Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bangy on December 08, 2016, 08:20:37 pm
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Building 2 5F2A's and ready to start wiring before the Turret boards go down.
1). The ground Bus shown going over the pots in Doug's illustration is made of what? Can I use the 20 Gauge bare wire used to connect Turrets?
2). The lasso depicted running from the ground buss to the RS174 cable I am guessing is to ground the shield of the RS174 cable, correct?
3). The 33K resistor between Tips of input Jacks and Pin 2 of V1 on Dougs schematic- Is this necessary in a two Input Jack application which has (2) 68K resistors on each Tip of each Input jack? It seems redundant (to this newbie) and is there only for a single input jack application. Im guessing (2) 68K's equal a 33K. A
Thanks for always helping this new guy out. Much appreciated.
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1 & 2 are yes. Don't know about 3.
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The 33K resistor between Tips of input Jacks and Pin 2 of V1 on Dougs schematic
If both the 68k's are going to V1's grid pin then you're all set. The point is to have a resistor there in series with the incoming signal to help eliminate unwanted junk signals sneaking in (I think:)
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Thanks guys and to clarify the ground buss is soldered to the back of the pots, as you see in a Gibson guitar harness?
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Yes, that's how doug does his preamp ground, make sure if you use that method that you get a good solder connection
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Thanks Shooter
I scuffed up the chrome plating and did what Lupe had done on my Fender 5F2A, soldering directly to the chassis. Is there any way with the multi meter to test a ground??? In other words can I see that the ground has been made?
Bangy
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You'd need a megger (sp?), I like the "tug" test, if I can tug on it, lift with it, ohm zero, it's probably good. I typically use solder tabs and bolt it to the chassis, but I use aluminum chassis
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soldering directly to the chassis
You can solder to a steel chassis but you cannot solder to an aluminum chassis. Use a ring lug and bolt to the aluminum chassis for a reliable electrical connection.
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Just while we're on the topic, is there a preferred location for a ring and lug type ground connection? In the P1 I built I used one of the standoff bolts for my chassis, but I've seen the Transformer mounting bolts quite a bit too. Is there a "rule of thumb" or does it vary heavily depending on where everything is located in each build?
Thanks guys
-Brett
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I generally have one ground lug for the power supply and power amp. I use a transformer bolt if convenient but I prefer to drill a hole near the PT for the power ground lugs. I drill a hole near the input jacks for the preamp ground lugs.
Sometimes I deviate slightly but always keep the power grounds separated from the preamp grounds.
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You were asking about testing ground, the best way is to put your DMM on 'continuity' and then put one end on the chassis far away from the connection point, and then go to the opposite end of the ground rail where it isn't connected and make sure you get good continuity. it shouldn't be much more than 1 or 2 ohms.
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oh and to understand the the 68k vs 32k etc, check out sluckey's write up on it, see page 8 here: http://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf)
~Phil
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> it shouldn't be much more than 1 or 2 ohms.
Specifically it must read the SAME as holding the probes together firmly.
There's always some resistance in meter leads. Some meters display that, never going below about 1 Ohm (perhaps typical for thin wire and a fuse). Some may auto-correct it (there may be a delta function). I have seen one which just rounded-down any low reading. Play with your meter and see what it says for probes held together firm, loose, probe points pressed to good clean metal, to dirty metal, etc.
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Thanks PRR and others. This is all makes perfect sense and I have applied this already and its working. I really could never have got this far without this forum. Doug has a great community going here.
Pressing on at my slower than snails pace. . .
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Pressing on at my slower than snails pace. . .
Slow is good, the sooner you finish, the sooner you start twitching, then thinking about the next one and the next one..... :think1: