a FBT 150 (or 120) restauration - italian 60's mojo :)

[nasdak]

Hi,
i'd like to share this one with you
just got a 1960's FBT 150, FBT is an italian brand.
It's labelled 150, but the guts are inded like a 120 :

[source=https://reho.st/medium/self/3689cca8f27fbb8bcc958a95cdb67a841b83dee8.jpg]Source[/source]

so what do we have here?
6X4 rectifier, grid leak bias (just like early tweeds), 2 inputs - input 2 is Hi and 1 is low, cathodyne PI, 2 6AQ5 as power tubes, cathode biased with grid controled tremolo! Trem has only speed, no intensity.

And Two 8" alnico speakers
eyeleboard wired.
Power Transformer is HUGE for a 15w ; output transf was DOA, had to find another one.

[source=https://reho.st/medium/self/f9a7dcdb290d0a6dec38af3b852b4029c3684ed1.jpg]Source[/source]
nice look isn't it ?

[source=https://reho.st/self/a2c69872c02eaa766d41a870b41a151ba21a1fa5.jpg]Source[/source]
the thing is loaded with tasty mustard caps

[source=https://reho.st/medium/self/c192348b5f2b3355e5f6732daa1d40f013645f62.jpg]Source[/source]
HUGE power transformer

what i had to do :
- filter caps checkout : the're ok and already have been changed, F&T caps
- cleaning and tensionning th esockets
- new preamp tube (crappy chinese JJ changed for NOS philips !)
- new OT (from an old reel to reel player, it's a 4ohms but it will do for the testing)
- new switchcraft jacks
- added screen grids resistors (470R)
- added a speaker jack for easier testing (dummy load...)
- somme bad solders fixes

voltages are ok with 310V on the plate - a bit high for 6AQ5s - but 70% dissipation.

and... it sings !
trem is very strong, clean good, but at full volume it's not that loud and SO great ! Reminds me of clapton cream sound (sound, not playing!)
here a small vid :

 

[Bendyha]

Nice..I have 3 FBT amps...in various states of repair, a 250 and 2 500's.
I envy you the simplisity of the 150, all you need, and no more, and no crazy tubes like the 500's.
As you say, huge transformers, you should see the size of the things in the 500.

 

[Wicked-T]

Sounds great, you did a great job on that amp.

 

[nasdak]

Trem sounds great but it lacks an intensity control. It was easy to add a intensity pot and fit it in the footswitch jack hole.
Now trem can be set from subtle to chopchopchop

The amps sounds great but might lack some bass, i'll investigate that, but maybe the're is not much bass to expect from the old small 8" speakers

 

[Bendyha]

I would be tempted to re-do the input stage in an attempt to get a fuller sound. Do away with the input .01µ cap, change the 4M7 grid-leak bias resistor to a more common set-up using a 1M resistor with the added cathode resistor of appropriate value, (not knowing the tube sort or plate voltage I can't hazzard a guess)
But as you mention, it could be the speakers....although the FBT 12" in my 250 is not at all bad. Is the cab open backed?

 

[nasdak]

Grid leak bias input stage is part of the amp, i don't want to change that, we see so much cathode bias amps here
It's open back as i got it without the rear pannel... A new one could improve the bass response.