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Post subject: Discrepancies in voltage from the schematic 68 PR
Posted: Tue Jan 08, 2019 3:08 pm
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My concern is the hot voltage, and I am regulating my AC voltage, I have 114 VAC going in.
I have around 408 VDC for center tap, schematic shows 420VDC, than it goes through a 1K 2W resistor and comes out 395VDC schematic shows 400VDC, than through a 18K 2W resistor to get 330 VDC, schematic shows 320VDC, than through 18K 2W resistor by the cap can and it gets 264.3VDC, schematic shows 240VDC. This 264.3VDC, makes my plates on all the other preamp tubes hotter than they should be. The other issue is Pin 5 6V6 Control Grid is -36.19VDC. Now the 1K 2W, and the 2x 18K 2W are new and in spec, the electrolytic caps have all bee changed, I am sure all of my resistor have drifted, I don't want to just start replacing them with only guessing, I do want to leave as much as I can original. Not sure where to start. Is it possible that the 2x 220K 5% resistor off of Pin 5 of the 6v6 could be the problem as I am getting -36.19VDC instead of -34VDC . Like I said I don't want to change them all. If you see any other thing out of wack let me know This is my other post on the PR
https://forums.fender.com/viewtopic.php?f=12&t=110747
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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Tue Jan 08, 2019 3:23 pm
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I don't see anything significantly amiss with your readings -- all seem within the tolerances that are common among the amps that Leo designed. OTOH if you start to see cook-off of any high-voltage components or red-plating of the tubes that would be cause for concern.

Arjay

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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Tue Jan 08, 2019 3:52 pm
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Retroverbial wrote:
I don't see anything significantly amiss with your readings -- all seem within the tolerances that are common among the amps that Leo designed. OTOH if you start to see cook-off of any high-voltage components or red-plating of the tubes that would be cause for concern.

Arjay

No nothing like that, I was poking around and had some concerns with the higher voltage, and wanted to make sure it was healthy to run that way, it sounds fine and its response seems good. I was checking out some older 6V6's I have, to see how they sound and where they sit in the bias range by the method of measuring output transformer voltage drop and resistance, and came to the conclusion none were close enough to be remotely matched, so I will continue using the matched EH 6V6 with the Mulard GZ34, until I pull the trigger on a pair of better matched RCA 6V6. the closest ones were 7.9, and 9.6 plate dissipation, and it sounded sterile and lifeless, one hot one cold, I am sure they will be fine in my 67 VC. So while I was in there I decided to poke around and was concerned. Thanks for reassuring me, sometimes I need that.
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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Tue Jan 08, 2019 5:28 pm
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Not a problem, I always enjoy helping out with vintage Fender amps.

HOOAH!

Arjay

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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Tue Jan 08, 2019 6:44 pm
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Pin 1 (plate) of V1a is the only reading slightly above specs(21.25%), you could add a resistor in series(inline) w/ the connection from the boards original plate resistor and the tube socket to reduce the voltage a bit w/o altering the originality other than a single solder joint. Maybe a 5k-10k 1/2 watt would drop the voltage enough. The rest looks good to me too.
Nice to see that someone else templates their work for posterity sake, surely makes life easier when looking for issues down the road. :mrgreen:
Edit: You could do some tube swapping but you will get different readings when tube swapping also so chasing exact numbers is extremely difficult. Remember when these specs were written up the NOS tubes we use today were actually NEW. The negative bias is set w/ the 27k fixed resistor in conjunction w/ the 25-50V capacitor, you don't have a measurement for it on your Schemo-highlights. Installing a bias pot is an option as well.
Revert to what Arjay said, if it sounds good and nothing is over distorting, oscillating or red-plating the amp is fine.


Last edited by sfceric64 on Wed Jan 09, 2019 6:23 am, edited 2 times in total.

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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Wed Jan 09, 2019 1:57 am
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There is nothing wrong with your amp voltages.

I never see a amp ( any manufacturer, any years ) with all voltages same as I see on schematic.

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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Wed Jan 09, 2019 11:55 am
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Retroverbial wrote:
Not a problem, I always enjoy helping out with vintage Fender amps.

HOOAH!

