18 watt jtm / 18 watt Lite II

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mehfuzhoss
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18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

Hello,

I want to make one of the mentioned amps. the lite will have a VVR for home use for power amp distortion at low level.

will the lite ii with a cascade mode sound better than the 18 watt jtm with a master volume? 'better' I meant fat and thick distortion.
please tell me which one would you choose and why!

Thanks
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Re: 18 watt jtm / 18 watt Lite II

Post by zaphod_phil »

The Lite IIb will be easier to build, while the JTM has better tone control options, and will likely give you more complex overdrive tones - I'm afraid I've never had my hands on on of those.
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Re: 18 watt jtm / 18 watt Lite II

Post by colossal »

This is tough to answer because everyone has their own definition of "the ultimate" or "better" distortion. What does that mean to you? What kind of guitars do you play? What kind of speakers and cabinets do you like and use? Do you prefer preamp or power amp distortion or a mix of both? Do you think AC/DC is the ultimate distortion or do you need higher gain?

Even with the answers to these questions, it's hard to recommend what might be the best 18W type amp. The Lite IIb is very simple to build, has an absolutely amazing sound, and is highly personalizeable/tweakable to suit the player's need. It can cover a wide variety of classic rock sounds from AC to ZZ :lol: Put a treble booster in front and:

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mehfuzhoss
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Re: 18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

Hi,
for the lite amp, is 290CAX will suffice? modification wise, what are the most effective ones? Tonally, I am very pleased how my last build sounded. I tried the Vox cut control that tamed the harshness of upper treble a little bit. Other than that, I wish the amp would break at 2/3!!! is this possible? please advise tweaks to achieve early break.

Thank you very much.
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Re: 18 watt jtm / 18 watt Lite II

Post by JMPGuitars »

mehfuzhoss wrote:
Tue 10/05/21 5:17 pm
Hi,
for the lite amp, is 290CAX will suffice? modification wise, what are the most effective ones? Tonally, I am very pleased how my last build sounded. I tried the Vox cut control that tamed the harshness of upper treble a little bit. Other than that, I wish the amp would break at 2/3!!! is this possible? please advise tweaks to achieve early break.

Thank you very much.
What's your B+ reading?
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Re: 18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

290CAX 315 0 315, with 115ma
I plan to add a VVR on the main B+ node for the entire amp scaling.
Is the 115ma current enough?

290PAZ has 290 0 290 with 140ma. This is 40 dollars more.

Please let me know your opinion.
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Re: 18 watt jtm / 18 watt Lite II

Post by JMPGuitars »

I was asking about your last build, since you think it breaks up late.

If you want it to break up like normal 18Ws, you need to be at 340 to 345VDC on your B+. If it's much higher than that, you'll get higher headroom, and later breakup.

If you go with 290-0-290, you'll have a lower B+ than with 315-0-315. You can user Zener diodes to adjust in either case.

As far as your current requirements, you need to determine what your tubes will draw.
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Re: 18 watt jtm / 18 watt Lite II

Post by geoff 1965 »

The 290CAX has both 630 and 550 secondary options so if you used the 550 = 275-0-275 you will be close to the preferred B+ Josh mentioned and 115mA is sufficient if you bias the el84’s to within the specs in the downloads 150 to 180R range.i’ve got 275-0-275 @ 120mA and biased between 150-180R the transformer stays nice & cool. The 290PAZ will give you a higher B+ but if you are planning to to use VVR then you can sweep between clean headroom and early breakup.
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Re: 18 watt jtm / 18 watt Lite II

Post by colossal »

350-345VDC is the target rectified DC voltage in these amps. When you go above, you're dealing with tonal changes (cleaner) and more stress on the tubes (bias has to be cooled to compensate). Listen to a demo of the 20W 2061 vs. the 18W 1974 for an example of this. When you go below the target voltage, the sound will get browner, less detailed and articulate, and break up sooner. This may be the direction you want to go in if you want early breakup.

The normal voltage gives that Marshally crisp "ping" to the attack. With a Les Paul and a 20W Greenback, I have never found these (Lite IIb type) amps to be excessively bright but very well balanced sounding. I have tried both the standard Volume and Tone control arrangements and added a Cut. The Cut was not really worth the knob with the standard Lite II Tone circuit, IME. I typically set the Tone for about 1pm with a 10% taper 500k audio/log pot and forget it. The rest comes from the guitar.
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Re: 18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

Amazing information. Thanks.
I dismatled my first lite build, it needed a better chassis. I used 290paz and the B+ was close to 400v as far i can remember. The amp sounded best clean and around 4/5 it sounded little pushed and gritty and my closed back v30 was boomy and loud. I could never got any OD.
Now i am thinking to rebuild the same amp and considering to use Vvr. If i go with 290paz again with vvr, i will get awesome and power scale down and turn up the volume to get thick od to distorted sound. Am i right on this?
And if i get 290cax with vvr, i will start with a lower b+ and can get OD early but will miss out the high voltage clean!

As per shared cathode, i had 130 ohms in my last build which pushed the el84s harder. This time i will experiment with upto 180 and what i can find.

The lite circuit is so simple there is not much to alter for a 'better tone'. So i will try 120k for more gain, alter cathode resistor and stick with 2.2k screen.

I have a 15 watt 8.5k 1760E as spare. Can you tell me how it might sound with this build?
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Re: 18 watt jtm / 18 watt Lite II

Post by Daviedawg »

Use the voltage management only on PI and PA for best results. That way you keep the full voltage on the preamp valves and the VVR works right down to quite a low level. Well worth the extra few components involved.

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Re: 18 watt jtm / 18 watt Lite II

Post by geoff 1965 »

I agree,also you don’t need the couplers on the input if you scale the phase & power section.
I’ve got a 1750EP “paper bobbin” on order from mouser for my ecl86 amp and at 15W 8K5 primary it will be good for el84’s & 6V6’s as well.
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Re: 18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

I have this vvr circuit and want to use it right after the Ez81 DC line so the 1M linear (1/4 watt Alpha pot) will lower the B+ going into the first filter cap 32uf/500V.

I can't source a 1 watt potentiometer. Is 1/4 watt 1M ok?
is the mosfet's body safe to be laid on the bare aluminum chassis, directly screwed on? or should I use mouser part: 532-4180 to stick it in between the chassis and mosfet's body!
is any additional heat sink required?
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Re: 18 watt jtm / 18 watt Lite II

Post by geoff 1965 »

Yes you need to use a silicon pad between the mosfet and chassis,the chassis acts as the heat sink and the pad helps transfer the heat and isolates the mosfet.you have to be careful handling the mosfet,use gloves or tissue to hold them.
Alpha make 1/2 watt linear pots.
you might find that vvr noisy,mosfet & zeners work best with filtered B+,the halls version has a dedicated capacitor.
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Re: 18 watt jtm / 18 watt Lite II

Post by mehfuzhoss »

is this Mosfet upto spec mouser part: 78-SIHP15N60E-BE3
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