PT VA

[lowell]

If I have 1 12ax7 and 1 6v6 and a B+ of 350vdc about how many watts should my PT be? NOTE: this is a preamp so there's NO output transformer - filaments are powered separately so exclude them.

Here are my calculations. Please advise:

12ax7 (Ra=100k) (B+ = 240v)
240v/100k =.0024A
.0024A × 240v = .6watts
.6watts x 2 =1.2watts (both triodes)

6V6 (Ra = 5k)
350v/5k =.07A
.07A x 350v = 24.5watts

TOTAL HT WATTAGE
25.7watts

These are MAX current draw. Is it safe to say I could divide this by 2 since MAX draw is only HALF of the sine wave? Could I use a 13VA PT?

Again this transformer is exclusively being used for the B+ only.

[Enzo]

Preamp? Then what is the 6V6 doing?

I'd quibble with your math. You apparently calculated the current if the 12AX7 plates shorted to ground. That is unlikely through the tube.

If your basic B+ is 350v, then all tube current ultimately comes through it, regardless of the plate voltages. SO if my 12AX7s are like most, I expect something up to 1ma per triode on average. And what of the 6V6? How much current do you expect there? 10ma? 20ma? I don't really think you will have a 6V6 in a preamp dissipating 24 watts. Add up realistic current draw and your 350v and calculate. It is class A circuitry is it not? SO draw will be essentially steady.

[Dave H]

For class A stages use the idle current in the VA calculations and use the max PSU voltage (350V) for both preamp and power amp. This will include the power in the dropping resistors (assuming the 240V supply is derived that way). To find the VA rating of the transformer double the total VA calculated for the amplifier to allow for the inefficiency of rectification.

Edit:
Enzo posted as I was typing again!
I see it's a preamp. I assumed it had a power amp because of the 6V6. Is the 6V6 driving a 5k resistor?

I calculate the VA as 350 x 0.0012 x 2 + 350 x 0.035 = 13. I'd use a 25VA PT.

[Mick Bailey]

I work on a generous 3mA per 12AX7. The 6V6 in class A maybe would pull around 40mA max for 100% dissipation. So my total VA comes to 350*(0.003 + 0.040) = 15.05.

Like Dave H, I'd go for a 25VA PT.

Are you using half-wave rectification?

[J M Fahey]

Lowell stated it's a preamp, not an amp with a Class A power stage, no audio transformers involved.
Maybe he's using the 6V6 and 5k resistor to drive a reverb tank?

As a first rule of thumb approximation, I'd consider all tube plates being resistive loaded, and with half +V voltage at idle, do the Math and get idle mA.
Then I'd multiply that current by 350V , since , as noted above, "everything" comes from there.
Even 240V at the furthest node is still dropped from original 350V .
Then I'd add 50% as safety margin, or pick the next larger size, and even so it will be a quite small transformer.
That 6V6 would take 175V/5000 ohms=35mA
Without too much Math I'd pick a 50mA transformer, or 18VA transformer (or the next up) and call it a day.
You found a 25VA one? .... fine.
It will probably have a 22x22 mm core , some 7/8"x7/8" or 19x25mm (3/4"x1") because those are generic lamination sizes and suitable for that power.


FWIW high voltage low current transformers are a bitch to wind: lots of turns of very fine wire, which forces you to go relatively slow and waste even more time, so up to a point a larger transformer may cost same or even less than a smaller one.

If you get one which can also feed the filaments, even better, the core will be larger,better used, less turns (inversely proportional to core area) and you avoid using 2 transformers, good for cost and size reasons.

EDIT: just curious and letting my mind ramble: is it a reverb add-on?
Or a hybrid power amp and the 6V6 drives a bunch of MosFets?
Both applications come to my mind.

[R.G.]

PT VA is not equal to the DC power out in a rectifier/filter setup. It has to be larger because Mother Nature's math insists that the PT is heated by the RMS rectifier current while the DC out is just the average DC current times the filter cap voltage, which is the peak of the incoming AC to be rectified.

What makes a difference is how you rectify and filter it. Full wave bridges are worst; PT rms current is 1.6 to 1.8 times the DC current out, depending on how big the filter cap is and how small the ripple is. Hammond tabulates this in the page "Design Guide for Rectifier Use" in their transformer and inductors booklet. It's a highly recommended page to read. Here are some quickie notes on capacitor input filter power supplies:

Half wave rectifier: transformer secondary VA = 3.57 x Idc
Full wave center tapped: transformer VA = 1 x Idc
Full wave bridge: transformer seconary VA = 1.6 x Idc

[lowell]

Thanks guys...so I wasnt too far off.
JM this is strictly a 3 stage preamp. No verb. Former version went into LM3886 PA. This one not so. 5K Ra resistive load capacitive coupling.

[J M Fahey]

Now we deserve some kind of schematic

[lowell]

Basically a tweed Fender Champ with a 5E3 tone control. Simple, effective. I know I know, why the hell would you "waste" a power tube like that? Well, thats where the tone comes from. and reason for the OP... I'm diggin the idea of a SMPS for the filaments. Oh almost forgot... Any suggestions on a toroid for 325-0-325 20VA?

[GainFreak]

AFAIK the most efficient rectifier for a toroidal PT is the bridge rectifier so going for a CT toroidal PT seems pointless.

[gui_tarzan]

I'm still trying to wrap my head around the 6V6 for a pre-amp.

[Enzo]

Well, it is a pentode after all, so I guess it can be used as one in a preamp circuit. It won't contribute the speaker sound or the transformer share of the tone, but you would get the pentodiness. Pentodism? Pentishness?

[gui_tarzan]

Pentabulous?

[g1]

Pentodial nature?

[J M Fahey]

Pentothal will make you sleep and feel nothing being done to you, even being cut in two with a bandsaw.