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freddymac406 21st April 2007 05:32 PM

Adding secondary of output transformer into cathodes of output stage ? Eico ST-70
Hi I have been restoring a Eico st-70 and have been happy with its performance. I want to try some enhancements to see if the performance / sound can be better yet.
1. I wanted to put the secondary of the output tranformer into the output chothodes and could not decide if it should be in phase or out of phase with the primary. If its out of phase that will introduce negitive feedback and run the risk of motorboating.
If its in phase you might get slightly higher power output output with higher distortion.
What is the correct way???
Also Macintosh made inagrated amp called the ma230 that looks like a copy of the eico's amp stage .
2. Are there any benifits to adding a CCS to the LTP phase splitter ?
Looks like it would add a new cap in the signal path but may also have better balance.

Thanks in advance for any thoughts on this.


SY 21st April 2007 05:56 PM

The phase splitter CCS is a very good idea for this amp. You need to balance the plate resistors and and the grid leaks of the output stage (that means you'll lose the balancing control and need to use matched output tubes).

I wouldn't go the cathode feedback route for this amp; what you pick up in output stage distortion reduction you lose in making the driver swing the extra voltage.

rdf 21st April 2007 07:08 PM

Re: Adding secondary of output transformer into cathodes of output stage ? Eico ST-70
Cathode feedback might not be possible. Being push-pull it requires two closely matched anti-phase returns from the secondary taps to the two output cathodes. The schematics show the 16, 8, 4 and C secondary taps. You need to find three adjacent taps such that:

- the two outside taps drive the speaker with a proper impedance match
- the centre tap can be grounded (removing the ground on the 4 ohm tap if it doesn't remain centre)
- the signals between the grounded tap and each outside tap are closely matched in amplitude and phase shift across a wide range of frequencies into the ultrasonic.

I tried this on a 7591 p-p amp (70 volt taps though, heavy return) which in its current config is down about 6 dB at 110 kHz into a resistive load. However at high frequencies the phase difference between the two feedback windings shifted more than 90 degrees and the feedback went positive. The 7591 makes a very effective 55kHz oscillator. Cathode feedback is much easier applied to single-ended outputs.

Re: the LTP, I may be missing something but if the DC currents remain identical why is a coupling cap required? Start up conditions?

Wavebourn 21st April 2007 07:15 PM

Use cathode feedback if you feel you need better woofer damping, otherwise leave it as is because as SY said you'll get less GNFB gain as the result, and who knows which distortions sound nastier in the particular amp, output stage's or driver's errors.

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