Hi folks ,
i've got an old FBT 500 amplifier on bench , power stage is made from 2 X EL503 in push pull
After some work to replace old sockets (main and output) and some caps , i powered it on and it works , but power output is low, i read 22W@8ohm and 35W@16ohm , also cross-over distortion is present from 15W , bias adjust cross over distortion only a little.
I think power tubes are gone , the thing is 40 years old ...
Since EL503 tubes are expensive , what i can use instead of ?
B+ is around 380V (schematic states 360V)
Would EL34 work here?
EL36 coul be a good substitute but i don't like the plate cap , is a bit dangerous if putted in wrong hands 🙁
i've got an old FBT 500 amplifier on bench , power stage is made from 2 X EL503 in push pull
After some work to replace old sockets (main and output) and some caps , i powered it on and it works , but power output is low, i read 22W@8ohm and 35W@16ohm , also cross-over distortion is present from 15W , bias adjust cross over distortion only a little.
I think power tubes are gone , the thing is 40 years old ...
Since EL503 tubes are expensive , what i can use instead of ?
B+ is around 380V (schematic states 360V)
Would EL34 work here?
EL36 coul be a good substitute but i don't like the plate cap , is a bit dangerous if putted in wrong hands 🙁
Changing the base is not that difficult and give me more options , also i don't know many power tubes with magnoval socket ... and most have the plate on top cap .
Maybe EL509 JJ ? If only i can find them here in italy cheap and easy 🙁
For 6L6 , i can be wrong but , plate load for 2 tubes in pushpull is about 2K5 for EL503 , 3K5 for EL34 5K6 for 6L6GC ...right?
6L6s seem too far from matching with actual Output Transformer...but i can test them for sure.
Anyway i prefer them too , they are a bit shorter and fit better in the case.
Maybe EL509 JJ ? If only i can find them here in italy cheap and easy 🙁
For 6L6 , i can be wrong but , plate load for 2 tubes in pushpull is about 2K5 for EL503 , 3K5 for EL34 5K6 for 6L6GC ...right?
6L6s seem too far from matching with actual Output Transformer...but i can test them for sure.
Anyway i prefer them too , they are a bit shorter and fit better in the case.
The Russian 6P3S-E is a better choice than the 6L6 and should be available to you, even off Flee-Bay. Sound very good for the price. What impedance are the output transformers?
I hate do dissapoint you but EL503 is rather unique. The problem is not the socket, that can always be adapted (even if it takes a lot of work) but it works into relatively low Raa and requires comparatively little voltage to drive it (about as much as an EL84).
To be honest I can't think of anything that could replace it directly although one may find something similar amongst sweep tubes. Another possibility, but dificult to adapt because of size and even more problematic socket, would be an RS1003. Each one of these can roughly substitute two EL34s and will last a LONG time (specified for 10000hr duty). But they are rare and expensive, and the socket is not easy to find.
To be honest I can't think of anything that could replace it directly although one may find something similar amongst sweep tubes. Another possibility, but dificult to adapt because of size and even more problematic socket, would be an RS1003. Each one of these can roughly substitute two EL34s and will last a LONG time (specified for 10000hr duty). But they are rare and expensive, and the socket is not easy to find.
We have the 6GB5 sweep tube, also same from Holland, Magnovar 9 pin socket, but top cap is needed. There is the 6JT6 sweep tube without the top cap, novar 9 pin, the skinny pins. I do not know if they have as low plate impedance for the same transformer drive ability.
I know , but owner can't afford new EL503s
Thanks , a nice tube!
You posted exactly the amp i have on the bench ... do you have the schematic? I've one for FBT 500R ...
Anyway , what if I use EL504 ? I've a lot of them from old TVs ?
And what about using EL36 ? I used them with regulated screens times ago ..
Thank you guys 🙂
I think i can go with tubes like EL504 with plate cap , i'll just put a grill on the back to protect from shock!
