DC for heater in Crack

[nick-seattle]

Today I ordered parts for converting AC heater supply in Crack to DC. I looked at the existing designs of heater supplies and use PSU Designer II to verify that all is good.

Has anybody attempted to do the same? Any advise? Here is what I came up with base on PSU Designer - Bridge 1N4001- resistor .33 ohm - capacitor 22,000uf 10V

Not sure if it's going to make a huge difference, but doing it is huge part of the fun

[Paul Joppa]

It will exceed the current rating on the power transformer. Look at the RMS transformer current; maximum is 3.5 amps RMS, and a DC supply will exceed that significantly. Since dissipation goes as current squared, even a small excess can drive internal temperatures too high for the materials. Shorts inside the transformer are a serious safety hazard, since the high voltage windings can short to something grounded. It's unsafe, don't do it!

The same transformer is used in the SEX, and is just barely capable of heating a pair of 6EM7s (total 2.1 amps DC). At 2.8 amps you will generate nearly twice the heat in that winding - enough to melt the insulation on the wire. DON'T DO IT!!

It also exceeds the average current rating of the diode, which is 1.0 amps (2.0 for a bridge, since each diode conducts half the time). They will just blow up, possibly shorting out and blowing the fuse. Tiny shrapnel is the main hazard there.

Best bet is what people did with the old original Foreplay - buy a separate wall-wart power supply rated for 6 volts at the desired current - 2.8 amps for 12AU7, more for some other tubes.

[nick-seattle]

And software gave up on me again. So much research for nothing, well now I will have a lot of 1n4001(s) (the software told me that they are rated at 6A, so they should have held). I have a PS for 6V, so I'll try that.

BTW, any advise on connecting rechargeable battery to use for cathode voltage? I tried it per Morgan Jones' book (Voltage seemed appropriate), but there was way too much hum.

I'm like a kid in a candy store - want to try everything!!!

[Paul Joppa]

For ratings, always get the data sheet. They are usually available form the vendor's catalog site (Mouser, DigiKey, Allied, etc.) and otherwise easy to find on the web.

[Paul Birkeland]

Hello Nick,

If you wanted to pud DC on the heaters in the Crack amp, you could get a wall wart power supply to do the job.

From our experience, the noise signature of the prototypes we developed showed that B+ ripple was the big contributor, hence the CRCRC power supply (and the mentioning here and there of adding chokes to the power supply).

-PB

[nick-seattle]

Got it. Thanks for your feedback. I'm having difficulties finding a decent 6v 3a power supply to power both 6as7 and 12au7. I tried just 12au7 heater, but that didn't generate significant improvement - the noise level is already quite low and I was hopping to get it lower.  I would assume that increasing capacitance in PS would smooth things out. Any recommendations on what that should be, so the amp eventually starts? Or should this cap be the soft start for Crack?

[Paul Birkeland]

A good upgrade to decouple the noise from the power supply in the Crack is the Speedball, more specifically the C4S loads on the 12AU7. 

After that, you would want to look into adding some inductance in the power supply.  I could redo the model that we used in the beginning to re-establish what we found to be the acceptable hum threshold if you'd like.   Going to a larger capacitor could actually elevate the noise floor if ESR is not also considered in the equation (I also tried to get away with this in the design phase).

-PB

[coca]

Nick, I wouldn't say that all of your research was for nothing. The Bottlehead crew just provided great education  at no cost,  as a result of your research. Keep on plugging,  and good luck.

Bernie.

[Grainger49]

Today I ordered parts for converting AC heater supply in Crack to DC.  .  .  .  

Not sure if it's going to make a huge difference, but doing it is huge part of the fun

The classic reason for this is to reduce hum.  So I would ask do you have hum?  If so, could it be from something other than the heaters?

[nick-seattle]

Nick, I wouldn't say that all of your research was for nothing. The Bottlehead crew just provided great education  at no cost,  as a result of your research. Keep on plugging,  and good luck.

Bernie.

Definitely appreciate all the feedback and the cost of parts and the knowledge is not wasted - they will got into PS for preamp that I'm building.

The classic reason for this is to reduce hum.  So I would ask do you have hum?  If so, could it be from something other than the heaters?

I have not idea if it's from something else, but hum is very, very low when used as headphone amp - much more when I plug Crack as my preamp. Ruled out the tubes. Put 2 chokes. Still low frequency hum. So the next step for me was to get DC for heaters, but it looks like that's a challenge with 3A that's required without going into a separate transformer, but that would make Crack quite immobile - since 4th transformer definitively will not fit inside

[Noskipallwd]

It's a shame your having a hum issue. I've never had any hum in my Crack. None when it was stock and none of the many mods, including the speedball, produced any as well.  Chasing hum sucks, it reminds me of a dog chasing his tail. Good Luck.

Cheers,
Shawn

[Laudanum]

It's a shame your having a hum issue. I've never had any hum in my Crack. None when it was stock and none of the many mods, including the speedball, produced any as well.  Chasing hum sucks, it reminds me of a dog chasing his tail. Good Luck.

Cheers,
Shawn

Me neither.  Really dead quiet.

[Beefy]

I have not idea if it's from something else, but hum is very, very low when used as headphone amp - much more when I plug Crack as my preamp.

I strongly doubt that the problem is amp-induced hum. A ground loop surely.

[Doc B.]

Sometimes the thing to look at is what has changed from the stock configuration. Possibly the extra sockets are picking up hum.

[BNAL]

I have gotten hum from bad solder joints.