help spec'n PT?

[Jonas]

HI

I'm looking for help understanding PT transformer specs

For 6.3V components (all lights and heaters) I calculate needing (minimum) 4.2A.  Should I multiply this number by a safety margin? For example 4.2 X 1.25 = 5.25A or is a rating of 4.2A adequate? Is it typical for the PT manufacturer to include safety margin for spec?

Second question, how do I calculate / understand the current rating for the HV winding?  For example, if I have a 270-0-270@150mA HV winding, how do I determine the 150mA rating is acceptable for a design?

thanks

[shooter]

I don't add a "safety" variable to the added up draw for the 6.3, it's a pretty steady state condition.
for the HV, calculate your PA tubes current draw for your "targeted" operation;


say max current at 360vdc, max AC signal is 37mA per tube.  times # tubes.  Add 2mA per pre tube,  Now round up since this is a dynamic value.

[66Strat]

Tube datasheets will provide suggested operating points, as well as, maximum ratings. My suggestion would be to use the tube datasheets for the tubes that you plan on using. Select your operating conditions from the applicable datasheet and then add up the currents as shooter suggested.

[shooter]

 
you did a much better job good English!

[HotBluePlates]

... how do I calculate / understand the current rating for the HV winding? ...

Most of the current supplied by the high voltage winding is used by the output tubes.  So you understand whether the high voltage winding has correct current & voltage when you learn to design a power output stage.

Prior to that, you look at comparable amplifiers & copy what they use.

   Look for the type & number of power tubes in the "similar amp"
   Look for the power output of the "similar amp"
   Find the ratings of the power transformer used in the "similar amp"

There are only a handful of common tube types, and only a handful of ways they have typically been used in guitar amps.  A number of vendors make replacement transformers for common amplifiers.  So you steal a known-plan.

[Jonas]

thanks everyone,

Is there a specific advantage to using a bias tap winding (55V or 70V)?

[thetragichero]

saves a resistor (or resistor and cap if no ht center tap)?
there was a time when iron (transformers, chokes) was cheaper to produce/spec out than large capacitance, so older designs have all sorts of things that might be odd considering current realities (capacitance is absolutely dirt cheap, big hunks of metal are not)

[HotBluePlates]

Is there a specific advantage to using a bias tap winding (55V or 70V)?

It makes creating a bias supply easier.

[Jonas]

thanks everyone 

Any recommendations on ordering a custom wound transformer built to spec?  i.e. Edcor?

[66Strat]

thanks everyone 

Any recommendations on ordering a custom wound transformer built to spec?  i.e. Edcor?

What specs do you want?

[sluckey]

Any recommendations on ordering a custom wound transformer built to spec?  i.e. Edcor?

I've never had a problem finding a suitable stock transformer for my projects. If you will tell us what you want to do with a PT I'm sure we can offer some suitable choices. But if all you want to do is just understand some PT design specs, the net is loaded with that kind of info. I would start with the Hammond website.

[Jonas]

thanks, appreciate you guys!!

I 'm familiar with Hammond just never used them.  In the past I have always followed know good designs; selecting what was available.  My new quest is trying to understand some of the why behind these selections. 

I'll keep the question coming and appreciate the links guys provide 

How does one determine fuse ratings for a PT?  I see Marshall used 1/2A (slow) for the HT, while Hammond equivalent HT shows max 150mA so why not 1/4A fuse?  Is this strictly for nuisance-blow avoidance or is 1/2A required to offset the operating dynamics of the windings (such as a huge current draw upon startup?) but still provides the short protection when a short occurs?

Same question for the mains fuse, the heater current draw in amps is cumulatively more than the fuse rating.  I not sure how the current demands on the secondary (6.3V) effect supply requirements on the primary (120V)?

[shooter]

(such as a huge current draw upon startup?) but still provides the short protection when a short occurs?

I believe that's pretty close to "why"
although I've found in the real world, most line side fuses refuse to die when expected, dead shorts maybe they work well.

[sluckey]

Same question for the mains fuse, the heater current draw in amps is cumulatively more than the fuse rating.  I not sure how the current demands on the secondary (6.3V) effect supply requirements on the primary (120V)?

1 amp of filament current does not equate to 1 amp of primary current. 1 amp at 6.3v is 6.3 watts or volt/amps (VA). So, 6.3VA on the secondary does equate to 6.3VA on the primary. But since the primary is running at 120V, the primary current required to produce 6.3VA is only 6.3/120 = .0525A.

[pdf64]

... I see Marshall used 1/2A (slow) for the HT, while Hammond equivalent HT shows max 150mA ...

Have a good look at the info https://www.hammfg.com/files/parts/pdf/290GX.pdf
150mA probably isn’t the max rated current. See the '213mA max'.
So the typical 700V@150mA means that when loaded at 150mA, the voltage will be 700V. At lower loads, the voltage will be higher, at higher loading, it will be lower. A typical 2xEL34 with a 3k5ish primary OT and a HT voltage of about 420V at max unclipped output will probably draw more than 150mA. And also probably more than 213mA.

[Jonas]

Thanks for clarifying!!

I've been looking at several different transformer spec sheets and see a few Edcor's with (2) 6.3V windings and this got me thinking....

One 6.3V winding used for preamp heaters and the other winding for output tubes.  For the preamp I was thinking of experimenting with DC heaters, or elevated AC, but how necessary is it to elevate the AC heaters for the power tubes, assuming the preamp has its own discrete winding? How much hum is induced by the power section?

[shooter]

How much hum is induced by the power section?

the AC hum is hopefully reduced in the PS
What do you want from your PS, Sag?, Crisp clean?
I build the later, in a SE I shoot for;
<4Vac at PA
<10mVac at V1

[Jonas]

Shooter, I've done a few amps with tube rectifiers, but I prefer the clean crisp PS from the 50w plexi.  I'm not sure how much preferential treatment I need for hum reduction in the power section or with the power tube heaters.  Any suggestions?

[shooter]

Any suggestions?

I stole this basic layout from a Marshal design.  It's overkill in PP amps, In SE it's always been hum-free.
you can eliminate the 100uf's and the 400 ohm and "start" there.  They were added to burn off some B+ to the PA and drop hum from 4v to mV at the PA.
I typically had a PA on a breadboard that I'd use for new amp PS builds.  I'd connect it up, tweak the PS then continue with build.