Arjay

Arjay Thank you for the reassurance, and your time, and I do appreciate all you do
sfceric64 wrote:
Pin 1 (plate) of V1a is the only reading slightly above specs(21.25%), you could add a resistor in series(inline) w/ the connection from the boards original plate resistor and the tube socket to reduce the voltage a bit w/o altering the originality other than a single solder joint. Maybe a 5k-10k 1/2 watt would drop the voltage enough. The rest looks good to me too.
Nice to see that someone else templates their work for posterity sake, surely makes life easier when looking for issues down the road. :mrgreen:
Edit: You could do some tube swapping but you will get different readings when tube swapping also so chasing exact numbers is extremely difficult. Remember when these specs were written up the NOS tubes we use today were actually NEW. The negative bias is set w/ the 27k fixed resistor in conjunction w/ the 25-50V capacitor, you don't have a measurement for it on your Schemo-highlights. Installing a bias pot is an option as well.
Revert to what Arjay said, if it sounds good and nothing is over distorting, oscillating or red-plating the amp is fine.

sfceric64, thank you for your comments and the time to look this over. It was the plate on V1 that had me a little alarmed mostly. I thought it would be good to make permanent notes to have a base line to start with for the future. Yesterday while pouring though my voltages I noticed I missed the negative bias voltage also, and made a note to do that next time I open it up. Lastly I reverted to what Arjay said, if it sounds good and nothing is over distorting, oscillating or red-plating the amp is fine.
stratele52 wrote:
There is nothing wrong with your amp voltages.

I never see a amp ( any manufacturer, any years ) with all voltages same as I see on schematic.

stratele52, I thank you for your assuring input and have now learned that I can add Fender Amps to the saying "close only works with horseshoes, hand grenades and vintage fender amps :lol:
Thank you all
mud


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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Wed Jan 09, 2019 7:46 pm
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I second, the "you are good to go" verdict. Unless you place rather complicated regulation circuits into a tube era amp --- you will never get precise voltages (compared to nominal values).

+/- 20-25% is a good ballpark. And this is at idle! Numbers can really jump, when you are honking on the thing.

BTW... how is the idle bias values on the output tubes?

:mrgreen:


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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Thu Jan 10, 2019 10:06 am
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BMW2002Ti wrote:
I second, the "you are good to go" verdict. Unless you place rather complicated regulation circuits into a tube era amp --- you will never get precise voltages (compared to nominal values).

+/- 20-25% is a good ballpark. And this is at idle! Numbers can really jump, when you are honking on the thing.

BTW... how is the idle bias values on the output tubes?

:mrgreen:
"+/- 20-25% is a good ballpark" This I did not know.
"BTW... how is the idle bias values on the output tubes? " Well that's what got me to open it up in the first place. I was trying to see if any of my older 6V6,s were close enough, especially these,
Imagewith a British made Mullard GZ34.
My tube tester showed they were close, but when I checked them in the PR using the bias method of measuring output transformer voltage drop and resistance, I came up with Plate Dissipation V6 = 7.9 w or 66.1% and V5= 9.6 w or 80.5% , than I tried it and it sounded lifeless, I switched V6 and V5, and V6 measured 10.1, I stopped there and put in the set off matched EH 6V6 I have, and I am looking at V6=8.4 w or 69.8% and V5=8.3 or 69.2% pretty close to the 70% mark, and it sounds quite nice
I was ready to add a bias pot to use the old tubes, but in the end did not add it. What I will do is.....save up my pennies to get a set of matched Vintage RCA's from one of the more reputable suppliers. Those other 6V6 I have will have to wait for the Vibro Champ to need a tube, as it only needs 1, but I think that they will be waiting a long long time as the RCA in there now I think are original to the 67 VC
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Post subject: Re: Discrepancies in voltage from the schematic 68 PR
Posted: Thu Jan 10, 2019 7:35 pm
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"I came up with Plate Dissipation V6 = 7.9 w or 66.1% and V5= 9.6 w or 80.5%."

Looks like the tube in V6 is tired. Time to put that holy black-plate RCA to pasture. :(
OTOH, V5 looks strong (often, if one of the push-pull pair is weak, the strong half will pull more current). Maybe, find another strong, quiet BP RCA 6V6GT to go with that strong tube? :)


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