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PostPosted: Fri Jan 04, 2019 4:04 am 
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Hey all... wishing you a happy 2019!

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: http://www.geofex.com.

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 parts inside your amp.

Firstly... here are the articles. No point me regurgitating what he has already said, as I'm not the expert!

https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1
https://www.premierguitar.com/articles/the-immortal-amp-mods-ndash-pt-2-1
https://www.premierguitar.com/articles/the-immortal-amp-mods-pt-3-1
https://www.premierguitar.com/articles/the-immortal-amp-mods-pt-4-1

I thought I'd share my progress and findings with these mods on my Trinity kit 18W PlexiV6 amp, 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:
Image

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.
Image

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

Briefly:
* 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/Farnell/Newark. Easy to mount in a chassis, and take the smaller 5x20mm fuses. You will need M2 (tiny) fasteners to mount these, they are compact!
https://nz.element14.com/littelfuse/05200003z/fuse-holder-5x20mm-10a-600v-qc/dp/2840063?ost=2840063&ddkey=https%3Aen-NZ%2FElement14_New_Zealand%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. Remember... Only use slo-blo fuses!

I'll update this thread if I make any progress with part 3 or 4. Best of luck if you wish to try this!
Any questions or comments welcome!

Cheers :)

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PostPosted: Fri Jan 04, 2019 7:43 pm 
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I'm just getting back into building since retirement. Seems like this would be a good post for the Dressing room forum, too as it likely applies to ALL tube amps and seem to be great preventative advice to avoid amp damage. Definitely mods 1 and 2.

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PostPosted: Tue Jan 08, 2019 11:56 am 
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Good work. There is a lot of it to make it bulletproof. Lots of fuses!!

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Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps


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PostPosted: Wed Jan 09, 2019 1:00 am 
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Cheers guys :)

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PostPosted: Mon Apr 22, 2019 6:39 pm 
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Joined: Thu Mar 21, 2019 12:34 am
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Location: Auckland New Zealand
Hi Corey
Looks like great work - I retrofitted some of these circuit protection mods on a Fender Vibrolux kit I built but didn't have the chassis real estate to fit them all in.

I've got an 18W Plexi kit inbound to NZ so looking at replicating your layout. A few quick questions if you don't mind:

  1. Is is all still working well with the fuse values you listed?
  2. Did you install MOV's across the output valve plates and the OT centre tap?
  3. What value MOVs did you use?

Thanks in advance,
Dougal


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