AC30 TBX mod with Hoffman board ; need some advice

[archaos]

I wanna install that kit in my '93 AC30 (Korg RI) instead of the cheapy & poor stock pcb, & because I have used the trem channel during +/- 30 mn these last 17 years .
Nonetheless I had a few questions before doing that as I'm not particularly skilled in electronics. My eyes are dead now because I have read & learnt the whole Hoffman instructions, & layout diagram for one week ; removed the chassis from my amp to see & compare it to the VOX/Hoffman/Weber schematics  (I'm curious too).

Here are my questions :

• What are used for R72 ; R74 ; R76 ; R78 ; R81 (82K) in the power amp section (VOX AC30 TB '93schematics) ?
• Does the green wire from the OT is connected to the ground ?
• What is used for the 50Ω R on the right end of the Hoffman board ?
• In my project should I install F1 ; F2 ; F3 ; F4 (like in the TBX) ? If so, how ?
• Could you explain me that : "Hook up the feedback wire. Soldered a white wire to one end of the FEEDBACK RESISTOR and solder the other end to the speaker tap you are going to use." ? I mean what's a FEEDBACK RESISTOR ? Is there one in the AC30 ?

Thanx for your answers,

P.-F.

[archaos]

OK, so here's the schematics of my amp :

[drew]

Can you find the places on your PCB that are marked R72, 74, 76, and 78?  I'm wondering if that "links fitted" statement means that there are jumpers (plain pieces of wire) in those positions instead of resistors.  If that's the case, then you don't need to worry about them.

It's hard to read that layout; is the R81 you're talking about the one that is in series with the amber lamp?  If so, there is no equivalent resistor on the Hoffman layout.  Hoffman uses a completely different pilot light setup than what you have:  one 6.3 volt bulb, powered by the heater wiring, like a Fender (shown on the lower right corner of the Hoffman diagram).  You would need to decide whether to convert your amp to that, or to adapt your existing dual, high voltage, neon lamp setup to the Hoffman layout.

The 10 watt, 50 ohm resistor on the Hoffman board substitutes for the parallel pair of 5 watt, 100 ohm resistors (R70 and 71) on the Vox layout.

The Hoffman layout doesn't have an equivalent for F2, the HT fuse.  If you want to add one, look at his 50 watt Marshall layout, you'll see a .5 amp fuse before the first 50uf filter cap.  As to the other three, I don't know what to tell you.  I don't think that incorporating fuses into the heater wiring was a common practice when tube amps were originally being manufactured, or is commonly done nowadays by amp hobbyists.  My guess is that Vox did this in response to modern day electrical codes and industry standards and practices.

You can ignore the sentence about the feedback wire.  The AC30 doesn't use negative feedback.  You can read this:  http://www.aikenamps.com/NegativeFeedback.htm to get an idea what negative feedback is, if you're interested.

One thing I'm wondering about:  is the Hoffman board actually suitable for installing in a real AC30, with the oddball chassis and the two rows of tubes?  Or is it just intended for use by someone who wants to put the Vox circuit in a standard, Marshall-style chassis?

[archaos]

Awesome ! Thank you very much, your answers are very helpful & useful for me.

Actually R72 through R78 do are jumpers each respectively near R73 through R79. I hope you'll able to draw conclusions from that, I haven't removed my PCB from the chassis (even it had been a real pain in the ass when I did it last week).

Do you think it'd be necessary to install fuses into the heater wiring again ?

Concerning the Hoffman AC30 board, indeed it's not so esay to install in a AC30 chassis, & I plan to hook it up @ 180° on the chassis.

[archaos]

I plan to swap all the H'man board stock caps circled in red above with SoZo vintage caps. I've read both John Gaynor (SoZo) & aikenamps tips & hints concerning "Where To Connect the Outside Foil on Capacitors", & here's what I infer from the caps (from the left end of the board to the right) :

• .047 foil connected  to the 1st 12AX7 PIN 1 cathode
• .1 ?
• .022 ?
• .022 ?
• .047 foil connected to the ground
• .0047 foil connected to the ground ?
• .047 foil connected to the ground
• .1 foil connected  to the 3rd 12AX7 PIN 1 cathode
• .1 foil connected  to the 3rd 12AX7 PIN 6 cathode

Am I wrong ?

[PRR]

> I mean what's a FEEDBACK RESISTOR ? Is there one in the AC30 ?

Connects from the output back to some earlier stage.

No, the AC30 does not use a feedback resistor.

[archaos]

Good morning guys !
Have (almost, as it doesn't work  ) finished my point-to-point wired AC30...& checked the valve data, here's what I got  :

pre amp tubes 12AX7 :

                                  V1     V2     V3
                              1   400   405    404
                              2      0       0       0
                              3      0       0       0
                              6   400   408    404
                              7      0    404       0
                              8      0       0       0



power tubes EL84 :

                                   V4    V5    V6    V7
                               3     0     0     0      0
                               7  409  409   409  409
                               9  409  408   409  409   

Help !!!

[sluckey]

Did you forget to connect the board grounds? Measure resistance from any tube cathode (pins 3 and 8 for 12AX7s, pin 3 for EL84s). All should show resistance readings.

[archaos]

Thanx for that quick reply. Actually my board grounds are connected to a ground star near the main transformer. But I'll measure the tube cathode resistance, & post pics this afternoon.

[darkbluemurder]

Good morning guys !
Have (almost, as it doesn't work  ) finished my point-to-point wired AC30...& checked the valve data, here's what I got  :

pre amp tubes 12AX7 :

                                  V1     V2     V3
                              1   400   405    404
                              2      0       0       0
                              3      0       0       0
                              6   400   408    404
                              7      0    404       0
                              8      0       0       0



power tubes EL84 :

                                   V4    V5    V6    V7
                               3     0     0     0      0
                               7  409  409   409  409
                               9  409  408   409  409   

Help !!!

Did you measure with the tubes plugged in? If you measured without tubes then you would not have any voltage losses because there is nothing that could cause any voltage drops.

Cheers Stephan

[archaos]

Yep, all measurements done w/o tubes.

[archaos]

OK, I must say some people here are masters whereas I'm hardly in the ground staff.

First of all, thank you very much for your help sluckey  , for, thanks to your advice I've found the breakdown in my circuit.

Here's what I've measured earlier :

                                                  V1       V2       V3
                                           3    1.58k   1.54k     0
                                           8    1.62k  57.4k      0

                                                  V4       V5       V6       V7
                                           3     52Ω      52Ω     52Ω    52Ω

I've deduced there was a problem with V3, therefore I've tested the resistor & got it was OK. Finally I've found I had a bad wire between the resistor & the cathode as there was no contact. Actually, my wire, a solid cloth one was broken near the pin inside the cloth. I've fixed that & then measured the resistance : both pins 3 & 8 readings on V3 show 51.4k.
After that I've installed all the tubes, & now it sounds fantastic !!!
It was my 1st tube amp project ever, & I recommend every AC30 KORG RI owner to replace the crappy stock PCB with which I've always got crackling noises with knobs, & whose cut knob didn't work correctly (it had always started to cut after the 12:00 position).
I think I'll certainly build a 5E3 project one of these days, certainly far easier than this one.

However that may be, some pics now :







Note that I've used SoZo vintage caps on the Hoffman board & Alessandro pots (made in Canada). The caps should "break in" within several hours, nonetheless the pots are just awesome compared to the stock horrible Omeg ones.

A view of the deffective wire circled in purple :



Thank you for your help guys, & particularly to you Doug Hoffman for all your informations, instructions, tips, schematics,...but above all for you way to be  :glasses10: