What Marshall clone to start with?

[Zedman]

Hello Forum, after talking with a couple of mates I'm now considering more fully what may be the best Marshall clone to make my first build based on.

I've now got a Marshall JCM900 4500 50w to use as a donor. It was a cheap amp that offered more in parts value than the purchase price so that will help bring the overall cost down. 

Initially I was gonna go with a Plexi style conversion but now I'm not so sure this is the best choice.

The reason I'm gonna convert the 4500 is so that I can learn more about amp mods effects on things like tone and sensitivity.
I want to be able to offer a mods service on top of the repairs I do and need to experiment to fully appreciate what is possible.
It appears that locally to me, there's more interest in modding JCM 800 series amps and later models. Plexis and earlier models are pretty thin on the ground around here. Keeping this in mind, it seems sensible to be starting with an amp similar to those I'm most likely to work on.

So, I'm hoping some of you more experienced builder and modders can offer an opinion on if starting with a JCM 800 is a good or bad idea. Additionally would there be any implications of what can be achieved using the OT and PT from the 4500. Ideally Id like to go from a 5881 based design to EL34 based.

Any advice you can give will be appreciated.You can probably tell, I'm a noob. 

thanks
Z

[Paul1453]

Let's start with the good news.

The 5881 (a beefed up 6L6) can use a 6K load in PP configuration, just like an EL34 can.
So you are most likely good using that OT with EL34s.

On the bad news side, mods.

Everyone seems obsessed with mods these days.
Classic amps sound the way they do because they haven't been modded.
MScaggs has a tag line "Everything affects Everything", and this is true.

Mods are also very subjective.
What you may love, I might not care for at all.
Then because "everything affects everything",
The mod I might love the sound of could have negative affects on other parts of the circuit.
I'm beginning to realize that simpler is better not only from a manufacturing standpoint but from a tone standpoint as well.

If you can get a minimum of components to a produce quality sound, that is best.
And Mr. Fender seemed to know that very well.

Of course a great sounding simple circuit can be modded to sound better.
But a mediocre sounding circuit will not be transformed into a great sounding circuit easily.
You know the saying "putting lipstick on a pig".

Mods are a very high level of circuit tweaking that few people are exceptionally good at.  IMO   

[Zedman]

thanks Paul1453, I can understand your points, it seems in escense that you subscribe to the KISS theory.

Im happy that it seems the OT would be able to be used.

cheers
Z

[Paul1453]

No problem.

I find that any mods I do to a circuit are not intended to improve on the sound an experienced engineer has designed,
but rather to get my maybe slightly different vintage parts to function properly in that design.

That is usually just a Power Supply/Voltage level adjustment to accommodate my parts.

My Army training drilled the KISS principle deeply into our minds.   

[p2pAmps]

My first amp build was a JTM45 and I am still in love with that amp.  Fairly easy build and very useful for all kinds of music and styles. 

[Zedman]

P2Pamps, I haven't played or heard a JTM45 for a quite a while but I remember they're good.

The situation I'm in is that Canberra, Australia, has a fairly small music scene. I'm returning back to doing repairs seriously after the best part of a ten year break. I wanna be able offer an improved service when I'm repairing amps so I need to gain more knowledge. Marshall JCM 800 series or similar seems to be popular here so I thought that would make a good starting point on which to expand my knowledge.

So I cant hear anyone sounding warning bells but then again this forum may not be very busy. Also I understand that as a noob I need to pull my finger out.

I cant see any Hoffman 2204a kits. CeriaTone has em but seem just a squeak more expensive than Hoffman. I guess now would be a good time to order from Britain if anyone had kits there, seeing as though the Brits have just voted for Brexit and the Pound Sterling has taken a massive hit.

thanks again Fellas
Z

[TIMBO]

Hey Mate, A good mate of mine here in Bris has everything you need.
http://www.evatco.com.au/product-category/tagboards-tagstrips-turrets/

[Zedman]

Hey Timbo, EVATCO are great! I only spoke to Kevin(?) a couple of weeks back. I used to get valves from WES but now use EVATCO exclusively.

I was aware they have lotsa components but didnt realise they did turret boards and such but now I see that they do! Thanks a bunch Timbo!!!!

I will check it out further however I am more inclined to buy a kit than spend time making up a special order. Its a time/ money thing.

Thanks
Z

[Zedman]

Hey Mate, A good mate of mine here in Bris has everything you need.
http://www.evatco.com.au/product-category/tagboards-tagstrips-turrets/

Hey Timbo, Ive just spent too much time looking for where to get parts and it seems that EVATCO gives the best overall price when purchasing and shipping to Australia. The parts are a bit more expensive but when international shipping is considered they come up trumps.

Time to go shopping!
Z

[SILVERGUN]

Here's some reading for you:
http://tone-lizard.com/the-ultimate-jcm800/

Also, check out the rest of his site as he tends to give good 'no-nonsense' advice and might fill in some blanks for you.

