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Help needed to upgrade the capacitor in OTL tube amp

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  1. telecaster
    Input impedance for the calculation of imput cap is the volume impedance gotcha! I now have pretty much all info to mod this great little amp, danke schöne meinen herren!
    I know that electro caps in the signal is the worst and I will switch them as soon as possible. We are not debating it here!
    Sadly my seller doesn't have 130µF in stock and the delay is two months, a downer... I think I will use two 100µF and two 30µF(to fit the case) per channel. Isn't it to detrimental to use four caps instead of two soundwise?
    Also I want to replace the last power cap but I fear funds are not allowing it. So is it possible to only add another film cap lets say 30µF at the end of the CRCRC filter and keep the benefit?
    Lastly i will change the elec imput cap to film ones, four 22µF is cheap enough!
  2. Zigis
    If you really use only 300 ohm phones, for output cap one 100 uF per channel is enough. 
    For input 4.7 uF good polypropylene film cap is enough, even for 10K pot, however in tube amps usually are 50K or 100K.
    Math is - corner frequency for input depend on input cap and pot impedance, for output - output cap and phones impedance. Larger impedance - smaller cap required for the same corner frequency. For 32 ohm headphones 470uF is the best, with 10x larger impedance 300 Ohm (Senn) 10x smaller cap - 47uF do the same job. Your 100uF cap is even extra.
  3. telecaster
    I want to  have a 2Hz -3dB point, to have a perfect 20Hz response, no compromise for that! That is why I need 265µF for the output cap.
    I have one question, can I replace the dual electro imput caps (2X22µF) for a single film 4,7µF? So I would need to change the resistor too?. How do I calculate its value?
    Also when I replace the last cap in the CRCRC power filter, can I switch the last elec 220µF cap unit for a parallel 2X50µF film? Would I also need to change the resistor between this and the preceding elec cap?
    Another question, isn't it better to bypass the power filter elec cap with something like film 2,2µF or higher value? I read a specialist saying that it's needed to have a better impedance curve without bump...
  4. jcx
    the Sound Science forum is the place to debate validity of listening protocols - I simply point out that there are reasons the Scientific/Engineering communities only respect Blinded tests of "audibility", with controls for level matching, frequency response - plenty of perceptual psychology, psychoacoustics refereed journals, textbooks explaining the the problems with sighted, "just listen", naive "I am mentally honest with myself and can suppress unconscious bias" claims
    if the OP really wants to know "what makes a difference" he should learn some of the issues, techniques for valid listening comparisons
    with the Figaro he has a great opportunity because the circuit is dual mono - he can change the parts in one side only and be able to compare with the unmodified channel - albeit in mono only
    this requires "blueprinting" the R,L channels for any other differences - tubes must match gm, mu, gain resistors to 1% between channels
    a few adapters, attenuators, some free SW and much of the required measurements could be done with a PC soundcard - even typically poor internal chipsets will get you a long ways towards having some technical ground to stand on
  5. Zigis
    With 100uF output cap and 300 ohm phones you got -3db at 5.3Hz, do you seriously think this is not enough? 
    For input cap formula is the same as for output, R is volume pot. 4.7uf is oversized in almost any situation (I know, you like it:)
    For volume pot 10K, -3db is at 3.39Hz, volume 50K - 0.68Hz, volume 100K - 0.34Hz
    Even with 1uF input cap you got 3.18Hz with 50K volume and 1.6Hz with 100K pot
  6. telecaster
    About the 2Hz -3dB point, I believe that the high pass fall off is really muting the bass from my not so deep HD650... I want to be sure I have the maximum freq bandwidth, so the 2Hz point is what I will stick too, even if I have to pay for 30/40€ more.
    About psychoacoustic:
    I learned to trust my feelings over the years not the numbers.. Of course I know a little about the scientific side of audio but I don't fall for the audiogoofy claims thats are plaguing this hobby. Years ago I grew up with a Meridian M2 active loudspeakers and I love them... When one blew up, I had to find an alternative and ended with a system that I didn't like at all, leading to listenning less and less music, two years later seeing my departure from being a music lovers to being a music listenner I took the money under the bed and shell out for a Meridian system... I was in love again!
