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where to get good 5AR4 's

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I am down to my last very old and worn out american made 5AR4's. am also using some old 5U4's I bought about a dozen JJ 5AR4's about two years ago. Am attemping to use them in Dynaco ST70 and Mark III. Had a 5U4 blow out a filament last night. Went through 4 of the new 5AR4. They arced and blew the incoming line fuse as they were warming up for the first time. Found and old RCA 5AR4 and a GE 5U4.. Put them in and they both worked fine. Did I get a bad batch of of the JJ's. Is there a better brand of new 5AR4's At some point I will run out of the old stuff and have to get some good rectifiers somewhere.
 
JJ needs to do something about their Octal production, YESTERDAY. The way things stand now, it's all garbage. :(

For a while, Chinese 5AR4s were OK. Then the sh*t started to flow again.

Saratov, Russia, made 5AR4s are probably the best of the current production.

Of all current production 5U4s, only the EH 'GB is any good.

5R4s are strictly a NOS proposition.

I 2nd the Jim McShane recommendation. No baloney from Jim.

Given the hassle associated with vacuum rectification, I suggest you switch the ST70 to SS B+ rectification and combination bias. A CL150 inrush current limiter will slow B+ rise down enough to allow the instant on bias supply to protect the "finals" against cathode stripping. A 100 Ω/330 μF. bias network under each O/P tube pair "eats" the extra Volts SS rectification yields, provides the "idle" current test point, and makes the single bias adjustment/channel a non-issue. BTW, EL34 matching requirements are relaxed a bit, when combination bias is employed. Close matching of gm is a must, but cathode current matching need be only reasonably close.
 
Went through 4 of the new 5AR4. They arced and blew the incoming line fuse as they were warming up for the first time.
Is your ST-70 power supply in good working order? I have one restored with new parts and JJ 5AR4 works fine so far. I did have a problem before the restoration though.
Here is a discussion on remedying the rectifier tube arc issues.
 
I had two in a row arc-over,with the stock supply. :(
I plan on adding the SS diodes on the plates,once I get a new 5AR4.


Yeowch! :eek:

So far my ST-70 has killed:

- "Dynaco" labled GZ34 (looks like an RCA innards).
- JJ GZ34.
- GE GZ34.
- Sylvania 5AR4.

Much Googling led me to the Sovtek, which survived two clients ST-70's so far.

I used a Sovtek 5U4G in mine before switching her SS, because that's all I had left on hand.

But I would suggest the series diodes bigtime! That PA-060 unloaded at turn-on can really ruin a tube's day with PIV.


Cheers!
 
But I would suggest the series diodes bigtime! That PA-060 unloaded at turn-on can really ruin a tube's day with PIV.

Yes , I agree . The most important parameter for a tube rectifier ( after
voltage and current ) is of course , a datasheet parameter called :
" Minimum Effective Impedance per Plate "

The value that is on the old datasheets ( GE,RCA,Sylvania,etc. ) , DOES
NOT WORK for the new production tubes , you need to consider an extra
safety margin , may be puting ( or adding ) series resistor between each
plate , and the transformer tap .

There is a formulae to calculate those resistors , but you can try to start
with a 47 ohms x 5 W wire ( only a guess , the correct is to do the calculations ) . it will help to avoid the arcing , for shure

That is my own experience .

Carlos
 
Wait,I think I was wrong.. One was with the stock PSU,then I decided to check the leakage of the (original) can cap,and it was plenty low.Then I installed the SDS cap board on a whim,and arced another one. Jim (McShane) offered to send me yet another replacement,but I told him not to worry about it,and have been using a 5U4 which has held up fine.I'm a little concerned about the extra filament current though,so I haven't been using the ST-70 as much.

Definitely time to add some diodes,and get another 5AR4,probably a Sovtek from Jim again. :)
 
Thanks One and all. I have ordered a bunch of diodes. I have 2 of the ST-70's and 2 of the Mark III's and one custom built mono block that all all going to get the treatment. All of them are recent rebuilds. All of the driver boards have been replaced to get rid of the 7199's. The Mark III's got the under chassis capacitor board modification. The ST70's got new 40-80-30-20 cans from AES (one of which lost 2 section yesterday. The ST-70 I am using on the subwoofer ( dual voice coil, one on each channel and a Y adaptor on the input) takes the hardest hits. LFE on action movies will just swamp the power supply. I have seen a good explosion scene on the dolby digital just suck a brand new 5AR4 up, arc, short out and blow the power fuse. I put one of triode electonics heavy duty power transformer on this amp . Boy does that make a difference and runs cooler to boot. Have avoided SS rectifiers to not go the cathode stripping problem. Standby switches are out of the question (only I could figure it out and the amsp are spread out all over the room by the speakers.) Thought about time delay circuits and relays but was getting complicated. The diodes on the plates is great. Basically the 5AR4 does not even need to rectify any more. The PRV and rectification is done by the diodes and the 5AR4 become a turn on standby switch. Best of all worlds and no need for a HV relay and timer circuit.
 
Have avoided SS rectifiers to not go the cathode stripping problem.


Only types with cathode sleeves, like the 5AR4, automatically protect against cathode stripping. When a filamentary type, like the 5U4 or 5R4, is employed B+ rise is almost as fast as it is with SS diodes. Even a few seconds delay is sufficient, when instant on "fixed" bias is employed. The negative voltage applied to the control grids provides an electrostatic shield for the cathodes. A CL-150 NTC (negative temperature coefficient) inrush current limiting thermistor neatly provides the requisite few second delay.

You mentioned that OEM Dyna power trafos run hot. If you SS rectify the B+, the 1st filter cap. can be kept on the small side, without adverse effect on the rail voltage. A smaller 1st filter cap. results in a larger conduction angle and cooler power trafo operation. Make the 1st filter cap. small enough, pile the energy storage up after the choke, and you don't have to use combination bias. The rail voltage will be the "same" as that produced by a 5AR4.
 
The ST-70 I am using on the subwoofer ( dual voice coil, one on each channel and a Y adaptor on the input) takes the hardest hits. LFE on action movies will just swamp the power supply. I have seen a good explosion scene on the dolby digital just suck a brand new 5AR4 up, arc, short out and blow the power fuse.
You'd be better off getting a couple (or one if connected in series) of plate amp like this for your subwoofer. Trying to get "tube sound" in subwoofer's frequency range is not going to give you much in return. It may be a waste of tubes.
 
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