Need to build JCM800 low watt/attenuator

[pjd3]

Hello and thanks for stopping by.

After 2 successful amp builds (Fender 40 watt and 20 watt Plexi) I'm feeling the dire need for a low wattage JCM800 combo amp. I gig quite regularly and as an old dog cover music from the 60's into nearly the 90's. So, theres a big call to emulate the tone/responce from the 70's and 80's regarding classic Marshall grind and lead tones.

A friend is giving me an old broken Carvin combo amp so I thought that a good cabinet for a low watt JCM800 amp with a single 12".

I'm really most interested is someone(s) can lead me to a high quality transparent DIY active/inductive attenuator for this amp. I'll probably start with a classic JCM800 preamp and add a few modification that are useful but still be able to default to the classic circuit (some mild EQ and gain mods maybe). I definately plan to install a PPIMV as I have found that very useful on my 20 watt Plexi amp.

I'm still on the fence on whether to outfit the output section with EL34's or EL84's. Of course I realize that there very well may be some benifit to using the big bottles but, I gig quite often at small pubs, restaurants and small venues where stage volume is something that needs to be complied to.

I don't need the amp to perform at conversation level volumes, but, it needs to sound nice at light to moderate stage volumes. I like the idea of a good high quality attenuator so that I can push the output section to a desired level.

So, thats really it in a nut shell. I want to get moving on this project and need to gather the best info a resources I can to maximize tone and authenticity and to somewhat minimize volume to accpetable gig stage levels.

thanks everyone! Exited about this one.

Best,
Phil Donovan

 

[Marcomel79]

https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/
best 100$ youll ever spend

 

[neikeel]

I wonder why you might need both an attenuator and PPIMV as well as the stock pre-PI MV.

You refer to a JCM800. If you mean a 1959 (NMV non cascaded preamp) JCM800 then you might need both. Now if you mean a 2203 (100w master volume with cascaded preamp) or 2204 (50w master volume cascaded preamp). Then you probably will not. JCM800 is a model line - not a specific amp.

Clarify what you mean and we can help.

 

[pjd3]

I just found what appears to be a popular thread on attenuators on this forum. Nice!

Best,
Phil Donovan

 

[pjd3]

Hi Neikeel, thank you but the elements you've asked me to clarify are really found in my post either explicitly or otherwise. When I think of a JCM 800 I think of the tone or probably what you refer to as a cascaded preamp with a cold clipper. I already have built and use a Plexi for cleaner tones and pedal platform so will be just attempting to add along a dedicated 2204 type amp, 20-50 watt, single channel. We can let me decide things like whether or not I like or use PPIMV and/or an attenuator. I've found that having options can help one develop and achieve subtle tone textures that you can't know what they are until they are those options are available for you to use. I'm good with having extra's. Its a one- off- amp so, I don't need to get overconcerned about "one or the other". I'll figure that out.
I'll be adding in a bunch of my own little mods I'm sure as I go. But I won't know exactly what that is until I get a classic 2204 type thing in the works. Just could use some tips from those that have needed similar solutions

thank you,
Phil

 

[pjd3]

Hey all,
I should probably add that I would expect to be keeping this simple and be using a single 8 ohm speaker. This meant to be for gigs and a quick uncomplicated set up to get playing at 9:00pm in the clubs without too much hastle! Stick a mic in front of it and get going! (and stick a second mic up infront of the clean Plexi or Fender amp, too) plus A/B switch.

Phil

 

[neikeel]

Ah, ok so 2204 type with cascade. That is important to clarify, there are lots of JCM800 amps!

I would just build one of those as it has a pre-PIMV that sounds pretty good with decent preamp tubes and make a good attenuator as linked to above to start to get the power amp moving (although that is a bit over-rated). You can use 6V6s instead of EL34s for less headroom or even just have 2204 front end and hook it into an 18w type power amp (you will need to experiment with dumping some signal between preamp and PI as you will not need to drive EL84s quite as much as EL34s).

 

[pjd3]

Thanks neikeel,

Those sound like very good options to keep in mind. I just got a scope for Christmas so, besides hopefully some ears, I'll have an extra diagnostic tool for looking at voltages at the grids to help assess gain staging if needed.

thanks very much!

Best,
Phil Donovan

 

[Gene Ballzz]

Hey @pjd3 ,
You must be the same Phil D. from over at The Amp Garage? If so, welcome to the party. This is a great place with some very knowledgeable and friendly folks! and yes, the Workbench is the home of really great attenuators!
Enjoy!
Gene

 

[JohnH]

Hi @pjd3
You sound like an ideal candidate for one if our attenuators! as linked above.

Best to read post 1 (which gets updated) to see all the basics, then skim though the later pages particularly from p110 on. Then join the thread with any questions. The design hasn't changed fundamentally in 2 or 3 years but it's very adaptable and there are a number of options and variations that are possible.

https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

There are now many dozens of builds described, and every one of them has been a success.

 

[pjd3]

Hello and thank you everyone for just being here sharing your opinions, views and experience. It is an absolutle invaluable resource for which I am very grateful for!

And yes Gene, I am the Phil D from over the Ampgarage site which has also been extremely supportive and helpful on my 2 previous builds which were very successful to the point of being my main gigging amps. I gig quite a bit for a 64 year old classic local rocker. I love good classic (and modern) guitar tone and did alot of research into each amp build and what might best go into each build, components and fabrication.

I have every intention of using the "attenuator" thread as a main resource for a reactive attenuator for which I Must build for this next and 3rd coming project. As a matter of fact, I am going to start another thread now on asking about upfront basics for my JCM800 2204 type amp. Really exited about this one!

Thank you all!
Best,
Phil D.

 

[Guitar-Rocker]

I have a slight hearing loss in the very upper registers from working in industrial environments. I have found that EL84's drive that hearing loss notch to extremes, such as nails on a chalk board, and have met several who also suffer from that same condition.

In searching for lower wattage amps I found that either the 6AQ5 (10watt--7 pin), or my new favorite the 6CM6 (14watt--9 pin) nicely fill in that void that the EL84 left.

Both of the 6AQ5 and 6CM6 tubes make outstanding amps, are plentiful and cheap, and hit that same tone of a big bottle tube.

The only caveat is hitting the right plate voltage with either. The sweet spot for the 6AQ5 is @ 290V on the plate, while the sweet spot on the 6CM6 is @325V.

Should you need assistance on either I would help.