    Back to the subject! I think I gave all wrong numbers about the values of the power filter caps, they are 820µF each, and the pairs located near the tubes are 220µF! Now I seriously think there are no imput capacitors and those 220µF caps are bypass caps ??!! Could you help me and tell me where is located the imput caps? That pair of 220µF is in parallel to the power caps if that helps...
    So all in all, about my 4.7µF imput question, I could use only one film cap instead of the CRC imput caps, if it is that?
  7. WALL-E
    Updated: full schematic with my multimeter reading.
    You quoted to my post on Darkvoice 339 thread but I think here on DIY I will continue the discussion.
    I drew scheme of our LF339 (incomplete) just to better understanding how it is built.
    As you see diagram below ther is NO imput capacitors at all,  to be honest only one capacitor is in signal pach output coupling cap and thank God. The two smaller electrolite working with resistor voltage divider part of power supply actually all EL are in power supply.
    I checked the voltage on cathode resitor is 90V so you don't have to be worry about SCR MKP 250V you chosen, 250V cap is more then enough.

    @ Originally Posted by telecaster I cannot locate a good seller for caddock resistors, I will then stick with Seta brand... Now could you lead me in calculating the correct value of resistors for the imput cap?

    Orginal value of cathode resistor is 430 ohm 50 wats so you have to use 3 resistor 1.3K ohm(1300 ohm) 20 Watt each in pararell = 433 ohm 60 Watt.
    C1,C2,C3 820uF
    C4,C5 220uF
    C6 130uf output coupling
    R5 430 ohm 50 Watt cathode resistor
  8. jcx
    false dichotomy - assuming you're human the results of perceptual psychology apply to you as well - "the numbers" for level matching, frequency response are necessary to establish a level playing field for effective listening for circuit, component differences
    one of the lessons from Science based psychoacoustic testing is that even 0.1 dB, ~1% SPL is heard as a "sound quality" difference - and not as "oh that's just because one's louder" - few can tell the difference is solely loudness until the difference is >1dB ~10%
    so to compare fairly by ear you have to match levels, and use some Blinding method to make sure the only information your brain can use is the sound entering your ears
    at frequency extremes the level match requirement gets much looser: http://home.provide.net/~djcarlst/abx_crit.htm
    the numbers in the abx threshold graph are useful to put upper bounds on "how bad" the frequency response can be without "sounding different" for the "uninteresting" reason that the level/EQ is off
    the required level matching does require measurement, "doing some numbers" - almost any soundcard or PC with analog line level in is adequate for this if you use the same channel for all measurements
    at the low end it appears that even 30-40 Hz 3 dB corner will not be "obvious" - but if concerns for subtle distortion drive you then as I mentioned above aiming for 5-20x lower corner frequency is a technical fix
    the above doesn't have anything to do with claims that something "must" sound  good just because the numbers are good - attending to these numbers are required for valid results from listening - whether you are gifted amateur, Golden Ears, Professional Musician, Sound Engineer...
    comparing magnitude, orders of distortion to guess relative audibility goes more like this:
    this doesn't address the (in)audibility of subtle capacitor dielectric distortion in a simple tube amp with 10000x higher 2nd order distortion from the tube, "simple" circuit's characteristics
  9. telecaster
    I agree jcx, I wasn't saying I don't believe in the science of measurement in audio, far from this. But I have no technical ability to do so. I am only an end user here!
    As for the tube distorsion, I happen to love it, and I would gladly hear it and not the electrolytic in the output. But as I have ordered some cheap bypass teflon and KP caps, I will first try to bypass those electrolytics with better caps and the filters too and see if I can improve things before going all out and going film only for the output caps.
