VTA ST120 Bias Resistors

K7JHG

Member
Greetings AKers.

I've had this amplifier for about 8 months and have blown each of the four 10 ohm 2 watt bias resistors at the rate of about one per month - each has blown at least once. The first time my rectifier tube flashed so I installed the rectifier diode mod and got a new 5AR4 tube. The rectifier tube has been good since until the last time the rectifier tube flashed again so I suspect it may need replacement.

All my voltages (quad cap, rectifier and KT88s) check out per spec.

My question is - it was suggested by the amp builder that I replace the four 10 ohm 2 watt bias resistors with 10 ohm 5 watt resitors. I can't seem to find 5 watt resistors with less than 5% tolerance so I'm thinking I might use 3 watters with a 2% tolerance.

Does this make sense. Will the increased resistor wattage, either 3 or 5 watt, save me from blowing reistors or could it possibly exaserbate an underlying problem.

Thanks,

John
 
If your amp is blowing 2 watt resistors , it most likely has some problem . I would have it looked at by someone that has a scope and signal generator and a proper dummy load . It may have parasitic problems , That poor 5AR4 is way overloaded , what do you have the outputs biased at

5 % 5 watt 10 ohm resistor are fine if that is what you want to to , just buy ten of them and select two matched ones with a digital meter .
 
For some reason you have output tubes running away and popping the 2 watt resistors which means a serious problem that needs to be remedied. Upping the wattage of those resistors is not advisable at all. That would be about the same as upping the value of a fuse because it keeps blowing.,..the next thing a transformer goes up in smoke!! Fix the problem at the source.......
 
Ok thanks. The builder is recommending upping the wattage on those resistors and the manufactuer is telling me probably not a good idea. The general consensus is not to up the wattage but identify the underlying problem which is what I always thought should happen. I am not an electrician and sometimes it seems like I have been looking at the underside of this chassis more than I have been able to enjoy the amp. I'm kind of at a loss here. I guess I could take the board out and send it to the builder to check out or should he have the whole amp to test.
 
What part of the world do you live in, your amp needs to be looked at as whole , not just the board .
 
Bellingham, WA about 90 miles north of Seattle. I suppose I could try and locate a technician but how does one know who to trust. I also hoped the builder might troubleshoot and fix a problem since I have had the problem from day 1 - wishful thinking?
 
Greetings AKers.

I've had this amplifier for about 8 months and have blown each of the four 10 ohm 2 watt bias resistors at the rate of about one per month - each has blown at least once. The first time my rectifier tube flashed so I installed the rectifier diode mod and got a new 5AR4 tube. The rectifier tube has been good since until the last time the rectifier tube flashed again so I suspect it may need replacement.

All my voltages (quad cap, rectifier and KT88s) check out per spec.

My question is - it was suggested by the amp builder that I replace the four 10 ohm 2 watt bias resistors with 10 ohm 5 watt resitors. I can't seem to find 5 watt resistors with less than 5% tolerance so I'm thinking I might use 3 watters with a 2% tolerance.

Does this make sense. Will the increased resistor wattage, either 3 or 5 watt, save me from blowing reistors or could it possibly exaserbate an underlying problem.

Thanks,

John
I'm coming in late, but when reading this thread i'm more convinced that you should ship it to bob at hifi4tubes.
Remove all tubes ( with a note of where they were mounted) , pack the tubes separately and ship it. Do talk to bob first.

Unless you have a very strong disturbance in your house ( radio amateur, radar nearby or similar, the problem is in the amp. And there is a problem.
 
Replace the resistors with pennies ... :D

I'd think you'll eventually be able to track it down to a bad solder joint or short at the quad cap. Worth checking for loose tube pins too - even though you've blown all four, a spike on one could spike another. A magnifying glass and an insulated hook to pull on wires while it's powered up with a meter to the B+ to see if you can spot a change should help track it down.

Luck wit it ... I know it can be frustrating, but that's half the fun! If you wanted easy, you'd be listening to solid state ...
 
K7JHG -- Assuming there are no issues from the build of the unit, there are conditions that can come together in these amplifiers that can actually promote output tube arcing, which you clearly have going on. The answer is NOT to increase the wattage rating of the 10 ohm cathode resistors, which are arguably already too large at a 2 watt rating, but to install Screen Stability Resistors to address the root of the problem, which I first suggested and later wrote an article about for AudioXpress here:

http://www.tronola.com/html/maximize_tube_life.html

Your unit already contains the usual limiting resistors for use when operating the unit in Triode mode. With a rewiring of the Triode/Pentode switch, these resistors could serve double duty, and solve your problem with virtual certainty when operating in the Pentode mode.

However, you should also realize that once an output tube has arced, it is prone to arcing forever more. Therefore, the answer is to install the modification, and a new set of tubes, after the modification is done.

