No load voltage readings with a toroid PT

joulupukki

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You don't need to increase the filtering, but you can if you want to since the transformer is slightly oversized for the load you're going to have.

The one thing you haven't mentioned is the bias supply. Since you won't have a center tap, and you'll be using a bridge rectifier, you can't just hang a reverse biased diode off the secondary. You're going to have to create a capacitor coupled bias supply.

cap_coupled_bias_supply2.jpg
Ah, right. I had yet to move on to that step. I wasn’t thinking I would use the AS-1T300 PT for the RR763V, but was wondering if this one would work: https://www.antekinc.com/as-05tc300-50va-300vx2-transformer/

My suspicion though is that it wouldn’t have enough DC current to handle a 12AX7 + 12AU7 + two 6V6 output tubes but I still need to figure out exactly how to calculate that. My best guess is that those tubes would consume around 100mA and it looks like that particular 50VA PT would only be around 84mA with the secondaries wired in parallel. I could be completely wrong. :)
 

2L man

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Ah, right. I had yet to move on to that step. I wasn’t thinking I would use the AS-1T300 PT for the RR763V, but was wondering if this one would work: https://www.antekinc.com/as-05tc300-50va-300vx2-transformer/

My suspicion though is that it wouldn’t have enough DC current to handle a 12AX7 + 12AU7 + two 6V6 output tubes but I still need to figure out exactly how to calculate that. My best guess is that those tubes would consume around 100mA and it looks like that particular 50VA PT would only be around 84mA with the secondaries wired in parallel. I could be completely wrong. :)
Possibly that 42mA x 2 is AC rating and when it is rectified and filtered the RMS voltage increase 1,41 times and RMS current capacity decrease 1,41 times so it comes 30mADC x 2. But on Antek pages they say that power can be exceed...

First calculate the VA of each coil and they should somewhat sum to total VA of a PT.

This 1,41 is "square root of two" and effect as multiplier or divider when sine wave is turned to rectified and filtered voltage and current.

When the CT rectified circuit is used it is better to limit DC current bit lower because peak current thru secondary halfs alternate and when they are "calculated" currents are double. This cause higher voltage drop on secondary halfs which heat the coil more but then during opposite sine wave half there does not flow current and coil can cool so CT somewhat regulate the VA power a PT produce.

When tube rectifier is used there comes more voltage loss and and peak current comes bit lower. Think how tube rectifier "flatten" sine wave peaks and current comes lower when it can flow longer time.

Power transformers can withstand some abuse! If for example the tube rectifier is changed to silicon diodes the filament power 10VA or 15VA does not build heat anymore and other coils can pass almost as much more power.
 

Lowerleftcoast

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My suspicion though is that it wouldn’t have enough DC current to handle a 12AX7 + 12AU7 + two 6V6 output tubes
Antek's data shows it handling more current so it is not that the transformer is in danger. It appears they have just picked an operating voltage to amperage number.
 

mountainhick

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My suspicion though is that it wouldn’t have enough DC current to handle a 12AX7 + 12AU7 + two 6V6 output tubes but I still need to figure out exactly how to calculate that. My best guess is that those tubes would consume around 100mA and it looks like that particular 50VA PT would only be around 84mA with the secondaries wired in parallel. I could be completely wrong. :)
Antek's data shows it handling more current so it is not that the transformer is in danger. It appears they have just picked an operating voltage to amperage number.

I'm confused. Help me out, more current than what?

ELECTRICAL CHARACTERISTICS​

Output Voltage Current
Outputs4x
Power50VA
300V42mA
300V42mA
6.3V2A
6.3V tap 5V2A

Total 84ma
 

Lowerleftcoast

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I'm confused. Help me out, more current than what?
Antek calls the PT a .05A at 300v. They display a load test at .06A at 295v. If we follow along with @2L man post #22...

To answer more succinctly, I ran across this post in the "Amp Garage" forum. Oct. 2020.

"I just completed a Princeton BFR over in the Fender sub-board using an Antek. I used the as-05t320-50va-320v part + a solid state rectifier and got about 375V on the B+ when biased hot. I had never used one before, but so far it's working great. You can check out the schematic in the "Donut Princess" thread. The basic power supply should work on anything with a pair of medium output tubes up to 6L6GCs. Even at full out into an attenuator with a hot bias, the PT never gets more than slightly warm."

