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Stephane Bureau
Reply with quote  #1 
After months of ordering parts from all over the world, I am now building the amp and I wonder if experienced builders could help me . I'm stuck . first, i chose 6P45C tubes which are bigger than EL509 , and also have 2 pins connected to the same part , example pin 1 and 8 are connected to grid 1. Do we choose one of the two ? I have the book Tubes and circuits and am using the plans plus the pictures builders have put on the net of the inside. I included one here and one channel schematic. In the book , Bruce tells us to wire all the power supply capacitor ground together on a post, but what I see on the picture is the plus side of one cap wired to the negative of the other , as a grounding post. Then the plus side of another going to that post. I don't get it. 
Also, if we look at the picture, we see that pin 1 and 6 of the EL509 are used, and on the schematic, they are not mentioned. Pin 6 is not supposed to be connected, I have octal sockets at home that don't have a terminal on pin 6. What is connected on pin 6 is the 0,47 uf capacitor and if i look at the schematic, we don't see that cap connected directly to the EL509 tubes. i saw little resistors on some El509 soldered from pin 5 to 6, not on the plan also. It is like the schematic and the pictures are not the same version of the Son of Beast, or I did not learn well how to read a plan. I would welcome some help, or my build will stay unfinished for eternity. Thanks.

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Kelvin Tyler
Reply with quote  #2 
Stephane. A couple of comments on your post.
First. Regarding the smoothing cap connections. If you look at the schematic of the power supply in the book you will notice that for the +170 V side the negatives of the caps are connected to ground, while on the -170 V side the positives go to ground. Thus the negatives on two caps and the positives on two others are essentially connected together. Just exactly how this is wired and the ground and T1 centre tap connection made is up to the individual constructor, but it is always safe to follow Bruce's advice.
Second. The 6P45C tube has a completely different base and connection pattern to the JJ EL509S. For the 6P45C I note that the tube diameter is some 8mm larger, the heater current is 0.5A higher and, of course, there is the top cap. No doubt you have made allowance for these factors in your build. Now pins 1 and 6 of the octal base have no connections to the EL509S tubes and can therefore be used as convenient anchor points. The 0.47 mfd caps you see connected to the pin 6 points are indeed coupling caps C4 and C5, while the resistors bridging pins 6 and 5 are the 100K grid stoppers R15-18. There are no free pins on the magnoval base of the 6P45C. In this application it should not matter if you use either pin 1 or 8 for the signal grid1 connection. For the duplicated pins 2 and 7, suppressor grid3, connect pin 7 to pin 9 which is the cathode. Either of pins 3 or 6 can be used for the screen grid2 connection.
Study the schematics very carefully and see if you can follow what I have written here. Eternity is not a concept I am familiar with. This is definitely a finite project; many have done it. Good luck.
Regards,
Kelvin
Stephane Bureau
Reply with quote  #3 
Thank you for answering me. Can you explain why do we connect pin 7 to pin 9 on a 6P45C. Is it because we use them in triode mode ? On the SOB picture, we see a white wire coming from pin1 of V5A. I don't see the utility.it's not a post to connect 2 other components. Also we see a green wire starting on pin 8 of V3B going to pin 1 of V2B, without going through a resistor like R15 and a capacitor like C4. For the heaters, I have a 160 VA instead of the 120 VA suggested, and my power transformer is a 1500 VA. It is so big that I will have to place it in a separate chassis. Maybe I can use it for the tube tester too, with the proper plugs , when I need it.
Kelvin Tyler
Reply with quote  #4 
Stephane. The suppressor grid3 on a pentode is normally connected to the cathode. Indeed, you will see that in the case of the EL509S it is internally connected to the cathode. It is the connection of the screen grid2 to the anode via the 100ohm resistor that puts the tube into triode mode. In the case of the 6P45C the grid3 is not internally connected to the cathode, hence the link from pin 7 to pin 9.
If you look carefully at pin 1 on V5A you will see a resistor linking to pin 6. This resistor is R14, 100K, which then connects via the white wire back to the A- bias point.
The green wire from pin 8 of V3B in fact goes to pin 8 of V4B. The green wire from pin 1 of V2B actually terminates at the right-hand end of the tagstrip where C4 is connected.
You really need to magnify the image to see these connections clearly.
Kelvin
Stephane Bureau
Reply with quote  #5 
Thanks a lot kevin. It helps me very much . My project is not in the back of the closet yet. Of course, I guess that the output power of this amp with Russian tubes should be much less than with JJ EL509 tubes, maybe 10 W / channel instead of 15. I read that the plate current going through the 6p45C is far from the other type.
Stephane Bureau
Reply with quote  #6 
Can you tell me please how to wire the A+,A- Bias. Each pot has 3 pins. Is the one in the middle the ground and we plug the + and the minus on the sides ?
Kelvin Tyler
Reply with quote  #7 
S. Look carefully at the schematic in the book.
The A+ and B+ bias voltages are taken from the wipers, centre terminals, of P1 and P2. Looking at the baccks of P1 and P2 the left terminals go via R35 and R36 to ground. The right terminals are joined together and go to the junction of R34 and Z5. This means fully clockwise rotation of P1 and P2 will apply the highest negative bias to the upper tubes.
The A- and B- bias is fixed and is taken from the quadrupler circuit at the R37/Z6 junction.
Make sure you understand exactly how this is depicted in the power supply schematic.
I would not worry about possible differences in power output between the 6P45C and EL509S at this stage. Both tubes have a cathode current limit of 500mA and can dissipate a total of 42W.
Kelvin
Stephane Bureau
Reply with quote  #8 
My project is going well thanks to you. i am near the first test of my first OTL amp. I am at the last step, the connection to the power transformers. i look at the pictures and the schematic and have trouble figuring where the current go from the wall to T1 and T2. Are they in serie or parrallel ? Also, on the real picture, I see the negative  side of C9 going to the right connector of L2 and C10 on the left side. Is it because the current go from right to left because it is the negative portion ? Thank you very much, i can't wait to test my amp with tubes .
Kelvin Tyler
Reply with quote  #9 
Stephane. T1 and T2 are in parallel. How they are wired depends on your mains voltage. Are you in France? If so, your mains voltage would be in the region of 220V. The primary windings of each transformer are in two parts. One part has blue and gray wires and the other violet and brown. This will be clearly marked on the labels. For 220V operation connect the violet and gray wires together and apply the mains voltage to the blue and gray. For 110V operation connect the blue and violet together and also the gray and brown. In that case connect the mains voltage to the blue and gray wires (or the violet and brown!).
I am not sure exactly what is your question regarding the direction of current flow through L2. I suppose, in a conventional sense current is taken to flow from + to -, so in the schematic it would flow from right to left. This is not really important in this context apart from the direction of resulting stray magnetic fields.
Take your time before you test with the tubes present. Check and recheck your wiring against the schematic. It is a good idea to fist power up without the tubes to check that all major voltages are present and correct. Make sure the +170V and -170V are present at the F2 and F3 fuses. I take it you have insulated top-cap connectors for the tubes!
Prudence et bonne chance!
Kelvin
Kelvin Tyler
Reply with quote  #10 
Important Correction. In my previous post I said, for 220V operation, '...apply mains voltage to blue and gray...'. This is clearly incorrect and should have said,'...apply mains voltage to blue and brown...'.
Sorry for that. It was about 6am here and was not at my most attentive.[smile].
Kelvin
Stephane
Reply with quote  #11 
Thank you again for your help. I am in Canada , so the main power is 120V. I have included pictures of my T1 and T2. The real Beast in that story is my power transformer. I could power 5 amplifiers with that. I will have to build a separate chassis with female 120V plugs, like an isolation transformer. I think it came from a medical isolation power supply. The  wire colors does not seem to be standard in transformers, because I have trouble following your instructions. Regarding L2 wiring, what is odd is that L1 appeirs to be wired from right to left and L2 the other way. For the thermistor, is it wired directly to wires from T1 and T2 at the same time ?

