1. please assist technical resolve SE tube amp drooped voltage condition.
2. vc508 older model, alterated as follows:
3. replaced ht rectifier to 3 ampere fast diodes type
4. added htdc snubber .1uf
5. added htdc electrolytic 1000uf x 450wvdc
6. added power switch snubber .005uf x 2kv ceramic disc
7. added line snubber .05uf x 250vac
8. added ac cooling fan stabilize PT thermal conditions
9. added shunt diode protection EL84 plate to cathode (same type as 3 above, 1kv x 3 ampere)
10. replaced OT using EDCOR 5k:8 x 15VA
12. schematic specifies 272vdc htdc but now measures 260vdc, a droop of 12vdc blamed on the EDCOR SE OT
13. this affects downline voltages for preamp section
14. yet tubes removed and energized deenergized, charged capacitors remain at 313vdc for week duration unattended, thus indicating no parasytic losses in circuitry.
15. is EDCOR voltage droop 12vdc SIGNIFICANT? amp plays rather sassy esp after 30 minutes, and plays even better when demanded continuously by guitar's signal. however, initial tone signature is dark voiced, later clearing into sassy wetness, on edge.
16. should i consult EDCOR on the droop 12vdc?
17. thanks in advance
Schematic says 272v, you measure 260v. What did it MEASURE with the old transformer? Unless we measured it we cannot assume it was at 272v then. Also we must verify what your mains voltage is during testing. a 4v shift in mains voltage should change B+ voltage by about 10v.
The schematic shows 60ma current through the tube, of which maybe 5ma is screen current. The schematic shows about 1.5ma for the two triodes.
What is the DC resistance of the old transformer primary? And what is the DC resistance of the new transformer primary? That might affect idle current.
For that matter, what is the DC resistance of the power transformer HV secondary? If your B+ is 313v unloaded, and even the schematic shows about 40v less than that with the 60ma load from the tubes, that indicated substantial resistance in the PT secondary. A shift in tube current will shift B+ voltage therefore.
COnsider the resistance of the PT secondary as a series resistance. Use Ohm's Law and that resistance to calculate what aamount of current must be flowing to drop voltage from 313 to 260, and then compare that to the 60ma tube current of the drawing.
Is this the same tube with both transformers?
You shunted the tube with a diode? Disconnect that and see if it changes anything.
And schematic voltages are not precision. This one doesn't, but many will have a note telling us that voltages are only approximate and can vary by 20%.
thank you enzo
1. i will obtain requested answers of enzo
2. will return and post answers
3. i requested carlos of crate parts whether thermal fuse embedded inside PT could cause resistance and consequential droop. will obtain dc resistance primary and secondary both PT and OT, both OT
EDCOR voltage data
Sunday 2013 February 10th VC508 EDCOR OT droop investigation voltages
1. Have not yet swapped in original SLM OT to obtain energized voltage data, standby
2. Here are static DC ohms: OT factory 140.8 and .6, EDCOR 121.6 and .5
3. EDCOR installed tubes removed DC volts: 314.8 bridge output, and opamp rails plus minus 7.63
4. EDCOR energized DC volts: 258.6 input, and 251.6 output
5. EL84 energized DC volts: 251.5 plate, and 248.9 screen
6. HT_AC: 111.9 and 102.9 (none CT)
7. HTR_AC: 3.114 and 3.115
8. EL84 cathode DC volts: 6.66
9. EL84 input grid DC volts: .055 and 1mVAC
10. ECC83 input grid DC volts: 0 and 0, and .9/.4mVAC
11. ECC83 cathode DC volts: 1.109/1.036, and .4/.4mVAC
12. PT 2nd'ary lifted AC volts: 227 HT_AC and 6.88 HTR_AC or 3.425/3.432 ref CT
13. Utility line 120Hz 120VAC
original posting item 11 missing
11. replaced power amp cathode resistor with identical value of ten watt handling (factory five watt cermet)
resolved droop: the radial 1000uf 450wvdc electrolytic was installed inboard chassis using crafter's extra tacky adhesive (solvent safe). the cylinder's sides interfaced with the pcb and the chassis wall, and here the adhesive was used. the chassis is electrical earth and such close contact with the cylinder wall of the electrolytic allowed for energy jump out of the casing wall thru the adhesive and into chassis steel earth. all manufacturers guard against such close proximity installation because unless specialized casing is used, to electrically isolate casing from contact with peripheral circuitry, energy does jump! the zinc protection of the steel chassis turned color from receiving the energy leaking out of the electrolytic. after i affixed hv insulation sheeting onto the steel chassis and reinstalled the electrolytic onto this protective layer, the droop vanished! the hv insul sheeting was scrounged from a trashed smps enclosure flooring. i would have never discovered the discoloration of the chassis zinc coating had i not removed the cap to test it out of circuitry for possible compromise. it is okay. there are no observable indications of injury to this cap.
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