Vox 730 Power supply

[Fifi79]

Dear friends,
nice to join your wonderful group; I'm Emanuele from Italy. I'm restoring a seriously damaged ampli "Vox 730" (one of models used by Beatles) where the power supply has been totally modified. Anyone can help me finding schematics?I haven't found anything regarding this ampli..

Emanuele

[vintagekiki]

Vox series 4 & 7 schematics
http://vintageamps.com/plexiboard/viewtopic.php?t=72679

[Robert M. Martinelli]

Hi Emanuele,
it will be very difficult for you to find some of the original components, i.e. the four germanium OA200 rectifiers, if you' re concerned with functionality only you' ll find modern replacements ( 1N4148 ) will work great too, of course, if you' re willing to do a vintage-correct restoration instead, you' ll have to search for the originals. As to the PT and OT, I hope they're still in good conditions, as it will be impossible to find the originals ( but it will be possible to find something suitable, maybe you'll need to do some adaptation work ).

Good luck with your restoration project!

Best regards

Bob

[vintagekiki]

Robert

OA200 is silicon ( NOT germanium ) diode.
OA200 is general purpose diode for 50V/ 160mA ; OA200 replacement with BA 127, BAY 18, BAY 44, 1N4148...49
I use 1N4002…4005 when I have not OA200 or recommended replacement

About OA200 diode look:

http://www.datasheetarchive.com/pdf-...DSA-88567.html Datasheet
and
http://english.electronica-pt.com/db....php?ref=OA200 Cross reference

About restoration
If you have not orginal output transformer, satisfactorily replacement is any 3400 - 4000 ohm / 50W OT for two tube 6L6 or EL34 ( Fender or Marshall etc ) or for four tube EL84 ( Vox AC30 ). For power transformer use any PT who gives + 350 to + 380V DC measured on first capacitor after rectifier ( like PT for Vox AC30; Marshall 1986 … )

[Robert M. Martinelli]

Hi vintageKiki,
sorry, something from my early "electronic" days made me remember the OA prefix was referring to Germanium diodes ( when I built my first radio receiver at the age of ten I used an OA 95 ).

I also beg Emanuele' s pardon, as I didn' t surely want to mislead him.....maybe I' m just getting old

Regards

Bob

[Fifi79]

Vox 730 Reply

Dear guys,
thank you so much for your reply! You have been really helpful. The amp works "almost" properly but the previous owner changed only the power supply circuit...so the main problem is that the EL84 tubes are almost burning (they are biased around 90% of maximum plate dissipation!). I saw the schematic posted on the link suggested by Vintagekiki (thank you!!) but they are sligthly different from mine (in my 730, the grid supply comes directly from the Output transformer).

I've seen this pictures:
http://www.photobox.co.uk/my/photo?a..._id=1300534953 . My 730 is identical!

The right schematic are the following:

http://www.photobox.co.uk/my/photo?a...to_id=94931062

but the picture is too little (should I order it?). I need only the value of the two big resistors (the first big one, after the rectifier bridge and the second "green" one...).

Thank you guys for your help. All the best!

Emanuele

[Robert M. Martinelli]

Hi Emanuele,
It seems like the previous owner attempted to obtain a sort of "triode" connection, ( screen grids and plates at the same potential ) so I strongly recommend to bring back things to their "original" status, connecting the screen grid supply right after R94 ( 470Ohm 1W at the top right after the main rectifier ) and put back in the screen grid current limiting resistors ( R95-R96-R97-R98, 100 Ohm ). This will take away some of the strain from the output valves and will also help protect the screen grids; as to the ( cathode ) bias, you' ll find the amp is biased way over the value you've calculated; if your +HT voltage is 330VDC, I expect the voltage on the plates ( er...anodes on this side of the ocean ) to be around 325 VDC, as some 5 volts will drop across the OT' s windings.

To check for the actual values, switch the amp off, wait for the caps to discharge, measure the actual value of R89 ( 63 Ohms on the schematic ) and take note of it; switch on the amp and let it sit for some minutes and measure the anode voltage and take note of it.

Now measure the voltage drop across R89, ( you' ll probably find your bias voltage to be around 12 VDC ) then divide the measured value by the actual value of R89 you measured before, this way you' ll get the sum of the quiescent currents flowing through the four tubes. Supposing the four valves have similar characteristics, dividing this value by four will give you the quiescent current flowing through one valve. Chances are you' ll find the quiescent current to be around 47-48 mAmps per tube, VERY hot, and very close to the way AC30s are biased ( about 47 mAmps quiescent ).

Now subtract the bias voltage from the anode ( plate ) voltage, this is the voltage drop across the valve under static conditions; you will probably find this value to be around 313-315 VDC.

Static plate dissipation will then be 315VDC*0,0475A=about 15W ( note that EL84s/6BQ5 are rated at 12W max static plate dissipation....Mr. Denney and his fellow designers were a bit rough on output tubes ).

I would also use a 5W resistor for R89 ( bias resistor ) as it has to dissipate 2,3W under static conditions, this is too close to its rating IMHO ( 3W ) and thus the resistor will heat up considerably, with the chance of thermal drifts, a 5W resistor will work at a much lower ( and safer ) temperature.

As to the OA200 replacement, the minimum suitable replacements would be 1N4004s ( 1N4002s and 1N4003 are not suitable because their voltage rating is 100 and 200 V respectively ), personally I prefer to use 1N4007s because of their higher voltage rating ( 1000V ).

