First, all the 6SN7s are running at pretty low current and the anode voltage is pretty low too. I don't think they can fully drive the output stage without significant distortion.
If you want to know the anode current of each 300B, just put a 1% metal film resistor of 1R value between the plate and the transformer (precisely where you have written the 324V anode voltage). Then you measure the voltage drop across the 1 ohm resistor: for each millivolt you get 1 mA. Be careful!
For 324V anode voltage, plate current should be rather high for -60V bias, possibly more than 100mA per tube....!!! (for the original 300B).
My advice is:
1) reduce the bias to -70V
2) if reducing the bias doesn't bring any change in voltage supply get a proper supply transformer. Buy a good 250-300 VA toroidal with shield. It's not expensive. I think you should get it with $40-50.
If you want to know the anode current of each 300B, just put a 1% metal film resistor of 1R value between the plate and the transformer (precisely where you have written the 324V anode voltage). Then you measure the voltage drop across the 1 ohm resistor: for each millivolt you get 1 mA. Be careful!
For 324V anode voltage, plate current should be rather high for -60V bias, possibly more than 100mA per tube....!!! (for the original 300B).
My advice is:
1) reduce the bias to -70V
2) if reducing the bias doesn't bring any change in voltage supply get a proper supply transformer. Buy a good 250-300 VA toroidal with shield. It's not expensive. I think you should get it with $40-50.
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I was listened for few hours without any distortion. Somehow I understand backward 
I though lower -bias will lower the current but it doesn't
@-50v, B+ 283v and 105mA
@-60v, B+ 324v and 80mA
@-70v, B+ 331v and 59.2mA
@-71v, B+ 340v and 57.2mA
I can touch the tubes with my finger at -70 without any problem. And the temperature drops under 150F.
-70v bias is fine, no problem with sound. As 6A3sUMMER mention, 1v drops 2 mA. Is it possible to go down more like -75 to -80 to keep the tubes cooler? Right now 12ax7 pin 1 is up to 108v an 6sn7 pin 2 is 244v. I think I can increase voltage of 12ax7 pin 1 to 120v buy replacing 100k2w to 80k2w but not sure if it would do anything different.
I though lower -bias will lower the current but it doesn't@-50v, B+ 283v and 105mA
@-60v, B+ 324v and 80mA
@-70v, B+ 331v and 59.2mA
@-71v, B+ 340v and 57.2mA
I can touch the tubes with my finger at -70 without any problem. And the temperature drops under 150F.
-70v bias is fine, no problem with sound. As 6A3sUMMER mention, 1v drops 2 mA. Is it possible to go down more like -75 to -80 to keep the tubes cooler? Right now 12ax7 pin 1 is up to 108v an 6sn7 pin 2 is 244v. I think I can increase voltage of 12ax7 pin 1 to 120v buy replacing 100k2w to 80k2w but not sure if it would do anything different.
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I was listened for few hours without any distortion. Somehow I understand backward
@-50v, B+ 283v and 105mA
@-60v, B+ 324v and 80mA
@-70v, B+ 331v and 59.2mA
@-71v, B+ 340v and 57.2mA
I can touch the tubes with my finger at -70 without any problem. And the temperature drops under 150F.
-70v bias is fine, no problem with sound. As 6A3sUMMER mention, 1v drops 2 mA. Is it possible to go down more like -75 to -80 to keep the tubes cooler? Right now 12ax7 pin 1 is up to 108v an 6sn7 pin 2 is 244v. I think I can increase voltage of 12ax7 pin 1 to 120v buy replacing 100k2w to 80k2w but not sure if it would do anything different.
The anode current of the 6SN7 is still too low at about 2 mA per tube. If you increase the anode voltage of the 12AX7 by 20V you will eat the voltage you have just gained reducing the anode current of the 300B but now you have more bias and can get more power!!
I suggest you study the Mullard 5-20 carefully.
Twenty-Watt Amplifier
At this page you also have a picture with measured voltages that I attach below.
I still think you need to change power transformer. You are not using the 300B efficiently and getting the maximum power with low distortion, in my opinion. You cannot tell this by listening because 1-2W average power is loud with decent speakers!!
Get some signal generator + oscilloscope at least or find some friend that can help you.
