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New build VOX AC30 clone...help, no signal

Transcrit

Member
Messages
17
Hi all need help please.

I built the vox AC30 clone almost identical to an Austrian guy's amp on the net...
http://homepage.univie.ac.at/markus.breier/php/amp_ac/the-plans/layout_e/


Checked everything 20 times it seems. Still no sound from my guitar on any channel. Only difference I did from his amp was I took a Hammond power transformer, output transformer and choke (vintage specs all). The output T has the exact same color wires as his. My choke is a 30H. The power T has one extra 'shield' wire which I have grounded to the mains earth. I left the extra primaries of the transformer unconnected and covered with tape and tucked away (230 vac here in Sweden).

Many volt readings are good but I don't really know what to be looking for as this is my first build. The incoming AC from the transformer is as expected for the high voltage 580 V, 6.3 V for the heater and 5V for the rectifier. the 6.3 has a central tap. (the 5v is listed 5 - 0 in the PT spects)

There is DC at the primaries of the Output T (some 390 measures against ground) but no sound is produced to my speakers except a small Hum. All tubes heat up fine so AC 6.3 is working its rounds.

I have tried swapping tubes around and changing rectifier tube. I have run out of ideas. There was initial hum and this was traced to loose ground. Checked under the board and all the wiring time and time again. heating wire works, lamp on, Tubes are heating up, not overheating. There is some interference nose as I measure the connections with my multi meter to the Ecc83 tubes (at the grid I think) the blue wire connection.

I can rin my sterio reciver radio through it an there is some radio coming out out the Celestian Vintage speakers but its quite distorted. Amp Volume and Master volume actually do function on this horrible radio signal.

This is my first Amp build so I am not too familiar with what to expect where in measurements for this VOX AC30 AMP version. I did have to relocate the Choke and first caps on the OT side (not as the Austrian drawing had them). I have rewired most things looking for bad solders. I have checked measured resisters etc.

Does anyone have a page of info to compare readings?


Would a ground loop have such non-working effect on the whole thing? (I ran a ground wire to take the 'shield' of the input signal at the input jacks and isolated the solder station near the input jacks with plastic. I don't really understand star grounding; but I understand the principles. I just made one chassis connection and ran all grounds to that (except the shield from the transformer, it goes to the AC ground/mains).

Any suggestions about what readings I should look for and where?

Best regards,

Hugh
 

Cirrus

Member
Messages
2,438
If your amp is the same circuit as a vox ac30, check out thevoxac30guide.com - that book has circuit diagrams, expected voltage readings at different parts of the circuit etc.

From your post it doesn't sound like you know how to troubleshoot the problem. You need to work through process of elimination really, since it's a DIY build it literally could be anything stopping it from working.
 

RussB

low rent hobbyist
Messages
11,158
Lots of detailed pics would help us help you.

What do you have for diagnostic tools?


Does your amp have the FX loop? If so, try plugging your guitar into the FX return
 
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pdf64

Member
Messages
7,659
There is DC at the primaries of the Output T (some 390 measures against ground)
That seems high; maybe the power tubes aren't drawing much current?
To check that I suggest you verify the resistance between cathodes and chassis ground (eg 68 or 270 ohms), and the control grids and chassis ground (eg 220k).
Then measure voltage at those points with the amp powered up.
 

Transcrit

Member
Messages
17
Lots of detailed pics would help us help you.

What do you have for diagnostic tools?


Does your amp have the FX loop? If so, try plugging your guitar into the FX return
I have a multimeter only (a nice fluke 177).

Just got a little touch of 266 VDC as I plugged mu guitar cable into the send of the FX loop. What's going on here? That's the DC at the power end...reads that on number 7 of the V3 there. 320 at 6 of V3.

I have wired the top of the C11 to the top of the R19 as I have no FX loop installed (the layout showed to do this on his site).
 

RussB

low rent hobbyist
Messages
11,158
Why do you have the FX loop jacks wired at all if you don't want the loop?
 

Transcrit

Member
Messages
17
That seems high; maybe the power tubes aren't drawing much current?
To check that I suggest you verify the resistance between cathodes and chassis ground (eg 68 or 270 ohms), and the control grids and chassis ground (eg 220k).
Then measure voltage at those points with the amp powered up.
330vdc from cathodes (pin 7) to ground and pin 9 to ground measures 321v on v4 v5 v6 and v 7 . Pin 3 has a power down resistance of 260 ohms and fired up 11 ish all tubes.

Crazy thing is that the cathode to earth resistance measures up from 1 to 1.5 M ohms and climbing (pin 7) when power is down and caps drained. I thought my multimeter might have been broken but its a good new fluke...it was stuck at autorange and sometimes climbing climbing from 500 k up to 20 M ohms (this is with caps bled etc)

What's going on guys?
 

Transcrit

Member
Messages
17
Why do you have the FX loop jacks wired at all if you don't want the loop?
I wired the thing like the guy's amp in the link (original builder). He had the loop (called passive in his layout) and claimed if it's not being used you can connect it as per diagram. (see link above).

I would use the lop of course but for testing purposes I just want to fire up the amp and not have to bother linking in a send/return cable.
 

Transcrit

Member
Messages
17
I don't know how to post images here. Seems like only an internet link is allowed. I try but the software is looking for a Url
 

pdf64

Member
Messages
7,659
the cathode to earth resistance measures up from 1 to 1.5 M ohms and climbing (pin 7) when power is down and caps drained. I thought my multimeter might have been broken but its a good new fluke...it was stuck at autorange and sometimes climbing climbing from 500 k up to 20 M ohms (this is with caps bled etc)
It's just the current from the meter (that's used to measure resistance) acting to charge up the cathode bypass cap.
If you leave it one it usually stabilises out; sometimes yes the autoranging kicks in and it gets stuck in a cycle.
Best thing when that seems to be happening is to switch the meter to manual range setting, to see if that calms things down.