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Stephane
Reply with quote  #12 
After posting picture of my power transformer, I noticed that on top we have 3 big gage wires and the rest of the wires are at the bottom with smaller gage. If it is primary 2 x 115V, shouldn't we get 2 sets of big gage wires on top ? Also, they say it has burgundy and violet wires, but I only see a violet one. Bizarre.DSCN2553.JPG 
Kelvin Tyler
Reply with quote  #13 
Stephane. Your 2x12V yellow transformer is as I described except they use Purple instead of Violet, so no problems there. Just wire for 115V.
Now for your large White 2x115V 'beast'. The thick wires on top are for the 0-29.2-34V winding (Black -Red-Yellow). You will not be using these for this amp. The label 'bu' on this transformer stands for Blue, not Burgundy. The Primary leads are those on the lower left. One pair are Black and Red, the other Brown and Blue. For 120V you must connect Black and Brown together and then Red and Blue to make one pair for applying the mains voltage. The Secondary wires are on the lower right. One pair is Brown and Blue, the other White and Violet The Blue and White should be connected together to form the centre tap which goes to ground as shown in the schematic. The Brown and Violet leads feed the rectifier. You will notice that the 34V winding accounts for half the potential VA capacity of this transformer so you could not in fact power 5 amplifiers, notwithstanding other complications which I will not go into here. All these transformer lead connections are best made on tag-strips by the way. Yes, the thermistor is connected to one of the mains inputs of both T1 and T2.
If the currents in L1 and L2 are in the same sense then the magnetic fields will be likewise. If in opposite senses then also the magnetic fields. Looking at your previous picture the currents are flowing in the same sense.
I lived in Ottawa while working at the NRC from 1961-4. Happy days, but very cold winters!
Regards,
Kelvin

Stephane Bureau
Reply with quote  #14 
My SOB is finished.I had some distortion on one channel and I realised that it was my used magnoval socket that were not doing  good contact with the pins. My filament trans is running hot. It is a 160 VA and It drives 8 x 6P45S tubes plus 4 x 12AT7. How do we calculate the current need ? 8 times 2.5 Amps plus 4 times 300 Miliamps ? Thank you for the help. Now I have to build another model !
Kelvin Tyler
Reply with quote  #15 
Stephane. Very glad to hear that your SOB build was successful.
With regard to your heater transformer running hot this is a common problem. I generally do not get worried if the temperature is not more than about 60C. That is just too hot to touch. If it sizzles when touched with a wet finger then it is certainly too hot and you would be better to replace it with one of a higher rating, perhaps a 200VA model.
Your power requirement is 6x2.5=15VA per output tube i.e. 120VA for 8 tubes, plus 12X0.15=0.18VA for each 12AT7 which is 0.72VA for 4 tubes. 160VA should ideally give you enough margin, but much depends on the quality of the transformer, its ventilation and, indeed, your actual line voltage. I take it is mounted on your main chassis along with the 6P45S tubes.
Regards,
Kelvin
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