New kit purchased today!

Get support and show off your MetroAmp JTM 45 kit builds.

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Fbiwaterman
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New kit purchased today!

Post by Fbiwaterman » Fri Aug 24, 2018 9:32 pm

Hello all,

It's been a journey to get here. Lot's of reading and questions, and thank you all for your responses. They were very helpful.

I purchased the Valvlstorm JTM45 kit with Metro-spec transformers and choke today. I also purchased the mustard caps and Carbon Comp resistors. The only things that I couldn't get were the KT66 tubes, which I found some matched valve art at Antique Electronics Supply, and the 100k 1/2w cc resisters were not in stock.

I'm doing a standard build with no MV (I purchased a Weber 100w load box) and no effects loop to start with.

Can anyone recommend where to get the 100k resistor?

Thanks, and I look forward to posting progress pictures and sound clips.
Mike

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neikeel
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Re: New kit purchased today!

Post by neikeel » Mon Aug 27, 2018 4:22 am

Welcome, good call on the kit and build, is it your first?

Personally I would not usually use cc resistors across the whole amp. You will see most have a mixture of carbon film (modern 1w ones are same size as old ones btw).

Equally I would avoid MO other than in the B+ line (so ok for 8k2 and 10k droppers in your amp and 1ohm bias).
Metal film I also use (much debate).

Suggest you implement Larry grounding from outset too.
Neil

Mpollard
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Re: New kit purchased today!

Post by Mpollard » Mon Aug 27, 2018 4:44 am

Aloha Neil,

Thanks for the welcome. While this is my first complete amp build, I’ve had several successes modding other amps, most recently my hotrod deluxe which now sounds incredible. My background is Physics, so I understand EM and I’ve built quite a few of my own measurement devices in the past. This is my first foray into tubes though. Reading everything I can and refreshing all of the great info I learned 20 years ago in college.

Is there a thread on the forum that you can refer me to so that I know where others have used the carbon comps with desirable results?

Also, MO? What is that?

I’ll look for the Larry grounding on the forum.

Thanks for the advice,
Mike

Mpollard
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Re: New kit purchased today!

Post by Mpollard » Mon Aug 27, 2018 5:07 am

MO....Metal Oxide. Got it

danman
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Re: New kit purchased today!

Post by danman » Mon Aug 27, 2018 7:32 pm

I agree with neikeel about not using them throughout the entire amp. They can lead to a noisy amp if used everywhere. If you do a google search for "carbon comp vs metal film or carbon film resistors" you will find thousand of pages of discussion over many forums on the topic. Many old amps including Fender used carbon comp exclusively throughout the circuit because they were easily available and cheap. Many think that the carbon comp resistor lends itself to a certain sound or mojo. I stopped using them because they do add noise to the circuit. Marshall almost always used carbon films in their amps over the years though a few carbon comp do show up from time to time. Piher and Iskra were the two top brands back in the day but scarcity and demand has driven up the price on those for people wishing to build an exact replica. They are still available on Ebay if you want to go that route though. You will need 1/2 watt for most of the circuit, but everything in the B+ rail should be 1 watt or higher. I like the carbon films that Valvestorm sells for use in my builds...http://valvestorm.com/Products/Components

Mpollard
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Re: New kit purchased today!

Post by Mpollard » Mon Aug 27, 2018 10:23 pm

In the JTM45 kit instructions/appendix, George gives some guidance on where the cc resistors should go. Do you guys agree with his recommendations? What I think I’m hearing from you guys is that the tone benefits are minimal, while noise increase is not. Should I just stick with the resistors supplied in the kit? Is it worth my time to experiment and build a separate turret board. My main goal is to get a great sounding amp that’s as close to an original as possible, without getting into a situation of diminishing returns. I’m a musician first and an engineer second in this case.

Also, one question regarding Larry’s grounding:
So, i think the way I understand it, I’m adding 5 or 6 new ground points/terminals to the chassis and tying specific ground lugs at each. I read in one thread that this eliminates the need to solder a ground bus to the backs of all the pots, and that the bass pot does not need a ground. Is this correct?

Thanks in advance for the advice!
Mike

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neikeel
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Re: New kit purchased today!

Post by neikeel » Tue Aug 28, 2018 3:35 am

Correct on both points.
Follow Georges suggestion
Implement Larry ground:
Image
Neil

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Re: New kit purchased today!

Post by Mpollard » Tue Aug 28, 2018 1:45 pm

Excellent! Thank you!

danman
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Re: New kit purchased today!

Post by danman » Tue Aug 28, 2018 7:28 pm

There are a few spots on the board where the turret spacing is too wide for the newer resistors to span. This is another reason to use the Allen Bradleys as their leads are much longer. Good luck with the build!!

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Re: New kit purchased today!

Post by RockinRocket » Tue Aug 28, 2018 9:55 pm

Don't use CC resistors for the inputs (68k) as shown in the metro guide this is the noisiest spot in the amp.
Noisiest spot + noisiest resistors = extra noise.

CC were used in some 45 because the lead length was longest in those spots. Cathodes and bias positions. Slope was also CC because Marshall already had 56k in the bias.

Mpollard
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Re: New kit purchased today!

Post by Mpollard » Tue Aug 28, 2018 10:15 pm

Thanks for all the advice! This is a fantastic forum!

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