Hi folks!
I have a problem: I own an old italian tube amp, brand FBT, a fender super reverb copy from 1965, 4 ECC83, 1 ECC82, 1 ECL82, 2 6L6 and a GZ34.
it's a very good sounding amp!
tubes were the original ones, all philips miniwatt, sylvania (branded GEC for the 6L6) and a brimar (mullard) GZ34.
Last year I changed the double filter cap (32+32uf 500Vdc) and I replaced it with an old but good LCR 50+50uf that worked well for a year, until I changed it with a J/J 32+32uf: turning on the amp my gz34 burned out, maybe for her 40 years, myabe for other reasons, I still don't know it.
I checked the power tranny and I found that there was something wrong with rectifier tube filaments (there weren't continuity between center tap and one of the two tap), so I preferred to replace it with a new one, built under original specs (or at least...I've paid for it!) and really oversized.
I tested the new tranny, voltages are ok, 6V, 5V and 350+350, all with center tap; I checked 6V filaments first of all, all tubes light on; then I checked 5V filament with a new JJ GZ34, it works.
the last step was to connect B+ secondary at the GZ34 (with the amplifier in standby), only to check voltage after rectifier: the tube arc'd!!
So I've tested with a GZ34 sovtek, it arc'd too!
damn, I read that this kind of new tubes could have this problem, ok, then I bought a GZ34 mullard NOS (small base, late '60) and a 5U4GB RCA (late '50), I tried with the 5U4 and the tube burned out!
so I tried with the mullard...the mullard too arc'd!!!!!
is not possible, voltages are right, no shorts, nothing...
so I tried to change the tube with a couple of 1N4007 and a 180ohm 20W resistor...the amplifier worked, the sound was very good but after 30min the power tranny became hot!
maybe I'm crazy: I played it for 2 times in 2 clubs, tranny became very hot but still working.
why?
where is the problem?
I repeat: I checked all the circuit, there is no shorts and the problem with the tube rectifier come only when 5V filaments and B+ is connected on it; without B+ tube light on regulary.
please help me!
Matteo
I have a problem: I own an old italian tube amp, brand FBT, a fender super reverb copy from 1965, 4 ECC83, 1 ECC82, 1 ECL82, 2 6L6 and a GZ34.
it's a very good sounding amp!
tubes were the original ones, all philips miniwatt, sylvania (branded GEC for the 6L6) and a brimar (mullard) GZ34.
Last year I changed the double filter cap (32+32uf 500Vdc) and I replaced it with an old but good LCR 50+50uf that worked well for a year, until I changed it with a J/J 32+32uf: turning on the amp my gz34 burned out, maybe for her 40 years, myabe for other reasons, I still don't know it.
I checked the power tranny and I found that there was something wrong with rectifier tube filaments (there weren't continuity between center tap and one of the two tap), so I preferred to replace it with a new one, built under original specs (or at least...I've paid for it!) and really oversized.
I tested the new tranny, voltages are ok, 6V, 5V and 350+350, all with center tap; I checked 6V filaments first of all, all tubes light on; then I checked 5V filament with a new JJ GZ34, it works.
the last step was to connect B+ secondary at the GZ34 (with the amplifier in standby), only to check voltage after rectifier: the tube arc'd!!
So I've tested with a GZ34 sovtek, it arc'd too!
damn, I read that this kind of new tubes could have this problem, ok, then I bought a GZ34 mullard NOS (small base, late '60) and a 5U4GB RCA (late '50), I tried with the 5U4 and the tube burned out!
so I tried with the mullard...the mullard too arc'd!!!!!
is not possible, voltages are right, no shorts, nothing...
so I tried to change the tube with a couple of 1N4007 and a 180ohm 20W resistor...the amplifier worked, the sound was very good but after 30min the power tranny became hot!
maybe I'm crazy: I played it for 2 times in 2 clubs, tranny became very hot but still working.
why?
where is the problem?
I repeat: I checked all the circuit, there is no shorts and the problem with the tube rectifier come only when 5V filaments and B+ is connected on it; without B+ tube light on regulary.
please help me!
Matteo
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