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Thread: 1960's Supro Thunderbolt scratch build copy

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    1960's Supro Thunderbolt scratch build copy

    I'm about to finish up construction and will be applying power to it sometime this weekend. Attached are the schematic I started with in PDF format and the other is the schematic that was actually built after adding a stand by switch, grid resistors, and a couple other details or two that were suggested by forumites.

    Is there anything that looks like a mistake/oversight ?

    All I have left to do is wire the heaters, grounds, and a few other connections. Whattaya think?
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    Bent Member Chuck H's Avatar
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    Looks OK to me. Looking at it I'm not crazy about the design but it seems to be a popular build. Some modern safety and reliability changes I might make would be to include an earth ground (not indicated in your schematic), a DPST mains switch, move the first filter to the other side of the standby switch and double up on the rectifier diodes (the schems I'm seeing on line use a bridge, rather than a full wave rectifier). None of this should alter the tone one bit.

    Were these used by some famous player or used on some popular tunes?

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    OK so I finished it out and brought it up slow. All looked ok so I took it off the limiter.

    Some notes:

    Good overdriven guitar sound but virtually no clean headroom, this is acceptable but wouldn't mind just a little bit of headroom

    tone control doesn't affect anything

    volume control is wired backwards (0 is max and 10 is quiet)

    low to moderate hum. Would like to quiet the amp a little if possible but *some* noise may be inherent in circuit

    Supply voltages
    D- 454
    C- 449
    B- 409
    A- 313








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    Quote Originally Posted by Chuck H View Post
    Looks OK to me. Looking at it I'm not crazy about the design but it seems to be a popular build. Some modern safety and reliability changes I might make would be to include an earth ground (not indicated in your schematic), a DPST mains switch, move the first filter to the other side of the standby switch and double up on the rectifier diodes (the schems I'm seeing on line use a bridge, rather than a full wave rectifier). None of this should alter the tone one bit.

    Were these used by some famous player or used on some popular tunes?
    I'm not really sure. I'm building this as a sort of surprise trade with a friend. He had mentioned loving these amps so this is what I chose to build as my end of the surprise. I'm building him an amp and he doesn't know what he's getting until it shows up at his door and he's building me a guitar that I won't know anything about until it hits mine. Nothing is critical.. this is all for fun.

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    Bent Member Chuck H's Avatar
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    The only noise that should be considered inherent to audio circuits is thermal hiss. Some tubes are hummy, and some hum is almost inescapable but it should be able to be kept low enough that it can be ignored. The hum is almost certainly because of the preamp tubes, the grounding scheme or both.

    I see that you have a ground buss bar. If it's connected to the chassis at both ends that can cause ground loops. Try cutting the bar at the nearest point where the preamp ends and the power amp begins. At the very least you should isolate the power supply grounds from the signal path grounds. And nothing should be grounded in more than one place. If you consider it, the buss bar grounds everything in two places.

    One thing that can help reduce internal hum coupling within the tubes themselves is elevating the CT of the filament wind on a DC voltage. Since you're already cathode biased you can use the power tube cathodes as a DC source. Since you wouldn't want a transformer or tube failure to couple the wrong things together you can unground the 6.3V wind CT, run it through something like a 22R. 1/2 watt resistor to the top of the cathode bias resistor.

    I've had several new tubes come out of the box with generous hum built in. Mostly the rusky 12--7 tubes.

    P.S. Very tidy build.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by Chuck H View Post
    Were these used by some famous player or used on some popular tunes?
    That's the lore. Telecaster + various Valco-made amps.


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    Last edited by bob p; 01-11-2014 at 10:58 PM.

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    Supporting Member loudthud's Avatar
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    Quote Originally Posted by mort View Post
    Is there anything that looks like a mistake/oversight ?

