That's definitely a bad idea. As already mentioned you should have separate supplies for each 6B4G tube. Best would be to use the two DC heater supplies for each of the 6B4G tubes and the unused 6.3VAC supply for the 6SN7 tubes.
Hi, I just finished building a 6B4G SE tube amp from a schematic I found online by Bill Howard. The issue is that the 6B4Gs keep red plating. I have rebuilt the cathode bias circuit twice and changed out the components as well. I decided to check double check the power transformer voltages and found that my power transformer was not 750V CT, but closer to 800V CT. The transformer was purchased off fleabay and is old stock. My question is how do I figure out the current draw of the circuit so that I can find a suitable dropping resistor? I would like to put the resistor on one of the legs on the transformer, before the rectifier so that I would not need such a large value.
There is a lot very wrong about that schemo. If you literally mean a 5U4G, then that 50uF reservoir capacitor us way too large, and will exceed the Isurge rating of the 5U4G. Even a 5U4GB likely will be sourcung too much current. The 5U4GB is one of the beefiest vacuum power diodes with an Isurge= 1.0A, that really isn't all that great. I did a project using the 5U4GB, and a 47uF reservoir capacitor was too much. I had to drop it to 34uF (two, 68uF/350V capacitors in series). Also, the voltage rating of the filter capacitors is too low, and are running on the edge, especially at power up when cathodes are cold and current demand low.
Secondly, are your heater supplies floating? If they're not,then the finals are running with zero bias. If you're trying to parallel heaters off the same floating power supply, then your cathode bias resistors are in parallel, and half what they should be. These DHTs really do need their own dedicated heater power supplies regardless of AC or DC operation.
If you need to drop the HV, and this is likely as many vintage PTXs were designed for a mains voltage of ~110V, whereas today AC mains run more like 120V. Have you checked your mains voltage? Sometimes, it runs higher than normal. I once did a repair job for a client who was complaining that the electric heaters of his dryer were always poofing. His mains voltage was 180V. No wonder they poofed so much! I advised he call the power company immediately to get that straightened out.
If you need to drop the DC rail: MOSFET Follies bottom of the page.
The overvoltage in the 6B4G. This valve in class A needs no more than 250 volts for 15 Watt of anode disipation. How much current you planned with this R on cathodes?
Each 6B4 needs its power supply for every heater. Separating they from the front end filamentary
I propose in keep out the in capacitor from the PS and you going to take the proper voltage and a better regulation of the current with a 5U4 feeding a choke.
Each 6B4 needs its power supply for every heater. Separating they from the front end filamentary
I propose in keep out the in capacitor from the PS and you going to take the proper voltage and a better regulation of the current with a 5U4 feeding a choke.
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There is a lot very wrong about that schemo. If you literally mean a 5U4G, then that 50uF reservoir capacitor us way too large, and will exceed the Isurge rating of the 5U4G. Even a 5U4GB likely will be sourcung too much current. The 5U4GB is one of the beefiest vacuum power diodes with an Isurge= 1.0A, that really isn't all that great. I did a project using the 5U4GB, and a 47uF reservoir capacitor was too much. I had to drop it to 34uF (two, 68uF/350V capacitors in series). Also, the voltage rating of the filter capacitors is too low, and are running on the edge, especially at power up when cathodes are cold and current demand low.
Secondly, are your heater supplies floating? If they're not,then the finals are running with zero bias. If you're trying to parallel heaters off the same floating power supply, then your cathode bias resistors are in parallel, and half what they should be. These DHTs really do need their own dedicated heater power supplies regardless of AC or DC operation.
If you need to drop the HV, and this is likely as many vintage PTXs were designed for a mains voltage of ~110V, whereas today AC mains run more like 120V. Have you checked your mains voltage? Sometimes, it runs higher than normal. I once did a repair job for a client who was complaining that the electric heaters of his dryer were always poofing. His mains voltage was 180V. No wonder they poofed so much! I advised he call the power company immediately to get that straightened out.
If you need to drop the DC rail: MOSFET Follies bottom of the page.
The caps in the power supply are rated for 375VAC, I thought that translates into twice that amount for DC duty?
Yes the power supplies are floating. I will connect each 6B4G to its own supply and use the 6.3V tap off the main power transformer for the driver tubes.
I am using a 5U4G, should I try a 5R4 as suggested in an earlier thread?
I found another choke I can add to the power supply as previously mentioned, but it is substantially large (at least 10 pounds) and is 10H @250ma. I don't know if I can fit it on the chassis 🙁
I am not too sure about the chokes I am using. They are vintage military oil filled can types.
I measured my AC voltage from my wall socket and am getting 121VAC. I suspect as stated above that my Xformer is rated for 110v in. I will fix my bias circuit this weekend and then hook the amp up to my variac and give it 110v and see if tubes red plate.
