Guitar Tube Amp

Step 23 Update, V0.2

Here's the entire schematic (V0.2), with a few changes...

-- The whole schematic is now a single graphic.

-- One of the power amp cathode bypass resistors has been reduced from 50uf to 20uF. Even less capacitance is an option...

The Screen Supply Switch

An additional source for the 6DG6GT screens has been added: the Edcor output transformer has a 40% screen tap. Now this is switchable, between the old PS B.3 tap, and the transformer tap.

A bit of research confirms that a pentode screen operating from a OT tap is running in ultra-linear mode. Although I see no evidence to support the claim "...a maximum power output equal or even greater than the strict pentode operation of the tube..."

In fact, it's definitely a softer, mellower, "woodier" sound (with some volume attenuation.) By alternating between the old and new schemes, the switch acts, in a way, like a simple boost.

Of course, using the screen tap on the OT shifts the current draw for the screens from B.3 to B.1....more current draw on B.1 reduces the voltage available on the B.2 for the preamp. This may also explain the volume drop.

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Alternative Power Supply

The PDF below includes an alternative power supply which uses off-the-shelf (Hammond) transformers in place of my own "scrounged" inductors. It utilizes a tube rectifier instead of the SS bridge--which is why the higher voltage (180V) PT works.

I haven't built it, so a little experimentation is called for....(in fact, read the pdf carefully, some modifications are suggested.)

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External Cab

I've been using the amp a lot lately with a 2x12 cab. There's a lot more headroom with the larger speakers / cabinet combination.

It's definitely bassier and louder, too. I still switch to the internal speaker when I want to drive the amp to the edge of instability (feedback, etc.), but it's a more vintage sound with the cab.