The umpteenth question about Standel 25L15

[kagliostro]

Which is the purpose of the 470R 3W resistor between the 15H Choke and the 20uF x 2 cap ??


that resistance is also in parallel with the choke ???


Thanks


K

[sluckey]

Probably doesn't matter because the rectifier tube can't conduct because of the 235Ω resistor in the filament circuit. That resistor limits filament current to only 21mA, so the 5U4 filament (which needs 3A) never gets hot enough to conduct. There can be no B+.

[kagliostro]

Ciao Steve


In one other version of the schematic that I've, the resistor to which you refers is in a different position

Franco

[HotBluePlates]

Probably doesn't matter because ...

... it doesn't exist in the original amp (to complete Sluckey's sentence a different way).

Oh yeah: there are other errors on that schematic as well; just because someone drew it up doesn't make it correct or authoritative.

[kagliostro]

Thanks for the reply HotBluePlates

...... just because someone drew it up doesn't make it correct or authoritative.

that's for sure


but don't you think it's worth checking out, since I do not have a secure version of the schematic ?


Franco

[HotBluePlates]

... but don't you think it's worth checking out, since I do not have a secure version of the schematic?

No. If the resistor were in a slightly different place, it would drop overall B+ voltage and create heat inside the chassis. As drawn (and as Sluckey said), it prevents the rectifier tube from getting heater current.

That resistor didn't exist in the original 25L15, so guessing what it does is meaningless... it shouldn't be there at all. If you want to actually build a 25L15, I'll put you in touch with someone who can provide 100% accurate information.

If you're not wanting to build one but just want to understand how it works, most of the small errors present won't stop you from seeing most of how it works.

[kagliostro]

I think I explained me in a bad way, saying

... but don't you think it's worth checking out

I didn't mean to give a try putting phisically a resistor in a circuit, I was curious to know if that resistor was really present on the circuit and more, if it was there really, which was the purpose

All starts from the idea of my friend DOM to realize a GU50 + KT88 SE amp

recently he had the idea to give to each tube kind a separated PS node and  he exposed this idea



as you can see he inserted a choke and a label saying choke or gyrator

Watching to that proposal I didn't agree at 100% and would like propose to him something like in the Standel

where there are two distinct choke for the anodes of power tubes from one branch and G2 + preamp on the other

So I give a look to the Standel schematics I've collected and I noticed that odd resistor and developed my curiosity

---

At the end this was my proposal for the GU50 + KT88 SE amp PS (see attached file)

Franco

p.s.: At the moment, though, I think the diode UF5408 is redundant in that position and not necessary or useful

---

If you want to actually build a 25L15, I'll put you in touch with someone who can provide 100% accurate information.

At the moment no .............. but ........ BTW, it happens that I get 4 NOS 1625 tubes with new sockets at the last Hamfest ........... never say never 

[HotBluePlates]

I see.

I think the person drawing the first of your 25L15 schematics intended to have the resistor after the output of the rectifier (but not in series with the rectifier heater, as it is drawn) but before the filter cap & choke. Drawn that way, all it does is reduce overall B+ due to voltage drop across the resistor. It would also harm the regulation of the power supply (how solidly it maintains voltage without sag) because the voltage drop across the resistor is unavoidable and varies with varying power supply current.

Where Dom has the choke drawn before the filter cap (I assume this is the first filter cap after the rectifier), it would cause the d.c. voltage at the first filter cap to be 0.9 * a.c. voltage peak supplied by the power transformer. With no voltage drop in the rectifier, a cap-input filter will try to charge up to 1.414 * peak a.c. volts, but the choke is always fighting the a.c. variation, and so you wind up with a much lower d.c. voltage.

A choke-input supply is fine in a class A amp, where you draw a fairly large idle current (the choke's inductance drops to zero when zero current is pulled through the choke) and also when the amp draws a fairly steady current from the power supply. These conditions help keep the supply voltage very stable.

Aside from the resistor which shouldn't be there, look again at your first 25L15 schematic. The power supply is cap-input (there is a filter cap before the choke), so B+ voltage is similar to what you'd otherwise expect in a guitar amp with the same power transformer. Then there is the first choke and a cap, all feeding B+ to the output transformer & tubes. That provides more-than-typical filtering for the output tubes.

It looks like the output tube plate voltage is being passed through a resistor, then to a filter cap, then supplied to the output tube screens. The wire at the top of this filter cap couldn't be correct, as it shorts out the second choke. It looks like the second choke takes the plate supply voltage, filter is further with the choke and following filter cap, and the resulting (probably very clean) d.c. is used to power the preamp.

Venturing a guess, this arrangement would make sense if the output tube screens had a d.c. voltage very much different than the preamp. It would make the most sense if the output tube screen voltage was lower than the highest preamp supply voltage.

Recall that you might use a choke instead of a resistor in a power supply where you want filtering (or isolation between power supply nodes), but a resistor with a high-enough value to provide the filtering/isolation results in too great a voltage drop when the supply current is drawn through it.

[kagliostro]

Many Thanks HotBlue Plates

the arrangement is still odd to me (but seems that is the same for you)

and this thing really looks like a typo

the other oly way I can think to this is an 1/2 choke input (instead of capacitor input or choke input) for G2

Franco

[HotBluePlates]

All I can tell you is I know with 100% certainty what was in the original 25L15. And that none of the 3 circuit arrangements you just drew (to understand the resistor which doesn't belong in your first schematic) are found in the 25L15.

The unfortunate situation is someone drew the schematic guessing what is in a 25L15 with the information they had available. They guessed wrong. 

[kagliostro]

Andrà a finire che prima o dopo farò un 25L15 ed allora ti toccherà penare per trattenere i segreti di cui sei a conoscenza
 

But at present you're lucky, I was only interested in the technical aspect from the point of view of the PS 


Ciao e Molte Grazie HotBluePlates per la pazienza e la disponibilità

Franco