Guitar Gear Forum

Hey guys,

Designing my first tube amp based on AX84 materials. Other than "tradition" and "brownout tone", is there a reason why SMPSUs are not commonly used with tube amplifiers? It seems like it would solve a whole hell of a lot of problems. Having a regulated voltage to B+ just seems like it'd make adjustments across the amplifier easier. Or maybe they are and I'm just not aware?

My goal is to piece together a Class A Single Ended ~10watt EL34 based amplifier. That requires a pretty heavy output transformer to handle the DC current, so I was thinking about saving weight and creating my B+ rail with a regulated supply from a charge pump circuit based around a MAX chip of some sort. Once I have the high voltage working (There seems to be plenty of material available), it's pretty simple to set your plate voltages and bias for each tube, because your b+ rail will hold the same voltage as you load it up (in theory). Just curious why this isn't done?

Anyway thanks! cheers
-Jonathan

Loads of DIY here. Sounds like an interesting idea - can't wait to see / hear what you make of it!

I seem to remember the Crate V-series using SMPS. It was plagued with catastrophic failures.

In general, I think there are two different issues with using a switch-mode supply in a tube amp.
1.) For the safety of your regulator, you have to dissipate a lot of extra heat in an environment that is already overflowing with tube-heated air
2.) Guitarists are very resistant to change: a well regulated supply is a very stiff supply! It can lose a lot of the things that make it inherently tuby sounding

It's possible to design around both of these. Using VVR achieves the first one (without even really attempting to!) and the second is irrelevant to anyone
who likes more modern gain tones.

Thanks! I'll definitely calculate dissipated heat. I'm hoping with the right MAX chip, I can have conversion efficiencies in the 80%-90% range.


And about the 'tubey' sound, that's what I figured. I lean towards cleaner sounds anyway, so it may not make a huge difference for my application.


I have more questions coming but thanks for the answers!

A brain dump of thoughts:

There was interest in using SMPS for tube amps over on sloclone. As I recall one of the guys there talked with an Chinese manufacturer about making some and they said it wasn't a problem. I can't remember if a sample got sent out or not but it didn't gain much more traction than that but it may be worth registering there to ask about it. There isn't any reason why they can't be tuned for high voltage, low current as opposed to their usual low voltage, high current application.

The Matrix VBA800 is a hybrid amp that almost certainly uses an SMPS for the power supply and that'll be used to power the ecc83 preamp too so it has been done.

Some guys over on either AX84 or music-electronics-forum did say that the hash/noise from a SMPS might cause issues with high frequency crap being injected into the signal.

Sag can be achieved though other means, such as undersizing the filter caps or using MOSFETs.

The typical tube amp Power supply is dynamic; it changes with the load on the amp, and (inadvertently) that is what makes a good tube amp sound "good". It's like ultralinear output transformers - they work well in audio reproduction applications because they are accurate - but they do not sound good in a guitar amplifier, because they are accurate.

Best Regards,

e