Nothing like having a description of something similar to what is being contemplated. (Now IDK if I would use it for a pair 6L6GC.) He claims to have 6V6s in this Princeton Reverb build.
*Slightly warm* to me means it can handle the current.
Take note he claims the AS-05t320 results in a B+ of 375V so, the AS-05t300will deliver a little less B+? I could see choosing either of these PTs for a 5E3-ish build.

For your 420B+ you may need more voltage.
 
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mountainhick

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Antek calls the PT a .05A at 300v. They display a load test at .06A at 295v. If we follow along with @2L man post #22...

To answer more succinctly, I ran across this post in the "Amp Garage" forum. Oct. 2020.

"I just completed a Princeton BFR over in the Fender sub-board using an Antek. I used the as-05t320-50va-320v part + a solid state rectifier and got about 375V on the B+ when biased hot. I had never used one before, but so far it's working great. You can check out the schematic in the "Donut Princess" thread. The basic power supply should work on anything with a pair of medium output tubes up to 6L6GCs. Even at full out into an attenuator with a hot bias, the PT never gets more than slightly warm."

Nothing like having a description of something similar to what is being contemplated. (Now IDK if I would use it for a pair 6L6GC.) He claims to have 6V6s in this Princeton Reverb build.
*Slightly warm* to me means it can handle the current.
Take note he claims the AS-05t320 results in a B+ of 375V so, the AS-05t300will deliver a little less B+? I could see choosing either of these PTs for a 5E3-ish build.

For your 420B+ you may need more voltage.

Thanks for the explanation. I remain a bit confused about the PT spec vs the combined max total from tube spec sheets, but doesn't matter much. Also a bit confused about Antek's own ambiguity

The empirical info and that chart provides enough info as a practical design starting point for anything I'll be doing! LOL!

I am glad you express doubt regarding a pair of 6L6GC!
 

Phrygian77

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I really don't understand why there is so much confusion about this stuff. They give you what you need to estimate the source resistance. From there you can model a bridge rectifier and input filter with your estimated load to determine the B+. You can further model the whole output section to see how much the supply sags and what the max RMS ripple current will be.
 

joulupukki

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@joulupukki do the math! The datasheet for the 50VA shows 310 unloaded and 295 at 60mA. That's an Rs of 250 ohms.
If I understood the concepts well enough, I would do the math. Pretty sure that’s where I’m lacking.

I really don't understand why there is so much confusion about this stuff. They give you what you need to estimate the source resistance. From there you can model a bridge rectifier and input filter with your estimated load to determine the B+. You can further model the whole output section to see how much the supply sags and what the max RMS ripple current will be.
Is there some place that teaches how all this is done? It sounds like if I knew how to do that it would be simple. :) What I find frustrating is that I find bits and pieces of what it seems like I should know about doing the calculations and models, but there’s always something left out of the info I find and so it still isn’t all clicking. Sorry.
 

joulupukki

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Antek calls the PT a .05A at 300v. They display a load test at .06A at 295v. If we follow along with @2L man post #22...

To answer more succinctly, I ran across this post in the "Amp Garage" forum. Oct. 2020.

"I just completed a Princeton BFR over in the Fender sub-board using an Antek. I used the as-05t320-50va-320v part + a solid state rectifier and got about 375V on the B+ when biased hot. I had never used one before, but so far it's working great. You can check out the schematic in the "Donut Princess" thread. The basic power supply should work on anything with a pair of medium output tubes up to 6L6GCs. Even at full out into an attenuator with a hot bias, the PT never gets more than slightly warm."

Nothing like having a description of something similar to what is being contemplated. (Now IDK if I would use it for a pair 6L6GC.) He claims to have 6V6s in this Princeton Reverb build.
*Slightly warm* to me means it can handle the current.
Take note he claims the AS-05t320 results in a B+ of 375V so, the AS-05t300will deliver a little less B+? I could see choosing either of these PTs for a 5E3-ish build.

