Wonderland/JM100 build

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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

markusw wrote: Sun Apr 23, 2017 11:15 amDon't you think it's possible that the "1.15M or 1.18M" has only a three band code like the 100k resistors?
Could be; that'd make it 1.1M.
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

interesting. I've been doing some reading on voltage dividers and with 1M and 68k you're get in 0.6% of the input voltage coming out. That's a huge loss, could someone explain why you'd want to loose 99% of your gain at that point?



Marcus
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

martin manning wrote: Sun Apr 23, 2017 11:20 am
markusw wrote: Sun Apr 23, 2017 11:15 amDon't you think it's possible that the "1.15M or 1.18M" has only a three band code like the 100k resistors?
Could be; that'd make it 1.1M.
that would be 0.58 % of the input signal being passed on so even quieter.


M
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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

Aaron wrote: Sun Apr 23, 2017 11:19 amThe resistor colours look like blue-grey-black-orange-red-gold. Which according to the 6 band colour code would be 680k, 2% tolerance with 100ppm temp. coefficient.
The standard 6-band code does not include gold as the last band. Regardless, the first 4 bands are the value, so I think it must be blue, grey, black, red, or 68k.
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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

norburybrook wrote: Sun Apr 23, 2017 11:24 am interesting. I've been doing some reading on voltage dividers and with 1M and 68k you're get in 0.6% of the input voltage coming out. That's a huge loss, could someone explain why you'd want to loose 99% of your gain at that point?
A 12AX7 requires only a small signal voltage to drive it from idle to cut-off, say 1.5V peak. If the gain is around 60, the output voltage is 90 peak. That's way too much for the following stage, so it would have to be knocked down by a factor of ~1/60 (0.017) to get back to the same level for the next stage. If there is a lossy tone stack following that will do it. If not you need a suitable voltage divider. Remember a typical 10% log pot loses 90% of the input voltage when it's set at noon, more if the output is loaded.

BTW, your math is off a decimal: 68k/(1000k+68k)=0.064, so 6.4%. That's like a 10% taper log pot set a bit below "2" like someone said above.
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

interesting stuff. you're right about the decimal being out I was working on a 10v input when doing my calculations :D

I suppose I could put a pot or a switch with a couple of different values there, be interesting to see how far you need to go before it overdrives the next stage. It might sound nice.....you could put a relay and have a footswitch with a pot and have it as a drive control and it would mean you'd have less reverb too when you were on the drive......


Right back to my cabinet making.....this thread still has legs.


Marcus
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

I've ordered some 'Dumble style corners' from my usual supplier and this time they've come countersunk. Never seen this type before.

Question; do I have to countersink the cabinet before tolexing so they will lie flush?



M
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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

Yes. Maybe. If it's Tolex then I believe you can do it after. If it's suede then do it before.
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

Right then....

Been busy with my drill and jig saw....yes that's the sum total of my tools..(oh I've got a detail sander too)....on the decking on the porch...no workshop!!!

so all that been said, here's the finished result. I need to add an aluminium edging strip to the cross bar and get the hair dryer on the grill as it's gone saggy, but apart from that I'm quite pleased with the results. Black tolex with the proper glue is a lot easier than suede cloth and spray on 3M.
DSC_1048.JPG
DSC_1046.JPG
DSC_1049.JPG
Thanks everyone, especially Martin , but there's been quite a few other's that I'm now indebted to, so cheers guys.

I'm particularly proud of this build as it's my first off piste effort as much of it it came from a donor Peavey combo.

I've learned a lot about voltages on this build;

FWBR to get the need voltage from the Transformer for the HT

voltage doubler to get the correct amount of bias Voltage

Voltage divider; the 1M-68k mystery.

and finally....check components before installing...EVEN DIODES.....it's a lot easier to check a component than spend a day trouble shooting to find a duff one.


Marcus
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Krinkle
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Re: Wonderland/JM100 build

Post by Krinkle »

Great job!!
telentubes
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Re: Wonderland/JM100 build

Post by telentubes »

Great job on the cab as well as the amp. Looks really good.
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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

Looks good, Marcus! You should really get a router, though. Next you need to learn about the frequency response of the coupling cap combined with the voltage divider. Recall we found an error in the layout, too.
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norburybrook
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Re: Wonderland/JM100 build

Post by norburybrook »

an error in the coupling cap?

I don't remember that, it has been a journey though.

So...how does the coupling cap work with frequency in conjunction with the voltage divider?


M


p.s. I did get a router but I didn't have the right round over bit so it only took the edge off and I had to go with the sander again.

I couldn't work out exactly what I needed from the bit specs (bear in mind we're all metric here).
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martin manning
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Re: Wonderland/JM100 build

Post by martin manning »

The error was the missing jumper on the power supply board.

In 100 words or less: There is more than just signal attenuation in the voltage divider. The coupling cap and the resistance to ground forms a high-pass filter that shapes low frequency response. The cathode bypass cap plays there too. The upper part of the divider is series resistance going into the following stage, which along with the inter-electrode capacitance and gain of the following stage forms a low-pass filter that shapes high frequency response. That's the reason for a bright cap on volume controls.
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erwin_ve
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Re: Wonderland/JM100 build

Post by erwin_ve »

Marcus, great job!
You did the last part real quick! Still surprised you can get it done with so little tools. Looks good and it sounds likewise I guess.
Erwin
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