GZ34 (5AR4) rectifier tube

Fongolio

Looking for a GZ34 (5AR4) rectifier tube in excellent shape for my Eico ST-70. Anyone have a spare they want to sell?

Kelvin

george daniel

Anything specific or just one that works?

Fongolio

george daniel wrote: »

Anything specific or just one that works?

A like new Mullard for $5.00 would be nice. One that works well. The voltage on my pin 8 is almost half of spec. and I think that's leading to problems with me biasing the output tubes. The output tube voltage at the bias points is supposed to be .38 volts and the best I can get with the bias resistors maxed out is about .10 volts. Everything else seems to check out fine except the voltage on the rectifier tube is at 230 when it's supposed to be 450 (aprox). I guess I could have a tranny issue but I'm starting with the cheaper stuff first. The amp still sounds fantastic, it's just not producing the full power it could. Any other ideas would be greatly appreciated.

mhardy6647

I'll check stock :-)

Fongolio

Thanks.

mhardy6647

I thought I had one or two used 5AR4s in some boxes of pulls/plinkers in da basement. Well - I have a fecal load of used 6SN7s, 6L6s, and 5Y3s... but no 5AR4s :-(

Sorry!

HB27

Sounds like your filter caps are leaking out causing the drop in voltage to the rectifier tube. Probably the first line cap on that one. IF they haven't been done I'd replace them and then your rectifier issues will be taken care of.
Just educated guessing without "hands on"

Make sure the solder joints are good on it. The last couple of Eico ST-70s I've owned had dozens of bad solder joints. One was so bad I sent it to Terry DeWick and he redid over 100 in that one. Bias issues and nothing else wrong with it other than lousy soldering.

Fongolio

Caps have all been replaced and I'm pretty sure the solder joints are good as I did them all and I've inspected them under magnification but I won't rule it out. The unit sounds like there is nothing wrong but I know I should be getting higher bias voltage at the test points. Maybe I've got a bad new cap. I think I'll check for that next. If the power transformer was going bad would it cause a decrease in voltage at the rectifier?

Maybe I should just not worry about it and enjoy the great sound it has. It plays loud and with authority when I want it to still but my OCD tendencies make me want to have it perfect.

I'm going to get another rectifier tube anyway just in case, so what brand do you all recommend besides the super expensive Mullards. Is there a good less expensive one?

HB27

Is the voltage checked with tubes in or out. Check with tubes out. I don't think the iron is going bad since it's playing fine. All caps changed? Even the Filter? I'm still thinking cap and that's where I'd search initially. Is it possible you burnt up a cap on install?Resistors could be a problem but I don't think so.
Without hands on and a meter I'm just guessing but I've done dozens of amps including a half dozen or so Eico ST-70s and what you're describing points me in that direction. But then again, anything at anytime can go wrong in electronics.
Next, I'd check with the Eico users groups at yahoo.com. Some brilliant Eico minds over there.
I'd stick with the Mullard or some good vintage USA 5AR4s. I don't have any luck with the re-issues for any length of time. I look at like this. $75 for a long lasting mullard or 4 or 5 $25 re-issues. The high buck Mullards are actually cheaper.

Fongolio

Well your experience HB27 tells me to recheck the caps, especially the one feeding the rectifier. I'll probably now save my money until I can afford the mullard. Several of the original Eico tubes in it are original made by Mullard, and if they're still working well after 50 years or so that's a ringing endorsement by itself. The rectifier has no identifiable markings at all so probably a replacement for the original. Thanks for your help and everyone else's.

HB27

I'd like to hear what you find on it. It's always good for future referrence. Good luck and enjoy the Eico.
Just out of curiosity, what was the voltage to the rectifier when you pulled out the tubes?
Harry

heiney9

The Mullard rectifier's from that time period might outlast all of us. It would be rare for it to take a dump, but it's certainly possible. I'd go with Harry's recommendation to check the caps as well.

IMO, the Mullard is the only 5AR4 to ever consider buying. I have (3). I use one and the other two are for rainy days or if I get an additional amp that needs a 5AR4.

H9

george daniel

I love that,,,, "rainy day Mullards'",,I think Brock has a plan.

Fongolio

The caps all check out fine. I haven't checked the voltage to the rectifier with the tubes out yet. I was really hoping to find a bad cap.

SCompRacer

IMO Holland made (Phillips) with double D getters are the best GZ34/5AR4. Either metal base (uber pricey) or Black base with metal base innards. My Holland made Black base with metal base innards are branded Mullard.

HB27

Did I read on Eico groups you changed out some resistors? IF so double check the resistor values.
I just got a bunch of resistors from Parts express that were bad. I also got some block resistors that were labled weird and didn't work. 2.7K was what I ordered and got 2K7. Needless to say I lost the tweeters output in my M7s.
It should have proper voltage and low bias is just as bad as high bias.