You can run both the 12AX7 at 120V and the 6SN7 at higher current if you have the following conditions:
A) 420-430V supply
B) you put a constant current sink in the cathode of the 6SN7. This way you get very good balance and let more current without eating voltage supply.
With 430V supply + CCS with EACH 6SN7 running at 5 mA you should be able to use 33K anode resistor and get some 150-160V anode voltage.
C)if want to have 420V-430V supply for the 6SN7 and 12AX7 you have 2 options:
1) use self bias for the 300B running at 350V/-75V for a total of 425V supply +some voltage drop in the OPT. This allows you to have a simple PSU with one voltage for the entire amplifier.
2) if you want to keep fixed bias, you need 3 voltages: anode voltage and bias for 300B + voltage supply for the 12AX7+ 6SN7.
Attachments
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If you run the 300B with self-bias you only need a good power transformer with 1 secondary for high voltage and then the secondaries for the 300B's filaments + preamp tubes heaters.
With this one, for example: AS-3T325 - 300VA 325V Tube Transformer - AnTek Products Corp
It's $40
You can get the supply voltage for the entire amplifier + 1 secondary per channel for 300B filaments.
You would only need a small transformer for the heaters of 12AX7 and 6SN7....or you could use SMPS AC/DC small module with good filtering.
With this one, for example: AS-3T325 - 300VA 325V Tube Transformer - AnTek Products Corp
It's $40
You can get the supply voltage for the entire amplifier + 1 secondary per channel for 300B filaments.
You would only need a small transformer for the heaters of 12AX7 and 6SN7....or you could use SMPS AC/DC small module with good filtering.
Thank you for your help 45, that explained everything. Before buy additional PS, do you think mine would work this way as in the image?
Can I take 310v to use for 12ax7 and 6sn7? Would it drive too much load on the transformer? This way, I can get 333v for 6sn7 and 142v for 12ax7. Then keep 300B at 344v plate and -70 cathode.
Either run full wave, half wave, or bridge for 12ax7 and 6sn7
Can I take 310v to use for 12ax7 and 6sn7? Would it drive too much load on the transformer? This way, I can get 333v for 6sn7 and 142v for 12ax7. Then keep 300B at 344v plate and -70 cathode.
Either run full wave, half wave, or bridge for 12ax7 and 6sn7
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Alashikata,
1. Your 110V bias supply is a Positive (+) bias supply.
Don't you need a Negative (-) bias supply?
For Negative bias, you need to rotate the bridge rectifier 180 Degrees.
2. 296VAC x (Root of 2) = 418.6V. (ignoring 2 diode drops of about 1.2V)
You need more than 296VAC to get 423VDC.
3. 310VAC - 296VAC = 14VAC (ignoring two diode drops of about 1.2V)
14VAC x (root of 2) = 19.8VDC
4. Stacked B+
418.6VDC +19.8VDC = 438.4VDC
And that is if it all works.
That schematic makes my head spin.
1. Your 110V bias supply is a Positive (+) bias supply.
Don't you need a Negative (-) bias supply?
For Negative bias, you need to rotate the bridge rectifier 180 Degrees.
2. 296VAC x (Root of 2) = 418.6V. (ignoring 2 diode drops of about 1.2V)
You need more than 296VAC to get 423VDC.
3. 310VAC - 296VAC = 14VAC (ignoring two diode drops of about 1.2V)
14VAC x (root of 2) = 19.8VDC
4. Stacked B+
418.6VDC +19.8VDC = 438.4VDC
And that is if it all works.
That schematic makes my head spin.
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Sorry, I forgot to rotate the image when I tried to put together, it's actually negative bias. The only thing I tried to rework the PS, not really touch the bias. I can use 413v wire to get 400v B+ but I think 350v B+ is fine. I got this amp for cheap but it has been mod pretty bad that caused the sound not right and hum too so I try to fix the PS again. So far got the hum fix.
i did check
296v => 426v....I will keep this for 300B. It would be around 344v and -70v
310v => 440v....and this for 12ax7 and 6sn7
Thank you 45 , 6A3sUMMER, and others in here.
Using 296v for 300B and 310v for driver stage didn't work, the voltage still shared with power tubes.
So I tried 413v, that brought the B+ for 300B up to 463v. Then I tried 10v-413v to get down 405v but still got high B+ that where B+ at 454v
Finally, I took 310v where B+ is 370v, 12ax7 plate is 134v and 6sn7 is 268v, I think it's enough.
