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Post subject: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 4:49 pm
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Hi everyone,

I have a 1962 Fender Super Amp that had a pretty clean recap job done on before I got it. However the amp has been running hot since I got it. I noticed the other day that they changed the resistors as well as the filter caps and that one of the four main resistors is a different value. I've been eyeballing the schematic and I can see it calling for 27K 1 watt resistors between the main power caps, but one of the ones I have is .27k(?).

https://onedrive.live.com/redir?resid=FC02FEE0909EE12D!8112&authkey=!AMiBOwZgB1h2YSA&v=3&ithint=photo%2cjpg

The third resistor of the four is a different value.

http://www.prowessamplifiers.com/schema ... _schem.pdf

Here is the schematic I've been trying to wade through. I'm also a little unclear about the two larger caps wired together. From what I understand if there wired in series the 2 350V would be safe up to 700v?

Thanks for the assist everyone!


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 6:19 pm
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According to the schematic the values of the resistors in the power rail should be:

1.) just left of choke = 4700-ohms, 1 watt. (yellow-purple-red)

2.) 27k-ohm, 1-watt (red-purple-orange).

3.) 27k-ohm, 1-watt

4.) 27k-ohm, 1-watt

These values read left-to-right on the PSU rail --- of the photo.

Kinda hard to tell from your photo, the exact colors. To be sure about the amounts --- with power OFF, pull one leg and measure the DC resistance with a good DVM. When you think the arrangement is correct --- measure the VDC of each branch point and see if it is close to the schematic's values, with amp ON, at idle.


Last edited by BMW2002Ti on Wed Apr 06, 2016 6:24 pm, edited 1 time in total.

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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 6:24 pm
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BMW2002Ti wrote:
According to the schematic the values of the resistors in the power rail should be:

1.) just left of choke = 4700-ohms, 1 watt. (yellow-purple-red)

2.) 27k-ohm, 1-watt (red-purple-orange)

3.) 27k-ohm, 1-watt

4.) 27k-ohm, 1-watt

Kinda hard to tell from your photo, the exact colors. To be sure about the amounts --- pull one leg and measure the DC resistance with a good DVM.


+1

It's possible that the last tech to service the amp mis-read either the schematic or the color bands of the suspect resistor.

It happens from time to time.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 6:33 pm
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Thanks guys! Its been a long week and I kept looking at it thinking something was off, but what?

It looks like the two big caps have been wired in series correctly, and the 4 resistors match the above list, but from left to right they are going 27k, 27k, 4700, 27k. I guess I need to swap one and three with each other?

Next challenge is double checking the coupling caps. Those were replaced with some of the tiniest Spragues I have seen yet. I need a magnifying glass for those, lol.

Thanks again!


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 6:34 pm
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OK... just saw the problem. He put in the first caps in series. They should be 20/600 caps IN PARALLEL (to equal total capacitance of 40mfd).

You need to pull the 100/350VDC caps and replace them with as near 20/600 you can find. They should be pointing with (+) in same direction --- unless he re-wired the doghouse circuit. The caps under the doghouse should look like the Deluxe Reverb arrangement.

Check the polarity of those two big caps on the left of your photo. And the wiring going to them.

Image

These prolly the closest caps to OEM value, for those first two positions:

https://www.tubesandmore.com/products/C-ET22-500-FT


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 6:46 pm
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It looks he did a common mod and wired the first 2 in series, so 50Uf, 700V total. He has wired the power to the negative of one cap, jumpered the 2 together, and also wired in 470k resistors as regulators (?) or bleed off. Then there is 27-27-4700-27 resistors left to right from the output transformer. I am guessing that should be 4700, then 3 27. I think that might be the problem as the power tubes (one especially) seem to be getting an excess of current.

If that doesn't solve it, I will have a look for some bigger caps that will fit under there.

Thanks!


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 10:13 pm
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50mfd maybe a bit too much for the GZ34. The 470k-ohm resistors are prolly equalizing resistor networks across those caps. You maybe getting a lot of in-rush for the GZ34. Just wire the PSU back to what is original. First stage in parallel. With proper dropping resistors between each stage.


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Wed Apr 06, 2016 11:18 pm
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BMW2002Ti wrote:
50mfd maybe a bit too much for the GZ34. The 470k-ohm resistors are prolly equalizing resistor networks across those caps. You maybe getting a lot of in-rush for the GZ34. Just wire the PSU back to what is original. First stage in parallel. With proper dropping resistors between each stage.


+1

Arjay

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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Thu Apr 07, 2016 12:18 pm
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After studying the schematic, pictures of successful cap jobs and original wiring, I think the resistors are in the right place. I yanked out the whole mish mash involving the first 2 caps and wired in 2 f & t 22uf 500v caps in parallel. Amp is now not redplating 5881 tubes and seems to be operating nicely. I had a few moments of an almost wooshing noise when not playing, but I should probably take the time to clean up the solder joins in the doghouse. Is it just my ocd getting involved, or should I switch all the caps to F&T if I have enough over here?

Thanks for the help!

https://onedrive.live.com/redir?resid=F ... hoto%2cjpg


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Thu Apr 07, 2016 12:50 pm
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F&T's are the best electrolytics IMO.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Fri Apr 08, 2016 7:27 am
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Seems correct setup. Best to check B+ and all VDC at branch points. Compare to schematic and layout.

Should be within (+/-)15-20%. But, most importantly the voltages should drop relative amounts between each successive cap/resistor/cap branch point. Highest just before choke and lowest at the end of the cap string. The swooshing noise could be coupling of stages, if the voltage drop is not correct between these PSU rail stages.

Nominal from choke ---> last stage:

448VDC -> 415VDC -> 390VDC (not 300VDC as on schemo) -> 370VDC -> 350VDC.


Good luck!


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Fri Apr 08, 2016 8:06 am
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Printed voltage specs rely on the specified line voltage. That usually isn't what is coming out of my wall socket.
Rated B+ also depends on proper bias settings,
I know you probably don't have one, but a Variac helps make sense of the PS string.
Adjust the AC voltage with the variac to obtain the rated B+ voltage regardless of bias effects.. Then the string of lesser voltages should line up with the specs..


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Fri Apr 08, 2016 9:30 am
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Thanks for all the tips and info guys. The next time I can get the amp on the bench, I want to check the bias, the plate voltage and then I will check the filter cap voltages as well. Its sounding incredible right now, and not killing a set of old Tung Sol 5881 tubes so that is huge progress. Its still running a bit hot but there are 9 tubes in there!

Leo was a genius I must say.


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Post subject: Re: Fender Super 6G4A filter caps - need the experts help!
Posted: Fri Apr 08, 2016 10:16 am
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Define "hot".

Remember -- this is a vacuum-tube platform and even under normal operating parameters, everything on that chassis will be too hot to comfortably handle with bare fingers. Absent visible red-plating, wisps of "magic smoke", weird aromas, or abnormally-high voltage measurements, the amp is likely running fine.

Arjay

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