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McIntosh bifilar output stage test proof
McIntosh bifilar output stage test proof
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Old 7th May 2016, 07:00 AM   #1
michaelsamra is offline michaelsamra  United States
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Default McIntosh bifilar output stage test proof

Ok I said I would do this test and I did..I talked to Doc Hoyer and he said you want to measure the total circuit of both output tubes because that's its function..The Fluke is measuring the RMS voltage in across both input grids and you can see that it is 325vRMS.There is 218.32vRMS across the plates which is being measured by the Instek and this is with a 1v signal coming in from the generator.In order for this output circuit to be a voltage amplifier,it would would have to increase the voltage at the plates vs the input grids as I will show you it does not increase.
On the Dynaco VTA70 we have 38.7vRMS going in across the grids and 673vRMS coming off at the plates with the same 1v input signal being used.In this circuit,it clearly does increase as a voltage amp should.
The last test I did was measure the output at the speaker terminals of the Mc30s without the 6L6gcs installed and it was 15.8v and when I put the output tubes back in,it went up to 18.3v but again,that is a marginal gain and it is due to the added current..All the tests were performed with the output tubes installed..I used 1v because I didn't need to drive the amp to full power for the test.
If any of you have Macs and do this test,you will come up with the same type result.
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Last edited by michaelsamra; 7th May 2016 at 07:06 AM.
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Old 7th May 2016, 08:13 AM   #2
jazbo8 is offline jazbo8
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But how about the cathode winding? They are also a part of the output stage.
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Old 7th May 2016, 08:48 AM   #3
michaelsamra is offline michaelsamra  United States
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Originally Posted by jazbo8 View Post
But how about the cathode winding? They are also a part of the output stage.
It's splits the load but it doesn't matter because you get the same reading on the cathodes as you do on the input grids even without the output tubes installed. The point is for this would have to be a voltage increase at the plates and there is not but on the Dynaco there is.Doc Hoyer knows these circuits like the back of his hand.He was friends with Frank McIntosh and Sidney Corderman and with Stu Hegeman over at HK..I think he knows what he's talking about.

Last edited by michaelsamra; 7th May 2016 at 09:01 AM.
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Old 7th May 2016, 10:00 AM   #4
jazbo8 is offline jazbo8
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Both the plate AND the cathode windings are coupled to the output via the OPT's core, so where does the energy from the cathode winding go? It can not just disappear, can it?
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Old 7th May 2016, 11:36 AM   #5
michaelsamra is offline michaelsamra  United States
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As I told you,when you measure the voltage on the cathodes with no output tubes installed,it 325rms on the grids and the same identical voltage is on the cathode terminals.So that voltage is already there even before you install the outputs tubes .That voltage probably stays on the cathodes and works as a cathode follower to help develop the added current for the output stage..I'm guessing there but I can ask Doc..Here is the thing,if we split the 325v that gives you 162.5 and if we were to double the 325 or double the 162.5 as a 2 to 1 gain,it still doesn't come out to numbers shown on the plates. All the voltage gain is done in prior stages and that's why we have such a high number on the MACs G1 grids and on the Dynaco it is very low. Bottom line,you get same RMS voltage at G1 and the cathodes with or without the output tubes installed. If the output stage had a 2 to 1 gain,we wouldn't need to boot strap the driver because the output stage would take care of stepping up the voltage as it does in the Dynaco.

Last edited by michaelsamra; 7th May 2016 at 11:42 AM.
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Old 7th May 2016, 12:09 PM   #6
pieter t is offline pieter t  Netherlands
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Quote:
Originally Posted by michaelsamra View Post
with no output tubes installed,it 325rms on the grids and the same identical voltage is on the cathode terminals.So that voltage is already there even before you install the outputs tubes .That voltage probably stays on the cathodes and works as a cathode follower to help develop the added current for the output stage..I'm guessing there but I can ask Doc.
It would be interesting if your Mr. Doc could chime in as he seems to be your guru.
One thing however is clear: you have no clue of how these unity coupling designs work, and you apparently do wrong measurements.
Measuring voltages at tube terminals without installing them?
The MC275 output stage acts like a catodyne phase splitter with equal loads in the plate and cathode circuit.
Unloaded this would give a voltage gain of 2; loaded in the practical case of the amplifier the voltage gain is about 1.5 as I measured already a couple of years ago (MC275 with shorted output transformer as discussed earlier at this forum).

Last edited by pieter t; 7th May 2016 at 12:25 PM.
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Old 7th May 2016, 03:44 PM   #7
smoking-amp is offline smoking-amp  United States
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Actual Tube Voltage Gain = (Vplates + Vcathodes)/(Vgrids - Vcathodes) = (436.6/106.7) = 4.09

Apparent stage gain (relative to drive signals) = (Vplates + Vcathodes)/(Vgrids) = 1.34

You cannot use just the plate voltage, this is not a grounded cathode stage. Some confused gurus I think.
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Old 7th May 2016, 04:00 PM   #8
kevinkr is offline kevinkr  United States
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McIntosh bifilar output stage test proof
I have worked on a lot of these amps in the past and owned a pair of MC-30 for many years and can confirm both pieter t and smoking amp comments are consistent with my experience.
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Old 7th May 2016, 06:23 PM   #9
jazbo8 is offline jazbo8
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Perhaps the title of the thread should be changed to "disproved"...
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Old 7th May 2016, 07:23 PM   #10
michaelsamra is offline michaelsamra  United States
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Quote:
Originally Posted by pieter t View Post
One thing however is clear: you have no clue of how these unity coupling designs work, and you apparently do wrong measurements.
You obviously didn't read what I said. I did this both ways for verification to which voltages exist on the output tube grids and cathodes with and without the output tubes being installed. The actual measurement was taken with the output tubes installed on both amps.

Last edited by jazbo8; 28th November 2016 at 10:55 AM. Reason: quotes
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