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what are your feelings about chinese 5AR4 tubes

boxoboom

boxoboom

Heath Nut
Hi guys and girls... I am repairing a Dynaco ST70 amp ... new mulitcan caps, caps etc... after doing this work ... used my variable ac power supply ... brought it up to about 60volts and it would blow a fuse....checked wiring caps etc... everything was fine... pulled the 5AR4 tube out and found that it was bad...ordered a Chinese 5AR4 tube ...put it in .... used my variable ac power supply to bring everything up .... with the new tube... it all worked perfectly... ran it for at least 12 hours yesterday.... turned it on today... sparks in the ole (new) 5AR4 distorted channels and then poof... it blew a fuse.... I was hoping forgetting more than a day off of this tube... I knew Chinese tubes are sort of crap... but only a day's use?? So what are your experiences with Chinese tubes... did I just get a bad one ... or are they getting cheaper by the minute....??
 
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The ST-70 is pretty hard on the rectifier, so it really should have a decent one or a solid state replacement. Anything that isn't good for a decent bit of abuse is not going to survive well. You might get some joy out of the "yellow sheet mod" but I wouldn't guarantee it.

I have a Chinese 5AR4 that I got with a cheap set of tubes from someone's ST-70. I think its used and I think it works, but I've never actually run it in anything. Not for any special reason, I just haven't had the need.
 
I've run a Chinese 5AR4 in a refurbed Dyna 70 with no problems, using the enhanced yellow sheet mod, the larger capacity quad cap (40/80/30/20), and surge supressors on the power transformer primary. All, imo, common sense measures.
 
Mine fried a JJ 5AR4. The Tad rectifier I replaced it with as held up nicely.
TAD tubes are Chinese, but built to be more rugged per TAD's specs. TAD's old supplier, they, used a brown base, is out of business at the moment and they had to go with a new manufacturer. They use a red base now. But you should still be able to find some of the older ones. Tungsol seems to get good reviews also.
 
I think the Chinese 5AR4 is a good tube. Just make sure to keep outoit bias current within spec, or else you will overload the tube. Also, since you just rebuilt the amp and it has never actually worked for long, but has ruined two tubes, I would consider you may have made a wiring error or the amp may have another issue which damaged the tubes.
 
You may have a bunk tube, you may have another problem, causing the power supply to pull too much current.
The ST70 really needs a second rectifier tube, but in order to keep the cost down they omitted it. Not only would they need 1 extra expensive tube, but also a socket and additional transformer winding current.

The ST70 stresses rectifier tubes, and you'll want a good one, if you're going to have it working all by itself...

Another thing to consider: UF4007 diodes will rectify as much current as 4x 5AR4s, for about a buck. 4x 5AR4 is going to cost you $200, just for the tubes!
In our modern sophisticated world theres a variety of ways to get DC out of Ac.
5AR4 tube rectifiers have got to be the least cost effective of all the possibilities I'm familiar with...
The search term "yellow sheet mod" should give you lots of data on the topic.
Personally I use squalid state diodes, and tube rectifiers depending on the goals and intentions of each individual project.
If you do go with squalid state diodes remember that your b+ will be slightly higher and you'll want to re-bias your tubes...
 
I had a Chinese one fail in Keg's ST-70, but the TAD seems to work fine. I also have run various Amperex, Mullard and US fat boy 5AR4s in there just fine. The fat boy is in there now as I like the look of the bigger glass.
 
Lavane said:
Mine fried a JJ 5AR4. The Tad rectifier I replaced it with as held up nicely.
TAD tubes are Chinese, but built to be more rugged per TAD's specs. TAD's old supplier, they, used a brown base, is out of business at the moment and they had to go with a new manufacturer. They use a red base now. But you should still be able to find some of the older ones. Tungsol seems to get good reviews also.
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The TAD red base units are fine. I used some recently in a pair of MK IIIs that I rebuilt and they survived my 24 hour burn-in test. However, I also always put solid state protection diodes on the sockets (1500V-rated, not UF4007/1N4007s) in case the vacuum rectifier croaks. 1000V rating of the *4007 isn't sufficient for the voltages present on the ST-70, so you'll either need 2 in-series with parallel caps to ensure proper load-sharing (Merlin has a good explainer here: http://www.valvewizard.co.uk/bridge.html -- look for "Two-Phase Rectifier"). Anyway, I keep a small inventory of 1500V 2A diodes around so that I don't have to bother.
 
