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Electrolytic in the feedback loop

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yup

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Joined 2002
#1
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
 
#3
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.
 
#7
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
 
#8
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.
 
#9
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.
 
#11
After hanging around on this forum for a few weeks my curiosity is peaked...

Is it possible to run a 6550 in the marantz 8b? The bias is individually adjustable for all 4 tubes so I can easily increase the bias, but I am not sure if there is something else that would need to be modified to try these.
 
#13
frank12,
Thanks for your offer to help. The schematic can be downloaded here:

Schematic



I like the sound of the marantz triode strapped so I was thinking that a tetrode 6CA7 or a 6550 may be the way to go since they do not have the additional suppressor grid. I just don't know if the 6CG7 will drive the 6550 properly in this configuration.

Thanks,
Jeff
 
#14
The 6CA7 is the US equivalent of the EL34, however it is a beam tube, and imo I have never heard one that sounded better than an EL34 - of course that doesn't mean they don't exist. 😀 A good 6CA7 should be just fine in the 8B

The EL34 was originally designed for good performance triode strapped, and in my experience triode strapped 6550 generally don't sound as good. (They are ok though)

The power supply in the 8B is not up to delivering significantly more plate current and therefore you cannot get what would amount to a significant increase in output power.

The bias voltage range probably will not allow you to adjust the bias to levels required for the appropriate plate current in the output stage which in this case is likely to be -60V or higher..

Next, even if the driver stage can swing the 120Vpp required to drive a 6550 effectively, its linearity will be significantly compromised. You may be asking it for almost 6dB more signal level at full power.

Lower transconductance and lower voltage sensitivity mean you will have both less open loop gain and higher drive requirements resulting in somewhat less margin for negative feedback, somewhat higher distortion and higher output impedance.

High frequency compensation will likely also be a little off.

**Good** amplifier design is often about synergy, the tubes, topology and component choices were made to optomize various design tradeoffs. Marantz had a definite reason for not using the 6550/KT88 which was readily available when Sid Smith designed this amplifier. I wouldn't push a valuable classic well beyond its design limits, although that is your decision as you own it.

Frankly I can't imagine why anyone would mess with an 8B at all, in its genre it is pretty damned hard to improve. (Other than the obvious need for parts replacements as they wear out.)

YMMV 😀
 
#15
kevinkr,
Thanks for the detailed insight.

I definately did not want to modify the amplifier beside trying out different tubes in different configurations (pentode, beam tetrode, and strapped triode) for different sound, so that helps me narrow down my choices. I have had this amplifier for 15 years and would like keep it for a long time (in working condition).

I am trying to make up my mind between the Electro-Harmonix 6CA7 EH (tetrode) and the SED SVEL34 (pentode) - comments anyone?
 
#18
I would have to do a little digging to find the specific reference, but Mullard does make reference to this fact in some of the early data sheets originally published for this type. (I have one) Curves for both triode and pentode operation were provided along with the comment that good linearity in triode connection was a design criteria for this type. In any event inspection of the triode connected plate curves published by Mullard show this pretty clearly, as do actual measurements of plate curves when triode connected.

It may be that this reference is also cited in the Mullard amplifier design manual that Old Colony Books republished some years ago.

The Marantz 2 and 9 actually can be switched from UL operation to triode with the flip of a switch. And imo the 9 in triode is pretty impressive driving at least older Quads..

Converting 8B's to triode mode was a fairly popular mod to these amplifiers locally. I fixed quite a few in the early 1980's.
 
#20
Tweeker's reference is a good one, I'm not sure I have the cited publication any longer, and have not been able to find either old ad copy or an exerpt from a technical manual to back my assertion on the web, but I'll keep looking when I get home.. 😉

It may be that it was an assertion I read in an old ad from an archived copy of Audio Engineering in which case I'll never find it.. 😕

Still I am pretty (relatively? not sure?) sure this is the case, and it is true that the EL34 has good triode curves. :smash:

I have several pdfs of the data sheets of varying vintages, most of which are also available at: http://www.tubedata.org/ Just click on search and enter EL34. The Philips and Mullard sheets are the ones of interest.
 
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