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 Post subject: Push Pull output transformer distortion
PostPosted: Mar Wed 25, 2020 10:39 pm 
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I have an RCA K105 that has been recapped, re-tubed, and works almost perfect - except for some bad audio distortion that I cannot determine the source of.. I think I have isolated it to the Audio Transformer - but want to be sure before I order an expensive replacement.. Question - the schematic says the secondary impedance is 2 ohms - and each half of the input impedance should be 300 ohms.. (600 total) - I measure 307 on 1 half and 272 on the other input.. Could this imbalance cause the distortion?? Any idea where to get a replacement?? (600 ohm total CT at 300 ohm, output impedance 2 ohm and 12 watt max output)
Any help or insight appreciated.. (p.S. physical 2 ohm speaker itself has been bypassed with a known good speaker- problem does not go away..)


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Wed 25, 2020 11:07 pm 
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The difference in resistance is normal; due to outer half having a greater length per turn - the difference in resistance is insignificant compared to the impedance of the winding. Imbalance in the output stage will create distortion - I think the two 6SF5 audio stages might be distorting if bias is off - the voltage divider for that would be pretty critical. If you have a scope, look at the plate waveforms on those with a modulated RF input - should be equal and undistorted. If the caps have been changed, check all the resistors around that stage.

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 12:03 am 
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As Tom said, the transformer impedance is much higher that you are quoting. More like 5000 ohms per side. The values you measure with an ohmmeter are DC resistances, not impedances.

The problem is most likely the bias on the output tubes. I didn't see the bias voltages listed in the RCA service data but you can look them up on the 6F6 specifications. Look for the bias with the plate and screen voltages that are closest to those in your radio. Measured from the chassis, both 6F6 should have the same negative voltage on the control grid.

Jay


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 12:46 am 
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Here is the schematic..
[url]http://www.nostalgiaair.org/Resources/download.asp?FN=\M0015504.pdf[/url]

If have checked the 6f6 grid and plate voltages ( -20 and 406) SB (-22 and 375) - Have also scoped the plates (dual scope) but not sure how to interpret what i'm seeing.. Assuming one scope lead is on OT CT (B+) and and other (-) on Plate and other scope lead on other plate.. Anything special i'm looking for?? the sine waves look identical but one side is a somewhat smaller voltage.. they are 90 deg out of phase.. Will double check voltages etc from the 6f6's.,. How (where do scope leads go) to check crossover distortion?

Thanks again for your help - I'm a learning!!


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 1:43 am 
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With that much difference in the plate voltages, I think you have a bad 6F6. Maybe both are bad since one seems to have high plate current and the other almost no plate current.

Jay


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 2:24 am 
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OOps - sorry, my reply was not very clear.. The "( -20 and 406) SB (-22 and 375) " was meant to say that BOTH 6f6 grid and plate voltages are the same (-20 and 406) - and they Should Be (-22 and 375) according to the schematic.. Sorry bout that.. Still not sure how to view crossover distortion on scope.. (where scope lead go..)


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 9:08 am 
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Greetings to Richard and the Forum:

With the measured value of grid bias you have, the tubes should be running closer to class A than the AB2 given in the RCA book which requires -26 volts bias. The fact that your plate voltage is higher suggests tired 6F6's. The cathode currents of the 6F6's are specified to be 25 mA on the Nostalgiaair schematic. You might wish to break the cathode connections of the tubes and measure the actual current. However, since the grid bias is low, the stage may be running close enough to class A for you to try the following: Pull one 6F6 and then the other, and see if the distortion is peculiar to one tube or the other. That may narrow down where it is coming from.

As far as cross-over distortion is concerned, I think the best place to scope that is on the output transformer secondary. However, with the grid bias that you report, I doubt seriously that you are hearing cross-over distortion. More likely clipping of the top of the output waveform. Try new tubes!

Regards,

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 11:43 am 
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If you have a tone generator (preferred), or signal generator, that can generate a good even sine wave, use this in conjunction, with the oscilloscope hooked to the secondary of the output transformer. The positive going peak should be the same amplitude as the negative going one. Voltage across the cathode resistors will tell you what the tube is drawing.

Check C42 if its electrolytic its positive to chassis.it will upset the bias if reversed & can induce hum. reversing can kill it.

Do not presume its the 6F6. If the wave form odd swap them & see if the peaks reverse. if it doesn't it may be a triode? The Oscilloscope should see the difference in waveform amplitude. I use one for calibration as it will show & trace distortion; ideal in this situation.

Marc


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 2:07 pm 
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Marcc wrote:
Voltage across the cathode resistors will tell you what the tube is drawing.


According to the schematic that I have access to, the cathodes are directly grounded.

Regards,

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 2:58 pm 
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Jthorusen wrote:
Marcc wrote:
Voltage across the cathode resistors will tell you what the tube is drawing.


According to the schematic that I have access to, the cathodes are directly grounded.

Regards,

He can measure the plate currents by dividing the voltage across each primary winding of the output transformer by the resistance of that winding.

Jay


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 3:55 pm 
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What is the ohms/volt rating of the meter in use and is it the same meter used to measure dc resistance of an inductor?

Some meters will LIE! Leading to incorrect conclusions...

Faith in an analog meter known to be accurate...

