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    5AR4 reliability in ST120

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    Will Power45

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    Join date : 2009-10-06

    5AR4 reliability in ST120

    Post by Will Power45 on Tue Oct 06, 2009 6:20 pm

    I have been using my kit built Stereo 120 for the last month or so. It is a great product. It sounds wonderful in the triode mode with my Reference 3A monitors. I am surprised how quiet it is. However last night the supplied Ruby rectifier tube lit up and blew the fuse. I replaced it with another Chinese tube and all is good. Is there a reliability problem with the Ruby rectifier. Should I bite the bullet and get a fancy NOS tube, go with a solid state plug in module or keep using the Chinese 5AR4 (Sino).

    Thanks,
    Will Power

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Tue Oct 06, 2009 7:26 pm

    Hi,

    The Ruby GZ34/5AR4 is a Chinese rectifier that has been run in and tested by Ruby in California before being shipped. The verified output is on a label on the base of the tube. Many guitar players use Ruby rectifiers. I have personally found the Ruby to be much better and longer lasting than the Sovtek or JJ GZ34's.

    The "best" GZ34 you can put in there is an NOS Mullard or a used Mullard from the 1950's or 1960's. They are (unfortunately) expensive at $50 - $150 a piece.

    A good alternative rectifier in the ST-120 (or ST-70 for that matter) is the Weber Copper Cap model WZ68. This is a solid state rectifier with a short warm up delay that mimics somewhat the warmup of a tube rectifier. They are only $22 + shipping online from Weber. I have used one in my own ST-120 for about 2 years now with no issues.

    Understand that in an ST-120 or ST-70 tube amp, the rectifier tube takes a lot of abuse with voltage surges and is the tube most likely to go bad.

    Another note > Everytime you change rectifiers you must check (and probably readjust) the bias settings.

    Bob

    jrethorst

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    Join date : 2009-02-01

    Re: 5AR4 reliability in ST120

    Post by jrethorst on Tue Aug 31, 2010 8:19 pm

    How can you tell when a 5AR4 needs to be replaced?

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Tue Aug 31, 2010 8:55 pm

    jrethorst wrote:How can you tell when a 5AR4 needs to be replaced?

    Hi,

    If you get LOW BIAS on all output tubes or NO BIAS on all output tubes then replace the 5AR4. Just because a 5AR4 lights up doesn't mean it is good. A more exact test would be to open the amp up and measure the DC voltage on pin #8 of the rectifier tube. On an ST-70 you should measure above 400 VDC from pin #8 to chassis ground. If the voltage on pin 8 is low (below 400 VDC) then either the rectifier is bad or the quad cap is leaking. Try replacing the rectifier tube first. If the voltage goes back up to a normal range (420 - 450 VDC) then the rectifier WAS bad. If a new rectifier doesn't do it then most likely the quad cap is bad. Replace the quad cap.

    Bob


    Last edited by Bob Latino on Wed Sep 01, 2010 5:07 am; edited 1 time in total

    j4570

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    Join date : 2010-08-30

    Re: 5AR4 reliability in ST120

    Post by j4570 on Tue Aug 31, 2010 9:45 pm

    Look for Used Mullards with other labels. I picked up an "HP" Mullard a few years back, though it was close to $50 I remember, it tested as new.

    Most are even more than that. Those Mullards ain't cheap.

    Corona

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    Join date : 2010-03-11

    Re: 5AR4 reliability in ST120

    Post by Corona on Wed Sep 01, 2010 10:21 am

    Use a mullard GZ37. They're cheaper than 5ar4's, look better and are more durable. I've been using one in my st-70 without issue for months.

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Wed Sep 01, 2010 5:52 pm

    Corona wrote:Use a mullard GZ37. They're cheaper than 5ar4's, look better and are more durable. I've been using one in my st-70 without issue for months.

    You can also use a GZ33 rectifier tube in the ST-120. A GZ33 draws more current than a GZ34 but the rectifier line on the ST-120 is rated at 5 amps and can handle the extra current draw with no problems. A GZ33 is a tall, coke bottle shaped tube. The nice thing is that you can still find NOS Mullard GZ33's for about half the price of a NOS Mullard GZ34. There is vendor on Ebay from Great Britain that sells NOS Mullard GZ33's for about $70 + about $7.68 shipping from GB. Not a bad price for a NEW Mullard tube. Check the link below.

