Pilot SA260: Low B+?

gullaz

Active Member
I acquired this some time ago, seller described it as "powers up but no sound". No wonder as the 33k 10w resistor
after the rectifiers was open. With my poor knowledge and skills I started the restoration.
New power resistor, new caps on the bias supply, and the first 40/80 can capacitor which was totally gone.
There is still a lot to do (first thing will be raplacing the selenium rectifier with silicon and the power tubes coupling capacitors) so I am not attempting to use it yet.
One thing that puzzles me is that after the rectifiers I measure only 405 volts dc instead of the 435 reported in Sam's schematics: I was thinking either weak 5u4gb rectifiers or (wouldn't be the first time) a mistake in Sam's sheet?
If any 260 owner read this I'd like to know what his/her B+ is.
 
What is your mains voltage relative to the nominal operating voltage?

What is the filament voltage on the tubes?
 
I operate this with a step down transformer (I live in Europe) AC out of it was almost 114 volts today.. Filament on the 5u4gb's was quite correct (almost 5v).
 
The nominal voltage for adjustment/specs appears to be 117V. So, operating at less mains voltage will result in less on the B+ and filament voltages too.

Also, bias set higher or other variables may result in some differences.
 
Most schematic voltages are +/- 10% in addition to what whoaru99 said, so you are in the ball park for voltage. One of the reasons for variation is the tolerance of the components in the circuit i.e. a 100ohm resistor may be anywhere from 90 to 100 ohms actual resistance given a 10% tolerance.
 
The nominal voltage for adjustment/specs appears to be 117V. So, operating at less mains voltage will result in less on the B+ and filament voltages too.

Also, bias set higher or other variables may result in some differences.
You are probably right, though I didn't think 3 volts less would have made this much difference in the high voltage supply.
I also have problems setting the bias (tends to be on the high side even with the trimmers turned all the way down) but this is probably a different issue concerning the negative grid voltage supply.
 
It should be roughly proportional, far as I know.

That's why I suggested checking the filament voltages. If they're all low too it could be just the supply voltage is lower than nominal.

As well, wouldn't low mains hinder ability to adjust bias more negative, compounding things?
 
Are the tubes properly biased? If they are drawing too much current, the B+ will be low. Could be weak selenium rectifier or leaky coupling caps. Original caps in these I believe were a paper type that is prone to failure. Mine had been re-capped sometime in the 1970s so my guess is they failed a long time ago. Still rocks those 70s film caps too.

If that doesn't turn up anything I can measure mine tomorrow for you at the same input voltage you're using.\

Oh, verify the 20 ohm resistors too. Mine were way off value. I replaced them all with 10 ohm and halved the original bias voltage numbers.
 
Actually I believe it's a mix of both problems: I get low negative voltage after the rectifier (77v instead of 85) so I can get to lower current enough in channel A only with the trimmer all the way down (probably selenium bridge started to be resistive).
Channel B instead, starts to draw too much current shortly after turning on, so my guess is one or both coupling caps are letting DC pass to the grids in that channel. So maybe, as you suggest, no use to measure voltages with the amp in such conditions.
Caps will be replaced as soon as I get the new ones from Russia.
I have also ordered 1% 20 ohms resistors to substitute the old ones on cathodes as well as some others which are slightly off.
 
A number of resistors in mine were off value.

The selenium had also been replaced with silicon in mine before I got it. I re-did the job as the original setup was very sloppy.

Probably worth replacing the rectifier and the coupling caps and checking again. If the new rectifier voltage output is too high to adjust with the bias pots, you may have to increase the value of R49, which is the 39K between the first and second bias filter cap. I don't think mine has been increased in value, but I'd have to go open it to confirm.

