Help needed to upgrade the capacitor in OTL tube amp - Page 2

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.

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

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

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

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

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.

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

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.

Quote:

Originally Posted by 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.

Quote:

Originally Posted by WALL-E

...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?

 

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