Vintage Asylum

RE: Sherwood S-5500ii power transformer woes? Bad filament secondary?

24.27.41.140

Triode_Kingdom

A

In Reply to: RE: Sherwood S-5500ii power transformer woes? Bad filament secondary? posted by jamesbythebay on May 30, 2024 at 10:35:56

"With the PT out of the chassis, the filament voltages, measured from the center tap, measure ~7.xVAC, and ~3.5VAC."

That's odd. It should measure about 3.15 each side (loaded), or maybe 3.5V each side unloaded. Have you checked to see if it might be shorted to the bias winding? Also be sure you're not measuring from one end of the winding instead of from the CT.

Follow Ups:

RE: Sherwood S-5500ii power transformer woes? Bad filament secondary? - jamesbythebay 12:12:38 05/31/24 (1)
Triode_Kingdom,

Thanks for your timely and most helpful response. Reading it, I had one of those head-slapping "Doh!" moments. Yeah, I was so focused on my belief that I was taking measurements from the correct leads that I was ignoring what the results were trying to tell me. I'm pretty sure that I mis-marked one of the leads when I replaced them all.

And just like that, I was talked off the ledge.

Re-checking my continuity tests, I found no shorts.

Measured voltages @115VAC in: B+ is 171VAC, Bias is 25.7VAC + 25.7VAC = 51.4VAC, Filament is 3.3VAC + 3.3VAC = 6.6VAC

Many thanks, Triode!

Btw, I'm doing a long overdue PS overhaul on this unit. Have you worked on this model before, and made any modifications to the PS? If so, would you mind sharing your results?

Cheers,
James
- RE: Sherwood S-5500ii power transformer woes? Bad filament secondary? - Triode_Kingdom 12:59:50 06/01/24 (0)
  I haven't worked on a Sherwood tube amp since the '70s, and I don't recall if any of those were the S-5500ii. It's not difficult to analyze the schematic to see where improvements can be made. Whether there's sufficient physical space for changes is another story. My approach to the B+ supply would be as follows:
  
  1. Increase the value of the doubler caps from 120uF to 220uF.
  2. Replace R62 33Ω with a 0.5H choke.
  3. Increase the value of C62-40uF to 100uF.
  4. Replace R64 470Ω with a 1H choke.
  5. Add a thermistor in series with the power transformer primary.
  
  Other revisions:
  
  Add a 220Ω carbon comp stopper resistor to each 7868 screen grid
  Add a 1Ω or 10Ω precision resistor at each 7868 cathode for measurement
  
  I'm sure something similar has been done to these at one time or another by people who specialize in this model. Again, there are likely to be significant space limitations, especially as these are so compact. Have fun!

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