Voice of Music 1448, voltages?

[Raul]

Yesterday, after a few year, I took off the shelf my VoM 1448 to finally complete the recapping. As it has no hum I left the filter can untouched, I already replaced the under-chassis electrolytics and some resistors in the past so this time I replaced the 4 ceramic 0.022 coupling caps. Also for space reason I decided to use the russian k73-16
The amp sounds very good, no noise, channels even, no problems, I checked all the resistors and they were all within spec, so I decided to measure voltages of tubes and can caps.
With my surprise for the 6EU7 and can caps they are definitely lower than the schematics I have (the original one that I bought from Gary of VoM Enthusiast years ago not a Sams), as my amp sounds fine I am wondering if there are variations of this amp or another schematics somewhere (Sams?) that show different voltages or if there is something more I need to check.
Here are my voltages (they were the same also before changing coupling caps):

6AQ5 (these are however within 15%)
pin 2: 10, 10, 10, 10 (schematics 11V)
pin 5: 280, 278, 280, 280 (schematics 250V)
pin 6: 184, 184, 184, 184 (schematics not showed)

6EU7
pin 6: V3: 82 (120), V1 V2: 100 (135)
pin 7: V3: 91 (120), V1 V2: 127, 115 (160)

Can Cap
triangle (30uf): 168 (245)
half circle (80uf) 181 (275)
Square (50uf) 176 (265)

Exit of the rectifiers: 284 (290)

Should I worry about something?

 

[FlaCharlie]

Hi Raul - The PS caps in the can may not be causing hum but they may also not be in spec in terms of uF. I would consider replacing them with individual caps mounted underneath. I'm not at home and can't look at a schematic but I would also check the values of any PS resistors. Some of them may have drifted high, especially if they are carbon comps. Wirewounds are usually fairly stable. The voltage coming off the rectifiers is fine so I would look for issues somewhere between there and the tubes and that would be the rest of the PS.

 

[FlaCharlie]

Raul - I got home and took a look at the schematic. This is a voltage doubler PS with two 100uf 200v caps in the doubler. One of those is in the small can and the other under the chassis. The voltage coming off those seems to be in spec (284v) but if you only replaced the tubular one and not the one in the can during your previous rebuild, I would put new caps in both postions. I would be tempted to replace them with at least a pair of 220uf 250v. All the voltages after that are lower than spec so I would suspect that the 1k 5w PS resistor might have drifted high. There should not be that much difference in voltage between the 6AQ5 plates (~280v) and the screen (184v). Looking at the Sams schematic for the 1428, the screen voltage is only 10v lower than the plate. So that resistor is likely out of spec.

Can caps might also be out of spec (as I suggested above) or that some of the tubes that are supplied by the can caps are drawing way too much current and causing the voltage to drop. So check the various cathode resistors to see if they are in spec.

Try some different 6EU7s too, I'm sure you have some others. If you replace the can caps it wouldn't hurt to go a bit higher in value and use higher voltage caps since the stock ones are rated at 350v and 450v parts might be easier to find.

 

[Raul]

Solved! the problem was the 1K 5W resistor that the factory or (maybe more probable) some previous owner replace with a wrong 10K .
Putting a correct 1K in that place changed voltages as follows, much closer to the schematics:

6AQ5:
pin 2: 15, 15, 15, 15 (schematics 11V)
pin 5: 268, 265, 266, 267 (schematics 250V)
pin 6: 258, 258, 259,259 (not on schematics)

6EU7
pin 3: V3: 248, V1: 234, V2: 0 (not on schematics)
pin 6: V3: 111 (120), V1 V2: 139, 133 (135)
pin 7: V3: 119 (120), V1 V2: 158, 160 (160)

Can Cap
triangle (30uf): 237 (245)
half circle (80uf) 260 (275)
Square (50uf) 265 (265)

Small Can Cap (100uf): 138 (not on schematics)

Exit of the rectifiers: 277 (290)