First project

[Phil_S]

I'm going to assume you have a 1 ohm sense resistor between cathode and ground to find 22mA or 26mA. If not, please explain how you are obtaining this info. It is normal for plate voltage and current to vary inversely. Yes, grid current should be deducted, but it is typically a small amount and not all that critical.

Maximum plate dissipation is 14W, so 70% of that -- a reliable rule of thumb -- is 9.8W per tube or almost 20W for the pair. So, I think your 22mA seems about right.

If you want to change the range of your bias circuit, there are a few things you can do. You want to be able to make your bias voltage colder (more negative).
--Change the 220K dropping resistor. 180K will supply more voltage to the diode, shifting the operating point (colder) of the circuit, but it will not change the range, which is controlled by the 10K pot.
--Reducing the 47K to 33K or 39K will supply a more negative bias voltage. The 15K to 47K+10K form 15:47 - 15:57 a voltage divider. Rotating the pot or changing the fixed 47K determines how much voltage is pushed to the power tube grids. The ratio of the divider determines how much of the available voltage is supplied to the tube grids.
--To change the bias voltage range, change the pot to 20K or 25K and reduce the 47K (to 33K or 39K) to keep the center point of the voltage divider reasonable.

[darefugee]

Thanks Phil. Yes I had a temporary 1 ohm resistor between the cathode and ground and measured the voltage drop across it with the Tektronix DM-44 which sits atop my 'scope.

I'm pretty comfortable with the bias setup as-is, just wanted to ask around to learn if anyone saw any red flags with the numbers. Thanks for your explanation of exactly how that circuit works - it'll likely come in handy for my next project, a Princeton Reverb clone using AO-29 iron. I have another AO-29 here with the smaller power transformer - ought to be just right for that.

The Princeton Reverb's AA1164 circuit is a little different, but the principles appear to be similar. In that circuit, the voltage divider is 22k and whatever portion of the vibrato intensity pot is dialed in, is that right? And changing the 100k resistor changes the voltage supplied to the diode?

[Structo]

By the way those Hammond power transformers have aluminum bell ends, you can polish it and make it shiny.

[Phil_S]

If the 1 ohm resistors are still in there, leave them in. If you need to re-bias, they will already be there and they do no harm. It is typical practice to leave them in.

Re: the AA1164, I didn't look, but there is more than one variation on the theme for bias circuits. Essentially, though, the divider is needed to fine tune the bias voltage, so it will be there one way or another. I think they call that one bias wiggle tremolo and I think you see why. You'll have to see if 100K is the correct value for your particular build. It really depends on the voltage supply and the bias voltage needed. The only magic to 100K or 220K is that it happens to drop the correct amount of voltage in that particular amp -- not really magic at all -- science!

[darefugee]

So I found a 1x12 Line 6 cab on craigslist to house my Plexi 6v6. The tolex is pretty clean and it sounds really good with the amp. I plan to hang the amp from the top panel, controls in rear. The other alternative is to cut the top at the rear and have the controls on top. But I'm afraid of butchering the cabinet so that's out.

Here's the amp and cabinet, I hope to mount it tomorrow.

[img:828:800]http://i1285.photobucket.com/albums/a59 ... f2b90c.jpg[/img]