Hello everyone. I found a pair of unmolested Mark IIIs and my plan is to keep the original circuit and just modernize components (resistors, caps, selenium rectifier to a diode).
It has been few years since I did this with two Stereo 70s. I'm looking for feedback on a few items. I don't want to create a huge debate about original circuit and improved circuits or this cap brand / that cap brand.
Questions:
Thanks in advance.
It has been few years since I did this with two Stereo 70s. I'm looking for feedback on a few items. I don't want to create a huge debate about original circuit and improved circuits or this cap brand / that cap brand.
Questions:
- Diodes across rectifier tube. Yes / No These will not be used daily and I did not do this on my ST70s.
- Feedback wiring. Leave as is or tap from the 4 ohm wire since I will be using the 4 ohm connection with my speakers
- Adding 100ohm 1/4 watt resistors (or was it 10 ohm) to the output tubes (pin 4) to the transformer. I know in the past, this was talked about as simple protection.
Thanks in advance.
1/ "Yellow sheet mod" - yes do it. It protects the GZ34 fom most abuse
2/ leave on the 16 ohm tap.
3/ screen stoppers , 100 ohn is a reasonable value. This protect the power tubes at no cost, especially if the B+ is increased.
4/ replace the Se rectifier with 1n4001 diode + replace the 2 50uF caps ( i think it is mentioned above)
5/ replace the can cap with a similar
2/ leave on the 16 ohm tap.
3/ screen stoppers , 100 ohn is a reasonable value. This protect the power tubes at no cost, especially if the B+ is increased.
4/ replace the Se rectifier with 1n4001 diode + replace the 2 50uF caps ( i think it is mentioned above)
5/ replace the can cap with a similar
To repeat, the diodes go in series with each rectifier section (on the winding side), not "across" it.
When replacing the selenium rectifier, make certain the diode is in the right way.
The cathode (banded end) goes to the winding tap. The anode goes to the negative capacitor lead.
This is a negative voltage grid bias supply, with the positive end grounded.
When replacing the selenium rectifier, make certain the diode is in the right way.
The cathode (banded end) goes to the winding tap. The anode goes to the negative capacitor lead.
This is a negative voltage grid bias supply, with the positive end grounded.
I've rebuilt and upgraded dozens of these (and ST-70, ST-35, etc.) for folks over the past 50 years...
For a simple rebuild:
The options are endless, but the above is what I've done for many of them and all have been running for many years without issue.
For a simple rebuild:
- PCB resistors, replace with Dale/Vishay RN60/70 1% (hopefully you have the newer glass epoxy PCBs)
- PCB capacitors, replace main 3 with Solen 630V PP film, small pF values with silver mica
- replace the quad cap with (perhaps) the Authenticap (still a quad cap, but much better quality), and has a higher voltage rating
- better wire and connectors... and tube sockets if you can get them
- upgrade BIAS supply with 75uF caps, film resistor resistors, new potentiometer, I use a 1N4007, as I have these in bulk from the 80's
- replace the remaining power supply resistors with Dale RS series, which can be found in 1% values
- personal note: I've never diodes with the 5AR4/GZ34 (or any other) rectifier, 1000's of Dyna amps (and many other brands) have survived a half century and more without them
The options are endless, but the above is what I've done for many of them and all have been running for many years without issue.
Old 5AR4s are fine (my 1970s Mullard is still going strong), but new ones are iffy and need the diodes.
I suppose I'm a bit more fortunate... being old and having collected vintage tubes for decades... all of my rectifiers are NOS, mostly Mullard, Amperex, RCA, Tung-Sol, GE and such... across 5Y3, 5R4, 5V4, GZ34, EZ81, etc. So... I've never had any issues with them.
The 1N4001 is only rated for 50 volts so you need at least the 1N4002, rated at 100 volts. I typically use the more common 1N4004 for these selenium diode replacements, which is rated at 400 volts.