My first Amp Mod/update

Dan0h

Dan0h

Well-known member
I’m pretty stoked with the results. Re-capped my 70’s tube amp. It still had the original filter caps, 50yrs old. Very out of spec. Moved the caps inside the chassis as well and added discharge resistors.(The thought of my kid being curious and touching something with 600volts caused the extra precaution.)
Also put the SurfyBear in the cab with a FS. This thing rules again!


8127D710-0B0F-4E47-877B-0EC18E2C0276.jpeg 78AB06FF-5B4A-40F1-983E-998FF619A020.jpeg 7619FF20-E843-46F2-8EDA-EEC39357ECF9.jpeg BA3620F5-3AD0-49B2-BBFC-12E126691075.jpeg
 
Paradox916 said:
Nice! Did it make a noticeable difference? I’m still afraid of messing with anything over 24v
Click to expand...
It did make a difference. It’s super punchy again. And extra quiet as well. I wish I would have done it years ago instead of just pushing this bad boy off to the side and leaving it off for so long. But I didn’t really know about the need to change the caps until recently.
 
Well done! It can make a huge difference!

Did you measure the dropping resistor values as well? If they are carbon film they should be fine but carbon comps can drift a huge amount, especially if they are inline with the B+. I once recapped a '65 Super Reverb which was not very loud, hummed and generally sounded a bit lame. Some of the 100K load resistors measured almost up to 1meg. As a result hardly any B+ was getting to the plates. Replaced them with 100K carbon film, plus all the electrolytics and the amp roared back to life. The owner couldn't believe all I did was replace caps and a few resistors. It sounded so different and so much better - and a lot louder.
 
HamishR said:
Well done! It can make a huge difference!

Did you measure the dropping resistor values as well? If they are carbon film they should be fine but carbon comps can drift a huge amount, especially if they are inline with the B+. I once recapped a '65 Super Reverb which was not very loud, hummed and generally sounded a bit lame. Some of the 100K load resistors measured almost up to 1meg. As a result hardly any B+ was getting to the plates. Replaced them with 100K carbon film, plus all the electrolytics and the amp roared back to life. The owner couldn't believe all I did was replace caps and a few resistors. It sounded so different and so much better - and a lot louder.
Click to expand...
Thank you, world of difference.
I had planned to replace the dropping resistors even bought the replacements per schematic, but it turns out when they built the amp they used larger filter caps than the schematic showed and also used larger dropping resistors. After reading up on things I decided to keep the dropping resistors they used as surprisingly they measured spot on still no drift at all. Schematic called for 1k’s they had 22k’s but schematic also called for 300v filter caps and they had 475v. Because the amp sounded amazing back in the day I didn’t want change their formula outside of the expired filter caps. All the other resistors checked out great too. This amp has had very low hours of use through the years. Hopefully that will change here soon. :)
 
When a schematic shows 300V filter caps that is kind of a lower limit - it's perfectly ok to use a higher voltage filter cap. It won't change the sound but it will protect the caps from blowing up!
 
You must log in or register to reply here.
Back
Top