I measured my AC voltage from my wall socket and am getting 121VAC. I suspect as stated above that my Xformer is rated for 110v in. I will fix my bias circuit this weekend and then hook the amp up to my variac and give it 110v and see if tubes red plate.
Would voltage regulator tubes be worth considering (0C3)? I have about 50 NOS (various types, not just 0C3) that I have been trying to find a use for.
You can use cathode resistor for output tube about 2 - 2.5 kohm, You can try that on the resistor +U must be about 150 v and current about 50-60 ma, and +U between plat and cathode must be about 300 V.
Use choke input power supply. Calculate what size choke you need for proper operation. A choke input filter will give you correct B+ for a cathode biased class A PP amp using your 6B4-G tubes and your transformer.
Ooops, you're building SE... Should work for SE also.
Filament hum will be a problem.
Ooops, you're building SE... Should work for SE also.
Filament hum will be a problem.
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Use choke input power supply. Calculate what size choke you need for proper operation. A choke input filter will give you correct B+ for a cathode biased class A PP amp using your 6B4-G tubes and your transformer.
Ooops, you're building SE... Should work for SE also.
Filament hum will be a problem.
😀😀😀😀😀 Input Choke the voltage will be the AC * 0.9 in your case, 375 * 0.9 = 337.5volts with better regulation of ripple. If use auto polarization you need 67 volts for 3000ohms of load charge. You going to take 270 volts across the 6B4G that is a reasonable idea and will take near of 3.5W of output.
For heaters, Use Rod Coleman regulator. Is a fantastic solution for DHT valves. 20 dollars, voltage+current regulation. Soft Start for long life expectancy
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Hi again. did you follow my advice in post # 13 ?
Again, do make sure you are using separate supplies for each output tube.
I won't go into all the detail of what is not ideal with this circuit. I just want to get your B+ down so your output tubes don't glow red...
Please advise. All the other "tweeking" advice from other posters won't help unless you get it running in a safe zone.
Ian
Again, do make sure you are using separate supplies for each output tube.
I won't go into all the detail of what is not ideal with this circuit. I just want to get your B+ down so your output tubes don't glow red...
Please advise. All the other "tweeking" advice from other posters won't help unless you get it running in a safe zone.
Ian
Hi again. did you follow my advice in post # 13 ?
Again, do make sure you are using separate supplies for each output tube.
I won't go into all the detail of what is not ideal with this circuit. I just want to get your B+ down so your output tubes don't glow red...
Please advise. All the other "tweeking" advice from other posters won't help unless you get it running in a safe zone.
Ian
Well, I got the filaments all wired correctly now; each 6B4G has its own DC heater supply and I have the two 6SN7s wired with the 6.3VAC tap from the power transformer.
I fired it up yesterday after making that change and it appears that only the right side power tube is red plating. The left side may red plate shortly after, but I am not sure because I turn the amp off before it has a chance.
I tried running the amp from my variac with 110V and the right side still red plates, but not as fast and not as bright.
I am wondering if the hum balance pot that I am using on the right side could be bad? I checked it before I installed it, but it is an old one that I had in my parts bin. I ordered two new 100 Ohm pots last night just in case.
I am going just take some voltage readings on the amp under load today and if the voltages are still high while under load I am going to take the advice and try the LCLC input. I am going to have to modify the chassis a little to accept the choke I found because its much bigger than I had thought and is probably closer to 20lbs 😱
I was thinking making a modified version of this power supply for SE use and of course using my current filament supply.

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ok so did you swap 6b4g's and still see the same tube red-plate? or is it the channel?
Check the Anode, Cathode and Grid voltages on your 6b4g's please report them here.
Idea: Maybe your 0.22uF coupling cap on one channel is not blocking DC.. or maybe there is a wiring error in one channel...
Check the Anode, Cathode and Grid voltages on your 6b4g's please report them here.
Idea: Maybe your 0.22uF coupling cap on one channel is not blocking DC.. or maybe there is a wiring error in one channel...
ok so did you swap 6b4g's and still see the same tube red-plate? or is it the channel?
Check the Anode, Cathode and Grid voltages on your 6b4g's please report them here.
Idea: Maybe your 0.22uF coupling cap on one channel is not blocking DC.. or maybe there is a wiring error in one channel...
OK, I swapped them and the tube red plates on both sides, but it will red plate on the right side first. I have a NOS Westing House tube that does not seem to red plate, but I am unsure as it is a strange tube in that you can see it light up at all, not even the heaters (but it does pass audio so I know it works), so if it does red plate, I cant tell. I am wondering if all of these tube are bad? I have four Sylvanias, one Ken-Rad, and one Westing house, and they have all red plated with the exception of the Westing House (but again not sure). I wonder if it would be beneficial to purchase two brand new 6B4Gs or 6C4Cs? I was going to buy some, but did not want to put them in the amp until I got it working. I tested all the tubes and they all check out strong on my tube tester, even after red plating.
