FI SONIC KT88 COMBO

[TIMBO]

Hi guys, Thought I'd share another vintage Aussie amp.
I have a few Fi Sonics that are normally a head and cab for the 30w plus amps.
So this one is as rare as, cause I've never seen one before this


I bought this as a chassis only and looks like it's been sitting on a shelf for sometime.


Some interesting wiring has been done??????
Gave it the onceover and the OT looks to be good and the calcs put it at 5k with a 15ohm load (also found a datasheet) for a pair of KT88.
There did not appear to have any resistors fried but the CT for the heaters has got rather hot and made the plastic insulation on the wire brittle and some has broken off.
It appears to have enough insulation left under the paper rap, this wire is much lighter gauge than the heater wires themselves.
My thoughts are to trim the CT off under the rap so it does not come in contact with the transformer casing and add an artificial CT.

[kagliostro]

Ciao TIMBO

Nice amp, interesting the 6BM8 reverb

Franco

[TIMBO]

Hi guys, I'm now at the stage of maybe righting some wrongs.

Having a close look at the KT88 output stage and comparing it to the Marshall major.

The Fi Sonic has neither a grid or a screen resistor.
I know these are added to reduce stress on the power valves, would adding similar values to the major make for a better design.

[shooter]

has neither a grid or a screen resistor.

I would put them on G1, from my understanding, more for induced noise from the outside world, but at the voltage used to drive the 88's, not sure that would be a problem.  On G2, you have a UL OT so that in itself helps "stabilize" some of the swing.  I think an R there would be more as a save the tranny "fuse" than "better" performance?

[DummyLoad]

add them: 100 ohm 2-3 watt for the screens; 1k-2k for for g1. 


the major used 250R at 5W for screens & 5.6k for g1. 



--pete

[TIMBO]

Thanks for the replies guys.

I have posted this restore http://www.guitargear.net.au/discussion/index.php?topic=48692.0  and there is some questions on some of the circuit wiring/components.

First up is the 10M leak resistors in the PI.

The 100k between the plates of V1
The 120k on V4
The extra 25u bypass cap on V4b

 

[sluckey]

First up is the 10M leak resistors in the PI.

That's OK. A LTP PI has a very high input impedance even if you use 1M resistors.

The 100k between the plates of V1

That provides the path for the 'dry' signal.

The 120k on V4

That looks like a schematic error???

The extra 25u bypass cap on V4b

Maybe 50µF is better than 25µF???

[TIMBO]

Thanks sluckey.
The 120k is a mystery, I have checked that many times and all wiring is original.
I'm hoping to get this one up and running over the weekend, hopefully it won't go BANG!

[TIMBO]

Hi guys, The old girl (date on Ecap 67) has fired up well.





Some chassis pics soon.

[sluckey]

The 120k is a mystery, I have checked that many times and all wiring is original.

Yes, it is. It would make sense if it connected from plate to plate. But as drawn V4B has a normal 100K plate load resistor connected to node C and the 120K is also a plate load resistor but connected to node D. That makes no sense! Are you sure it's connected as shown?

[TIMBO]

Hey sluckey, This is the resistor in question


As you can see the solder is dull much the same as other joints and it's a carbon comp (the only one)

I'll remove it and take some more voltages.

[trobbins]

Perhaps just to raise D and E up a titch (perhaps E was under 200V).

[TIMBO]

Hi guys, Some tiding up.






Some tweaking of the trem and reverb is still needed.

[TIMBO]

Hi guys, tweaked the reverb as there was way to much, so much that you could hear the tank sizzle.
I have done this mod to another of my Fi Sonic and the reverb is plentiful and gets that SURF at the top end of the dial.

[TIMBO]

Hi guys, Some more progress.


Rewired trem circuit as the original wasn't great. This circuit is in other Fi Sonic amps.

The reverb rewired with the control pre recovery stage, work much better, more usable reverb.

New LDR/NEON as the old ones were stuffed. The original setup had these mounted on a 4pin socket. Not having a 4pin plug,I found a replacement and a nice aluminium can to house the LDR/NEON(I'm assuming this is what they were thinking)




With most Fi Sonic's they had a removal lid, makes working on them great.

[kagliostro]

Nice idea for the LDR/Neon cabinet 


Franco

[DummyLoad]

cool stuff! 


--pete

[TIMBO]

It's been a while......



I have a choice of speakers

These are in most of the Fisonic cabinets along with 12ux50 speakers
There is also a 12UX50 I have seen as well
The info on the 12u50 I believe they are more a bass speaker and the 12UX50 is a PA/guitar


These Etone's have also been found in some cabs and are a guitar speaker.
I believe they are rated at 30w and the 12U50 are 50w
I'm sure the amp will push 60+ watts.

[tubeswell]

Those ETones were also made in NZ back in the day under license (from Celestion/Rola) in NZ’s old Plessey factory. I’ve got the complete 1960s Rola spec sheet saved somewhere - including the ‘12U’s

[TIMBO]

Hey mate, Any info would be great.