    All I have left to do is wire the heaters, grounds, and a few other connections. Whattaya think?
    I usually wire up ground first. It's the most critical node. I recommend spliting the ground as shown in the attached edited schematic. I can't see what you used for filter capacitors, it may not be possible to split the ground if all the filter caps are in a single can. Each ground symbol is a separate connection to the chassis. It looks like the red/yellow wire from the transformer is grounded at a transformer bolt. Ground the first filter caps and the output tube bias resistor there. Add more ground lugs if you need to. The big ground wire for the preamp that grounds at the input side of the chassis is close enough to the input jacks to be considered one ground connection. Ground the A and B filter caps somewhere along that wire.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by loudthud View Post
    The big ground wire for the preamp that grounds at the input side of the chassis is close enough to the input jacks to be considered one ground connection. Ground the A and B filter caps somewhere along that wire.
    But only have the buss wire connected at the input side. Not the PS side AND input side, as it appears to be now.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Supporting Member loudthud's Avatar
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    Quote Originally Posted by Chuck H View Post
    But only have the buss wire connected at the input side. Not the PS side AND input side, as it appears to be now.
    Yes, the big buss wire will be the quiet ground. It should be only connected to the chassis near the input jacks. The A and B node filter caps ground to that buss wire, they are isolated from the hum on B+ by the 27K 2W resistor. The big buss wire has to be disconnected from the noisy ground in the power amp. The power amp grounds to the noisy ground of the power supply at the transformer bolt. This is not an ideal ground, but it's how old Fender amps are grounded.

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    I'll take a look at the grounding and adjust it as you suggested. It's not a ton of noise but if it can be cleaned up a little then great.


    Found my mistake on the tone control. Forgot to ground and solder a couple nodes from the tone control circuit (visible in the first two pics). Easy fix.


    What would I change if I wanted to increase the bass response by just a little teeny bit. I like how the amp has a nice growl without that woofy bass sound in it but just a smidge more bass response would be nice.

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    And here's a set of operating voltages taken just now. I can't quite remember how to calculate actual wattage (plate dissipation?). I did put in the sense resistors to make it easier, I remembered that much at least.

    V4- 29mV across 1 ohm resistor
    3- 466v
    4- 185.2v
    5- .31v
    8- 15.6v


    V3- 28mV across 1 ohm resistor
    3- 465v
    4- 182.6v
    5- .68v
    8- 15.6v

    V2
    1- 225.9
    2- .037
    3- 1.99
    6- 251.2
    7- .097
    8- 2.54

    V1
    1- 249.1
    2- .08
    3- 2.62

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    Bent Member Chuck H's Avatar
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    Well that input capacitor has a knee of -3dB at about 100Hz. -4dB at 82Hz. Bumping that up to .022uf should do. But since this is the earliest place to affect the LF it also means the LF here will suffer the most clipping, increasing the woofy. The LF has a pretty straight shot through the rest of the circuit so there's not much to be done other than choosing a speaker with strong bottom end. Increasing the first filter could tighten the bass back up a little. With the diode rectifier there's no need for it to be as small as it is and the 20uf value was probably a matter of cost and availability back in the day. At least double it. I'd go 100uf if it were my call. IMHE it won't detriment the tone to increase just the first filter value. You might notice less 120hz noise though

    You could also change the taper on the tone pot so that the perceived bass is greater. If you have a linear pot change it to an audio pot. If you have an audio pot change it to an A10 audio pot.

    Another thing will tighten the LF would be a zener diode across the power tube cathode resistor. It won't add LF but it will help tighten the LF you've added when amp is clipping. Use a value chosen as follows...

    Get the amp to where it starts to show obvious clipping on the wave form (if you have a scope) Not a full square wave. Just a little flat spot. With the amp operating like this, measure the voltage on the power tube cathode. Choose a zV one volt higher than your cathode voltage. If you don't have a scope just measure the voltage at idle and choose a zV three or four volts higher. The rating should probably be about 5W for the diode. This rating IS available in a DO package so mounting is easy.

    What this does is allow the amp to operate as cathode biased up to the point where it starts clipping harder and then the bias morphs to fixed. Thin puts a ceiling on any cathode bias squish from continued bias shift and keeps the amp a little tighter.

    If you did all four things, change the input cap, change the tone pot, increase the main filter and add the diode to the bias circuit, I'm certain you would notice a marked improvement in bottom end responsiveness. Maybe not much actual increase in bottom end. You'll need a bigger OT and the right speakers for that.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Bent Member Chuck H's Avatar
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    I think you have your PS nodes connected cattywhompus. With the power tube screens on node B and the preamp on node C.

    EDIT: None of what I said in the previous post applies before the voltages are corrected and the amp is reassessed.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    I think you have your PS nodes connected cattywhompus. With the power tube screens on node B and the preamp on node C.

    EDIT: None of what I said in the previous post applies before the voltages are corrected and the amp is reassessed.
    In the initial schematic they lettered the nodes weird. to keep things easier for me to follow from their schematic to mine, I just kept their nomenclature.

