Audio Asylum Thread Printer

Get a view of an entire thread on one page

FAQ / News / Events

Tubes Asylum

Newbee ST-70 questions

128.218.5.86

[ Follow Ups ] [ Post Followup ] [ Thread: Display Email Next ] [ Tubes Asylum ]

Hi all,
I'm just getting into tubes, and having a great time listening to my ST70 and building a bottlehead Foreplay. I have a couple questions that I would like to get answers to, if anyone here would be so inclined.
The other day, I had one of the power tubes on the ST70 go cherry red, and a buzz started to come out of the corresponding speaker. The unit was not playing, it had been on for about 20 minutes to warm up and I just hadn't put anything on yet. It was plugged into an old Parasound SS line amp (which, by the way, has had some grounding problems in the past, if that might make any difference here). I unplugged everything, and other than plugging the ST70 directly into the surge protector, I've done nothing to correct the problem, and the unit has worked fine since. (Bias tests right on.) Reading through the archives, I've seen that the "cherry" is sometimes caused by a failing coupling cap, and I'm set to order the bias repair kit from Triode Electronics, but I'm wondering: can I damage the amp by running it in the meantime?
Also, since I'm ordering the kit, I might order some extra tubes to experiment with. I've got National EL34s in there now. Any recommendations on others to try?
Thanks so much for whatever advice you can provide!

Follow Ups:

Thanks, I appreciate the advice! (nt) - M. Lucky 09:12:51 01/29/04 (0)
nt
Re: Newbee ST-70 questions - Steve O 16:34:59 01/28/04 (0)
You queried: "...but I'm wondering: can I damage the amp by running it in the meantime?"
The answer is yes and no. Red plate output tubes are an indication that something is seriously wrong. Although there are numerous causes, some already discussed, the net result is that the glowing tube(s) is drawing far too much current. This condition may very quickly damage the glowing tube and may also damage the rectifier, OPT and PT if allowed to continue.
If you feel compelled to operate the amp w/o determining and fixing the cause, as long as there are no glowing tubes or popped fuses you're probably OK doing this. You must literally keep an eye on the amp when powered up so that you can shut it down if the problem occurs again. Do not do an unattended warm-up.
If this were my amp (even considering that it's just an ST70), I would wait and get the new parts and be sure that it's fixed before anything other than diagnostic operation.
Re: Newbee ST-70 questions - Russ57 13:11:30 01/28/04 (0)
The easy question first. For EL34's I would buy new production and save my money. I like the one's by EH as they have bit more top end energy but many like Svetlana. They are both gret tubes for the money.
When an EL34 glows red in your amp it's because the tube is passing too much current. To control current in a tube you need to make the grid negative with respect to the cathode. This is done in your amp by grounding the cathode and applying a negative voltage to the grid from the bias supply. The grid is also connected to the phase splitter through a coupling cap. SO, if the bias supply fails...or a coupling cap allows DC voltage to leak through, either can cause that condition. But far and away the most common reason is a poor connection between the tube and tube socket. That is why the problem is often cured by reseating the tube or often seems to correct itself.

Russ

This post is made possible by the generous support of people like you and our sponsors:

You can not post to an archived thread.