Let's build a tube preamp

[Passinwind]

@agedhorse beat me to it but a 2 diode full wave rectifier on a transformer with a CT is equivalent to a bridge on a non-CT transformer in terms of the rectified voltage. In fact, for a unipolar power supply like in a tube preamp, using a bridge with a CT transformer doesn't make any sense because it will give you a bipolar one and you don't need a negative supply here. The 0.9 factor is the DC equivalent of the RMS AC voltage before any filtering. Once you add filter caps, the DC voltage will be very near the peak AC voltage minus any diode drops, so ~1.4x the AC RMS voltage. Since the transformer is rated for 330VAC at 140mA, it's going to be higher than that if loaded less, which is a given here since 2-3mA is a typical 12AX7 current, so 465VDC is on the low side of what you'll see on the HT here. It will probably be more like 500V+. Not a deal breaker but you'll need significant power resistors to drop it to a usable level. I typically like ~350VDC for a HT supply in a 12A_7 based preamp. Typical bias currents for 12AX7s are less than 3mA so even if you did 5 stages at 3mA you're only looking at 15mA. Let's round up to 20mA for fun. With 20mA, in order to get 350V from 500, you'd need 7.5k of series resistance capable of dissipating 3W. There are lots of 5 or 10W power resistors around so that's not a big deal. Given that much series resistance you could easily do CRCRC filtering and not bother with the choke at all. So the first thing to figure out is what tubes you're going to use and how you want to bias them so you can get a bead on how much current you need from the supply. That's usually how you decide on a PT but there's no reason we can't work with what you have. That said there are lots of pretty cheap options out there from places like Edcor or Triad Magnetics. @beans-on-toast put me on to this one last year:

my last tube preamp build

It's a basic 120V primary, 230V secondary toroid at 110mA, which gives you about 325V at 110mA but more like 370V at the 25mA I'm drawing out of it. Doesn't have a filament winding but you could buy a Hammond 166 for that. You'd still be looking at under $50. Edcor has a bunch of tube PTs that are around $50 as well.

Good advice. If you're only drawing 5mA from a 140mA trafo it probably won't be into self regulation mode much if at all without some form of extra loading. One compelling reason not to start at 500vDC is that you then need higher spec/more expensive/larger caps that you would if you started with something like this: my last tube preamp build or the next step up, which IIRC is 117-0-117. One of those plus this: Invalid Link Removed worked very well for me in Invalid Link Removed.

 

[Mark Reccord]

For a heater regulator, a simple Zener diode with pass transistor is a simple, effective compromise.

Yeah, this can work as well.

What is a pass transistor?

See this link for a better explanation than I can type. Essentially, you use an NPN transistor as an emitter follower with the emitter voltage set by a Zener diode connected between the base and ground. The emitter voltage will always be ~Vbase-Vbe, where Vbe is about 0.7V (i.e. a diode drop).

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[beans-on-toast]

What is a pass transistor?

Here is one way, figure 3 works well.

Transistor-Zener Diode Regulator Circuits


Edit: I see that Mark covered it.

 

[rufus.K]

Yeah, this can work as well.



See this link for a better explanation than I can type. Essentially, you use an NPN transistor as an emitter follower with the emitter voltage set by a Zener diode connected between the base and ground. The emitter voltage will always be ~Vbase-Vbe, where Vbe is about 0.7V (i.e. a diode drop).

Invalid Link Removed

Invalid Link Removed

Great stuff. My reading list is getting big.

 

[Mark Reccord]

Good advice. If you're only drawing 5mA from a 140mA trafo it probably won't be into self regulation mode much if at all without some form of extra loading. One compelling reason not to start at 500vDC is that you then need higher spec/more expensive/larger caps that you would if you started with something like this: my last tube preamp build or the next step up, which IIRC is 117-0-117. One of those plus this: my last tube preamp build worked very well for me in Invalid Link Removed.

With such a small load, the transformer's secondary voltage will be very close to its unloaded value. We could easily figure out what that would be if they provided the regulation spec for the transformer. Wonder if they would provide that if asked?

 

[Passinwind]

With such a small load, the transformer's secondary voltage will be very close to its unloaded value. We could easily figure out what that would be if they provided the regulation spec for the transformer. Wonder if they would provide that if asked?

It wouldn't be all that hard to just measure it with a dummy load, but my vote would be for just starting with the right tool. Link Removed

 

[beans-on-toast]

With such a small load, the transformer's secondary voltage will be very close to its unloaded value. We could easily figure out what that would be if they provided the regulation spec for the transformer. Wonder if they would provide that if asked?

The Hammond specs are at full load. They usually don't publish unloaded specs, the engineers have run tests for me to provide voltage and current data with various loads. They are very helpful.

