Problem with tube rectifier!

[matteodic]

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

[Albert Kreuzer]

Hi Matteo,

it didn't arc before you put in the new JJ 32+32µF can, did it?
And afterwards it arced with every rectifier tube you tried?
I strongly suspect the electrolytic. It may not show a short with your meter, but it probably pulls too much current under HV conditions.

I assume the power tubes are good and correctly biased.

HTH
Albert

[matteodic]

Hi Albert, thanks for the reply.
no, cap is good, the problem is when amp is in standby, this means that only work filaments (6V and 5V) and HV tap on GZ34, nothing else.
standby switch is right after gz34 without any cap between them.

[matteodic]

Hi guys, surfing on the web I'm finding a lot of infos about rectifiers, and I've some question about filament windings : I'm grounding both center taps, is it right for 5V filament too?
I've seen a lot of schematics of amps equipped with a GZ34 that don't have a center tap.
I don't have a schematic for my amplifier, I know really little about it, but I found the schematic of this amp without reverb, it's very similar to mine, it has HV tap connected to pin 2 and 6 of rectifier, and 5V heater (center tapped and grounded) connected to pin 4 and 8, then pin 8 goes to B+.
I never seen a scheme like this, is it correct for a GZ34?

[matteodic]

This is the power supply schematic, amp is an "FBT 300" with a GZ34:


This is the entire scheme:

[Rob Mercure]

Matte,

The second schematic you provided is in error! Grave error! You cannot ground the center tap of the power transformer high voltage secondary and also ground the center tap of the rectifier 5V secondary and not expect a short circuit once the rectifier tube warms up. If this is the way the circuit is actually wired I can't beleive it worked for over 20 seconds. The power transformer rectifer 5 V secondary centertap should be connected to the first 32 uf filter cap - it should be the only part of the rectifier circuit connected there!

Funky circuit - has a true "phase inverter" with a single stage dedicated to 180 degrees phase flip.

Are you in Italy? I notice that the power transformer has multiple primaries for different line voltages - any possibility that the line input has been wired incorrectly (problem, or in addition to other problems?) - as voltages have risen over the years it may need to be tapped down a bit.

[Enzo]

Yep, there is you problem, that 5v center tap must NOT be grounded. If you ground it, then teh rectified B+ voltage is shorted to ground.

[matteodic]

yes I'm italian, during '50-'60 there were a lot of companies that built very good musical instrument, with the new transformer I choose to have only one primary for european voltage, the older one had primaries for all the countries.
the only thing I don't understand is why tube in that schematic si connected in that way... when I purchase this amp, it was untouched and the rectifier was connected correctly. in the older power tranny there were 3 center tap grounded, I didn't know that 5V center tap must not be grounded so I thought that the third one was the 5V ct.
when I checked continuity between 5V filaments and this common tap, tester didn't read nothing so I was sure tranny was damaged! and now I have a brand new transformer (more or less 100$...)
ok guys, thanks a lot for you preciosu help, I will let you know if this amp will work or if another rectifier will die!

[matteodic]

here's a photo of my amplifier:


this is the faceplate with controls:


and this is the back with circuit board:


I recapped the reverb circuit, it has an old big GIBBS spring tank behind the board, but not well working.
I use a couple of big 6L6G fivre from 1958 that sound amazing!

[matteodic]

Hi folks! problem fixed!
well, I didn't know that 5V center tap must not go to ground, I desoldered it and my amp is now playing with a nos mullard as before!
the only "problem" (?) I have now is this: power tranny becames hot. not hotter than before (I had 5V tap grounded as I said before), but...HOT!
amp still sounds good, but I don't know if is a real problem or a common thing for this kind of application.
I own an old fender bassman head (aa864 circuit from 1964) that is taking dust in my garage (fbt sounds better!), last night I've played with it and after 2hs I realized that in this amp too power tranny becames very hot!
is it normal?

[Robert M. Martinelli]

PT Power rating ?

Hi Matteo,
you stated the PT is not original, I know it' s a dumb question but have you checked the total power rating and the power ratings of each secondary of your PT?

The 5V secondary for the rectifier tube' s heater should be able to deliver at least 1,5 amps without sagging, the HT section should be able to go up to 250 mA, maybe a little more, without any noticeable drop ( of course you' ll have some voltage dropping across the rectifier, especially when playing loud ), and the 6,3 V secondary for the filaments should handle at least 3 Amps....
Try to check voltages when the tranny becomes very hot, I expect them to be somewhat lower becouse the wire resistivity increases with temperature and the tranny' efficiency decreases, but if the voltage drop is 5% or worse I suspect your tranny could be undersized....An amp like yours needs at least 150 W ( 165VA considering a 90% efficiency ) to deliver 50 W RMS on a 8 Ohm load, check the "iron pack" dimensions to see if they "fit the bill".

to evaluate the PT temperature, keep in mind that if you can keep your hands on it you' re under 50 degC, if you can keep them on it for a sec or so you' re around 60 degC ( still an acceptable value ) but if you' re higher than that something is probably wrong.

Hope this helps

Best regards

Bob

[thetechguy]

Problem with ht rectifier

Thetechguy, (Paul) Just a comment,that you may have caused damage to the power transformer,when the rectifier centre tap was wired to chassis ground in error. Due to past experience of working on high voltage power supplies,was the correct value h.t. capacitor of correct voltage working,and also probably overlooked,the ripple current rating for the "reservoir"section--usually identified on the can of the capacitor-by a coloured spot under connecting tag. Finally,was the capacitor new/unused?--or a "known-good" component, if it has not had h.t. applied for a time,this should have been gradually brought up to full working h.t.voltage,over a length of time by a resistor and series milliammeter,with preferably a variable high voltage supply-(D.C. ONLY--NEVER USE A.C.)--Capacitor breakdown is most likely,in a short space of time,if it was lain around unused without being connected to h.t.voltage for some time (more than a year or two--if a "new" component.)