ST-120 Rectifier Options?

I have an ST-120 on preorder and looking at options for the rectifier circuit.  I want to use a solid-state rectifier and will install the time delay relay option. I don’t care for the look of the solid-state substitutes above the chassis.  Is it possible to just connect the heater of the rectifier tube for looks and connect two 1N4007 diodes below the chassis for the rectifier function?

Just follow the yellow sheet mod and you can use tube or solid state rectifiers without any further mods.

Just make sure if it’s a tube that you use the correct tube rectifier.

corndog71 wrote:Just follow the yellow sheet mod and you can use tube or solid state rectifiers without any further mods.

Just make sure if it’s a tube that you use the correct tube rectifier.

Thank you for your response. If you follow the Yellow Sheet Mod the rectifier tube is still in the circuit and I didn't think this would eliminate the voltage sag??

The drop in voltage from the tube will still be there and is desirable.

Whatever you plug into the socket comes after the diodes on the socket.

Sounds good. That’s what I will do.

If you are using TDR, you can eliminate the socket and just wire two diodes in, bypassing socket completely.

For reference, the Weber WS1 is just that, two diodes.... you can also add a "TDR" with a Thermistor, also a little voltage sag. The Weber WZ68 is two series diodes per side, with a delay and 36ohm dropping resistor. So the short story is with a power resistor in the 0-120ohm range you can tune the B+, to emulate tube sag, or just go straight no sag. The yellow sheet mod adds diodes, eg if applied and you use a SS rectifier you could have 2-3 series diodes per side. Just verify B+ is in line with specs.

Series diodes drop a little more voltage. Many people do this on purpose, I do not see the need nor benefit, especially if diodes are mismatched. Just more to fail. Others may have a different opinion. So if you are going SS all the way, get good high rated diodes and use one per side.

I do like the flexibility of having tube or SS. I have my TDR board installed in my ST120, but bypassed and have successfully used solid state rectification with the thermistor delay. For my M125, I use the Weber WZ68 or a JJ GZ34. All these have the yellow sheet mod with FR207 diodes, and despite my comment above about series diodes, it happens in this case.....

Just connect to the HV legs off top of my head pins 4-6 (HV) to 2 or 8 (DC) not rectifier heater. The transformer does have a heater tap for the rectifier (5V), this tap will not be used in this case. Just a thought, with the TDR and yellow sheet mod installed you could easily make a very low profile "rectifier" that is simply wire, using the yellow sheet diodes for rectification. Best of all worlds?

I think Roy should chime in here.

Correct me if I'm wrong but, the "yellow sheet mod" is done to protect the tube rectifier in the event that the amp is power cycled too quickly. The mod prevents the still charged caps from sending current back through the tube rectifier and killing it.

My ST120 uses a weber WS-1t (thermistor) and the "yellow sheet mod." I have the auto bias board. One of it's functions delays full power to the output tubes. Prior to the AB, I used a Tung-sol re-issue 5AR4 and had no issues with it.

New2tubez, that's correct. The diodes protect the tube. They're also rectifiers. When used with tubes in the yellow sheet mod, they are in series to the rectifier "diodes" in the gas. You still get tube sound, as you have sag in B+ and whatever magic happens in the gas.

I thought about my previous off the cuff response... let me offer an alternate. To the original question.

Find a plug in you like? and... are you sure you don't want tubes?

Maybe build the kit, with the TDR board installed. Try tubes, try a solid state rectifier. a JJGZ34 is cheap and solid state plug ins are cheaper.
With Solid State There is no "desirable" tube voltage sag. You get a higher B+. Which some say is good.

Listen. Enjoy. Try. If you wanna get rid of the rectifier and socket, you can absolutely once you decide.

You can certainly do the yellow sheet mod although you have no tubes to protect if you are using solid state and with the TDR board (which both delays and resets timer if cycled).

