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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 1:54 am 
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Joined: Nov Tue 01, 2016 7:33 pm
Posts: 23
Great! I will get those resistors and replace. Could you expand a little on the bias relationship of the PIs?

Thanks martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 2:45 am 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
Vacuum tubes set up for amplification or as a cathode follower, in this case to invert a phase, will have some negative voltage bias applied to the grid of the tube. In the phase inverter cathode follower section (pins 1,2 and 3) you will observe the schematic voltage at pin 2 is 130V, and at pin 3 is 131V. Here the grid is at a voltage that is 1 volt less than that of the cathode, so the grid bias is -1V. The absolute voltages are all positive wrt the chassis ground but the important point is that the grid voltage always be negative wrt the cathode.

If the bias voltage is too negative, which is what I am seeing in your voltage measurements for both tubes, the tube will either not be operative at all or, the cathode current (to the plate) will be cut-off where an input signal is of sufficient negative voltage to drive the grid into cutoff which will cause the output to clip (go flat like a square wave) or if the current is not fully cut off, some other type of distortion may result. In any event, for proper operation the bias voltage relationship should be near to -1V with the absolute voltage perhaps within 5% or thereabouts of what is indicated on the schematic. Another possibility is if the bias voltage is too close to 0V, an input signal of too great a positive voltage could cause the grid to draw current which would lead to faulty operation as well.

When the PI is operating properly, the input signal to the amp which is inverted is picked up at the cathode, pin 3 on the schematic at R116 (known as the bias resistor). As the input signal to the follower at pin 2 goes positive for instance, the tube current will increase causing a drop in the plate voltage at pin 1, which is registered as an input to the grid of the amp tube - this voltage will be falling. At the same time this increase in tube current in the follower will raise the voltage drop at R116 causing the voltage at the cathode to rise which is registered as an input to the grid of the 2nd amp tube - this voltage will be rising. When the input signal to the follower goes negative all of the foregoing is reversed. Accordingly, the voltages registered at pin 1 and pin 3 of the follower will be 180 degrees out of phase with each other. This can only happen if the follower is correctly biased for its operation.

It was a bit of a treatise, but I hope that answers your question.

Two other questions I have for you before you tear up the PI sections is what are you using for the tubes, new or used, which brand, and are they checked as having good emission? Take a good look at those pots R108 and R109 to be sure they are functioning ok, have you tried to adjust them? If not ok, they should probably be replaced.


Last edited by radiomania on Nov Mon 07, 2016 1:01 pm, edited 1 time in total.

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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 9:15 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10448
Location: Powell River BC Canada
Can you set your scope to add input 1, to input 2 ?

In this mode the signal from one probe will be algebraically added to
the signal from the other. Both inputs will have to calibrated the same.

You will see only one trace on the scope.

If the signals from V8 pin6, and V9 pin6 (use AC coupling) are
added to each other, the scope trace should reduce to zero, at the
correct setting of PHASE INVETER ADJUST LEFT.

Also, another issue. If your signal source, that provides the 200 mV
aux signal has a grounded power cord, which grounds the
shield of the signal cord. Your output meter across your load
resistors must not have a grounded side. (were you using the scope ?)

See inset. There is a chance of a ground loop causing mayhem
at high power levels.



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Attachment:
Fisher 500c speaker system.JPG
Fisher 500c speaker system.JPG [ 37.48 KiB | Viewed 2464 times ]

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 3:52 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
Posts: 23
Great info, Thanks! Much of what you describe is happening here, so seems we are in the right track.
The present tubes are new JJ and good tested Phillips ( probably not the final ones) another Philips of the pair has failed. I will check for matched balance and correct accordingly when things look better. The pots were cleaned in place, and work apparently fine in place as judging from the smooth equal increase in resistance from wiper to one end.

Thanks Steve for the tip, next time I will use the add mode.
Regarding grnd loops: My load box preserve the floating configuration . The scope however shares the gnd , that is why I try not to use the dual mode when figuring out problems, although i have a second scope ungrounded that i plan to use when circumstances warrant, one for each channel.

Thank you for wisdom Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 9:14 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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Hi: Checked res of both PIs within spec. (R 113&115 read 180k 20% off schematic)
Changed tubes , repeat v meas V12 1--305V, 2--116 V 3---118 v
V13 1-- 298, 2--121v 3--124vdc

Caps are new 47uf/50v
pots seem ok

Now rt ch grids dephased but unstable , hum/ oscil is back!left grids in phase.

I never had such a headache in many previous 500-C amps, the only significant difference is that this is the first time I replace OPTs for the smaller 400 opt s. Transformer problem??

martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 10:48 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
Hi. Not sure what you meant by"20% off schematic"? Are you saying that the measured resistance of R113 and R115 was found to be 20% off the schematic value of 180K, ie they were both out of spec.? According to the service manual, the tolerance for those two resistors is to be 5%. So, I'm thinking you changed them and the tubes? The voltage results which you are showing now are much improved with -2V and -3V bias but this is still somewhat high so you may want to try a different set of 12AX7s again. Plate voltages at 298V and 305V are OK, should be good. What are the contacts like on the tube sockets? Are they good?

I checked the GE manual for the 12AX7 and with a plate voltage around 180V above the cathode, which is the case here, a -1.5V bias can be interpolated from their data table - so this makes sense with the voltages quoted on the schematic. I trust all the other resistors R101, R106, R103, R114, R111, R121, R122, R126, R127, R128 and their corresponding counterparts in the other channel have checked out OK then? You may even want to take a closer look at C83 and C81. I know ceramics rarely if ever fail, but they have been known to go bad, it may be worth a look.

