• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Poor man's 300B amplifier

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#81
Globulator said:
Oui, j'en ai un, quelque part ici, un ampli chinois bon marché modifié 'Sweet Peach'.
C'était une triode, le son était nul - principalement à cause des capuchons d'entrée recyclés, ce qui est intéressant - je n'avais jamais réalisé que les capuchons pouvaient supprimer les aigus en ligne LOL, c'était bizarre.
Mais je voulais plus de puissance, et tester une idée... pas de GNFB sur l'OPT.

À mon avis (limité LOL), le mode Triode est simplement une pentode avec un peu de feedback intrinsèque - c'est-à-dire que le gain est plus faible mais l'impédance de sortie est plus faible, la distorsion est plus faible : tout ce que nous attendrions d'un feedback local.

Mais GNFB dans un ampli à lampes n'a aucun sens pour moi, puisque Thorsten Loesch (dans un article de l'IIRC) a souligné qu'aux fréquences extrêmes, l'OPT (transformateur) a de toute façon un gain moche, donc tout le GNF martelait les médiums et ignorait les bords. , et la bataille de la « marge de phase » avec cette lente masse de fils, de fer et de capacité parasite créée pour un « plaisir » et une misère sonore sans fin.

Mon ampli est donc une pentode complète avec régulation de décharge de gaz pour la grille d'écran (tamponnée via un MOSFET car le régulateur de tube à gaz avait une impédance trop élevée pour les demandes du GU50) avec une grande quantité de retour de l'anode GU50 vers le tube pilote.

Il existe probablement une meilleure façon d'organiser ce retour en utilisant uniquement des résistances, mais je me suis retrouvé avec un capuchon à film gras qui semble bien fonctionner.

Donc mon OPT est assis seul, discutant avec les haut-parleurs, tous les retours sont locaux sur le tube d'entrée et durs entre le GU50 et son tube pilote.

Le son est sans effort. D'un amplificateur bien plus gros. Pas de difficulté notable plus que le monstre à semi-conducteurs de 150 W/canal de classe A de 45 kg qu'il a remplacé.
Ouvert, doux, détaillé, musical : alors je l'ai laissé comme ça. En plus les tubes à gaz sont très jolis🙂
Je peux donc recommander une pentode GU50 SE avec uniquement du feedback autour des 2 tubes derrière l'OPT.

Une chose contre laquelle je mets en garde est d'utiliser des prises bon marché pour le GU50. Le tube peut survivre verticalement, mais placez-le debout pour le peaufiner - donc le GU50 est horizontal et ils se fissurent autour de la base - j'ai perdu 4 tubes de cette façon. Les prises Sweet Peach semblaient correctes, mais si vous achetez les vôtres, utilisez les prises de poubelle soviétiques appropriées comme l'OP de ce
Click to expand...
Hello,
Thanks for your reply and sharing your experiment .
Regards
 
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#84
Globulator said:
Yes I have one, on here somewhere, a modified 'Sweet Peach' cheap chinese amp.
It was triode, the sound sucked - mainly due to the recycled input caps, interestingly enough - I'd never realised caps could remove treble inline LOL, it was bizarre.
But I wanted more power, and to test an idea... no GNFB over the OPT.

In my (limited LOL) view Triode mode is simply a pentode with a bit of intrinsic feedback - i.e. gain is lower but output impedance is lower, distortion is lower: all that we'd expevct from local feedback.

But GNFB in a tube amp makes no sense to me, since Thorsten Loesch (in some article IIRC) pointed out that at the frequency extremes the OPT(ransformer) has lousy gain anyway, so all the GNF was hammering the midrange and ignoring the edges, and the 'phase margin' battle with this slow mass of wires, iron and parasitic capacitance made for endless 'fun' and sonic misery.

So my amp is a full pentode with gas discharge regulation for the screen grid (buffered through a MOSFET as the gas tube regulator had too high impedance for the GU50's demands) with a large amount of feedback from the GU50 anode to the driver tube.

There's probably a better way of arranging this feedback using only resistors, but I ended up with a fat film cap which seems to work fine.

So my OPT sits alone, chatting with the speakers, all feedback is local on the input tube, and hard between the GU50 and it's driver tube.