    NOS Mullard GZ33 on Ebay

    Bob

    thevic24

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    Re: 5AR4 reliability in ST120

    Post by thevic24 on Wed Sep 01, 2010 5:57 pm

    I have been using a NOS mislabeled Philips fat boy gz34 in an ST70 that I have and it has been solid 100%.

    It seems to me, so far, that the new production GZ34's are not that great. In my 2 ST70's they seem to do ok but in my ST120, they never last long at all.

    60wpc is a lot for a single GZ34 to deal with though it will but it dose tax the gz34. I just installed a Weber CC gz68 which is equal to 2 gz34's.

    BIG night and day difference in head room and sound at high volumes and much stronger bass output.

    -Vic

    thevic24

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    Re: 5AR4 reliability in ST120

    Post by thevic24 on Wed Sep 01, 2010 5:59 pm

    Bob Latino wrote:
    Corona wrote:Use a mullard GZ37. They're cheaper than 5ar4's, look better and are more durable. I've been using one in my st-70 without issue for months.

    You can also use a GZ33 rectifier tube in the ST-120. A GZ33 draws more current than a GZ34 but the rectifier line on the ST-120 is rated at 5 amps and can handle the extra current draw with no problems. A GZ33 is a tall, coke bottle shaped tube. The nice thing is that you can still find NOS Mullard GZ33's for about half the price of a NOS Mullard GZ34. There is vendor on Ebay from Great Britain that sells NOS Mullard GZ33's for about $70 + about $7.68 shipping from GB. Not a bad price for a NEW Mullard tube. Check the link below.

    NOS Mullard GZ33 on Ebay

    Bob

    Hey Bob,
    What would the differences be between the GZ34 and GZ33 in the ST120?

    When I was running a 5U4GB in the ST120 I still had great sound but much less power. How would the GZ33 compare?

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Wed Sep 01, 2010 6:13 pm

    Hi Vic,

    A 5U4 in the ST-120 does give a higher voltage drop and you will not get full power out of the ST-120 with a 5U4 in there just as you said. A GZ33 has a lower voltage drop - much the same a GZ34. Your amp should sound about the same with a GZ33 as a GZ34 in there for a rectifier.

    As you mentioned in an Email to me, the Weber WZ68 solid state rectifier works out better in the ST-120 IF you play the amp loud a lot. At lower volume or even moderate volume levels there really any difference in sound between different rectifiers.

    Bob

    jrethorst

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    Re: 5AR4 reliability in ST120

    Post by jrethorst on Wed Sep 01, 2010 11:39 pm

    Which one of these rectifier tubes would be best in a Mark 3? Any difference for triode mode? Any difference when using EL34s?

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Thu Sep 02, 2010 5:16 am

    Hi,

    I like the Weber WZ68 Copper cap because it will flow more current than any tube rectifier. As for the GZ34 or GZ33 - either would work fine in a Mark III. The use of a 5U4 in either a Mark III, Mark IV or ST-70 will restrict overall power.

    I wouldn't use EL34's in a Mark III. IMHO you would be pushing them too hard. KT88's or 6550's are better output tubes for a Mark III.

    Re: triode mode. Not sure ... I would try different rectifiers in there and listen for myself.

    Bob

    erlingt

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    Location : Denmark

    Re: 5AR4 reliability in ST120

    Post by erlingt on Thu Sep 02, 2010 8:25 am

    Is the GZ33 a too heavy load on the power transformer in the "Latino" ST-70 kit, or is it usable her too?

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Thu Sep 02, 2010 8:53 am

    Hi Erling,

    Yes, you CAN use a GZ33 or GZ37 rectifier in the VTA ST-70 amp kit with the upgraded PA-060 power transformer (or PA-521 120/240 volt power transformer). I have done this before with no issues other than the power transformer will get a little warmer using a GZ33 or GZ37 in there as your rectifier.

    A GZ34 draws about 1.8 amps while a GZ33 and GZ37 each draw about 3 amps. The 5 volt AC rectifier line on the VTA ST-70 kit is rated at 4 amps (and rated 5 amps on the VTA ST-120) so it (they) will handle a GZ33 or GZ37 with no problem. Personally, I would not recommend the use of a GZ33 or GZ37 in a stock ST-70 with an original PA-060 power transformer.

