A British guy named Grusvenor has published a complete guide to servicing Vox amps, containing never-available-before diagrams, layouts and schematics.
Makes you wonder what happened to quality control at the manufacturing stage. Great fault finding Stuart, as always. Thoroughly enjoyed that one, thank you.
Stuart, A fine example of why I so enjoy your videos. The way you talk through the troubleshooting process is wonderful. The use of the schematic to explain the process is enlightening. The way you do this has taught me much. Thanks, Mike
The AC-30 is my favorite Vox model. It is a shame that the factory Q.C. was horribly irresponsible. Great job getting it sorted, Stuart. I admire your sense of saving the customer a possible 75 quid for a full exploratory. The mod worked well. Thanks for posting. Cheers!
Great work Stuart.. I think Vibrato & Tremolo are great effects to have built into an amp and I'm sure the customer will have been delighted that you managed to fix them at a sensible cost. You can get a very adequate trem pedal for £20 theses days so I completely see your point of accounting for the time/cost/value equation her. It works now.. Job done !
You must fill that amp with helium first. Then it's easier to lift. 😂 BTW, don't do it like Zeppelin did. They filled those airships with hydrogen and everybody knows what happens if hydrogen catches fire... And because of the quite low working frq of the tremelodramatic oscillator, I usually use larger (50-200uF) cathode bypass cap value than that standard 25uF because it at least theoretically gives higher gain to that osc triode at those very low frequencies.
Yeah - and with the temperatures these Vox AC30s run at, it WILL catch fire😆. I've spent this weekend servicing my AC30 CC2 with new valves etc. and I'm APALLED at how hot it runs. Time for some resistor changes.... 🪛😆
@@NotMarkKnopflerSome say that you just have to play it loud all the time because then at least some of the idle plate dissipation power of that EL84 quartet goes to speakers. 😁 And if VOX AC 30 output stage really was a true class A amplifier (which it is not), those hot biased output tubes in fact would go significantly colder when you crank it up.
Valve Wizard/Merlin Blencowe---> "A visual frequency / rate indicator: There is one useful modification that can be made to this otherwise ancient circuit- LED biasing. By replacing the cathode resistor with an LED we eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop. A red LED will be suitable in most cases, providing about 1.6V bias, although other colours can be experimented with of course. Additionally, because the oscillations extend to cut-off, the LED will flash in time with the trem' signal giving a novel and useful visual indication of the frequency, and that the circuit is actually working! The circuit [right] shows this, using an ECC81 this time."
I only have a VERY Basic understanding of this stuff, so pardon me for asking........... What was that "Hum" trim pot adjusting..... Heater voltage.? Some kind of feed-back loop.? Phase inverter output.? I really have no idea. Not even sure if i have seen that before 🙂 Thank Yiou
Why did they use a 6 positions rotary switch for 3 speeds available ? Maybe (if it would have been connected) would it make different configs of resistors to achieve 6 speeds ? We’ll never know. Good job anyway 👍
Yes it's super weird isn't it? Best guess is that these swicthes are often sevral ways and you have a little plastic bit you insert to cut it down to 2,3,4 or whatever you want.
Odd, as you say, that all three tracks were going nowhere. PCB board to board fingers never soldered properly from new, perhaps (and a design accident waiting to happen).
The speed selector should be a three way switch. It seems like it definitely isn't in this amp. So probably the main problem comes from that. By the way, it is also worth checking the cathode of the rectifier tube, because the B+ is not connected to the right leg at the factory. (Instead of pin 8, it is incorrectly connected to pin 2.)
Hi yes it's odd but you would have thought that ONE of those switch positions would have worked at least, and probably three. Those switches are made to be multi way but you can select the number by a sort of spigot thing (hard to explain). The switch definitely worked as I tested the back. Ah well, an odd one.
@@stuartukguitarampguy5830 Odd indeed. I had initially surmised (guessed) that the spigot was never fitted or fell out of its hole, or an incorrect switch had been fitted, but those wouldn't necessarily explain why none of the positions connected to the resistors.