The sound is effortless. Of a far, far larger amplifier. Not noticeably struggling more than the solid state 150W/channel class A 45kg monster it replaced.
Open, sweet, detailed, musical: So I left it like that. Also the gas tubes are very pretty 🙂
So I can recommend a GU50 pentode SE with only feedback around the 2 tubes behind the OPT.

One thing I would caution against is using cheap sockets for the GU50. The tube may survive vertically, but stand it up for tweaking - so the GU50 is horizontal and they crack around the base - I lost 4 tubes that way. The Sweet peach sockets seemed Ok, but if buying your own, use the proper soviet dustbin sockets like the OP of this thread!
Click to expand...
Hello Globulator,
Do you have any distortion measurements?
 
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#85
Globulator said:
Yes I have one, on here somewhere, a modified 'Sweet Peach' cheap chinese amp.
It was triode, the sound sucked - mainly due to the recycled input caps, interestingly enough - I'd never realised caps could remove treble inline LOL, it was bizarre.
But I wanted more power, and to test an idea... no GNFB over the OPT.

In my (limited LOL) view Triode mode is simply a pentode with a bit of intrinsic feedback - i.e. gain is lower but output impedance is lower, distortion is lower: all that we'd expevct from local feedback.

But GNFB in a tube amp makes no sense to me, since Thorsten Loesch (in some article IIRC) pointed out that at the frequency extremes the OPT(ransformer) has lousy gain anyway, so all the GNF was hammering the midrange and ignoring the edges, and the 'phase margin' battle with this slow mass of wires, iron and parasitic capacitance made for endless 'fun' and sonic misery.

So my amp is a full pentode with gas discharge regulation for the screen grid (buffered through a MOSFET as the gas tube regulator had too high impedance for the GU50's demands) with a large amount of feedback from the GU50 anode to the driver tube.

There's probably a better way of arranging this feedback using only resistors, but I ended up with a fat film cap which seems to work fine.

So my OPT sits alone, chatting with the speakers, all feedback is local on the input tube, and hard between the GU50 and it's driver tube.

The sound is effortless. Of a far, far larger amplifier. Not noticeably struggling more than the solid state 150W/channel class A 45kg monster it replaced.
Open, sweet, detailed, musical: So I left it like that. Also the gas tubes are very pretty 🙂
So I can recommend a GU50 pentode SE with only feedback around the 2 tubes behind the OPT.

One thing I would caution against is using cheap sockets for the GU50. The tube may survive vertically, but stand it up for tweaking - so the GU50 is horizontal and they crack around the base - I lost 4 tubes that way. The Sweet peach sockets seemed Ok, but if buying your own, use the proper soviet dustbin sockets like the OP of this thread
Click to expand...
Hello,
Please :
Does the current of G2 increase beyond 30ma?
What is the Opt?
What is the transformer load?
What's the max value of the anode current?
Regards
 
#87
Marco audio,

Many different operating points of a given tube have tradeoffs . . .
Output Power; Distortion; Damping Factor; Tube Life; Efficiency; (and Looks, Shapes, and Price).

Check out Page 4 of this 300B link, and see the tradeoffs listed:
we300a_b.pdf

Then consider that there are many different operating conditions, with similar tradeoffs for:
KT66; EL34; 7591; 6V6; 829B; etc. . . . you name the Pentode or Beam Power output tube that you want to use.
Then pick the relative tradeoffs that mean sense to you, in order to meet your favorite wants and needs:
Output Power; Distortion; Damping Factor; Tube Life; Efficiency; (and Looks, Shapes, and Price).

Similar tradeoffs exist for single ended; push pull; parafeed; etc.

Decide; Build; Power-On and test; then . . .
Have Fun!
Enjoy Listening!
 
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#88
revintage said:
No reason to apologize. I anyway enjoyed it. My hifi-french is acceptable after having spelled through all volumes of L’Audiophile, that I subscribed to through the years.
Click to expand...
Hello,
Those were the days Altec Vot ( i still remember the first time hearing them 35 years ago with a 2A3 single ended in Rue de Belfort) Hiraga, Anzai srpp, Isoda)
Some of these publications contained more information than most complete threads here haha.
Of course they were making money too but not in the same way some sponsors do here. Some of them will sell you a goldplated coffee filter after they convinced you it will give you first quality coffee.
Even some Vietnamese will go to that famous American coffeeshop just to be seen while going to a small shop will give you perfect coffee like in the photo for a $
Greetings Eduard
P.s by the way there are serious audiophiles in Vietnam too
 

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