Marshall 10 watt combo clone

[Diabolical Artificer]

Sorry to hear your down in the dumps Rob, send us a PM if you need any help. Re your posted schematics in post #38, all the 1k intensity pot is doing is altering the bias level. the original has a lk from wiper to gnd,this in parallel with the pot mucks about with the law of the pot, IE changes it from a lin to log sort of law.

The modification as followed is terrible, wire the pot wiper to one end else if the wiper lifts the valve with have no bias. Might be an idea to play with this circuit trying different pots, EG log and lin types as well as a combo of pot and fixed R to gnd. I'd use a good 3W ww type.

Andy.

[Murphyv310]

Hi.
Having repaired umpteen Marshall amps during my years in "the trade" I have come to the conclusion that although the bands revel in them they are really not that wonderful.
OK I suspect I'll be shot down in flames, in a service engineers point of view and someone that has built many valve amplifiers they dont cut the mustard.

I'll explain why. They are well known for hum, noise and pick up. One issue is the method of wiring and component layout along with cheap materials. I've encountered those with the black tag boards, they are paper based and prone to leaks and carbonisation, for an amp that is not exactly cheap it penny pinching.
Layout is often poor, wiring can be long and loose, and the quality of build especially in the 60's sought after offerings very bad.

Looking at some of the circuits as well you often say..... why on earth do it this way, coupling caps are also low in value so LF response is poor and phase shifts high. No NFB, no UL output, small transformers in some amps.

Bands seem to like distortion and that's what these amps do (not all of them though) .
I have to say I prefer Peavey, Fender, Blackstar & VOX.

If you want to build this amp I honestly would say that seriously look at the circuit and improve it. Use good quality transformers, think about going Ultra Linear.
The ECL86's are good enough and have been used in many high quality amps, the downside now is the price and availability, The EL84 is a good valve and there are perfectly good Russian equivalents that you can use. The ECL86 triode is half an ECC83 so you can use them, admittedly that's two extra bottles but that will improve reliability somewhat, EL84's are cheaper than ECL86's unless you go for cheap ones like Lamba etc.
The addition of a pre-amp is a worthwhile inclusion and you could add in some NFB to improve stability etc.

[Bushbaby]

Hi Trevor,
Thanks for your comments.
Ed Dinning has built me some wonderful transformers which I am sure will be a massive improvement over the originals.
I do recall when I lived in Hertfordshire and a neighbours (close friend) son was having issues with his Marshall DS201 amp, I took this to a Marshall employee I knew in Milton Keynes to review. The first thing he did was add extra solder in places in the circuit board as he indicated that during manufacture, the least amount of solder was used per board to keep costs down!
I do agree these earlier amps (I have a 64 WEM Custom 15) uses really thin internal cables and I am sure that this was to reduce costs.

When I started to collect parts for this, I did select the best possible components e.g. I do have a range (more that 2) ECL86s & ECC83s (including NOS Telefunken, Mazda, Mullards etc).

Best Regards

Rob

[Bushbaby]

Hi All,
I have been woking on the circuit board layout using the DIYLC software (which I think is excellent and easy to use) to design my circuit board. I've had a lot of help and valuable guidance from Trevor and Andy (I cannot thank these members enough) as well as the transformer builds from Ed Dinning. As I couldn't get the spec of the original power and output transformer, I asked Ed if he would build these from those used within the Marshall 18 watt model. With the power transformer, the rectifier power leads were NOT included as a rectifier valve was not used in this circuit. Anyway, this is my first attempt in designing the board with initial modifications recommended by Trevor (many thanks again).

Best Regards


  1930 circuit board rev 4 Apr 21.pdf (Size: 182.46 KB / Downloads: 10)
  18 watt transformer.pdf (Size: 30.15 KB / Downloads: 6)    

[Bushbaby]

The circuit board pdf doesn't always work 1st time with Adobe Acrobat reader so you may have to retry (several times!). I don't know why??

Rob

[Murphyv310]

I've suggested to Rob that there should be limiting resistors before the rectifiers and also fuses to protect the mains tx. Also recommended a double pole switch and twisted cables to and from said switch to keep hum levels as low as possible.