I believe if the PT has 350v AC, it would be perfect. I can live with it at 310v will leave this amp as it for now. May be build another one with better PT.
Using 296v for 300B and 310v for driver stage didn't work, the voltage still shared with power tubes.
So I tried 413v, that brought the B+ for 300B up to 463v. Then I tried 10v-413v to get down 405v but still got high B+ that where B+ at 454v
Finally, I took 310v where B+ is 370v, 12ax7 plate is 134v and 6sn7 is 268v, I think it's enough.
I believe if the PT has 350v AC, it would be perfect. I can live with it at 310v
will leave this amp as it for now. May be build another one with better PT.
Are 370V and 463V voltages with tubes in? If you can get them you did something wrong. IF you can get them why don't you use separate supplies? 370V for the 300B's on one side and 463V for the 12AX7+6SN7. That's what you need!
If you can get these 2 voltages with tubes in you have done it! The only other thing you will need to adjust is how those 463V are partitioned between the 12AX7 plate and the 6SN7s: keep the anode voltage of the 12AX7 at 120-130V even with 463V B+ and then you only have to work on that 27K cathode resistor.....that is what kills the 6SN7 stage. You need high value to keep the balance but this results in low anode current....put a constant current sink there.
You can make a simple constant current sink even with a small pentode.....
I will post something about this later but you can find lots of information in the forum if you search CCS for LTP tail etc...
If you can get these 2 voltages with tubes in you have done it! The only other thing you will need to adjust is how those 463V are partitioned between the 12AX7 plate and the 6SN7s: keep the anode voltage of the 12AX7 at 120-130V even with 463V B+ and then you only have to work on that 27K cathode resistor.....that is what kills the 6SN7 stage. You need high value to keep the balance but this results in low anode current....put a constant current sink there.
You can make a simple constant current sink even with a small pentode.....
I will post something about this later but you can find lots of information in the forum if you search CCS for LTP tail etc...
Antek is pretty good deal but 325v it not enough. These voltage below is with tubes in the amp. Hope you don't get dizzy like me with all these #
First, I took the schematic online and someone in here posted too so I don't even know if the B+ is 406v or not since I have compared the schematic where it's exactly like mine. However, i found that other people have been used this schematic to build EL34 PP amp so the voltage may not correct for B+ in this design.
The PT on mine has AC:10v, 296v, 310v, and 416v wit primary 110v. While I am in US where we have 120v outlet so I use variac to get it down to 110v so both AC heater stay at 5v and 6.3v. It's impossible to get B+ 406v at those voltage available. Unless using tube rectifier 5U4GB at 404v or 416v, then I think it would get around 420v for B+. But this is PP so but rectifier tube may not enough.
Another problem is the case is too small to add another PT. Probably a best way is to get a new 300B PT with 350v and 370v.
Actually, this schematic has pretty good sound if someone decide to build 300B PP then, you can decide what kind of PT needs to run at 400v.
So far I have listened and compared to mark levinson, the sound is pretty much the same except warmer with tube especially the bass.
Here was my test with all the voltage available and I think AC 310v is good enough even though the 12ax7 and 6sn7 are lower.
First, I took the schematic online and someone in here posted too so I don't even know if the B+ is 406v or not since I have compared the schematic where it's exactly like mine. However, i found that other people have been used this schematic to build EL34 PP amp so the voltage may not correct for B+ in this design.
The PT on mine has AC:10v, 296v, 310v, and 416v wit primary 110v. While I am in US where we have 120v outlet so I use variac to get it down to 110v so both AC heater stay at 5v and 6.3v. It's impossible to get B+ 406v at those voltage available. Unless using tube rectifier 5U4GB at 404v or 416v, then I think it would get around 420v for B+. But this is PP so but rectifier tube may not enough.
Another problem is the case is too small to add another PT. Probably a best way is to get a new 300B PT with 350v and 370v.
Actually, this schematic has pretty good sound if someone decide to build 300B PP then, you can decide what kind of PT needs to run at 400v.
So far I have listened and compared to mark levinson, the sound is pretty much the same except warmer with tube especially the bass.
Here was my test with all the voltage available and I think AC 310v is good enough even though the 12ax7 and 6sn7 are lower.
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My C R filters are under rate at the moment, it's too hot to test at 400v AC Already order new one so I can try 400v again later.
