The Amp Garage

A GZ34 has an internal resistance of about 175 ohms, so adding that inline with your SS diodes should get you in the ballpark. As jp suggested, you might like more or less, so feel free to experiment. Note that this resistor will dissipate 7 or 8 watts, so use a 15 or 20 watt flameproof resistor, an...

I use a MOSFET switch to switch the NFB in and out on my 18watt/trex, because the 18watt side sounds dull with NFB and the trex side is out of control without it!

http://mhuss.com/18watt/schematics/MosfetSwitch.gif

--mark h

I think the issue is that 12A*7s have a fairly low heater-to-cathode maximum (~70 volts IIRC), and the cathode of the top half of the cascode is a bit more than this! The 6DJ8 is great for cascodes, and has been used in a few hi-fi circuits that way, but tends to microphony.

--mark h

The "spare" pin to use on octals (for screen Rs, etc.) is 6. This is N/C on 6V6, 5881/6L6/KT66, EL34, and 6550/KT88.

Also, it' s safe to connect pin 1&8 on all of these (and somewhat necessary for the EL34 )

--mark h

I work from home a couple of days a week, and my office is by the front door. When the UPS guy comes, he walks most of the way up the sidewalk and *chucks the package* onto the step, about 6 feet away, and runs. When the FedEx ground guy comes, he puts the package down by the door and rings the door...

Allynmey wrote: Cant wait to see "Brand X"

Perhaps Ty would dig it .

Allyn, I think you're mixing up your Xs

Kings X --> Ty Tabor
Brand X --> John Goodsall (Phil Collins on drums)

--mark h

The issue with potentiometers as plate loads is the *voltage*, not the wattage. There's precious little insulation and clearance inside to be keep hundreds of volts away from the cover (and metal shaft!) Even if you don't electrocute yourself, you could have quite a fireworks display as the HV power...

A plate-driven tonestack is 'less ideal' (whatever that means) then a cathode-driven one, because it has a higher output inpedance. That said, all mid-to-late period Fenders used plate-driven tone stacks, as did most other vintage amp manufacturers. Grap Duncan's free tone stack calculator ()http://...

The older Mesa Boogies (Mark II era) used these as well. There *shouldn't* be any audible effect during normal operation (i.e., they should be in the nonconducting state 100% of the time), but I still prefer to just put an Ampeg-style 250 ohm resistor across the output.

--mark h

The treble control in a Fender/Marshall style tone stack is traditionally linear taper.

--mark h

I think cherry would be appropriate.

--mark

I do wonder about the master volume affecting the bias. However, another issue is the bias DC being across the pot. Pots with DC impressed across them tend to get "scratchy" quicker. The PI MV shown in loverocker's post above is basically the "Rich mod" with a couple of extra caps to keep the bias D...

markd wrote: There are two more 80uF caps under the power supply board(that's why it's up so high) that are in series with the two above the board.

Odd that they would be 450v caps then. Evan at silly high voltages 350 each would be more then enough.

--mark h

Yes, the input cap is 80uF, and that is theoretically too high for a GZ34, which (according to all the old data) has a 60uF max. I guess the rectifiers in a Komet don't last a long time?

--mark

The extra tap is called a bias tap, and is typically 40-60vac with respect to the center tap. The other way of doing bias (just dividing down the full secondary voltage) works just as well, and doesn't require a special transformer.

--mark