Electrolytic in the feedback loop

[yup]

Hello,

I am in the process of doing some preventative maintenance on my Marantz 8b (replacing electrolytics, tubes and anything else I find too far out of spec)

I noticed that there is a 15uf 6V electrolytic in the feedback circuit hooked to the cathode of the 6bh6 - (C3A &C3B in the wiring diagram)

Marantz 8B wiring diagram

Was this electrolytic just becuase of it's large value?

should I replace it with a film cap or does it need to be polarized in this aplication?


If anyone has any experence with this amp or has any suggestions on specific items I should check carefully, I would welcome your comments


Thanks,
Jeff

[Tweeker]

You can always replace polar caps with nonpolar, the reverse is not always true.

[Eli Duttman]

Jeff,

No doubt about it, the 15 muF. parts are in the signal path. Something better than a "vanilla" 'lytic IF it can be found is indicated. I checked Michael Percy's catalog looking for a Black Gate or even a Nichicon KZ without sucess. You may have to settle on something like a low ESR Panasonic part.

[rdf]

Is 15 uF so critical a 22 uF wouldn't do the job?

http://www.partsconnexion.com/catalo...ctrolytic.html

[gingertube]

Hint - hint
One of my favourite caps - wherever I can get away with the voltage rating, like for C3a C3b, is the Blackgate NX 22uF 6.3V - US$2.25 each in the PartsConnexion catalogue.
Cheers,
Ian

[ilimzn]

At 16uF, you could rather easily find a 16uF 100V foil cap (Solen, Audyn...) which will easily otperform any lytic. That is, if it fits mechanically.

[yup]

Thanks for your resplies. I have planty of room for a solen cap, I guess I will give that a try and see how it sounds...


I have another question about two adjustable capacitors (C12A & C12B) at the output transformers. I am not quite sure what these are used to adjust. The service manual make no real mention of them. I measured them in the factory positions (red paint still intact) and one measures 340pf and the other 440pf. Would these ever need to be adjusted over the life of the amplifier or were they set based on some parameter in the transformer that probably would not change? I have a scope, signal genrator, etc... just not sure what to look for (I have mostly SS experience)

Thanks,
Jeff

[Brian Beck]

I agree with “ilimzm” that you might try a film cap there instead. But I would avoid significant changes to its value. In this feedback amp there are 4 low-frequency poles in the loop (three caps and the OPT) that were carefully chosen to maintain a safe phase margin. If you change any one of them by very much, in either direction, you risk low frequency instability, even motorboating. It’s hard to say for sure without a thorough circuit analysis or by an accurate simulation. A 16uF cap will be safely OK. Even a 22uF may work fine, but I would carefully measure low frequency response with a scope at that value or with any other value that is well above or below 15uF, looking for peaking in the response at sub audio frequencies.

[Brian Beck]

These varable caps are part of the high frequency feedback compensation network. The desired settings might conceivably slightly change if you replaced the EL34s, but the settings are probably more determined by the OPT characteristics. My advice – don’t touch them if they’re still at fixed by at factory settings by red paint. If you are adventurous, put square waves through the amp and monitor with a scope on the output (load the OPT with a dummy resistor). Adjust the caps for sharp edges with low or no overshoot. Try different power levels.

[ilimzn]

I don't think the tolerance of the value used with a film cap will be a problem, electrolytics are not very precise to begin with, and certainly back then it would be asking a lot expecting +-20% tolerance from them.