McIntosh MC 30 low humming& hissing sound help!!!

[redhog]

Hi
My son and I are restoring a JBL Paragon speaker system and McIntosh MC 30 amps that has been in my family since about 1957. The MC 30 amps serial number 18746 & 21253 were rebuilt at Audio classics. The C-4 pre amps changed out for MC31V pre amp. The system sounds great, fantastic, and wonderful. BUT I am getting a very low hum/hiss from the speakers with the sound tuned down low or all the way. I haven’t been able to adjust it out with the gain or hum adjustments control on the amp. I have separated the components with no luck it seems to be the amps and both sides of the speaker system. I am not sure how much noise I should hear if any. Any suggestions you have would be very helpful.
RH

 

[skippy_ps]

Weclome to AK, Red.

Of course, hum and hiss are two different things. Some obvious things to try are turning off or disconnecting the source, turn off the pre and substituting a different amp(s) and see if the noise is still there. Just process of elimination.

It would seem that the most likely sources of hum are your amps - from both channels?

BTW, the Paragon is one cool speaker! Post some pics.

Murray

 

[redhog]

Hi skippy_ps
Thanks for your suggestions. I will try another amp next. .Eliminating each component put me right back to the MC-30 .It is nice to have this system running again after 22 year in storage. Please see the attached pics let me know what you think.
RH

 

[dewickt]

With the MC30 gain all of the way down try the hum balance on the 30, no effect ? next pull out the first 12AX7, if it get quiet you have found the problem. Each small tube may be checked this way, if you get to the last 12AX7 and still have hum you have a weak bias supply filter or possibly a bad main filter though if replaced a short time ago it should be good.

 

[kcin]

Wow nice speaker,

I'll bet you'll be hard pressed to get much quieter than what you have now unless something, as Terry suggested is wrong. That's one sensitive speaker and the inherent noise of the amp will come through as function of your total system sensitivity.

With another type of speaker your may not notice any noise at all.... if you have hum though..filter caps and bias supply are good bets.

Good Luck

 

[skippy_ps]

That Paragon looks brand new as do your amps. Nice work. :thmbsp: Gonna be hard to get that speaker on a bookshelf, tho.

Some good tips from Terry about checking out the little tubes.

Murray

 

[redhog]

:tresbon: Thanks I am not use to getting so much help. At this point we are down to pulling tubes as dewickt advised. Hooked up another amp a Denon. The hum was minimal barely audible. I think kcin is correct the speakers are very efficient and will exaggerate any noise. When switching out the amps the quality of the music dropped substantially. After re-attaching the MC-10 amps that wonderful natural sound came back every instrument was clear clean and sharp. The MC-10's are so well matched to the Paragon drivers on these early units from 1957. Please keep the suggestion coming I will follow your advice. Interesting that the hum adjustment on the amps is higher when turned all the way down and all the way up. The middle position is the best ????? Here is a pic of the MC-10 with the original mono amps C-4 audio compensators. I was advised that the C-4 are not worth refurbishing. Dewick do have any feeling on that????
Thanks again
RH

 

[Victor]

Here is a pic of the MC-10 with the original mono amps C-4 audio compensators. I was advised that the C-4 are not worth refurbishing. Dewick do have any feeling on that????
Thanks again
RH

If you play 78s then it would be great to have the C4 compensators in your system so you can compensate for pre RIAA curve records, otherwise I would agree the C4s are not worth the effort. On the other hand if you updated the C4s with some modern parts you may like the results and they would allow you to set the gain on your MC30s to the quietest setting and use the C4s for volume control. Also they would look great in your vintage system.

Victor

 

[dewickt]

MC30 hum control is set up to null out the hum at the half way spot if all tubes are perfect and have no filiment to cathode leakage, a slight bit in one tube will cause the humm null point to change slightly.

 

[jcmjrt]

You system looks to be in beautiful condition. And it's especially cool that it's all from within the family. I hope you get that hum run down and fixed so that it can sound as good as it looks! :thmbsp:

 

[kelsa]

From your attached pics it looks like you have the MC30's touching each other. Maybe spacing them out a bit will decrease the hum.

 

[CarlV]

I was advised that the C-4 are not worth refurbishing.

I would have to disagree there. A salesman tell you that?


Carl

 

[redhog]

You guys are great very helpful!!! The C-4's are the originals with this system. They use to sound great. I was considering getting a second opinion on fixing them even if they are not used so often. Carl great com back on the salesman’s comment. Unfortunately it was a McIntosh authorized service center. They may have been looking at cost vs. performance using another pre-amp?? I have a very nostalgic reason for fixing them, they are impressive. Maybe I can do it for reasonable money. Any suggestions!!!!!
The original MC-30 manual doesn’t address hum control. Dewickt comments reflect exactly what I am seeing for controlling hum.
Attached is a 1955 review from Down Beat magazine on the MC-30 and Audio Compensators. I found this with the manuals
RH

 

[CarlV]

From my point of view your C4's would be kept. I have the C8/C8S combo and they are really really great sounding even at almost 50 years of age.

That is very special to have an original combo like yours. I am jealous!

If you wish to tackle the restoration on your own there are many here that would help you through it.


Carl

 

[R-Y-R]

Attached is a 1955 review from Down Beat magazine on the MC-30 and Audio Compensators. I found this with the manuals
RH

Can you please enlarge that a bit so its readable? Would love to print that out and frame it.

 

[AudioClassics1]

That was probably me that talked about the C4. They can be repaired, however it is an expensive repair. If they are sent to us we can call with an estimate and take things from there. Most of the time, people are not interested in repair when the unit is worth less than the repair cost, however if someone wants them repaired still...well who are we to argue! Bringing another McIntosh back to life is not a bad thing to be doing!

 

[redhog]

R-Y-R

The forum limits the size of the attached document. If you send me a private message with your email adress I will be happy to send it to you. If their is another way to enlarge the image and send it to you let me know
RH :yes:

 

[Victor]

If I concentrate on the article I can just read it, pretty cool. Thanks for posting it but you may try to enlarge it and post it as 2 or 4 images instead of one, this may make it easier to read.

Victor

 

[redhog]

That was probably me that talked about the C4. They can be repaired, however it is an expensive repair. If they are sent to us we can call with an estimate and take things from there. Most of the time, people are not interested in repair when the unit is worth less than the repair cost, however if someone wants them repaired still...well who are we to argue! Bringing another McIntosh back to life is not a bad thing to be doing!

Hey Ryan
I am pretty sure your tech got a good look at the C-4 pre-amps Perhaps you could send me a PM on what you think I am looking at with the C-4 repair

 

[redhog]

If I concentrate on the article I can just read it, pretty cool. Thanks for posting it but you may try to enlarge it and post it as 2 or 4 images instead of one, this may make it easier to read.

Victor

Victor
Try this size and let me know
RH