Harmony 420 gain stage

[ok_state_blues]

I have a Harmony 420 amplifier that I would like to modify by utilizing the unused triode from V1. What would be the simplest way to up my gain for guitar use and live out my early Zepp day dreams? The amp is stock and looks to be prime candidate for tinkering.

[Dave]

Look in the mods and projects section on the EL34 homepage. There is a mod there called the hotswitch mod. Some variation of it will work in your amp.


Dave

[ok_state_blues]

In looking at that, how would I make that permenamt and not switched. Also, is it necessary to have another pot?

[passaloutre]

My Thunderbolt has the following mod for extra gain stage:

https://www.premierguitar.com/gear/ask-amp-man-modding-a-supro-6420-thunderbolt

It works really well and I like having the option to choose stock or extra distortion via two jacks.

[ok_state_blues]

Would parallelism the V1a&b be a good option? Just 3 jumpers for more grit?

[passaloutre]

Parallel is certainly easy to do, and adds a measure of grit, like jumpering a two channel amp. And, as you say, just three wires. Loop

The series connection has substantially more distortion, more like an overdrive pedal, without being substantially more work.

[ok_state_blues]

To help me get my mind around this, I am referencing the weber smokin joe 2. I think this will help me accomplish what I am going after. It seems similar to the Hot switch minus the actual switch and the schematic is helping in my copy and paste quest.

[sluckey]

Weber calls that a cascode preamp but I'm not so sure. The top triode seems to just be a cathode follower. No idea how much gain (if any) that will add. Merlin explains a cascode stage here.

Just looking at the H420 circuit I think you may get all the extra gain you want simply by removing that .1µF cap from the junction of the two 100K resistors feeding V1 pin 1. Easy to do and certainly worth a try.

[tubeswell]

Simple step to bumping up the gain a knotch is to parallel the spare V1 triode with the other V1 triode using jumper wires across the socket pins.


Otherwise, insert a stage between the vol and the PI (but that’s a little bit more work)