Firts build! Some questions and a few ideas in mind


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Nov 30, 2021
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Hello, World!

I have a some questions and a few ideas for my firts build, sort of... I recently bought a Harley Benton GA5 head, which is basically an Epiphone Valve Jr. with an EQ knob. It sounds great, but it could be even better. I had thought about building an amp from scratch and someone had suggested an AX84, and given the similarities, I will mod this one.

I like the simplicity of this single-ended 5W tube amp. It has an standard marshall tone stack wtih one (bass) knob labeled as EQ. I recently replaced the EQ board with a single 15k resistor (ala Valve Jr) and repurposed the 1MA poti as a master volume. Now I can get the natural OD at any volume!

Now I want to change the grid block, as I think that is not optimal. The first two resistors (68k & 68K) are arranged as a voltage divider, resulting in 50% of the input signal loss! The Epi has a 1M resistor instead, resulting in only 6% loss. I was thinking of replacing the 68K with a 1M and moving it after the bridge, which will result in virtually no signal loss. This should be better than removing the resistor completely as I saw in another post. (See schematic for details)

I will rewire the whole thing and upgrade the components, which should result in a better sound and ease of modifications down the road. Ultimately, I will try to add a second pre-amp tube (ala AX84 HO or Marshall Class 5) to have some extra gain.

What do you think? Is there anything that could go terribly wrong, something that I have overlooked?



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