FBT-250 / Italian tube amp / Mod request and advice

FrodeK · Post by **FrodeK** » 11 Nov 2020, 00:33

Hi everyone!

I am looking for some advice here. I recently bought this amp. The brand is called FBT, model 250. It is reported to be from 1977, Italian made. It has a separate pre-amp and main stage.

Main stage has 2x el84, plus 1 x ecc83 (phase shifter?). Preamp with tremolo has 2 x ecc83.

It has two channels. Normal has high and low input, no tremolo.
Guitar channel has high and low input with tremolo. The high input is broken. I need to replace a resistor, jack input and solder again.

It sounds very warm and nice and the tremolo is very nice and juicy!

Any possible mods to be done?

1. Can I make it a one-channel amp?
2. Can I make the pre-amp a little more hot?

The schematic for the preamp for FBT-250 LB does not apply to this model.

Manfred · Post by **Manfred** » 11 Nov 2020, 22:42

Personally I think it would be a pity to modify the amp.
This amp contains the rare old Mullard capacitors, which have become very expensive in the last years.
Also the internal wiring looks like new.
If the amp has a strong humming noise I would replace the electrolytic capacitors.
Such large radial capacitors are available here:
https://www.die-wuestens.de/eindex.htm

To make the preamp a bit hotter I would use a good booster between guitar and amp.

CheapPedalCollector · Post by **CheapPedalCollector** » 12 Nov 2020, 01:25

I concur.

If it happens you just don't like the amp, sell it to a collector and buy an amp you do like.

ppluis0 · Post by **ppluis0** » 12 Nov 2020, 01:29

Hi FrodeK,

I completely agree with Manfred's suggestion about internal condition of this amp, so leave in their actual status is the best option.

After study the schematic you posted I found a (surely, drawing) error:

The point "C" at the power supply is to feed the preamp and tremolo tubes with siutable high voltage to work and NOT have to be connected in any way to pin 3 of each power tube !!!

Please check that with the amp unplugged after wait some time to allow completely discharge the filter caps.
That point at the supply (point "C") must appear isolated to pin 3 in the EL84's

Another drawing (or manufacture ??) error are the 6.3V line to feed all the heater tubes that appear floating respect to chassis ground.
Check if exist continuity from any heater pin to ground or if that winding in the power transformer is really floating.

In any way, you can modify the actual heater configuration with one of the suggestions from this site: http://www.valvewizard.co.uk/heater.html

(I'm are fan of the "heater elevation" implementation)

To make the amp run "hotter" my best advice to you is to build an external stompbox named VALVECASTER explained in this site: http://beavisaudio.com/projects/valvecaster/ that runs a tube at low voltage, use inexpensive condensers and low wattage resistors, and it's safe to have placed in your pedalboard.

Cheers,
Jose

FrodeK · Post by **FrodeK** » 13 Nov 2020, 08:50

Manfred wrote: ↑11 Nov 2020, 22:42 Personally I think it would be a pity to modify the amp.
This amp contains the rare old Mullard capacitors, which have become very expensive in the last years.
Also the internal wiring looks like new.
If the amp has a strong humming noise I would replace the electrolytic capacitors.
Such large radial capacitors are available here:
https://www.die-wuestens.de/eindex.htm

To make the preamp a bit hotter I would use a good booster between guitar and amp.

Thanks for this info. I was not aware that this amp contained those capacitors. I am quite new to all of this. My knowledge of parts, schematics and such is quite limited, but increasing by the minute. This has become my new hobby during covid and I learn a lot from forums etc.

I have decided to leave the amp as it is, replace the tubes and keep it safe. It sounds great.

FrodeK · Post by **FrodeK** » 13 Nov 2020, 08:52

CheapPedalCollector wrote: ↑12 Nov 2020, 01:25 I concur.

If it happens you just don't like the amp, sell it to a collector and buy an amp you do like.

The amp is great. And I will keep it as it is.

FrodeK · Post by **FrodeK** » 13 Nov 2020, 09:38

ppluis0 wrote: ↑12 Nov 2020, 01:29 Hi FrodeK,

I completely agree with Manfred's suggestion about internal condition of this amp, so leave in their actual status is the best option.

After study the schematic you posted I found a (surely, drawing) error:

The point "C" at the power supply is to feed the preamp and tremolo tubes with siutable high voltage to work and NOT have to be connected in any way to pin 3 of each power tube !!!

Please check that with the amp unplugged after wait some time to allow completely discharge the filter caps.
That point at the supply (point "C") must appear isolated to pin 3 in the EL84's

Another drawing (or manufacture ??) error are the 6.3V line to feed all the heater tubes that appear floating respect to chassis ground.
Check if exist continuity from any heater pin to ground or if that winding in the power transformer is really floating.

In any way, you can modify the actual heater configuration with one of the suggestions from this site: http://www.valvewizard.co.uk/heater.html

(I'm are fan of the "heater elevation" implementation)

To make the amp run "hotter" my best advice to you is to build an external stompbox named VALVECASTER explained in this site: http://beavisaudio.com/projects/valvecaster/ that runs a tube at low voltage, use inexpensive condensers and low wattage resistors, and it's safe to have placed in your pedalboard.

Cheers,
Jose

Thank you so much for this detailed insight!

Most of it is above my skill-level I am afraid, but I am interested in learning. I will get to it all, eventually.

The amp works well, so I guess the wiring can't be done completely wrong? I will investigate the C point and see if it is in any way connected to pin 3 in the el84's. There is no continuity between the heater pins and the chassis.

--

The VALVECASTER looks great. I am tempted to try a build. Thanks for the link!

deltafred · Post by **deltafred** » 13 Nov 2020, 10:54

Good to hear that you are not going to modify the amp.

It would be a different matter if someone had already been in and hacked it about but such a pristine example deserves to be left stock.

Obviously change anything that needs it, out of spec resistors, electrolytic capacitors that are drying out, leaky film capacitors, and it is worth sourcing some spare valves (tubes) but other than that just enjoy using it.

For some light reading on amplifiers see the download in this post -
viewtopic.php?p=283110#p283110

ppluis0 · Post by **ppluis0** » 13 Nov 2020, 12:53

FrodeK wrote: ↑13 Nov 2020, 09:38 Most of it is above my skill-level I am afraid, but I am interested in learning. I will get to it all, eventually.
There is no continuity between the heater pins and the chassis.

If you haven't experience to work inside a tube amp is better to contact a qualified technician. There is dangerous voltages inside !!!

FrodeK wrote: ↑13 Nov 2020, 09:38 The VALVECASTER looks great. I am tempted to try a build. Thanks for the link!

It's a simple and not so much expensive project, and add more options to your sound.

Cheers,
Jose

CheapPedalCollector · Post by **CheapPedalCollector** » 13 Nov 2020, 21:16

I think you've made the correct decision.