DC Heaters for V1 & V2

[big_teee]

Saw this on another site, and I nabbed it. lol
No one knew who the author was and no one had tried the circuit.
I'm looking for a way to tame the front end of a cranked up Marshall 2204.
Will this work and be suitable for a 2-(V1,V2) 12AX7 Circuit.
[ATTACH=CONFIG]15588
Thanks in advance,
Terry

[tedmich]

DC heaters are pretty straightforward, and favored by tube'ophiles BUT will do very little to "tame" a raging guitar amp unless its a very specific 60-120Hz hum in the preamp. Dropping a lower gain tube is likely the best way to tame things easily
tube gain factor
12AX7 100
5751 70
12AT7 60
12AY7 45
12AV7 41
12AU7 19

Id try an AT in either V1 or V2

[mac dillard]

Why wouldn't you just hook them up to a 6 volt lantern battery to see if that is what your problem is?

[big_teee]

Why wouldn't you just hook them up to a 6 volt lantern battery to see if that is what your problem is?

I was thinking of incorporating DC Heaters for Preamp in a new amp build. I have the 2204 where I can live with it, but want to improve the next high gain amp I build.
Does anyone want to take a stab at how that particular circuit works.
I figured there would lots of interest in a different layout.
Thanks,
Terry

[tedmich]

the circuit could definitely work, but I'd have to SPICE or breadboard it to know; where did it come from?
I've always been partial to the arrangement PV uses on their XXX/JSX/6510 amps with 24VAC heaters for some tubes (in series) and then 24VDC for the sensitive tubes (also in series) this all comes off the same tap for the fixed bias (voltage doubled) and gives the +/- 15-24VDC for the op amps too!

[jmaf]

I built three amplifiers the past 3 months, all 3 with DC heaters on all 12ax7's and regular AC heating on the power tubes.

I used separate secondaries for the power tubes and the 12ax7's - the PT's normally @ 6.3 VAC and the preamp tubes at 12.6 VAC. I then rectify this and regulate it via a LM338. I wanted to use a LM350 but found none in the local market - I tested the LM338 with 5 tubes(my amp uses just 3) at 12.6 volts and it gets warm at most, with a small heatsink.

The ripple on the DC heaters is negligible with all 3 tubes on. The LM338 has shown to be an excellent regulator - I let my test rig burn in overnight and the next morning the tubes were still on perfectly, voltage had varied one tenth of a volt and the regulator was only warm.

Using the chassis as a heatsink, I take the postive regulated output from the chassis and the negative from the rectifier + filter cap. The heaters look fancy with just one wire - the other taken from the chassis, via a bent pin on the socket.

The results? I think it was major overkill and totally not worth it. The hum levels are very low indeed, but I've achieved approximate results by just elevating the AC similarly to your schematic. So if I may give you my opinion after this recent experience of mine: not worth the effort of the regulator, rectifier, extra wiring, etc. The single wire heater wiring does look fancy, it'll probably catch the eye of a tech who opens it, but apart from that I totally would've built it using AC if I could - it would have made the project cheaper, saved me a few days and some space on the turret board.

[Chuck H]

I'm wondering what the advantage is to elevating the filament supply when it's not AC??? And if there is no advantage then whomever designed this circuit didn't know it. So, what else don't they know??? And why use their design???

If there is some reason to elevate the supply (that I don't know about) then I think it's a nice, simple and eloquent circuit. As mentioned, without SPICing it or building it we can't know the actual voltages. Of course current requirements aren't indicated and do change (increase) with the bridge rectified supply. Also mentioned is the a more regulated supply that is typical of DC preamps.

[Wombaticus]

I'm wondering what the advantage is to elevating the filament supply when it's not AC??? And if there is no advantage then whomever designed this circuit didn't know it. So, what else don't they know??? And why use their design???

If there is some reason to elevate the supply (that I don't know about)...

Elevation of DC heaters is still desirable if the circuit includes cathode followers or cascodes, etc., that run the risk of otherwise exceeding the maximum heater-to-cathode potential permissible for the tube.

