Carbon film is probably fine in most positions.. theoretically the circuit will be quieter with metal film.
George, hoping to get your thoughts on the compensation cap value.
I've tried 120pF, 270pF, 390pF and 470pF.
As you increase the value, the square wave gets nicer.
The issue though is that beyond 120pF, the amp distorts at about 3W of output.
I think this could be similar to an issue rkinze mentioned with new 12AT7's, although I've tried 3 different NOS types, a Siemens & Halski, Brimar and GE 6201 and all of them have the same issue.
I've also tried various output tubes JJ and NOS types, as well as both UL and Pentode connection, with the issue the same.
Technically, I can keep the amp with 120pF as I can drive the amp to clipping without observing this distortion, but the square wave is pretty ugly.
George, hoping to get your thoughts on the compensation cap value.
I've tried 120pF, 270pF, 390pF and 470pF.
As you increase the value, the square wave gets nicer.
The issue though is that beyond 120pF, the amp distorts at about 3W of output.
I think this could be similar to an issue rkinze mentioned with new 12AT7's, although I've tried 3 different NOS types, a Siemens & Halski, Brimar and GE 6201 and all of them have the same issue.
I've also tried various output tubes JJ and NOS types, as well as both UL and Pentode connection, with the issue the same.
Technically, I can keep the amp with 120pF as I can drive the amp to clipping without observing this distortion, but the square wave is pretty ugly.
Attachments
I still have the instability issue as demonstrated above.
I have a different thread with more detailed information, the latest in this post: https://www.diyaudio.com/forums/tub...easurable-distortion-makes-5.html#post6373967
If anyone has an idea of where to start looking, please let me know.
Thanks!!
I have a different thread with more detailed information, the latest in this post: https://www.diyaudio.com/forums/tub...easurable-distortion-makes-5.html#post6373967
If anyone has an idea of where to start looking, please let me know.
Thanks!!
If I'm not using a pot, can I replace the 220K resistors across the input to something like 50K ?
I'll give this thread another shot, as most of my questions above were left unanswered.
With UL and feedback the input sensitivity is about 0.5V for max output.
Is it OK to replace the 12AT7 with a 12AU7 ?
My DAC outputs 2V so I'm throwing away alot of signal and don't have much usable range on the pot.
With UL and feedback the input sensitivity is about 0.5V for max output.
Is it OK to replace the 12AT7 with a 12AU7 ?
My DAC outputs 2V so I'm throwing away alot of signal and don't have much usable range on the pot.
itsikhefez-- not being a smart aleck- why not go ahead and try it? What's the worst thing that can happen?
Yea agreed, I am going to go ahead and do that. Still wanted to hear from someone with the expertise if there any issues of doing this.
I don't trust my ears enough to know if this is can cause some subtle problem with the amps operation.
I don't trust my ears enough to know if this is can cause some subtle problem with the amps operation.
Tubelab says he blows things up so we don't have to. I doubt that you'll blow up any tubes switching out the 12AT7.
I've rolled SO many different 12AU7 types into my SPP.
My favorite is the 6414.
The 12AT7 Baldwin Japan Blue Tip is really nice too.
My favorite is the 6414.
The 12AT7 Baldwin Japan Blue Tip is really nice too.
Yes, any resistor less than 220K is fine as long as your source can drive it. I have used 10K with a solid state source.
I have also tried lots of different tubes in the 12AT7 socket. Some work, some don't. Any tube that has the same pinout should do something, and not blow any parts. Something with largely different characteristics may need some resistor value changes.
I wouldn't use a 12AX7 since they don't like to run over 2 to 3 mA.
Hey all, I'm just putting the finishing touches on a new P-P build. Edcor 25w opts, EL84s, probably going with UL wiring. Need to order a few more parts and contemplating the C101/C201 feedback cap as well as the R104/R204 resistors.
Please help double check my thinking in this department. Have read in previous posts that this amp likes around 6dB feedback (set by the resistor). The capacitor helps with oscillation and square wave performance - and it is a balance between the two. I've recently fought a modified Dynaco MkIII in this department and hopefully learned a couple lessons. If I've learned correctly, the bump on the descending side of the sine wave (and corresponding FFT extremely noisy) is oscillation and requires a change to the feedback cap.
Sooooooo, If I'm on the right page so far, something in the vicinity of 120 pf should get the job done. If I may ask, what values of the feedback caps & resistors are working out for people?
Thanks in advance!
-- Pete
Please help double check my thinking in this department. Have read in previous posts that this amp likes around 6dB feedback (set by the resistor). The capacitor helps with oscillation and square wave performance - and it is a balance between the two. I've recently fought a modified Dynaco MkIII in this department and hopefully learned a couple lessons. If I've learned correctly, the bump on the descending side of the sine wave (and corresponding FFT extremely noisy) is oscillation and requires a change to the feedback cap.
