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Another Eico ST 70 thread

Ok I didn't realize it had internal calibration (duh) I am surprised I had to sync it. Oddly enough the external generator had less noise than the internal one. I'd say it works right once I switched to DC coupling. I'll have to move back to bench and open. Then I need a game plan. Tomorrow , maybe the weekend. Thanks. The noisier image is the internal The quieter the external
 

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I am getting to the edge of my knowledge at this point,I have seen a thread about tuning a Scott integrated for best frequency response,but I can't seem to find it,What I have now are only an educated guesses,I hope Dave G. pops in and corrects my info.
As a start I would check that the amp is flat.
Connect the sig gen to pin 7 or 2 on V5(12AX7) You will need a 8 ohm,10 watt resistor(at least) on each output,connect the scope across the resistor.
The schematic doesen't specify which channel is left or right.

I am not sure if there will be a grounding problem,as the common is not ground on this unit,and if the sig gen is grounded,there may be feedback.
I know there is a mod to make the commons a ground by grounding the common and changing the feedback loop, but I have not really studied it.

You would set the sig gen to 100Hz sine wave and adjust the voltage untill you get 2.8v RMS across the resistor(1 Watt) (I use 100 hz sine because my dvm is not accurate at higher frequencies) ,then switch to 10khz square wave and view the output. Hopefully you have a decent looking square wave.
 
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Thanks Scott, The limit of my knowledge hasn't been in sight for a while so thanks for getting me this far. I already made a mod as part of the project which made ground common and moved the feedback (to 16 ohm tap) so that's not an issue here. Thanks again.
 
NJ If you ended up using the schematic as posted back in post #19, that is a modification I came up with years ago to address not only the (un) common ground system, but also a less than stellar HF stability performance from the stock design.

The stock design always had the feedback coming from the 16 ohm tap, and the modification leaves it there, but allows for grounding the Common output lead as opposed to the 4 ohm tap of the original design, with the feedback level of the new configuration being corrected to the original amount by way of replacing a 100 ohm resistor in the cathode circuit of the 12AX7 AF amplifier stage with a 47 ohm unit. The HF stability of the design is improved significantly with the addition of the step network shown in the plate circuit of the same tube.

If you measure the response of this modified circuit, you should find that it will be ruler flat within the 20- 20 kHz audio band, and in fact, flat up to the 70-80 kHz range with in +/- .5 db, with excellent stability.

Dave
 
OK I built to # 19 post except no CCS. I used stock 18k value r. So (again doing this with free minute on the phone which makes it hard to see and type). I'll look at that link and see if it makes sense. Thanks Scott and Dave
 
Funny I was following Kevin's post from the beginning
I never made the connection. What would be a good input voltage? Approx what would be output voltage at speaker terminals? Is there a standard set of test frequencies?
 
Your input voltage is determined by your output voltage,you are trying to get 1 watt at the output (2.8v@8ohm)a 1k & 10k signals are good frequencies
(square wave)
 
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I'm not clear on that. I know my signal generator signal can be set to a specific voltage. I was thinking somewhere between .5-2v (for some reason 1.5-2v stick in my head for input signal levels. Maybe testing might be different). I suspect at some point you can overload your input. Then I assumed the gain would determine the output voltage. The frequency is clear. I'm sure before I'm done I'll check more . I just figured if I hit a few choice ones early I would pick up anything glaring, before I went further
 
You adjust the input so that your output equals 1 watt, so that you have no chance of overloading.with the volume control all the way up the input may only be .5v depending on the gain of the whole amp,at the input of the 12AX7 the input may be different depending of the gain on the power amp section.
 
Actually I made a mistake,the output should be 2.8v for 1 watt @ 8ohms I don't know how I made that mistake,but I did. 8v would be 8 watts
 
ok still on the learning curve.There are two places on scope to set for DC coupling. One of my meters doesn't read voltage well above about 10khz. I cant get a clean square wave at 20hz generator to scope. All that being said I have learned some things and open other questions to why. Lets say at 70hz definite slope towards zero from both peaks which gets more pronounced as the frequency drops. At 20 hz horizontal peaks look like worms squiggling. no fixed line or curve at peaks.
That being said pretty flat 20khz, 10khz,1khz 500hz, 100hz. Not sure what happening at bottom.
 
partial
 

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rest down to 1khz. obviously one channel looks better than other. in the next day or so i'll try 100 and under again . sorry I can't shoot video of moving worm at 20hz
 

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All I know now is channel 2 looks better than channel 1 and I have some low end issue. I did set bias and balance before test and it had reasonable time to warm up.? 2.8v output at 8 ohms (assuming my meter is accurate at the different frequencies), tone controls centered
 
While the feedback caps and/or step networks could be slightly tweaked a bit to limit the amount of overshoot, the 10 kHz square waves otherwise look very good. Even the characteristics of the same model transformer can vary somewhat from unit to unit, and more particularly from run to run, and even more particularly from one vender to another. None the less, they look very good otherwise. I wouldn't adjust anything unless the frequency response sweep showed otherwise.

Two other points: (1) Testing at 20 kHz only points up the falling response characteristic of the OPT, as testing at that frequency really requires an OPT of much greater response than most mortal made transformers possess. For this reason, 10 kHz is generally considered the standard test frequency, testing the response of the circuit to at least 100 kHz, which is typically plenty good enough for vacuum tube work. (2) It is generally best to first test the response of the power amplifier section and control section separately. Otherwise, you have no idea where a response issue might reside. Therefore, check them individually first -- then you can check them as a complete audio unit to check for overall response.

Dave
 
before I drop out I have a couple of questions. Is there any issue to connecting tone generator to both channels at once (input). Is there any interreaction? I just nicked a small section of leads from 8 ohm tap to my load resistors to connect scope and vtvm . Is there a better (neater) way to do it? Right now i'm just sticking a piece of insulated wire with the ends skinned to the middle of the female bncs of the sync jacks. I ordered a male to male bnc jumper. That's all there is to it right? would an analog meter read voltage better at the high and low and frequencies?
 
Last item I have the 10k (I believe they are ac balance pots) on the 6SN7s attached to 2 27k resistors with the wiper attached to "U" of the power supply (see post # 19) I has forgotten about them. They were set by eye to center but I imagine there is a set up procedure to them? (I don't believe it has much (if any) to do with present situation, but still they should be set).
 
Are the traces from pin 2 and 7 of the 12AX7 or the AUX inputs?
As Dave said,10k looks great!
The AC balance can certainly affect the scope traces.I'm not sure of the procedure to set it on the Eico,But on the Citation II you set DC balance with no signal,then put a 60hz signal through the amp and adjust the AC balance for equal current from a pair of tubes
 
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