Is this correct schematic ?
It is easy to check if output tubes are faulty or not.
Just adjust the bias and measure the anode current of both tubes.
Then look if they are equal and match the tube data.
You should reach min. 100 mA anode current when the bias is adjusted to low end.
(but the maximum you can use depends on the used anode votage...)
In the attached schematic the supply and G2-voltages are exceptionally high, 365 V and 350 V. The recommended maximum by the manufacturer is 300 V.
Most of american sweep tubes are not "happy" with screen voltage higher than 200-220V, but europian tubes like EL500, PL500, EL504, PL504, 6P36S and 6P44S works good (and long!) with 350-360V.
Here is the pictures of the same amp with 6P44S, and schematisc of whole amp with new bias circuit.
Here is the pictures of the same amp with 6P44S, and schematisc of whole amp with new bias circuit.
Attachments
Last edited:
Time to pull sweep tubes out of the box and use alligator clips !
Don't have the correct schematic , the model is exactly FBT500 (no R suffix , nor bass-organ) , but is quite similar from what i see , to what artosalo posted , the only difference is mine has tremolo.
Don't have the correct schematic , the model is exactly FBT500 (no R suffix , nor bass-organ) , but is quite similar from what i see , to what artosalo posted , the only difference is mine has tremolo.
After looking the spec's of different types I came to a conclusion that soviet 6P13S is more closer equivalent than EL36.
http://www.mif.pg.gda.pl/homepages/frank/sheets/113/6/6P13S.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/113/6/6P13S.pdf
Done some test with EL34 and 6L6GC (owner is a guitar player ) , power is about 20W with 200mA of current drawn at full . I tried a 16 ohm load on the 8 ohm tap , power goes to 30W with 170mA at full , so work better but still not good ...
I was expecting more difference between EL34 and 6L6GC due to the "low" load thinking El34 would match more easily , reality shows quite same results in tems of power.
Now , not having 6P13S , i will try EL36, EL504 and EL500 .
i'm a bit worried with EL36 beacause of its 12W Pa ..
Also have some 6DQ6B tubes to try.
I was expecting more difference between EL34 and 6L6GC due to the "low" load thinking El34 would match more easily , reality shows quite same results in tems of power.
Now , not having 6P13S , i will try EL36, EL504 and EL500 .
i'm a bit worried with EL36 beacause of its 12W Pa ..
Also have some 6DQ6B tubes to try.
6p13s are cheaply available, 4 pcs. at 14 euros including shipment.
EL36 6GC6 Beam Tetrode 6P13S 4 New | eBay
Unlike advertized, it is essentially different than EL36. Pa + Pg2 = 16 W max.
EL36 6GC6 Beam Tetrode 6P13S 4 New | eBay
Unlike advertized, it is essentially different than EL36. Pa + Pg2 = 16 W max.
Last edited:
Last evening i tried a pair of Philips miniwatt EL36 , with 150V on screens ,biased at 35mA each , i get 50W @ 8 and 16 ohm , load seems correct! Current reach 220mA at center tap input ...
I never measured current qith original EL503 tubes but i'll check , anyway i think 220mA at 330V is ok for 50W output.
Now i want invesitigate in power supply , at idle i get 370-380Vdc , at full drops to 320-330V.
I never measured current qith original EL503 tubes but i'll check , anyway i think 220mA at 330V is ok for 50W output.
Now i want invesitigate in power supply , at idle i get 370-380Vdc , at full drops to 320-330V.
hello ,
it is possible use EL502 , me looking datasheet to EL 503 and 502 , the power anode is différencial to 7 W : EL 503 = 27 PA ; EL 502 = 20 PA !
possible solution 😉
it is possible use EL502 , me looking datasheet to EL 503 and 502 , the power anode is différencial to 7 W : EL 503 = 27 PA ; EL 502 = 20 PA !
possible solution 😉
- Status
- Not open for further replies.