The two Edicron branded valves are Soviet 6P3S, which is a copy of the early 6L6. The maximum dissipation of either of those types is 19W. Much more than that and they will start to show a red plate, with the possibility of thermal runaway due to positive grid current. Not many modern 6L6s are comfortable at USA made 6L6GC or UK KT66 anode dissipation.
Nice that fender provides biasing pots along with an output to attach a multimeter. It makes me wonder why they skimp on the cheapest of components which in this case resulted in soldering extra resisters in parallel to bring the bias voltage up in-line with the JJ valves. As always an informative video from Stuart, in the workshop at Reading Guitar Amplifier Repairs.
Just curious, sometimes I don’t like to think too deeply on certain things, I should know the answer. It seems to me, if I want to pull 2 output tubes on my nephews Marshall amp to cut total volume by 3 dB, being fixed bias - it shouldn’t need a re-bias? Whereas a cathode biased amp would probably need different cathode resistors to get the correct idle current. Is this correct?
@@stuartukguitarampguy5830 thanks, why pull the outside tubes? I figured pulling the two inner tubes, might get more cooling potential between the outer tubes
Hello Stuart. Thanks for sharing your experience with this amp. Quick question: I have the same one, and when opening the circuit, I've seen that R216 (just between 2 bias test slots) has been replaced, leaving traces of burn. I see on ua-cam.com/video/iMptc01aQB4/v-deo.html that yours has been replaced too and may show similar burn traces, can you confirm? Maybe a factory weakness affecting all "The Twin Red Knobs"?
Hi, I have the same amp. So using the endpoint to biasing the tubes works fine but not so good to balance it? I also have only two power tubes used (I removed the inside two). When biasing I have to get 40mV using to end points or this reading if for 4 power tubes? Thanks!
@@stuartukguitarampguy5830 The endpoints in the rear of the amp where you put the digital voltimeter in your video and the balance shows a value of 6 not 0 as Fender recommends. Thanks
@@fernandodelarasillasiri3368 Hi Struggling a bit to understand your question. It's been a while simce I did the video so don;t have all the details to hand now.
What’s the big deal with printed circuit boards? Especially since all those tubes are mounted into actual chassis mounted sockets and from there hand wired to the circuit boards. That’s much better than most amps today where the the tubes mount directly to the boards! Not to mention those early 80s hand wired fender concert amps are a spaghetti mess! If you want anything other than a real simple circuit, printed circuit boards are the way to go
My personal view is there is no difference in sound between a pcb and hand wired. How can there be really? I think it's just that many guitarists like 'vintage' e.g. they will pay a fortune for NOS tubes and so on. My only problem with PCBs is that sometimes they are hard/impossible to lift out to change components. E.g, just worked on an Engl Powerball - all pcb and virtually impossible to repair as they have mounted them track side up so you can't see or get to any components!
@@stuartukguitarampguy5830 I think there can be differences in tone due to all the capacitance between all the hundred and thousands of traces. But I also think if you wired a circuit that complex using hand wired. You would run into the same issues among more noise issues. Whereas, you kept a really simple circuit like a champ into a pcb - it would sound no different than hand wired. Where I think people get stuck is when they have a 6 channel Marshall pcb amp. And one of them is supposed to be a super lead circuit. In that case it doesn’t sound the same because it’s a super lead circuit compressed into 1/6th the area vs a traditional hand wired
There's nothing wrong with pcb so long the amp is designed properly. However, the manufacturer's will use the cheapest (lower cost) components in the amps. This in itself is OK, but in most cases the components break down. It's good for the repairman, but it surely leaves a bad taste with the owners of the amp. Cheers
The absolute incredible amount of detail you incude in this video is admirable indeed, Excellent lesson for us
Thanks I'm glad you enjoyed it.
The two Edicron branded valves are Soviet 6P3S, which is a copy of the early 6L6. The maximum dissipation of either of those types is 19W. Much more than that and they will start to show a red plate, with the possibility of thermal runaway due to positive grid current. Not many modern 6L6s are comfortable at USA made 6L6GC or UK KT66 anode dissipation.
