My first foray into the KT88 world

Discussion in 'Tube Audio' started by mroboto, Jul 6, 2018.

  1. mroboto

    mroboto Super Member

    Messages:
    1,433
    I have a few vintage solid state amps and receivers that I bought between years 2000-2010 but almost exclusively listen to my tube amps for a few years, they sound a little less "congested" to me, + I can roll tubes to find the better tone and wamness-softness combinaison.

    I restored some solid state stuff, replacing all electrolytics, trimmers, but upgraded the tube stuff with decoupling capacitors.

    I will do some upgrades on this one too, as I do to almost everything that I own. if this is an easy fit, I will add one or two 4.7uf 630V CBB capacitors + one 0.1uf 1000V Ceramic disc directly to the big B+ cap, and will look to add some of the right values to other electrolytic caps for exemple: a 4.7uf 100V CBB to a 25V electrolytic, and maybe add some Ceramic disc caps there too.

    There is someting like 14 electrolytic capacitors inside this amp and it's all point to point, cannot find an easier work to do.

    This will tend to get the amp very fast , dynamic and silent.

    I will bypass all electrloytics with decoupling capacitors except the ones between the rectifier and the choke, and the coupling caps.

    Sure it will sound fine.
     
    Last edited: Jul 10, 2018

     

    Please register to disable this ad.

  2. mroboto

    mroboto Super Member

    Messages:
    1,433
    I received the amp today, rather quickly from DHL as I ordered it nine days ago, works and sound as it should, very happy.

    The only issue is that one side cover of the choke vibrates and make loud noise, a dip of glue can do the trick, but it's not very elegant, maybe I will add a thin layer of synthetic foam between metal parts.

    On one side, the bell cover is tight, and the other is loose, don't know if this can be unmounted and tightened easily, I must find a way to solidify that bell cover.
     
    Last edited: Jul 9, 2018
    Tubeglowpio likes this.
  3. mroboto

    mroboto Super Member

    Messages:
    1,433
    I forced from the bottom to the top both covers, and that seems to have tightened them up, no vibrations.

    If they move, and vibrations come back, I will simply force them to the top again and add some glue on the bottoms of the covers to keep them up and tight.
     
    Last edited: Jul 9, 2018
  4. sKiZo

    sKiZo Hates received: 8642 Subscriber

    One more comment on a thermistor ... I usually use a CL90 inline with the incoming AC. Yes I know the amp probably exceeds the amps rating of the thing, but I've never had one fail yet. If one were to fail, it'd fail open anyway. Just make sure you leave as much air clearance as possible around it as these suckers DO get HOT.

    [​IMG]

    One other fun part about thermistors is they never open up entirely. That's a good thing with older amp designs that usually feel more comfortable at VAC lower than what the power companies tend to supply nowadays. I use a bucking transformer to feed a bit over 117vac to mine, and the thermistor drops that to around 116vac. B+ stays nominal in the lower end of the range, so good to go.
     
    Tubeglowpio likes this.
  5. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Messages:
    8,986
    Location:
    Macedon NY
    I wouldn't exceed the current rating of the thermistor - it's a fire hazard! This probably draws a little over one amp, so CL-90 will be OK.

    Another way to get a slow B+ start is to use a 5V4 or 5AR4 rectifier - the former is rated for 175 mA, which should be enough. Either will raise the B+, won't bother the KT88 unless it's running at max dissipation already. But the inrush thermistor will limit transformer inrush too, making it easier on the power switch.
     
  6. mroboto

    mroboto Super Member

    Messages:
    1,433
    I verified with my IR thermometer the temperature of the sides of the plates of the KT88, not the sides where you see holes and heaters, but the two other sides.

    I see 350°F on one side of one tube , and 305-310°F on the other side of the same tube, identical for the other tube.

    My amp is cathode biased.

    What are the temperatures of your KT88 when properly biased ?
     

     

    Please register to disable this ad.

  7. sKiZo

    sKiZo Hates received: 8642 Subscriber

    A lot of it depends on how much air circulation you've got. Proximity to other hot components could easily give you different temps on the same tube. If they pack the tubes and iron tight on the chassis as they're prone to do, it will certainly have an effect. I did a custom chassis for what's basically a Dynaco ST kit and by spacing the tubes and drilling the chassis plate for vent holes,

    [​IMG]

    I was able to drop the temps a good 80 degrees compared to the stock setup. ANY thermistor or fusistor is a fire hazard if you don't give it enough clearance. The bottom plate is also vented, so I get a good chimney effect, with the hot tubes drawing cool air up from underneath.

