Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MaJiG on October 03, 2013, 04:04:14 am
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I am reasonably experienced at repairing/modding tube amps, but have a really bizarre amp mod in my shop. It is a 5e5a Tweed Pro clone. The owner bought it long distance and when he got it, he was unhappy with the "anemic" sound.
I am attaching a partial schematic of the power tubes. I have not included the heater wiring for simplicity.
[1] There is a 100 ohm 1/4W resistor from each the heater pins of one of the 6L6's to its cathode - between pin 8 and the 1 ohm series resistor to ground. The cathode will have a few millivolts on it and thus is not ground, but this I cannot gronk. Any idea why someone would put these here?
[2] Each 6L6 has a 22 ohm 2W resistor in series with the its screen grid and the CT of the OT. Are these snubbers to quell oscillations? That was all I could think of.
Bizarre.
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22 ohms resistor is for run tubes cooler . Most Fender Blackface use 470 ohms resistor. I' ll put those to replace 22 ohms.
Heater with two 100 ohms going to same place !!! There must be no voltage feeding these heater ?
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The 100 Ohm resistors are probably left-over from an attempt at elevating the heater, but in this case, they are just loading down the heater supply, so should be removed. But neither the heater nor the small screen resistors should result in an anemic sound. Have you checked the tubes, bias, etc.?
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Ok I understand it have wires (not show ) bring 6.3 AC volts to these heater .
IMO the 100 ohms are a artificial center tap because power transformer do not have a heater center tap . Yes ?
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[1] There is a 100 ohm 1/4W resistor from each the heater pins of one of the 6L6's to its cathode - between pin 8 and the 1 ohm series resistor to ground. The cathode will have a few millivolts on it and thus is not ground, but this I cannot gronk. Any idea why someone would put these here?
Those 100Ω form an artificial center tap that references the filament circuit to ground and is helpful to reduce filament induced hum. They should be fine connected as is, but I'd connect them directly to ground rather than to the cathode in this case.
[2] Each 6L6 has a 22 ohm 2W resistor in series with the its screen grid and the CT of the OT. Are these snubbers to quell oscillations? That was all I could think of.
Those 22Ω resistors limit screen current (not much though) and are an improvement over the original 5E5A which had no screen resistors.
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It is a 5e5a Tweed Pro clone. The owner bought it long distance and when he got it, he was unhappy with the "anemic" sound.
The original feeds the output tube plates after the choke. This means that the choke must be rated to carry the high plate current. Does this clone have a big choke that's up to the task?
If unsure about the choke you can move the OT center tap to the other side of the choke, but leave the screen connection as is. Does this help with the anemia?
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Sluckey;
The 100 ohms are connected to the ground via 1 ohms resistor . 1 ohms resitor did not change anything . 100 +1 =101 ohms +/- 10 %
Cathode is connected to the ground too via 1 ohms (fuse resistor ? )
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maJIG , I never use 1/4 watt on tube amps , 1/2 watt at least.
I don't know why 1 ohms 2 watts resistor on cathode, I mean if AB class , no need those or are they a fuse ?
Leave the 100 ohms there you need them but put some 1/2 watts
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Cathode is connected to the ground too via 1 ohms (fuse resistor ? )
The 1Ω resistors are provided to easily measure cathode current when checking idle dissipation to determine desired/proper bias setting.
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Cathode is connected to the ground too via 1 ohms (fuse resistor ? )
The 1Ω resistors are provided to easily measure cathode current when checking idle dissipation to determine desired/proper bias setting.
+1 I was sleeping :BangHead: I used that on all my amps
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[1] There is a 100 ohm 1/4W resistor from each the heater pins of one of the 6L6's to its cathode - between pin 8 and the 1 ohm series resistor to ground. The cathode will have a few millivolts on it and thus is not ground, but this I cannot gronk. Any idea why someone would put these here?
Convenience.
This thing is built tweed-style, right? Same smallish chassis, pilot light, etc?
On the original amp, only 1 wire ran to each tube's heater; in this case, pin 7 of the output tubes. Pins 1 & 2 were connected together and then to ground at the chassis right by each tube socket. Grounding pin 1 made sense because metal-shell tubes had pin 1 connected to the shell, and in the mid-50's there was a very good chance you'd be using those tubes (even if glass versions were available).
The pilot light and PT heater winding were also grounded. The PT winding at the chassis itself near the PT, but the pilot light holder was grounded in an odd fashion: one of the two lugs for electrical connection to the light was spun around and soldered directly to the frame of the holder, thereby grounding that lug and that side of the pilot light.
Most people now feel it's better to run 2 wires instead of one, so the connections to the chassis are not there in modern wiring. Further, because the amp had no heater center tap or 100Ω resistors back in the day (the unbalanced wiring gave a ground connection, so a tap wasn't required, though maybe not the best plan), there is no ready-made place to put those 100Ω resistors now.
The previous builder probably decided to have the 1Ω resistors from cathode to ground to measure output tube cathode current. And since there was already a good connection to ground, they used those same points (output tube cathode) as the grounding point for the 100Ω resistor which didn't exist in the original amp.
I did almost the exact same thing once in a 5E3 build. In my case, I forgot to include the 100Ω resistors until after the amp was built. Too much work to undo what I'd done just to make a new board to have a place to put those resistors. I solder one leg of each into a ring terminal, then soldered the resistors to the heater pins on the input 12AY7. That way I didn't have to undo any wiring.
I could have just as easily put them on the output tubes (for a free positive d.c. reference) or to the top of a 1Ω resistor in a fixed bias amp like you have seen.