Hello again! I did a little poking around on the subject but could not find anything satisfying. So for a quick background, novice here. I bought a mystery Eico ST 70, slowly recapped with all new electrolytic and film caps. New and tested good output tubes all other tubes were tested go/no go on my tube tester. First service steps in the manual say to set balance on channel 1, then bias, then balance on c2 and then bias. Simple enough but when I probe for both balance and bias I get erratic readings. For example, bias should be 0.38 and my meter reads easily between .2 and .4 for each channel. For balance, it says it should be 0V or close to it, it is jumping between -0.1 and .3.
One one hand I could say maybe the bias and balance pots are bad or just dirty, corroded from age, but all 4? I have no issues getting new ones but just wanted to poll the community on any other potential things I should take a look at?
Thank you!
One one hand I could say maybe the bias and balance pots are bad or just dirty, corroded from age, but all 4? I have no issues getting new ones but just wanted to poll the community on any other potential things I should take a look at?
Thank you!
The bias must be undertaken under no signal conditions.
Could be dirty pre sets but a twiddle usually sorts that out.
The bias cannot go into negative current! -0.1?
Could be dirty pre sets but a twiddle usually sorts that out.
The bias cannot go into negative current! -0.1?
Some amplifier engineers used fixed adjustable bias: Higher power output, Lower distortion, No bias shift with sustained 30 foot organ pipe notes.
*&#$@! Darn those complex and interdependant adjustments!
Other amplifier engineers used Individual Self Bias for each output tube.
Simple, reliable, no adjustments required.
Both amplifier topologies can sound excellent.
. . . Nothing more to say.
*&#$@! Darn those complex and interdependant adjustments!
Other amplifier engineers used Individual Self Bias for each output tube.
Simple, reliable, no adjustments required.
Both amplifier topologies can sound excellent.
. . . Nothing more to say.
That negative reading was the balance the bias never went into negative @JonSnell Electronic
I have not checked grid voltage @euro21 what do you recommend I am looking for?
I have not checked grid voltage @euro21 what do you recommend I am looking for?
First of all, short the power amp part inputs (or remove ECC83, 6SN7 tubes).
Check the -21V existence on balance potentiometer slider pin.
If it exist, check the -17V on each power tube grid (pin 6).
If its exists and the voltage is stable, but in TP point the voltage not 0.38V (and cannot be set), check the anode voltage on power tube pin 3 (435...440V).
Check the anode voltage stability.
Check the -21V existence on balance potentiometer slider pin.
If it exist, check the -17V on each power tube grid (pin 6).
If its exists and the voltage is stable, but in TP point the voltage not 0.38V (and cannot be set), check the anode voltage on power tube pin 3 (435...440V).
Check the anode voltage stability.
Alright thank you for that info! I was actually looking over my work and noticed that the original caps, for some reason were different on each bias pot. For some reason they had a 16uF and a 50uF, so let me first take care of that, not sure if that could make a difference but surely they should be the same especially since that is what the schematic calls for. Here is a photo of what it was, surely odd that they would be different...
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Okay so here are some measurements, first off, on the balance pot, there are three connections they all read the same, about 12.8 VDC.
Next, pin 6 about -17.7 on one and -18 on another.
Pin 3, 431.2 and then another was 432.6
Now, I did these measurements with and without 6SN7 tubes, without tubes, everything very stable, with tubes, very unstable. Is this telling me there are issues with the 6SN7 tube? They did test good on my tube tester but this was just a go no go test.
Next, pin 6 about -17.7 on one and -18 on another.
Pin 3, 431.2 and then another was 432.6
Now, I did these measurements with and without 6SN7 tubes, without tubes, everything very stable, with tubes, very unstable. Is this telling me there are issues with the 6SN7 tube? They did test good on my tube tester but this was just a go no go test.
Doesn't really tell you about the 6SN7's, just that the issue isn't in the output stage. Have you grounded the amp input as suggested above? You could be getting some sort of signal/oscillation from a number of upstream sources. Make sure the volume control is all the way down and the source selector is on a line level input.
Best to insert shorting plugs into the power amp inputs for bias testing.
The preamp line stage could be noisy, and this would not be reduced by the setting of the volume control.
So if it is still noisy with the plugs in place, look at the input tubes or the power supply.
The preamp line stage could be noisy, and this would not be reduced by the setting of the volume control.
So if it is still noisy with the plugs in place, look at the input tubes or the power supply.
Make up two RCA plugs with the two terminals shorted together, and plug them into the amp inputs.
This places a short across the amp inputs, so external noise cannot enter.
Then if there is still noise, you know it is internal to the amp.
This places a short across the amp inputs, so external noise cannot enter.
Then if there is still noise, you know it is internal to the amp.
Excellent, thank you I will try that, sorry I was thinking they were asking me to ground the amp main better, sorry just confused.
Is that what is meant when they say above to ground the amp input?
Is that what is meant when they say above to ground the amp input?
Sure. A better term would be short.
The amp was well designed, so it's unlikely its grounding system could be vastly improved.
The amp was well designed, so it's unlikely its grounding system could be vastly improved.
Thank you. Okay and a little further clarification, if I have all of that noise or instability in the one side and the other channel is fairly stable, would it still point ot possibly the way my bias was set up without the shorting plugs? I will repeat again (just made the short plugs) but also just want to see what else I could look at, like you said maybe power supply or?
If only one channel is noisy, the power supply, which is shared, is ok.
The bias circuit does hae separate pots and filtering, so those could be bad.
Or the input tubes or their sockets. Or their associated resistors, especially the plate resistor.
Swap the 4 small tubes after the volume control between channels and clean their socket pins.
It's a tossup whether the tubes or dirty sockets are more likely to be the noise source.
But at this age, the resistors or solder joints could also be in the game.
The bias circuit does hae separate pots and filtering, so those could be bad.
Or the input tubes or their sockets. Or their associated resistors, especially the plate resistor.
Swap the 4 small tubes after the volume control between channels and clean their socket pins.
It's a tossup whether the tubes or dirty sockets are more likely to be the noise source.
But at this age, the resistors or solder joints could also be in the game.
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Eico ST-70 is integrated amplifier, no external input for power part.
https://community.klipsch.com/uploads/monthly_10_2013/post-24628-13819487856846.jpg
The quick method is selecting "Low Filter" switch to ON, and shorting ECC83 grid leak resistors (470k, R33, R72), thus power amplifier input is grounded.
https://community.klipsch.com/uploads/monthly_10_2013/post-24628-13819487856846.jpg
The quick method is selecting "Low Filter" switch to ON, and shorting ECC83 grid leak resistors (470k, R33, R72), thus power amplifier input is grounded.
Okay dumb question it there a good way to clean the socket pins aside from just spraying some deoxit down there?
Hi again, okay thank you I have my input set to Tape/Aux and my grounding plugs in Tape 1 and Tape 2 on the back.
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Don't do that, this is HV circuitry. Never spray cleaner directly into a tube socket.
Use a lintless towel and wipe the tube pins with deOxit D5. Do not wipe off.
insert the tube into its socket several times to wipe the contacts with the cleaner. Then wipe off the tube pins.
Repeat for each of the 4 tube stages following the volume control. You can also clean the output tubes/sockets later.