Power Supply: Big and Manly or Small and Frail?

[The New Steve H]

When I started putting amps together, I got the idea that sag was important. I thought you had to be careful about using a big PT and caps, because it would make you lose compression and sustain.

It sort of looks like I was wrong. I ran an amp set up for 15 watts (PT, OT, and caps) using only two tubes capable of a total of 7 watts, and the amp sounded great. I replaced the 5Y3 with a big 5AR4, and it sounded much better.

I am starting to think compression and sustain come mainly from the pedals and guitar, and that the best thing is to go ahead and use a stiff power supply.

I have a bunch of stuff lying around, and I'm thinking of putting a two-tube 7-watt amp together. I need a PT. I can either order the same PT I already have (which works great with two or four tubes), or I can look for something smaller and a little cheaper. I am thinking I might as well stick with what I'm using, since I know it works well.

So: stiff or wimpy?

[R.G.]

It's easy to dial in any degree of sag you like.

Sag from the transformers is due to the resistance of the windings. Likewise, sag from rectifiers is from the effective "on" resistance of the tube's V-I curve. You can't make a wimpy power transformer and rectifier stiff, but you can sure make a stiff one saggy by putting in the appropriate power resistors.

Swapping caps is easy enough too.

Go try it and see what you like.

[ToneMerc]

Well many of the vintage amps that we love have or had very "wimpy" power supplies, no better example than a 5F6A Bassman. IMHO, stiff power supplies are great for Metal chunka chunka and fast single note jazz runs, but are dead
for soulful stuff.

TM

[The New Steve H]

Sounds like the safe thing is to jam a big PT and caps in there and then mush it up later if necessary.

[JMFahey]

Well, that's the most versatile solution.

The point is that using a poor supply, specially very small caps , and a small transformer which means thin, high resistance wire plus a very lossy rectifier tube which only adds to the equation, means that +B sags *a lot* , which means you are adding a very cheesy *compressor* to the mix, specially one which "breathes" a lot, in the Recording Studio language that is.

Now that which is horrible on Recordings, may be used as an effect by a Guitar player by adding some "expression" ... truly "modulation" , not unlike a skillfully used Volume pedal but automatic in this case.

FWIW I have added exactly that to my SS Op Amp based preamps and result is quite acceptable ... and no tubes involved.

This is a >30 y.o. amp, sag and compression are very evident, in fact slightly exaggerated; plain vanilla guitar straight into the amp, of course.
https://www.youtube.com/watch?v=FNehpDVExU8

[pdf64]

If using a tube rectifier, most info sheets specify a minimum supply impedance, ie in series with its plates.
This is required to restrict the peak plate current, and is just as important as the reservoir cap value.
The 2 characteristics should be in balance, such that as the ac side of the supply impedance is reduced, so should the reservoir cap value too.
I think that most power transformers intended for use with a tube rectifier, especially those of vintage design, incorporate a degree of resistance into the HT winding, in order to accommodate this requirement.
It will result in a hotter running transformer than one which is designed without consideration for it, all else being equal.