So far B+ at 339v has no problem, listened for couple hours last night, didn't have any issue. I am not sure about the long term.
I have seen other 300B SET design at 320v, 350v , 375v and the s6sn7 from 210v to 300v. Is there a problem later if I don't run B+ at 400v?
Already order new one so I can try 400v again later. So far B+ at 339v has no problem, listened for couple hours last night, didn't have any issue. I am not sure about the long term.
I have seen other 300B SET design at 320v, 350v , 375v and the s6sn7 from 210v to 300v. Is there a problem later if I don't run B+ at 400v?
You have to look at the effective plate voltage of the 6SN7 in your amp which is not remotely close to 200V....that's the problem together with really low anode current at 2-2.5 mA max per tube into an effective load of about 23K.
Put simply, 2.5mA peak into 23K will be quite distorted and equals 57.5V peak which is less the the bias voltage of the 300B. This means that you are not able to fully drive the power stage.
Thanks 45 for all suggestion. I have been listened over 50 hrs and compared with Mark Levinson 532 and I am really happy with it. The bass is better than ML 532. I am using the PT that can get B+ max at 355v. As I remembered when I bought it as used, B+ was around 320v.
This circuit is pretty decent, so this is my summary and everything that someone may want to build it, it's more simple than other 300B PP that I have seen.
This is the original schematic without any voltages info, the one floating on internet with voltage maybe for EL34 and KT88 tubes where I have seen they used the same driver stage.
It's used 300v for 300B and 400v Half Wave for 6sn7 and 12ax7
I revised the PS and got its up to 320v for all. I couldn't understand why the original schematic did the Half Wave which it didn't do any different in voltage for 6sn7 and 12ax7 because it used the same secondary. This PT has option for 0v, 10v, 296v, 310v, 413v @ 110v primary. So I took advantage of 115v to push 310v to 320v to get B+ at 355v. The 404v or 413v would give B+ over 455v so I can't use it. So far I am happy B+ at 355v
I tested between 58mA and 62mA, the sound is pretty much the same so I decided to stay at 58mA to keep the 300b tube cool.
Here is the info if someone wants to do points or using the PCB
The schematic floating around internet. If you want to use this voltage, you need to have PT with 370v
PCB: This is 12ax7 that's the same as my amp
Goldox GOX 300B Push Pull Circuit Board, 12AX7 + 6N8P Buis Printplaat, gebruikt Voor 300B Buizenversterker, LG37|Transformers| - AliExpress
Goldox 12AX7 + 12SN7(6N8P) Push Pull Circuit Board Voor 300B 2A3 Buis, 12SN7 Links En Rechts Omgekeerde Push Pull Uitgang, LG97|Transformers| - AliExpress
PCB: This is 6sl7 version, I have ordered adapter to test with my amp later.
12SL7 + 12SN7 Push 300B 2A3 Push Pull Board, Het Geluid Is Royaal En Losse, vol Moed LG97A|Transformers| - AliExpress
There is a hum problem. However, I am not sure if the pcb design or the PT. I had to remove the 6.3v CT and use 2-100 ohm resistors to remove the hum. Otherwise, 300B, 6sn7, and 12ax7 all use AC filament heater. No hum after using resistors. Earth ground gives hum as well if connect to Ground but good if screw it to chassis near by.
Good luck anyone decides to to build this amp.
This circuit is pretty decent, so this is my summary and everything that someone may want to build it, it's more simple than other 300B PP that I have seen.
This is the original schematic without any voltages info, the one floating on internet with voltage maybe for EL34 and KT88 tubes where I have seen they used the same driver stage.
It's used 300v for 300B and 400v Half Wave for 6sn7 and 12ax7
I revised the PS and got its up to 320v for all. I couldn't understand why the original schematic did the Half Wave which it didn't do any different in voltage for 6sn7 and 12ax7 because it used the same secondary. This PT has option for 0v, 10v, 296v, 310v, 413v @ 110v primary. So I took advantage of 115v to push 310v to 320v to get B+ at 355v. The 404v or 413v would give B+ over 455v so I can't use it. So far I am happy B+ at 355v
I tested between 58mA and 62mA, the sound is pretty much the same so I decided to stay at 58mA to keep the 300b tube cool.