 

[agedhorse]

A pass transistor regulator is, simply put, a current amplified shunt regulator, where the shunt regulator (Zener diode) sets the regulated reference voltage and the pass transistor acts as a series regulator that is controlled by the shunt regulator acting as a voltage follower (current amplifier).

Very simple, effective, inexpensive and can be made virtually indestructible with a few additional circuit components.

 

[beans-on-toast]

The TL431 is a bit of a step up. It can be configured with a transistor for higher current applications.

http://www.ti.com/lit/ds/symlink/tl431.pdf

 

[BassmanPaul]

Using a bridge on a CT'd secondary allows you to use a pair of series connected filter caps and use that CT to balance the voltage across each You don't need to put balancing resistors across them. Saves parts and is a little more elegant.

 

[BassmanPaul]

i'm afraid I haven't kept up with the thread.

If we are still discussing tubes, I like to start at the input stage. A 12AX7 et-al makes a nice little variable gain stage by feeding the out from the first section, pins 6,7 & 8 for lowest noise, into the in of the second. A feedback loop from the second plate back to the first cathode controls the gain. Another candidate is an EF86 where gain can be controlled by feedback from plate to the control grid. The former scheme I've used a lot. It's in the little pre-amp I use ever time I play and in a bunch of guitar amps I've built or modified. The second is one of my next projects.

 

[rufus.K]

i'm afraid I haven't kept up with the thread.

If we are still discussing tubes, I like to start at the input stage. A 12AX7 et-al makes a nice little variable gain stage by feeding the out from the first section, pins 6,7 & 8 for lowest noise, into the in of the second. A feedback loop from the second plate back to the first cathode controls the gain. Another candidate is an EF86 where gain can be controlled by feedback from plate to the control grid. The former scheme I've used a lot. It's in the little pre-amp I use ever time I play and in a bunch of guitar amps I've built or modified. The second is one of my next projects.

In the beginning of this thread, I wanted to use up the parts I have lying around. The thread us evolving organically, and that doesn't seem like it's going to go that way. That's fine though, we'really getting some good info on the table. I would still like to keep it in the 12ax7 and 12ax7 domain .

I can't wait to get to the first stage. Draw or find a sketch showing what you're thinking and post it so everyone can chime in. I'm trying to explain things to the thread subscribers who havent any experiance, so it's helpful when folks do that as well.

 

[rufus.K]

I just found this in my parts bin. It should be acceptable for heaters, yes?
If I run the primary in series with just 120v input, I should get half the output on the secondary, yes? It should still have enough current available after that, and it readily available.
http://catalog.triadmagnetics.com/Asset/F-212Z.pdf

 

[Mark Reccord]

The Hammond specs are at full load. They usually don't publish unloaded specs, the engineers have run tests for me to provide voltage and current data with various loads. They are very helpful.

All of the manufacturers spec full load but very few will quote regulation or no load voltage. I've also found Hammond and also Plitron to be very helpful when you go to them with questions. I still think they should provide at least the regulation because at least then you can quickly get an idea of what the voltage might be at various loads.

Using a bridge on a CT'd secondary allows you to use a pair of series connected filter caps and use that CT to balance the voltage across each You don't need to put balancing resistors across them. Saves parts and is a little more elegant.

With the transformer we're talking about here, that would give us a B+ of ~925V at full load....

 

[BassmanPaul]

With the transformer we're talking about here, that would give us a B+ of ~925V at full load....

Well, were we not looking for some headroom??

 

[Old Garage-Bander]

And DC for both heaters and B+. Diodes are cheap...

If DC heaters, how 'bout a heater polarity switch?

 

[cfsporn]

Cool. SS rectifier it is. Can you sketch up the DC conversion for the class please (for the heaters)

A bit late, but better late than never! Please excuse my horrible handwriting and I encourage anyone to give me feedback and/or criticism.

 

[rufus.K]

If DC heaters, how 'bout a heater polarity switch?

hmm never seen that

 

[beans-on-toast]

With transformer manufactures I find that sometimes they have the regulation data, you just need to ask for it. Other products where they don't, they are will to measure it and provide it. Hammond has always been good to me.

I prefer to source the proper transformer rather than drop the voltage through some means. One way people do it is with a zener connected to the center tap of the high voltage winding. The voltage across the zener is the amount that the B+ drops. The diode often needs to be mounted in a fairly big heat sink and higher power zeners can be expensive. A more cost effective alternative is to use a power mosfet with a lower power zener on the gate to control the voltage. You still need a heatsink for the mosfet. Changing the zener allows you to change the B+.

Better to find the right transformer.

 

[rufus.K]

A bit late, but better late than never! Please excuse my horrible handwriting and I encourage anyone to give me feedback and/or criticism.

What is output voltage then?
Do DV heaters need to be 12v, or can they also be 6v?