I am going to check my wiring again, but I have tried to sets of .22uf coupling caps.
Will hopefully get some voltage readings this afternoon after I finish my "honey do" list and rig up some dummy load resistors.
OK, so I managed to get a voltage reading while under load and I am only getting about 260v B+ 😕 And the weird thing is that the voltage was steadily dropping. I am stumped at this point. In retrospect, I should have purchased new components instead of what I had on hand.
OK, I swapped them and the tube red plates on both sides, but it will red plate on the right side first. I have a NOS Westing House tube that does not seem to red plate, but I am unsure as it is a strange tube in that you can see it light up at all, not even the heaters (but it does pass audio so I know it works), so if it does red plate, I cant tell. I am wondering if all of these tube are bad? I have four Sylvanias, one Ken-Rad, and one Westing house, and they have all red plated with the exception of the Westing House (but again not sure). I wonder if it would be beneficial to purchase two brand new 6B4Gs or 6C4Cs? I was going to buy some, but did not want to put them in the amp until I got it working. I tested all the tubes and they all check out strong on my tube tester, even after red plating.
I am going to check my wiring again, but I have tried to sets of .22uf coupling caps.
Will hopefully get some voltage readings this afternoon after I finish my "honey do" list and rig up some dummy load resistors.
This is beginning to look more and more like plate current run-away. I've seen this myself with gassy tubes, even if there wasn't enough to produce the purple glow.
"I have a NOS Westing House tube that does not seem to red plate, but I am unsure as it is a strange tube in that you can see it light up at all, not even the heaters (but it does pass audio so I know it works), so if it does red plate, I cant tell".
Are you sure you actually have 6B4s? The originals were DH triodes with thin filament wires. Back in the '50s, Sylvania, especially, began to sell 6AV5 power pents as "6B4s" by internally connecting the screen to the plate. (The 6AV5 is an IDH power pentode with a fat cathode.) That makes a pseudotriode as the screen doesn't simply go away. Over volting the screen will still cause the same sorts of problems you see in pents operated as pents, and the 6AV5 has a limited screen voltage spec. That includes plate current run-away.
This is beginning to look more and more like plate current run-away. I've seen this myself with gassy tubes, even if there wasn't enough to produce the purple glow.
"I have a NOS Westing House tube that does not seem to red plate, but I am unsure as it is a strange tube in that you can see it light up at all, not even the heaters (but it does pass audio so I know it works), so if it does red plate, I cant tell".
Are you sure you actually have 6B4s? The originals were DH triodes with thin filament wires. Back in the '50s, Sylvania, especially, began to sell 6AV5 power pents as "6B4s" by internally connecting the screen to the plate. (The 6AV5 is an IDH power pentode with a fat cathode.) That makes a pseudotriode as the screen doesn't simply go away. Over volting the screen will still cause the same sorts of problems you see in pents operated as pents, and the 6AV5 has a limited screen voltage spec. That includes plate current run-away.
Wait....I thought the purple glow was normal? Every one of the Syvanias emit purple glow.
Also the 6B4Gs that I am using have the big coke bottle shape and not the 6AV5 shape...could they still be 6AV5s?
Wait....I thought the purple glow was normal? Every one of the Syvanias emit purple glow.
There are two kinds of glow. One is a sky blue that's caused by cobalt contamination if the glass and/or the plate alloy. You often see it just above the plate, opposite the holes in the plate used by the assembler to wiggle the grid(s) into place. This glow is quite harmless, and makes for increased glowey bottle goodness.
The other glow is much more violet in colour, occurs between the anode and cathode (looks like a thyratron in action) or clings to the glass, and it will either disappear and/or follow your finger like one of those plasma globes. This is the bad kind of glow from excessive gas. Even if your VTs are gassy, you may not see it, but you sure will notice it. especially with fixed bias, as your plate current won't settle down.
Take a close look inside your 6B4s. If you see a cathode, and a screen grid, then you have rewired 6AV5s. The original 6B4 is a DH triode.
There are two kinds of glow. One is a sky blue that's caused by cobalt contamination if the glass and/or the plate alloy. You often see it just above the plate, opposite the holes in the plate used by the assembler to wiggle the grid(s) into place. This glow is quite harmless, and makes for increased glowey bottle goodness.
The other glow is much more violet in colour, occurs between the anode and cathode (looks like a thyratron in action) or clings to the glass, and it will either disappear and/or follow your finger like one of those plasma globes. This is the bad kind of glow from excessive gas. Even if your VTs are gassy, you may not see it, but you sure will notice it. especially with fixed bias, as your plate current won't settle down.
Take a close look inside your 6B4s. If you see a cathode, and a screen grid, then you have rewired 6AV5s. The original 6B4 is a DH triode.
These definitely have the bad purple haze glow....FML

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