EVJ PT Can i do this?

[BackwoodsJoe]

Wondering if i could rectify the (unused)12V-0-12V taps and use it to power two EL34 filaments in series? THX, Joe

[zaphod_phil]

Why would you rectify the heater supply for a power tube? That would be pretty pointless. Anyway, I reckon this 12-0-12V supply probably won't have the current capacity for EL34s, as it's probably for the SS FX circuitry you find in other Epiphone models. However, you can run a single EL34, 6L6, KT88 or 6550 from the VJrs heater supply.

[Mooreamps]

Why would you rectify the heater supply for a power tube? That would be pretty pointless.

One would do that to reduce the 60 cycle hum induced from the filaments, M!F!.

[zaphod_phil]

Hmmm, yes, I understand what the basic reason might be, but whether it would really buy you much could be debated. Especially if you consider the amount of filtering you would need to get all the ripple out of the current it takes to power EL34 heaters. It would probably be easier to elevate the power tube heaters to around 30V. In amps with even numbers of power tubes, you can also use heater phasing to get the hum to cancel out.

It did also occur to me that if you wanted to make use of both sides of the 12-0-12V secondary winding for providing 12V power, full-wave rectification would be one way of doing that. You would of course get ((12X1.41)-0.7)V = 16.27V, which you would then need to regulate down to 12.6V. As a result you would waste some of the power as heat.

[veets]

While the idea of DC for filaments is a good one, that is, to reduce hum, it's usually true that DC filaments cause more noise than they quiet. Sticking with well constructed AC is much better.


And you may notice that I'm able to post here without resorting to calling someone as intelligent and helpful as zp a M!F!

[zaphod_phil]

Yeah, a real strange comment, coming so soon after I asked people to cut Gary a little more slack.

I wouldn't place any big bets on my being intelligent, though! But sometimes being an EE does help you look more deeeply and analytically into issues. That being said, I had to un-learn a lot of stuff when I started getting into guitar amps. Basically, heater-induced hum is much more of importance in preamps, where you have high levels of gain, causing the smallest sources of noise or hum to be amplified. In reality, heater hum in power tubes isn't really much of an issue if at all. And certainly DC elevation to 10V or more above cathode voltage will really kill it.

[paulster]

From my experience DC heaters aren't necessarily any quieter than DC-elevated AC heaters unless you've got some dreadful wiring layout or a badly laid-out PCB. I mention the dreadful layout as the original poster was referring to the VJ which certainly meets these criteria.

Regulated DC heaters are a different matter though, and I've found that a seemingly quiet amp with AC heaters can have its noise floor reduced quite substantially by adding a well-regulated and filtered DC supply. You need an awful lot of filtering, exceptionally fast diodes and a regulator that can handle a fair chunk of current at a very low dropout voltage to make it work, so it's not the sort of thing you can generally just bootstrap into a build.

I use it as standard now for preamps via a drop-in PCB card, leave the power tubes on AC and there's not a trace of heater-induced hum in any of my builds. It definitely makes the amps more usable for recording which, IMO, is a good thing.

[tunghaichuan]

For the VJ Voodoo Baby rebuild that I did a while back, I found that it was necessary to use DC heaters. I used an eyelet board from Ken at Turretboards.com.

http://www.diycustomamps.com/valvejunior.htm#vjrebuild

I tried all the normal stuff: two 100 ohm resistors to ground, connecting the 100 ohm resistors to the EL84 cathode, but I couldn't get the low level hum out of the amp. Out of desperation, I cobbled together an unregulated DC heater circuit just like in the stock VJ circuit, and viola, the hum vanished completely. I used a bridge rectifier with the same specs as the stock one and a 10,000uF/16v cap, which is double the size of the stock value. The amp turned out to be much quieter than my personal modded VJ.

My $.02.

tung