Vox V125 Assistance?

[Helmholtz]

A 4xEL34 OT should have a primary impedance of 1.6k to 2k. From this the voltage transfer ratio (step-down ratio) calculates as 14 to 16 referring to the 8R secondary. So 6.6V at the 8R secondary should show as around 100V at the full primary.
Did you verify the 6.6VAC when connected to the secondary? If it drops significantly, you might have a winding short, which hardly shows in the DCR.

To verify, do the neon flash transformer test.
https://music-electronics-forum.com/...413#post735413

[vintagekiki]

So what did the 6.6VAC supply actually measure when it was connected to the OT secondary winding?

To measure the transmission (winding) ratio of OT, but did not give the voltage measurement results.

[wcieslik]

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?

Measuring 6.59Vac at the input. and yes measuring 97Vac across the whole winding.
which gives me winding ratio 14.7 and Imp ratio of 216...??

[wcieslik]

A 4xEL34 OT should have a primary impedance of 1.6k to 2k. From this the voltage transfer ratio (step-down ratio) calculates as 14 to 16 referring to the 8R secondary. So 6.6V at the 8R secondary should show as around 100V at the full primary.
Did you verify the 6.6VAC when connected to the secondary? If it drops significantly, you might have a winding short, which hardly shows in the DCR.

To verify, do the neon flash transformer test.
https://music-electronics-forum.com/...413#post735413

ok.. This matches up with what I'm seeing ...
6.6Vac at the ohm secondary produces 97Vac across the full primary.
So probably good to go ?

Thanks folks...

[wcieslik]

Measured results for those still interested ...

Hmm...
Using the Plate Resistance I found on a random data sheet (3400 ohms at 480v plate voltage)
I get the Secondary taps on this OP Transformer are :
RED = 16 ohm
White = 8 ohm

Not 8 and 4 as the cct diagram suggests....
Which means I've been using the 16 ohm tap into an 8ohm speaker set up, for 30 odd years !! (Who Knew !!)

But wait (there's more)...
If I use a plate resistance of 1600 ohms or 2000 ohms as suggested for EL34's in quad setup, then the calculation seems about right...
RED = 8 ohm
White = 4 ohm
as expected.

I think I need to research more data sheets w.r.t the EL34 plate resistance at 480v plate voltage ...

[pdf64]

Yes, as you’ve worked out, parallel push pull pairs will halve the load impedance calculated for a regular push pull pair. So it’s all working out as expected. But (as I understand it) you’re getting a good sound and power output, how could it not?
What’s the random data sheet you refer to, as it seems somewhat extreme (unless screen grid voltage is from a lower supply)?
EL34 anode resistance and idle HT voltage aren’t the relevant things here. As pentode anode resistance is generally much higher than the chosen load impedance and for AB fixed bias amps, the screen grid and anode voltages at full load will sag from their idle levels.
Typical EL34 AB fixed bias guitar amps rely on their HT sagging about 100V at full load, compared to its level at idle

[Helmholtz]

3.4k is a typical plate load impedance (Zaa) found in EL34 literature for class AB operation and a (loaded) plate supply voltage of around 400V.
With higher plate voltage it is recommended to increase Zaa to 4k.

Datasheet values are for one PP pair.
Two pairs in parallel, as here, want to see half the impedance.

[vintagekiki]

... ...

[wcieslik]

Thanks folks,
Very much appreciate all of the comments coming back.

pdf64 , and Helmholtz ah ha !! (the penny drops!), Two pairs, in parallel !!! Of course! Why did I not see that !!!
The random data sheet was one of several EL34 spec sheets I downloaded from different manufacturers. (Amperex I think, and I used the Class AB two tubes, wasn't sure how to handle the difference in the supply. That's how I arrived at 3400ohms) FWIW, See attached

vintagekiki
Thank you (again). You have the best diagrams ! That's exactly what I was after, and trying work out manualy ! (in case I needed to replace it).

I have a working V125 again !!
(having replaced all caps and tube sockets, some resistors, hum balance pot, and a bunch of wiring I didn't like.)

I ran it up 'sans EL34's and through a variac to a dim bulb to the amp, with a 1KHz signal in and a 8 ohm 25W dummy load, but all volumes set to '0', and brought the AC volts up slowly, while measuring B+. All good !
Powered it all down and did same again with new matched EL34's fitted, and let it sit for a while.
Then same again with speakers attached instead of the dummy load, and checked the volumes and distortion working OK... All Good !

Then did same again still with variac, but removed dim bulb.
And finally just the amp plugged into the mains ...
Check all the voltages that are marked on the circuit diagram and all are well in the ball-park and stable.

and had a quiet play. How much have I missed that valve sound !!!
Distortion sounds terrible, (but it always did).
Breakup seemed to be quite early ... but no unexpected noises or crackles

Whilst rebuilding I added 1ohm 1% resistors to each of the EL34's, so last step was to adjust the bias.
Found it was running super hot (70-80ma), so dialed it right back (35-45ma)...

Very pleased so far ...
Although I don't understand why I see approx. 10mA difference in the bias current from one half of the pairs to the other half. (Is that to be expected ? Is that because the plates, pin1 and pin6, of V4 run at approx. 5V different voltage ?)

I haven't verified these voltages yet ... I'll do that today ...

Thanks you all again, this has all been very educational !

[vintagekiki]

Whilst rebuilding I added 1ohm 1% resistors to each of the EL34's, so last step was to adjust the bias.
Found it was running super hot (70-80ma), so dialed it right back (35-45ma)...

http://web.archive.org/web/20120507060924/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Common Power Tube Bias Charts

http://web.archive.org/web/20110621211343/http://diyguitarist.com/Images/BiasChart-EL34.jpg