Litany of ST-70 questions

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Litany of ST-70 questions

Postby kingmatt » Fri Oct 22, 2010 10:46 am

Its been about a year since I built a Triode ST 70 kit, and along with a PAS-3 that I rehabbed, its been fantastic and scary reliable. I have been tinkering with it a bit as of late, and have some nagging issues with it.

The first one is a very slight hum when anything is hooked up to the PAS other than my iPod (or nothing at all). I'm reasonably sure this is some sort of ground loop issue, as when I ran the ST off of my solid state receiver's preamp out, I got the same hum. When I have the PAS hooked up with nothing or the iPod connected to it, it is whisper quiet. The second I hook up just one channel of a device that is plugged into the wall, I get the hum, even with the volume turned all the way down. When I built the amp, I left the ground terminal disconnected in the IEC socket. The PAS has the stock 2-prong power cord without ground. I tried grounding the IEC socket in the ST to chassis and it made the hum much, much worse. It also hums even when the PAS is powered off.

My second issue is I wanted to swap out my phase splitter tube on the Triode EF86 board to a 6N6P that I got off of ebay for another project, just to see how they sound. Strangely, when I plug one into the socket, the heater doesn't come on. Do I need to rewire the heater circuit to get these tubes to work?

Also, after killing another 5AR4 rectifier tube by firing the amp up while the tubes were warm, I decided that it might be wise to add some diodes under the rectifier socket and find a bit more robust tube. I have a 5U4BG that I got with a bunch of other tubes. Can I use this with the diodes and not have any issues? If worse comes to worse I do have a Webber Copper Cap that has been faultless, but I like the look of tubes.

Thanks in advance to anyone who has any input. DIYT is a wealth of information.
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Postby antiquekid3 » Fri Oct 22, 2010 2:09 pm

I had a similar problem with a shortwave receiver a few years back. Try hooking the ground back and switching the hot and neutral wires. You might have them backwards. It certainly caused a lot of hum in the radio when I had it backwards, at least.

I put two diodes in my ST-70 and it seems to be working just fine. If you use a 5U4, you will have a greater B+ voltage, which may be a problem.

EDIT: I'm wrong about the 5U4. It has a greater voltage drop, thus lowering the B+ voltage.

Kyle
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Postby battradio » Fri Oct 22, 2010 3:29 pm

antiquekid3 wrote:. If you use a 5U4, you will have a greater B+ voltage, which may be a problem.

Kyle


Hi Kyle , there is greater voltage drop with a 5U4GB than a 5AR4 , so the B+ will be lower
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Postby Shannon Parks » Fri Oct 22, 2010 6:13 pm

Were you grounding the chassis with the safety ground at first when this problem occurred? I.e. I've never had luck running unbalanced amps with safety grounds.

I would suggest first using an inrush current limiter on the primary side of your transformer before moving away from a stock 5AR4. Sure, tube rectifiers die eventually, but something like a GE CL120 will be a boon.

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Postby kingmatt » Fri Oct 22, 2010 11:10 pm

First off, I'd like to thank everyone for their replies.

First, to answer Shannons's question, I never connected the ground lug of the IEC connector and still got hum. Thinking that may be an issue, I attached a wire from there to a chassis ground point and the hum got much much worse. So I disconnected it.

How would I connect the inrush limiter? From the IEC socket to the input side of the transformer? I'm still learning about this and apologize in advance for my density when it comes to electronic stuff.

Would the lower B+ cause performance issues? I also read somewhere they use slightly more heater current. Does the Triode PA-060 have the stones to drive it?

Anyone have any experience with running ECC99/6N6P tubes? I'm kind of flummoxed by the heaters not coming on. I can't imagine I've gotten 4 bad ones.
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Postby Shannon Parks » Sat Oct 23, 2010 4:53 am

kingmatt wrote:How would I connect the inrush limiter? From the IEC socket to the input side of the transformer? I'm still learning about this and apologize in advance for my density when it comes to electronic stuff.


You are correct. ;)

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Postby burnedfingers » Sat Oct 23, 2010 4:54 am

quote:
Also, after killing another 5AR4 rectifier tube by firing the amp up while the tubes were warm,


I've done this hundreds of times with my Dynaco's and Quicksilver amps and NEVER lost a 5AR4. Put diodes in series with the AC feeding the 5AR4 and you will not loose another one. No need to use a different rectifier tube. I would mention to stay away from chinese and JJ 5AR4's. The Sovtek brand hold up well.

My Quicksilver KT88's and 8417 amps used to have problems will rectifier life but since installing diodes in the power supply I have used them for years without loosing another rectifier tube. The Quicksilvers happen to have a large filter cap(above design limits) that seems to raise hell with rectifier life. Note* These amps have dual 5AR4's.
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Postby antiquekid3 » Sat Oct 23, 2010 3:21 pm

Oh, that's right. So I guess the only downside is a faster conduction time with the 5U4 in the case of an ST-70?

Kyle

battradio wrote:
antiquekid3 wrote:. If you use a 5U4, you will have a greater B+ voltage, which may be a problem.

Kyle


Hi Kyle , there is greater voltage drop with a 5U4GB than a 5AR4 , so the B+ will be lower
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