    Bob

    erlingt

    Posts : 15
    Join date : 2010-06-23
    Location : Denmark

    Re: 5AR4 reliability in ST120

    Post by erlingt on Thu Sep 02, 2010 11:53 am

    OK, thanks Bob. I think I will get one - very much like the retro look of it Very Happy

    And if it's durability is better, then that's a added value.

    Dynalover

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    Location : Waterloo, Ioway

    Re: 5AR4 reliability in ST120

    Post by Dynalover on Wed Sep 22, 2010 9:25 pm

    Bob, that is good advice about checking the 5AR4 in circuit. (CAREFULLY!)

    Have a totally, completely restored yet stock circuit ST70. Run it from a variac and lop off about 5 volts from whatever is coming out of the wall, usually 121VAC. I have two rectifier tubes, one a Mullard, the other Tung Sol.

    Both measure 404VDC at pin 8 under this arrangement, is this acceptable, or should be seeing 420-450VDC??

    Goal here is simply to run the power transformer a bit cooler and quieter, @ around 117 versus the 121+ during off peak hours. The lower incoming AC voltage does of course require rebiasing the outputs, noticed have to rotate the bias pots pretty far to get the specified 1.56VDC

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Wed Sep 22, 2010 10:03 pm

    If you are only getting 404 VDC B+ off pin 8 of the rectifier tube on an ST-70, your overall power will be down slightly. Nothing really wrong with that as long as you feel that you have enough power for musical peaks in your audio system. With only 404 VDC B+, you won't get 35 WPC out of your amp. An ST-70 should have about 430-440 VDC off pin 8 of the rectifier tube to get full power out of the amp.

    Bob

    Dynalover

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    Re: 5AR4 reliability in ST120

    Post by Dynalover on Wed Sep 22, 2010 10:33 pm

    OK, reads 441VDC when running from standard line voltage here. One of the rectifiers was purchased as NOS from a reputable antique tube vendor. Also have an old X top "mullard" that was only supplying 375VDC and the power transformer started groaning. Guess its time to pitch that one! Thanks for the advice Bob, this is good to know, particularly since rectifier tubes, as I understand it, cannot really be tested, at least on an emission type tube tester.

    GP49

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    Re: 5AR4 reliability in ST120

    Post by GP49 on Sat Oct 02, 2010 2:25 am

    Bob Latino wrote:
    I wouldn't use EL34's in a Mark III. IMHO you would be pushing them too hard. KT88's or 6550's are better output tubes for a Mark III.

    With all due respect, I disagree.

    As long as you're using original Mullard EL34s, there is no problem, other than lower maximum power output, with using them in a Mark III. The genuine Mullard EL34s have a higher maximum plate voltage rating than a 6550 or a KT-anything. You still have to adjust the bias properly FOR THE EL34 (not the Dynaco-specified "1.56 volts" which is for the 6550/KT88) but once that's done the EL34s will be perfectly happy and you will have a 50 watt amplifier with the sound of the EL34 tube, which is different from that of the 6550 and KT88...and why many prefer the sound of the Stereo 70/Mark IV to the Mark III.

    And what is a Mark II, but a Mark III with EL34 tubes and a series resistor in the power supply instead of the choke (which many people have installed in their Mark II as an upgrade anyway)?

    HOWEVER, if you use Russian or Chinese or Yugoslav or East German EL34s, you won't have the safety margin in an original Mullard EL34, as they just aren't made as well.

    Bob Latino
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    Re: 5AR4 reliability in ST120

    Post by Bob Latino on Sat Oct 02, 2010 8:18 am

    GP49,

    You have a point there. The original Mullard EL34's were very durable and could withstand very high voltages well and *could* be used in a Mark III BUT ...

    1. Very few Mullard EL34's from the 1950's and 1960's still exist and those that do are quite expensive either NOS or even used. Probably 98% of all Dynaco amps in use right now are not using the original Mullard output tubes or NOS Mullard EL34 output tubes. My comment was directed at the current crop of EL34 output tubes, which as you mentioned are not as well made as the original Mullard EL34's

    2. As you mentioned there will be a slight loss of power if you use EL34's in a Mark III.

    3. Dynaco recommended the use of KT88's in the Mark III - not EL34's

    4. At the bias setting mentioned in the Dynaco Mark III manual, each output tube got 70 milliamps of current which is pretty hot by todays standards for an EL34 or a KT88. The combination of higher voltage AND higher current would punish any current EL34 in a Mark III.

    Bob

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