Yes it happened from a cold dead start.... once the caps were formed I saw no reason to use the variable power supply even tho it was still plugged in it ... was still set at 117volts... just turned it ...the power supply on... and the sparks happened inside the tube....sort of like when I first plugged it in with the old tube... same result...

6DZ7 said:
It happened from a dead cold start...? Or had the amp been run at all and warm and did you do a quick off/on when it arced over? Running the variac at 117v or upping it to above 120v or on the wall power then?

And did you stay with the original can cap values, 30/20/20/20?
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OT a bit... Several people I know over the years went to a solid state rectifier on their ST70

I have no idea if doing so changed the SQ
 
and on the other hand ... has anybody used a solid state replacement rectifier in stead of the tube for this Dynaco st70.... I know they are available for a lot less money than the tube ...but I have also read that because of the higher voltage that it will supply... it may blow the selenium rectifier that is in there also.... so as I have never used one... just sort of curious as to some of the caviots that I might run in to... so any and all issues or comments will be welcome....as I might use one of those instead of a tube... buying tubes can be sort of expensive if all I am going to get only a day use out of it....!!
 
boxoboom said:
and on the other hand ... has anybody used a solid state replacement rectifier in stead of the tube for this Dynaco st70.... I know they are available for a lot less money than the tube ...but I have also read that because of the higher voltage that it will supply... it may blow the selenium rectifier that is in there also.... so as I have never used one... just sort of curious as to some of the caviots that I might run in to... so any and all issues or comments will be welcome....as I might use one of those instead of a tube... buying tubes can be sort of expensive if all I am going to get only a day use out of it....!!
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Replacing the selenium Rectifier with a silicon diode is one of the easiest and least costly upgrades you can do.
There are lots of "how to" videos on how to do it if you do a google search.
 
using a solid state rectifier in place of the tube will not damage the selenium bias rectifier.

A proper solid state rectifier will have a resistor to simulate the voltage drop in the tube anyway. At power-on the voltage will be higher but once the tubes start to conduct its not a problem. Where you have to watch is the voltages on the can cap. The voltage they see at power-up will be higher than what they see with the tube rectifier.
 
I use SS rectifiers with resistors and a thermistor built in with my M-125 mono blocks. I may try one in my ST-70, but I would install higher voltage caps in place of the cap can if I did go with that Copper Top rectifier.
 
gadget73 said:
using a solid state rectifier in place of the tube will not damage the selenium bias rectifier.


That's a good point about the not damaging the selenium rectifier, since it is powered by a separate transformer winding and does not derive it's voltage from the high voltage B+ circuit.
If boxoboom is doing a refresh on an old vintage ST70 and not replacing the Selenium rectifier, I would think he could be doing a more thorough job for very little expense by making sure the bias rectifier is a new silicon one.
In fact if he is experiencing high current draw thru the GZ34, a failure of the bias circuit could be at the root of the problem.
Low bias voltage will create higher current to be drawn at the EL 34's.
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Not a seperate winding, just a tap on the HV winding.

As usual I'll be the standard malcontent and suggest the selenium really isn't as much of a problem as it's made out to be. It works at a very low current level and its pretty reliable doing that sort of work. Exception is if the cap(s) have shorted and it gets abused horribly. They don't like that very much. Silicon is cheap and easy but I don't consider it to be quite as much of a critical change as many do.
 
Opinions? Does anybody think Psvane 5ar4s are of better quality than standard Shugang typical grade? I wonder if TAD sell this tube for their standard 5ar4
 
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