5V tap enough to power DC heaters and relays?

[Hardtailed]

The build I'm working on will have 5 preamp tubes and is sort of a high-gainer.
Since the Marshall 100W PT I'm using only has 7A on the 6.3V tap (with 6A just for the power amp, that's a bit short) I was thinking about adding an extra small transformer to power V1 and V2 as well as the relays.

Since I'd be running DC for those 2 tubes, I figured a 5V transformer would be more adequate since it gives 7V after full-wave rectification. That's less voltage to bleed away as heat.

Does that make sense?

Hammond has a nice 5V 3A transformer for not too much money.

Also, to get it down to 6.3V, do I really need a voltage regulator?
We need to remove 0.7V, and current will be 600mA, so a 1.2 ohm 1W resistor will do. With some heavy filtering (say 4700uF, bypassed by a smaller film cap), should be stable enough? Right?

As for the relays (would be run on a parallel line from the heater so they don't affect the voltage drop accross the resistor), I guess 7V is within the operating range of a 6V relay.

Any help/comment appreciated

[tedmich]

check Antek, they have nice toroids for dirt cheap
Antek - AN-0506

you can hid small ones in the case too.


6VAC-6.4VDC is possible, but most regulators need ~2v over to regulate (not "drop out"). 5VAC-6.4VDC is a little harder what with diode voltage drop, but being a little under doesn't hurt too much and some recommend it. No reason to settle for 5VAC if you need 6.4VDC, unless you have a tube rectifier 5VAC tap that's doing nothing... (plenty of 5VDC relays out there too!)

[Hardtailed]

check Antek, they have nice toroids for dirt cheap
Antek - AN-0506

you can hid small ones in the case too.


6VAC-6.4VDC is possible, but most regulators need ~2v over to regulate (not "drop out"). 5VAC-6.4VDC is a little harder what with diode voltage drop, but being a little under doesn't hurt too much and some recommend it. No reason to settle for 5VAC if you need 6.4VDC, unless you have a tube rectifier 5VAC tap that's doing nothing... (plenty of 5VDC relays out there too!)

Oh yes, I had forgotten about the voltage drop accross the rectifiers, so I may get the exact voltage I need without too much trouble.

My question was mostly about whether I needed or not the regulator, assuming I can get the voltage close enough without one.
In which case a 5V transformer would be sufficient.

The 6.3V 4A is actually 2$ cheaper than the 5V 3A (14$ at AES), but I just felt that the 6.3V (8.8V rectified) would simply waste power for no useful reason. However, if a regulator is better, then the voltage headroom is useful.

[defaced]

If you don't need the regulator, you don't need it. The heaters are just resistors, so load won't be dynamic or overly voltage sensitive (like TTL/CMOS), so a regulator is kinda overkill IMO.

[Joey Voltage]

If you don't need the regulator, you don't need it. The heaters are just resistors, so load won't be dynamic or overly voltage sensitive (like TTL/CMOS), so a regulator is kinda overkill IMO.

mmmmmm..... A lot of people go to great lengths to achieve quiet DC run heaters, in which regulation is only part of (passive RC filters can only smooth so much). It's one thing you really don't want to skimp on, and DC can actually make things worse if you don't know what you are doing, or good at trouble shooting noise.

Now, if you fully rectify a 6.3V tap you will get 8-9V unloaded, which might be just enough if you plan on running your heaters at 5VDC from a low drop out regulator, but what happens if you happen to be using the amp in a place with below normal wall AC?... It happens. I would honestly say screw it!, and just elevate your AC heaters, which in my experience in building super high gain is just as quiet, and takes less to *properly* set up.

[Enzo]

I can't imagine why you'd need a regulator. The current draw is as stable as anything can get in a tube amp. You can drop a volt with a resistor if you need to, or add a diode. The half a volt junction drop would be as good a way as any.

AC heaters POTENTIALLY cause hum when current flows from heater to cathode. If you elevate the AC heaters to a higher positive voltage than the cathodes, no such current flows.

The alternative to that is DC heaters. Well, AN alternative anyway.

6.3VAC is a signal of 17.8v peak to peak, so rectifying things with a reasonable amount of filtration results in millivolts of ripple - a tiny voltage peak to peak.

And recall that it is the flow of electrons from Heater to cathode that injects this hum. And that only can occur when the cathode is more positive than the heater.

[wizard333]

Whats the most elegant way to elevate the heaters?

