Vox AC4TV - on the bench - Fixed - Let's talk about the circuit now.

[LooseChange]

I'm working on a Vox AC4TV (Schematic attached).
Sound not getting through.
Trouble seems to be the second half of the 12AX7...
I used a listening amp and I get sound on the 12AX7 pin 2 but not pin 1 and 3.
The B+ feeding the second half of the 12AX7 is 287v
Pin 1: 279v, pin 2: 0, pin 3: 5v
Obviously pin 1 is wrong but the 220k is good.
I think there is a bad solder joint and I was hoping someone could tell be what the voltages around the components connected to pin 2 and 3 should be.

I ask for your help because the PC board is tight and hard to work with out of the cabinet.

Thanks!

[VMS]

Maybe C6 is bad.

Gain stage looks like some kind of boot-strapped pseudo split-load stage.   

[LooseChange]

C6 is good. I get signal through to pin 2.
Yeah, I don't know what you call that but you would think with that arrangment there should be some voltage on pin 2?

[phsyconoodler]

Try measuring for AC voltage.Plug in your listening amp and measure the AC at pin 7 and then at pin 2 with the pots all the way up.You my have a broken volume or tone pot which will block the signal from getting through.

[LooseChange]

I get signal at pin 2 and pin 7. Checked the Volume pot... All good.
No signal at pin 1 and 3.

[stingray_65]

Dumb Question here Dan,

Is the tube bad? did you try a replacement?

What about the socket? Re-tentioned it?

Ray

[stratele52]

Loosechange , have you tried a new 12AX7 , you don't write nothing about that , I guess not.

I just read the answer of stingray , he is right .

Have you a bad solder or socket contact pin 1 ?

[stingray_65]

I just read the answer of stingray , he is right .

LOL! NEWBS! they crack me up

Ask my Ex she'll tell you how right I am.

JK stratele JK

Ray

{edit-- quote-tag fixed --PRR}

[LooseChange]

Tube is good. Pin one tests good for solder joints.
What I am looking for is an approximation for correct voltages at pins 1, 2 and 3.
Thanks guys!

[LooseChange]

Trace was broken between R12 and R8! Fixed.
Thanks for letting me talk through the problem here on the forum.

That circuit around pins 2 and 3... What's it all about?

Another wattage control to look at... It's an attenuator between the OT and the Speaker.

[DummyLoad]

C15 shorted?

--DL

EDIT: never mind... lol!

[stingray_65]

Trace was broken between R12 and R8! Fixed.
Thanks for letting me talk through the problem here on the forum.

It sounded like a mechanical issue to me, something simple. I know your troubleshooting skills are way superior to mine, and I never meant to imply you don't know your basics.

I'm glad you found it! Broken traces are difficult to see and often will measure continuous until some heat or current flows though them.

Ray

[LooseChange]

No problem.  I forget to do stuff all the time.

Not impressed with the amp. Not much bottom end and a bit thin. It does saturate pretty easily with the power switch set to 1/4 watt.

[Geezer]

Another wattage control to look at... It's an attenuator between the OT and the Speaker.

At 1st glance, appears to be a type of phase cancellation (like a typical Matchless crossline MV) but on the opposite side of the OT.

I wonder what the 2x 10uf electolytics (series/opposing polarity) do? Maybe something to do with EQ'ing the output when the attenuator is in use?

I see the amp doesn't impress you, but how does the attenuator sound....IOW, does it seem to change the "tone" much when the power is lowered, or does it seem to retain the same crappy-ness, just at a lower volume? 

G

[stratele52]

I just read the answer of stingray , he is right .

LOL! NEWBS! they crack me up

JK stratele JK

Ray

{edit-- quote-tag fixed --PRR}


Sorry Ray I don't understant your joke . I'm french speaking.... but it look funny . Can you write in other words  ?

[Boots Deville]

Where that attenuator network is, is where you'd find a Zobel Network, which is not that uncommon in amps with EL84s. That's just a non-polarized cap and small value resistor in series between the OT secondaries, but I can't quite get my head around this circuit to determine if this is a variation of that or not.

What is the plate voltage on the EL84?  Wasn't there talk in these parts around Vox running relatively high numbers in their EL84 plates?  I wonder if it would sound better at a lower voltage.

[OldHouseScott]

The attenuator is just a voltage divider feeding the speaker. The back-to-back caps provide a slight high-end lift to brighten the signal when attenuated.

The circuit around pins 2 and 3 is also a shelving circuit to cut some bass at that stage. I would expect this amp to have lots of mids and highs, and not much low end as you noted. The bypass cap on V1a is shaving lows as well.

[LooseChange]

Plate voltage is 300v.

[fretts]

The attenuator is just a voltage divider feeding the speaker. The back-to-back caps provide a slight high-end lift to brighten the signal when attenuated.

The circuit around pins 2 and 3 is also a shelving circuit to cut some bass at that stage. I would expect this amp to have lots of mids and highs, and not much low end as you noted. The bypass cap on V1a is shaving lows as well.