Another Eico ST 70 thread

[nj pheonix]

Thanks Dave and Scott. I found ac balance adjust instructions. They're part of Tronola article. I didn't have a scope or signal generator before so I kind of turned a blind eye to them before. I do have a little question on the instructions. I understand the hook up and the voltages. Where its saying set for minimum distortion, is this a visual read on the scope waveform or do you need a distortion meter. My take is just the scope image. If not I'll set with a voltmeter. Sorry I missed last question .(aux inputs)

 

[dcgillespie]

Steve's thought was to use a THD test set to make the adjustment.

Dave

 

[nj pheonix]

Thanks Dave, I'll go the meter route. I can't justify a thd set

 

[nj pheonix]

Just an observation. My son had been away and in keeping with the mad scientist credo. I felt the need to wow him with some waveforms. I noticed down around 25hz it seemed output tubes growing brighter. (Just the heaters not red plating also one of the pair of the driven channel 7591 seemed to be oscillating. Because I'm not sure I'm getting a real square wave at this freq. (Even direct from SIG gen to scope. I'm not sure what amp is seeing and what its trying to do. The fact that the tubes seemed brighter makes me Leary and for now I'll stay away from lowest freqs
In next couple of days (weekend looks crazy and its not likely I'll get to then) I'm going to try to test in sections. 1 channel at a time. Pre., Tone, Amp
Maybe well find something telling. I'll keep notes comparing channels (one looked better than other). In any event, excellent learning experience. I'm thinking about buying a Tek Rag 101 ( its a cheap chinese wein bridge. But decently spoke of) and replacing SIG gen. I fear the gen is part of the problem.

 

[dcgillespie]

What you saw was almost certainly the screen grids glowing.

As designed in the Eico ST-70, the screen grids in the output tubes are already operating at absolute maximum dissipation ratings at high power output levels for normal sine wave (i.e. speech and music) signals.

With a square wave signal however, the on time duty cycle for the tubes is much greater than with sine waves, causing the tubes (and therefore, the screens as well) to draw significantly more average current. At high power output levels then, the combination of increased average current flow on top of design conditions that already operate the screen grids at absolute maximum ratings under normal high power conditions caused the screens to overheat and glow.

High power output testing using square waves can certainly be done, but such tests must be mindful of the extreme stress this can place the amplifier under in doing so.

Dave

 

[s-petersen]

Since you seem to have a pretty flat amplifier(with the 10k sig)You can see if the tone controls affect the the imbalance you are observing at the low(25hz) and high(20Khz)to get the best and balanced and flat response.to center your tone controls.I don't remember if the knobs are splined or keyed on the shaft,if they are splined,the simple way would be to pull the knobs off, adjust the controls for flat,and reinstall the knobs to indicate flat.

 

[nj pheonix]

Thanks Scott. Its an oddball control. Concentric shafts . Middle small half moon. Outside something like 2 Cs facing each other. The ST 84 link you gave me had some instruction on adding resistance between the wiper and the side terminals to center. Let me try and get the bugs out and I'll give it a shot. I'm surprised how flat it looks centered now. At least the high freqs. I'll run the tests to see where issues are. Try and get the two channels matched and flat as low as reasonably possible.

 

[nj pheonix]

I may have discovered something valuable. I set ac balance pots (I regret now not putting in multi turn pots, very tough to zero). Let me digress a second. I thought it interesting you could audibly hear tone with purely resistive load. What am I actually hearing? Back to the point. I noticed tone was most quiet when ac balance pots zeroed. That being said , I rechecked waveforms as yesterday. Both channels evened out. I never checked how equal balance was between channels . As it turns out they're nearly dead on. I have to admit when checking at 100hz and 30 hz I had bass set maybe 1/3 of the way between center and +1. This is what it looks like. 20 hz is still pretty ugly with slopes to the middle from the peaks at left. No fluctuations like yesterday.

 

[nj pheonix]

the others

 

[s-petersen]

Those wave forms look very good AFAIK If you were hearing noise/tone while playing with just the resistor,you were likely hearing the output transformers"sing" I could hear it in the amp I was just experimenting on.
On mine it was most apparent around 2-3k