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Another Eico ST 70 thread

Is the oscillation regular?,is it once a second,faster?,slower? can you see the speaker cone moving in and out? I'm trying to see if it's feedback or drift.
You could try disconnecting the feedback loop to see if that is related.

Amp design is a little beyond my expertise(just ask my Citation II) I have to get back to that when time permits.(long story)
 
Maybe I can give it a shot tomorrow, Promised my wife i'd do something for her. I tried an experiment . I pulled 6sn7 on that channel and I could bias fine sooooooo? something down that path. Thanks guys for sticking with me
 
suggests feedback or low frequencies from earlier sections. I have to go also,
try different inputs,disconnecting feedback loop.

Later,
 
Scott you are correct . Lifted NFB lead on problematic channel and bias is stable. I'm not sure where to go from here.
 
Upon looking through my work I found ,(what I believe) to be an unrelated mistake but i'll correct either way before I change anything else. From the grids of the 6sn7 (pin 1 & 4) tied together with a 1M resistor then to ground with a .47uf cap. It seems I used a .047uf cap. I don't have them. I ordered some today. Not sure what that would affect. But it wouldn't explain why one channel is good and one bad, unless the circuit is so balanced that a slight difference in the 2 is enough to push it over.
 
if you are using the schematic from post 99,without the ccs (I think) that would be a problem,do you have a resistor to ground from the cathodes of the 6SN7?

18K 1w on original schematic .25 uf for the grid to ground.
 
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You are correct, I have an 18k 1w to ground from pin 3 and 6 tied together, just like the original Eico drwg. I am using (or at least I thought I was ) the .47uf for the grid to ground, which in fact turned out to be .047 (at least part of my problem, my post 106). Dave G said that could be causing my issue. I had nothing to approx. that value in my parts bins. So I went to R.S. this afternoon and picked up 2 to 2 packs of .22uf Which I put in parallel with the .047 on each side (I did order .47s), so now I have about .487uf in that position and I noted a couple of things.
First the low level hum is gone.
While hiss is still present its lower, it wasn't much to begin with (its interesting that as you turn the level up (no source) the hiss dimishes to nothing by midway. I'm sure someone could explain that
Last it corrected bias issue substantially. If I hadn't seen it rock solid at first I would think its right now. Fluctuation is now with .01 vdc which is probably acceptable I still there is a little oscillation going on because without the feedback its rock solid.
 
I think you're right . I'd bet Jon Y is the Jonathon Yaeger who I did the build from (the schematic of the amp and the PS). I think I have the hum beat, that cap issue. I even have to say the hiss is super low (volume at zero) and it fades out as you turn volume up (no source, i'm not just talking signal overpowering it) I think i'm pretty close. When I get the .47uf I'll swap them in and see where I am. I think i'm pretty close now (if I didn't screw up so much this would be easier. I like to think I have better focus, just a lot going on here 15 hours a week commuting for work on top of work.getting up at 3:45 every day, not at my best when I get half an hour 45 min at the end of the day. (that's my story and i'm stickin' to it)
 
ok I thought I was good, Lost all bass in one channel, bias was good , I tried removing caps I added but still no bass in that channel. tomorrow is another day. Wish I had a scope and a tone generator. Maybe I need a day or two away from this one.
 
Sometimes taking a break is the best thing to do,A fresh set of eyes will do wonders.

I make beer and soda cans(industrial mechanic)
 
Ok I'm back in the game. I replaced the .047uf caps with the proper value .47uf caps. I can bias to a point (maybe i'm nit picking) left to right test points with face of amp facing me .374-.381, .371-.381, .370-.380, .371-.380 (those are fluctuations i'm seeing on digital meter. I'm sure if I pull 12ax7 of common amp or the problematic channel 6sn7 from before the bias becomes rock steady with no fluctuation). Very close. I think there is still a minor oscillation which is controlled better now that the caps were replaced. I have bass on both channels. I think there's a dead spot in one channel of the level ( volume)control. At very low volume it seems I don't have one channel or its much lower. Slightly up they're even , I think that was throwing me off. It also seems like it has a lot of gain. (a lot of volume (loudness, High audio level?)) early on in the the rotation in the volume control. I'm pleased its functioning close now. I would like to put the oscillation issue to bed. I don't know if it would make a difference, I was going to add a 5pf cap to the 120pf cap in the feedback circuit at the otf tap (its on order). Where do I go from here?
 
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Scott, Thanks for the links.
I don't know whether its because amp has warmed for a while or (what I think) I was rolling 12AX7s in the AF position and the tone positions but bias has locked up to within .002 volts of rock solid which to me is dead on sometimes it doesn't fluctuate at all. I have to look at the loudness article and see if I have the pieces. Loudness is next on the agenda . Jon has the first of the EFB modules in the pipeline. I'm on line for mine. At the risk of jinxing myself , I think I am now where I thought I was before (if that makes any sense. )It seems to be working well and could use some burn in time and some house keeping. Just for the record, I replaced the 120pf caps with higher voltage ratings (at the OTF tap , feedback circuit). Also after post 115 in hopes of quieting bias power I switched the electrolytics back to the values in Jon's supply which I changed to Eico spec when I was having problems getting the bias working properly (which didn't solve my problem then, but we covered that) but I left it then , until now.
 
Trying to get a grasp on Loudness control mod (fix). I read thread of link Scott sent. I'm posting portion of schematic effected, Drawing of switch as it exists and as I understand it should be when I'm done (sorry about quality of drawing no cad or drafting skills), and page describing changes.
 

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According to the text all you do is add the 2 jumpers and you are done.At least if they are wrong,it is easy to undo.

If I understand it correctly it would look like this on the schematic:
 

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