Which brand GZ34 to choose?

Hi all,

Have just gone through another power tube meltdown in my ST120, and lost the GZ34 along with it. Have a Weber WZ68 for backup, however I like the look of glowing tubes. In the past I have used Sovtek, J/J, and Ruby rectifiers. At present I have just installed my last J/J. I had two for backup, put one in with the new power tube duo and it promptly went up in a flash in just over 24 seconds. Rechecked one channel at a time with another GZ34 and found no problems with either side separately, and then both together with no problem. You just can't tell if you have a weak tube even when new.

Need to purchase some backup GZ34's (Again), and was interested in your thoughts regarding your experiences with the different manufacturers.


ArlanB


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Sorry to hear about your repeated failures. I have have had good results using the $13 Sovtek 5AR4 from guitarcenter.com in my Mark IIIs. I am pre-rectifying with forward 1n400* ahead of the 5AR4. I have about 30uF on the rectifier side of the choke.

Mullard GZ34 1958-1882. They look and sound great.

jdm

Thanks for the suggestion. Actually I found a pair of NOS Mullard GZ34's a week ago in a local music store with a service department for electronic instruments. The service man had them hidden away from the general stock of tubes in the store. I was able to pick them up for $50 each. Now I have to build up the courage to use them and hope that I do not have another major power tube failure. Sure eats up the rectifiers.



ArlanB

make sure you've got the diode mod on your rectifier socket (see post somewhere in this forum), and that you aren't going into more than 30uF.
Although they are rated for 250ma, most modern tubes will probably fail if you consistantly run them at more than 200ma.
The VTA drive circuit uses about 12ma, so you've got just under 50ma available for each output tube.

$50 for NOS Mullards, If he has more at that price let me know, I'll give you a finders fee to buy them for me...


Sal

Sal:

Sorry to tell you that when I bought the Mullard GZ34's, I asked if he had more, but was informed that I got the last of his stash.


ArlanB

tubes4hifi,

I have one of the first 3 VTA ST120 kits from Bob with a stock VTA Driver board with no capacitor mods. Wonderful kit!
I made a search of the forum for your reference to a diode modification for the rectifier socket and came up with a mod
proposed by mantha3 that placed a diode between pins 4&5 and 6&7 of the rectifier socket for MKIII, MKIV, and ST70s.
Is this the one which you are making reference to? If not, could you steer me to the correct one, and explain what the advantage is?

ArlanB

Arlan,
Oh well, you got a great deal on the NOS Mullards.

Here is the diode mod Roy was talking about.

Regards,
Sal

thanks Sal, yes, that's the diode mod, anyone running a power amp with a rectifier tube should do this simple mod!!

Sal,tubes4hifi,

This is the mod that I located during my search, however, I am not sure just what this modification will do for the power supply. I need help to understand what is happening here along with the advantage to the circuit.

Thanks in advance.


ArlanB

Just think of it as pre-rectifying the AC to DC before it goes to the plates of the 5AR4 tube but with the 5AR4 tube installed with the diode mod, you still get the benefits of slow start of the high voltage and the inherent voltage drop of the 5AR4 tube.

Sal

it also increases the peak voltage capacity of the rectifier, from 1500v to maybe 2500v, and puts most of the impact of a power glitch on the cheap silicon diodes rather than on the tube instead, it's cheap protection, like a spike snubber.

Well the one thing it dose is make the TUBE AMP sound more like sand-amps... the GZ34 in my MK3 is 50years old cant blow them up...even with the best diodes you lose some of the Tube sound... but it may save your GZ34s...give it a shot.. you can get 3a1000v at rat-shak..for $3.or less... an the need for all the Tube sound i can get,still workin on it..


Thanks Bob for the site..

zx, tubes4hifi, Sal'

If it can act as a kind of buffer for 5AR4 from the voltage spikes created when the power tubes short out, I will try anything to help limit the damage cascade to just one tube.

Arlan B

Arlan,
The diode mod will not act as a buffer if an output tube shorts. One way to save the 5AR4 tube is to use a low wattage 10 ohm resistors on the cathodes of the KT88's, if a tube shorts and pulls to much current, the 10 ohm resistor will burn out (hopefully) and save the 5AR4 tube. You may want to speak to Bob about that as I am not sure how many watts (or is it watt) he uses on his board.

Sal

PS: IMHO, the diode mod will not make the amp sound like a solid state amplifier. There are many tube amps out there with solid state rectification.

good point Sal, 100ma thru a 10 ohm resistor is still only 0.1 watt, so it's gonna take more like 1/2 amp to burn out a small resistor,
but I agree that using a small wattage resistor here is better than using a 1 or 2 watt resistor.
Of course the safest idea is to use a .1 amp fuse on each output tube, but then someone will argue that it affects the sound.
(well, yeah, so does a resistor). Personally if I was running $100 tubes, I'd use a 25c fuse on each one.
Some people have bad luck, I've only burned out one output tube in the past ten years.

Sal wrote:
PS: IMHO, the diode mod will not make the amp sound like a solid state amplifier. There are many tube amps out there with solid state rectification.

What has made tube amps "resemble" solid state in some ways for me: very high reservoir capacity in the power supply, more so than solid state rectification.

Sal

PS: IMHO, the diode mod will not make the amp sound like a solid state amplifier. There are many tube amps out there with solid state rectification.

You say so, just got a set of old Mk3 kits 50years old.Sale why the guy took them to a shop the pull gz34 out an put ssDiods in after 50years the owner sead thay sounded like a $10.sand amp.
An yes i have tube amps with sand rectification but the power tranfourmer has no center tap, so you make your Neg.leg for the amp .I see this as a + for the sound in some tube amps,but all tubes add to the sound in the dyanco Amps,But all one has to do is try it for there self.. Parts are at rat-shak,Less than $5.
If you cant hear a diff. Sands for you,so less see $100.for a pr of ONS gz34s or $5.for sand?
I no a lot of people that think tube amp sound best,but would never have a tube amp.

Thanks Bob for the site

You might be missing the point, the 5AR4 tube is still in the power supply circuit so you still get the benefits of the voltage sag and B+ delay. In the diode mod you are not eliminating the 5AR4 tube as what was probably done to your Mark III's.

Sal

The high V is what kill the sound,The B+ went from 490v with the old GZ34s to 525v the 35+v i get this


yes I have done the mod years a go,an just worket on a ST70 amp for a guy that had this mod..An he felt that With just the GZ34 it sounded better with out the Sand..but all this is just my 2cent.
I think If you dont go you wont NO.. Glad you like the mod.


How about this if were going to go SS diodes an GZ34s,put one more diod in an get the 5vAC heater out of the B+DC.Just come off pin 8 before the choke an cap with the diod?

Thanks Bob for the site

Kevin Deal at Upscale Audio has some NOS Dutch-made Philips 5R4s, they ain't cheap ($55) but will blow the doors off of the Weber ss rectifiers, IMHO. I've got two (out a batch of 4) that will approach 1.2 on the M-125s; the other two come  in at about 1.0.