@@vintageguitaramp_guitarteacher , interesting that you were using something close to the DADGAD tuning so popular for Celtic influenced fingerpicking in the 60's/70's: it put me in mind of Bert Jansch, John Renbourn, Martin Carthy, Martin Simpson, Richard Thompson and so on. By the way, you rarely seem to have any tuning issues with the guitars you use the most and I'm curious if you do all your own setups on them.
I do like the sound of the AC-50.. Nice to see you do another thorough job on an amp well worth saving and sending out with the other 60's lumberjacks to do a bit of logging. ( or gigging if you prefer).
According to the 12 page brochure reprinted on the Vox AC50 website, the (missing) CZ4 Brimistor (basically a high-power thermistor) measures 1180 ohms at 20°C room temperature, 340 ohms at 50°C, and 5.5 ohms at the maximum 1.2 amps of current. They can reach 250°C at their maximum electrical rating! These Brimistors are uncommon and perhaps somewhat costly nowadays but might be replaceable by a thermistor of suitable ratings. It's worth noting that the placement of the the brimistor in the Vox schematic might not be optimal because while the series resistance of the cold Brimistor will drop the surge and charging voltage to the filter caps, and reduce plate voltage/current to the output tubes, it will allow an exaggeratedly high bias voltage through to the output tubes during the warmup period because the tubes aren't conducting yet and the transformer B+ isn't yet being dragged down by the output tubes, not until the Brimistor heats sufficiently to go to low resistance and pass full output current. The application notes say that you can install the Brimistor inbetween the center tap of the transformer B+ secondary and ground, which might actually be a better method since it will also limit the initial voltage and current to the negative bias supply by roughly the same ratio as the amp warms up and the Brimistor comes up to full operating temperature and reaches it's lowest resistance. I would hazard a guess that a Vox AC50 with the Brimistor in place might sound a little strange for the first 10-15 minutes or so of operation because excessive bias voltage will screw up the operating point of the output tubes (abnormally low plate current) until the Brimistor heats up fully.
Hello. I reckon it would be due to cost why they stopped fitting them. I have put an 80 ohm Thermistor on the AC wall source. That has cured the over volting. It's only rated at 1.6 amps. I couldn't find one above. The fuse is 3.15 slow blow so I don't know how long the Thermistor will last. But it's cured it for the time being, till I find something better. Take care.
@@vintageguitaramp_guitarteacher , here in the States, some 1950's hifi and TV's had *surgistors* added to them to drop the mains input turn-on voltage until the units had warmed up. Solid-state thermistor and brimistors didnt exist then; Surgistors were a wire wound resistor/ heating element that added 50 to 100 ohms in series with the mains until the heat caused a bimetallic strip to bend and close a connection to shunt the resistor out of circuit, like a heat-activated relay.
I thought Brimistors worked opposite to a Thermistor? Not letting current through at the beginning. IDK, I have four AC-100s* on my list, so I snapped up the one Brimistor I came across some years ago, and haven't seen one since. *Mike Zaite made a thing called a Z-80 that filled the niche, but he quit making those recently. One left at Peach, if you beat me to it.
Hello Simon. I tried that. The problem here is that is no centre tap on the HT tapping. And no separate winding for the bias voltage. So there is already DC were they have tapped on to rectifier. They have used that circuit to reverse it. but it affecting the HT voltage. Take care Simon.
The sound of these JMI Vox's are incredible but the maintenance is costly. I recently moved on a '64 AC-10 twin that I just couldn't get a great sound from, they are super loud too and the weight is unbelievable. A lot of amp techs are no where near as thorough as yourself Martyn and maintaining them turns into a regular costly ritual. Can't argue with the sounds though!.
Back when this amplifier was built-in the late 60s your operating wall voltage was probably 220-230VAC at most. It's my understanding that everywhere in the UK and much of Europe now runs at 240VAC which can easily bump up the DC B+ by an addiitional 10 to 15%, another 40 or 50 volts. Under these circumstances, some adjustments to the power supply seem prudent.
