Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwp on March 15, 2011, 09:22:04 pm
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I just finished my 1997 version bumble copy, it came out great. I used a 17" chassis (JTM45/18W size) undrilled, used standup JCM800 50 watt iron, is a massive PT, and used every bit below for board. with 10 control pots, I went with an open front head cab design, and located the input jacks on top, reach in, right in front of the V-1 preamp tube. revolutionary I say, but feel free to use....(lol)
I also decided to try two single input jacks instead of the typical Fender parallel 68K's. Per the layout I used one 22K, put the 1M to ground on it, then the second a 38K. Why does the 38K actually sound brighter? Is it because I didn't put a 1M to ground on it? And we need the 1M, correct? but could you please explain why again.
I would post a pic but I just viewed dude's "sold number eighteen" cabs, am alittle embarrassed about the cab it is in now. Work in progress, but the idea is open front, highlighting iron and tubes, finished cab to follow.
Thanks for your replies in advance, dwp
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OK here's a pic. Tolex forthcoming, or open top 2X12 Combo....
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google grid return resistor and you will come across some info. It makes the input more stable, without it you could get plate cutoff. Don't take my word for it I'm no expert.
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I'm not understanding. A schematic would be helpful. It's odd because a 22k in series with the input should have less hi frequency attenuation than a 38k. This is why I'd like to see the actual wiring. I am not catching something.
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no schematic regarding the input jack. It's the standard Rumble type 22K resistor, single input jack to V1-2 with 1M to ground. then I just added another single input jack to V1-2, 38K resistor, both are shorting jacks to ground, but they are not tied together at the jack. only one 1M, on the 22K jack. I'll confirm the value of the 38K next time I open it up, but I thought it would be the lower input, it is not....
Schematic for the Base Build I got from Doug's info page, the 1997 disected version.
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If I am understanding this correctly, when plugged into the jack with the 38k series resistor, the 22k is in series with the 1m grid leak which is effectively a 1M load on the guitar. When plugged into the jack with the 22k series resistor, the 38k is in parallel with the 1M grid leak and thus putting a 38k load on the guitar which is a lot. The 22k jack should be much quieter
or (since you said the 38k is brighter and not louder)
either 1 way the 22k and 38k are in series which = 50k and will attenuate hi freq &/0r they are in parallel which (counting on thumbs) is about 12k, very little hi freq attenuation. A picture is worth 1000 words.
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not in series since the odd input jack is shunted to ground when not used. 1M though to ground behind the shunt. two entirely seperate jacks. or is that the issue? the shunt to ground? hmmmmm. will have to go ponder that somemore....
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I don't know if I'm getting this right but if they are both connected to pin 2 when you plug into one say the 38K the other shorts and you have 22K to ground. Try plugging into one and use a cord attached to nothing plugged into the other so the jack doesn't short and tell me what you hear.
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A picture is worth 1000 words.
+1
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If you simply can't find a crayon and a scanner, then put an ohm meter on a gitar cord, plug into each input, see what you see.
If I am guessing right, one way shows 1,022K and the other way shows 55K. 55K directly across a pickup will suck-out the treble.
If you only plug-in guitars, the 1Meg is not essential for playing. Grid current flows through the 5K-250K DC resistance of the guitar. When not-plugged-in, lack of either 1Meg or some resistor shorting to ground will "float" the grid, tube won't work. You don't care if you are not plugged-in, but plugging-in will yank the grid back to earth with a POP. The worst-case is some external box (such as a pedal) which lacks any DC path on its output. Then lack of 1Meg or similar will float the grid, tube is off-bias, and sounds poorly, sick, or dead.