For your 420B+ you may need more voltage.
Very interesting. Good find. Yeah, maybe the AS-1T350 - 100VA 350V would be better for reaching the correct B+.
 

mountainhick

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I really don't understand why there is so much confusion about this stuff. They give you what you need to estimate the source resistance. From there you can model a bridge rectifier and input filter with your estimated load to determine the B+. You can further model the whole output section to see how much the supply sags and what the max RMS ripple current will be.

Learning pathways are different in different people.

Consider yourself lucky! Some of us do not have the mathematical mind that you do. I think you and some others here have some kind of basis learned earlier in life, or just that you are good at this type of thing naturally. In my 60s now I am fully aware of my inability to learn by certain pathways.

I can come to understand a lot of math, but it does not come from reading text, or verbal explanations. It is not a matter of inability to understand, it is the medium of communication. It is common for me that if I already understand the subject, I also understand the explanations, but it is also a bit of a catch 22 that if I don't understand the material first, I don't understand the explanation.

I do so appreciate all your contributions! Thank you for trying with us morons! I am sorry that you are frustrated.
 

joulupukki

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Agreed. I so appreciate all of you amazing and generous folks here in helping me and others learn and understand more so we can enjoy the journey of amp building! Had I known that I would have loved doing this so much I would have tried to take a few more EE classes while in college. The two I did have to take have definitely helped a lot of my basic understandings of circuits and such, so I’m grateful for that. Maybe part of my problem is that I code all day long building an iOS app and by the end of the day when I want to get my math brain on, it’s fried. Haha.

My good friend started up a little band where they do some classic rock n roll gigs. His old Fender Solid State Princeton 65 was acting up and so he was looking to get something else. He asked me if I could build him something for under $500. After looking at different options, and by the time I were to add reverb, tremolo, etc. I let him know it wasn’t really possible to stay under that price and also include a speaker, cab, and all the things needed. This was even with me being willing to build & tolex whatever cab(s) needed. I ended up finding a used Boss Katana 50 mk II on the used market here locally for $200 and he’s really pleased with it.

I took apart his Princeton 65 and discovered the issue (audio would occasionally cut out) was a bad solder joint on one of the power resistors. The amp “works” again but man, what a junky amp in comparison to the hand-wired Princeton Reverb (Mojotone) that I built! :)

Anyhow, after all that, I am still itching to figure out a lower-cost way to build a good blackface clone – and likely without the reverb & tremolo since a pedal could be used for that. That’s where the idea for building Rob’s RR763 6V6 came into play and I started wondering which of Antek’s toroid power and/or output transformers I might be able to use to build it with (lower cost, actually available, and also way less hum than an EI PT - as I discovered with my 18W TMB build).

I’ve got this 300V 100VA (Antek AS-1T300) toroid power transformer. I’d like to figure out at some point what to use that for and I know it’s not going to work for a RR763 since B+ needs to be so high. Maybe I could build a Plexi 6V6 at some point. But that‘s for another time. I guess I could build another Princeton with it sans reverb and tremolo.

But, the quest is still on for me to more fully understand all of this stuff. When I get more time, I’m gonna try to learn how to use one of these modeling apps Phrygian77 mentioned.
 
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Phrygian77

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@joulupukki and @mountainhick

You don't necessarily need an app. You can use Merlin's method of estimating the source resistance and then use his chart to estimate the B+ and the ripple current based on the source/load ratio.

I'll walk through simulating it in LTspice first using the specs for the 50VA 05TC300 from their datasheet. I'm not going to do a complete tutorial on LTspice. I'm just going to go through a few models to show how we can simplify this down to Merlin's method.

Hopefully @andrewRneumann will check this for accuracy.

1662998015064.png



The first thing we need to do is calculate is the turns ratio. Antek gives an input voltage of 120 Vrms and an unloaded output voltage of 310 Vrms.

Vpri = 120
Vsec = 310
Turns ratio = Vpri / Vsec

120 / 310 = 0.38709...

The impedance/inductance ratio is the turns ratio squared.

Inductance ratio = (Vpri/Vsec)^2

0.38709... ^ 2 = 0.14984...

In LTspice we'll create 4 inductors with mutual inductive coupling to simulate the two primary coils and the two secondary coils. We don't know what the actual inductance values are for those coils, and we don't really need to know for a simple model. We can just use an arbitrary high value of 100 henrys for the primary coil, and calculate a value for the secondary coils using the inductance ratio.

