VOX Beatle
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
plenty of documentation on these amps,
so the Beatles needed more power to overcome the screaming so they went from AC 30's to amps that were louder but blew up often, causing the Beatles to retire early from live shows, thank you VOX!
http://geofex.com/Article_Folders/Thomas%20Vox/Beatle%20Preamp%20Boards%20Documentation.pdf
so the Beatles needed more power to overcome the screaming so they went from AC 30's to amps that were louder but blew up often, causing the Beatles to retire early from live shows, thank you VOX!
http://geofex.com/Article_Folders/Thomas%20Vox/Beatle%20Preamp%20Boards%20Documentation.pdf
1 K resistor in series with the base provides volt meter reading in milli-amps, set dual supply to 4 volts for the collector and bring up base current till we reach 450 ma collector current, divide 450 by volt meter reading, 450/5.55 and we get a DC current gain of about 81.
transistors read 75 75 81 and ....51. dang, probably have to replace all 4 with a matched set of modern parts with modern bandwidth that will oscillate at modern frequencies,
or not. hfe might equalize out at 120 watts output?
boiler plate has a ser # that starts with 12, = 1969 according to>
http://www.voxshowroom.com/us/amp/beat_head.html
oh wow, parts list cross and tech data>
http://www.geofex.com/Article_Folders/Thomas%20Vox/Thomas%20Vox%20Replacements.pdf
transistors read 75 75 81 and ....51. dang, probably have to replace all 4 with a matched set of modern parts with modern bandwidth that will oscillate at modern frequencies,
or not. hfe might equalize out at 120 watts output?
boiler plate has a ser # that starts with 12, = 1969 according to>
http://www.voxshowroom.com/us/amp/beat_head.html
oh wow, parts list cross and tech data>
http://www.geofex.com/Article_Folders/Thomas%20Vox/Thomas%20Vox%20Replacements.pdf
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I remember fixing one very similar to that years ago ,a Vox Conqueror I think it was ,tonewise a bunch of crap really, spitty ,nasaly and downright nasty if you drove it balls to the wall ,still I guess as a collectors item its still worth a fix up job .
Heres a cool blues tune from Johnny Winter where he uses a similar Vox amp ,slightly ironic he's playing somewhere called the Tubeworks in Detroit with knock off transistorised Vox .
https://www.youtube.com/watch?v=PRQzWTMtSIA
theres a few more clips to be found from the same performance on youtube
Heres a cool blues tune from Johnny Winter where he uses a similar Vox amp ,slightly ironic he's playing somewhere called the Tubeworks in Detroit with knock off transistorised Vox .
https://www.youtube.com/watch?v=PRQzWTMtSIA
theres a few more clips to be found from the same performance on youtube
oh happy day, just found some old Motorola MJ21194 NPN's that we salvaged from a QSC rack mount power amp, one of those amps with 10,000 transistors and 4,000 2200uf/100 V caps,
and QSC did a good job of matching, like 80 - 81 - 80 etc!
good for 250 volts and 16 amps, 3055 can work but has suspect Vce ratings,
and QSC did a good job of matching, like 80 - 81 - 80 etc!
good for 250 volts and 16 amps, 3055 can work but has suspect Vce ratings,
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Groovy test jig ,
120 watts into 2 ohms ,thats impressive enough for the time. Maybe its just an illusion Im under ,but I always found transistor amps based on complimentary pairs ,even though less powerfull ,all things being equal ,survived output clipping better than bigger set ups
Jennings was a genius ,but I think some of his licensing deals landed him in a world of pain afterwards ,not to mention thousands and thousands of American players who thought they would sound like the Beatles with these things
120 watts into 2 ohms ,thats impressive enough for the time. Maybe its just an illusion Im under ,but I always found transistor amps based on complimentary pairs ,even though less powerfull ,all things being equal ,survived output clipping better than bigger set ups
Jennings was a genius ,but I think some of his licensing deals landed him in a world of pain afterwards ,not to mention thousands and thousands of American players who thought they would sound like the Beatles with these things
this guy really tears into it>
http://geofex.com/Article_Folders/voxamp/voxprot.htm
http://geofex.com/Article_Folders/voxamp/voxprot.htm
Thats a great resource for anyone with one of these things ,the guy really knows his stuff .
Ahh, so its a transformer in the driver stage ,was a common topo way back in the day alright .
