A nice little valve amplifier

[Bigun]

Having finished (mostly...) a big speaker with 15" single full range driver I now want to build a new amplifier to drive it.

I believe every DIY nut should have a 2A3 SET at some point in their journey and this thread is where I get mine. I know it's a 'been there done that' kind of amplifier but the devil is in the details and I hope some of you folk with share your knowledge as I progress.

Things I have decided on:

Output tube: 2A3 [1]
Output transformer: Sowter SE05 2.5k primary into 8 ohm
Input tube: 12AX7 [2]
Topology: Loftin-White


[1] and maybe I will also want to try 6C4C, 6P31S...
[2] and maybe I will also want to try 6G7, 6SF5, 6SL7...

I'll work up a schematic or two.

[JRKO]

Originally Posted by Bigun

Having finished (mostly...) a big speaker

big? you could most peoples ENTIRE stereo setup inside it

Driving it with a teenyweeny wattage toob amp sounds like fun!



Subscribed

[Bigun]

Originally Posted by JRKO

big? you could most peoples ENTIRE stereo setup inside it Driving it with a teenyweeny wattage toob amp sounds like fun! Subscribed

Yes, when visitors see this thing it takes a bit of explaining

So, low wattage. How do we get big sound ? - I believe one key thing is the power supply - it has to be designed not to hold the amplifier back, when it needs juice then it should flow freely.

To this end I have been advised to design a power supply which has low-ish DCR components and good transient response. Yet simple.

[disco]

Pictures, please!

You need to control that large, flappin' diaphragm.
It takes current, or stay away of the resonant frequency.

[sippy]

Subscribed.
Why isn't it built yet?

Yes, when visitors see this thing it takes a bit of explaining

I'd say: "Do you speak in stereo" (or 21.7 for that matter!)

[sippy]

Sorry, this thread is about the amp, not the glorious speaker you created.

[Bigun]

Well let's see what we got...

The speaker was designed to be free of cross-overs and other encumbrances and the amplifier should be approached the same, where it makes sense to do so.

The direct coupled approach means no transformers in the signal chain except at the output. Not saying trafo's are bad, but they are not a 'straight' wire and good one's cost real money.

Also means no capacitors. The component everyone loves to hate but we have them everywhere. In a SE Class A amp you can't fully eliminate them too easily and usually we have at least one in the power supply. This being the infamous 'last cap' which the signal current must flow through to complete it's circuit.

Electrolytics are the most unpopular because 'they' say the sound is bad, but I see a benefit in eliminating a technology that is known to have a limited life span and hence avoid having to replace them down the road. We can eliminate electrolyitics and design the amplifier to last a lifetime. To do that we need to avoid needing large caps without generating excessive ripple from the power supply. We do that using the Loftin-White topology (the secret sauce being the cap + resistor connecting the cathodes of the two types) and by using a supply with good regulation.

Key elements of the power supply owe their origins to several sources but it was a gentlemen called Jeff Medwin that put me on this particular path. This being the low-DCR and low energy storage approach. The low DCR approach is desirable because we would like to arrange for the impedance of the power supply to be much less than that of the amplifier. The 2A3 has a plate resistance of 800 ohm (nominal) so we'd like to achieve something around 1/10th of that, or at least close. We don't want to resort to large electrolyits remember and simplicity means no heat generating shunt regulators here. This favours SS rectifiers and a big power trafo. The filter design also optimizes for high regulation and good transient response. If we size the caps well, we may be able to do the whole thing without electrolytics.

[chrish]

I would say your power supply goals support the use of a regulated supply.

[tomchr]

I'd move the fuse to the primary. Add a soft start circuit if the fuse keeps blowing. Right now the fuse is in the signal path. The impedance of a fuse is highly non-linear and dependent on the current through the fuse. The Loftin-White topology saves the day as it makes most of the signal current flow through the cathode-anode cap, but still.... The fuse will also not protect anything in case one of the reservoir caps short out and blows (this is actually a rather common failure mode).

~Tom

[Bigun]

Thanks Tom, that's a good point - my thought was to have something to protect the rectifiers and secondary in case of something nasty. I will include a fuse in the primary for sure.

To explore a little further the potential benefits of this approach to the B+ supply I did a quick comparison between two options using the PSUD simulation tool - Jeff uses this approach to fine tune the component values in his designs. I'm not that familiar with PSUD but it sure is simple to use and fast compared with using LTspice. Jeff's approach has been controversial amongst some, but it does appear to offer less ripple without large inductors or large capacitors and if that's the case I'm all for it.

On the right (attached image), I set up the proposed design. I used Hammond's data sheet as guidance for the rms voltage on the secondary of the 363CX. I don't have a model for the SiC rectifiers but I think we're close enough with the 1N4007 for now. I introduced a step in the current draw at 1.5 s after turn-0n so that I could see something of the transient response.

On the left, I set up something closer to what I used on my current CELLINI amplifier. This uses a large choke, the so-called Critical Inductance needed for good regulation at the design current level and it ensures the diode conduction angle is relatively large. This puts less stress on the diodes and the transformer. Of course I had to use a higher secondary voltage in order to achieve the desired B+ voltage after the filter and I increased the DCR of the secondary to reflect the likelihood that the higher voltage would mean more wire and more resistance. The total capacitance in both cases is the same, but in CELLINI I used a lot more; I don't think the L-W needs so much.

What do I see ?

The more conventional approach (left) has around +/-1V of ripple. It takes 50ms to respond to the step change in current and doesn't settle that well.

The proposed scheme (right) has around +/-0.15V of ripple. And it's response to the current step looks a little cleaner, maybe slightly faster too.

Food for thought.