Refurb: 50s Sano Excelsior 25W

[xtian]

I found two leaky caps, the red Astron (notorious!) type. This totally sorted my tone stack--no more scratchy pots, and finally I have a good, stable voltage on that last triode that was mysteriously low.

Now, of course, on to more problems--there is lots of bacon-frying hissing and popping. Guess I need to replace more of these ancient carbon comp resistors. I read that the plate load resistors are a good place to start?


Also I'll have to address the hum. The grounding scheme is Flying Spaghetti Monster style. Lots of ground wires strung all over the place like a plate of noodles. Everything is grounded to everything everywhere!

[tubeswell]

Wow! tidy lead dress under the bonnet! What a score!

Re: the popping and spluttering - did you try different tubes (before you go replacing those resistors)?

[gingertube]

The noise will come from the higher value resistors with the most current through them - so yes start with the anode load resistors. You may need to go on to look at cathode bias resistors later but the anode is the right place to start.
Cheers,
Ian

[Phil_S]

+1 on replacing the plate load resistors. Frying bacon is a pretty sure sign. Good job on finding the leaky caps.

[xtian]

I replaced the load resistors, and some others that looked like eroded sand sculptures… This really helped. No more bacon sizzling.

This amp came with three identical inputs, each going to one triode, then mixed out to the tone stack. I disconnected one of these three, so there are only two functional inputs. One standard, and one with paralleled triodes, like in my 18watt build. Strangely, they sound identical to me.

Only remaining problem is hum. The power section is reasonably quiet (I can unplug the preamp from the power section easily, because it's a co-ax run, terminated with an RCA plug). But I've got a nasty ground loop as soon as I plug in the cable. I tried lifting the cable's shield on one end, but the hum is actually worse this way! Hum. My nemesis.

[Phil_S]

I'm looking at the picture of the input section and there is much that bothers me about it. It is really hard to know the source of hum and you don't say if it is 120Hz or 60Hz.

At the top of my list is relocation of the yellow heater wires. They are rudely mixed with the signal wires, passing too close -- maybe touching -- to not be a problem to generate 60Hz hum. I've attached a picture showing the heaters in the air. I'd do them like that, keeping them well away from other wires.

Aside from the heaters, as you observed, there are all manner of circuit ground "violations." I see:
-Buss wire on the input jacks in contact with the chassis
-Input jacks grounded -- not isolated from the chassis
-All manner of parts grounded to the socket tabs -- I think I might make a list and see if anything sticks out. I might even think about reworking the whole ground scheme to feed to a single node on the chassis, the probable spot being the ground lug on the terminal strip between the tube sockets. See if you can get all home runs to that point -- might be messy and overload the lug though. Then, I think maybe bonding the two chassis with wire braid or use the outer mesh of coax wire. I'm not sure what I'd do, to be perfectly honest!

I don't care for the B+ supply resting on/wrapped around the heater supply. Is there anything you can do to re-route the heater supply so it isn't next to the bottle side of the tube? Curiously, I might consider a small metal tube (conduit) for those heater wires, and then I might zip tie the B+ supply to the outside of the conduit on the side away from the tubes.

I think I've got some shielded twisted pair wire that will do the job nicely for the heaters, but it is 22ga. For a pair of 6SL7's at the end of the run, I think 22ga should be OK. If you PM me your name and address, I'll be happy to put a foot or two of it in an envelope and mail it to you. This will be simpler than jury-rigging a conduit! Then, you can zip tie the B+ wire to it.

[xtian]

Thanks very much, Phil. Your observation about the heater wiring is well taken. I'll mess with that and see if it makes a difference.

[xtian]

I replaced the co-ax run from the preamp to the PI (from the control panel to the main chassis). In order to test the grounding scheme, I left the shield unconnected on both ends. The hum is bad when both ends are left ungrounded. Connecting either one does nothing to improve the hum. Connecting both reduces the hum by about half.

Isn't this weird? I learned to connect only one end of the shield for sensitive signal runs, like from the input jack to the first triode.

Another disappointment, Phil: I moved the heater pair away from the B+2 wire, no change. I went the extra mile, too: the main umbilical from the panel to the chassis carries AC, heater pair, and B+2. I rerouted B+2 totally outside that umbilical, and…no change. In fact, with a length of B+2 wire waving in the air, I wasn't able to induce any extra hum by moving that wire close to other stuff.