Makes sense to use some form of limiting in the earlier stages to get distortion too ,no point in cooking a nice set of blue bulldogs ,one of the meatiest sounding 12's of all time .From what Ive heard some of the old cabs that went along with that series did actually contain Alnico drivers ,always worth keeping an eye out for .
Ahh, so its a transformer in the driver stage ,was a common topo way back in the day alright .
Makes sense to use some form of limiting in the earlier stages to get distortion too ,no point in cooking a nice set of blue bulldogs ,one of the meatiest sounding 12's of all time .From what Ive heard some of the old cabs that went along with that series did actually contain Alnico drivers ,always worth keeping an eye out for .
link above says add more heat sink, so we hacked up the salvaged sink from the QSC amp and screwed it down with some #10 s,
should pick up 0.528749 degrees per watt, a fan might no be bad also if we can find a place to kludge it in,
and wound some turns on a 10 ohm resistor to put in series with the speaker line, hash filter,
got some thermal shut offs but they are rated at 150 C so no good on that,
got some new emitter resistors, a recap job and set the driver or 3.2 volts on emitter, trimmed the pwr transistor bias divider resistors to get about 25 mv offset which creeps downward as the amp sits,
almost the same circuit as the Acoustic 150 with the exception that this is plus/minus.
should pick up 0.528749 degrees per watt, a fan might no be bad also if we can find a place to kludge it in,
and wound some turns on a 10 ohm resistor to put in series with the speaker line, hash filter,
got some thermal shut offs but they are rated at 150 C so no good on that,
got some new emitter resistors, a recap job and set the driver or 3.2 volts on emitter, trimmed the pwr transistor bias divider resistors to get about 25 mv offset which creeps downward as the amp sits,
almost the same circuit as the Acoustic 150 with the exception that this is plus/minus.
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still working on this Vox Beatle,
all channels working except the Normal Channel,
almost done with that, had to bring in some 2N5485 fets and some 2N5088 transistors from Tayada, 3 bucks for 5 free ship not bad, and they look real,
got to staring at the schematic, wondering why the waveform is nice through the first two stages, then it turns into square and Viola! i had been messin with the overdrive circuit, and that circuit rang a bell, "where have i seen this before?"
does this circuit look familiar?>
all channels working except the Normal Channel,
almost done with that, had to bring in some 2N5485 fets and some 2N5088 transistors from Tayada, 3 bucks for 5 free ship not bad, and they look real,
got to staring at the schematic, wondering why the waveform is nice through the first two stages, then it turns into square and Viola! i had been messin with the overdrive circuit, and that circuit rang a bell, "where have i seen this before?"
does this circuit look familiar?>
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analogguru
Well-known member
Vox "invented" the Fuzz-Face - the (US-made) Vox V816 Distortion Booster was up for sale before the Fuzz-face. And yes, they used a similar circuit already in the T60 from the early 60's.
Bo Deadly
Well-known member
I don't think I've ever liked the Fuzz fully bypassed though. Swap out that 1K with a trimpot and dial it back a little by default.
^^good idea!
we already lowered the input voltage by using a 15 K in the bottom of the 100K / 100K voltage divider as the input to the distortion circuit was getting slammed way hard.
we already lowered the input voltage by using a 15 K in the bottom of the 100K / 100K voltage divider as the input to the distortion circuit was getting slammed way hard.
Bo Deadly
Well-known member
Honestly I'm not so sure that's a great idea because I think high source resistance is desirable. Backing off the Volume pot on the guitar can sound a lot nicer. The Fuzz sounds a lot better with HF cuts at just about every opportunity. That's why I like the 100p in there. But as it is, source resistance is 50K into a grounded emitter which is going to present low input impedance. So if you swap with 15K on the lower leg, that will make the source impedance like 13K. A backed off Volume pot on the guitar is going to be more like, well mmm, 50K. If you really want to reduce the voltage, maybe do 220K and 68K.CJ said:
good point, although this circuit is being driven by a transistor instead of a guitar, we will try it out as is and see if it is too muddled, got a 2.2 uf in place of the 10 uf now,
Bo Deadly
Well-known member
Yes, it is being driven by a transistor but it is very plausible that they deliberately chose a relatively high Z voltage divider. There are other Fuzz-like circuits that use large series resistance on purpose (sometimes using a pot). My feeling is that it has to do with the sound of the blocking distortion caused by the grounded emitter clipping and charging the coupling cap.
