PT VA

[Justin Thomas]

Sounds like it should be the name of all those digital guitar "amps..." Or at least a patch. (yawns) Zzzzzzzzzzz...

Justin

[potatofarmer]

Here's the Hammond link RG mentioned: http://www.hammondmfg.com/pdf/5c007.pdf

I've been tweaking something similar recently - I'm using a Kustom Defender 5H as a bass preamp. One 12AX7, one EL84. Initially I tried the "Garnet Herzog" method - using the stock OT and a pot across the dummy load for a line out, but the original OT was designed for guitar (if that) so it wasn't passing enough bass. I considered using the EL84 as a cathode follower but I wanted to get some gain out of it and also didn't want to worry about elevating the heaters.

Eventually I settled on a resistive load for the EL84 and a Triad TY-250P isolation transformer, using a 220k resistor to drop the voltage into a 5k pot for the line out. Like-a this:



Parts values are based on whatever I had in my bins that was close enough to load line predictions. It delivers a little more than 1VRMS into a 1M input impedance. Decently low enough output impedance for the matching transformer to be reasonably linear, but the rest of the preamp is based around a Matamp topology. Pretty quiet with the iso transformer as far away as possible from the PT.

If I didn't have this thing sitting around assembled and unused I would've just used an ordinary preamp pentode, but there you have it.

[potatofarmer]

Oh almost forgot... Any suggestions on a toroid for 325-0-325 20VA?

I've had good luck with Antek transformers. This one would be more than sufficient, if you can tolerate using a bridge rectifier. Includes plenty of heater current too: AS-05T320 - 50VA 320V Transformer - AnTek Products Corp

[J M Fahey]

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?

Interesting idea

What will you drive with it?

[lowell]

JM ive built one... Sounds incredible IMO. I like classic rock tone. Ala Bad Company's debut record, or Neil Young. Power amps are plentiful, and SS ones are cheap and class D is especially nice and light. Saving good coin on doing away with the OT and honestly IMO I dont miss the mojo that people speak of regarding OT-speaker interaction.

Please clarify your question. What am I driving what with?

[lowell]

That might be perfect thanks!

[lowell]

Overdriving a 6v6 sounds better than a 12ax7 IMO.

[J M Fahey]

Agree, triodes are softer which is fine for Hi Fi but pentodes clip harder , I like bite in my sauce.

My earlier question was because I thought you had basically built a power amp stage , transformerless and resistive loaded so it would need reamping, but a power stage nevertheless, and guessed you might use it after some multieffects pedalboard or, say, a POD , to give it "tubeyness".

But now i see it's a (simple) full amp designed to be used on its own (still reamped of course).

The sound you achieved is the sound I love.

Real clean or buzzsaw is easy for SS , so I don't even care to use glass for such purposes; where it still can't be beaten is in the grey middle area, classic Blues/Rock sound, the Keith Richards sound, etc. , in general the area where you do not switch channels or need pedals but control everything with pick pressure and guitar volume.

Samples??????

[lowell]

EXACTLY. I'll post some samples when it's done. I sold the original prototype years ago. 😀

[lowell]

Oh and worth mentioning is I added a jfet input stage at about a gain of 3. Allows for a real balls-out sound.

[Wombaticus]

You've made a mistake here R.G. by failing to compare apples to apples. Assuming that you have some specific target voltage in mind, full wave bridges are by far *better* than full wave CT. If you compare, say, a 300VAC non-centre tap PT w/ full wave bridge recto to a 150-0-150 VAC centre tap PT the centre-tap does indeed look better --- until you have the unhappy realization that you're only going to get half of the rectified voltage out of the centre tap arrangement than you would from the non-centre tap. So you need to double the centre-tapped PT to 300-0-300 -- a full 600VAC across the secondary -- in order to get approximately the same rectified voltage as you would from the 300VAC. This is intuitively evident by the fact that the centre tapped arrangement is only effectively using half of the secondary at any one time, whereas with the non-centre tap, the whole secondary is always engaged.

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

[Enzo]

When I read that, I did not see him specify voltage, so in the context I assumed the transformers would be chosen for the same application, ie. a 300v non-tapped one for a bridge or a 600v CT for the other.

[Wombaticus]

When I read that, I did not see him specify voltage, so in the context I assumed the transformers would be chosen for the same application, ie. a 300v non-tapped one for a bridge or a 600v CT for the other.

As would most people, and that's the problem in a nutshell. An ideal centre tapped PT with full wave recto producing equivalent rectified voltage and delivering the same load current as a FWB setup will *not* have 1/2 of the total current (per secondary leg) of the non-centre tapped PT! Rather, *each* leg will have 1/sqrt(2) -- right around 70%, -- of the current that would flow in the non-centre tapped PT scenario.

[Gregg]

I consider using a CT PT with almost double the weight and the power compared to a single bridge rectified winding as completely absurd unless of course you're into tube rectification. There are plenty of 100W amps around running a 200W PT iron with the secondary delivering enough juice for real 100 Watts. I'm not very much into theory but my personal experience is a 350-360V/400-450mA secondary is more than enough to cover most current requirements for up to 100W RMS.
What's the point then of using a 700V CT 350Watt iron just because theoretically it could be a bit better than a single winding (bridge rectified)?

[Tom Phillips]

I consider using a CT PT with almost double the weight and the power compared to a single bridge rectified winding as completely absurd unless of course you're into tube rectification...

Yes that would be absurd. But we need to consider the design differences for a transformer made to be used with a full wave center tapped rectifier vs. one made to be used with a full wave bridge rectifier. Basically the weight and size of the transformer is determined by the power it needs to supply.
There will be twice the turns of wire on the transformer designed with a 350-0-350V secondary compared to one designed with a single 0-350V secondary. However, the wire size used for the 350-0-350 V secondary will be smaller than that used for the 0-350V secondary IF both transformers are made to deliver the same amount of overall power AND they are properly designed. Both transformers could be essentially the same size and weight. The wire used in the 350-0-350 secondary can be smaller because the duty cycle is 50% of the duty cycle in the 0-350V secondary for our case of equal power requirements from the power supply.

I though this would be easy to explain but I feel writers block so I'll stop here.

Tom