Vox V125 Assistance?

[wcieslik]

Thanks Man... but yeah ... been there ....

It would seem mine is a lead cabinet with a Bass cct board and modified somewhat.

Thanks anyway.

[wcieslik]

Well here we are again...
2020 is fast wrapping up and yes ... I did have the V125 working, reassembled, (still working), and then back on the shelf since then. (was that really 2017 ?!?)

Jamming with some loud friends and my little VT20X was not cutting it so ... hey! grab the V125 from storage.

Click! phooph ! (back to the VT20X for the jam)...

One of the EL34's in the V125 is arcing at HT switch on, and thankfully, blowing the HT fuse. ...
Back to the drawing board.

At all of the EL34 sockets, (with tubes out) :
Pin 5 ~= -38.5v
Pin 4 ~= 482 <= range <= 490 (seems to drift up and down over time)
Pin 3 one pair at 490 and the other at 486

Oh .. and 4 x EH's on order ....
(never did find an owners manual ...still looking though !)

[wcieslik]

Damn.. forgot to ask my question ...
I know the heaters are working, 'cause I can see 'em light up !!!)
But I get no DC voltage at the heater pins (2 and 7)... So assume the heaters must be AC ?

How best to measure these :

• ACv meter direct between pins 2 and 7 ?
• or ACv meter between pin2 and chassis and then pin 7 and chassis ?

I'm a little worried about grounding something I shouldn't ....

Cheers,
WIt C.

[wcieslik]

Sussed it ... 6.5Vac between pins 2 and 7 on all sockets. All good there !

[wcieslik]

I have re-traced and updated my cct diagram for my V125 combo.
The updated diagram is within my PDF document, found at :

• http://www.lawleycottage.id.au/vox/index.html ...

The result is a diagram that matches what I have, and highlights the differences to the original cct.

The input section is drawn in BLUE and differs entirely from the original CCT.
I have also re-typed all of the component values and numbers for easier reading ...

RED components and joins are those that that were found to be almost the same, (but slightly different).

I note that the ONLY cct's I can find are horrible pdf's of the original hand drawn one...
Hopefully mine will prove to be a little easier to read...

Cheers,
Wit C.

[wcieslik]

Nearly done...
Have replaced :
All capacitors except 4 x ceramics
All power resistors
All valve sockets
Hum balance pot (and moved it )
All EL34's
re-drawn the cct and added a parts overlay, and removed the copywrited(?) images from my AsBuilt document.

• http://www.lawleycottage.id.au/vox/index.html ...

[Pedro Vecino]

Howdy All,
If anyone has had any experience with amp and it's shortcomings, if any would be appreciated.
Matt

A defect of this amplifier is that the aluminum chassis tends to be deformed by the mass of the transformers. On some occasion I have had to put steel plates and squares riveted around them to reinforce it after recovering its original shape.

[wcieslik]

That's odd... the chassis in mine is steel, (with wooden end blocks) ? Maybe combos are steel and heads are aluminum ?
Even with the steel chassis, mine is still a bit flimsy. I can feel the flex in it, each time I turn it over or around to get to the other side...

[Pedro Vecino]

The ones I tried were heads. Despite being from a long time ago, they should be made of aluminum since I consider it impossible that I could recover their shape with basic tools if it were made of steel. The huge longitudinal cut in one face of the chassis has something to do with it.

[wcieslik]

Finally got around to examining the AND/OR switch on the back of the combo more closely, and worked out that the :

• OR position uses the 8 ohm tap and disconnects the internal 8 ohm load if you plug in an external cabinet. (which should be an 8 ohm cab).
• AND position uses the 4 ohm tap and parallels an external 8 ohm cabinet with your internal 8 ohm load, to provide 4 ohm total load. (or halves your output speaker power if you have no external cabinet)...

added/updated my ongoing PDF doc at http://www.lawleycottage.id.au/vox/index.html

[Helmholtz]

What do you mean with internal 8R load?
R23 is a 820R resistor used as a base load to protect the OT from no-load condition.

An 8R load resistor would have to be huge.

[g1]

What do you mean with internal 8R load?
R23 is a 820R resistor used as a base load to protect the OT from no-load condition.

By 'internal 8R load' I think he means the combo's own internal speaker.

[wcieslik]

Thanks G1, yes...
Just to clarify, I did mean the internal speakers, which provide an 8 ohm load. (Two x 4 ohm speakers in series).

[wcieslik]

Still working on my V125 Climax...
While it's all opened up, I want to verify that the OP transformer is OK. and also work out the winding ratio.

My DC resistance check seems OK (for what it is)...
Primary CT to Green = 26.6 R
Primary CT to Grey = 21.5 R
Secondary (8ohm tap) RED to Blue = 0.4 R
Secondary (4ohm tap) White to Blue = 0.3 R

No shorts to chassis or casing.

In an attempt to work out the winding ratio, I put a 6.6Vac 500mA signal across the 8 ohm tap on the secondary (Red to Blue) and measured the AV volts at the Primary, and was surprised to find it was only 48Vac on each side.
This was much lower than what I was expecting. (and works out at a winding ratio of approx. 7.3)...

Can anyone advise me as to what the expected winding ratio is, for the OP transformer ?
If anyone has replaced one, what did they use ?

Many thanks in advance.

[pdf64]

So what did the 6.6VAC supply actually measure when it was connected to the OT secondary winding?
The primary voltage of interest is the class A area of operation, ie across the whole winding, so it looks like would that be 96V?