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 Post subject: MC30 choke question
PostPosted: 16 Jan 2021, 06:36 
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Hi,

Does anyone know why the McIntosh MC30 amps had chokes on the anode of the power tubes?
I don't have any of these amps but looking at the circuit they sort of jump out at me. :)
Is there anything special about the construction of these chokes?
I assume they are small RF types but was there a problem with the circuit in some way to require this measure to be applied?
Choke shown in the pic.

Regards
M. Gregg


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 Post subject: Re: MC30 choke question
PostPosted: 16 Jan 2021, 13:19 
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M. Gregg wrote:
Does anyone know why the McIntosh MC30 amps had chokes on the anode of the power tubes?
My suspicion is that this was a fix because of the method of feedback used.

Normally, when the feedback is taken off the speaker windings, the equivalent distributed capacitance seen by the loop is large enough to suppress any feedback at higher frequencies which would lead to oscillation. However, the MC-30 amps use a separate winding for feedback. If the distributed capacitance to this winding was too small, the resonance or positive feedback could lead to oscillation problems at high frequencies. A simple (and inexpensive) answer to this problem is to choke the forward path at high frequencies such that |1-Aß|>1 at high frequencies.

This was most probably done this way as a cost savings measure rather than going with a more expensive output transformer winding approach.

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 Post subject: Re: MC30 choke question
PostPosted: 16 Jan 2021, 13:45 
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Hi,

thanks for your answer, have you ever tried putting chokes in the anode?
Just wondering what the effect was.
This is the spec which is again strange because the two chokes are different values on the same channel OK the MC30 is a PP amp.
But this should create an imbalance in the output tx freq response?

I used to build PP but now its only SE but its still interesting. :)

Regards
M. Gregg


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 Post subject: Re: MC30 choke question
PostPosted: 17 Jan 2021, 12:46 
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Hi,

Just wondering, would these inductors work in the cathode with lower voltage applied instead of the Anode?
I had a look around on the net and its easy to make an air cored small inductor giving about 2uH.
I didn't want to use ferrite I have some cores that would do it in a smaller package but that's not the point.
Layers are 16 turns under and 14 turns over.
So in the interests of listening testing...I might give it a go just for fun of course. :D
Just a moment of madness.

Regards
M. Gregg


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 Post subject: Re: MC30 choke question
PostPosted: 17 Jan 2021, 17:09 
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Hi,

Now I know why McIntosh fitted them.
I just fitted them on the cathode of the SE and you can hear or should I say not hear what they are doing.
Its obviously early days yet but as always I left the soldering iron on ready to rip them out.
But they are staying for an extended listening test.
The effect sounds a bit like a Zobel network but its less intrusive and doesn't change the character of the amplifier.
Its a bit difficult to explain as usual things can change over extended listening.
This is just a test on my equipment and it may not have the same effect on other circuits.(YMMV)

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M. Gregg


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 Post subject: Re: MC30 choke question
PostPosted: 18 Jan 2021, 10:51 
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Hi Matt,

Its interesting when you start looking at output inductors, they appear in quite a few SS designs to give stability with capacitive loading.
Perhaps McIntosh were ahead of their time maybe and thought panel speakers were one of the considerations.
Or as you say a fix for the type of FB. I still find it interesting.

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M. Gregg

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 Post subject: Re: MC30 choke question
PostPosted: 19 Jan 2021, 05:12 
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Hi,

Some more research led to parasitic suppression.
Its been used for many years in tube equipment and anode suppressors were sometimes built using a resistor with an inductor wound over the top shunting the resistor. As soon I saw this I vaguely remember from the distant past seeing this on some PA equipment in a factory.
It took some more reading and the images of some old equipment flickered in my mind, you know that old black and white image that slowly forms in your mind. Its still quite vague as to why this was used on audio PA :confused:
Perhaps it was the large factory horn PA covering quite some area.
Its an old vintage technique used to keep the circuit stable, but was used in RF and VHF suppression.
Just thought I would post some info that was under the radar from times past.
Now its even more interesting I will have to look for some of the early info on cinema audio. :)

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M. Gregg

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 Post subject: Re: MC30 choke question
PostPosted: 22 Jan 2021, 09:41 
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Just for fun,

I thought I would try making what was used in the past as a plate inductor.
In this case its a 3 watt carbon resistor with 14 turns wrapped around it and connected in parallel.
Its a bit of a cheat because I'm connecting this in the cathode.
But its interesting that there is a change in how much HF is reduced.
There is less reduction of HF with this design.
I'm not sure at this point if its the resistor dissipating the energy or the reduction to half the number of turns.
Reading up on the resistor being inside the inductor others say it reduces the "Q" of the inductor.
So another listening test I think.
NB its interesting looking at RF design on the net, that to reduce parasitic problems you want low "Q".
But for a perfect inductor you need high "Q".
Apparently you can also reduce the "Q" by changing the material used for the coil.
And its not what you would expect.

Regards
M. Gregg


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 Post subject: Re: MC30 choke question
PostPosted: 24 Jan 2021, 07:27 
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Hi,

Just for interest.
I prefer the air cored output inductor in preference to the anode inductor wound around the resistor.

Just for anyone interested this is how I made the air cored inductor.

Output inductor.

I made the inductor using some 18 AWG enamelled copper wire.
Get a cross head screwdriver with a 1/4 shaft and wind 16 turns onto it then lift the wire on top of the existing turns and wind another 14 turns on top of the 16 turns you have already. So you wind up the screwdriver then back down the screwdriver making a two layer coil.
Then remove it from the screwdriver and heat shrink.
I didn't want to use ferrite I don't like it in the signal path.
My past experience with putting ferrite on audio conductors gave some strange results.

I'm still researching other material but nothing new there. :D

Regards
M. Gregg

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 Post subject: Re: MC30 choke question
PostPosted: 10 Apr 2021, 08:10 
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Just a new listening test,

I thought I would try a modification to the 2uH inductor.
This time using 12turns under and 12 turns over to lower the inductance slightly.
But also using silver plated copper wire with PTFE insulation.
The leads are shorter and stripped to identify the start of the first layer to keep both inductors in the same orientation on both channels.
I also wanted to "Damp" the coils so I wound the first 12turns on a 1/4 inch cross head screwdriver.
Then added a turn of masking tape over the first layer, then wound the next 12 turns over the tape.
So I had 12turns first layer then one turn of masking tape followed by another 12turns over the first layer as before.
I then heat shrink over the outer layer and epoxied the ends of the heat shrink leaving the inside as an air core.

The result is a "warmer" sound very clear with detail.
No spit or high frequency glare.
Just waiting for the initial "burn in" if it exists, but the change is probably the solder joints changing "maybe" :D

So just a listen to compare between the different types.

NB this also has a marked effect on the sound of different connections of output tubes in particular in pentode operation.
I might try wax filling the inductors, but its another maybe.

Regards
M. Gregg


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