    The nomenclature not withstanding, do you still think I hooked them up wrong? I was very careful to not make a mistake there but perhaps I did....

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    Bent Member Chuck H's Avatar
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    Your voltages would indicate that it's wrong. You need to check to be certain one way or the other.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Oh I see, you're right. I made a mistake from the initial drawing to mine, that 270 ohm resistor off the plate of V2b should get B node, not C node.

    Thanks. Will fix right away....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mort View Post
    Oh I see, you're right. I made a mistake from the initial drawing to mine, that 270 ohm resistor off the plate of V2b should get B node, not C node.

    Thanks. Will fix right away....
    I think you meant 270k. I hope you're not using a 270 ohm plate resistor there.

    That's not all that needs changing. Your screen supply should be from node C in your schematic. You screen voltages posted above are too low.

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    That's not all that needs changing. Your screen supply should be from node C in your schematic. You screen voltages posted above are too low.
    Power tube screen supply is definitely from node C with 470K resistors (with actual read of 510K). Should I reduce the value? I had noticed the screen supply dropped down into the 180's

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    yes 270k is correct*

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    Mort,
    I may be misreading the photo/schematic but:
    In the third photo you posted it appears the cathode bias bypass cap for the output section is located right next to the 200 0hm 10W bias resistor.The heat coming off that resistor will shorten the life of that cap considerably.
    It would be preferable to use a cap with longer leads to allow considerable airspace between those two components. Your bias cap will love you for it.
    SG

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mort View Post
    Power tube screen supply is definitely from node C with 470K resistors (with actual read of 510K). Should I reduce the value? I had noticed the screen supply dropped down into the 180's
    Screen grid resistors should be 470 ohm. Not 470k. That WOULD explain the low screen volts. Sorry I missed that on the first post!!!

    If you have a pair of 1K resistors you should use those instead. Better for modern tubes.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    Screen grid resistors should be 470 ohm. Not 470k. That WOULD explain the low screen volts. Sorry I missed that on the first post!!!

    If you have a pair of 1K resistors you should use those instead. Better for modern tubes.
    Sweet. I'll see what I have. Is 1W ok for these resistors?

    I corrected the B/C node goof up and the amp sounds a little more full without going into woofy territory. Not sure if that would be expected but it helped.




    SG- I will address that too. I should be able to get it at least 1/4" away or more...

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    Bent Member Chuck H's Avatar
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    1W would work but 3W would be better because these resistors get hot and derate.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    I have 1W on hand and will test with them and get heavier resistors this week. About to run a test with the 1K 1W in...

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    Oh wow, there it is. Now there's tons of punch, more volume, more clean headroom etc.

    Still not woofy, which is good, and plenty of bass response. So great!

    Here are the latest working voltages... I'm not cooking my tubes now am I ??


    V4- 69mV across 1 ohm resistor
    3- 444
    4- 437 (441 before screen resistor)
    5- .253
    8- 36.79

    V3- 65mV across 1 ohm
    3- 442
    4- 436 (439 before screen resistor)
    5- .190
    8- 36.64



    And here's a photo of what the build looks like. My friend doesn't come to this website so I hope it doesn't get back to him before he receives the amp




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    Bent Member Chuck H's Avatar
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    That's amp+ right there! Is that pink paint on the chassis or is it just the photo/lighting??? Enough curly maple to cap a Les Paul!

    BTW, re the screen grid resistor rating... The current through that resistor will never reach a half a watt so the 1W rating is fine. Maybe better if we consider that a tube short often results in a blown screen grid resistor. In this case the screen grid resistor is a fuse protecting other components. You really don't want it to NOT blow. So just leave the 1W's in there.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    I painted the chassis pink because I love Shell Pink on old Tele's and I kinda hope he's building me a pink Tele. Won't find out for another few weeks though.

    Plus he's just gonna have to live with whatever I choose for him.. and it's all for fun so pink it is !!

    As for the screen resistor... is there anything unhealthy about going back up in value some? When it had the high 470K in there it was very hairy and I liked some of that. Now it's very bold with alot of backbone and less hairy and I would like to get the right mix of hairy and bold.

    Also, would someone remind me how to calculate the load on the tubes so I know that they are biased correctly not too hot not too cold?

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    Oh and btw I found that figured maple for about $1.90 a board foot. I was so giddy at the price that I bought enough to do 3 amp heads !! lol

    Next time I go back to that town I might just buy an entire truck load. For their market they consider the figured maple to be trash wood and they sell it for a dirt cheap special price just to get rid of it !! I couldn't believe my ears when they told me that !!!