Once upon a time, I found this link helpful:
http://www.dynakitfriends.com/downloads/files/jcm800-schema.pdf

[TIMBO]

I have a mate that has been pestering me to build him a JCM so GO FOR IT!!!!!1

[Zedman]

thanks for the links Mr SILVERGUN, a BOM definately helps. 

Mr Timbo, parts will be on order today. When my order is done I can shoot ya a copy with the part numbers listed, it may save ya a bit of time in the future. NB its gonna be a parts list for JCM900- JCM800 conversion so wont include all parts.

I appreciate the encouragement and help Fellas.

Z

[PRR]

> 5881 ...can use a 6K load
> it seems the OT would be able to be used.

Within limits....

The key "match" is the PT to the OT.

PT gives a voltage up-to some current.

OT imposes a load, which is a relation of voltage and current.

In UK (and AUS?) they call tubes "valves". A very apt word.

Within limits, different water valves will pass about the same water between a supply and a nozzle. If you are washing sheep, with a 3/4" nozzle, a 1" valve may work. A 2" valve may work. A 1/4" valve won't pass enough water to wash the sheep.

The different water valves will need different handle positions. In electronic valves, this leads to different "bias". EL34 and 5885(6L6GC) are different yet similar. Both will wash sheep similarly, after you allow for different biases.

There are limits. Two water valves may be similar on the outside but one may have slipperier inside passages. At lower B+ and higher current, a pair of EL34 will pull 3.2K load and a 6L6-family valve is generally happier nearer 4K load.

[Zedman]

Thanks Mr PRR, your'e right, down under we call tubes valves. I can't understand why others insist on incorrectly calling them tubes. 

I can understand your analogy. It appears that you suggest that the PT and OT may work however the match may not be optimal.

I'm unsure now if I will be creating a monstrosity that when completed wont actually sound like a standard JCM 800 2204a.

Luckilly I got the 4500 for close to the cost of a wooden case here in Canberra. One way or another I will work something out. I could end up making a Plexi as an option. At very worst I can resell the amp and start from scratch with nothing lost but my time.....drats! 

cheers
Z

[sluckey]

FYI, there's a big difference between the JCM-800 2204 and the JCM-900 4500. That's the PT. The 900 uses a PT that puts out low voltage for the B+ supply and so it uses a full wave bridge rectifier (FWB). This in turn requires a different bias supply. You will have to build the 900 FWB power supply and bias supply to go with your 800 circuit. Compare the two schematics to see exactly what I'm talking about.

There's no problem doing this. Just be aware that if you buy a premade turret board for a JCM-800 2204 (such as Ceriatone's 2204) you will have to make some modifications to the board and probably need to add a terminal strip for the extra components in the bias supply. Of course if you will make your own board, just incorporate the needed changes directly onto your board.

[TIMBO]

I think he should build a JCM 60 or a JTM45
The PT has a 40v supply for the bias.http://www.classictone.net/40-18075.pdf
I did a mod to the OD and sounds great

[PRR]

> you suggest that the PT and OT may work however the match may not be optimal.

You got the PT and OT from the same amp? Then they ARE designed to work together, by a guy who did such design for a living.

The valves, if "big" enough, are a secondary issue.

Below overload there may be almost no difference 6L6 or EL34, after bias-check.

Past overload, 6L6 and EL34 overload somewhat different, but the ear is the best judge of that. Wire the socket to take either tube (note EL34's G3) and make bias testable and adjustable.

[Zedman]

thanks PRR, thanks for your comments, maybe I didnt articulte well enough.

What I meant was that the PT and OT will be matched but not necessarily matched so well to changing to EL34 rather than the 5881 which are originally in place.

I'm now doing more research and can see that the EL34 draws a higher heater current. If the PT cant provide this level of current within the heater winding is it possible to add an extra small transformer as an additional supply?

I had planned on adding test points for bias monitoring and realise that bias would need adjustment.

From memory at one stage in the early 90's Marshall amps could be modded from El34 to 5881, so I guess with the PT and OT that was in place in those amps, conversion of output vales could be done with component substitution. I get a feeling that the 4500 came out after this, so it may be that the PT and OT are of a different spec.

Time to continue research.....

cheers
Z

[Zedman]

thanks Mr Sluckey and PRR, I have now reviewed both schematics side by side and see your point. On the weekend I was enthused and have a turret board from Evatco ordered, so some mods or tag strip will be in order.

After researching I can appreciate that the characteristics of the OT for a 2204a and 4500 are almost identical though the mounting style is different, which is no problem.

Next point of call is getting my head around the PT.

I appreciate the help Fellas, your pointing me in the right direction so that I can research properly.

Parts are on the way and a mate who is a good guitarist has agreed to do some recording of before and after.
Hopefully I can get to work on the amp soon however its winter here now and working in the garage is bloody freezing. Today our maximum was 8 degrees Celcius and when I rode my bike to work was zero! I know its not as bad a it gets in other places but hate the cold.

Z