Marshall Origin 20 Guitar Amp Repair Follow Up Video on the Bias and Power Scaling

This follow up video on the Marshall Origin 20 Watt Guitar amp is about the bias and the power scaling, and to check if I had made a mistake when biasing this amp. It turned out that I hadn't made a mistake but I had missed out some useful information which was pointed out by a watcher on the channel. In this video we go through what I should have put in the first video. More problems with this amp were also pointed out in the comments form one of the best guitar amp techs on on you tube. This amp seems to have world full of design problems.

[@PsionicAudio]

I just looked at my notes - it was R7 that burned on the Origin 20. Shouldn’t burn when things in the amp are going right, but it’s right on the edge. 17.47V drop for 2.03W at idle. Thanks for the kind words.

[@vintageguitaramp_guitarteacher]

Thanks Lyle I will log that one for the future. Take care.

[@matthewf1979]

I think that your assumptions about biasing the amp on the high power setting is the correct way to do it. It's nice to see what the dissipation is on the lower settings though. I normally don't take in amps like this anymore, but it's nice to know everything about them....just incase I feel sorry for an unlucky young guitar player. Thanks for being thorough, as usual.

[@vintageguitaramp_guitarteacher]

Hello Matt, thank you. Yes, they can be tricky to repair. I think I got lucky with this one. take care.

[@faultlessguitarsandamps1116]

Glad that Lyle watches your channel . According to Lyle R7 is 150R 2Watt . I haven't been able to find a schematic myself . But I had an Origin 20 in a few weeks ago and replaced it with a low mass 5 Watt . The fault symptoms were quiet crackling noises , and R7 had not failed yet , it still measured 150R . But I guess it wasn't enjoying being baked . The 150R looked like a 100R resistor as the green band had burnt to black and a cursory glance wouldn't have appeared to be anything wrong .The amp hasn't come back , so seems to have been the problem and the cure . By the way , the main board is fairly easy job to remove and work on .And I can confirm the dissipation of this resistor is over 2 Watts . And my bias calculations were the same as yours and Lyles . Many thanks to Lyle , as I wouldn't have been able to figure this out in s a month of Sundays .

[@vintageguitaramp_guitarteacher]

Hello. That's great info, thank you. Lyle posts some great videos. Email me anytime if you think I can help you with anything. Are you in the States or the UK? Thanks for watching and take care.

[@rgbplumbinghilton]

Lots of Marshall purists knock this amp. It is far better than most at this price point and with a nice speaker in, it sounds great. The power scaling is a bit of a sales carrot but does what it is intended to do. I have the 50watt combo and have gigged it a few times. It sounds at its best in half power and gain and master cranked. It cleans up nicely and is harmonically pleasing. I have used a fair few amps that cost more and are not as good as these. Stock speaker is a tad brittle sounding so I have a 1981 Celestion sidewinder in mine.

[@vintageguitaramp_guitarteacher]

Hello. I am sure it's true that the Marshall purists do knock these amps. I am sure that isn't easy to read when you own one of these amps. I have to say that I don't use Marshall amps. These amps do sound okay and I am sure changing the speaker makes a difference, and if that gives you the sound you want then you are on a winner. What has been documented in this video is the poor manufacturing and poor design. The testing that I and all the other techs have done are based on facts. The truth of it is these amps are not made to last, and they are not made to repair either. I suppose that is understandable from a marketing perspective. But that is unfair on the customer who shells out hard earned money. I try to see the good in every amp that comes on to my bench. These amps look good and they do sound okay. Enjoy your amp. You don't have to justify to anyone why you bought it. Just bear in mind there are issues in the way these amps have been designed. Most amp techs are in there 50s and 60s and 70s. They have seen a time when amps were manufactured properly and were a work of art. Watching that crumble away saddens them. We don't do it for the money, it's an art. We live it. Thanks for watching and take care.

[@rgbplumbinghilton]

@@vintageguitaramp_guitarteacher it's not hard reading for me. I wanted a loud cheap amp to gig so my Vintage gear can stay at home. It is not the only amp I gig and I know it won't last forever. I accept that. I was just getting across that it sounds decent. Marshall have made some right dogs over the last 20 years with the dsl and Tsl models. The boards in those are far worse than these.

[@S_Bellew]

@@vintageguitaramp_guitarteacher - sound is subjective. I absolutely like how the amp sounds. The PCB quality of the Origins is better than that of the Studio amps. To begin with, the Origins have cleanable metal Alpha pots whereas the Studio amps have those sealed plastic things for pots, which are very fragile and impossible to maintain. So much for your "tech consensus".

