NAD ( modded JCM800 content )

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griff10672 said:
according to everything I've read ...... the 6550's when in a " 50 watt " amp like in a 2204 ..... should be treated as 25 watts each when biasing ... because that's what the transformer is designed for ....
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Do you have any references for that you can toss my way? I haven't been able to find much in the way of a technical explanation of this beyond "my tech told me so." I would think biasing the 6550's like 6550's isn't going to cause the amp to put out more than 50W, because other things will limit the output. And the little extra current draw at idle seems so tiny as to not matter to the power transformer. But, I haven't looked too closely into this and might be missing something.
(Edit: one possible thing, from some more digging around after posting, is that the so-called strain comes from asking the amp to provide enough power for the tubes to dissipate 49W amongst themselves, if biasing to 70% of 35W, or 58.8W if 70% of 42W. But, I haven't worked out all the details on if/how that'll actually affect the amp.)
griff10672 said:
I also have a Ceriatone 2204 running EL34's ...... they both sound great ..... but the 6550's just add some friggin push and girth the EL34's don't ....
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Yeah, there's just something there with the 6550's. Nice big open chord/power chords sound like they're going rrRAWWKkk! At least from what I can tell from @Bxlxaxkxe's demos.
 
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Mike Oxhuge said:
Awesome build! How does it compare to the stock 2204?
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It's got more gain ..... it's a lot tighter due to some of the mods to the circuit .... no flub ....

with a Superdrive or a Chiron in front of it you can get pretty close to some modern tones with it

but as a old school metal amp this thing just friggin slays
 
7704A said:
Do you have any references for that you can toss my way? I haven't been able to find much in the way of a technical explanation of this beyond "my tech told me so." I would think biasing the 6550's like 6550's isn't going to cause the amp to put out more than 50W, because other things will limit the output. And the little extra current draw at idle seems so tiny as to not matter to the power transformer. But, I haven't looked too closely into this and might be missing something.
(Edit: one possible thing, from some more digging around after posting, is that the so-called strain comes from asking the amp to provide enough power for the tubes to dissipate 49W amongst themselves, if biasing to 70% of 35W, or 58.8W if 70% of 42W. But, I haven't worked out all the details on if/how that'll actually affect the amp.)

Yeah, there's just something there with the 6550's. Nice big open chord/power chords sound like they're going rrRAWWKkk! At least from what I can tell from @Bxlxaxkxe's demos.
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yes exactly what I read ...... so you're not " taxing " the power transformer ..... there's also strong opinions out there to treat them as 30 watts each ...

there isn't a lot of difference in settings between the two ... so I just aimed in the middle ...
 
Even if you were basing it on 35 watts, you're still at 54% which is still great. I would have done the same.

What were you seeing on the screens?

I watched the video Lyle Caldwell did on a '69 Park 150 he worked on 4 years ago. That would be my desert island amp! It was a hybrid cathode and fixed bias design. They used a cathodyne PI, but it had a 12au7 follower behind it to drive the KT88s. It had a huge Partridge OT and the PT might as well been a cannon ball. That amp sounded MASSIVE!
 
Sixstringflannel said:
The emblem on the amps is so dope dude. I love great brands! Digging the rest of the aesthetic choices as well!
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if I were to do anything different .... it might have been white faceplates instead of gold .... but I think the gold looks OK .... and kind of shows it's a Marshall of some type .... IMO anyway
 
griff10672 said:
according to everything I've read ...... the 6550's when in a " 50 watt " amp like in a 2204 ..... should be treated as 25 watts each when biasing ... because that's what the transformer is designed for ....
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Yep.
Just to be safe, in a Marshall anyway. Vs a Fryette/VHT, that wants to see 50-60ma...those transformers are designed around a 6550/KT88.


Congrats on the amp! Looks killer.
 
griff10672 said:
my experience with doing pedals has definitely made it easier for me to build an amp .... from soldering skills ... schematics ... grounding schemes .. etc

I've been considering doing some PCB's for amps .... but I really love the turret boards ... some amp companies mix the two which I think looks sick ....
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Sure wish you’d do some amp pcb’s.
Something that’s not already out there would be great.
 
tpruitt said:
Sure wish you’d do some amp pcb’s.
Something that’s not already out there would be great.
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Unfortunately ..... just like pedal circuits .... I don't feel like there's much room for anything actually new and ground breaking out there .... so like the pedals ..... I'm just trying to build a higher quality mousetrap .... but I'm definitely always learning more and more with each build ....

once I'm done tweaking this one a bit more ...... I'll put it up for sale .... and hopefully sell it to fund the next one ...

