5U4GB

[huggygood]

I would like to have an opinion on the 5u4gb.
I had bought two recent production (EH) and the two died the same way in two different amp (heating cut), more, I think they give a flat sound and without relief.
I would like to have an opinion on the current production (JJ among others) and on the old production (without that I need to sell an organ to buy it)
thank you

 

[huggygood]

and also an opinion on 5μ3C sevtlana 8 filaments

 

[Eli Duttman]

Going to keep on eye on the EH variant, as they previously have been decent.

On the NOS front, look for GE. GE 5U4GBs are good and cost less than TungSol or RCA.

 

[kodabmx]

I prefer the winged C from Soviet production, however I prefer UF5408 to kenotrons...

 

[huggygood]

I came across a nest of 5μ3c, I took 4 as a sample to try and see if the seller is serious.

 

[huggygood]

thanks Eli but i don't like a EH production

 

[6A3sUMMER]

huggygood,

If you have 2 rectifiers die, is it possible that the amp is exceeding the rectifier's specifications?

How close is the filament voltage to 5V?

etc.?

 

[TonyTecson]

^ my thoughts too...it would be helpful if you can investigate why it failed...
tubes don't just die, there has got to be a reason..

 

[huggygood]

2 different amplifier ...
but my mains voltage is a bit high here.
after, in both amp (different so) it's the same failure, heating that fails while my 5 v is ok

 

[6A3sUMMER]

If your filaments are at 5V that is OK.

Other factors might be the problem:
A capacitor input filter that has too many uF.
Too large of a current draw.
Too little resistance in the B+ secondary DCR, at the same time as no resistor between the rectifier and the first cap.
Too high of a voltage from the B+ secondary.
Capacitor with too low of a voltage rating.

How about posting a schematic for us to look at, power supply and output stage (to estimate the current draw).

 

[huggygood]

I think you're right about the 1st capacitor.
on one of the two, the original capacitor makes 80uF and when I recapitulated it,
I only had a 100uF layout and I think it's too big for these modern rectifier.
I knew I had to change to a smaller value, and put a bigger one in b1 b2 b3

 

[Lingwendil]

Definitely too big of caps!

 

[TonyTecson]

i used 10 ufd/630vdc film caps in the first cap after the rectifier, then choke, and a big cap after the choke....

 

[Eli Duttman]

The 5U4 can comfortably handle 33 μF. in the 1st position.

 

[huggygood]

ok,I order

 

[huggygood]

psu schematic

 

[TonyTecson]

^so that psu has a soft start circuit by way of the 3k 10 watt series resistor with the 5U4 cathode to the 80ufd cap and is in parallel with a relay contact that closes after a delay, this way the caps gets charged albeit slowly......

i also used this trick in my 6c33c single ended amp build...

 

[Zoran]

I think you're right about the 1st capacitor.
on one of the two, the original capacitor makes 80uF and when I recapitulated it,
I only had a 100uF layout and I think it's too big for these modern rectifier.
I knew I had to change to a smaller value, and put a bigger one in b1 b2 b3

47uF is some max value for the 1st cap to 5U4GB. And also depend on R coming to anodes... Try to calculate R value measuring Your HV transformer. Example is on 6X4 rectifier datasheet, if I am remember well...
Cheers
.
BTW I am running 5U4GB by FIVRE in Quad II monoblocks for years. Rear and really good that fivre rects.

 

[TonyTecson]

100 ohm per plate from the datasheets...so if your traffo has less than 100 ohms per leg, then the balance can be filled up using an external resistor.....but this practice i seldom see...

 

[6A3sUMMER]

Yes, the switch and 3k resistor will help.
But then why did the two rectifiers die?
How about the secondary voltage, and the current draw?

5U4GB:
40uF,
Plate to Plate; Effective R; Load
600V; 21 Ohms; 300 mA
900V; 67 Ohms; 275 mA
1100V; 97 Ohms; 162 mA

Current is probably OK with the four 7868 tubes as a load, they are similar to 7591, just a different package.