[@stevencochrane115]

Good job Martyn! We can all learn something new every day.

[@vintageguitaramp_guitarteacher]

Thanks Steve. take care mate.

[@henrikreiersen837]

The general consensus is that they built these with el34's because that's what people want to see in a Marshall. It's not a bad sounding amp, but it is a gimmick and it would have been much better with el84's or 6V6's. Cheaper also

[@vintageguitaramp_guitarteacher]

Hello Henrik, That's the feeling I got reading up on it all. It's crazy. Still it seems to make for good videos. Thanks for watching and take care.

[@S_Bellew]

That's a very narrow consensus. I absolutely hate the el84s, they sound more like pre-amp tubes, and the 6V6s don't have the mids of the EL34s, full power or not. I have a 15W DSL with a pair of 6V6. They are not good for power amp distortion.

[@fiddlix]

Seems like a flawed way to do power scaling imho. Excellent video. Thank you 🙏.

[@vintageguitaramp_guitarteacher]

Hello I will give that video from Headfirst a watch as soon as I get a min. Should be an interesting one. Take care David.

[@fiddlix]

@@vintageguitaramp_guitarteacher you too my friend.

[@MichaelSmith-rn1qw]

I'm glad you did this follow up video! Did you measure the actual resistance of the 150 ohm cathode bias resistor? In my one cathode biased amp (1979 Vibro Champ), the 470 ohm cathode bias resistor had drifted up in value to 545 ohms. It had been in the amp for 40 years before I changed it not long ago to get my plate dissipation down to a more reasonable level.

[@vintageguitaramp_guitarteacher]

Hello Michael. Yes I did. I t was on the money. They do drift sometimes. I do find the cement resistors don't drift as much as other types. Thanks and take care Michael.

[@itsonlyme9938]

I have seen output transformers for sale specific for EL34 and with 3.5 k plate load @ 450 V but when driven hard the anode voltage drops. Modulus O/P transformers for EL34 are 3.3k primary I am guessing that its done that way allowing for the voltage drop on the anode and allowing full output power to be achieved.

[@vintageguitaramp_guitarteacher]

Hello, that's good info. I buy a lot of parts from Michael at Modulus. I have some of his output transformers. They are great. He winds some of them himself. thanks for watching. Take care.

[@itsonlyme9938]

Some bathroom silicon sealants are corrosive and have acid of some sort in them.

[@vintageguitaramp_guitarteacher]

I will bear that in mind. take care.

[@stevehead365]

The power scaling is a compromise on impedance matching, hopefully it is well matched on the highest setting. Neat trick using g2 voltage for power scaling, does it sound decent?

[@vintageguitaramp_guitarteacher]

Hello Steve. Someone left a comment saying the output transformers are 5.4K and he had tested two of them. So that is well matched on the high setting. It's miles out on the low. Some people have said they sound best in middle setting, but not great on the low. That's just what I have picked up browsing the net. Thanks for watching Steve, and take care.

[@HandWiredAmps]

I tested 2 of these Origin 20 transformers (2022 model year) and got 5.4K

[@vintageguitaramp_guitarteacher]

Hello. That's brilliant, thank you. I thought they might be around 5K. Thanks for watching and take care.

[@user-dz1fq2di1g]

😄😎

[@stevepi1]

Maybe I'm missing something and this is a stupid question but what is the point of power scaling anyway? A 100 watter with a master volume will work down to bedroom level and sound fine. Power consumption? This thing is relatively low powered to begin with, not running on batteries, and overall usage time for a guitar amp won't be great either so is it really worth reducing?

[@HandWiredAmps]

It's an option that sells amps. People love options. Marketing to the bedroom player. On this amp, I personally never found the lower settings useful, but some people think the middle position sounds better. The sound is different with power scaling than a master volume (be it pre or post phase inverter), whether you like that sound it's up to you. Now there are some implementations of power scaling that are quite useful, but it really depends on the amp.

[@vintageguitaramp_guitarteacher]

Hello, good info there, thank you. Take care.

[@vintageguitaramp_guitarteacher]

Hello Steve. Power scaling is a different sound to Master Volume. I have to say that I am not fond of power scaling and I don't really like the way it is done in this amp. As @HandWiredAmps says, it's all down to marketing. Thanks for watching and take care.