Build Your Own Clone Message Board

Depending on the role of the extra 12AX7 (read: if it's function, not tonal), it could be replaced with a high voltage MOSFET and provide the same function as the tube.

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-Mike

The screen resistance can affect the tone. It adds a bit of compression and darkens the tone a tad. It's suggested to lower the cathode bypass capacitance if you object to the difference.

Oh, and the higher the resistance the higher the compression effect.

Schematic for the 470R/2W (or value of your choice) screen resistor mod.

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"It’s your soldering."

And refer back to THIS POST from Keith on how best to install the screen resistor.

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Here's a modification of the main Champlifier layout diagram, showing the output tube screen resistor modification as per Keith's installation recommendation:

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Just take your time and be careful, you'll be fine. The instructions are good and the project is straightforward. I would recommend implementing the amp stock first, getting it right, then modding it.

Noticed the grounding scheme of this amp is rather interesting. Anyone experimented with a more "modern" scheme?

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http://www.pdfelectronics.com/

all the build reports I've read claim how quiet the amp runs....to what end with a more "modern" grounding scheme?

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Everyone likes sausage. Nobody wants to see how it's made.

There is a few things that can be done to make it even quieter and improve the grounding scheme.

There is some updates you can do to make the amp more safe and comply with electrical safety standards.

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http://www.pdfelectronics.com/

but it's already quiet. this implies no noise.....

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Everyone likes sausage. Nobody wants to see how it's made.

Just to clarify what I've experienced with my own Champlifier build:

The amp is very quiet, but not absolutely silent. Once the volume is turned up past 10, there is some slight hiss that can be heard. From what I've read, that is thermal noise that originates in the amp components themselves. What I hear absolutely none of, however, is hum, which is what I associate with grounding issues.

I suspect that because the Champ circuit is low gain, low power and extremely simple in layout, less than "state of the art" grounding practice is perfectly adequate.

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

When I said quiet in my earlier post, no hum is what I meant as well.

What ever I'm not going to argue this.

I'm just saying there is some things that could be improved on the grounding scheme to make it even more quiet (if that's possible ) and help improve reliability in the long run. Also a few safety things.

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http://www.pdfelectronics.com/

Hey Chris - arguments or whatever aside, I'd like to know what you're hinting at too. Are you speaking to the usual dual star grounding scheme with power and preamp ground points, back of the pot bus ground, isolating the input jacks, etc, or something else? And what about the safety things? Seems better to detail those than just hinting at them.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

For a simple amp like this the two ground system (plus one safety ground) is a good system like you mentioned Morgan.

One ground point for low current ground's - preamp
One ground point for high current grounds - output section, PSU

I'd leave the input jack un-isolated that's fine for this low gain amp. However I would not use the input jack as a ground point like it is currently. The jack is going to come loose over time as you plug and unplug your instrument in hundreds of times. As the jack loosens the ground connection becomes compromised and then problems will just come poping up. In general it is never good to use a mechanical part as a ground connection or count on it being stable and lasting.

So what I'd do (and many other amps do) is put a little #4 hole an inch or so away from the input jack and have a terminal lug there for preamp ground connections. This terminal lug will be held down with a #4 bolt and ideally a locking nut. It wont come loose over time and will provide a very good connection to the chassis.

Now for this ground point I'd bring R4, C1, R6, R8, C7 and the ground connection of the Volume pot. These are all low current grounds and belong together. They are sensitive signals and should not be mixed with the power amp signals.


For the power amp I would do the same thing and install another terminal lug. The terminal lug must not use the PT bolts for mounting. To this ground pot I'd bring R9, C4, C5, C6 and the PT CT's for the HV winding and heater winding.
*If you really want to try to eliminate 60 Hz heater noise btw you can bring the heater CT to the output tube's cathode. Simple mod, no adverse effects on tone and easy to reverse.

There is a lot to say and read up on grounding in amplifiers but I'll just say
1. It's not good to use the chassis as a conductor for grounding
2. It's not good to mix low ground currents with high ground currents
3. Secure connections are key


Now the safety stuff
These are based standards and protocols set in North America not me. They are to be followed in consumer electronics to protect the user. So this isn't me being anal or anything
But anyways...
1. The PT mounting bolts should not be used as ground points or mounting points. Just because old Fenders did something does not mean it's right
2. The AC mains safety ground needs it's own dedicated ground close to the AC in point. Not on a PT bolt and not with the heater CT.
3. The primary live wiring should be fuse then switch not switch then fuse as the layout and schem show.
4. Wire nuts are a big no no. Heat shrinking and tying off is better. Ideally the unused wires could terminate on a terminal strip somewhere.
5. If it were me I'd add a 2W 220K bleeder resistor to drain residual charge from the filter caps when the amp is powered down. This is just personal preference through.

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http://www.pdfelectronics.com/

Here's RG's article on star grounding for the uninitiated: http://www.geofex.com/Article_Folders/stargnd/stargnd.htm

I'm usually of the 'if it ain't broke don't fix it' camp, but on the other hand best practices should be promoted. And I def agree with Chris on the wire nut thing. I'm surprised that they keep popping up in amp kits.

One question that I've always had in the back of my head regarding separating the preamp and power amp grounds, and star grounding in general, is how does it matter? If both the pre and power grounds are going to the chassis, isn't it same thing, electrically, as tying them together at the same point? Same thing as if you were to star ground the whole amp but not isolate the input or speaker jack. Or is it the distance that matters? Are these interference signals so small that they will not conduct far through the chassis?

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

What I mentioned isn't star grounding.

It's actually not the same. The distance does matter.
This article explains grounding in tube amps very well, better then I can. If you can read through or skim I highly suggest to.
http://valvewizard2.webs.com/Grounding.pdf

The main idea is keeping currents of the same magnitude together and not mixing them up, this leads to cross talk and noise.

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http://www.pdfelectronics.com/

From that valve wizard article:

Wow - that hits right close to home!

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MasterDelayer/Reverbrador/Ampaholic/TopJacker