[Hardtailed]

If I get this right, 6.3VAC once full-wave rectified can still be a little short on headroom to run a regulator, so might as well not bother with it.

So I was thinking: maybe stay with the 6.3VAC (it's cheaper and has more current capability, win-win) and use the headroom to implement a "C-R-C" filter and double the diodes (in serie) for safety. The R in the power supply will be tailored to get the voltage as close as possible. This should be clean enough compared to just one capacitor.

I did think about varying wall voltages which mean the end voltage won't always be 6.3VDC, but come to think about it: this is what happens when running the heater AC and nobody seems to worry about it. Not to mention how Mesa's Bold/Spongy switch changes the primary tap and therefore reduces every voltages in the amp (it sounds like scheisse too… but that's just an opinion

I would run the relays in a parallel node to the heaters node, just so switching relays in and out wouldn't affect the filaments operation.

As for my thinking for going with DC heaters on V1 and V2, I figured since I have to rectify it for my relays and I have to use an extra transformer, might as well go all out. My lead channel is somewhat inspired by the Mesa Mark IV, and this amp runs V1 on DC (and doesn't use a regulator, I think). I used to have this amp and it was still pretty noisy to be honest, but it had way too much gain for my needs!

Thanks everyone for the thoughts, this is all very interesting

[Enzo]

Yes, the mains voltage varies. Some nights it is exciting viewing watching the meter. I don;t know why, but around here, the mains voltage bounces. Just a little amplitude variation at a steady 2Hz or thereabouts. Not much, but visible on a scope. I often wonder - is it a beat note interracting with the rest of the national grid? Is it a reaction to some phase correction stuff at a local substation?

In any case, at 120v mains, your 6v will vary in proportion. 1/20 proportion. SO if the mains drops 10v, your heaters drop half a volt. 5v on the mains and your heaters move a quarter volt. And I wouldn;t worry about that. You coulod make an argument that that variation would affect teh emission. Maybe so, but while the heater voltages are changing at 1/20th the rate of the mains, your B+ is going crazy. If B+ is 480v, then each volt of mains variation nets you 4 volts on the B+. SO if the Mains went 10v astray, your B+ would change a whopping 40v. Now which thing would you worry about?

Not sure what you mean by for safety running two diodes in series. It is an easy way to drop extra voltage, that extra diode, but at 6v, your diodes are not under any stress at all.

You know, you are not stuck at 6v for those heaters. You can wire them up as 12v heaters. SInce you are buying the transformer to run them, they don;t have to be compatible with the other tubes. ANd they do draw half the current at 12v. And you could rectify that higher voltage and regulate it down for relays a lot easier.

And now even further. Two tubes? Wire them in series for 24v. A 20v transformer rectified would give you 28vDC, plenty of headroom if you wanted to regulate the 24. And 24v relays are common. You could run the two tubes in series - 24v at .15A - off a 24VDC supply. Or for that matter, just run them off 24VAC, a 24v transformer, and you can still make any number of relay supplies off it at the same time.

Does your MArshall PT have a bias tap? Or does it make bias from the high voltage?


And how to elevate AC heaters? Use either the center tap or a pair of resistors, and connect that to a positive supply in the amp. In fact, you could just connect one side of 6VAC to some +V. If there is a bias tap, you can add a rectifier pointing the other way and filter it - a nifty V+. You could make a voltage divider off the B+. One I always liked was when you have cathode biased power tubes, use that voltage. COnnect the 6VAC CT to the power tube cathodes. The connection draws no current, won;t affect the power tubes.

And I am not suggesting it, but I am fascinated when I see a circuit where they wired a preamp tube heater AS the power tube cathode resistor.

[Hardtailed]

Now which thing would you worry about?

I had an amp with a regulated B+ once, unlike old-school amplifiers which sound good most of the times and bad some times, this one always sounded bad (way too stiff). So I learned not to worry about mains stabiliy!

Not sure what you mean by for safety running two diodes in series. It is an easy way to drop extra voltage, that extra diode, but at 6v, your diodes are not under any stress at all.

I have this habit of always putting 2 diodes in serie per side, extra insurance if one fails. Didn't think about the fact that 6V means they won't be working really hard! On the

You know, you are not stuck at 6v for those heaters. You can wire them up as 12v heaters. SInce you are buying the transformer to run them, they don;t have to be compatible with the other tubes. ANd they do draw half the current at 12v. And you could rectify that higher voltage and regulate it down for relays a lot easier.