HB27

Fongolio wrote: »

Well your experience HB27 tells me to recheck the caps, especially the one feeding the rectifier. I'll probably now save my money until I can afford the mullard. Several of the original Eico tubes in it are original made by Mullard, and if they're still working well after 50 years or so that's a ringing endorsement by itself. The rectifier has no identifiable markings at all so probably a replacement for the original. Thanks for your help and everyone else's.

Just touching the old Mullards will quickly erase the ink on the tubes and nothing is left QUICK.
Did you test the 5AR4??? IF it tests strong there's no reason to replace it.
Check the tubes out good. I know that's the first simple step and I took it for granted you already did that. Look at the guide pin on the rectifier tube. IF it's a pin hole it's most likely a vintage Mullard.

Fongolio

Problem is I don't have access to a tube tester and where I live there's not likely to be one for about 300 miles. Some very faint writing I found on the 5AR4 reads Great Britain so it could be Mullard. Yes, I replaced all the resistors and I'm really thinking the problem could be on one or more of them or a solder joint. I'm going to really go through it tomorrow. What's weird is that it's both channels so if it is a bad solder joint or resistor it has to be where both channels and/or all four tubes are fed and I've looked and looked and so far nothing. Man this is frustrating!!

heiney9

IIRC, some of the Japanese Matsushita 5AR4's say Gt. Britain instead of made in England. Is there a slight raised X on the top and does the guide pin have a hole in it?

Fongolio

No raised X but yes to hole in guide pin. There is some heat coloring to the tube around the top and I'm not talking about the normal heat shielding type chrome color. The sort of multi color stuff you see when chrome has been heated. A little bit around one part of the base where the glass and the plastic meet. Is this normal?

Fongolio

One more thing. The tube is NOT sealed. If I blow on it through the hole in the guide pin (insert joke here) air escapes from between the glass and the plastic base. Is that normal?

HB27

Keiko wrote: »

Don't think so, but I may be wrong. Hence, vacuum tube.

IF the vacuum seal was broken the tube would be white coated in the glass. The bases can be loose from the glass and still be fine. I have a nice collection of old "Coke Bottles" and some are loose. I use speaker edge glue to tighten them back up.
Rectifiers run hot but the Mullards usually keep a nice even chrome on the top with a touch of brown edge
Ya should get a tester if you're running tube gear. Sorry, bottom line there. It's a must have. It doesn't have to be high buck or fancy.
One bad tube can/will short out resistors and it's possible it's where your trouble is. Not saying it is but possible.

george daniel

Fongolio wrote: »

One more thing. The tube is NOT sealed. If I blow on it through the hole in the guide pin (insert joke here) air escapes from between the glass and the plastic base. Is that normal?

Thats hardcore--dedicated --- this man has Platinum Club Membership-


dunno if it's normal-- hell,, we are not "normal",, but when I get home,,I'm gonna blow thru my Mullards--

heiney9

Never toked on a tube before. Be sure to wait until it's cooled :biggrin:

If it doesn't have the slight impression of the X on top and it has a guide pin hole (considering it hasn't been added) then it's probably a later Mullard type rectifier. They made a few different types.

The Japanese copy doesn't have the guidepin hole, but there are some unscroupulous sellers out there that are good with a drill and add the hole.

H9

mhardy6647

Don't Bogart that tube, my friend
pass it over to me...

I wouldn't worry about the loose base, as long as the envelope is still evacuated.

Fongolio

As I was doing it (blowing on tube) I knew there was a funny to be found. I truly am a tube newbie. Thanks guys for making me laugh when it's been very frustrating trying to track down this problem. I guess before I spend any more money I should get a tube tester. I checked a whole bunch of the resistors and the signal path last night with the multi-meter but could find no problems. Just a thought. Could this problem be coming from a weak power transformer?

Fongolio

Hey Mike, I got your PM. Thanks man.

heiney9

Fongolio wrote: »

As I was doing it (blowing on tube) I knew there was a funny to be found. I truly am a tube newbie. Thanks guys for making me laugh when it's been very frustrating trying to track down this problem. I guess before I spend any more money I should get a tube tester. I checked a whole bunch of the resistors and the signal path last night with the multi-meter but could find no problems. Just a thought. Could this problem be coming from a weak power transformer?

I believe our own Dr. Cilantro will test tubes for $5 ea. Might be a better option than buying a tube tester and then making sure it's properly calibrated, etc. I don't have one.

H9

Fongolio

I lucked out! I just found a guy locally that has a tube tester and as it turns out he owes me a favor. I was at a local auction house a year or so ago and we were both bidding on a box of records (all good stuff - Led Zep, Sabath, Stones) and I won the biidding. He came up to me afterward and asked if I'd sell him one of the titles. It was one I didn't care about so I told him he could have it for nothing. He was very happy and I bought some karma that just came back to me today. Going to hook up with him and test all the tubes on this amp. Yay!!!

george daniel

well I want all of ya'll to know that I came home from work,,and toked-tooted my Mullard.You can toot a Mullard if you hold your mouth just right. I just read my reply,, this is wrong in so many ways,, lol. Mullard tooters.

heiney9

Hope there's no Mercury in these old tubes