Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on May 29, 2012, 11:21:02 am
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Hi, got a silverface here and I can't bias the power tubes hot enough. The bias trim pot and resistor values are just fine, ( 10 k and 27 K ) so is the bias capacitor. When turned fully clockwise ( hottest ), the bias pot gives me minus 60V , which is too negative. At this voltage, the tubes draw 3mA each. Screen and plates are around 495V, so I'd like to bais at 40mA each 6L6 . Thanks for help.
Colas
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Decrease the 27KΩ to 15KΩ, or increase the 470Ω to 1KΩ. You may have to try some other values to get the exact range of negative voltage you like.
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Sluckley a parfaitement raison. Ton voltage de plaque va alors baisser et tu devras en tenir compte avec le nouveau bias . Car actuellement to bias dois être très froid = voltage de plaque plus élevé.
Sluckley +1
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If the bias balance has been changed to a bias adjust, the change to a 15k should do the trick.
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Oubli ce que j'ai écrit Colas j'ai regardé le mauvais schéma. Un Silverface a un pot de balance, pas un pot d'ajustement . Tu peux facilement modifier le pot de balance en un pot d'ajustement. Ou , comme le mentionne Ed_Chambley diminue la 15 k et moi j'ajouterais d'augmenté plutôt la 1.8 K a la sortie du transformateur
Right Ed_Chambley and what about higher value on the 1.8 K ?
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Oubli ce que j'ai écrit Colas j'ai regardé le mauvais schéma. Un Silverface a un pot de balance, pas un pot d'ajustement . Tu peux facilement modifier le pot de balance en un pot d'ajustement. Ou , comme le mentionne Ed_Chambley diminue la 15 k et moi j'ajouterais d'augmenté plutôt la 1.8 K a la sortie du transformateur
Right Ed_Chambley and what about higher value on the 1.8 K ?
I do not know where you would find a 1k8 resistor anywhere in a AB763. Please enlighten me. I love new ideas. I can only assume you are speaking of subbing in a 1k8 in place of the 470ohm prior to the diode. It would work, but you would be on the cold side. Optimally, I would want my pot to adjust from about 34ma to 44 ma with a plate voltage around 460. Also you begin leaving the "blackface" schematic. I am assuming what we have here is a silverface in mid blackface conversion. It is not really a AB763, but if it has a 27k on the pot it is not an AB 763 either. Whatever works. It is just the way I have always done it.
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ce que je ne comprends pas, c'est que ça fonctionnait et là ça ne fonctionne plus. comme si une résistance avait changé de valeur d'un coup.
merci, thanks for help.
Colas
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Ed_Chambley
Ab763 is a blackface S R . The 1.8 k is on the AA1069. Yes on Blackface , it is 470 ohms and Siverface , it is a 1.8 k
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I found many schematic there ;
http://ampedia.redbeartrading.com/datasheet.php?amp_id=151 (http://ampedia.redbeartrading.com/datasheet.php?amp_id=151)
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ce que je ne comprends pas, c'est que ça fonctionnait et là ça ne fonctionne plus. comme si une résistance avait changé de valeur d'un coup.
merci, thanks for help.
Colas
Tu veux dire quoi au juste; que tu arrivais a avoir le bon bias hot a 40 ma et que là tu ne l'as plus , seulement 3 ma ? Et tu as -60 volts ?
Si tu n'est pas capable d'avoir un voltage négatif de -44 volts ou environ et que tu as autour de 60 volts comme tu le mentionnes , ton potentiomètre de balance ( car c'est ça que tu as n'est-ce-pas ? pas un un ajustement de voltage comme les Blackface ? ) est peut-être défectueux. Son branchement à la résistance de 15 k est ouvert , ou la résistance n'est pas bien branchée ; soudures.
Si tu as -40volts et toujours 3 ma de bias ; tubes defectueux peut-être.
Tu as quel schéma ? moi le AA1069,
Bon travail
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anyway, this amp seems to have been modified to blackface specs but not totally..........
I replaced the 27k gnd resistor from outer bias adjustment ( not balance ) pot leg by a 4.7k to give the proper negative voltage. I have now a range from -22V to -65V or so that let me bias the tubes at 40mA, just as desired. I must be lost. It worked fine and all of a sudden, no bias. Did I dream that or was it another silverface ? In another life ?
I'll try to solve my own ( memory ) issue.......<
Colas
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mon cerveau a perdu quelques bits d'information en cours de route.........prob. que les lampes ont toujours été très ''froides''. Je me suis mis à vérifier le bias suite à un changement de GZ34 et j'ai vu pourquoi cet ampli-là ne sonnait pas comme il aurait dû. Merci. problème réglé mais je ne sais pas trop ce qui s'est passé !
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Good news , Bravo .
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Ed_Chambley
Ab763 is a blackface S R . The 1.8 k is on the AA1069. Yes on Blackface , it is 470 ohms and Siverface , it is a 1.8 k
Yes, I thought from the description someone was attempting to blackface it. I am a super reverb nut and am very familiar with AA1069. It is the best used amp you can buy IMO. I've got 4 super reverbs at the moment. One of them was an AA1069. Still a very easy amp to blackface. I changed a few caps and resistors, made a bias adjust. Installed 2 Weber Alnicos in the top and 2 Weber ceramics in the bottom. Man it sounds great and I don't have to worry about dragging it around. They are very versatile amps. It is not "collectable". I think Marsh has put together a kit to blackface these with step by step instructions. And while it still aint a true blackface, it is really close.
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Yes , those silverface are great amp
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mine is a AA1069 and I have performed slight ''enhancing'' mods that gives it a very nice ''edge''. No pedal, it sounds awesome. Huge signal caps, built like a tank. Can't be compared to a Marshall with 20 X 1/8 w resistors and tiny caps by square inch on a PC board, practically impossible to service !
Colas