Time Delay Relay Installation - ST-120

Greetings all. Its been some time since I've posted on the forum = all has been well until recently.

I switched from using a GZ34 tube rectifier to a Weber WZ68 a couple of years ago after having problems blowing bias resistors and damaging my KT88 output tubes. After switching to the Weber WZ68 rectifier and bumping up to KT120 output tubes I have not had any issues until recently. My power started cutting out and it turned out that the Weber WZ68 was fried - literally. I looked inside the WZ68 and the ceramic cap was toast!

So I started inquiring about replacing the Weber rectifier. As it turns out Ted Weber actually recommends using the WS1 copper rectifier for this amp - Bob? The WS1 doesn't have the two ceramic resistors, considerably less voltage drop and supposedly doesn't get as hot as the WZ68. Regardless, I decided to stick with the WZ68 because that is what Bob recommends.

I also decided (not sure why I waited on this) that I should have the time delay relay installed WHICH HAS MORE TO DO WITH THE REASON FOR THIS POST.

So I ordered the time delay relay (TDR) kit form from Roy and got it put together - that was a fun little project. When it came to installing the TDR I ran into some challenges.

BOB -all the photos of the TDR are of the old yellow board with no markings. The instructions and photos are all well and correct but the white writing and markings on the new green boards are a bit confusing when it comes to connecting the HV and SW red wires from the XFMR and rectifier. Please note that the HV and SW markings on the green board show the HV hole in the middle on one side of the relay and at the top on the other side when in fact the HV wires should go in the middle holes on both sides as shown on the old yellow board illustrations and the directions Roy provides.

I went with my gut on this, after looking at the traces on the bottom-side of the board, and all checked out OK including the voltages.

I'm up and running and a happy camper once again. I guess I should expect to replace the Weber WZ68 again in another couple years of hard use. Or may I should try the WS1 copper rectifier????? Bob?

John

While I simply prefer a tube in the rectifier hole for many reasons, I have at times quite successfully used a WS-1 without problems in my ST-120, and still do, as the first one I bought is quite alive. With the TDR, there's technically no reason the WS-1 should be a problem. It indeed runs much cooler than the WZ68. I've melted a couple of those too....
BUT - (isn't there always a BUT?) you will be increasing your B+ voltage by a good bit - seems to me mine runs literally 20-30 volts higher. And bias needs to be carefully regulated too, as it will be equally higher.
Seems to me if your house AC runs a little low like mine (always between 116 and 118), this increased B+ may actually give you a nice ballsy presence without compromising your output tube longevity. In other words, carefully check your B+ on the driver board test point. Target is 370 to 410 VDC. I suspect that above 400 is pushing the envelope-thingie.

Thanks Kentley and Bob.

Kentley - My line voltage runs high about 121 VAC so I use a variac set to about 118 VAC. I'm gonna try using a SW1.

Bob - I agree the TDR is easy to build and install and the instructions and schematic are clear. It wouldn't hurt to add a note letting the builder know that the HV/SW markings on the green board are misleading. I don't know how to post an image so I will email you and Roy a photo of the board. The SW/HV markings are clearly misleading.

Probably not a big deal for the experienced. If you follow the directions and schematic that Roy provides you will wire it up correctly. I looked at the traces on the underside of the board and all things considered was pretty confident that the HV holes were the second holes down on both sides of the relay but I was a little confused by the markings on the board. Thanks for acknowledging I wasn't totally off on this Bob. Keep up the good work and great support.

John