Refurb: 50s Sano Excelsior 25W

[Structo]

Is it loose enough to peel it back a ways?

As far as injecting glue in, I'd be afraid of not getting it all stuck down.

I wouldn't use CA, it sets way too fast.

Man those are some old school carbon comps.

Be sure to measure those, they usually drift upwards by quite a bit.

[xtian]

Is it loose enough to peel it back a ways?

No.

[gingertube]

xtan,
I do a lot of antique amp restores.
To keep the "looks" of the amp I generally do not remove the old multisection cap at all. Just remove the connections from it an leave it in place. Replacement single caps are so much smaller that you can wire them point to point under the chassis. You may need to add a tag strip to take the cap connections, if so use one of those 3 tag things with the centre ground tag screw, a tube socket mount bolt is a good place to add it without having to have another hole/bolt.
Cheers,
Ian

[Phil_S]

Peeling paper covering? Hmmm...that's a tough one. I might be tempted to try some diluted water based wood glue and a small paint brush (art type, not interior wall type). Apply sparingly. Press down. Hope it holds.

One problem with glue is that new glue won't stick to old glue. Use a space age product that doesn't have this drawback. Weldbond is a product that comes to mind. You'll probably find it at your local Ace Hardware. I love the stuff.

Heat (hair dryer) may allow you to peel more, but I'm not sure that's a good idea.

[xtian]

Here's the main chassis fixed up. Added a new 1/4-inch jack for the speaker out (to replace the dodgy RCA connector).

Moving on to the pre-amp chassis next.

[xtian]

Well, poo. Got it working, but overall volume is very quiet. Voltages are weird. 63 volts on one plate of the PI and 178 on the other. Actual circuit doesn't match either of the likely schematics. Don't know what to do next.

[Phil_S]

Not weird. I recognize we don't seem to have the exact schematic, but I think you are working with a cathodyne PI. Those voltages seem about right. In the cathodyne (split-load, concertina) PI, it is the current on each half that should be about equal.

Can you post plate, grid, cathode voltages for all tubes, plus screens on the power tubes?

[xtian]

Wall AC: 118
PT secondary: 378 x2
B+: 430
B+1: 367

Preamp 6SN7
3 cath: 10.0
2 plate: 206
1 grid: 0
6 cath: 5.9
5 plate: 30.6
4 grid: 6.0

PI 6SL7
3 cath: 0.9
2 plate: 63.5
1 grid: 0
6 cath: 70.3
5 plate: 178
4 grid: 50.3

6L6
4 screen: 430
8 cath: 23.3
3 plate: 420
5 grid: 0.1

There is an RCA patch cable that runs from the preamp to the main chassi and to the PI. I plugged in my iPod, and I can get much louder output from the amp. So now I'm going to look at that low voltage on the plate of the preamp.

[xtian]

Preamp 6SN7
5 plate: 30.6

I swapped the two preamp tubes, and this low plate voltage remains.

[Phil_S]

First, I think I'd look at the DCR of the plate load resistors for the 30.6v and 63.5V sections. Then I'd look for something else that isn't right. Check DCV across the plate load resistor (find the voltage drop).

To get a good DCR reading on a plate load resistor, one end needs to be out of the circuit. Pulling the tube should do the job so you can avoid unsoldering it.

[xtian]

See photo. There are four 100K resistors that feed voltage to the four plates. Right in the middle of these four resistors is B+2 = 282vdc.
Voltage drop across these 100K resistors is 75, 75, 82, 250.
Respective plate voltages are 205, 206, 198, 30.6.

I pulled the tubes to try to measure the plate resistors. See photo: the south-east one measures 100K, but the three others measure about 50K. I think this is because of the other components in the circuit.

My 6L6s are sure putting out a lot of heat. I'm actually using RCA with black metal jackets. Perhaps these aren't meant for 420v plate voltages? (Yes, I see this is true: 6L6 rated to 360v, 6L6GC rated to 500v. I'm swapping tubes now!)

[xtian]

Woah. I started to dial the Bass control, and suddenly voltage on the plate in question jumped up to 120v, and the volume got much louder. Guess I know what to troubleshoot next!

[Phil_S]

What is different about the 30V plate that might make that voltage appropriate?

[xtian]

I bypassed the tone stack entirely. Voltage on the suspect plate went to 50 (odd, because with the tone stack in place, and the Bass control at 0, the voltage was up to 120!). Also, I got a LOT of hum.

I'm suspecting those red coupling caps in the photo.

[Phil_S]

If the PI tube has one gain stage and the cathodyne PI, can you take the output from V1 for input to the gain triode? Bypass everything on V2.