In order to cause oscillation, some of the output needs to get back to the input (think shoving a mic in fron of a speaker), For oscillation to occur, the gain in that 'loop' (i.e. input to output, then the tiny portion of that output which gets back to the input) must be greater than one. If less than one, the feedback will gradually die away to nothing. So no oscillation. If greater than one, it will oscillate. Changing the phase seems to have an effect on the size of that small sniff of output whichh finds its way back to the input, hopefully reducing the 'loop ganin' to less than 1 and getting rid of the oscillation.. Hope that helps!
@@stuartukguitarampguy5830 So the power tubes output is some how "finding a path" back to the preamps input to create a LOOP? I'm just confused how the output signal from the output tubes is finding a path back to the input to cause oscillations
Hi Wayne Yes you've got it exactly. Think about it. The average guitar gives out ten THOUSANDTH of a volt. The output tubes deliver about FIFTY volts. You only need the tiniest sniff of that 50V to find it's way back to the input and the amp will oscillate. Keeping high gain amps stable is quite a feat if you think about it! How will it find it's way back? Youve got wires radiating the signal, pcb tracks with resistance between them (albeit high - but it doesn't take much). That sort of thing.
@@stuartukguitarampguy5830 Vibrato is wiggling the string with your finger - so change in pitch. Tremolo is the regular cyclic change in amplitude. Leo Fender really messed everyone up lol
A British guy named Grusvenor has published a complete guide to servicing Vox amps, containing never-available-before diagrams, layouts and schematics.
Ok any idea where to get it?
@@stuartukguitarampguy5830 Grosvenor The Vox AC30 Guide.
"All amplifiers are oscillators and all oscillators are amplifiers"!
Ah yes but which is which??.... :)
The ones incorporating positive feedback and a loop gain >1 are oscillators.
Makes you wonder what happened to quality control at the manufacturing stage.
Great fault finding Stuart, as always. Thoroughly enjoyed that one, thank you.
Thanks for the great feedback.
Stuart,
A fine example of why I so enjoy your videos. The way you talk through the troubleshooting process is wonderful. The use of the schematic to explain the process is enlightening. The way you do this has taught me much.
Thanks,
Mike
Thanks Mike.
Splendid as usual 🎉
Thanks!
Young Stuart! The man!
Thanks!
The AC-30 is my favorite Vox model. It is a shame that the factory Q.C. was horribly irresponsible. Great job getting it sorted, Stuart. I admire your sense of saving the customer a possible 75 quid for a full exploratory. The mod worked well. Thanks for posting. Cheers!
Thanks Alex I'm pleased you enjoyed it.
Hey Stuart, a bit of pest that one..Love the way you get around these nasty little faults..Great job as usual..Interesting video..Ed..UK..😀
Thanks Ed!
Great work Stuart.. I think Vibrato & Tremolo are great effects to have built into an amp and I'm sure the customer will have been delighted that you managed to fix them at a sensible cost. You can get a very adequate trem pedal for £20 theses days so I completely see your point of accounting for the time/cost/value equation her. It works now.. Job done !
Yes it's nice to have it working again.
Jolly good!
Cheers Paul.
You must fill that amp with helium first. Then it's easier to lift. 😂
BTW, don't do it like Zeppelin did. They filled those airships with hydrogen and everybody knows what happens if hydrogen catches fire...
And because of the quite low working frq of the tremelodramatic oscillator, I usually use larger (50-200uF) cathode bypass cap value than that standard 25uF because it at least theoretically gives higher gain to that osc triode at those very low frequencies.
Yeah - and with the temperatures these Vox AC30s run at, it WILL catch fire😆. I've spent this weekend servicing my AC30 CC2 with new valves etc. and I'm APALLED at how hot it runs. Time for some resistor changes.... 🪛😆
@@NotMarkKnopflerSome say that you just have to play it loud all the time because then at least some of the idle plate dissipation power of that EL84 quartet goes to speakers. 😁 And if VOX AC 30 output stage really was a true class A amplifier (which it is not), those hot biased output tubes in fact would go significantly colder when you crank it up.
