Custom Marshall Style Build...PT/Tube Question

[bradberry00]

Hey guys,

I am going to be building a JCM800 inspired build based very much on the Weber 8CM100 Schematic: http://www.tedweber.com/media/kits/8cm100_schem.jpg.

However, I want to run KT88's so I will be changing a few things. For example 100k to replace 220k grid leaks, lower bias resistor value, etc.

I will also be running the Hammond 373HX http://www.hammondmfg.com/pdf/EDB373HX.pdf and the Hammond 1650T 120W OT. Will this power transformer allow me to use KT88's to at least most of their ability (as opposed to the stock JCM800 with 6550 that needs to be biased as if it was running EL34)? I know KT88s can be demanding so I picked this transformer for its higher current rating. I want to build a Marshall 6550/KT88 JCM800 that works a little better than the ones that came to the US in the 80's.

hopefully this question is somewhat understandable.

-david

[g1]

as opposed to the stock JCM800 with 6550 that needs to be biased as if it was running EL34

Not sure what you mean, can you explain this part further?

[bradberry00]

The jcm800 that ran 6550 had a very under spec power transformer for 6550s in both voltage and current.

[g1]

It was an EL34 design that had 6550's subbed in for better reliability (and probably less warranty claims).
But I don't think the bias tied into this in any way.

[pdf64]

The jcm800 that ran 6550 had a very under spec power transformer for 6550s in both voltage and current.

I don't see how the PT secondary voltage/s can be seen as under spec? The design doesn't appear to push EL34 to their limiting values, so no obvious reason that should change with the move to 6550.

Regarding the current rating, perhaps power supply sag may be seen to play a significant part in the Marshall thing? A stiff B+ may make it into a different amp.

With these Marshalls, it may not be a good idea to set 6550 / KT88 to 70% static plate dissipation, as that may result in excessive static B+ current draw, potentially overloading the PT. So yes, with biasing it's always a good idea to consider both the power tubes' and the PT's capabilities.

A classic equivalent is guys fitting 6L6GC to their DR and adjusting their static dissipation up near 70%; the difference being that's a customer mod, whereas Marshall did the 6550 change. But then I'm not aware that they are recommending a bias procedure, so a good degree of technical competence is required to do it, which would hopefully take the PT thing into consideration.

[Chuck H]

To get more to the OP...

Those transformers will work fine. They are generously over rated for the task You'll have no need for 9A of filament current, and Hammonds tend to run high here anyway, so expect your filament voltage to be in the 7+V range unless you take steps to adjust it or use a regulated DC circuit. At 700V CT and a full wave rectifier on a quad of big bottles you probably won't get beyond 300mA on the most demanding peaks so at 400mA rating you're in overkill territory. And like the filament circuit, this rated voltage is current dependent so you can expect Vp to be high as well. Which may not be a bad thing considering what I perceive as your goal. The extra 5V winding is icing on this very heavy cake. Likewise, the enormous OT is overkill. These are hi fi OT's. Operating exclusively in the guitar frequency range isn't very demanding. A tech at Hammond told me it usually worked out best to half rate for a guitar amp with the 16XX series. I do this all the time and the OT's are typically about the same size or larger than what is typically used in production amps.

Your note that Marshall used under rated iron is an interesting observation. I don't think any Marshalls, including the 6550 models, were ever considered weak, saggy, soft, low powered or otherwise under rated for guitar amps. Quite the contrary.

Consider this... You're design ideology seems to presume that this amp will be producing 120W full time at all frequencies. This is so far from actual operating conditions as to be absurd. In the end, I would expect the amp to be VERY strident and hard sounding, if loud. Perhaps if you wish to use it between sets for a very loud music track or as a backup P.A. Maybe the best JCM800 bass amp ever made!!! Guitar amps actually need a little inefficiency built in because they are tone production tools, not tone reproduction tools. The Marshall amps, JCM800's included, have about as little of this built in deficiency as any designs. That's why they were the go to metal amp in their day.

If, at this point, you still choose to build the amp like this my best advice is to include room in the budget for a hand truck. Someone's going to need to move the thing around.

[bradberry00]

Thanks everyone for your comments.

Though I want a JCM800 style build, I am not necessarily looking for a Marshall sounding build. This will be its own thing. I will say I am not a blues or classic rock player. I play more experimental and heavily detunes music. I want my amp more hi-fi, as I get most of my dirt from pedals and want everything to stay clear with little "sag". This amp will maybe be the JCM800 version of the Sunn Model T, A superbass with a hi-fi/ultralinear power section.

Another Option of PT would be the Hammond 373FX that has 325-0-325 @ 460ma.

[cbarrow7625]

So, why bother with a tube amp at all? If you want a nice stiff power amp solid state sounds like it may be a better match for your djenting (is that a "word"?).

How about a nice light weight tube pre amp / oversized, but still small and lightweight, clas D power amp (Blasphemy!! Heresy!!)?

You've got me interested to understand what it "really is" you want to accomplish (sorry, my inner product manager just doesn't turn off). Just trying to dig a little deeper out of curiosity.

[bradberry00]

Djenting, no. Who mentioned that? My goals are a big powerful amp, that's loud, has balls and is unique. There are plenty of fine transistor amps, but it's not what I want. I don't mean this in negative way, but I really just asked if a couple things would work. The idea of a good sounding amp is subjective. I want an amp that is in the middle ground between a guitar amp and a bass amp.

[pdf64]

Just to expand on Chuck's point, Hammond PTs are specified at full load; at lower loads, the secondary voltages will rise. So it may be best to use the smallest one you can get away with, or else you may have higher than expected voltages to deal with.
And the 16XX series OTs are generally rated within 1 dB at full power down to 30Hz, so even if the power into them is doubled, they should still manage to be within 1dB down to 60Hz, which is plenty even for downtuned guitar.

[cbarrow7625]

Ha, ha. Sorry. I actually have very little idea what proper Djent is. Just being stupid. My bad.

Thanks for taking the time to reply. Like I said, just curious

Chris