What about the control grids, terminal #2, resistance to chassis and static voltage?

It usually works out best to post images on to a hosting site like photobucket and then link to it.
 

pdf64

Member
Messages
7,659
Just got a little touch of 266 VDC as I plugged mu guitar cable into the send of the FX loop. What's going on here? That's the DC at the power end...reads that on number 7 of the V3 there
From the schematic http://homepage.univie.ac.at/markus...ontent/uploads/2012/11/AC30_Schaltplan_V2.pdf the fx insert loop looks to be between 2 coupling caps and doesn't have a dc reference to ground; a high value resistor, eg 1M to 10M, would normally be used.
Without that, the Vdc at the fx loop node between the caps may drift up. I guess that the V3 terminal 2 grid may be at ~100V, so it may drift up that high.
Immediately something with a dc path to ground is connected, eg meter, guitar, the Vdc on the loop should be pulled to to 0Vdc.

It would be useful to show the Vdc / Vac (as appropriate) on each tube socket terminal, eg http://bmamps.com/Schematics/marshall/Marshall_jtm45_pa_45w.pdf
 

Transcrit

Member
Messages
17
1) Best thing when that seems to be happening is to switch the meter to manual range setting, to see if that calms things down.

* tried that...no go.



2)What about the control grids, terminal #2, resistance to chassis and static voltage?

control grids, terminal #2 for V4, V5 is 33.8 ohm and V6, V7 is 35 ohms. Static's about 3 MVAC

Will post images up soon but they are exactly like the link's images. See link top. Guy has great images posted.

Can't for the life of me see any thing I did different to him. I even done some moderations but changed them back to his set up when amp didn't fire up. He wired the pots of the master volume wrong and admitted it so that's one thing he did wrong.

My shield on my PT I wired to mains ground. He didn't have a shield.

I will post a chart with readings on all tube pins
 
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RussB

low rent hobbyist
Messages
11,158
Just got a little touch of 266 VDC as I plugged mu guitar cable into the send of the FX loop. What's going on here? That's the DC at the power end...reads that on number 7 of the V3 there. 320 at 6 of V3.

So you have 266 volts on the FX loop jacks? Please double check that. That is a deadly situation! If that is the case, unplug that amp and do not turn it back on until you have a tech look at it!

I have wired the top of the C11 to the top of the R19 as I have no FX loop installed (the layout showed to do this on his site).

But you DO have a loop installed, and it looks like you're putting DC into the SEND jack?


The purpose of plugging your guitar into the FX RETURN jack is to bypass the pre-amp and see if you get signal to the power amp.
 
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Transcrit

Member
Messages
17
So you have 266 volts on the FX loop jacks? Please double check that. That is a deadly situation! If that is the case, unplug that amp and do not turn it back on until you have a tech look at it!




But you DO have a loop installed, and it looks like you're putting DC into the SEND jack?


The purpose of plugging your guitar into the FX RETURN jack is to bypass the pre-amp and see if you get signal to the power amp.
Yep, 264 VDC measured through the guitar jack. Stung my hand too. (Sorry, had to turn on the amp again to give you that 'double check´. PDF 64 even has theory for the pull of voltage...

No I'm super careful but I just did not see that coming. Guess I'll have to warn the Austrian too.

I posted a volt chart for the tubes on a website alone with some pictures of the sneeky amp. http://hughcurtis.weebly.com/
 

jay42

Member
Messages
7,055
Yep, 264 VDC measured through the guitar jack. Stung my hand too. (Sorry, had to turn on the amp again to give you that 'double check´. PDF 64 even has theory for the pull of voltage...
You have a dangerous mis-wire. find the mistake with the thing unplugged. Since the amp isn't melting down, you could remove all but the rectifier tube and look at voltages again. It's probably too late now, but I hope you used a unique wire color for B+. I use Red.
 
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pdf64

Member
Messages
7,659
If it stung then there's a problem!
And it may be the same reason that the amp in't working right.
Get busy on the voltage chart.
 

Transcrit

Member
Messages
17
If it stung then there's a problem!
And it may be the same reason that the amp in't working right.
Get busy on the voltage chart.
I posted a volt chart for the tubes on a website (see the downloadable document entitled voltage chart) along with some pictures of the amp in question. http://hughcurtis.weebly.com/

Where should I run the ground reference to rid me of that DC in the FX insert? Wouldn't this cause a loop since the jacks are grounded already?
 

RussB

low rent hobbyist
Messages
11,158
I have wired the top of the C11 to the top of the R19 as I have no FX loop installed (the layout showed to do this on his site).

I'm re-posting this because I think it's where your problem is. You DO have a loop installed, and you have the jumper. Remove the jumper, test for DC on the FX jacks again.

Next would be to completely disconnect the FX jacks from the circuit.
 

RussB

low rent hobbyist
Messages
11,158





Vacuum tube voltage chart VOX AC30 clone

Tube Pin 1 Pin2 Pin3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9
V1 141 0 1.04 AC heater AC heater 138 0 1.1 AC heater
V2 158 0 1.04 AC heater AC heater 137 0 1.1 AC heater
V* - - - AC heater AC heater - - - AC heater
V3 321 265 297 AC heater AC heater 320 265 299 AC heater

V4 - .003 11.2 AC heater AC heater - 330 - 324
V5 - .003 11.2 AC heater AC heater - 330 - 324
V6 - .003 11.2 AC heater AC heater - 330 - 324
V7 - .003 11.2 AC heater AC heater - 330 - 324
V8

V9 - 326vdc18vac - 285 vac - 285 - - 337 17.4VAC
 




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