Valve Wizard/Merlin Blencowe--->
"A visual frequency / rate indicator:
There is one useful modification that can be made to this otherwise ancient circuit- LED biasing. By replacing the cathode resistor with an LED we eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop. A red LED will be suitable in most cases, providing about 1.6V bias, although other colours can be experimented with of course. Additionally, because the oscillations extend to cut-off, the LED will flash in time with the trem' signal giving a novel and useful visual indication of the frequency, and that the circuit is actually working! The circuit [right] shows this, using an ECC81 this time."
Very interestihng thanks!
I was screaming -Valve Wizzard! lol. You can even leave the LED in, no sound issues@@stuartukguitarampguy5830
I only have a VERY Basic understanding of this stuff, so pardon me for asking........... What was that "Hum" trim pot adjusting.....
Heater voltage.?
Some kind of feed-back loop.?
Phase inverter output.?
I really have no idea.
Not even sure if i have seen that before 🙂
Thank Yiou
Not feedback. I'm not sure on this exact amp but usually the general idea is to even up the bias current between the two power valves.
Why did they use a 6 positions rotary switch for 3 speeds available ? Maybe (if it would have been connected) would it make different configs of resistors to achieve 6 speeds ? We’ll never know.
Good job anyway 👍
Yes it's super weird isn't it? Best guess is that these swicthes are often sevral ways and you have a little plastic bit you insert to cut it down to 2,3,4 or whatever you want.
Odd, as you say, that all three tracks were going nowhere. PCB board to board fingers never soldered properly from new, perhaps (and a design accident waiting to happen).
Yes maybe that board was inserted but not soldered. Under test they got lucky and it just made a contact, only to fail after that.
The speed selector should be a three way switch. It seems like it definitely isn't in this amp. So probably the main problem comes from that. By the way, it is also worth checking the cathode of the rectifier tube, because the B+ is not connected to the right leg at the factory. (Instead of pin 8, it is incorrectly connected to pin 2.)
Hi yes it's odd but you would have thought that ONE of those switch positions would have worked at least, and probably three. Those switches are made to be multi way but you can select the number by a sort of spigot thing (hard to explain). The switch definitely worked as I tested the back. Ah well, an odd one.
@@stuartukguitarampguy5830 Odd indeed. I had initially surmised (guessed) that the spigot was never fitted or fell out of its hole, or an incorrect switch had been fitted, but those wouldn't necessarily explain why none of the positions connected to the resistors.
STUART, Why does reversing the output transformers Primary winding will fix power amplifier oscillation problems?
It reverses the phase of the signal and that can affect the loop gain and hence the propensity to oscillate. That's the best I can do!
@@stuartukguitarampguy5830 What do you mean by Loop Gain?
In order to cause oscillation, some of the output needs to get back to the input (think shoving a mic in fron of a speaker), For oscillation to occur, the gain in that 'loop' (i.e. input to output, then the tiny portion of that output which gets back to the input) must be greater than one. If less than one, the feedback will gradually die away to nothing. So no oscillation. If greater than one, it will oscillate. Changing the phase seems to have an effect on the size of that small sniff of output whichh finds its way back to the input, hopefully reducing the 'loop ganin' to less than 1 and getting rid of the oscillation.. Hope that helps!
@@stuartukguitarampguy5830 So the power tubes output is some how "finding a path" back to the preamps input to create a LOOP? I'm just confused how the output signal from the output tubes is finding a path back to the input to cause oscillations
Hi Wayne Yes you've got it exactly. Think about it. The average guitar gives out ten THOUSANDTH of a volt. The output tubes deliver about FIFTY volts. You only need the tiniest sniff of that 50V to find it's way back to the input and the amp will oscillate. Keeping high gain amps stable is quite a feat if you think about it! How will it find it's way back? Youve got wires radiating the signal, pcb tracks with resistance between them (albeit high - but it doesn't take much). That sort of thing.
Not to split hairs but it’s actually a tremolo, not vibrato 😉
It's both on this amp but I always get confused which is which. Tremelo is the amplitude shift. Tremelo the frequency shift.
@@stuartukguitarampguy5830 Vibrato is wiggling the string with your finger - so change in pitch. Tremolo is the regular cyclic change in amplitude. Leo Fender really messed everyone up lol
Ha ha, yes thanks. The way I can sometimes remember it is that violinists use vibrato, not tremelo.
Please correct the tremolo to tremolo ....