Rectifier tube over voltage

[Karl5150]

My CD player runs through an old Scott amp that recently went silent, no smoke, sparks or smell and all tubes glow normally. I replaced the 5AR4 rectifier tube but no change so I started poking around under the hood.
The 5AR4 feeds a 4x20/450V (originally equipped with a 4x20/475V) cap-can that I suspect may have gone south. 
The schematic shows 430V from the tube to the can, but both tubes are feeding it 540V. This seems awfully high, even taking into account BHs +/-15% voltage check instructions. Does this point to something amiss in the transformer?
I ordered a replacement 4x20/475 can, but I don't want to install it if I need to deal with the transformer first.
Thanks for any help with this.

 

[mcandmar]

Sounds to me like the amp is not drawing any load from the power supply, are your output tubes glowing, check for open circuits on the anode and cathode etc.  You really need a variac at this stage as a 450v cap isn't go to last too long at 540v. Personally i would start with resistance checks before applying power again, then bring it up slowly on a variac to check voltages.

 

[Karl5150]

Thanks for the quick reply. The amp was running fine, moderate use over the last couple of years (since building my BH SEX & Stereomour II) and regular use over the holidays with no issues.
All tubes glow normally. The cap-can usually supplies 410/405/325 and I get nothing off of the taps.
The SS rectifier is also pushing more juice. Would different preamp tubes (5751 vs 12AX7) allow the power side to draw more since the amp uses negative feedback throughout?  (This is just me wondering out loud, I am in waaayyyy over my head at this point)
Thanks again.

 

[mcandmar]

I'm confused, are you getting 540v at the capacitor, or nothing?

 

[Karl5150]

I'm sorry if I wasn't clear: The lead off the rectifier tube to the cap reads 540V at the cap. The other taps of the can-cap read virtually nothing.
You were spot-on about checking resistance, one of the 55 year old wire wound resistor bricks is toast.
Like I said this is beyond me, is this the cause or a symptom?
Thanks again.

 

[mcandmar]

When you say toast, is it open?    If so that may explain the high voltage readings as the circuit is now disconnected, but the real question is why did it fail in the first place.  Wiring short, transformer short, or a faulty tube would be my suspicion.

Can you point out which resistor failed in the schematic?

 

[Karl5150]

Thanks Mark (correct?)
It is the 80 ohm/10W and shows as an open load.
To be honest, while that part of the circuit is virtually the same, not only is this a slightly different model but it has been modified. It is a kit version that the tone, balance and other sound shaping circuits have been bypassed in an effort to raise the sonic bar. (Not by me)
The rectifier tube will be replaced, as will all the resistors in the neighborhood and the 4x20 electrolytic.
The thought of the transformer being the source of trouble scares me.

 

[Doc B]

I'll offer a suggestion - recap the amp. Scotts often had interstage coupling caps that leak like sieves after all these years and it's pretty common to see blown transformers and dead output tubes because of this. Whenever I think of Scott gear I think of Major Winchester from MASH. I sold David Ogden Stiers a big pile of Scott gear back in the 90's.

 

[Karl5150]

Thanks for weighing in Doc. The amp has been recapped as well as having had the selenium rectifier replaced. The only original caps left are the ones in the circuits that have been bypassed, unfortunately along with all the wiring that goes with them.

 

[Natural Sound]

I'll offer a suggestion - recap the amp. Scotts often had interstage coupling caps that leak like sieves after all these years and it's pretty common to see blown transformers and dead output tubes because of this. Whenever I think of Scott gear I think of Major Winchester from MASH. I sold David Ogden Stiers a big pile of Scott gear back in the 90's.

"Know this. You can cut me off from the civilized world. You can incarcerate me with two moronic cellmates. You can torture me with your thrice daily swill, but you cannot break the spirit of a Winchester. My voice shall be heard from this wilderness and I shall be delivered from this fetid and festering sewer."
--Charles Emerson Winchester III

 

[Doc B]

Great line. He is a very nice guy, and a major audiophile and music lover. He showed up incognito with a few days stubble and a rumpled baseball cap at a swap meet where I had the Scott gear. Somehow looked vaguely familiar, then he wrote me a check and I saw the name. Told him I was a fan and he gave me that "please don't let on to people that I'm here" kind of look. At the time he was commuting to a production in BC from his home in Oregon. He invited me to sit in his new van into which he had put a very nice sound system and, IIRC, we listened to a recording of a group that he had conducted.

Bypassing a cap will not remove the short if it is leaky. It has to be removed and replaced. A bad output tube could also take out that PS resistor so you might want to check them as well.

 

[Karl5150]

I meant that the tone, balance etc. circuits, where the original caps are left, are no longer active. New wiring has been run to make the amp a simple power, volume & source affair, it's just that the wire, resistors and caps are physically still in place.
I had a power tube go red several years ago (replaced the pair) with no other issues 'till now. I don't have access to a tube tester, but a new quad of el84s would be cheaper in the long run than a repeat of this.

It was nice of you to be a good fan and let him continue on incognito. 

 

[Doc B]

OK, then it's pretty much down to a short somewhere after that fried resistor. Quite possibly it's the final power supply filter cap section that is bad. Definitely want to replace that cap along with the 80 ohm resistor before another power up.

 

[Karl5150]

Thanks all for the replies.
I took the advice and checked all the resistors and replaced all the WWRs, the 4x can-cap and the rectifier tube that included a tweak of adding a couple UF4007s in-line.
Fired it up on the bench, checked the voltages, and heard the familiar hum with my ear next to the sacrificial speakers.
It's back on the rack and Dave Grohl & company sound pretty d*#n good!
Thanks again.