• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Dynaco ST120 rectifier

calman46

calman46

AK Subscriber
Subscriber
I have Bob's ST 120 kit (6550 quad) driving the top end of a pair of Von Schweikert VR4 gen II . Its about 2 years old and the Chinese 5AR4 went . I've been reading about rectifier tubes for this amp so I need some recommendations . I'm considering the GZ34/5AR4 new Mullard . What do you think ?
 
Register to hide this ad
Get the Sovtek GZ34/5AR4 instead. Basically the same tube and at 1/2 the cost or less. I'm running a Sovtek in my TA-600 and then in my X-101-B for 5 years now, and it's a good tube. The new Mullard is a Russian tube and isn't any better than the SOVTEK. Both made in the same plant. If you want a Mullard, go for OLD STOCK, not Russian.
 
I've used the Sovtek 5AR4/GZ34 in Scott gear with no issues, but no experience with Dynaco. Agree the ST120 with 6650's would probably want more than the 5AR4 could give over the haul.
 
Shadowdog said:
http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod
Click to expand...
This is a good mod, putting SS diodes in series with the tube rectifier to avoid catastrophic failure if the tube allows backwards current flow.

Just to avoid any confusion... somebody else in this thread recommended "going solid state" and this mod is not a replacement for a tube rectifier, only added protection to be used with a tube rectifier. Soldering in diodes to replace one would look different, or you could buy an octal-base SS device which plugs right into the 5AR4's socket.
 
I found this...
On another web site it has been suggested a better solution is to use one forward conducting diode on the rectified output of the GZ34 (inserted between GZ34 pin 8 and the first filter can cap). Doing so eliminates potential diode switching hash noise from a pair on the transformer output. The single diode on the output is always conducting in normal operation. And in the same way blocks reverse current arcing on AC power interrupt short cycles. I'm about to start a new ST-120 build and will probably try the single diode approach for starters. I have some nice beefy 3A 1000V diodes (UF5408) that should do the job. At a current of 400mA the Vf (forward voltage drop) across the diode is about 1.0V so the power dissipation is 0.4 watt worst case. Should be no sweat for this device. So I'll probably install the diode on the GZ34 pin 8 (anode) to pin 7 (cathode band) and then attach the filter can wire to pin 7. Scott
So the "going solid state" mod is just for protection when the tube fails or does this mod prolong the rectifier tube ?
Then there is this the SS option ...
https://www.tubesandmore.com/products/solid-state-rectifier-5ar4-5u4-5y3-tubes or maybe the Webber WS1vs WZ68 is a better choice and add Bobs delay circuit or a stand by switch .
It seems to me that the smart money is on complete SS rectification. The GZ34/5AR4 tube is over worked in this amp . I need to do my homework on the SS mod.
I also want to make checking the bias better . I'm also thinking on making a meter plug/ socket to check and set the bias "hands free". A switch on it would nice to change what pin is connected to the meter .

Tube or SS rectifier that is the question .













I
 
Last edited:
I built and installed the delay circuit and I am using a WZ68 Copper Cap rectifier that I have modified with a number of ventilation holes on my VTA ST120. Works great. Previously, I had lost several GZ34 rectifiers. Both of these combined run about half the cost of a NOS Mullard.
 
eickmewg said:
I built and installed the delay circuit and I am using a WZ68 Copper Cap rectifier that I have modified with a number of ventilation holes on my VTA ST120. Works great. Previously, I had lost several GZ34 rectifiers. Both of these combined run about half the cost of a NOS Mullard.
Click to expand...
Ive been thinking about this with my VTA ST-70. Maybe use a thermistor also. Would the WZ68 work with the ST-70? No problems with the GZ34, yet though.
 
On the Webber site he recommends the WS1 for stereos etc. as there are no resistors in it ,so no extra heat and no "SAG". Webber can add a thermistor to the WS1 for a" few seconds of warm time". I don't think a few seconds is enough , Bobs board is defaulted to 17sec. I'm looing into the WS1 with TDR kit.
 
calman46 said:
On the Webber site he recommends the WS1 for stereos etc. as there are no resistors in it ,so no extra heat and no "SAG". Webber can add a thermistor to the WS1 for a" few seconds of warm time". I don't think a few seconds is enough , Bobs board is defaulted to 17sec. I'm looing into the WS1 with TDR kit.
Click to expand...
I was just looking at the WS1. If I try this the TDR kit will be added for sure.
 
The relay kit came and looking at it , I'm thinking about mounting the red &green power LEDS inside the unused pin holes of the power out sockets . So they will be visible . Anybody foresee any problems with this ?
 
I can see the reflection of both the red and green led's through the ventilation slot at the rear of the rectifier socket between the left output and the power transformers. That is enough for me. Besides, you can hear the relay click when it sends the high voltage on to the rectifier socket.
 
Last edited:
Bob asserts that "The two tube heater legs are rated for 5 amps each instead of 3 amps each..." This is, of course, the 6.3V heaters, but if he increased the amperage of the 6.3V heaters it is also possible he increased the amperage of the 5V heater too. He would have to confirm this. So, if the 5V heater can handle 3A then you have a first class rectifier tube available to you for about $10 each in huge supply. I speak of the 5V3A. I see these all the time on epay for around $10 and they are the most rugged rectifier you can get. I standardized all my builds to use the 5V3A.
 
At 415ma steady-state output, that would certainly be an upgrade over the 5AR4 and its 225ma steady-state output rating. Up to 6.6 amp transient peaks too, which is pretty stout. The problem you have is a drop in output voltage. Off the top of my head I want to say a 5AR4 is listed for 17 volts or so, this one shows 42 volts at 350 ma per plate.
 
gadget73 said:
At 415ma steady-state output, that would certainly be an upgrade over the 5AR4 and its 225ma steady-state output rating. Up to 6.6 amp transient peaks too, which is pretty stout. The problem you have is a drop in output voltage. Off the top of my head I want to say a 5AR4 is listed for 17 volts or so, this one shows 42 volts at 350 ma per plate.
Click to expand...
There is always a catch, eh? ;)
 
You must log in or register to reply here.
Back
Top Bottom