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RG Keen - the Immortal Amps Mods - (18W Content!)

Posted: Fri Jan 04, 2019 8:12 pm
by Cdog
Hey all... had this project ticking away while, might be interesting.

For those of you who don't know RG Keen is quite a well-known electronic guru with a really great website for guitar amps and effects pedals. LOTS to be learned here:

Anyway, I've been meaning to perform some of his suggested 'Immortal Amplifier Mods' from his Premierguitar columns dating back in 2008. The idea is, all Tube amps fail periodically as tubes or other parts wear out... some mods to make them 'soft fail' without burning other expensive (particularly Iron) parts inside your amp.

Firstly... here are the articles. No point me regurgitating what he has already said, as I'm not the expert! ... fier-mod-1 ... ash-pt-2-1 ... ods-pt-3-1 ... ods-pt-4-1

I thought I'd share my progress and findings with these mods on my Trinity Amps kit '18W PlexiV6', as it might help some here who wanted to try the same. There has been research, trial and error involved! (Perhaps you too can benefit from the pile of dead fuses and time invested.)

I've done part 1 and part 2 already with success. Part 3, I'm going to try next... (I have MOV'd the AC inlet) and maybe... part 4 one day.

Here's a pic of the changes inside:


Firstly... I added the 2 rectifier 1N4007 diodes as suggested. I used a tag strip, as you can see near the EZ81 socket.
Then I fused every transformer line.

The values took some trial and error. Here's what I have been using, with no issues so far:
F1 (EZ81 Filament) - 1.6AT
F2, F3 (Both 290V windings going to EZ81) - 1AT
F4,F5 (All other Heaters, fuse each winding) - 6.3AT
F6 (B+) - 300mA

* Choosing the values means reading the datasheets of your Valves and determining the current required for normal operation.
* I'm using all JJs, 1xEZ81, 3x ECC83S, 2xEL84 and 2x6V6S.
* F2 or F3 blew at 6A when I turned the amp on and flicked the standby on too quickly. I'm trialing 6.3A there now.
* Only use slow-blo fuses. (That's what the T means!)
* Fuse both windings if the secondary is centre-tapped and referenced to ground!

Finally... the parts! I found these really handy panel mount fuse holders, from Element14. Easy to mount in a chassis, and take the smaller 5x20mm fuses. You will need M2 (tiny) fasteners to mount these, they are compact! ... d%2Fsearch
For fuses, I use regular 250V 5x20mm Cartridge fuses, even for the B+. I got a mix of ceramic and glass types, whatever was cheaper. If you try this, remember... slo-blo fuses!

I'll update this thread if I make any progress with part 3 or 4.

Cheers :)

Re: RG Keen - the Immortal Amps Mods - (18W Content!)

Posted: Tue May 14, 2019 6:04 pm
by tubeswell
The rectifier protection diodes are a no-brainer.

As for the heater fuses, remember that filament resistance is really low when the filaments are cold (at startup), so the filaments will draw quite heavily for the first second or so, and then current draw will decrease as the heat kicks in. This will vary with the type of tubes used. (The more current the filaments are designed to draw, the higher the startup surge, and the more robust the fuse needs to be).