Electric Guitars/power valves


QUESTION: hi . what's the difference between overall sound of ECC82(aka 12AU7) & ECC81(aka 12AT7) , at power stage of low wattage all tube amps ?
p.s. : Marshall's making 50th anniv. 1w amps and all of them use ECC82 as power valve ; except JMP1 which uses ECC81 , and I wanna know the reason and the result !

ANSWER: Hi, Eric!  Here is the answer to your question, direct from the man who designed the amp; James Marchant of Marshall Amplification's R & D staff:

Your question is quite difficult to answer in direct terms.  There is no ‘tone bible’ that we could turn to say what the tonal effect of each valve.  The story has a little more to do with the more human nature of the engineering team here at Marshall.

The project started because a Jon Langstaff and I started looking at the best techniques to design low power push pull amps.  We had heard competitor’s low power pp amps and thought we could do better.  Jon was looking at slightly higher power amps in the 5 watt region and I looked at 1 watt power as I had a bit of previous history playing with 1 watt designs before I joined Marshall.  As it transpired, Jon was given a project that needed his full attention and he had to concentrate on that, whereas I still had some pure research time available, so my 1 watt was completed.  The amp I built was tested with the ‘won’t get fooled again’ riff as I thought this was definitely a ‘big amp’ sound and if it could nail that, then I would be on to a good thing.

Once it was built, I showed the amp to Chris George, our product demonstrator.  He really liked it and decided that we should use it as a basis for the 50th anniversary celebrations with one tuned to each decade, and each designed by a different engineer.

Jon had moved on to take charge of our Eden bass brand, so Steve Dawson, Santiago Alvarez and I were asked to submit suggestions for the different models.  It was clear to me that the amp I had designed was a bit of a 60s / 70s crossover, and since Steve’s formative years were in the 70s (his first concert was Led Zep touring LZ IV!) then I shifted it more into the softer more bassy 60s.

So Steve proposed the 70s JMP, I showed the 80s (I have owned an JCM800 master volume since the mid 80s), and Santiago took the modern high gain stuff, the 90s and 00s.

I asked Steve why he used the 12AT7 in single ended mode and he said it was to save on valves as it would not need a phase splitter.  This does not work of course as you need two 12AT7s to get the 1 watt required!  It is more likely that, as he had recently designed the Class 5, he had already been thinking about the design of a 1 watt – he had in fact mentioned that he wanted a Class 1.  The upshot of using two output valves is the same as the difference between a 50 watt and a 100 watt which is an added texture that seems to be present when output valves are paralleled.

I used the 12AU7 because all classic Marshalls are push pull and I feel this is part of the sound.  I specified the output transformer by conducting experiments with whatever I had lying around and seeing what I liked.  What I ended up with was quite a low reflected impedance, but which has a bit of a compression and sag as the 12AU7 struggles to provide the current required.  Santiago also decided to use the 12AU7 in push pull mode, but he started off from the drawing board and came up with an impedance 3 times that of my designs!  This makes the power amps stiffer and quicker, which is suitable for the high gain models.

So you can see that it is not just the choice of valve, but also the way it is used.  It is worth adding that there are several changes I made in the power amp between the 60s and the 80s so that even though both use the same transformers, they have quite different responses – particularly dynamic responses.

I hope this helps.

Best regards,

James Marchant
R & D Designer

---------- FOLLOW-UP ----------

QUESTION: hi again and thank you very much J . here we go :
1- few years ago there were a few low wattage amps out there , but today there are some remarkable . the best of them is " Marshall Class 5 " !
2- the first gen. of ' Class 5 ' was a real amp . no master volume ! superb ! the pure tube saturation ! beautiful ! but there was a big problem , a huge problem !!! rattling !!! so the second gen. ( called C5-01 ) came out . no rattle ! surprise ! the best amp ever made .
3- economy is more important than pure art ! so Marshall discontinued ' Class 5 ' and started to make a limited edition series , 50th anniv. 1w models .
4- all of these 1w amps are push pull , but JMP1 . it's Class A , single ended , with an extra 12AT7 , for another valve stage . I really prefer the topology but the sound isn't good enough . not comparable with ' Class 5 ' . [ I'm not sure if 12AU7 could sound better but I think it sounds more like EL84 , which sounds perfect for low wattage amps ]
5- actually I'm looking for a ' Class 1 ' as James mentioned . a real ' Class 5 ' sound in a 1w amp ; more usable in bedroom for some guitar crazies like me .
6- are they gonna make a new ' Class 1 ' ? ' JMP1 ' isn't a real ' Class 1 ' !
7- 50th models are good amps but they look better than they sound , so collectioners would be more satisfied with'em than guitar players !

thank you

Hi, again!  I don't know of any current plans to introduce a new Class 1.  The 50th Anniversary Models were the big push for those.  I agree with you, though; they look better than they sound.  What I'd suggest is sending Marshall's R & D guys a letter with your feedback on their low-watt amps and what you as a player would like to see.  The R & D gang really does pay attention to player feedback and you never know - your ideas may strike a chord and have something new created!

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J Ross Smith


Find at www.jrosssmith.com! Over 20 years of guitar playing, teaching, building, and modifying; have worked as a touring musician, studio session guitarist, engineer, guitar tech, and guitar teacher. Registered member of ASCAP. Registered member of Freelancers Union. I have a working knowledge of music theory and styles, and a taste for all types of music and instruments. If you have a favorite guitar player or style, chances are I share it! If you have a question I can't answer, I'll rely on experienced and knowledgeable people I know to get the correct information for you, and I hope I can help inspire your playing style and tone. Promo photography by Sebastian Castillo at Castillo Photography, San Diego, CA.


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