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Thread: Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

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    Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

    New member saying hello and hoping to tap into the collective wisdom of this group.
    I am a newbie at this, so please forgive any totally lame statements on my part.

    I have picked up an older Bogen CH8 PA and I would like to mod it like Terry from D-Lab did in this video:
    https://www.youtube.com/watch?v=sMfYSLZj4hc

    I have email Terry about his conversion but he has not replied yet.

    I am getting ready to order the parts, but I have a question about the filter cap can.
    In the schematics that I can find online, the caps are: 5 µF@400V, 5 µF@400V, 5 µF@400V, 5 µF@400V. However, my cap can is: 20Mfd @450V, 20Mfd @450V, 20Mfd @450V, 20Mfd @450V (see pic)
    And it looks like the Fender AA 764 schematic uses: 20µF @450V, 20 µF @450V, 20 µF @450V
    Question 1: Is Mfd the same as µF? (I think it is, but just want confirmation)
    Question 2: Is the schematic for the Bogen incorrect for my amp in terms of the filter capacitors? Or has the cap on my amp been changed.

    Any tips or suggestions?

    Relevant information: I plan on replacing the 6SF5 with a 12AX7 and if I can figure it out, rewiring the 6SJ7 for a 6SL7 preamp tube like in the video.
    Power transformer: Thordarson TS-24R01 Secondary 325V-0V-325V
    Output transformer: Merit A2904

    Thanks.
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    Last edited by tboy; 11-27-2017 at 04:40 AM. Reason: fixed link

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    Welcome to MEF!

    I would consider 5uF to be undersized in a HV tube amp power supply filter. Either your Bogen has been upgraded to 20uF, or the design changed, but the schematic didn't. Either way, it is an improvement. If you want to use the original 4 slot mounting insulator to mount the new can, you could get a 20/20/20/20@450v can, and jumper two of the 20's to make a 40uF for the first node for a little better filtering.

    Mfd is the same as uF, both express microfarads. In dealing with caps, especially in high voltage situations, the tolerances for capacitance or uF is pretty loose, and not highly critical, so the difference between 20 and 40 uF isn't that much. However, the voltage rating is more important. Always use a rating at or higher than designed. For instance, this four cap 20uF can is rated at 475v, where the original is 450v, making the replacement an improvement, especially with today's higher wall voltages. https://www.tubesandmore.com/product...f-electrolytic

    And if you mess up your first amp, I know a guy....

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    It's weird, because it WAS working fine.....

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    You really don't need to use a filter cap can. Electrolytic capacitors are much smaller and more accurate than 50 years ago. You could use individual capacitors. It's cheaper than a replacement can cap. You can also "stuff" them in the old can if you want, but I'm sure there's plenty of room in the box either way. Since you are changing and rewiring tube sockets anyway, I would recommend using all new ceramic sockets. They are very inexpensive. I would also consider using a turret or tag board. If it's your first build, point to point construction can be maddening.

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    Last edited by olddawg; 11-27-2017 at 04:10 AM.

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    Randall, thanks for the info. I am gong to give it a shot and I will keep you posted. If I get in too deep, I may need to take it to that "guy" you know. He has done some great work for me

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    Last edited by tboy; 11-27-2017 at 05:19 AM. Reason: typo

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    Those are good ideas olddawg. I am looking at using an eyelet board.

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    Since this amp uses a 6L6 single ended... you might want to look at a few Princeton schematics. If the pilot lamp is run off the filament voltage, if you run a neon or LED lamp off of the power switch, you might even free up enough heater current for a Princeton Reverb build. If you use an eyelet or turret board it is easy to mount all of the power supply filter caps on one side. You can also move the OT to the other side if the chassis to give you more room.

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    Last edited by olddawg; 11-27-2017 at 10:25 AM.

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    Have a CH8 that I converted

    I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
    I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
    So - did you get that amp done?

    BMac

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    "And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids "

    The reasoning behind the lower screen voltage is to limit the overall amplifier output.

    After all, it is only a PA amp.

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    Quote Originally Posted by bmccowan View Post
    I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
    I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
    So - did you get that amp done?

    BMac
    As was stated before.. look at some Fender Princeton schematics. I doubt the conversion is too complex to copy their power supply architecture for the output stage. I would probably gut it and just build a Princeton if the PT specs are close...