I did think about going 12V. One thing is transformers choice (as far as Hammond is concerned, and those are cheap and easily sourced to me, being in Canada) is better and cheaper for 6.3VAC. The 3A one is about 12$ and I like headroom in a PT.
On the other hand... I do have 1A 12V wall-warts gathering dust around the house: 2 12AX7 and about 5-6 relays shouldn't draw much more than 500mA at 12V... kind of worried about trusting something out of a cheap wall-wart though.

Also, I've found this circuit in another thread to get 12VDC out of 6.3VAC, sounds like fun (see attachment)

Does your MArshall PT have a bias tap? Or does it make bias from the high voltage?

It has a dedicated bias tap.
I was planning on elevating using a voltage divider on the B+ line, is there any advantage/downside to going with the bias tap? I assume it's all the same, except you have less voltage to drop?

I have a 8W 6K6 based "baby Marshall" where I connected the 6.3VAC line's artificial ground (dual 100ohms resistors) to the power tube cathodes, nice trick! But this build will be fixed-bias.

Many thanks

[Joey Voltage]

I can't imagine why you'd need a regulator. The current draw is as stable as anything can get in a tube amp. You can drop a volt with a resistor if you need to, or add a diode. The half a volt junction drop would be as good a way as any.


And recall that it is the flow of electrons from Heater to cathode that injects this hum. And that only can occur when the cathode is more positive than the heater.

Depends on what we are talking about. If we are talking about just supplying two stages, you probably don't need the regulator, but I would argue that DC would be unnecessary in this situation as well, and will most likely not offer any upper hand in noise performance, when compared to AC elevation and proper layout/lead dress.

Now if we are talking about DC heaters as a whole, My experience with them in my own use, in commercial amplifiers, and people who have asked me to trouble shoot simple RC DC heaters, and even regulated circuits dictates otherwise. and care should be taken regardless or not the supply is regulated. so much so that I don't think it is worth it in most cases. The heaters demand a lot of current, so the supply will potentially generate a lot
of noise regardless.

heaters POTENTIALLY cause hum when current flows from heater to cathode. If you elevate the AC heaters to a higher positive voltage than the cathodes, no such current flows

Very true! thermionic emission, essentially what you have is a diode, if you raise the heater potential enough it cant conduct. but the other source of noise is coupling to sensitive areas like high impedance nodes typically the grids.

is a signal of 17.8v peak to peak, so rectifying things with a reasonable amount of filtration results in millivolts of ripple - a tiny voltage peak to peak.

Well whats reasonable? I have it on good record that every commercial amp that I have owned that utilizes simple RC decoupled filaments@ 6.3V still hums badly. Also, somebody I trust quite a bit pointed this out to me, rectifying AC to DC can potentially be hrder on the PT

[Joey Voltage]

You know, you are not stuck at 6v for those heaters. You can wire them up as 12v heaters. SInce you are buying the transformer to run them, they don;t have to be compatible with the other tubes. ANd they do draw half the current at 12v. And you could rectify that higher voltage and regulate it down for relays a lot easier.

Hell why stop there!, you can use a series parallel arrangement, and wire them for 24V. or 4 in series for 48V. You can even run the 2 power tube heaters in series, and use 12V for everything.

[Enzo]

Well, I was merely responding to the question asked.

He already had the power tubes covered, he was asking about adding an auxilliary transformer for the two preamp tubes.

[Steve Conner]

Yes, the mains voltage varies. Some nights it is exciting viewing watching the meter.

Meter party at Enzo's place!

I've had reasonable success using voltage multipliers on heater windings. I once tried a full-wave doubler with Schottky diodes on an unused 5V winding, and that gave a little under 12V DC.

You can also hook up a multiplier to your 6.3V winding even as you're using it to drive your heaters. It's a little trickier to do without disturbing the balance to ground, but I figured out a circuit with three capacitors and a bunch of diodes that produces about 12V DC. I've posted it somewhere on the forum before.

So with either of these you could run preamp tubes wired for 12.6V heaters.

DC heaters are only worth it if you're going for Mesa Boogie levels of gain, IMO. And I always thought the DC should be regulated. If you're going to the trouble of having DC heaters, why settle for just getting rid of "most" of the ripple? Surely you want to squish it all. And yes, a huge filter cap will stress the transformer more, so you'll have to oversize it.

[Enzo]

Meter parties are the current rage...