    It's not super high grade AAAAA or anything but SO WHAT

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    Bent Member Chuck H's Avatar
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    I'm sure at some very high screen grid resistance there must be potential for problems, though I don't know what sort. 470k would be very high. I might be inclined to turn the amp over as the stock circuit since your friend reports that's his bag. I can't say how high you need to go before it's a bad idea but I'm sure you could experiment with values up to 22k. Past that I might be concerned about additional decoupling because of the higher resistance between the screen and the filter. 22k is nowhere near as high as 470k, but I wouldn't try that again.

    It's possible the amp would sound sweeter at a lower Vp. With the screen circuit corrected what is your plate voltage? Did you match the PT specs for this design?

    At the Vp listed above I would think that a 200 ohm shared cathode resistor may have the tubes running too hot. Measure the cathode voltage and divide by the cathode resistance. This is your current. Multiply that number by your "plate to cathode" voltage to get your "watts at idle" figure.

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    Quote Originally Posted by Chuck H View Post
    I'm sure at some very high screen grid resistance there must be potential for problems, though I don't know what sort. 470k would be very high. I might be inclined to turn the amp over as the stock circuit since your friend reports that's his bag. I can't say how high you need to go before it's a bad idea but I'm sure you could experiment with values up to 22k. Past that I might be concerned about additional decoupling because of the higher resistance between the screen and the filter. 22k is nowhere near as high as 470k, but I wouldn't try that again.

    It's possible the amp would sound sweeter at a lower Vp. With the screen circuit corrected what is your plate voltage? Did you match the PT specs for this design?

    At the Vp listed above I would think that a 200 ohm shared cathode resistor may have the tubes running too hot. Measure the cathode voltage and divide by the cathode resistance. This is your current. Multiply that number by your "plate to cathode" voltage to get your "watts at idle" figure.

    Voltages after the screen resistor correction are listed in the post with the photos. I put 1 ohm resistors in series with the cathodes and get 65mV and 69mV across them, which should give me the current of 65mA and 69mA. Plugged into your equation should be 28.09W on V4 and 26.35W on V3 ??

    Here are the voltages again.. For the Plate/cathode voltage I subtracted the cathode voltage from the plate voltage and then multiplied by the mV across each 1 ohm resistor.


    V4- 69mV across 1 ohm resistor
    3- 444
    4- 437 (441 before screen resistor)
    5- .253
    8- 36.79

    V3- 65mV across 1 ohm
    3- 442
    4- 436 (439 before screen resistor)
    5- .190
    8- 36.64


    Looks like I might be a little hot. Upping the screen resistor value will drop the idle wattage, correct? (since I'm interested in upping anyway)

    And as for the PT(and OT), I just used what I had on hand which was a 1966 Fender Bandmaster transformer set (original). I figured that going from a 2x6L6 amp to another 2x6L6 design would be problem free.

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    I really don't think you should get nutty with the screen grid resistors. Something like 2.2k would be about as high as I ever see in a design. That wouldn't decrease dissipation enough for the correction you need.

    Since you already have a cathode resistor the 1 ohm resistor is sort of redundant. You can use the cathode resistor to figure the shared current. Provided that resistor is on spec your shared current is 183mA!!! That's 91.5mA per tube!! If you subtract the cathode voltage from your plate voltage we get a working voltage of 406.4Vp. If we multiply the current (per tube) by the working voltage it shows your amp is dissipating 37 watts per tube!!! I suspect either your cathode resistor or your 1 ohm resistors spec low, but even if we use your figure of roughly 67mA per tube that's still 27 watts per tube. You need to get that down brother! I don't know what you have on hand but I think a 330 ohm cathode resistor would be about right. Or...

    If you have another 200 ohm resistor you could put it on the HV winding center tap before ground. That would simulate a rectifier tube somewhat adding sag and softening the attack envelope. Way better idea than giant screen resistors. This will also drop the Vp to about 400. Which gives you a working voltage of about 365. In this case a 250 ohm cathode resistor should be better. What values of big resistors do you have on hand?

    And you never answered my Q about the PT spec for that circuit.?. I'm in serious doubt that the original design spec'd a 200 ohm cathode resistor with over 440Vp.