I had to replace my bias pots too. They had dead spots and cleaning didn't fix it.
 
mlvI67.jpg
An update: I continued slowly this restoration attempt; my approach has been kind of "bite and run" as my skills are very limited and the layout of this amp is kind of a nightmare to me: really crowded and imho lacking logic in some spots.
I started by subsituting R15, 16, 20, 35, 36, 40, as well as the four 20r cathode resistors; all of them were way out of specs.
Also C9, C10, C16, C17, C14 and C7 in fear they were leaking dc. Things got better in the negative bias supply but still the output tubes started to draw more current after a few minutes. In the end I decided to get rid of the selenium bridge and put on a couple of 1n4007.
I can now bias the el34 correctly, current is steady and also B+ voltages look correct now (as gadget73 said too much current draw on the el34 kept B+ on the low side)
Unfortunately, the amp still does not work properly: there is no hum or noise when playing but my cones start to oscillate badly if I turn the volume up a bit (autooscillation ?). Don't really know where to look next... Maybe worth changing those big carbon resistors?
I put a couple of picture of the work done so far and a scan of the schematics; I'd really appreciate some advise.View attachment 968332 B- supply.jpg
 
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have to agree, its not the easiest amp to work on. Too much stuff, not enough space, and a far from ideal layout.

A number of the other resistors in mine were out of spec. Worth measuring them at least. I wouldn't go whole-hog replacing them until you know they are the issue though.

What are you using for a source and a preamp? Just wondering if maybe something is wrong there, and not in the 260. What you describe is usually known as motorboating, and it can come from a number of things. Bad filter caps will do it but I believe you have replaced those?
 
Well, first I tried with its sp210 which is working perfectly with my sa232; in this case there also was hum (so probably a problem in the power supply dedicated to the preamp?) as well as the oscillations... after that I have hooked it up to a separate preamp known to work fine (a Hegeman Hapi2). In this case everything was quiet but the oscillation was still there.
Only the first bigger can was replaced, the small one seems to work fine (but you never know for sure), furthermore I am not sure I am able to replace it with all that mess of resistors attached to it.
Replacing most of the resistors would be maybe the best thing but I don't think I'm up to the task...
new.jpg B- supply.jpg
mlvI67.jpg
 
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try clipping a cap across the sections of the can you have not replaced. If the problem stops, you know where to look. anything from 10-47uf is fine as long as its high enough voltage, value isn't super critical for this.

I made a sort of buss bar for mine to tie the resistors to. The cans I used didn't have the same terminal setup as the original so a little creativity was required in order to make it all work. Its possibly not the neatest layout, but it does get the job done. What I used for the small can was single section, so the individual cap underneath is to replace that. As you can see, there are a lot of new resistors in there. The caps between the small signal tubes are K42Y Soviets, the old blue ones are the 1970s Arco caps that were in there when I got it.

B+ supply.JPG
 
Nice idea using a bus bar to connect all these 68k resistors, looks neater than the original layout.
I also noticed you substituted the two 15k parallel resistors after the first can with a single bigger (7.5k?) one.
In a few days I'll have some time and try to parallel a couple of caps to the old can as you suggested and see if anything gets better. I also intend to check the other resistors starting from the 5.6k's in the feedback circuit.
 
I also noticed you substituted the two 15k parallel resistors after the first can with a single bigger (7.5k?) one.

yeah it was largely for space reasons. I didn't see the point in wasting the little bit of room I had to work with. Same reason for the little buss strip. The caps I went with have pretty small terminals, so it just wasn't going to be possible to tie a bunch of stuff to it. They're snap-in caps meant for PCB mounting.
 
Hi-FiGuy: You're right, it was a typo: the actual value is 33r.

Resuming this thread because I wanted to update:
After renewing the PS caps and substituting all the 68k plate resistors the amplifier is now working fine.
The 68k carbons had drifted values dramatically (up to 100k and more). all.JPG
I'd consider the work done if not for a couple of things:
1)One of the new cans I used is larger than the original and it touches the power transformer so it gets quite hot; I already had this in a drawer but in time I will order one of the right size.
2) Most urgent, the mains cable needs changing but since the location of one of the wires is impossible for my skills to reach, in the underside of a very crowded connector, I will have someone do that for me.
The job is kind of messy but at least I made sure nothing is shorted.
One thing I noticed is that the power transformer gets quite hot: after a couple of hours 130/140 fahreneit (55/60 C)
Should I be worried about that?
 

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That temp is perfectly fine for the transformer to operate at. The idea of "hot" for a power transformer doesn't begin until temps rise over 160F, with safe operation (assuming nothing is wrong) up to as much as 200F -- although I hardly recommend operation at that temp!

Dave
 
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