First time I've seen the inside of an early AC-10.. Rare birds.. Great work you've performed in keeping things as near to original as would allow to get it working properly again. Many thanks for videoing this Stuart.
Great video Stuart!. Very informative. You’ve got a very clear way of explaining how everything works. Thanks again for the advice and for the excellent job on my AC10. The amp sounds great now!. And at a very reasonable cost. I can highly recommend your services!.
Nice work Stuart..😀 I like that you're always mindful of costs, when you work in an amplifier..The clients must be happy with your charges. Unnecessary component installation will keep the customers returning..! Great job, and info.. Regards Ed..UK.😀
I have the ac10's big brother the ac30. Easy amplifier circuit to understand and even easier to work on and maintain. A real work horse of an amplifier.
Great video as always, Stuart, thanks for sharing. Agree that cathode bias is typically used with low power amps, there are a few that use bigger octal valves (albeit still at low power) for example I have a commercial amp that has a pair of EL34s in parallel class A cathode bias for about 12 watts (it is going for a lower volume Marshall sound) and I have a cathode biased (single ended class A) amp that has a bias pot and can use all the usual octal power tubes - to be fair, I built it and it is currently rocking a 6550 because I had one spare... I'm a big fan of the EF86 as a preamp tube, Dr Z uses them a fair bit and I think some of the Matchless amps did too as did the Blackstar Artisans. They can be microphonic though.
ECF82 triode-pentode is directly equivalent to the American 6U8; the other nearly identical American tubes that can substitute are 6EA8/ 6GH8. Same exact pinout, with slightly different gains or electrical characteristics. All of these were commonly used in classic tube hifi equipment such as Scott, Fisher and others, and they are being reproduced these days as well as a good amount of new old stock still being available. The EF86 pentode is also known as the 6267, or Z729; *not an easy tube to find nowadays*, if you want one that is quiet and non- microphonic anyway. Pentodes were once commonly used as microphone preamp here in the States, starting with the octal based 6J7, with a grid cap on top (a classic, historical mic preamp tube for broadcast and recording studio use, still popular today), superceded by the 6SJ7 (no top connection), and finally the special low noise 5687 mini 9 pin tube, somewhat similar to the EF86 but unfortunately with a different pin out and lower gain. Some Gibson guitar amps used the 5687, and they were often seen in commercial mic/music mixers and paging amps for restaurant/ business background music and industrial paging. The 6AU6 is a 7-pin pentode that you might see in guitar amps as well (Silvertone/Danelectro, for instance).
@@stuartukguitarampguy5830 , I looked at the AC-10 schematic; the EF86 is the input preamp for the vibrato channel, but it is the pentode half of the ECF82/6U8 that forms the vibrato oscillator. The triode half of the ECF82 is the mixer tube where the "normal" channel enters the amp, therefore the vibrato channel has much more gain. Pentodes typically have far more gain than triodes, and less internal capacitance, making for better high frequency performance, and great for building oscillator circuits.
Hi Stuart. The Fender Bassman AC568 (and other Fender AC568 circuits) use 2 6L6GC tubes and are also cathode biased, using a pair of 150ohm, 7 watt resistors. It also is grid biased, using a 10K linear pot and some additional resistors. Very unusual Hybrid "Fixed/Cathode Biased" circuit. I don't think Fender stayed with this circuit revision for very long. My 1968 (or 1969) Fender Bassman happens to have this circuit.
@@stuartukguitarampguy5830 If you would like an AB568 schematic, let me know and I can email you one (pdf), but you can probably find one online if Paul McCartney's "Get Back" Bassman amp ever finds it's way into your shop.
EZ81 aka 6CA4 rectifier. Those Mullard EL84's might be perfectly okay ---- I would have tested them and tried them in the amplifier. They're probably more rugged by half than most modern replacements.
20:00 You might be interested to know that the early 60s Ampeg B15 bass amp ran a pair of 6L6GC output valves and was cathode biased. Later versions were grid biased, increasing the output power rating from 25W to 30W.
@@christianboddum8783 Hi Christian. Actually there are two meanings and they are pronounced differently. slAver (with a long ayyy sound) would be an old term for someone who ran slaves, e.g. ran a slave ship. Slaver (sl'ah'ver) has a short 'ah' sound. It means to visibly drool over somthing tempting! A similar word is 'slober'. Neither are used much these days (like many, many words are dropping out of use. I wonder if it is the same in Denmark?).
I'll have to disagree about old caps, they can explode and cause a fire too, also if they short they can take the power transformer out. I've seen all these things happen in my 30 years of working on amps. I'd rather replace a $20 cap than later a $300 power transformer.
@@stuartukguitarampguy5830 , if you're not replacing the main filter caps ---- the filters for the output tube plates and screen being the most likely to fail catastrophically ---- then adding a fuse (250 to 500 milliamps should do) from the centertap of the HT secondary to ground is a good idea (mandatory under the circumstances if ya ask me). You can put it inside the chassis using an in-line fuse holder with leads, making it easily reversible/ removable, and you won't have to drill any holes. It will save the power transformer from destruction if a filter cap shorts. By the way, many modern amps, especially those with EL84'S, also add an HT fuse to the CT of the output transformer primary, probably because today's modern EL84'S are prone to fireworks when run right at or beyond their maximum ratings. An internal HT fuse would be a good idea.
