18 watt JTM45 quesstion

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mac dillard
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18 watt JTM45 quesstion

Post by mac dillard »

I'm getting started on a 18 watt 45 JTM build. Using the schematic and the layout from the downloads section. A couple of questions and observations. Don't know if these drawings get up dated but if they do- the titles for the bright and the normal are reversed. ie: bright is over the normal section and normal is over the bright section on the schematic. Some resistor questions...I haven't tried to run the actual calculations but: ever PP EL84 amp I have ever built the cathode resistor was a 5 watt. Could probably get away with a 3 watt. There is no notification on the schematic which indicated what? Probably would calculate out to be 0.9 watt actual. The 10K dropping resistors in the power rail, R27, 28 and 35, show 1 watt. I would thing at least 2 watt.. Good to be back. I'm old be patient with me. And for Phil, last time I was here we were on about the original layout for the Superlite- did not forget about it just kinda let it slide when I couldn't find it. I do have the layout from 2007 from Maccabee. Title is CR18WPlexi-Layout.
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Re: 18 watt JTM45 quesstion

Post by JMPGuitars »

If you look at one of my build docs, it shows the same general rules. The first resistor between A and B (usually a cap can) is 5W. Then the following resistors on the B+ nodes (decoupling) are 3W. The power tube cathode resistor is 5W, and I use 1W everywhere else. If 1W isn't available in specific value I need for a lower voltage section, I'll use .5W.

You'd probably guess that the B+ resistors are mostly overkill to avoid heat and other issues.
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mac dillard
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Re: 18 watt JTM45 quesstion

Post by mac dillard »

Thanks for the get back. So the lay out is correct and the schematic is correct in form just have not added the resistor wattage. I do the same thing on the resistor watts. Being using 1 watt in place of 1/2 watt for years. Over kill, probably, but it is what I am comfortable doing. I'm going to build the JTM 45 reason I was asking. I usually follow the schematic instead of the layouts..this is going to be an interesting build for me. I'm stripping out an old 18 watt lite 2b (Silvatone, if you remember him) for the chassis and transformers.. Going to build it as a head. My chassis will be 6 1/2 inch wide. I usually build in an 8 inch wide chassis. It will all fit if I can just handle the soldering in the tight places. (Things are harder when age catches up to you)
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Re: 18 watt JTM45 quesstion

Post by JMPGuitars »

I can't comment on the layout or schematic, I've not reviewed them. That being said, it probably wouldn't be there if it didn't have its merits. I'm sure others have built it too.
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Re: 18 watt JTM45 quesstion

Post by mac dillard »

I’m referring to the amp wrong. I actually mean the 18 watt jtm. Not the Marshall JTM 45
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Re: 18 watt JTM45 quesstion

Post by JMPGuitars »

Yes, I assumed you meant this one: storage/18wattjtm.jpg

Still, I haven't reviewed it. If you see something odd, ask away.
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Re: 18 watt JTM45 quesstion

Post by mac dillard »

Question: The PI cathode resistor R18, 470 ohm or 820 ohm ?
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Re: 18 watt JTM45 quesstion

Post by JMPGuitars »

For typical 18W it's 820Ω. I use 470Ω for 36W.
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Re: 18 watt JTM45 quesstion

Post by mac dillard »

Thanks. Shows 470 on schematic and layout.
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Re: 18 watt JTM45 quesstion

Post by JMPGuitars »

That could be fine. It's not a typical 18W circuit, so you should expect to see differences.
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