[Phil_S]

I replaced the co-ax run from the preamp to the PI (from the control panel to the main chassis). In order to test the grounding scheme, I left the shield unconnected on both ends. The hum is bad when both ends are left ungrounded. Connecting either one does nothing to improve the hum. Connecting both reduces the hum by about half.

Isn't this weird? I learned to connect only one end of the shield for sensitive signal runs, like from the input jack to the first triode.

No, not weird. I think you've got the wrong principle in mind. You only connect one end of the shield to make that mesh part of the chassis. When you connect both ends, it creates a ground loop. In this case, you are strapping the two chassis together. You will need to attach both ends of the outer mesh to each respective chassis.

Another disappointment, Phil: I moved the heater pair away from the B+2 wire, no change. I went the extra mile, too: the main umbilical from the panel to the chassis carries AC, heater pair, and B+2. I rerouted B+2 totally outside that umbilical, and…no change. In fact, with a length of B+2 wire waving in the air, I wasn't able to induce any extra hum by moving that wire close to other stuff.

Well, like I said, it's hard to know. At least this suggests it's not the problem. I don't like this two chassis design. I have never dealt with one and am baffled by it.

Fire it up and start chopsticking it. Maybe something will become more obvious.

It is probably time to think about how to correct the ground problems on the preamp chassis. You know the power amp chassis is OK because unplugged the PA didn't hum. You can start by unbolting the input jacks and leaving them in the air free of contact to the chassis.

[xtian]

No, not weird. I think you've got the wrong principle in mind. You only connect one end of the shield to make that mesh part of the chassis. When you connect both ends, it creates a ground loop. In this case, you are strapping the two chassis together. You will need to attach both ends of the outer mesh to each respective chassis.

No, I get that. And here, I didn't want to connect both ends of the shield in the co-ax, because there is ALREADY a dedicated ground wire that is part of the umbilicus. So I KNOW I'm creating a ground loop by connecting both ends of my co-ax shield. But that's how it wants.

It is probably time to think about how to correct the ground problems on the preamp chassis. You know the power amp chassis is OK because unplugged the PA didn't hum. You can start by unbolting the input jacks and leaving them in the air free of contact to the chassis.

May have misled you. The power section by itself, with it's input grounded so it's at its quietest, has the system's hum. And at this point, I can live with it…because...

The pre-amp's HISS is LOUD. As a test, I ran my iPod into the power amp's RCA jack. No hiss, gets plenty loud. So I think this hiss is definitely coming from the preamp. And I replaced every damn component! Except that I replaced the old plate load resistors with carbon comps. Maybe I have to go to metal film?

Anyway, the BIGGEST problem, now that I reassembled the whole combo, is that the preamp 6SN7 tubes and the 6SL7 PI are microphonic, and feed back now that they're mounted in the vibrating chassis. I read that newer 6SN7's might not be as microphonic?

The takeaway is, I've made great strides, but I expected to invest 4 hours, and now I'm at 12, and this amp is barely playable. I'm learning a lesson about old combos. The speaker is the best thing in this box--really, it's quite nice--but I don't have another cab to put it in. Guess it's best to let it sit for a while. New Rocket Dog cabs coming from AmpMike to take my attention away from this beast.

[martin manning]

One wire only connecting the two chassis, and make sure the connections are clean and tight.
Center tap on the filament circuit? If not, then use two 100R's to ground in the power amp chassis.
Are jack and pot bushings being used to make ground connections? If so make sure they are clean. You can attach a wire to a good ground and touch it to the jack frames and pot backs that are supposed to be making grounds; maybe you can find a bad one that way.
I don't worry about mingling primary and secondary PT leads, but twist the filament leads together. Attached is a picture of my ODS... if this doesn't hum, why worry?

[xtian]

Thanks, Martin!

Hum has moved to the bottom of my worry list. See above.

[Phil_S]

... I expected to invest 4 hours, and now I'm at 12, and this amp is barely playable. I'm learning a lesson about old combos.

Sorry to poke fun here, but this is as rich as it gets. When you plunge into an old amp like this, it's got to be a labor of love. You could be on this for weeks or months, a little at time. Go with the flow.

It must have taken me 4 months to solve the problems on my Gibson Minuteman -- my very first tube amp. A genuine rat's nest. I got it done and I knew absolutely nothing when I started on it. That was almost 10 years ago. BTW, I'm old enough now that I can withdraw my IRA with no income tax penalty. If I can do this stuff, anyone can.

Good luck!

[xtian]

Tell it like it is, brother.