Hope this helps

Best regards

Bob

[Fifi79]

Vox 730

Dear Bob,
thank you for your precious suggestion (ma sei italiano come me :O?).
The previous owner didn't modify the circuit...looking at the pictures of the link I posted before I learnt that there are two kind of Vox 730. In my version, the supply of the screen grids come out directly from the coil of the Output transformer and also there are the 100ohms resistors before the screens. The main problem is that the, measured after the HT bridge on the capacitor without load the Voltage go around 415V (Maybe also the HT power supply coil is different..) and I need to know the exact value of the two resistors in series of this schematic (I don't know how to download it by this website):

"http://www.photobox.co.uk/my/photo?album_id=23598354&photo_id=94931062"

because the previous owner only changed this two resistors oh HT supply...

Furthermore, thank you for your suggestion regarding the biasing of this amp. In addition, just to let you know, differently from the original schematic where R89 collects the cathodes of all four EL84, in my 730 there is a cathode resistor for each couple..

I'm wondering to hear your comments..thank you so much for your help

Emanuele

[vintagekiki]

Emanuele

Unknown resistor on your pics http://www.photobox.co.uk/my/photo?a..._id=1300534953 between diodes BY100 and C56/32uF/450V is Brimistor CZ4 similary as in schematics for Vox AC50’65 ( http://www.voxamps.co.uk/downloads/circuits/ac50_1.jpg )
A Brimistor is a Positive Temperature Coefficient (PTC) thermistor. In an amp they are used to limit the B+ inrush current when the unit is first turned on. In most cases the CZ4 can be replaced with a 22 ohm 6 – 10 watt resistor.
The replacement is based upon the amount of current draw and the hot resistance required by the circuit as in schematics for Vox AC30’89 ( http://www.voxamps.co.uk/downloads/c...s/ac301989.jpg )
Information about Brimistor CZ4 is found on http://www.wizard-labs.com/land_of_vox.html
Uknown resistors on your pics http://www.photobox.co.uk/my/photo?a...to_id=94931062 is most likely two resistor in series 22 ohm 6 watt as in schematics for Vox AC50’76 ( http://www.voxamps.co.uk/downloads/circuits/ac50_2.jpg )
And now possibly is combination Brimistor CZ4 in series with 22 ohm 6 – 10 watt resistor.
So if Brimistor in your amp is bad on http://vintageamps.com/plexiboard/viewtopic.php?t=72679 is found schematics for Vox series 430/730'66 poweramp without Brimistor & resistor.

PS: Look Visitor Messages in your profile.

[Robert M. Martinelli]

Hi Emanuele,
Now I understand ( thanks VintageKiki ) that you probably have an early issue of 730.

I recall that Vox had reliability problems on their early versions of the AC50 which had an automatic ( cathode ) biasing system as well. I don' t know about the 4- and 7- series amps reliability record ( after all, they're so rare I don' t think it' s possible to do any statistics ), but it could be that they had similar problems and Vox ( or Triumph electronics ) added the 470 Ohm resistor in series with the +HT to lower it and improve reliability.

As to the bias resistors, the layout actually makes no difference as long they provide the required voltage on the cathode, you could have a single 63 Ohm, or two 126 Ohm ( one for each couple ) or four 252 Ohm ( one for each valve ), the only difference would be the power rating, as one resistor would have to take the sum of the four tubes' cathode current, two resistors half of that, and four resistors the current from a single valve.

I'm not so keen as VintageKiki on Vox Hybrid amps ( though I own a "mega-super-rare" UL4120 I restored myself, the model Paul McCartney used recording "Revolver", "Sgt. Pepper's".... and live on the "Magical Mystery Tour" ), but I would like to give you some measuring hints you' ll find to be valid regardless of the amp you' re servicing - to measure your +HT voltage ( and all the other operating voltages ) you'll need everything in its place. Measuring with no tubes in your case makes no sense, e.g. your +HT voltage will be higher because no current is flowing, and if you measure the bias voltage across the cathode resistor(s) with the tubes out, you' ll get no reading ( no current=no voltage drop across it ) - also, never operate the amp without a load, if you' re concerned about the volume, use a dummy load of the same impedance.

If you find that your +HT voltage is well over the rated 330 VDC with the tubes in, cross-check the heaters' AC voltage, it should be 6,3 VAC, if that is on the high side too, chances are your mains voltage is a little high, in this case I would recommend to switch the voltage selector on the 245 VAC tap, if you have one ( telling the truth, I keep it this way at all times on my 1964 AC30 to make things safer ).

Hope this helps

Best regards

Bob

P.S. - and, yes, I' m Italian too!

[Fifi79]

Vox 730

Thank you guys!!!
Finally, repaired my Vox 730. I'll tell you my experience...At the beginning, I tried many resistors in order to have something like 330V on the central tap (obiouvsly taken with tubes) of Output transformer (as suggested by VoxRules, this is the correct voltage in order to have a plate dissipation of around 15W/El84...very warm but I guess is one of secrets of Vox sound..) I used a series of 51ohm (7W) + 220ohm(20W) resistors until I found around 340V. The amp was very good but (the two resistors were dissipating a wide heat..although much the dissipating power on each resistor was lowen than maximum power dissipating of the two resistors). Finally, I could try a Thyristor very similar to Cz4..this kind of device decrease the resistance when the temperature grow up...so..the resistance starts from around 3.3Kohm and goes down until 50ohm when is really hot. The amp now works properly..thank you for all your suggestions and sorry for my bad english (I hope to improve it eheh). All the best!

x VintageKiki: I'll reply you now!
x VoxRules: Grazie mille per i preziosi consigli!

Emanuele