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Rectifier tube 5AR4 vs. 5U4GB?

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scott17

Member
Joined 2009
Are you saying that the CL150 is placed in series with the CT of the 5VAC and the first PS filter positive connection? I am using the 5U4GB and taking the HT from the 5VAC CT. I have a 30uF-630V 1st cap, 10H choke, and four 100uF-450V caps in a series-parallel configuration with balancing resistors to achieve a 100uF can with increased working voltage. I know that my filters can handle the initial high voltage surge, but I'm using a standby switch anyway by switching both sides of the HT AC to the rectifier, just for extra safety you know.

I guess my question is: Will the use of the CL150 further eliminate the need for the standby switch?


Eli Duttman said:
A CL150 between the rectifier's filament and the PSU filter is (IMO) better. B+ rise is slowed down a tad and a bias supply (if present) still turns on immediately. If the 5 VAC winding has a CT, that's THE place to take the B+ from. B+ from pin 8, when types with cathode sleeves are employed. B+ from a CT, if available, when directly heated types are employed.

A word to the wise, a 5U4 can't tolerate as large a 1st filter cap. as a 5AR4 can. Be cautious.
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Nikon1975

Member
Joined 2009
I am always a bit surprise when I see people commenting on the sound of different rectifiers, as I think that what they are actually listening to is a different operating point of the output valve, unless the voltage is not adjusted with a variac. Am I correct ?

I use a 5R4GYA or 5AR4 with the tubelabSE and a 5AR4 for the SimpleSE, as they are sharing the same PSU. there is around 35 V of difference and the 5AR4 comes up much slower.

D.
 
kevinkr

kevinkr

Administrator
Joined 2004
kta-hifi.net
Paid Member
Nikon1975 said:
I am always a bit surprise when I see people commenting on the sound of different rectifiers, as I think that what they are actually listening to is a different operating point of the output valve, unless the voltage is not adjusted with a variac. Am I correct ?

I use a 5R4GYA or 5AR4 with the tubelabSE and a 5AR4 for the SimpleSE, as they are sharing the same PSU. there is around 35 V of difference and the 5AR4 comes up much slower.

D.
Click to expand...

It may be a combination of different operating point due to higher/lower B+, but it may also be the result of a significant change in the internal resistance of the rectifier. IMHO the audibility of different tube rectifiers in an amplifier is a good indication that the supply is actually a significant part of the audio path which is not necessarily the sign of a good design, but not always entirely avoidable either.
 
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Nikon1975

Member
Joined 2009
Let's see if I understood something:
I took the specs of the 5AR4 and of the 5U4GB (just to make an exercise)
I see that the 5AR4 has a plate impedence of 160-200 ohm per plate. The 5U4GB has an impedence between 21 and 67 ohm. So let's assume 180 ohm for the 5AR4 and 50 ohm for the 5U4GB. I guess I have to multiply by two, so I get 360 ohm for the 5AR4 and 100 for the 5U4GB.

So, if I am thinking right, I would expect the reduced impedence of the 5U4GB to allow for a faster recovery of the reservoir capacitor after it is partially discharged for a transient.

I have to say that the 5U4GB has a though requirement on the minimum resistance of the transformer, so this eats up most of the benefit.

If I just replace the rectifier, and keep all the rest the same, I am changing the output voltage and the impedence of my power supply, and this should be what gives the difference in the sound.

Is it correct ?

I am asking as I want to make some test on the power supply, that in my case can power three different amps I have, starting for the simplest configuration to end up with a regulated PSU, and I would like to have control and understanding on the parameters involved.

Thanks,

Davide
 
hartze11

hartze11

Member
Joined 2008
kmaier said:
You're correct on both counts... and yes, the B+ will come up before the input and output tubes heat up. This will give you a surge on the filter caps (no load) so be sure the caps can handle the surge. I prefer to use either a 5V4G or 5AR4/GZ34 as they typically have a good 10 second delay before B+ builds up and it builds slowly. Your current draw is high for a 5V4G so I'd stick with the 5AR4.
Click to expand...

Sorry to resurrect this thread, but I had my last 5AR4 do weird things (too much hum), so I'm thinking about using the solid-state rectifiers that I built in...

Won't the solid-state rectifiers also put a lot of B+ out before the tubes warm-up?

Any recommendations? Thanks! Dave
 
Rod Coleman

Rod Coleman

Member
Joined 2004
Yes, the SS rectifiers will apply B+ instantly, and this may give problems.

The modern-production GZ34/5AR4 seems to be less than perfectly reliable, too - so many of us here use "damper" (efficiency) diodes - either 2 tubes in a full-wave rectifier, or paired with (eg) UF4007 SS rectifiers in a hybrid bridge.

The 6CJ3 and PY500A are examples of good rectifiers in this position. They not only delay the application of B+, but also cause it to appear very slowly across 30 seconds - meaning you don't get unpleasant surge currents, as well as protecting the signal-valves while their heaters/filaments warm up.

There's lots of different dampers to choose from - but please use Duncan's PSUD2 to find out the repetitive peak current in your power supply, to make sure the damper can handle it. But suffice to say, the diodes named above can cope with 500Vdc supplies with 50uF directly after the rectifier, and still be working years later. The reliability of dampers is much better than 5AR4s!
 
Z

zman01

Member
Joined 2011
Sunil,

Any particular reason you want to use the 5U4G?

I am not an expert, but from memory I recall that with a 5U4G tube you'll drop the voltage by another 30 volts vs the 5AR4 and you shall have to limit the first filter cap value to a maximum of 40uf (others please correct me if I am wrong).
 
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