FBT 500R help

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catosalsa

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Hi! Found a vintage tube guitar amp top at a flea market, and bought it. Didn’t work as it should, low signal and loud ground hum.

Started looking at it and found a few caps etc  I need to replace. Was also a bit confused how the ouput jack and transformer was wired. See picture.

From the tranformer theres brown, white and black cables. Black was not in use. Brown was soldered together with green from the circuit to pin 3 on the xlr. White was soldered to pin 1 and pin 2 together with the black cable from the circuit. Is this Really the right way to connect the output jack? Do i need to find a schematic to fully know what the transformer cables are or can i find out in another way? Thanks!
 

mjrippe

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What make and model is the amp?  The only vintage guitar amps I know that used XLR output jacks are the Ampeg fliptops (B-12, B-15).
 

catosalsa

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mjrippe said:
What make and model is the amp?  The only vintage guitar amps I know that used XLR output jacks are the Ampeg fliptops (B-12, B-15).

I dont thinks its xlr in original, another, probably some vintage style jack has been replaced. What made me suspicious was that it looked like pin 3 was used as instead of pin 2.
 

catosalsa

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mjrippe said:
What make and model is the amp?  The only vintage guitar amps I know that used XLR output jacks are the Ampeg fliptops (B-12, B-15).

Model is an italian made FBT 500 R2
 

catosalsa

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Cleaned a few scratchy pots, wired the output jack and tried the amp again. Tremolo is working, reverb aswell. The sound on both channels is not very good though. Sound like a weird compression/gating going on, distorted and quite a loud constant hum. The caps are old, maybe replaced sometime in the past as the values on all of them is different compared to the schematic. I guess caps are the problem here. I have attached the schematic here and have a question on the caps maked with blue. They are multisection caps. Ive never ordered multi section caps before and cant find any in the three web shops I normally order from. Do they go under any other name where I can find them?

Cap marked red is leeking, so will replace this one at least.
 

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Khron

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Just disconnect them in case you want to keep the look of the cans. But either way, replace them in the circuit with "singles", preferably of reputable brands (Panasonic, Elna, Nichicon, United Chemi-Con, Rubycon), sourced from a reputable supplier (not eBay or China).

Pretty sure no one makes "multiple" electrolytics anymore nowadays.
 

peterc

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CE Manufacturing and a couple of others still make multi section caps, but they are not cheap.....

https://www.tubesandmore.com/products/capacitors?filters=Brand%3DCE%20Manufacturing
 

catosalsa

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Khron said:
Just disconnect them in case you want to keep the look of the cans. But either way, replace them in the circuit with "singles", preferably of reputable brands (Panasonic, Elna, Nichicon, United Chemi-Con, Rubycon), sourced from a reputable supplier (not eBay or China).

Pretty sure no one makes "multiple" electrolytics anymore nowadays.

thanks! Should I go for the values stated in the schematic or the the same asthe caps in the amp?
Shematic is dated 1972, the caps are dated 1973 so pretty sure theyre original.
One can is 170+47uf (450V)
The other can is 100+47 (450V)
Schematic says:
200uf 500V
40uf 500V
40uf 500V
100uf 350V
 

abbey road d enfer

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catosalsa said:
thanks! Should I go for the values stated in the schematic or the the same asthe caps in the amp?
Shematic is dated 1972, the caps are dated 1973 so pretty sure theyre original.
One can is 170+47uf (450V)
The other can is 100+47 (450V)
Schematic says:
200uf 500V
40uf 500V
40uf 500V
100uf 350V
You should not worry too much about absolue values. Lytic caps of the era were often +/-20% tolerance or more, and value drifted significantly over the years. With modern caps, you can safely use a little less than nominal value. For example, using 150uF for 170 would not be a big issue.
On the contrary, increasing the value of the 1st smoothing cap can increase stress on the transformer and rectifiers.

 

catosalsa

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question on a capacitor value.
there are two capacitors in parallel wired to the to el503s. marked "arc 01.52 2200/10 1000V". In the schematic it says 4.7k. 4.7k means 4.7nF, right? Are these parallell caps 4.4nF? Does 2200/10 mean 2.2 nF?
 

catosalsa

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Follow up..

I got a tip that the 4,7nf caps at the EL503 often get bad in these amps. That was the case also with this one! Now working!

A master pot would be great to add. Anyonr have an idea how and where to put it in the circuit?

 

abbey road d enfer

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catosalsa said:
question on a capacitor value.
there are two capacitors in parallel wired to the to el503s. marked "arc 01.52 2200/10 1000V". In the schematic it says 4.7k. 4.7k means 4.7nF, right? Are these parallell caps 4.4nF? Does 2200/10 mean 2.2 nF?
Looks like. 4.4k instead of 4.7 won't probably make much difference.
 

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