YMMV

Chas

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 5:24 pm 
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Ok guys - thanks for all your tips and ideas - it gives me some things to try and increases my knowledge.. Will get back when I have investigated (tried) your inputs.. I'm finding that isolating distortion in a PP audio amp ain't all that easy..


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Thu 26, 2020 7:45 pm 
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recapped, re-tubed, and works almost perfect - except for some bad audio distortion that I cannot determine the source of.. I think I have isolated it to the Audio Transformer - but want to be sure before I order an expensive replacement.

I wonder how the distortion was isolated to the output transformer? Are you sure it's not introduced by other stages?

What test equipment can you use to work on this? An audio generator and a signal tracer come to mind as useful things to have.

Have the resistor values been checked? Are the voltages on the other tubes reasonable?

one scope lead is on OT CT (B+) and and other (-) on Plate and other scope lead on other plate.

This is a safety concern. It's not clear to me what the function would be of the three scope leads mentioned. Is the frame of the scope grounded? For sure, you don't want to be operating the scope with B+ on the frame.

dual scope doesn't tell us much. There are lots of different ways to get a scope to be dual. You would want a differential input on any scope with B+ as reference.

Ted


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 12:39 am 
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Whilst the cathode comment still applies (my bad) that is back biased so the current draw will be from the voltage across the back bias, which is usually the bias voltage of the Output tubes, measured centre tap to chassis: Chassis positive. Most of the time you do not need an earth with the Scope.

This is a transformer set there is isolation between the AC supply as it should not be connected to chassis in any way. The only Utility wire to chassis would be ground / earth.

The output transformer secondary has no connection to ground. Same concern as Ted: The earth wire of the scope is never connected to b+. That is liable to short it. Serious damage can result.

The reason for placing the scope lead (one channel only) on the Output transformer is that you see the whole signal going to the speaker, which can also be implicated in distortion. The earth and probe can be connected to the speaker (which must remain connected).

So I believe that both Ted & I are both looking for the result from a clean sine wave going into the amp and what it looks like at the speaker. That will tell us something. If its a Signal Generator it can be on the antenna, or as for IF alignment & keep the signal down. Tone still has to be a good sine wave into the amp. If its not we need to find out why.

Oscilloscopes are not, despite their looks, the easiest to use, as you have to be able to interpret what it is showing you. But it is the best device, applied well, to sort this. So lets go back a step & hook it to the speaker as per comment & see what we get.

Marc


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 4:57 am 
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JnTX wrote:
With that much difference in the plate voltages, I think you have a bad 6F6. Maybe both are bad since one seems to have high plate current and the other almost no plate current.

Jay


the DCR of the primary of the output transformer looks to be normal.

i would replace the 6F6 tubes and see what that does. they could be grossly mismatched or something in one (or both) is drawing too much current and running away.

somewhere in the troubleshooting, i would check the cathode current and make sure one (or both) is/are not running away.

in my experience, the 6F6, especially the older metal ones, can be very finiky.

steve

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 10:06 am 
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Steve said "finiky". first time I ever have seen that spelling... ! you lost the c... its finicky .

Anyways, I agree the output transformer (OPT) is probably not the issue.

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 10:50 am 
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+1 with Mark, I am in no way convinced at this point that its the transformer.

As an example: I refer to a national SW5 Ok it has a transformer couple: But! Its audio output was absolute crap. The signal to headphones off of the driver sounded perfect & looked perfect on the Oscilloscope. Transferring the Oscilloscope probe to the output transformer (as noted before) saw a positive going wave around twice the amplitude of the negative one (bad tube). If one of the 6F6's, or the triode feeding it is not working, or is substantially down. That is what you can expect to see on the oscilloscope, at the speaker of your set and confirm very simply, that there is a problem.

This is the reasoning behind "Matched pairs". In the majority of basic push pull circuits, if both tubes are not behaving in a similar fashion: You get as noted by Jay, a mismatch & you get distortion.

The other thing as a practical exercise in applying the Oscilloscope, as a fault finder, for real. If it shows the distortion described? Touch nothing, save for swapping the 6F6's into the others socket & see if the sag reverses. If it stays the way it was, Its likely one of the Triode's, or its circuit not the 6F6's.

It's always the dodgy set that is the educational one.

Marc


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 4:21 pm 
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oldradioparts wrote:
Steve said "finiky". first time I ever have seen that spelling... ! you lost the c... its finicky .

Anyways, I agree the output transformer (OPT) is probably not the issue.

aaaah, the "c".

my spell checker could not come up with the korekt splleng. i was mobile and did not look it up.

lol.

steve

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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Fri 27, 2020 11:16 pm 
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+1 on the spell checker, treacherous, and I am always amazed at how many basic words it can't fathom & the bizarre ones proffered.

Marc


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 Post subject: Re: Push Pull output transformer distortion
PostPosted: Mar Sat 28, 2020 12:29 am 
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you know what the "automatic word suggestion" speller said one time when i texted my wife ?

i told her that "i am going to the gay sex shop to get hot dogs".

all i texted was that "i am going to the store to get supper". i was in a hurry and i never looked at the text before i sent it.

i turned off the "automatic word suggestion" feature immediately.

:shock:

steve

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