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 Post subject: Tube push pull amplifier question
PostPosted: Nov Fri 04, 2016 4:16 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
Posts: 23
Hello vacuum tube fans! I am new to A.R.: Finished restoring a stereo push pull amplifier and I find a 20 mV 60 hz
contamination in only one of the 4 pwr grids. not caused by tube problem or pwr supply fault, without input signal,
the hum is eliminated by pulling out the other c0ntralateral tube, not the tube of the problem. Filament circuit ok.

Thanking you in advance Vintelectra


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Fri 04, 2016 4:43 pm 
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Location: Annapolis, MD
Welcome to the forum!

Can we see a schematic?

If it goes away when you pull another tube, it could just be that you're changing the loading on a filter....or maybe changing the operating point of the P-P pair. Try swapping the problem tube with another one.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Fri 04, 2016 8:50 pm 
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Thank you for taking your time Mark; Pics attached. Fisher 500-c with replaced OPT with the fisher 400 opts
Tubes have no effect since I swapped for another new set of 7591s and also swapped between them.

additional abnormal findings are: Very low power output in both channels. Sine wave into drivers but square wave into power grids. V-R analysis oK, new driver 12ax7 s Thank you: Martin

( for some reason cannot attach pics or avatar. I am new)


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 1:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3293
Location: Seattle WA US
Have you replaced or changed anything in the 500C output circuits other than the transformers ?

If not, are you sure that you have not exchanged the plate leads on each output transformer resulting in positive feedback, rather than negative feedback?
-Chuck


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 6:22 am 
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Joined: Jan Tue 31, 2012 2:17 am
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Location: Westland, MI, USA
While you are checking connections on output transformers you may also want to verify correct values for R120 and R85 of the filament supply to the power amp are 220ohm each and, R108 and R109 inverter balance are each properly adjusted.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 8:51 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11559
Location: Powell River BC Canada
The hum on only one grid on a 7591 might be caused by a contamination
or perhaps carbonization of the socket in a past event, i.e. loss of speaker
load at high volume. The grid is pin 6 on base 8KQ while the filament is
pin 7. Leakage across that path could induce 60 Hz hum.

Loss of load can cause a flash over at the socket of an amplifier.

_________________
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 Sat 05, 2016 2:17 pm 
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Location: Annapolis, MD
vintelectra wrote:

( for some reason cannot attach pics or avatar. I am new)

Posting pictures:
viewtopic.php?f=2&t=167677

For the avatar, look at the settings in your profile.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 3:33 pm 
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Thanks for all the interest. I will work on the pic mark.
to k7mcg : I restored the whole unit . The amp has the grid mod for new 7591 s, recapped with Russian pio audio caps, driver recapped. I test swapping plate leads for that question and causes bad oscilation, so that should be OK.
However when I cut the nfb the output power rises. This affects both channels , but the hum is only in one grid.
PS has normal ripples. restored.
to Radiomania: I will work on that and get back. When you say properly adjusted meaning?
to radiotechnician: Nothing on inspection but will do measurments.

Martin KI4JRY


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 8:36 pm 
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Location: Sunnyvale CA
vintelectra wrote:
Thanks for all the interest. I will work on the pic mark.
to k7mcg : I restored the whole unit . The amp has the grid mod for new 7591 s, recapped with Russian pio audio caps, driver recapped. I test swapping plate leads for that question and causes bad oscilation, so that should be OK.


I would urge you to replace the PIO capacitors with conventional film capacitors from a reliable vendor (Panasonic, etc). The russian army surplus capacitors are not quality parts that you can rely on and unless you have carefully screened them for both leakage and value, I wouldn't trust them to work the way you expect and certainly not for long.

Brett


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 8:55 pm 
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OK I am back; The fil resistors 220 ok. the phase balance pots ok, fine adj pending when amp functions. All leads double checked, Resistive analysis same as similar good unit for comparison :? I wonder if the transformers themselves are not the problem, although I checked all terminals for continuity and the color code matched with the original 500 c tr

Still working on pics. my pics have a ; large kb


Thanks Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 10:53 pm 
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Joined: Jan Tue 31, 2012 2:17 am
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Location: Westland, MI, USA
The phase inverter adjust controls the drive signal to each side of the push-pull amp by adjusting the cathode follower section; it adjusts the grid bias (if my interpretation of the schematic is correct). Nothing about it in the service manual except the indication on the schematic for the operating voltages of V12 and V13 (see 12AX7 pins 1, 2 and 3). Assuming the 12AX7s you're using are checked good, as a start, adjust the pot to set pin voltages: pin1= 310V, pin2= 130V, pin3= 131V as indicated or as close to those values as can be best realized. They don't give a tolerance in the schematic wrt voltages but 5% is probably reasonable. In any event there should be a grid bias of -1V on the follower section of the 12AX7. Alternatively, you will want to check the actual level of the drive signal being realized on each grid of the 7591s to verify and readjust accordingly.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sat 05, 2016 11:39 pm 
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Thanks.correct if I am wrong: I usually adjust the phase pots when everything locks good ,by equalizing the grid signal level of pwr tubes in both channels. But here we are talking of a Big problem. Here the input signal pwr grin is a good 1.5v but distorted to a square shape, and the output varies between a max of 20 wt per ch to a lot w of 4 wt . It was looking for causes of this that I came across the grid hum in one tube,and the distortion in the driver of both ch.

Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 12:32 am 
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Posts: 363
Location: Westland, MI, USA
Its OK to do it that way. I was suggesting to be sure that the driver tube pin voltages, and the driver tube bias made sense along with a balanced drive signal to each final. If the phase pots are out of adjustment the cathode follower section of V12/V13 may not see proper bias creating insufficient drive or distorted drive signal.

If I understand this all correctly then, there is a problem with one of either the left or right hand channels. On the channel that is problematic, what you are seeing is a square wave drive signal to the power tube grid rather than a clean sine wave. Is that a square wave input to one power tube or both power tubes of the push-pull pair? I would ask that if you have a digital data scope can you capture the image as a file and post it for a look. Once you have said square wave from the phase inverter, I am suspicious that what you may be seeing is a clipped sine wave which is going to look like a distorted square wave. Bias could be the issue but so could a bad component. What voltages are you seeing on pins 1, 2, 3, 6, 7 and 8 of the 12AX7 phase inverter of the problem channel?


Last edited by radiomania on Nov Sun 06, 2016 12:45 am, edited 1 time in total.

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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 12:43 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11559
Location: Powell River BC Canada
The square wave could be the input of the scope being overloaded, or
to much drive level from audio oscillator.

_________________
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 Sun 06, 2016 12:50 am 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 363
Location: Westland, MI, USA
Yes, hadn't thought of that. Was presuming test equipment properly set up.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 3:52 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
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]Redefining problem: Both channels affected. Lw pwr out ( under 1 wt).Driving both ch simultaneously 200 mV @ aux.
V/R V12 & V13 ok. scope load box etc ok
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Pic#1 signal outLeft ch
pic#2 signal out Right ch
pic# 3 signal @ pwr grid Rt with hum?oscil
pic#4 pwr 2nd pwr grid RT ch
pic# 5 pwr grid left
pic# 6 2nd pwr grid Lt ch
pic#7 Input signal @ driver input rt ch
pic#8 input signal @ driver input lft ch
pic#9 schematic


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 8:25 pm 
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Replaced coupling caps for audiophiles : hum/osc in the one grid Gone! Thanks BRET.
Low pwr out and distorted signal in (pic) persists.
nice sine at nfb when vol up

martin
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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 9:07 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 363
Location: Westland, MI, USA
What I notice about those traces, aside from the obvious distortion evidenced in the signal to all the power grids, is the input to the second power grid tubes (both channels) doesn't appear to be inverted; it should be 180 degrees out of phase with the signal input to the grid of the other (ie first) output tube. If its not inverted then there will be no push-pull operation of the power tubes and output power would be impacted significantly. This info. though, will be lost with a single probe trace. Can you get a dual trace from your 2215 by running ch1 as input to power grid and ch2 as input to 2nd power grid of the same channel, at the same time for comparison.

As to the distortion there is clearly a problem with both drivers, left and right. Usually, in problems common to both channels I think power supply, but since you have rebuilt it and checked its voltages and ripple, lets rule that out for the moment. Since the driver was recapped there may be a component problem. Only caps here are C68 and C70, otherwise all other components are resistors. What caps were used to replace C68 and C70? Aside from the caps, I see R101 and R104 are 470K, followed by R113 and R115 at 180K. High value carbon comp resistors should always be suspect for going off value. R101 and R104 can be checked in circuit. R113 and R115 would need one lead isolated for a proper check. Of course, there could be problems with other parts in the circuit but whatever is going on appears to be common to both channels.

Also, could you post the voltages that you found on pins 1,2,3,6,7,8 of V12 and V13 if you have that recorded.


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 10:48 pm 
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Joined: Nov Tue 01, 2016 7:33 pm
Posts: 23
Good pick-up! grids IN phase. Pic #1 pwr grids rt ch. #2 pwr grid left ch(one very weak

V12: #1-318----#2-116 #3--120 #7-- 0 #8-- 1.1vdc

V13: #1-257---#2--120 #3--168 #8--1.4 vdc


Thanks Martin


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 Post subject: Re: Tube push pull amplifier question
PostPosted: Nov Sun 06, 2016 11:36 pm 
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Joined: Jan Tue 31, 2012 2:17 am
Posts: 363
Location: Westland, MI, USA
OK. Now we're getting somewhere. No inversion and both tubes V12 and V13 do not have the correct bias relationship on the cathode follower section, pins 2 and 3, which should be around -1.0v vs. the -4V and -48V which you are showing for V12 and V13 respectively, not to mention that the absolute voltages you are reporting are way out of spec. even considering a 5% tolerance. The cathode follower sections are likely not operating at all, esp. V13, and the signal at the grid of the amp tube could be that which is leaking through, not sure.

So, presuming the power supply is OK, I would suggest checking resistance values around these two tubes including the two adjustment pots which may have a poor or unreliable contact wiper. Myself, when I rebuild a tube amp I scrap all the carbon composition resistors as they are not reliable after 50 or 60 years of service as many will have gone off value too much, or will have become noisy. As a replacement I use 2% tol. metal film and flameproof resistors in all such rebuilds. Yes, it is a lot of work but I have never had a post rebuild problem. If that sounds extreme, and if you want to keep the absolute vintage character of the amp then try new 5% tol. carbon comp resistors and verify their value before installing them.

Let us know how you are getting on.


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