    I had a hard time comparing sounds of different tubes, but only by hear and analysis I have achieved some findings. Going back and forth is really easier but my auditive memory is not too shabby, I am a musician.
  10. telecaster
    Thank you a lot WAAL-E!! I now see a lot clearer in this amp! I was sure I responded last night but here it is again! Thanks for the schematic! I'm glad the SCR 250V is right because nothing else fits in there hehe!
    So there is not imput cap? Isn't it strange or a genius of a design? Do you know what are the two white boxes sticking to the sides of the amp near the driver tubes?
    I have bought some 2200pF teflon caps and 0.5µF KP caps in order to experiment with bypassing, a lot of fun in sight!
    Thanks again for the diagram! I ask Yuking team about it, but they never responded...
    I have something that I need to ask :
    What is are the two soldering card for and what are the components on them?
    Also in Keph modded 339:
    What are the diodes for (on the soldering card next to the 2000pF capacitors)? I assume the tiny caps are for decoupling the power filter? The diodes are needed if I want to bypass my power filter too?
    Also how is called what is used to solder the resistor network? I need to order that part but I cannot find it unfortunately.
    BTW thanks for the calculation of the resistor but I remembered my high school days yesterday and found this as well. Appart from that my resistor is 470ohms 50w, not sure why he change the value except that availability of parts...
  11. kevin gilmore
    The schematic has errors.
    The grid of V2 is driven from the plate of V1.
    There is a resistor between the cathode of V1 and ground.
    There may also be a cap across this resistor.
  12. jcx
    any V1 cathode bypass would count as a "signal path" C if you're obsessing over Cap quality
    discrete diode bridges may be fast recovery - nowadays Schottky are available at tube B+ voltages - Schottky shouldn't need snubber C - only standard recovery diodes really need local RF bypass - questionable to be concerned here when the unshielded tubes sit out in the open
  13. WALL-E
    I drew it because looking on the picture of amp is difficult to descramble what is what?
    This diagram is incomplete I omitted a couple of elements I don't wont posting full schematic on public forum that would be illegal I guess?

    @There may also be a cap across this resistor.
    I think you mean local feedback, no there is not. It is pure SE design, the V2 twin power triode(two triodes in each glass envelope) is in parallel mode (not shown in  schema) to handle more current and reduce half the output impedance to better match with lower impedance load.
  14. telecaster
    I added up all the prices for the caps I want to change and the resistors too, I came up with 230€, it's a little steep right... That is for the top of the line vishay BC electro, and 265µF of output coupling SCR MKP per channel. Oh and every elec will be bypassed by it's SCR MKP too. I am sure the price to pay won't transform the 339 radically, but I expect a good step up sonically. Oh and as a bonus I have shop teflon and KP as bypass experimentation!
    WALL-E, thanks for the drawing I understand way better now! May I ask you what you want to upgrade in the 339?
    I have an advice to ask though, I wonder if those MKP is any good for output coupling duty : Would they be good as I never heard anyone using them as such...?
    They have 3.5mOhms ESR, huge voltage rating which is good I understand, they are MKP of course, and I can have a good price.
    Downside is it doesn't fit inside the central black cover, so I thought I could tie them together, markings hidden down, with the lugs facing toward the sides symetrically and have 8 units like that sticking between the two black covers remaining in the open air. For sure I'll have to find a way to secure them to the case. Oh the and total height of this monster cap would be 9cm, so it would be 1 or 2cm higher than the black box... Thinking about it, it's not that pretty, but that way I could pack high voltage MKP which otherwise I would have to stick with 250V MKP, it's the only way...
  15. jcx
    in the US at least, for any commercially sold product, absent binding contract secrecy terms - anything you can determine by any level of inspection, reverse engineering, is considered fair game - equivalent to being published
    the only restriction is that you cannot publish a copyright protected exact "mechanical" copy of the manufacturer's user, service manual drawing without permission - if you derive it yourself, hand draw or enter in your own schematic software is it yours to publish freely
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