    Oh. And speaking of fire hazards, I'd also consider an exploding rectifier a fire hazard. I've heard the same about the felt bumpers that I added to steady the tubes. Ain't losing any sleep over that either.
     
    Bill Ferris likes this.
  8. Tubeglowpio

    Tubeglowpio Active Member

    Messages:
    322
    Location:
    AZ
    Let's just start putting asbestos around all components:D
     
  9. mroboto

    mroboto Super Member

    Messages:
    1,433
    Good to know.
     
  10. sKiZo

    sKiZo Hates received: 8642 Subscriber

    Fire hazard? HA!

    [​IMG]
     
    autoanalog and Tubeglowpio like this.
  11. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    7,083
    Location:
    northern cal.
    Many of these kits from China use stainless steel chassis. Drilling holes in them can be difficult. I burned up several bits and one even welded to the chassis. Adding a separate selector is a good idea.
     

     

    Please register to disable this ad.

  12. dowto1000

    dowto1000 Active Member

    Messages:
    188
    Absolutely STERLING advice. Bravo.

    And on a SE amp, assuming you have a preamp, or some earlier method of volume control, ( I "eliminated the line stage, and JUST KISS-use a quality attenuator " ) you want to ELIMINATE the errors in that design, I can easily spot:

    1) a CRUMMY sounding pot, on the input of the amp, degrades the audio signal at the lowest level the signal has inside the amp. NO pots, on inputs of amp.

    2) Likewise, eliminate the RCA input jacks, whenever they are inadvertantly positioned by the unknowing manufacturer, at the rear of the amp.

    Re-position the RCA input jacks within two inches, maximum, of the amp's INPUT tube socket. Use high quality wire, I like 19 AWG or 18 AWG high quality SILVER, less than 2 inches long, on the hot, and doubled-up, 2 inches, to ground.

    In a SE amp,, one must " get started on the right foot ", with the RCA jack placement. FREE stuff.

    Dowto1000
     
  13. dowto1000

    dowto1000 Active Member

    Messages:
    188

    Current Inrush LIMITERs are SMART additions for long term relaibility !!!!! How about a CL-90, with a 0.1 or so, 1000 VDC cap across it??

    Dowto1000
     
  14. dowto1000

    dowto1000 Active Member

    Messages:
    188

    Its BEST, IMHO, for all to come on at once . I always use a CL-90 and a H.V. film cap across it.

    Dowto1000
     
  15. mroboto

    mroboto Super Member

    Messages:
    1,433
    I installed 3 thermistors this morning, (120 ohms) NTC 120D15) it's similar to CL-90) one on the AC input, and one on each of the rectifier inputs.

    The plates of the tubes are now at 340°F on one side, and 295°F on the other side of the tube.

    I want the output tubes to last for years and years, and I think this is on the way to.
     
  16. mroboto

    mroboto Super Member

    Messages:
    1,433
    I am about to add a 4.7uf 630V CBB bypass caps to the big 330uf 450V electrolytic, and something caught my attention.

    Please, look at this pic, clic on the link at the right of "Document Joint" to see full size image.

    https://www.cjoint.com/c/HGkriBpp0qs

    The small capacitor at the right of the big one seems to be tied directly to a black wire of the PT.

    This is not on the schematic, this is the only capacitor that is not on the schematic.

    Schematic.jpg

    Maybe to isolate the center tap of the PT from the ground ?

    The chassis is not grounded with the 3rd prong of the power cord, and I hesitate to take it grounded.
     
    Last edited: Jul 10, 2018

     

    Please register to disable this ad.

  17. peterh

    peterh AK Subscriber Subscriber

    Messages:
    1,840
    Location:
    Gothenburg,Sweden
    The glass in inpermatible for IR, thus the temperature you measure is the temperature of the glass.
     
  18. mroboto

    mroboto Super Member

    Messages:
    1,433
    Sorry to be skeptical, but I do not think the glass reach 340°F, that would lead in instant burn to the skin to touch for a 1/10 or even 1/20 fraction of a second

    I made a quick search on the internet to see what people say about that and it seems that the simple answer is "that depends".

    But, maybe the glass can be a part of the infrared thermometer temperature calculation.
     
    Last edited: Jul 10, 2018
  19. mroboto

    mroboto Super Member

    Messages:
    1,433
    I still haven't installed any bypass capacitors and the amp sound punchy in the bass frequencies, mids and highs are good for a stock, still not hot-rodded amp.
     
  20. mroboto

    mroboto Super Member

    Messages:
    1,433
    So, why don't you put two thermistors in parallel - in series - with two others thermistors in parallel.

    That would nearly double de rating, while saying a good resistance, will heat less, will be a little bit more resistive, and your amp will run cooler.
     

Share This Page