FBT Bass Organ .......... Conversion ? - (EL503 Power Tubes)

[kagliostro]

Yesterday, at a small Ham Fest, I got (really on the cheap) an old italian amp, an FBT Bass Organ (very few info on this amp)


This isn't my amp, but is the same model






The amp isn't in good shape and a big restoration work is needed

the question is about the power tubes (very difficult to be find and expensive if found, the EL503)



http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL503.pdf

I've (at least) 4 used EL503 but I'm wondering if it will be better to convert the amp to a less expensive and more easily available kind of tubes

John Chambers converted a Geloso PA amp from EL503 to EL36 and a friend of mine, following Chambers instructions, converted an FBT 1200R the same way

http://www.chambonino.com/modify/mod1.html

http://www.chambonino.com/modify/giumod1.html

Thinking to convert the power section (as to have a more easily manageable amp), the question is

The EL503 has a very high sensitivity, 23mA/V when tubes like EL34 has "only" 11mA/V, and other tubes, like 6L6 are around a low 6mA/V

so ... do you think it will be better to modify in some way the PI arrangement ?

--

OK, I forgot, is to be considered also that I haven't yet measured the OT impedance (I don't know if they used the 2.4K aa indicated on the tube datasheet because the datasheet indicate a B+ of around 265V and the FBT schematic indicate a 365V B+, 100V more) and this will be an important thing to consider on the choice of a new kind of power tubes (as soon as I can I'll measure te turn ratio)

Franco

[PRR]

> I'll measure te turn ratio

Do that. It is important. Also the B+; 365V is past the EL503's rating, but we also see "315V" 6V6 with 400V and 440V in guitar amps, so it may be correct.

I'd consider EL34 because the FBT may have been designed around '34 until tube-TV sales fell and the '503 became available very cheap. Or 6L6GC because its heater power is more-like EL503.

You may need more bias; or maybe not. B+ and OT Z will be enlightening.

The preamp sure looks more like guitar than traditional organ. Some extra heavy treble-filtering which could be reduced.

[kagliostro]

Ciao PRR

I measured the OT, firstly I was thinking to a burnt OT because from red wire and CT I measured 191.8 ohm of resistance and between the green wire and CT I measured only 100 ohm

but when I applied a voltage to the secondary (14.23v) the voltage on the primary were very close each other (112.3v and 112.1v)

so the turn ratio is (224.4 / 14.23) 15.769501 that squared is 248.67716

the reflected impedances that results with 4 - 8 - 16 ohm speakers are

4  ohm = 1K (rounded)

8  ohm = 2K (rounded)

16 ohm = 4K (rounded)


--


BTW ... the EL503 were audio or TV tubes ? I was thinking they were audio tubes .......

Franco



p.s.: With 365v B+ and -20.5v Bias, which can be the approximated output power ?

[kagliostro]

Before the measures and now some pictures













Franco

[DummyLoad]

Che è bello guardare!


cool score franco! 


--pete

[kagliostro]

[DummyLoad]

40W EL-34


--pete

[kagliostro]

Yes Sir, that is the opportunity I'm considering (with a bit more of load, around 4K)

Franco

[terminalgs]

A strike against the EL34 is the 1.6A 6.3V draw.  the EL503 is 1.05A. It seems unlikely that the engineer would spec the PT to cut it too close to the total required for the circuit with EL503's, but if the PT's 6.3v capacity is unknown, it is unknown.   and extra 1A is a lot to guess on.   


Those are massive transformers tho..,
 

[kagliostro]

If the conversion will be done, a toroidal transformer will be added for the Power Tube requirements

Franco

[DummyLoad]

If the conversion will be done, a toroidal transformer will be added for the Power Tube requirements

Franco

bring it up on a variac to the rated line voltage. with all preamp tubes and el34s installed, measure fil. voltage - if it holds at 6,3v then you're go to go.


--pete

[kagliostro]

Very good council 

Thanks Pete

Franco

[drgonzonm]

I would not be concerned about your voltage to the plates.  While the spec sheet shows a typical application, look at the limiting values, Vao =550v, Va =300v
and that is design center ratings.  Its too bad, none of the sheets found on Franks Tubes,  show absolute limits.  Compare design center numbers to absolute values, specs on other tubes. 

As you aware, the hotter you run the plates, the shorter life

The following link has a discussion on three different rating systems,  http://www.tubecad.com/december2000/page17.html  Considering that the el503, was designed for TV service, a conservative rating is called for, after all, no one wants their TV to breakdown during the Superbowl or Association Football WorldCup,

Drgonzo

{EDIT - removed extra dot in URL --PRR}

[kagliostro]

Ciao Drgonzonm

Thanks for the link (but it didn't work for me )

However I don't matter if the amp runs the EL503 overspec


(to me it is an Audio Frequency Tube, not TV Tube, see datasheet http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL503.pdf)

My concern is only about the availability and the excessive price of those tubes

http://www.ebay.it/itm/Valvole-EL503-U33-5C2-/222606569821?hash=item33d462b15d:g:kuAAAOSwLnBX8sec

so the wondering about the conversion to a cheaper and more easily available tube

Franco

[DummyLoad]

29W anode dissipation limit.


--pete

[PRR]

> Thanks for the link (but it didn't work for me

Fixed. (extra dot.)

