220k or 470k grid resistors?

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7n7
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220k or 470k grid resistors?

Post by 7n7 »

Hello,
Two of the 18watt minimalist schematic versions show different grid-to-ground references...one is pair is 220k and one pair is 470k. I'm sure this isn't a mistake, but what is the reason?
Thanks!
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dartanion
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Post by dartanion »

The difference is the amount of signal voltage that hits the power tubes grids. The 220K will be a lower voltage than the 470K. What does this equate to tonally? Well, the 220Ks will give you more headroom / later breakup than the 470Ks.
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Post by zaphod_phil »

The very neatly drawn layout you posted on AX84, http://frankolson.com/www46frankolson46 ... 6html.html , seems to be for a kind of hybrid between a Minimalist and a Lite III. The latter was an experimental design that was rarely built. It uses the Vox 220k resistor values in the power amp, so you will get more of the Voxy clean chime and less of the Marshall grind. So not really the definitive 18W tone, but maybe that's what you like. :)

One thing I'd like to ask is how are you doing your grounding scheme, which isn't shown on the layout drawing? Same with the power supply. You need to have a good, reliable grounding arrangement.

BTW, if you want to have the Lite III's fixed tone control, you may still do best to experiment with a 500k pot, to find the best combination of values for the two resistors.
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Post by 7n7 »

Well, I've currently got 2 main star ground points, connecting/clamped to the chassis via one leg of the OT... one ground point for the preamp section, and one for the power amp.. though, I'm switching it up a bit at the moment, so I can keep the center taps away from the preamp section...
any suggestions?
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klingo
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Post by klingo »

Hello,
In my 18wlite i have the 470p/10nf caps and the tone pot position is following the vol pot position(more or less)....so isn't a dual 500KA vol/tone pot a good idea?
are you guys experiencing the same thing? may be my 18wlite or my guitars strangely voiced. :)
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Post by rjgtr »

7n7 wrote:Well, I've currently got 2 main star ground points, connecting/clamped to the chassis via one leg of the OT... one ground point for the preamp section, and one for the power amp.. though, I'm switching it up a bit at the moment, so I can keep the center taps away from the preamp section...
any suggestions?
I'd suggest grounding the preamp and input jacks as close to the input jacks as possible on a single grounding lug. Avoid a bus bar across the pots, but otherwise using a bus bar is fine. Of course, ground the power chord as close to where it enters the chassis as you can and separately from any other ground. Ground the power supply and power amp together. It is generally a good idea to put the power/output ground point as close to the ground spade of the first filter cap. It seems to reduce nose and hum more.

Don't use the transformer bolts as ground points. Use separate screws. Transformer bolts can work loose over time and the musician then becomes a grounding point.

This type of scheme will keep the power and output section away from the preamp. The general rule I use is to get the power/output grounds closer to the power chord ground than the preamp ground, and make sure the preamp ground is more distant from the power/output ground than the power/output ground is from the power chord ground. This seems to avoid any polution from the power/output ground into the preamp.
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Post by klingo »

+1 with rjgtr way of grounding!
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Post by Hywelsollis »

Would it be possible to use a dual gang pot to take you from 220k to about 470k? A sort of "Vox to Marshall and anywhere in between" control.
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Post by Phil_S »

I'm not so good with switches, so double check what I say. I think you can do this with a DPDT toggle. Wire a pair of 470K in parallel (that's 235K) and use the toggle to lift one pair out of circuit. Ganged pot sounds too complicated to me, with all the need to lift one set to get the other in circuit. If I referenced the wrong kind of switch, well you were warned! Anyhow, I'd do it with a toggle switch.
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Post by dartanion »

You guys are essentially talking about a PPIMV. Look in the downloads section under the mods area. This varies the gridleak value by paralleling a 1M resistor with a 1M pot on each side, thus a dual ganged pot.
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Post by rjgtr »

+1 on what dartanion says. I've used the PPIMV and it is a great way to control the drive of the output stage.
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Re: 220k or 470k grid resistors?

Post by bvayling »

On grounding, it seems Josh and others suggest keeping the preamp section ground connection close to the inputs. This seems to make sense, in that the weakest signal‘s ground has the shortest path.

I am building a stock style GDS combo tremolo with a bus wire at the top of the board. Right now per Josh’s guidelines the chassis connection is near the inputs.

On Richie’s layout, the bus chassis connection is on left, near the power section. Are there two schools of thought on this?

I can still change it!

I know jack but I’m good at following directions!
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bvayling
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Re: 220k or 470k grid resistors?

Post by bvayling »

Sorry, meant to post this in grounding area.
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