Re the over voltage on the caps,, I don’t suppose there’s room there to wire two 400v or 300v caps of half the value in parallel ? Dunno if that might work ? ( you can tell I’m no technician) 😂😂😂
Well, if you want to solve that over voltage problem with two capacitors, they both should be double the original capacitance in value, and connected in series, not in parallel. And equipped with voltage balancing resistors.
Hello. That would work but there is nowhere on the amp to mount them. The pre-amp smoothing is also over volting because when there is no current going through the amp, all the caps see the same voltage. There is no room to mount more caps with those either. Even if there was room, I can't be drilling holes and modding a vintage amp. The customer wouldn't like that. If it wasn't a vintage amp and there was room to mount other caps then that would have been the perfect solution. I have a few other ideas. Thanks for watching and take care.
Ingenious capacitor securing, much better than the tape wrap method 👏🏻
Hello, thank you. I was lucky I had those in stock. They were a wrong order some months ago. Thanks for watching and take care.
Man this guy can play really well
Hello Michael, than you. Take care.
That's some sweet playing, even sweeter than usual for you! 🙂 I don't remember seeing you play a Strat before.
Hello, thank you. It's tuned in to DGDGAD. I have had the Strat since I was 16. Bought it new at the beginning of 1981
@@vintageguitaramp_guitarteacher , interesting that you were using something close to the DADGAD tuning so popular for Celtic influenced fingerpicking in the 60's/70's: it put me in mind of Bert Jansch, John Renbourn, Martin Carthy, Martin Simpson, Richard Thompson and so on. By the way, you rarely seem to have any tuning issues with the guitars you use the most and I'm curious if you do all your own setups on them.
Nice work and tasty licks! Always a pleasure to watch and listen. 👍👍
Thanks Paul. Take care.
Jus came across with your channel ... greetings from Mexico City.
Hello. Greeting to you too. Thanks for watching and take care.
I have a 68 one of these. Great sounding. Much fatter and warmer sounding than the ear blisteringly bright AC30
Hello. I agree, they are a much richer sound that the 30s. Take care.
I do like the sound of the AC-50.. Nice to see you do another thorough job on an amp well worth saving and sending out with the other 60's lumberjacks to do a bit of logging. ( or gigging if you prefer).
Hello Pete, ha ha, thank you. He's not quite okay yet. Still working on this over volting of the caps at turn on. Take care Pete.
Martyn, I've just heard Mrs Blenkinsop is sending you her TV to be fixed. You're up for the challenge, so I know you can fix it!!
Hello Dave Noooooooooooooooooooooooooooooooooooooooooooo!!!!!!!!
According to the 12 page brochure reprinted on the Vox AC50 website, the (missing) CZ4 Brimistor (basically a high-power thermistor) measures 1180 ohms at 20°C room temperature, 340 ohms at 50°C, and 5.5 ohms at the maximum 1.2 amps of current. They can reach 250°C at their maximum electrical rating! These Brimistors are uncommon and perhaps somewhat costly nowadays but might be replaceable by a thermistor of suitable ratings. It's worth noting that the placement of the the brimistor in the Vox schematic might not be optimal because while the series resistance of the cold Brimistor will drop the surge and charging voltage to the filter caps, and reduce plate voltage/current to the output tubes, it will allow an exaggeratedly high bias voltage through to the output tubes during the warmup period because the tubes aren't conducting yet and the transformer B+ isn't yet being dragged down by the output tubes, not until the Brimistor heats sufficiently to go to low resistance and pass full output current. The application notes say that you can install the Brimistor inbetween the center tap of the transformer B+ secondary and ground, which might actually be a better method since it will also limit the initial voltage and current to the negative bias supply by roughly the same ratio as the amp warms up and the Brimistor comes up to full operating temperature and reaches it's lowest resistance. I would hazard a guess that a Vox AC50 with the Brimistor in place might sound a little strange for the first 10-15 minutes or so of operation because excessive bias voltage will screw up the operating point of the output tubes (abnormally low plate current) until the Brimistor heats up fully.
Hello. I reckon it would be due to cost why they stopped fitting them. I have put an 80 ohm Thermistor on the AC wall source. That has cured the over volting. It's only rated at 1.6 amps. I couldn't find one above. The fuse is 3.15 slow blow so I don't know how long the Thermistor will last. But it's cured it for the time being, till I find something better. Take care.
@@vintageguitaramp_guitarteacher , here in the States, some 1950's hifi and TV's had *surgistors* added to them to drop the mains input turn-on voltage until the units had warmed up. Solid-state thermistor and brimistors didnt exist then; Surgistors were a wire wound resistor/ heating element that added 50 to 100 ohms in series with the mains until the heat caused a bimetallic strip to bend and close a connection to shunt the resistor out of circuit, like a heat-activated relay.
I thought Brimistors worked opposite to a Thermistor? Not letting current through at the beginning.
IDK, I have four AC-100s* on my list, so I snapped up the one Brimistor I came across some years ago, and haven't seen one since.
*Mike Zaite made a thing called a Z-80 that filled the niche, but he quit making those recently. One left at Peach, if you beat me to it.
Great video as ever, thanks for sharing. I'm no tech or expert but I would opt for changing the bias circuit. fwiw.
Hello Simon. I tried that. The problem here is that is no centre tap on the HT tapping. And no separate winding for the bias voltage. So there is already DC were they have tapped on to rectifier. They have used that circuit to reverse it. but it affecting the HT voltage. Take care Simon.
The sound of these JMI Vox's are incredible but the maintenance is costly.
I recently moved on a '64 AC-10 twin that I just couldn't get a great sound from, they are super loud too and the weight is unbelievable.
A lot of amp techs are no where near as thorough as yourself Martyn and maintaining them turns into a regular costly ritual.
Can't argue with the sounds though!.
Hello Keiran. This one sounds good, but it took some sorting out. Should be good for a few years now though. Take care.
These AC50's seem to be more affordable or less expensive that the vintage 30's. I know the Beatles used these amps for a time. I want one!
Hello Ron. There isn't many of them about now. I bet there is hardly any in the states. Take care Ron.
Back when this amplifier was built-in the late 60s your operating wall voltage was probably 220-230VAC at most. It's my understanding that everywhere in the UK and much of Europe now runs at 240VAC which can easily bump up the DC B+ by an addiitional 10 to 15%, another 40 or 50 volts. Under these circumstances, some adjustments to the power supply seem prudent.
I don't think that was the case in the UK. I will have to check that one. I will ask my mentor and let you know. Take care.
Wow! look at the oxidisation on the back of those pots. I'll bet they're a tad noisy. Nice amp though Martyn.
Hello Steve. Considering the state of them, they were not to noisy. Most of the noise came from the leaky caps. Take care mate.
10:15 what's that gunk coming out of the power transformer? 5A fuse suddenly becomes an issue...
Hello. It's wax. I have seen this on most of the Vox amps I have worked on. Take care.
Re the over voltage on the caps,, I don’t suppose there’s room there to wire two 400v or 300v caps of half the value in parallel ? Dunno if that might work ? ( you can tell I’m no technician) 😂😂😂
Well, if you want to solve that over voltage problem with two capacitors, they both should be double the original capacitance in value, and connected in series, not in parallel. And equipped with voltage balancing resistors.
Hello. That would work but there is nowhere on the amp to mount them. The pre-amp smoothing is also over volting because when there is no current going through the amp, all the caps see the same voltage. There is no room to mount more caps with those either. Even if there was room, I can't be drilling holes and modding a vintage amp. The customer wouldn't like that. If it wasn't a vintage amp and there was room to mount other caps then that would have been the perfect solution. I have a few other ideas. Thanks for watching and take care.