DC Heater Supply In A Wreck Build

[RJ Guitars]

Hey Folks,

Has anyone done DC heaters on a Wreck build? I have been thinking about this to see if I can take a little more hum out of the Express or Liverpool circuit. At first I wanted to be pretty simple minded about the whole thing and just make a little DC board but then I thought of something else... Since they make these really handy and cheap little DC voltage regulators it occurs to me that you could do the heater voltage power scaling thing in addition to any noise reduction for a double benefit.

I found one other thread heading down a similar path - https://tubeamparchive.com/viewtopic.php?t=17779

Does anyone know what DC heating does for a guitar amp as far as the impact on tone? I know the HiFi Guys really swear it does bad things but they also work really hard to get rid of some other very cool sounding tube amp artifacts.

rj

[funkmeblue]

alessandro does it in his bloodhound

[Clyde]

RJ, if you're getting heater noise, V1 is probably the best place for dc heaters. I'm sure they're won't be any tonal differences (YMMV). For just the one tube, you can go without a regulated supply and do a circuit quite similar to the one you posted, but increase the filtering substantially. I use this setup on my amps with 4700uf. Doing a regulated supply is quite an undertaking because of the amperage involved.

[paulster]

I've done it on my Wreck builds but you need tightly regulated DC to make it better than AC. Poorly implemented DC will yield more noise than AC, and the Wrecks are great builds with their lack of interstage attenuation to highlight it.

I run the entire preamp on DC, and use 30,000uF of filtering before my regulator to give plenty of headroom for it not to drop out of regulation when the wall voltage sags.

Tonally it won't hurt it because it'll still be a low impedance source around cathode potential. You're more likely to impact tone by doing a DC-elevated supply which will put the heaters at a different potential to the cathode.

[Mark]

Have you identified all the sources of noise in the amp?

I wouldn't assume the noise to be all heater noise, is there any noise not being cancelled at the output?

I'd think if you turned the amp quickly off you'd hear the heater noise stop while the power supply discharges. Thus less noise and the amp continues to amplify.

[RJ Guitars]

In this case I am working up a design for an upcoming Liverpool build and I have a 5V-3A rectifier tap on my power tranny that some have suggested could be used as a DC heater supply to the preamp tubes... so this is an exploration of that option. In my most recent Express build I don't really have a lot of hum, but still audible in a quiet room. I am just curious if it can be tweaked out altogether and wanted to explore the good and bad of a DC heater supply.

I am using the recently built Express amp with an isolation cab to get the volume down to a reasonable level. The overall reduction in volume makes any hum almost inaudible although I have found that certain tubes hum louder than others. As you might expect before I knock the signal down with the iso cab, it has more hiss than hum but I won't try to conquer that one yet...

I've done it on my Wreck builds but you need tightly regulated DC to make it better than AC... I run the entire preamp on DC, and use 30,000uF of filtering before my regulator to give plenty of headroom for it not to drop out of regulation when the wall voltage sags...

That sounds like a lot of capacitors and capacitance - how much is enough if we assume the local power is reasonably stable?

Have you identified all the sources of noise in the amp...

I have not done any tests to isolate hum sources - That is a good suggestion before I put measures in place to conquer the wrong problem!

Thanks for the input, I appreciate learning a few things from you guys that have already been down this path before.

rj



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[billyz]

I have done it a few times and it never made any audible improvement. Ground layout and lead dress gave the lowest noise and hum.
Most hifi amps use regulated DC supplies on the heaters, they regulate everything.

[keewee]

Hi RJ, Hey I've used the ac to dc circuit from Steve Ahola's (blueguitar.org) website, works for me. It's basically a full wave rec. bridge 25 amp, you can increase the 3700 uf cap I use a 4700. If your power line voltage varies you may get some hum. Beats the dc regulator and all the heat they generate. The voltage comes in higher than 6.3 but within tolerance. I couple the d.c. heater with good transformer placement (using the earphone trick) and a real good 7025 tube in v1, I like the rca nos. but, I just like to experiment with stuff.

[paulster]

I've done it on my Wreck builds but you need tightly regulated DC to make it better than AC... I run the entire preamp on DC, and use 30,000uF of filtering before my regulator to give plenty of headroom for it not to drop out of regulation when the wall voltage sags...

That sounds like a lot of capacitors and capacitance - how much is enough if we assume the local power is reasonably stable?

I did a SPICE model for my board when I was designing it and 30,000uF was ideal for up to 5 or 6 preamp tubes in real-world conditions with the wall voltage sagging by 10%. That was my headroom calculation to ensure it never dropped out of regulation, because if it does then the noise is going to be worse than the AC hum you're trying to replace.

Then I came up with this little drop-in solution:
https://tubeamparchive.com/viewtopic.ph ... pcb#119846

The rule of thumb is that 10,000uF will give you 10% ripple for each amp drawn. So with 3 tubes you're just under an amp so you can expect 10% ripple on your 6.3Vdc supply with 10,000uF.

When it comes to calculating how much is enough you need to calculate the voltage that your cap will be charging to, at the lowest point when you factor in the ripple. This is the minimum voltage you'll be feeding the regulator. You can influence this by over a volt by choosing Schottky diodes for minimal forward voltage drop vs. regular 1N4007s. Then you need to see what your regulator's dropout voltage is for the load you'll be drawing and ensure that you have this plus 6.3V on the input side.

That'll give you precisely no headroom for the wall voltage changing, so then you need to give yourself a bit of wiggle room on the input because wall voltage is always a moving target.

It's definitely tougher to do right than just to do AC well, but the results are great. I've got a high-gain 100W amp which can run 5 preamp gain stages when it's in uber-metal mode(!) and there's not a hint of hum from it when it's dimed. There's absolutely no way you could achieve that on AC heaters, how ever carefully wired.

[RJ Guitars]

...It's definitely tougher to do right than just to do AC well, but the results are great. I've got a high-gain 100W amp which can run 5 preamp gain stages when it's in uber-metal mode(!) and there's not a hint of hum from it when it's dimed...

That is very Impressive!