Nice that fender provides biasing pots along with an output to attach a multimeter. It makes me wonder why they skimp on the cheapest of components which in this case resulted in soldering extra resisters in parallel to bring the bias voltage up in-line with the JJ valves. As always an informative video from Stuart, in the workshop at Reading Guitar Amplifier Repairs.
Just curious, sometimes I don’t like to think too deeply on certain things, I should know the answer. It seems to me, if I want to pull 2 output tubes on my nephews Marshall amp to cut total volume by 3 dB, being fixed bias - it shouldn’t need a re-bias? Whereas a cathode biased amp would probably need different cathode resistors to get the correct idle current. Is this correct?
Hi. Yes that's correct. You can pull 2 out of the 4 (suggest you do the outside ones) and you don't need to rebias.
@@stuartukguitarampguy5830 thanks, why pull the outside tubes? I figured pulling the two inner tubes, might get more cooling potential between the outer tubes
@@voxpathfinder15r Yes either will work.
Hello Stuart. Thanks for sharing your experience with this amp. Quick question: I have the same one, and when opening the circuit, I've seen that R216 (just between 2 bias test slots) has been replaced, leaving traces of burn. I see on ua-cam.com/video/iMptc01aQB4/v-deo.html that yours has been replaced too and may show similar burn traces, can you confirm? Maybe a factory weakness affecting all "The Twin Red Knobs"?
Hi. Sorry it's been a while now since I worked on the amp and I can't recall the details of that resistor.
Hi, I have the same amp. So using the endpoint to biasing the tubes works fine but not so good to balance it? I also have only two power tubes used (I removed the inside two). When biasing I have to get 40mV using to end points or this reading if for 4 power tubes? Thanks!
Hi Fernando, not sure what you mean by 'end points'? Anyway, if you aim for abouty 40mA per tube, that would be close enough.
@@stuartukguitarampguy5830 The endpoints in the rear of the amp where you put the digital voltimeter in your video and the balance shows a value of 6 not 0 as Fender recommends. Thanks
@@fernandodelarasillasiri3368 Hi Struggling a bit to understand your question. It's been a while simce I did the video so don;t have all the details to hand now.
Who designed these amps??
Gawd knows. SOme schoolboy possibly??
40ma draw for 2 tubes per side. 1 tube will draw half, 20ma! Dont bias 1 tube at 40ma.
What’s the big deal with printed circuit boards? Especially since all those tubes are mounted into actual chassis mounted sockets and from there hand wired to the circuit boards. That’s much better than most amps today where the the tubes mount directly to the boards! Not to mention those early 80s hand wired fender concert amps are a spaghetti mess! If you want anything other than a real simple circuit, printed circuit boards are the way to go
My personal view is there is no difference in sound between a pcb and hand wired. How can there be really? I think it's just that many guitarists like 'vintage' e.g. they will pay a fortune for NOS tubes and so on. My only problem with PCBs is that sometimes they are hard/impossible to lift out to change components. E.g, just worked on an Engl Powerball - all pcb and virtually impossible to repair as they have mounted them track side up so you can't see or get to any components!
@@stuartukguitarampguy5830 I think there can be differences in tone due to all the capacitance between all the hundred and thousands of traces. But I also think if you wired a circuit that complex using hand wired. You would run into the same issues among more noise issues. Whereas, you kept a really simple circuit like a champ into a pcb - it would sound no different than hand wired. Where I think people get stuck is when they have a 6 channel Marshall pcb amp. And one of them is supposed to be a super lead circuit. In that case it doesn’t sound the same because it’s a super lead circuit compressed into 1/6th the area vs a traditional hand wired
@@voxpathfinder15r Useful points, thanks!
There's nothing wrong with pcb so long the amp is designed properly. However, the manufacturer's will use the cheapest (lower cost) components in the amps. This in itself is OK, but in most cases the components break down. It's good for the repairman, but it surely leaves a bad taste with the owners of the amp. Cheers