[leonardo@argento.pro.br]

Hey friends,
How you’re doin’?

I need help here… just want to confirm that I have the Friedman Smallbox schematic - without any mod.

Anyone had experience with that amp?
I really love it but it’s impossible to find one in Brazil.

Cheers!!

[MaVo]

Hello,

From the gutshots I‘ve seen, the power supply has different values for the electrolyte capacitors. Namely 2x 32uF around the Choke and 32uF and 16uF between the dropping resistors. The intention of this supply is to be spongy and fast, yielding a low time constant, therefore the relatively low component values of the capacitors. The rest of the circuit seems right to me.

[Shorty85]

Sorry to bring that up again... I am going to build a Smallbox 50 Clone and studied this Schematic and there is one Point that I personally would change:

The standby switch should be placed AFTER the first filter capacitor as shown in my attached diagram.

Why?
If you switch the cap in hard after the transformer has fully powered up, the inrush current into the cap is very high because the output impedance of the secondary winding is relatively low. This places high stress on the cap and reduces its lifespan, degrades its capacitance, and increases leakage current over time.
If you charge the first electrolytic already during power-up of the primary side, it charges much more slowly because the voltages and currents must first build up in the transformer during the switch-on moment. This protects the cap.

The second cap is connected after the choke anyway, so it charges slowly regardless.

I will implement it as shown in my diagram for my build, and I believe this is also the standard configuration in most amps - switching after the first cap.

[carlitz]

Sorry to bring that up again... I am going to build a Smallbox 50 Clone and studied this Schematic and there is one Point that I personally would change:

The standby switch should be placed AFTER the first filter capacitor as shown in my attached diagram.

Why?
If you switch the cap in hard after the transformer has fully powered up, the inrush current into the cap is very high because the output impedance of the secondary winding is relatively low. This places high stress on the cap and reduces its lifespan, degrades its capacitance, and increases leakage current over time.
If you charge the first electrolytic already during power-up of the primary side, it charges much more slowly because the voltages and currents must first build up in the transformer during the switch-on moment. This protects the cap.

The second cap is connected after the choke anyway, so it charges slowly regardless.

I will implement it as shown in my diagram for my build, and I believe this is also the standard configuration in most amps - switching after the first cap.

If you like it, DO IT

[Helmholtz]

If you charge the first electrolytic already during power-up of the primary side, it charges much more slowly because the voltages and currents must first build up in the transformer during the switch-on moment.

Do you have any evidence supporting this theory?
I doubt it makes a significant difference regarding reservoir cap inrush current.

Apart from that, switching AC causes less arcing in the switch.
There's a reason why Marshall changed from DC switching to AC switching in the 70s and never went back.

[carlitz]

In my opionon, switching DC, like in the Mesa Boogie Amps, makes only sense, if you want to safe money for a 2-Pole switch.

The better way to switch is like Helmholtz wrote, switch the AC part.

Makes also repairing much easier, as no capacitor will be charged and you will not get electrified during repair ...:-) expierence from a friend !

[Shorty85]

My thoughts are based on this model here:
https://www.elektroniktutor.de/bauteilkunde/tr_real.html

The equivalent circuit has C1 at the input, which first needs to be charged — the voltage can’t just “jump.” With L1s, the current can’t jump either... the whole thing follows from that. And so my theory was that, on the secondary side as well, current and voltage can’t just jump, and therefore it should be better if the capacitor is charged through this whole chain, rather than everything already being charged and saturated at that point.

But after thinking about it twice, I’m not so sure anymore if that’s actually correct. Especially because the effect might be very small. The best would probably be a real measurement... but maybe I’ll run an LTSpice sim at some point to get a better feel for it.

But yeahh... you may be right that this does not matter in reality in particular regarding the switch on spark...

[Helmholtz]

My thoughts are based on this model here:...

Effective input capacitance of a PT is a few nFs (just measured one having 2.7nF).
Mains source impedance is around 0.5R.
I leave it to you to calculate the time constant.

[Shorty85]

Ok, then I was really on the wrong track there... it was a long night — or a short one, depending on how you look at it    ... thanks for your input! I’ll leave it and go with the AC switch!

