Increase of 299C power

Discussion in 'Tube Audio' started by oldman55, Jul 22, 2017.

  1. Retrovert

    Retrovert AK Subscriber Subscriber

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    kvflyer's comment about risk of hanging 100 uF filter capacitor off a rectifier is spot on.

    To answer oldman55's question about sizing the initial capacitor, the reason has to do with how the filtering stage works: the filter capacitors charge only when the applied voltage exceeds the stored voltage. The larger the capacitor the more charge it holds and the more stable the voltage will be when charge is drawn down, so the shorter the charging time to replace the deplete charge. (The remaining charge is roughly at the same voltage as the charging source.)

    So the primary capacitor is going to directly load the rectifier and transformer. This is why capacitive loaded rectifiers and transformers have shorter lifespan, or catastrophically fail relatively early on: the capacitor's short charging cycle pulls more current for a shorter time period. The bigger the capacitor the greater the charge stored, so the greater the current pulled. That increased current creates damages the rectifier tube and overheats the transformer windings.

    The CLC or CRC input filter adds the middle stage, L or R, to isolate the secondary capacitors (on the right) from the rectifier and transformer. The first C is kept small to not stress the rectifier and transformer, and the second C is made larger to better filter and supply current, and it takes longer to charge because of the L or R isolating it from the first capacitor. The DCR of the middle slows the charging rate to something which will not damage the rectifier.

    The rectifier specifications dictate the maximum loading by specifying the maximum first-state capacitance. These limits, will clearly set forth, are far too often honored more in the breach.
     
  2. oldman55

    oldman55 Well-Known Member

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    554
    I used JJ 32/500 all the way. Paralleled 32s for the first 60 uf, and then the next two 30 uf and finally for the 20s at the end of the line.

    For the four 75/75s on the bias side, Hayseed 75/100s in a can.
     
  3. kvflyer

    kvflyer AK Subscriber Subscriber

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    According to the Amerex tube data sheet, the MAXIMUM (Not design center) capacitance for the input filter is 60 uFD. Note, that is maximum. That is why you typically see a 40 uFD capacitor first in a resistive (not inductive) power supply. I am in Australia right now, so I can't walk out into my garage and look at my RCA RC-30 tube manual.
     
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  4. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    Could the transformer voltage be excessive (501v in lieu of 430v) due to only one channel operating? How do I figure out where the issue of the non-operating channel occurs? Before the recap, transformer voltage was 430v.

    The first filter cap is designed for 60 uf and the old cap measured 89 uf so I dont think that 64 uf is out of line as it is still within tolerances.

    I am really going to have to find a schematic that matches this unit If I am going to have any kind of success in tracking down the problem. Most of the power supply schematics are correct but all other circuitry is not. Checked HH Scott and hifi-engine so far. SN 165401.
     
  5. Tubologic

    Tubologic Well-Known Member

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    592
    The "Ceracaps" used in later generation Scott amps are actually high quality MYLAR dielectric and are quite reliable. Sometimes they could go open if you subject them to mechanical stress (repeated moving and bending of wires,etc...) but (electrical) leakage is very rare. Replacing them is not mandatory.
    And for all those who claims they're NOT Mylar, just have a look below:

    ScottCap.4.jpg
     
  6. oldman55

    oldman55 Well-Known Member

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    554
    Well its too late for that now.

    I could sure use some help finding a schematic for this 299c. Has both bias and balance pots.
     
  7. Retrovert

    Retrovert AK Subscriber Subscriber

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    The fact that exceeding the capacitance (and thus peak current) ratings for the tube did not cause Chernobyling is not a ringing endorsement for exceeding the tube's ratings as a design practice. As I explained above, the peak current to charge a larger capacitor will exceed the rectifier cathode's limits and damage the rectifier; it can also overheat the power transformer.

    To find your over-voltage problem, replace the actual amplifier circuitry with two resistors, each equal to one channel. That simulates the load. Now you can work on the power supply to stabilize the voltage without knowing if unloading the supply increases the voltage.
     
  8. Retrovert

    Retrovert AK Subscriber Subscriber

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    The polyester (tradename mylar) capacitors should be replaced.

    PET has problems like large dielectric absorption (charge is absorbed into the dielectric, then slowly migrates out, acting to average voltage levels causing distortion) and positive temperature coefficients which cause capacitance to rise with temperature. Some are oil-filled, some not, and the properties have changed with age. This is not the 1950s or 1960s. We have better options from modern film capacitors which are inexpensive and spot on in tolerance, without temperature or DA problems.
     
  9. Tubologic

    Tubologic Well-Known Member

    Messages:
    592
    No need to search very far, all the (old) Scott schematics can be found here in the AK Database:

    http://akdatabase.com/AKview/thumbnails.php?album=89&page=2

    Three variations of the 299 are included but it looks none have a bias pot (only balance). The 299D has both bias and balance pots (2 per channel,4 in total) and should be closer to your circuit. Worth checking.
     
    Last edited: Aug 11, 2017
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  10. oldman55

    oldman55 Well-Known Member

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    554
    Could you elaborate on this? Size of resistor and location?

    Checked them all again. It has a power/bias supply just like the 299C-C1 sub 4:
    power supply.jpg
    But then it is totally different from there onward. For instance, there are no schematics that I can find that have 470k and 270K ohm resistor along with a .1uf cap coming together in the vicinity of pin 6 of the power tubes:
    Mine:
    7591 base.jpg

    299C-C1 schematic:
    power tube.jpg

    And it has a 4uf/250v ecap near the driver like some later models.

    Any help would be appreciated as I am spinning my wheels right now.