Here is the info if someone wants to do points or using the PCB
The schematic floating around internet. If you want to use this voltage, you need to have PT with 370v
PCB: This is 12ax7 that's the same as my amp
Goldox GOX 300B Push Pull Circuit Board, 12AX7 + 6N8P Buis Printplaat, gebruikt Voor 300B Buizenversterker, LG37|Transformers| - AliExpress
Goldox 12AX7 + 12SN7(6N8P) Push Pull Circuit Board Voor 300B 2A3 Buis, 12SN7 Links En Rechts Omgekeerde Push Pull Uitgang, LG97|Transformers| - AliExpress
PCB: This is 6sl7 version, I have ordered adapter to test with my amp later.
12SL7 + 12SN7 Push 300B 2A3 Push Pull Board, Het Geluid Is Royaal En Losse, vol Moed LG97A|Transformers| - AliExpress
There is a hum problem. However, I am not sure if the pcb design or the PT. I had to remove the 6.3v CT and use 2-100 ohm resistors to remove the hum. Otherwise, 300B, 6sn7, and 12ax7 all use AC filament heater. No hum after using resistors. Earth ground gives hum as well if connect to Ground but good if screw it to chassis near by.
Good luck anyone decides to to build this amp.
I couldn't find PS and Bias PCB anywhere but you can use these or do point to point
Bias PCB
EL34 KT88 300B 4-way Grid Negative Pressure Adjustable Circuit Board Push-Pull | eBay
Universal PS PCB, this should work
DIY PCB Board Tube Amp Universal PS Power Supply HV with Neg Bias KT88 EL34 EL84 | eBay
Bias PCB
EL34 KT88 300B 4-way Grid Negative Pressure Adjustable Circuit Board Push-Pull | eBay
Universal PS PCB, this should work
DIY PCB Board Tube Amp Universal PS Power Supply HV with Neg Bias KT88 EL34 EL84 | eBay
2 identical Goldox amps landed on my bench. They are the KT88/EL34 variants but the PCBs look the same.
I think you might get better performance replacing the first drop resistor with a choke and replacig the stacked filter caps for the 6sn7 driver with panasonic 500v square film caps, ditto the 12ax7 filter caps, that way it saves a lot of B+ bypassing with small valueand you get a more coherent tone.
I think you might get better performance replacing the first drop resistor with a choke and replacig the stacked filter caps for the 6sn7 driver with panasonic 500v square film caps, ditto the 12ax7 filter caps, that way it saves a lot of B+ bypassing with small valueand you get a more coherent tone.
Thanks jlw, I have seen exactly same schematic that has been used with KT88/EL34. I believe there is a company sell that amp in U.S. Did you modify anything else?
The chassis is too tight to put the choke in so I added another small RC (60r) in front to reduce AC ripple and rewired the PS where B+ is 389v now (was 340v from previous owner), I think he tried to fix the hum problem and convert to DC filament too but couldn't get rid of the hum). I changed everything back to AC filament, there is no more hum. l I replaced 22uF for 12ax7 where it was 100uF. The original PS was using Bridge for B+ and Half Wave for Driver so I rewired similar as the PS for KT88
The reason I reworked the PS is because the PT is 110vAC so I don't want to run with Variac, it 's little bit warm to run at 120vAC so I could get B+ at 389v and Grid Bias at -82v. So far it looks great on the scope where I estimate around 16w/channel. The mid is so good except the bass is not strong enough but it's alright. So far it has been working great. I did use adapter with 6SL7, it sounds better than 12ax7.
The chassis is too tight to put the choke in so I added another small RC (60r) in front to reduce AC ripple and rewired the PS where B+ is 389v now (was 340v from previous owner), I think he tried to fix the hum problem and convert to DC filament too but couldn't get rid of the hum). I changed everything back to AC filament, there is no more hum. l I replaced 22uF for 12ax7 where it was 100uF. The original PS was using Bridge for B+ and Half Wave for Driver so I rewired similar as the PS for KT88
The reason I reworked the PS is because the PT is 110vAC so I don't want to run with Variac, it 's little bit warm to run at 120vAC so I could get B+ at 389v and Grid Bias at -82v. So far it looks great on the scope where I estimate around 16w/channel. The mid is so good except the bass is not strong enough but it's alright. So far it has been working great. I did use adapter with 6SL7, it sounds better than 12ax7.