Vox ac50

[Stevem]

Anyone know about how long the partially cathode biased version of the ac50 was made?

[Long Distance Call]

Steve,
As far as I can remember and after looking at some if my old notes, the 2 input one channel AC50 was issued in late 1963 and showed up in the catalogs in early 1964. It had a GZ34 rectifier and an 800 ohm common cathode resistor on the EL 34.

The AC50/2 came out later in 1964. It had four inputs and two channels, a GZ34 and an 800 ohm common cathode resistor.

Next was the AC50/4 MK II, which is basically the same as the AC50/2 with a 470 ohm common cathode resistor on the EL34's. The grid bias voltage supply was changed from single feed (traditional Fender Marshall etc.) to a dual supply with each tube having it's own bias pot.

In January 1965 they came out the AC50/4 MK III, with diode rectifier and a separate 47 ohm resistor on each EL34.

I can post any of the schematics if you want them.

LDC

[Stevem]

Sure the schematics would be great, thanks!
I recall playing the mkII back in the day and it sounded good,that's why I do not understand why the Fender silverface somewhat cathode biased amps got such a bad rap back in the day, as to me what I found from playing them other than the drop in wattage was the sound of those Utah speakers used was a bit too far removed from the sound of the Jensens they used to come fitted with!
In fact the only Ciramic magnet Utahs I ever liked the sound of the ten inches that came in super reverbs!

[RWood]

I'd love a MKIII layout in PDF if anyone has one.

[pdf64]

As far as I can remember and after looking at some if my old notes, the 2 input one channel AC50 was issued in late 1963 and showed up in the catalogs in early 1964. It had a GZ34 rectifier and an 800 ohm common cathode resistor on the EL 34.

The AC50/2 came out later in 1964. It had four inputs and two channels, a GZ34 and an 800 ohm common cathode resistor.

I think that the early tube rectified AC50 were purely fixed bias, grounded cathodes.
The big ~800 ohm resistor being a shared screen grid resistor (which then splits off to 2 x 100 individual screen grid resistors, similar to arrangement used on the JTM45).
With an LTP phase splitter.

The later mixed bias design used a floating paraphase http://www.valvewizard.co.uk/paraphase.html

Strangely, if memory serves correctly, both types sounded and responded quite similarly.
I think this may be down to them having the same pre-amp and an open loop power amp.

edit - google found http://www.voxac50.org.uk/ which answers the OP.

[Roe]

As far as I can remember and after looking at some if my old notes, the 2 input one channel AC50 was issued in late 1963 and showed up in the catalogs in early 1964. It had a GZ34 rectifier and an 800 ohm common cathode resistor on the EL 34.

The AC50/2 came out later in 1964. It had four inputs and two channels, a GZ34 and an 800 ohm common cathode resistor.

I think that the early tube rectified AC50 were purely fixed bias, grounded cathodes.
The big ~800 ohm resistor being a shared screen grid resistor (which then splits off to 2 x 100 individual screen grid resistors, similar to arrangement used on the JTM45).
With an LTP phase splitter.

The later mixed bias design used a floating paraphase http://www.valvewizard.co.uk/paraphase.html

Strangely, if memory serves correctly, both types sounded and responded quite similarly.
I think this may be down to them having the same pre-amp and an open loop power amp.

edit - google found http://www.voxac50.org.uk/ which answers the OP.

interesting - is there a schematic showing ac50s (or 100s) with a paraphrase PI?

[pdf64]

This looks close to the old fixed bias type http://bmamps.com/Schematics/vox/Vox_ac50.pdf
and this is the mixed bias version, with adjustment for each EL34 as noted by LDC http://bmamps.com/Schematics/vox/Vox_ac50_1.pdf
On the former, the channels are in opposing polarity, whilst on the later they're in the same polarity!

[sluckey]

On the former, the channels are in opposing polarity, whilst on the later they're in the same polarity!

Would you explain this comment? Thanks...

[pdf64]

The earlier version with an LTP phase splitter feeds the normal channel into the LTP '+' input, the brilliant channel into the '-' input, similar to a Watkins Dominator / Marshall 18 watt.
So if the same signal is fed into both channels, certain setting would act to cancel itself out.
Whereas the later paraphase version mixes the channel outputs together with 220k resistors (similar to a twin channel BF Fender) and feeds the mixed signal into the phase splitter.

[sluckey]

Got it. Thanks...

[Long Distance Call]

I have these three AC50 schematics to throw into the pile for comparison. I mentioned a Mk. III schematic in an earlier post, but I'm pretty sure I was looking at two different versions of the same schematic. At least I think that's what happened...

Best, LDC

[pdf64]

Brilliant, thanks for sharing, I've never seen the proper Vox drawings for the tube rectified models before.

[pdf64]

Back to the OP, it looks like AC50 (mixed bias type) may have been in production in the 70's, during the Dallas-Arbiter period http://bmamps.com/Schematics/vox/Vox_ac50_1976.pdf
Some more info about the AC50 development / JMI period at http://www.voxshowroom.com/uk/amp/ac50bb2.html
If you really want to know, it may be worth asking the question over at http://vintageamps.com/plexiboard/viewforum.php?f=2

[Long Distance Call]

Here's the schematic for the Mk IV Os/163 that was mentioned on the Voxshowroom.com page that pdf64 posted. I did not include the MK IV in the earlier post because it's not considered to be one of the golden era JMI amps, rather it's from the Vox Sound LTD. period that followed.

This morning, I re-noticed that there are actually two different OS/053 schematics, one labeled AC50/2, and the other one labeled AC50/4 MK II. I've already posted the AC50/2, so I'm posting the AC50/4 MK II here now.

Also, I'm not so sure there was ever an AC50 made, starting with the first AC50 OS/44 in 1963, that did not have a negative grid bias voltage and a cathode resistor(s), the only variations being the size and number of cathode resistors and whether it's a single feed (early) or dual feed (later) grid bias. but, ya never know...

All the best,
LDC

[pdf64]

I'm not so sure there was ever an AC50 made, starting with the first AC50 OS/44 in 1963, that did not have a negative grid bias voltage and a cathode resistor(s)

I've only had chance to open up a couple of tube rectified AC50s, but both were purely fixed bias, grounded cathodes on the EL34, as per the schematics
Thanks a million for those schematics!