    Helping track down where the dead channel died would be a help also. And could this be related to the jump in voltage or just coincidence?

    old and frustrated

    One more minor detail I should mention: when I fired it up, I had the bridge rectifier pos to pos until I saw a resistor starting to smoke.
     
  11. Retrovert

    Retrovert AK Subscriber Subscriber

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    Disconnect the power supply and drive a resistor to simulate each channel.

    That resistor should be sized to pass the identical B+ current load that is normally presented by the amplifier.

    You now have a power supply driving a resistor and can stabilize that without trying to find shorts, opens, etc. elsewhere in the amplifier.

    BTW: best to disconnect the heaters to the tubes. You do not want to have heater on without B+ applied as this causes barium orthosilicate to form on the cathode, permanently reducing its emissions, although this is a function of time.
     
  12. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    Is it possible that once both channels are working that the voltage will drop down to normal? If so, shouldnt I take the approach of finding out why one side is not working?
     
  13. Retrovert

    Retrovert AK Subscriber Subscriber

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    First step is to ensure power supply function is correct for both sides.

    You have too many variables in your equation. Simply and solve.
     
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  14. primosounds

    primosounds AK Subscriber Subscriber

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    have you checked your rectifier tube? You should get a SS replacement for the 5ar4 which in conjunction with a variac allows you to do testing at less than full voltage. This allows you to not stress your tube or components while verifying if your rebuilding is proceeding correctly.
     
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  15. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    After replacing blown fuse, put in new rectifier tube. Put in a 33ohm in lieu of 18ohm on the first dropping resistor on bias supply. Running it off the variac at 97vac, all the voltages, HV and bias, were in line. But still only one channel. Then an angry hum started and I shut it down.

    Without schematics, dont know if I'll ever solve.

    Are there any single components that could fail in the preamp or driver circuitry that would cause that channel to go completely silent aside from a resistor to ground?
     
    Last edited: Aug 12, 2017 at 1:26 PM
  16. Retrovert

    Retrovert AK Subscriber Subscriber

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    Even without schematics this is simple and requires only some patience:
    (1) Disconnect both channels, load power supply with dummy load (resistors), ensure it is stabilized and properly working.
    (2) Add ONE channel. This avoids loading which can drag down B+. Debug.
    (3) Swap channels, so only the second channel is being tested. Debug.
    (4) Add both channels. Debug.
    (5) Enjoy tube sound.​

    It appears that all of the documentation you need is available via the link above posted by Tubologic:
    The file list is:
    HH Scott 299 Operations Manual.pdf
    HH Scott 299 SAMS Schematic.pdf
    HH Scott 299 SAMS.pdf
    HH Scott 299 VTV12 (w-schematic).pdf
    HH Scott 299A Schematic D-299-C1 Sub 6.pdf
    HH Scott 299A Schematic SAMS.pdf
    HH Scott 299B Schematic SAMS.pdf
    HH Scott 299C Schematic D-299C-C1 Sub 4.pdf
    HH Scott 299C with 7199 Service Bulletin & Schematic D-299C-C1 Sub 0.pdf
    HH Scott 299D Schematic D-299D-C1 Sub 0.pdf​

    Which seems to include everything you would need.
     
  17. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    Unfortunately, none of the schematics fit my unit. For reference, I use pin6 on the power tubes to see if any schematics match up (other than power supply which is a 299C).

    I believe my channel problem is a signal issue inside the unit as the right speaker makes the same angry hum just as the left speaker when testing pin 2 of the corresponding 12ax7s (V2 and V102). Otherwise dead silence on right channel.
    Hopefully voltages will work out once both channels are amplifying signal. For now, running it at 97 vac keeps the voltages in line.
    Also noticed that after heating up for awhile, the balance on the bias of the right side tubes was way off (-7 and -19) and had to be rebalanced.

    Edit: Also, would it be advisable to add screen resistors? Will be in the area adding some test jacks anyway.
    recapped.jpg
     
    Last edited: Aug 12, 2017 at 6:02 PM
  18. kvflyer

    kvflyer AK Subscriber Subscriber

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    At the risk of sounding as though I am barking, please get it working before making modifications. Remember, it worked when it left Maynard, Massachusetts.

    Try the 233 schematic. It is very similar to some 299C amplifiers. There were no less than 7 different 299C variants. So, no surprise that yourse is different.
     
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  19. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    I think it is a 299C power supply section with the remainder being pretty much a 233.
    As all I did was swap out caps and resistors, I am at a loss as to what the problem is after going back and checking each one; So I was going to get the mechanical modifications done to keep things moving along until I figure out how to troubleshoot the issue. I know that it is amplifying on each channel, just one not getting signal down the line.

    Edit: And it also worked before I started swapping parts.
     
    Last edited: Aug 13, 2017 at 11:08 AM
  20. oldman55

    oldman55 Well-Known Member

    Messages:
    554
    Checking all of the solder joints, a "used" radial film dipped capacitor fell apart apparently from prior metal fatigue from bending the leads to different positions on a previous project. Replaced so now both channels are good and sound is good (some hum at present).
    However, voltages are sky high at 117 vac.

    On the HV rail:
    Design This unit
    430 512
    300 331
    270 295
    255 283

    On the bias:
    at silicon rectifier is -59 vdc
    at 12ax7 heater it is -47 vdc vs design of -40
    pots will only adjust bias to tubes to -23.5 vs spec of -20.
    I changed the first dropping resistor to a 33 ohm in lieu of the original 18 ohm. Maybe I need a 47 ohm?

    But I dont know what to do with the 525 vdc from the transformer. Screen resistor drops their voltage down to 490 vdc in lieu of 410 (or 425 if using the 233 schematic). Guess a couple of CL80s will help a little but not near where I need it to be.
     

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