[PRR]

> to me it is an Audio Frequency Tube, not TV Tube

It was probably a TV sweep tube, with huge cathode, re-designed as an audio tube with less cathode and bigger plate.

There is nothing quite like it. _I_ would use 6L6/KT66 with 2X load impedance to get ~~40W. Sure beats that price to EL503!

(Or use EL34, but the large heater demand may be a problem.)

[kagliostro]

I agree with you PRR

My choice of an EL34 instead of a 6L6 (with the heater problem to be solved) is due to the 11mA/V sensibility respect the 6mA/V of the 6L6, the original tube (EL503) has a very high 25mA/V

However I think that something in the PI is to be adapted to the lower gain tube also if EL34 has a higher gain respect to the 6L6 tube

Franco


p.s.: Thanks, the link now works correctly

[DummyLoad]

so, have you measured the turns ratio of the output transformer?


--pete


EDIT - I should have read reply #2 - doh!   

[kagliostro]

Yes Sir

I measured It

Franco

[DummyLoad]

having second thoughts... consider the 5881s - russian 6p3s-e is still common in your part of the world and still not out of reach price wise - the pluses are all, 1) no issues with filament winding rating. 2) you run 16ohm load (2 x 8ohm in series) for ~4kRa-a load. 3)  with 360~ to the plates and g2 your bias will need to be around -29V to -32V give or take. 4) should make around 40 to 45 watts. 

mod the concertina and it'll drive a pair 5881 just fine... have a look at the fender super 5F4 - it uses a concertina to drive 6L6G with -40V bias. the 5F4 concertina has 330V B+. copy it. your "C" node should be about the same. leave the 470K bootstrap R, change the 33k plate and cathode Rs to 56k and add 1.5k auto-bias R. replace the electros, retube it and you're done.

--pete

[DummyLoad]

looking at the schematic attached: i see that power node "D" isn't connected anywhere. could be it's a just another RC filter stage just to get the E & F rails where the designer wanted them, or it could be an error on the schematic? franco, have you done any reverse engineering on the amp yet?

please review attached for clarification.

regards,

--pete

[kagliostro]

 

I'll give a look to that Pete

Franco

[kagliostro]

I examined the circuit, not very accurately, but till the node D I examined the PS circuit

(I didn't had much time to dedicate to this thing, may be tomorrow I'll do a better job)

What I've seen is that the e-cap part of the PS is in very bad shape and some capacitors are missing

The amp has 2 chassis, one for PS and Power Section on the bottom and one for the preamp on the top

the two chassis are joined via a 6 pin connector (similar to a tube socket) but only 5 wires are present

the node D is connected to the 6 pin female connector on the bottom chassis



but on the male connector to the pin that correspond with the D node, nothin was ever connected (no wire and no residual tin on the pin)

May be a node for a different version of the preamp ?

Franco

[PRR]

> 11mA/V sensibility respect the 6mA/V

I would not worry about the numbers.

There's two stages of gain, and a tonestack. Then another stage of gain before the power stage. This alone will normally be plenty for guitar.

And there is a gain-of-2 mixer which you could modify for more gain.

I'm sure the driver has enough max output for any audio power tube. The cathodyne is fed very nearly the same B+ as the screens. The stage which drives it is biased a little toward low plate voltage, change 2.2K to 3.3K or more and it will be more centered.

[DummyLoad]

BTW, in the opening post, the pic you posted of the FBT has RCA black plate 6L6GC. best sounding 6L6GC ever made.


--pete

[kagliostro]

@ PRR

I'm sure what you say is as you say, I wasn't thinking strictly to the drive signal level, in my naivety, something told me that to maintain

a bit of the original character (tone if you prefer) of the power section the new tube will be better to have the higher Siemens as possible

but this is only a sensation without any other element to be considered

@ Pete

you have the eye on it , I did not realize that the head of the photo I posted was already converted

BTW

with that B+ (365v) isn' too low a load of 4K for the 6L6GC ? To me it must be around 6.6K and this is too far from the original 2K / 8ohm set of the OT

Franco

[drgonzonm]

....
BTW

with that B+ (365v) isn' too low a load of 4K for the 6L6GC ? To me it must be around 6.6K and this is too far from the original 2K / 8ohm set of the OT

Franco

Shift the reflected impedance by changing nominal load of the speakers.  I don't remember which way you want to shift, whether you use 16 ohm load or a 4 ohm load.  My head hurts thinking about it

[kagliostro]

EL503 have a load of 2K in that amp with an 8 ohm speaker

To use EL34 I must connect a 16 ohm speaker for 4K reflected imperance

To reach 6.6 K is not easy and implies the use of a "not standard" speaker

Franco

[DummyLoad]

EL503 have a load of 2K in that amp with an 8 ohm speaker

To use EL34 I must connect a 16 ohm speaker for 4K reflected imperance

To reach 6.6 K is not easy and implies the use of a "not standard" speaker

Franco

yes, that's a given, you need 4K for either 6L6/5881 -or- EL34.

6L6 will deliver about 18W into a 4k load; the EL34 will deliver about 30W into 4k load. el34 bias ckt should not need to be modified, for 6L6 it may need to be modified.

try both, see how you like it. 

i don't think the extra load on the PT from the EL34 filaments is going to be an issue. if it is, then use 5881/6L6GB and live with 18-20W, or like you say, add an auxiliary filament trans for the EL34s.

--pete