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    No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.

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    Quote Originally Posted by Chemtooler View Post
    No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.
    What exactly is meant by “too strong”? There is just voltage and current. It shouldn’t matter unless your B+ is too high or it can’t deliver enough current. If the PT is more “robust” it usually just means less sag. I put a PT meant for a 30(ish) watt amp in a 20(ish) watt amp the other day. It may be a little stiffer but it sounds fine and is reliable as hell.

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    Some of these old amps were built around specs for the metal 6L6{nothing}.

    Design center values were:

    Plate Voltage 360V
    Screen Voltage 270V
    Plate Dissipation 19W

    The metal shell is connected to pin 1 so designs like this usually have pin 1 on the socket grounded. Some old Fender amps are wired like this. You see it on the layouts.

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    Quote Originally Posted by Chemtooler View Post
    No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.
    Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
    I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
    I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.

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    Quote Originally Posted by bmccowan View Post
    Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
    I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
    I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.
    There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.

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    Quote Originally Posted by olddawg View Post
    There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.
    Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
    Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
    After the tweed era - all Princetons were push pull 2-6V6.
    Brownface Princetons - push pull, 2-6V6.
    Blackface & Silverface Princetons - push pull, 2-6V6.
    All Princeton Reverbs - push pull, 2-6V6.
    Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
    The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
    Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
    Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
    What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.

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    Quote Originally Posted by bmccowan View Post
    Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
    Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
    After the tweed era - all Princetons were push pull 2-6V6.
    Brownface Princetons - push pull, 2-6V6.
    Blackface & Silverface Princetons - push pull, 2-6V6.
    All Princeton Reverbs - push pull, 2-6V6.
    Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
    The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
    Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
    Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
    What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.
    I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.

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    Quote Originally Posted by olddawg View Post
    I thought the issue was a "weird" power supply problem? My point simply was to compare some similar schematics and modify the power supply more conventionally if the old PA setup is causing the issue.
    And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.

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    Quote Originally Posted by bmccowan View Post
    And my point was simply to correct some of the Princeton info you provided, so that Chemtooler would not spend effort looking for what does not exist; that's all. I get stuff wrong sometimes and others correct it - no offense taken. I have that Bogen CH8 - the voltage is higher than desired for the circuit and Bogen scrubbed off voltage with a couple of big wattage resistors to ground. In fact according to their schematics, they did that for several of their smaller amps. Why? I can only guess that with the huge number of models Bogen produced, they tried to standardize as much as possible on power transformers. That left some models with transformers designed for larger amps. Following a fender power supply schematic, would not fix that, unless you also replaced the PT itself. In many cases, you are right; follow a well know power supply/filtering architecture and you will be ok - but not for this one.
    No offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.

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    Quote Originally Posted by olddawg View Post
    No offense taken. I’m obviously not an expert on all of the Champs and Princetons. For some reason I thought there were single ended production Princeton Reverbs. Guess I was wrong, probably confused it with some custom builds as you said. That said, there’s always more than one way to Egypt. With a high B+ I would be tempted to use a 6L6 and a couple of big zeners. But as you say... The OT might be an issue.
    "More than one way to Egypt." I've never heard that before - that's funny. I've never tried the zener route, but know of it. The CH8 uses a 6L6. I have dropped the voltage more than I need to to protect the tube, but I like that "brown" sound you get from lower voltage. And hey, if you are interested in checking out all the various Fender amps, check out the Fender Amp Field Guide; The Fender Amp Field Guide There are a few mistakes but its pretty interesting and comprehensive.

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    Supporting Member Randall's Avatar
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    This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (440v - 39.8v) x .0427A = 17.1 watts, correct?

    If this is correct, does 931 ohms seem like a high value for a cathode resistor?

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    Last edited by Randall; 02-05-2019 at 07:37 PM.
    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    High? Low? Who knows? Does it work, sound good? That is the issue.


    Does a different set of 6L6 act the same?

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    Supporting Member Randall's Avatar
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    It sounds about right. It's a single ended 6L6 Champ build, so no sets.

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    It's weird, because it WAS working fine.....

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    Oh, it is single ended? You know that will be running class A, right?