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    If you look at the original schematic that Sean Weatherford compiled with my help and others help, (S6420.pdf above) the B+ with the solid state rectified version is right around 425V. Your voltage is a bit higher, so the proper cathode resistor value should be a bit higher too. With the stock amp, a 200 ohm resistor puts the bias up around 80% to 90% of the max dissipation for the 6L6GC. Sovtek 5881's actually sound great in these amps because they like to be biased hot. In any case, with a stock amp and a 200 ohm cathode resistor on the power tubes, tube life is shorter than many fixed bias amps such as your typical fender. With a stock amp, changing to a 240 ohm cathode resistor cooled the tube bias down, but made the sound noticeably less sweet. You will probably have to experiment around to get to a good resistor value.

    A couple other things....

    - the originals used ceramic coupling caps throughout the amp. Using film caps is generally considered to be a better route to go, but that may get rid of some of the character of the original design.
    - You will notice if you look at vintage amps that they either don't use a screen grid resistor, or use a 470 ohm, or a 1k, but almost never much larger than that. Read the attached document, specifically section 8 and they talk about optimum screen grid resistor sizes. The main thing to keep in mind is that you want some protection for the screens, but not so much that you start to lose power.
    - You can take a look at a stock Supro Tbolt in the attached pics and the way they made the amp.

    Greg
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    Quote Originally Posted by soundmasterg View Post
    If you look at the original schematic that Sean Weatherford compiled with my help and others help, (S6420.pdf above) the B+ with the solid state rectified version is right around 425V. Your voltage is a bit higher, so the proper cathode resistor value should be a bit higher too. With the stock amp, a 200 ohm resistor puts the bias up around 80% to 90% of the max dissipation for the 6L6GC. Sovtek 5881's actually sound great in these amps because they like to be biased hot. In any case, with a stock amp and a 200 ohm cathode resistor on the power tubes, tube life is shorter than many fixed bias amps such as your typical fender. With a stock amp, changing to a 240 ohm cathode resistor cooled the tube bias down, but made the sound noticeably less sweet. You will probably have to experiment around to get to a good resistor value.

    A couple other things....

    - the originals used ceramic coupling caps throughout the amp. Using film caps is generally considered to be a better route to go, but that may get rid of some of the character of the original design.
    - You will notice if you look at vintage amps that they either don't use a screen grid resistor, or use a 470 ohm, or a 1k, but almost never much larger than that. Read the attached document, specifically section 8 and they talk about optimum screen grid resistor sizes. The main thing to keep in mind is that you want some protection for the screens, but not so much that you start to lose power.
    - You can take a look at a stock Supro Tbolt in the attached pics and the way they made the amp.

    Greg

    Thanks for the info ! It's amazing that I would get some advice from one of the very people who's schematic I found and copied.

    I'll play around with the cathode resistor value and see how it goes.
    \

    And Chuck I really appreciate your help too. I'll have time and parts to implement a couple more adjustments by the weekend and I will definitely post up the results.,


    Thanks guys !!

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    Ok I'm getting back around to working on this and was planning on upping the value of the cathode resistor to decrease plate current a bit and realized that I had actually installed a 270 ohm 10W, instead of a 200 ohm like my schematic showed. I had read a note somewhere about using anywhere from 200-330 and used a 270 but forgot to reflect that on my drawing.

    The above comment about losing tone above 240 has me wondering what the amp would sound like if I dropped the value down to 200 or 220(sounds great now but maybe it could sound even better?). I expect lowering the value would increase plate current so now I need another strategy of decreasing current.

    I guess one way would be to raise the grid resistor values but as mentioned already that may not be a great idea. ... how should I go about this?

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    Quote Originally Posted by mort View Post
    I guess one way would be to raise the grid resistor values but as mentioned already that may not be a great idea. ... how should I go about this?
    Bad idea. Don't do it. 47k is already high and one side has a 470k bias resistor, which is also high. If you look at the spec'd grid circuit impedance you'll see that 517k exceeds the maximum grid circuit resistance already. Besides, the current limiting wouldn't be enough to matter much unless the tube were trying to draw grid current. About the only way to keep the cathode resistor low and the tubes cool would be to get tubes that tend to run cooler (the net difference in tone should be about the same) or reduce plate voltage, which really complicates matters.

    Incidentally, your mistake on the cathode resistor value explains how you and I got different bias calculations.

    I wouldn't try decreasing the cathode resistor value if I were you. No matter what you read. That's running the tubes dangerously hot and any possible tonal benefit would be short lived at best. Momentary at worst.

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