Finally, be aware that the use of a rectifier tube -- ANY rectifier tube -- even with protection diodes installed ahead of it -- will not let this amplifier develop anywhere close to its rated power in both channels at the same time. With such excellent transformers and overall design (besides these two concerns), you really should consider using SS rectification for this unit.

Does your unit have either the Time Delay Relay board and/or the power supply cap upgrade installed? These can be a factor as well.

Good luck with it!

Dave
 
I would also look at the speaker impedance- is it too low for the amp? While the speakers are a dynamic load that changes with frequency, too low of an initial impedance might be the ultimate source of the problem. But even if that is the case- D.C. Gillespe's suggestions should also be included in any changes as well.
 
Finally, be aware that the use of a rectifier tube -- ANY rectifier tube -- even with protection diodes installed ahead of it -- will not let this amplifier develop anywhere close to its rated power in both channels at the same time.

?

tungsol-kt120s-002.jpg


Mullard GZ37 has plenty of current capacity in the ST120, even driving a quad of KT120's comfortably. Granted, a GZ68 copper cap is a whole bunch cheaper ...
 
Bellingham, WA about 90 miles north of Seattle. I suppose I could try and locate a technician but how does one know who to trust. I also hoped the builder might troubleshoot and fix a problem since I have had the problem from day 1 - wishful thinking?

If you have trouble logistically with Bob, you could try Roger Hug in Seattle - he's an absolute wizard with tube gear (especially EICO) although he's getting along in years. I'm pretty sure I have his phone number somewhere. PM me if you want it.

-D
 
Bob does come to this forum from time to time.. But I would post your issue on his forum as well as this one. The more input the better. Bobs forum - http://dynacotubeaudio.forumotion.com/f2-basket

I have an ST120 and it is nearly 3 years old with no issue like this.

I think you do want the resistor leads as short as possible on these resistor spots... I would stay in the spec and not do the 5W as this may just push the real issue to another weak link in line.

When you build an ST120 you have excellent build instructions from Bob. I built mine.

I would flip that amp over and just review each step from start. weak solder spot? stuff like that. If you really just go step by step and review each step I think you could be done in 1 hour or 2. I bet you spot something. Bob also mentions in the build instructions some details where you want resistor leads as short as possible.

If you build an ST120 right it should not have this issue and work so long it still runs 40 years later like the ST70s we see running today that are 40-50 years old
 
PS - These amps are hard on a GZ34 and you may want to run a new fresh one. You also may have gotten a bad KT88 tube
 
sKiZo -- I'm not sure where you are getting your information from, but the GZ37 is rated for 250 ma, which is likely being taxed to 85+% of capacity just operating under quiescent conditions (no signal) if the output tubes are biased to 50 ma each. If this is what you mean by "...has plenty of current capacity......even driving a quad of KT120s", then this rectifier tube will in fact handle THAT load. But that is not what my statement said that you high lighted. It said in part "ANY rectifier tube.....will not let this amplifier develop anywhere close to its rated power in both channels AT THE SAME TIME".

The point being that the amplifier is offered as a 120 watt amplifier, but using a rectifier tube, it is incapable of producing this power level. At a 60 watt RMS power output level in each channel, each output stage is attempting to draw about 300 ma of current from the power supply, plus about 10 more ma for the driver circuitry. This equates to 620 ma of steady state current draw required to produce 60 watts out of both channels at the same time (120 watts RMS total). If you try that with your GZ37 installed, it will quickly arc over, almost surely destroying the tube.

For low power output levels, a rectifier tube will be well taxed, but quality pieces will likely survive. However, for those who have inefficient speakers and need this amplifier to actually developed the power it is advertised to deliver, only SS rectifiers are up to that task.

I hope this provides a clearer understanding of my statement for you.

Dave
 
Isn't that just the old instantaneous peaks argument? Real world performance vs the lab? How often does a full load condition occur, and for how long? That's what all the extra capacitance is for - to store energy to handle those ... points out another shortcoming of the original Dynaco designs, in that they didn't have sufficient reserve capacity, bringing to mind the argument as to what "sufficient" entails ...

Given that, the GZ34 is rated at 160mA, and the GZ37 at 250mA. Way I understand it, that makes the GZ37 capable of restoring the reserve quicker without working as hard?

Either way, it looks way cooler than a solid state replacement, and after all, what's more important? :D

PS ... the WZ68 solid state tops out at 450mA - admittedly much greater than the bottle rectifiers, but based on your numbers, still falls short. So, once again, when is enough enough?

As a real world example, I can drive my somewhat inefficient 89db speakers at 8ohms to sustained 100+ SPLs regularly without a hiccup. Without any rectifiers blowing up. Also without the hard clips the same levels would produce with my McIntosh 200wpc/400wpc instantaneous solid state monster. Good enough for me anyway.