Should I remove the image or delete the post altogether so it doesn’t confuse others? What do you think?

[Volka]

Moin...

"...and never went back."  Naja, nicht so ganz: hab hier gerade einen JTM 45 (Reissue) von 1990 : da ist (war bis jetzt  ) der Standby-Schalter zwischen GZ34 und erstem Ladeelko!
Nu gibt's keinen Standby mehr !!! Völliger Blödsinn: wir haben eine GZ34 die den Amp gaaanz sanft hochfährt, und als Mute-Funktion ist ein Standby-Schalter an der Stelle der Schaltung so, als würde ich beim Auto zum Bremsen jedes Mal die Zündung (den Motor) ausschalten.... und dann wird wieder und wieder auf die 5AR4/GZ34 von diesem oder jenem Hersteller geschimpft! 

Gruß
Volka

[Shorty85]

Sehr spannend! Hab auch schon überlegt ob ich eine GZ34 anstatt der Dioden nehmen soll. Beeinflusst das den Tone noch mal?

Ich suche halt nach dem klassischen Marshall Sound Super Lead 1979 bis hin zu den late 80s - daher dachte ich der Smallbox 50 wäre die ideale Plattform für erste Experimente in Eigenregie.

@Moderator: Wenns hier zu sehr ausartet mit Disskussion hier bei den Schaltplänen, bitte verschieben oder was sagen... ich will nicht gleich den Forums-Knigge crashen in meiner ersen Woche hier 
Aber wird mein erster Selbstbau - ich bin für jeden Input Dankbar gerade.

Rockige Grüße
Shorty

[Shorty85]

Deutsch oder Englsich, was ist denn hier besser? Ich mach mal Englisch wieder, glaub das verstehen dann mehr:

I have another question for the experts. I've now drawn my own plan based on photos and the schematic. Everything is clear EXCEPT for the 15uF 500V capacitor in the HV branch. I was about to simply install two of them as shown here in the diagram.

However, I then noticed in photos that only one 16uF appears to be installed, presumably buffering both 12AX7s (all 4 triodes)... is that possible, or is the second one installed at a different location?

How would this affect the sound if one were to use two as shown in the schematic? Or would a multi-section electrolytic capacitor be better? Does that make any difference?

Image of the part I'm referring to is attached.

Thanks in advance.

[Helmholtz]

Schaltplan sieht korrekt aus. Vgl. auch hier:
https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-50W-2204S-Schematic.pdf

Aus Stabilitätsgründen sollten nicht mehr als 2 Gainstufen an einen Versorgungsknoten (also Filter-C) angeschlossen sein.
Auf einen Dropper muss immer ein Filter-C folgen.

[Shorty85]

Hi Helmholz,

danke für die schnelle Antwort. Hab auch schon andere Pläne angeschaut und gesehen dass es eig. die übliche Beschaltung ist.

Um dem ganzen auf den Grund zu gehen hab ich jetzt noch mal den ganzen Abend das Internet durchforstet. Auf fast allen Fotos die behaupten einen Friedmann Smallbox 50 fotografiert zu haben findet sich nur ein Cap... und offenbar sind beide Vorstufen Röhren daran angeschlossen.

Ich hab auch noch ein Layout gefunden von jemandem der behauptet das sei von einem Friedmann Smallbox 50 abgezeichnet.

Beides im Anhang.

Ich werde beide Caps vorsehen in meinem Layout aber definitiv mal testen wie sich der Sound verändert mit und ohne den Cap. Evtl. ist es ja genau das was das Mojo bringt? Wenn hier die Spannung instabil wird sollten die Röhren früher sättigen oder?

Für den Bau mach ich dan hier im Forum mal einen separaten Thread auf.

Rockige Grüße

Anhang (Bild eines Smallbox 50 von innen):
https://marshallforum.com/threads/smallbox-50-cliff-jack.132917/

[Helmholtz]

Instabilität bedeutet Schwingneigung.
Kann u.U. erst auftreten, wenn der Elko nicht mehr ganz frisch ist.