[trobbins]

Most people find that AC supplied heaters with suitable heater wiring, and a humdinger pot with wiper to ground achieves all the hum reduction that can be garnered when the pot is positioned for nulling the hum contributions that leak to the grid of the input valve.

Elevating the DC voltage level of the humdinger causes an increase in the leakage resistance between heater and cathode, and hence can lower heater induced leakage in to the cathode bias circuit - although this rarely achieves a noticeable benefit unless the valve has a relatively low leakage resistance for starters, and the cathode bias impedance at mains frequency is relatively high.

These techniques have been well know for more than 60 years, but were typically only found in older amplifiers that were at the hi-fi end of the market, due to the cost of extra parts, and peoples awareness of noise levels (most sound sources probably had much more inherent noise and hum than the amp itself).

Unless the DC rectification of the heater winding is done sympathetically, or via a separate transformer, then an amp can end up with more noise than original condition. So recommendation is to firstly replace the fixed humdinger, or CT direct connection to ground, with a pot and trim the hum.

Ciao, Tim

[Chuck H]

Elevation of DC heaters is still desirable if the circuit includes cathode followers or cascodes, etc., that run the risk of otherwise exceeding the maximum heater-to-cathode potential permissible for the tube.

Ahhh, right. Since DC heaters are mostly used in high gain guitar amps, which often include cathode followers... Got it. Makes better sense now.

[Chuck H]

Just looked at the schematic again... Isn't that circuit going to produce about 8 volts with that bridge rectifier (after the forward voltage drop)? And what is the effect of the false CT before the bridge? Does that just act like a load and reduce the voltage back down to range? Won't that further reduce current capacity in a most inefficient way? I think I'm liking the regulator idea better.

[big_teee]

Just looked at the schematic again... Isn't that circuit going to produce about 8 volts with that bridge rectifier (after the forward voltage drop)? And what is the effect of the false CT before the bridge? Does that just act like a load and reduce the voltage back down to range? Won't that further reduce current capacity in a most inefficient way? I think I'm liking the regulator idea better.

Take a look at this link and the scematic at the end of the Manual.
They are using a bridge and grounding the - side.
http://www.vhtamp.com/manuals/VHT-Sp...tra-Manual.pdf
If that would work it would be simpler.
I wouldn't need the Led they have in the circuit.
Terry

[defaced]

The results? I think it was major overkill and totally not worth it. The hum levels are very low indeed, but I've achieved approximate results by just elevating the AC similarly to your schematic. So if I may give you my opinion after this recent experience of mine: not worth the effort of the regulator, rectifier, extra wiring, etc. The single wire heater wiring does look fancy, it'll probably catch the eye of a tech who opens it, but apart from that I totally would've built it using AC if I could - it would have made the project cheaper, saved me a few days and some space on the turret board.

Very insightful. I have found similar results I my high gain turret builds where the heaters are away from the circuit itself. In a PCB amp where the heaters run very close to the circuit components, regulated DC heaters makes since, but I've yet to cross that bridge personally. I think this is an instance where the build technique and layout dictates the schematic.

[jmaf]

In a PCB amp where the heaters run very close to the circuit components, regulated DC heaters makes since, but I've yet to cross that bridge personally. I think this is an instance where the build technique and layout dictates the schematic.

I've never built a PCB tube amp, so that's a good tip there to remember when I get to making one. Also, as you said, lead dress and organizing components properly in the amp are much more important than DC heaters IME.

[trobbins]

In that circuit, the midpoint of the AC winding, and the midpoint of the heater DC voltage, will closely match each other due to the symmetry of the bridge. The midpoints of the resistor dividers are connected together, and only noise and divider imbalance currents should flow between those two dividers. There is likely to be a benefit in connecting both midpoints to ground, or the elevated DC as shown, rather than just connecting the midpoint of the AC winding to the elevated DC, as it is likely to reduce rectifier noise passing through the DC powered heaters in to the grids and cathodes of the input tubes (the 25uF cap shunts that noise to ground).

The rectified DC voltage available to the heaters is likely to be less than 6.3VDC due to the voltage drop of the pn diodes. 1A shottkys are likely to get a DC level closer to rated, although I've found that heater AC voltage levels are usually on the high side in many amps and so that would help.