Bit of an excursion from the OP's target theme here, so that's all I got to say about that ...
 
Hardly trying to take away from the coolness factor of the GZ37 tube, but simply state the facts as they can ably be demonstrated to be. Maintaining accuracy then for any others following along:

1. Within the allowed voltage rating (which covers the scenario of this amplifier), the GZ34 is also rated for 250 ma with a cap input filter from its Rating Chart, with most folks interpreting that figure to be 225 ma, as that is the available madc output of the closest "Typical Operation" values given in most tube manuals. The 160 ma figure you cite is for operation beyond the 475 vac limit (per plate) that generates the 250 ma maximum current rating, which is not applicable to the scenario this amplifier represents. The GZ37 and GZ34 then are nearly identical in the safe current levels they can handle in scenarios applicable to this amplifier.

2. The storage capacitance of the power supply has little to do with the issue, since the caps do certainly store significant energy to help cover the high current peaks -- but then they basically draw the same amount of current they provide to replenish themselves again, getting this current from.....the rectifier tube. Therefore, the net change in operating current drawn from the rectifier tube changes little as the storage capacitance is increased -- except at turn on of course. All current that the amplifier ultimately operates from is obtained from the rectifier -- be it to actually operate the amplifier circuits, or keep the filter caps charged.

3. If the WZ68 is limited to 450 ma of current, that is almost certainly not due to the SS rectifiers employed, but rather some internal fuse of current limiting device.

If the GZ37 works well for you that's great, as I have no desire to invoke you to use otherwise. But just because the GZ37 is your preference doesn't change the fact that it is still the weak link in the amplifier relative to the power it is designed to produce, ultimately limiting its performance to less power output than it's advertised to be capable of.

Dave
 
I'm in the Seattle area. I may be able to help you to diagnose this if you're willing to drive down here sometime. PM me if you are interested.

You should never install larger cathode resistors. Those are your tube's fuses. Dave's advice on the adding some screen resistors is very good advice.
 
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Isn't that just the old instantaneous peaks argument? Real world performance vs the lab? How often does a full load condition occur, and for how long? That's what all the extra capacitance is for - to store energy to handle those ... points out another shortcoming of the original Dynaco designs, in that they didn't have sufficient reserve capacity, bringing to mind the argument as to what "sufficient" entails ...

Given that, the GZ34 is rated at 160mA, and the GZ37 at 250mA. Way I understand it, that makes the GZ37 capable of restoring the reserve quicker without working as hard?

Either way, it looks way cooler than a solid state replacement, and after all, what's more important? :D

PS ... the WZ68 solid state tops out at 450mA - admittedly much greater than the bottle rectifiers, but based on your numbers, still falls short. So, once again, when is enough enough?

As a real world example, I can drive my somewhat inefficient 89db speakers at 8ohms to sustained 100+ SPLs regularly without a hiccup. Without any rectifiers blowing up. Also without the hard clips the same levels would produce with my McIntosh 200wpc/400wpc instantaneous solid state monster. Good enough for me anyway.

Bit of an excursion from the OP's target theme here, so that's all I got to say about that ...

Very appropriate point. How often does a full load happen under normal music listening....NEVER. If it did adding SS rectification is not going to solve the problem. The fact of the matter is tube amplifiers sound like total dung if driven that hard and actually long before you reach full load. In the grand scheme of things for most normal listener playing music at reasonable levels you use about a 1/4 of an amplifiers full load power. You might reach above that for some hard transients but those usually only last for a wink of your eye. If you are using more than that you should buy a more powerful amplifier. Tube amplifiers perform and sound there best at the lower half of there maximum output where (if designed properly) make the least distortion. Headroom is key my friends.... stay the heck away from the maximum output of your tube amp will net you the best sound.

You can test of your amp is running into problems fairly easily while playing music. Simply test the B+ output at the rectifier and see if it holds steady. If it starts to sag drastically then you are indeed taxing the power supplies ability to keep up.

I bet 99% of the users out there with 5AR4 rectifiers in an ST-70 never experience any voltage sag while listening to music in the real world. If you want to see at what sound level the rectifier sag starts I suggest some ear plugs ;)
 
All points are appropriate! No doubt that a single rectifier tube may serve the needs of many with this amplifier. But equally valid is the fact that the likes of Fisher, Scott, Altec, Eico, and Pilot (to name a few) all felt the need to use dual rectifier tubes to provide adequate capacity for their early stereo offerings of similar or even less advertised power output levels, with none of these pieces being particularly known for eating rectifier tubes.

Its a beautiful amp you have, and I'm sure you enjoy it immensely!

Dave
 
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