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Old 1st December 2009, 12:38 AM   #211
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You likely have the CFB hooked up backwards. This raises the distortion instead of lowering it, and will buzz in UL mode because the gain is higher in UL mode. This seems to occur with EL34's more than other tubes because they have more gain.

The solution? Swap the two secondary wires on the OPT. These are the green and the black wires on my old Transcendars. Just exchange the green and black wires with each other leaving all other wiring the same. Do the same for both channels.
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Old 1st December 2009, 12:50 AM   #212
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Thanks George.


I'll give it a try tomorrow.






Nik
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Old 1st December 2009, 10:32 PM   #213
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Tried it...... Didn't work.



So I started tracing wires. Everything was right except for the two pink wires going to the UL/Triode switch. They were backwards. Switched them and the amp works perfect now.

And sounds Phenomenal!!! I don't know what I was thinking when I parted this thing out!



Click the image to open in full size.



Nik
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Old 1st December 2009, 11:25 PM   #214
rknize is offline rknize  United States
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Cool...thought that sounded familiar as i did the same thing at one point.
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Old 10th January 2010, 12:30 PM   #215
Junior is offline Junior  Singapore
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Quote:
Originally Posted by nikolas812 View Post
Can anyone tell me the idea B+ for EL34's???



I have just switched resistors. With the 560ohm resistor. I was getting 490 on the B+

I just installed a 680ohm resistor and I am now getting 409 on the B+


If it needs to be lower I have a 750ohm resistor on hand.




TIA


guys,
pls bear with me for my silly question (again).
am planning to use both KT888 and EL34 for my SSE build.
understand that to do that cathode bias resistor needs to be changed to accomodate the valve used. may someone kindly let me know the resistor value to use for KT88 & EL34. or 560 is good enuff for both tube?

am considering to use a mini binding post to change the resistor value or might be incorporate a switch (with the resistors mounted on a veraboard).

thanks.



Nick
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Old 10th January 2010, 04:14 PM   #216
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I have no problems running EL34's on a the 560ohm resistor...

I've never had any plates start glowing on me and they always sound fine..


In the end though. I've always stuck with the KT88's. They have always sounded better to me.. That being said. Its always nice to know you can throw a EL34 in there from time to time...


Good luck









Nik
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Old 10th January 2010, 04:39 PM   #217
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Quote:
Originally Posted by nikolas812 View Post
I have just switched resistors. With the 560ohm resistor. I was getting 490 on the B+

I just installed a 680ohm resistor and I am now getting 409 on the B+

If it needs to be lower I have a 750ohm resistor on hand.
These voltages don't quite make sense. You can measure B+ at the lead on the primary side of the output transformer (usually red). Or, you can measure B+ at the power supply on C2 or on one side of the choke L1.

With a larger cathode resistor (i.e., 680 ohm instead of 560 ohm) you will have a greater voltage drop across it. This greater bias voltage will cause the output tube to draw less current at idle. Less current draw will cause your B+ voltage to increase.

Also, there is a big difference between 490 volts and 409 volts. I would be surprised to see that much of a change just by swapping 560 ohm cathode resistors for 680 ohm resistors. On my Simple SE (with my particular power transformer), I might expect to see about 440 volts with 560 ohm cathode resistors for KT88 power tubes. If I were to change the cathode resistors to 680 ohms, I would expect an increase in B+ to somewhere around 453 volts or so.
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Old 10th January 2010, 11:48 PM   #218
Junior is offline Junior  Singapore
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hi nik,
u mean that you had been using 560ohm for both KT88 and EL34
if that is the case, then i need not purposely added a switch to switch resistor to run both tubes. but still will implement mini binding post for the Cathode Bias Resistor just in case...



Hi Ty,
With my configurations:
Pwr T - Hammond 374BX (375-0-375 @200VA)
OPT - 1629SE (6.5k impedence)

what will the recommeded respective Cathod Bias Resistor value to run KT88 and EL34 based on text book calculation?

thanks
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Old 11th January 2010, 12:12 AM   #219
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Quote:
Originally Posted by Junior View Post
Pwr T - Hammond 374BX (375-0-375 @200VA)
OPT - 1629SE (6.5k impedance)

what will the recommended respective Cathode Bias Resistor value to run KT88 and EL34 based on text book calculation?
I have the same exact power transformer. In my area, the line voltage tends to run 120VAC or higher. With KT88, I prefer to use a 560 ohm cathode resistor. For EL34 or 6L6, the plate dissipation tends to be a little too much and I prefer to use a larger cathode resistance - 810 ohms was convenient, and works well for me. I kept my original 560 ohm cathode resistor, and added another 250 ohm in series. It's pretty easy for me to put a shorting jumper around the 250 ohm resistor if I want to run the bigger tubes.

Your rectifier tube will make a difference, too. I prefer to stay with the 5AR4, mostly for the desirable slow start characteristics. I have tried the 5U4 as well, which drops an extra twenty or thirty volts. The lower B+ from the 5U4 might let you get away with using the smaller cathode resistor, even with the EL34 / 6L6 type tubes.
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Old 11th January 2010, 04:27 AM   #220
Junior is offline Junior  Singapore
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since i am into KT88 and EL34, will try out the resistor value as you had suggested. will try to implement it using a switch as i do not wish to unscrew and screw to change the resistor value when swapping tubes.

thanks Ty for the informative inforamtion.
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