here is the AC4 + vibrato layout drawn by Dwinstonwood



Here the redrawn (by me in Visio) board of the AC4 + vibrato (I'll go on with the whole layout and I'll post the .vsd file for those who can be interested to mod it)



here the AC4TV layout drawn by Sluckey and with a bit of mod by me (cropped, only Board)



attached the .vsd file of the AC4 + vibrato (Board only at the moment)

Franco

[kagliostro]

I'm really stupid

I was aked about the attenuator and I remembered only about the photo I had

here is what really give an answer to the question

Franco

[walkman]

In the schematic posted, at the input there is a 22p to ground at the junction between 10k from the input jack and a 1M grid leak. Does that form a low pass filter with 3db down at 796hz? Won’t that muffle the sound quite a bit or is something else going on?  I prefer a bright amp and to use my tone one controls to take a little off if needed.

[kagliostro]

The purpose of that cap is to form a filter that block RF noise, being able to use at the input a smaller resistor than the usual 68K, this preservs the integrity of the input signal

Franco

[dwinstonwood]

I've started using them on my builds, too. I learned about it in Merlin's Preamp book. That input resistor is supposed to be the noisiest one in the amp. I absolutely hear a real difference in hiss by using a 10K resistor and small cap to ground. I wire both directly to the grid pin on the tube socket, using a small tag strip.

[walkman]

Ok just rechecked the low pass calculator I used… 
796khz yep  maybe a low flying jet ✈️.

[PRR]

> there is a 22p to ground at the junction between 10k from the input jack and a 1M grid leak. Does that form a low pass filter with 3db down at 796hz?

Yes, at 796KHz as you later corrected.

But there is always 100pFd at the grid of a 12AX7 gain stage, see "miller feedback". Another 22pFd is small potatoes. 68K+100pFd is about 23Khz; negligible effect on the highest useful string harmonics, but >25dB down at the bottom of the AM radio band. I've lived in sight of a 50KW tower and I know it gets in everywhere.

[dwinstonwood]

My 10K/250pF = 63.6620kHz.

Is the idea to get is just above 20kHz? a 500pF would be 31.8310kHz.

Thanks.

[PRR]

> just above 20kHz?

Try it. (Get a 10-pack of 100pFd or 250pFd and try one, two, several parallel.)

There's no response past 6kHz on "guitar" speakers, because the inharmonic overtones of a steel string are very dissonant. However Fender differs from most older amplifiers by having electronic response far past 5kHz (despite <5kHz speakers). I dunno what he heard in there but can't argue with success.