As to the tubes themselves, if problems still persist after checking and changing resistors, being certain that all there is good and your tubes check out good, you may wish to try a good used version of OEM 12AX7 tubes for this amp. I once encountered a very difficult problem with an amp that was cleared up by using OEM tubes. Since then, I only use USA or Canada manufactured tubes.

As for the transformers, what prompted you to change them for those of the 400c? The output tubes 7591 and 7868 (400c) appear to have nearly identical characteristics across the board including identical plate to plate impedence so, at the moment, I'm not sure why this swap would not work. Again, you will need to be sure applied voltages and biasing are correct for the power tubes. I note in the schematic for the 400c they use a 330pf cap C61 at the 16ohm output terminal of the output transformer to ground as part of the feedback, otherwise the power output circuits are basically identical.


Last edited by radiomania on Nov Mon 07, 2016 11:51 pm, edited 1 time in total.

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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Mon 07, 2016 11:32 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
Posts: 23
Rather than going by the schematic, I measured each resistor related to v12 7V13 matched the color code to their measurement: R 113 &115 measure 180 k but the color code says 160k. I tend to ignore this discrepancy because the schematic says 180k as well as you suggested ( did not express myself correctly) I may need to extend the resistor
search more peripheral .

Signal tracing suggest the problem starts at plate of both fist triodes, showing the scalloping distortion, while having a nice sine at grid of first triode. I was in the process of cutting the nfb return and see what effect it has. I will chec c 83 & 81.
I changed and tested tubes (new) . I cant believe a brand of tube can resolve this gross problem.

Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 12:00 am 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
Color code for 6 is blue, 8 is grey. Maybe color has faded/changed with age? In any event, sounds like you're making progress. Likely a tube brand would not [solve the problem], but where the brand is manufactured might. As I said, I have experienced this once. The Japan version of the tube (tested good) would not function properly in the circuit vs. the USA tube. This was only realized after doing extensive filament voltage and current characteristic tests in laboratory fashion and creating a graph of the results, which, once I saw them, explained the problem. Since then, I've been cautious about the tubes I use. In other words, "tests good" is open to question.

BTW, what about those new 47uF caps? Since you have isolated the distortion problem to the first section of PI tube, and if resistances, voltages check out, and the tube is good, it doesn't leave much left by elimination except for those or maybe the feedback network C81, R117 etc. You might want to try disconnecting the FB to see if it changes the PI first stage output at the plate.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 1:00 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
I should have thought to mention this earlier. Something else worth a look are the solder joints in the PI circuit. You may want to desolder/resolder anything that hasn't already been done. In a full rebuild this usually take care of itself.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 1:41 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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Thanks! will do and let you know.

martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 4:26 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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Making progress: Replaced neutralizing caps.: Pic 1 rt ch grids ,no osc, clean sine. Pic2 left ch grids distortion on one ,
very weak & not 180 Deg on the other.

martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 4:53 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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Attachment:
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Solved lft ch: Wrong coupling cap connection!
You made the key suggestion to look at the neut caps: Thankyou!
I will redo audio measurements and fwd to forum.

Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 9:59 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
It looks really good. AR is an excellent forum and it was a pleasure to be able to help out with this one. I'll have to keep my eyes open for a 500c now!


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Tue 08, 2016 10:59 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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The best forum! Full audio measurements coming up. Thanks! Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Wed 09, 2016 7:59 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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Audio measurements :Values are per ch, both essentially similar:

Max audio pwr:27 wtts rms
Cross talk: 50dB
S/N: 50 dB
THD: 0.5%
Tube noise @ aux max vol59mV. Hum O
Sine-Triang- Square waves excellent

now the bummer: Freq response +/- 2dB: 20hz-----12khz. ??

Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Wed 09, 2016 9:03 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10448
Location: Powell River BC Canada
The frequency response is stated at 10 dB below reference distortion under I H F specs.

What power level did you measure distortion at ?
Attachment:
Trickier than a bag of snakes.jpg
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Here is the test definition:
Attachment:
I H F  A  201.jpg
I H F A 201.jpg [ 224.79 KiB | Viewed 2409 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Wed 09, 2016 10:59 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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I will have to read the Ihf standard many times and slowly. My method is very crude and probably invalid, but if used as comparison with what I get usually with 500 c transformers, this respond better in lows, and very poorly on highs. The readings I get usually is around 40-40khz.
I realize that THD is vol. dependent,I crudely set it at hearing level.(although I confess that I haven't yet hooked to speakers, but Set it at low low pwr.

Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Thu 10, 2016 2:28 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 264
Location: Westland, MI, USA
According to that, a valid test point for the amplifier output needs to fall within a bounded range for each frequency measured. The upper level of the boundary for output being defined as 10db below the point where distortion occurs, the lower level of the boundary for output being defined as 20db above the point of residual hum and noise. That forms a pass or fail criterion at each frequency. When those two points meet a measured frequency the test is over.

It would be helpful to see 3.2 and 3.7 noted in the test description of 3.8, although I think these reference levels for distortion and hum/noise may be freely chosen for the test. For instance if you were to allow 1% THD vs. 0.5% then your frequency response should be wider than you have measured.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Thu 10, 2016 3:30 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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I did not get 3.2 and 3.7 Only got 3.8.
Will repeat measurement at 1% THD Thanks


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Thu 10, 2016 7:50 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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GENIUS!! At 1% THD I get +/- 2dB 50-36Khz on left ch and 100-38Khz on rt. I can live with that .

I have the audio measurements handbook of Bob Metzler which I need to scrutinize,

Thank again Radiomania! Martin


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