1663000295188.png


Using the inductance ratio, we calculate a value for the secondaries:
100 / 0.14984... = 667.36111...

We create two 100H inductors, L1 & L2, to simulate the primaries, and two 667.311H inductors, L3 & L4, to simulate the secondaries. We give these mutual inductance with the K statement (K1 L1 L2 L3 L4 1). We also need to add the DC resistance for each coil. So we create resistors in series with the inductors using the values from the Antek datasheet (note that in LTspice you could simply add the series resistances to the inductor properties instead of creating separate resistors).

We add a voltage source for the 120Vrms 60Hz mains input. In LTspice this needs to be Vpeak, not Vrms.

Vpeak = Vrms x sqrt2

120 x 1.41421... = 169.70562...

Lastly, we need a load to simulate. Going by the datasheet example, 295Vrms output with a 60mA load. Using Ohm's law:

295 / .06 = 4916.66666...

Adding a 4916.67 ohm resistor for the load, and running a simulation (0ms to 100ms = 6 cycles):

1663005157056.png


Vrms: 298V
Irms: 60.609mA

The voltage is off by about 1% compared to the datasheet, most likely because we are not modeling any losses.


Since we're probably going to be using the primaries and the secondaries in parallel, we don't really need to model all of the coils, so we can simplify the model by combining them. The turns ratio doesn't change. We just need to divide the series resistances in half.

1663006449307.png


Running the simulation we get the exact same results:

1663006553647.png


Vrms: 298V
Irms: 60.609mA


Now, moving on to Merlin's method found here:


In that article, he describes how to calculate the source resistance of a transformer as follows.

The source resistance is equal to the resistance of the secondary coil, plus the resistance of the primary coil when 'reflected' to the secondary:
Rs = Rsec + Rpri/(Vpri/Vsec)^2
Where:
Rsec = measured resistance of the secondary winding.
Rpri = measured resistance of the primary winding.
Vpri = RMS primary voltage (e.g., mains voltage).
Vsec = RMS secondary voltage measured with no load.



We already did part of this calculation, which is finding the impedance/inductance ratio.

120V / 310V = 0.38709...
0.38709... ^ 2 = 0.14984...

The DCR of each primary is 22 ohms. In parallel that's 11 ohms. We take the combined resistance of the primary and divide it by the impedance ratio, which gives us the reflected impedance of the primary.

11 ohms / 0.14984... = 73.40972... ohms

Now we add that value in series to the resistance of the secondary. Since each secondary coil is 235 ohms, that's a parallel resistance of 117.5 ohms.

117.5 ohms + 73.40972 ohms = 190.90972 ohms

We can get rid of the inductors in our model, and simply use a series resistance of 190.91 with a voltage source. The voltage source will be the unloaded secondary voltage. In LTspice, this needs to be Vpeak (310 x sqrt2).

1663008332571.png


Running the simulation we get practically the same result, accounting for rounding errors.

1663008408779.png


Vrms: 298.01V
Irms: 60.612mA



So far, all we've done is simulate an AC load to show how we can model just a transformer. That's all I have time for at the moment. I'll post an update with rectification and filtering, and we'll compare it to Merlin's source/load ratio chart and calculations.
 
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mountainhick

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Thanks @Phrygian77

Mmm, going to have to read that when my cognition is a little better. It flags this time of day. Not only learning pathway stuff at work, also the damn cancer drugs... and the cancer for that matter... and the COVID vax I got yesterday. Feeling a bit wiped. At least I was able to slot a new bone nut on my Gretsch and play a little this afternoon!

Sorry for the digression, I will study this.
 

andrewRneumann

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Those models look stellar. I dropped your numbers in Falstad and came up with 296VAC for the loaded voltage. It all checks out nicely.

I especially like the last model--the Thevenin one with just a voltage source and a resistor. That's really all you need isn't it!
 

joulupukki

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@Phrygian77 wowsah! Talk about a deep dive. :) Very cool explanation! Thank you for sharing this. I got distracted replacing my single coil neck pickup with a humbucker and playing it a bunch tonight and now it’s late.

I, like @mountainhick (hope you can start feeling better soon!), feel like I need to have all cylinders firing to fully understand this. I’ll review it again more in the morning.
 




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