Sooooooo, If I'm on the right page so far, something in the vicinity of 120 pf should get the job done. If I may ask, what values of the feedback caps & resistors are working out for people?
Thanks in advance!
-- Pete
Hi sgtnoah! For my OPT's (Hammond) 120pF was not enough to make the square wave nice... at the same time, over ~400pF caused some high frequency oscillation (which was cured with a small cap across the screen connector from the OPT)
I kept the standard 5.6K (IIRC) resistor.
So.. some experimentation may be needed here
I kept the standard 5.6K (IIRC) resistor.
So.. some experimentation may be needed here
itsikhefez, thanks! New Edcor iron should be here in a few weeks. It will be fun finishing this project up!
Heater voltages
Bought some premium tubes for my working PP had been using JJ the Millard el84s red plate quickly which caused me to recheck readings. My heater voltage is 6.5(sort of hot) rectifier heater 5.1 volts( just hot) using a Hammond 272jx I have a plate voltage of 343 measured at l-pri-1. How can I lower my heater voltages? I think the tubes I bought were junk they red plated I my HealthKit sp14 also
Bought some premium tubes for my working PP had been using JJ the Millard el84s red plate quickly which caused me to recheck readings. My heater voltage is 6.5(sort of hot) rectifier heater 5.1 volts( just hot) using a Hammond 272jx I have a plate voltage of 343 measured at l-pri-1. How can I lower my heater voltages? I think the tubes I bought were junk they red plated I my HealthKit sp14 also
Your heater voltages should be fine.
Try putting a 5u4 in the rectifier slot.
I run a 5u4 for everyday use, I have a Matsushita GZ34 when I drive 4r speakers.
I've used JJ, Sylvania, Canadian Marconi & Polam EL84's and have never had red plate issues.
I AM running the optional choke, oil filled motor run cap & Hammond 272JX PT.
Try putting a 5u4 in the rectifier slot.
I run a 5u4 for everyday use, I have a Matsushita GZ34 when I drive 4r speakers.
I've used JJ, Sylvania, Canadian Marconi & Polam EL84's and have never had red plate issues.
I AM running the optional choke, oil filled motor run cap & Hammond 272JX PT.
Attachments
Your heater voltages are within 3% of spec. Your line will likely vary much than that throughout the course of the day..
When you say hot, do you mean temperature? That's kinda how thermionic emission works.
When you say hot, do you mean temperature? That's kinda how thermionic emission works.
I *think* I just bought all the parts from Digi Key for the SPP except for tube sockets and tubes/transformers.
I've downloaded my order as a csv file which can be uploaded to the digi key site, this might save someone some time ordering in future. However I replaced all the Xicon resistors (sourced from mouser originally in the tubelab parts list with DigiKey equivalents as Mouser required minimum orders of 200), in addition some of the capacitors in the parts list were end of life and replaced with equivalents so it's not exactly the same but as close as I could get it and doesn't require two suppliers. Total cost was €58 so I got free shipping too.
In addition I bought the following from Don Audio in Germany:
Edcor XPWR066-240 Power transformer
Edcor CXPP25-MS-7.6K
2x JJ ECC81 / 12AT7 Gold Tube
4x JJ EL84 / 6BQ5 Power Tube JJ Electronic
1x JJ GZ34S - 5AR4 Tube
6x Tube Socket Noval Ceramic Print 9-Pin Gold
Only thing missing (I think!) is the octal tube socket as it wasn't in stock in Don Audio. Hope this is useful if anyone in Europe/EU is trying to build this.
I'll update with my progress, if anyone has any suggestions please let me know.
I've downloaded my order as a csv file which can be uploaded to the digi key site, this might save someone some time ordering in future. However I replaced all the Xicon resistors (sourced from mouser originally in the tubelab parts list with DigiKey equivalents as Mouser required minimum orders of 200), in addition some of the capacitors in the parts list were end of life and replaced with equivalents so it's not exactly the same but as close as I could get it and doesn't require two suppliers. Total cost was €58 so I got free shipping too.
In addition I bought the following from Don Audio in Germany:
Edcor XPWR066-240 Power transformer
Edcor CXPP25-MS-7.6K
2x JJ ECC81 / 12AT7 Gold Tube
4x JJ EL84 / 6BQ5 Power Tube JJ Electronic
1x JJ GZ34S - 5AR4 Tube
6x Tube Socket Noval Ceramic Print 9-Pin Gold
Only thing missing (I think!) is the octal tube socket as it wasn't in stock in Don Audio. Hope this is useful if anyone in Europe/EU is trying to build this.
I'll update with my progress, if anyone has any suggestions please let me know.
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