I simply just love building stuff
 
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OIP (1).jpg
 
7704A said:
(Edit: one possible thing, from some more digging around after posting, is that the so-called strain comes from asking the amp to provide enough power for the tubes to dissipate 49W amongst themselves, if biasing to 70% of 35W, or 58.8W if 70% of 42W. But, I haven't worked out all the details on if/how that'll actually affect the amp.)
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Came across something today that again makes me doubt this is an issue. The spec sheet for a 2204 says it can do 90W at 10% distortion.
1762641274807.png

https://www.drtube.com/schematics/marshall/2204specs.gif

Which isn't news, we already know that Marshalls can put out significantly more power when clipped hard. But it made me think... if a 2204 is actually capable of putting out ~90W of power, why is the tubes idling at a hair under 50W total an issue for the transformers? We all agree that running the amp cranked, where it's pushing ~90W, is running the amp hard. But that's almost twice the power we're talking about idling at. So until I dig further, I suspect it's not an issue whether you bias the 6550's like 6550's or EL34's. But, I'm not certain and would appreciate a more technically experienced person chiming in.
 
griff10672 said:
Unfortunately ..... just like pedal circuits .... I don't feel like there's much room for anything actually new and ground breaking out there .... so like the pedals ..... I'm just trying to build a higher quality mousetrap .... but I'm definitely always learning more and more with each build ....

once I'm done tweaking this one a bit more ...... I'll put it up for sale .... and hopefully sell it to fund the next one ...

I simply just love building stuff
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Have you looked at Jason's (evolve diy/Headfirst) project boards? His ST1 and generic project boards are pretty customizable. I think you can build an Alta 100 off the generic board.

I bought the Merlin boards to eventually build a Wizard clone. Those projects will be on my back burner for a while, but I can say they are nice high quality PCBs
 
MadAsAHatter said:
Have you looked at Jason's (evolve diy/Headfirst) project boards? His ST1 and generic project boards are pretty customizable. I think you can build an Alta 100 off the generic board.

I bought the Merlin boards to eventually build a Wizard clone. Those projects will be on my back burner for a while, but I can say they are nice high quality PCBs
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I've got a Headfirst ST-1 board hanging around ...... I opted for the turret board configuration on this one ....

I also have a Headfirst Wizard MTL board ...... that's one I want to build next .... but picking out transformers for it have me a little stumped ...

I need to do some more research on the Wizard
 
7704A said:
Came across something today that again makes me doubt this is an issue. The spec sheet for a 2204 says it can do 90W at 10% distortion.
View attachment 422988
https://www.drtube.com/schematics/marshall/2204specs.gif

Which isn't news, we already know that Marshalls can put out significantly more power when clipped hard. But it made me think... if a 2204 is actually capable of putting out ~90W of power, why is the tubes idling at a hair under 50W total an issue for the transformers? We all agree that running the amp cranked, where it's pushing ~90W, is running the amp hard. But that's almost twice the power we're talking about idling at. So until I dig further, I suspect it's not an issue whether you bias the 6550's like 6550's or EL34's. But, I'm not certain and would appreciate a more technically experienced person chiming in.
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I'm actually going to try some KT88's in it tonight .... I've been tweaking it for a couple of weeks now ... really happy with it so far
 
griff10672 said:
according to everything I've read ...... the 6550's when in a " 50 watt " amp like in a 2204 ..... should be treated as 25 watts each when biasing ... because that's what the transformer is designed for ....
Click to expand...

This is correct. And ultimately, it doesn't matter. As long as the tubes aren't red plating, you're fine. You want the bias hot enough to get rid of crossover distortion and cool enough not to damage anything. Anywhere in between in fine as long as it sounds good to you. And really, there isn't that much of a sonic difference anywhere within that range unless the amp is on 10 and the power section is severely distorting.
 
griff10672 said:
I've got a Headfirst ST-1 board hanging around ...... I opted for the turret board configuration on this one ....

I also have a Headfirst Wizard MTL board ...... that's one I want to build next .... but picking out transformers for it have me a little stumped ...

I need to do some more research on the Wizard
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When you get working on the MTL board let me know how it goes. I've looked at the schematics and layout; it looks a bit daunting for my level of experience. I'm sure I'd be able to get ot done, but it's more complicated than what I've built so far. By the time I get to it I'm hoping I'll have done enough where it won't seem as bad.
 
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