View Full Version : Reducing Amplifier Feedback in a Almarro A318B

12-02-2020, 10:16
I've been playing around with my Almarro amplifier again to try and extract more performance from it.

Here's the changes I've made so far:-

Replace the 1 ohm heater resistors to get all the heaters to work
Replace melted heater supply cable on left channel
Cleaned up melt damage from left 6CCB tube
Replaced all the signal capacitors with Jupiter copper foil types
Removed the channel switcher (makes 3 much easier)
Replaced All signal wiring with Neotech silver wire
Replaced the volume control with a 250K EIZZ Gold Switched Attenuator (with a 1M ohm shunt)
Replaced the rca sockets with some quality copper plated ones

This time I decided to experiment with the amount of feedback the amplifier uses. Feedback is set by a 22 ohm resistor and 1.5K feed from the cathode leg of the 6H9C tube. By default feedback is 5.4db, I soldered another 22 ohm resistor in parallel with the existing one with the intention of halving the negative feedback (see attached circuit).

Initially I listened to some jazz, Lee Morgan and Lou Donaldson at the Savoy, the horn instruments did sound more life like but as I played more up beat music something sounded odd although I could not put my finger on it. Initially I thought the bass was missing but after playing some reggae it was clear it was still there but perhaps not so well etched. Next I played an album I knew inside out The Smith's Peel Session album, on some of the quieter tracks it was as though you were along side Morrissey as he sang but it was while listening to 'Reel Around The Fountain' that things sounded really odd. It was as if the band had never played the track, timing was off, it was almost as if a cover band were playing it. Perhaps I over did it on reducing the feedback?

Anyway I removed the change and the Smiths returned to form. Perhaps I'll try again some time with a smaller change.

12-02-2020, 10:35
I know if you google Lampizor ( I think that’s how he spells it ) he suggests that there should be no negative feedback. There are various opinions of this I removed it on a el34 amp I had and I was happy with the result. According to one of his webpages its one of the first thing that he does.

Not sure if this really helps but just another view.


Light Dependant Resistor
12-02-2020, 11:15
The valve base wiring I hope has been tidied up, Soldering in the image is very poor.
If not suggest you carefully remove wiring to the base by referring to the schematic , obtain some solderwick to remove excess solder
on each of the tabs, then tidy up the wire ends. Your soldering iron for this type of work needs to be ( depending on tip used )
approx 350 degrees C

Your soldering should when finished be nice and shiny, with no loose wire ends - and look professional.

12-02-2020, 13:04
Chris - I think the photo is showing the "melt damage", before the modification and repairs were made.

12-02-2020, 14:13
The damage has been fixed and cleaned up as best I could. At the end of the day the amp is a play thing for me to have fun and fiddle about with.