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    I've had to go from 470R to 1K on the cathode of a single ended fender before. No big deal when considering how much the line voltage has gone up from 117V, and also that I wouldn't subject a modern tube to the hotter conditions they used to get away with.

    edit: but that was 6V6, and that was to get it down to max. dissipation.
    17W would be considered cold for 6L6 single-ended I think.

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    Here's a bias calculator. You should be able to go 30watts diss with a6l6gc.
    https://robrobinette.com/Tube_Bias_Calculator.htm

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    "17W would be considered cold for 6L6 single-ended I think."

    I always get confused with the class A and dissipation. What would be a good target to shoot for with a 6L6GC?

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    Less than 100% say.

    If it sounds good now, why fight with it? results matter more than anything I can calculate on a napkin.

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    Education is what you're left with after you have forgotten what you have learned.

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    Bent Member Chuck H's Avatar
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    What they said^^^^^^^^^^^^^^^^^^^^^

    Also, the max dissipation @ voltage and for a given load is what you'd be aiming for WRT idle dissipation for class A I think. That is, just because the tube is rated at 30 watts max doesn't mean that a standing idle of 30W is ideal at 440Vp into the load provided. The idea here is center bias when conducting with a nod to a safe idle dissipation. I'm sorry I don't know the formulas, but it can be done pretty easily on a scope with a big rheostat as the cathode resistor. Chances are that at 17W dissipation idle the tube will be cutting off ahead of saturating. That means you're not getting maximum efficiency. But as Enzo pointed out, if it sounds good you're done. It's often the case in single ended guitar amps that a center bias during conduction can't be achieved without an idle current over max dissipation. I think this might be because of high plate voltages. High WRT class A operation anyway. And it's also not uncommon for single ended amps to be dissipating more at idle than when conducting signal. Which is just saying the same thing in a different way I guess.

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    Quote Originally Posted by Randall View Post
    This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (400v - 39.8v) x .0427A = 17.1 watts, correct?
    17.1W is correct but there's a typo. It should read (440v - 39.8v)

    What does the power supply look like now compared with the original schematic in post #1?
    The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.

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    Last edited by Dave H; 02-05-2019 at 03:08 PM.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Dave H View Post
    What does the power supply look like now compared with the original schematic in post #1?
    The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.
    Hot for a 6L6 single ended?

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    Supporting Member Randall's Avatar
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    "17.1W is correct but there's a typo. It should read (440v - 39.8v)"

    Yes, thank you. I fixed it.

    I found a bad supply node resistor. It was a 10 ohm where a 1000 ohm should have been, and was distressed to the point it read 968 ohms in circuit, tricking me into thinking it was the 1K it was supposed to be. During unsoldering, one leg fell away. So now that it is corrected, and I have realized I forgot two things, A: this is class A operation so that whole 70% thing is not in play, and B: 6L6GC is 30 watts, not 25 watts. So now I am back to the original 470 ohm cathode resistor.

    Here's where I'm at now.

    Cathode resistor = 464 ohms
    Plate voltage = 402.5 v
    Cathode voltage = 31.6 v
    6L6GC = 30 watts

    This has this tube idling at 25.2.watts, and that is where I am going to leave it. Once again, trying to fix a problem in my own head only.

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    Last edited by Randall; 02-05-2019 at 08:29 PM.
    It's weird, because it WAS working fine.....

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    Quote Originally Posted by g1 View Post
    Hot for a 6L6 single ended?
    Randall said it was running at over 100mA and burning out 6L6s.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Dave H View Post
    Randall said it was running at over 100mA and burning out 6L6s.
    Oh, ok. We are talking about different points in time.
    I was talking about with the cathode R changed to 931R and idling at 17W.
    I thought you were also speaking of the 17W iteration but you were talking about when he received the amp. Thanks for the clarification.

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    Quote Originally Posted by g1 View Post
    I was talking about with the cathode R changed to 931R and idling at 17W.
    I should have guessed that I agree 17W and 42mA cathode current is low for 6L6 but it looks like Randell has fixed it. If the OT impedance is 7k like the marked taps on the chart below then I think he has it about right.

    Click image for larger version. 

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    Supporting Member Randall's Avatar
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    I think the bad node resistor was skewing things under load. It seems to be settled in now. I'll leave it to the owner/builder, a smart guy, to decide where to take it from here if he wants to adjust the cathode resistor/current.

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    It's weird, because it WAS working fine.....

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