@@stuartukguitarampguy5830 , the reason you don't see those amps with shorted caps that have lead to blown transformers is because, among your client base anyway, such amps have become doorstops, gathering dust somewhere because the blown amp is considered too expensive for the owner to justify fixing, or sold off to somebody else with deeper pockets or sufficiently determined to do whatever is necessary to bring it back to life. Only an in-demand, professional gigging musician with money, or a wealthy corporate lawyer or hedge fund manager who dabbles in guitar at home, is likely to pay the cost of installing a repro transformer with the period-correct specs, or to have a dead vintage transformer rewound. I don't mean to sound harsh, but such people probably don't constitute a large part of your client base; they likely take their amps to a full-service, "do whatever it needs for the long-term" tech, rather than a "get it working for now and we'll hope for the best" type of service.
@@stuartukguitarampguy5830 More common in American and Japanese amps because of low quality parts chosen I would imagine. Lots things were designed for and sold at department stores. Some "high end" manufacturers use substandard parts these days as well.
I've got a 64 Ac10 Twin, excited to watch this and compare to mine!
First time I've seen the inside of an early AC-10.. Rare birds.. Great work you've performed in keeping things as near to original as would allow to get it working properly again. Many thanks for videoing this Stuart.
Glad you enjoyed it Pete.
Great video Stuart!. Very informative. You’ve got a very clear way of explaining how everything works.
Thanks again for the advice and for the excellent job on my AC10. The amp sounds great now!. And at a very reasonable cost.
I can highly recommend your services!.
Cheers Mathew.
Thanks for this, Stewart. I’m just getting into amplifier build/repair, and your videos have been really insightful.
Thanks Danny.
Thanks for the mini biasing tutorial. Very well explained. I've always struggled with the naming of the two bias schemes also.
Yes weird isn;t it? No wonder people get confused!
Nice work Stuart..😀 I like that you're always mindful of costs, when you work in an amplifier..The clients must be happy with your charges. Unnecessary component installation will keep the customers returning..! Great job, and info.. Regards Ed..UK.😀
Chers Ed.
Im with ya Stewart on the terms doesnt make sense to me either but i now know what they mean . Heck i was a land surveyor for 30yrs .
I have the ac10's big brother the ac30. Easy amplifier circuit to understand and even easier to work on and maintain. A real work horse of an amplifier.
Nice amp! Yes I think I've worked on a couple in the past.
That AC-10 has an ungodly tone! It's biting high-mids react very well with single coils. Well done, Stuart.
Yes I really liked the sound of it.
Nice one Young Stuart!, I know I'm an old school (An old git!) but I hate all these new "near unrepairable" circus board amps!!
It's all to do with cost of production and they don't really care about repairabilty
Great video as always, Stuart, thanks for sharing. Agree that cathode bias is typically used with low power amps, there are a few that use bigger octal valves (albeit still at low power) for example I have a commercial amp that has a pair of EL34s in parallel class A cathode bias for about 12 watts (it is going for a lower volume Marshall sound) and I have a cathode biased (single ended class A) amp that has a bias pot and can use all the usual octal power tubes - to be fair, I built it and it is currently rocking a 6550 because I had one spare... I'm a big fan of the EF86 as a preamp tube, Dr Z uses them a fair bit and I think some of the Matchless amps did too as did the Blackstar Artisans. They can be microphonic though.
Interesting stuff, thanks for sharing.
I like it! Very NICE!!
Yep, wouldn;t mind one myself.
Nice piece of gear.
Yes I offred him £50 for it and he wasn't interessted!
ECF82 triode-pentode is directly equivalent to the American 6U8; the other nearly identical American tubes that can substitute are 6EA8/ 6GH8. Same exact pinout, with slightly different gains or electrical characteristics. All of these were commonly used in classic tube hifi equipment such as Scott, Fisher and others, and they are being reproduced these days as well as a good amount of new old stock still being available. The EF86 pentode is also known as the 6267, or Z729; *not an easy tube to find nowadays*, if you want one that is quiet and non- microphonic anyway.
Pentodes were once commonly used as microphone preamp here in the States, starting with the octal based 6J7, with a grid cap on top (a classic, historical mic preamp tube for broadcast and recording studio use, still popular today), superceded by the 6SJ7 (no top connection), and finally the special low noise 5687 mini 9 pin tube, somewhat similar to the EF86 but unfortunately with a different pin out and lower gain. Some Gibson guitar amps used the 5687, and they were often seen in commercial mic/music mixers and paging amps for restaurant/ business background music and industrial paging. The 6AU6 is a 7-pin pentode that you might see in guitar amps as well (Silvertone/Danelectro, for instance).
