Capacitor ratings

[EniGmA1987]

It looks like there are two 160v capacitors soldered to the tube sockets and some 250v capacitors soldered to the power side of things. What is the lowest voltage rating that can be used in place of these capacitors and still be safe? Could I use some 100v rated capacitors in place of the 160v ones on the tubes? I am looking at some Elna Simlic II RFS capacitors as well as some Mundorf E-Cap MLytic Bi-Polar's, but both those capacitors are only rated up to 100vDC

Also, I have heard that increasing the capacitor rating for the ones attached to the tubes could result in a little bit better low end response. Is that true? If for instance I was able to get my hands on NOS Blackgate 150uF capacitors would that give better bass response than the stock 100uF?

[orson]

If you want to be safe you should go with the given voltage ratings. Sure there is a certain headroom but it is there for a reason.
For the coupling caps there is a table somewhere here in the forum with the low end frequencies depending on capacity and resistance of the headphones. But as long as your headphones are in the 300-600 Ohms range and they are not able to go lower than 10Hz you are fine with the stock value.
(Disclaimer: I am not a technical specialist and have my knowledge only from reading other threads.)

[Paul Birkeland]

What is the lowest voltage rating that can be used in place of these capacitors and still be safe?

100V electrolytics would be borderline.  As the 12AU7's age, they will push the 6080 cathode voltage up past 100V.  160V is a good rating here.

Could I use some 100v rated capacitors in place of the 160v ones on the tubes? I am looking at some Elna Simlic II RFS capacitors as well as some Mundorf E-Cap MLytic Bi-Polar's, but both those capacitors are only rated up to 100vDC

I think 100V film caps would be a better choice than 100V electrolytics.  I wouldn't use the Elnas or Myltics in this position with that voltage rating.

Also, I have heard that increasing the capacitor rating for the ones attached to the tubes could result in a little bit better low end response. Is that true? If for instance I was able to get my hands on NOS Blackgate 150uF capacitors would that give better bass response than the stock 100uF?

This depends a little bit on your headphones.  You might move your -3dB point from 5Hz to 2Hz, but having a larger capacitor at the output will bring other operational issues that are not at all worth the tradeoff.  The 100uF value was carefully chosen in the design, and we advise against going any higher.  (Though you can go lower with high impedance headphones)

[EniGmA1987]

Thanks for the reply.

100V electrolytics would be borderline.  As the 12AU7's age, they will push the 6080 cathode voltage up past 100V.  160V is a good rating here.I think 100V film caps would be a better choice than 100V electrolytics.  I wouldn't use the Elnas or Myltics in this position with that voltage rating.

Guess I wont use the 100v rated ones then since I want to stay safe and not make things explode at some point. lol. That was what I was wondering really, if the voltage ever would push up past that 100v mark. I dont really want to use film caps though because of their size and also price. I have some other things planned and it wont fit with massive caps in there. Im also not really looking to make this into a $1000+ amp with my mods, so trying to keep the capacitors under $40 each.

This depends a little bit on your headphones.  You might move your -3dB point from 5Hz to 2Hz, but having a larger capacitor at the output will bring other operational issues that are not at all worth the tradeoff.  The 100uF value was carefully chosen in the design, and we advise against going any higher.  (Though you can go lower with high impedance headphones)

Unfortunately my new headphones are 62 ohm phones, not ideal at all for the Crack. One of the things I have planned though is adding a 2nd output for low impedance headphones that will raise the resistance. Not the most ideal solution but I like these new headphones and Id rather try this out then look for either a new amp or new phones. I will be using some Z-Foil "naked" resistors so that they dont have any inductance in the resistor and these are some of the best sounding resistors out there, so hopefully it will work out well. Ill do this mod first to see how well it works. If this does work out, then ill probably go with some of the Blackgate's in 44uF size and maybe try out a 0.1uF Mundorf SilverGold Oil bypass. If needed, then ill add another 22uF to the Blackgate's

[Paul Birkeland]

Unfortunately my new headphones are 62 ohm phones, not ideal at all for the Crack.

I'd give the amp a try as-is, you may be completely happy with the results. 

Resistor modding is pretty tough, as the resistor really needs to be inside the ear cups of the headphones, or at the headphone end of the cable.  Additionally, there is likely a small optimal range of resistance that may work well with your headphones, at least until you plug them into a different amp, at which point you may really want to remove those resistors.

-PB