Down the line(?) you can certainly connect diodes below the hood to replace the tube and any plug in when using a TDR board. Id recommend beefy diodes like FR207. Ive torn down enough Webers, they mostly diodes. as is the yellow sheet mod. In short, in answer to your original question Yes.

I would be sure however that if there is a socket where a rectifier should go, it should work as advertised or be non functional. EG if you want to eliminate the plug in, don't use the socket in any other way than it was intended. Rather, use DIY skills and solder to create your rectifier circuit (2 diodes in place of the tube socket, tube and the yellow sheet mod) under the hood.

Its a great product fun amp to build and sounds amazing. Good luck. Hope this is helpful.

Bob

Rec wrote:Series diodes drop a little more voltage. Many people do this on purpose, I do not see the need nor benefit, especially if diodes are mismatched. Just more to fail. Others may have a different opinion. So if you are going SS all the way, get good high rated diodes and use one per side.

Diodes have a voltage drop across their P-N junction when they are forward biased. Putting multiple diodes in series simply means there  are more P-N junctions and therefore more voltage drop. It's actually (IMHO) a better way to drop a few volts than adding a resistor. And putting diodes in series INCREASES the peak reverse voltage they can withstand.

There is nothing inherently wrong with using diodes in series as long as they are properly rated for the application. In tube audio gear I use the UF5408 fast diodes; they are rated to carry an average of 3 amps of current (150 amps for very short term peaks) and have a reverse voltage spec of 1000 volts. If I need higher reverse voltage ratings I simply use two in series to give me a 2000 volt limit. As well the UF series of diodes has less switching noise than the "1Nxxxx" series of diodes. Finally, the UF5408 sells for 50 cents each or so - so skip the 1N diodes and get the UF instead.

Rec wrote:

Maybe build the kit, with the TDR board installed. Try tubes, try a solid state rectifier. a JJGZ34 is cheap and solid state plug ins are cheaper.
With Solid State There is no "desirable" tube voltage sag. You get a higher B+. Which some say is good.

Listen. Enjoy. Try. If you wanna get rid of the rectifier and socket, you can absolutely once you decide.

You can certainly do the yellow sheet mod although you have no tubes to protect if you are using solid state and with the TDR board (which both delays and resets timer if cycled).

Down the line(?) you can certainly connect diodes below the hood to replace the tube and any plug in when using a TDR board. Id recommend beefy diodes like FR207. Ive torn down enough Webers, they mostly diodes. as is the yellow sheet mod. In short, in answer to your original question Yes.

I would be sure however that if there is a socket where a rectifier should go, it should work as advertised or be non functional. EG if you want to eliminate the plug in, don't use the socket in any other way than it was intended. Rather, use DIY skills and solder to create your rectifier circuit (2 diodes in place of the tube socket, tube and the yellow sheet mod) under the hood.

Its a great product fun amp to build and sounds amazing. Good luck. Hope this is helpful.

Bob

Thanks Bob. I will install the TDR board and can bypass it to try a tube rectifier.

Jim McShane wrote:

Diodes have a voltage drop across their P-N junction when they are forward biased. Putting multiple diodes in series simply means there  are more P-N junctions and therefore more voltage drop. It's actually (IMHO) a better way to drop a few volts than adding a resistor. And putting diodes in series INCREASES the peak reverse voltage they can withstand.

There is nothing inherently wrong with using diodes in series as long as they are properly rated for the application. In tube audio gear I use the UF5408 fast diodes; they are rated to carry an average of 3 amps of current (150 amps for very short term peaks) and have a reverse voltage spec of 1000 volts. If I need higher reverse voltage ratings I simply use two in series to give me a 2000 volt limit. As well the UF series of diodes has less switching noise than the "1Nxxxx" series of diodes. Finally, the UF5408 sells for 50 cents each or so - so skip the 1N diodes and get the UF instead.

Thank you for your input. I will order some UF5408 diodes per your suggestion!

I'm agreeing with REC and Jim