Interesting thanks. I think they use a pentode here to get the vibrato to work as it has that extra grid.
@@stuartukguitarampguy5830 , I looked at the AC-10 schematic; the EF86 is the input preamp for the vibrato channel, but it is the pentode half of the ECF82/6U8 that forms the vibrato oscillator. The triode half of the ECF82 is the mixer tube where the "normal" channel enters the amp, therefore the vibrato channel has much more gain. Pentodes typically have far more gain than triodes, and less internal capacitance, making for better high frequency performance, and great for building oscillator circuits.
Hi Stuart. The Fender Bassman AC568 (and other Fender AC568 circuits) use 2 6L6GC tubes and are also cathode biased, using a pair of 150ohm, 7 watt resistors. It also is grid biased, using a 10K linear pot and some additional resistors. Very unusual Hybrid "Fixed/Cathode Biased" circuit. I don't think Fender stayed with this circuit revision for very long. My 1968 (or 1969) Fender Bassman happens to have this circuit.
Interesting thanks Michael.
@@stuartukguitarampguy5830 If you would like an AB568 schematic, let me know and I can email you one (pdf), but you can probably find one online if Paul McCartney's "Get Back" Bassman amp ever finds it's way into your shop.
EZ81 aka 6CA4 rectifier. Those Mullard EL84's might be perfectly okay ---- I would have tested them and tried them in the amplifier. They're probably more rugged by half than most modern replacements.
I did test them and they were fairly low emision. WOrth replacing I think. Customer took then away with him.
20:00 You might be interested to know that the early 60s Ampeg B15 bass amp ran a pair of 6L6GC output valves and was cathode biased. Later versions were grid biased, increasing the output power rating from 25W to 30W.
Interesting thanks.
Hi Stuart. What a beautiful amp. Are you planning on joining the 2 beatles and make a new album.
I wish!
I agree with not changing caps for home use, but for Giging musicians, change everything, can’t afford having your amp go down in the middle of a set.
Agreed. This customer plays occasionally at home, never gigs.
@@stuartukguitarampguy5830 The lovely condition of this amp says home use only! :)
Drool!!
Slaver....
@@stuartukguitarampguy5830 ? Slaver, what does that mean. I'm Danish, I might need to add a new word to my vobaulary... ;-)
@@christianboddum8783 Hi Christian. Actually there are two meanings and they are pronounced differently. slAver (with a long ayyy sound) would be an old term for someone who ran slaves, e.g. ran a slave ship. Slaver (sl'ah'ver) has a short 'ah' sound. It means to visibly drool over somthing tempting! A similar word is 'slober'. Neither are used much these days (like many, many words are dropping out of use. I wonder if it is the same in Denmark?).
I wish you were closer to me. I have some equipment you could use.
What do you have?
I'll have to disagree about old caps, they can explode and cause a fire too, also if they short they can take the power transformer out. I've seen all these things happen in my 30 years of working on amps. I'd rather replace a $20 cap than later a $300 power transformer.
Hi SCott. Interesting. I've never sen either of those in thousands and thousands of amp repairs.
I do take your word for it though.
@@stuartukguitarampguy5830 , if you're not replacing the main filter caps ---- the filters for the output tube plates and screen being the most likely to fail catastrophically ---- then adding a fuse (250 to 500 milliamps should do) from the centertap of the HT secondary to ground is a good idea (mandatory under the circumstances if ya ask me). You can put it inside the chassis using an in-line fuse holder with leads, making it easily reversible/ removable, and you won't have to drill any holes. It will save the power transformer from destruction if a filter cap shorts. By the way, many modern amps, especially those with EL84'S, also add an HT fuse to the CT of the output transformer primary, probably because today's modern EL84'S are prone to fireworks when run right at or beyond their maximum ratings. An internal HT fuse would be a good idea.
@@stuartukguitarampguy5830 , the reason you don't see those amps with shorted caps that have lead to blown transformers is because, among your client base anyway, such amps have become doorstops, gathering dust somewhere because the blown amp is considered too expensive for the owner to justify fixing, or sold off to somebody else with deeper pockets or sufficiently determined to do whatever is necessary to bring it back to life. Only an in-demand, professional gigging musician with money, or a wealthy corporate lawyer or hedge fund manager who dabbles in guitar at home, is likely to pay the cost of installing a repro transformer with the period-correct specs, or to have a dead vintage transformer rewound. I don't mean to sound harsh, but such people probably don't constitute a large part of your client base; they likely take their amps to a full-service, "do whatever it needs for the long-term" tech, rather than a "get it working for now and we'll hope for the best" type of service.
@@stuartukguitarampguy5830 More common in American and Japanese amps because of low quality parts chosen I would imagine. Lots things were designed for and sold at department stores. Some "high end" manufacturers use substandard parts these days as well.
Vox liked to run el84s like flogging a bunch of rented mules. Makes it difficult for the lowly repairman this day and time.
Ha ha! Yes I'm pleased to have dialled them down a bit.