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K12G Questions
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Old 4th November 2018, 09:56 PM   #41
HollowState is offline HollowState  United States
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The easiest way to determine phase is to just power it up on completion and listen for a loud squeal. If you get it, then it's phased wrong because it's oscillating. Simply reverse the plate & screen connections on the output tubes, meaning one side to the other. Or reverse the coupling capacitors to the grids. So leave the transformer leads long till you're sure they're phased correctly.

Hammond said to connect the secondary windings together to get the most efficiency through better coupling. The lower 4 ohm winding could, in theory, be used as a tertiary winding for feedback. The advantage would be some isolation from the effects of speaker impedance variations. But I would go for the better coupling efficiency.

Going UL mode still keeps the screens connected and drawing some current so overall voltage level should be pretty much the same. If it increased at all it will only be a small around and well within acceptable tolerance.

I like the chassis.
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Old 5th November 2018, 04:39 AM   #42
jazbo8 is offline jazbo8
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K12G Questions
@JTCamp - it’s against the Forum Rules to start multiple threads on the same topic, your threads have been merged.
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Old 5th November 2018, 01:27 PM   #43
JTCamp is offline JTCamp  United States
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HollowState, thank you for your response. I'm trying to think about the layout as I go along but it's tough not to get stuck on all the little details. The chassis is pretty cool. I bought it from a vendor. It's anodized aluminum but it's modular; the panels go together with screws into captured nuts. Unfortunately there isn't electrical continuity between the panels... so i'm working on that too.

jazbo8, my apologies. I won't let it happen again.

Thanks for all the help,

John
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Old 6th January 2019, 03:27 PM   #44
JTCamp is offline JTCamp  United States
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All

I completed the work last night and am happy that there is no hum what so ever. The sound is pretty good and there isn't any popping when I turn the power off. I remember the old setup doing this...I'm not sure what changed. However, I not excited about the temperature of the tubes and the PT.

My first question is about the tube temperature. Can someone explain to me how to determine if they are running too hot? I think this is called tube biasing and/or dissipation. I have a meter and am comfortable checking but I'm not sure how to do it. I have running plate and cathode voltages etc if that would help.

My second question is about the PT temperature. I'm going to try to get an amperage reading on the secondary B+ winding. I had been concerned that the 180 mA that the transformer is rated for wasn't enough. Once I get the reading, I'll post back.

Thanks for looking and please let me know what you think.
John
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Last edited by JTCamp; 6th January 2019 at 03:30 PM.
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Old 6th January 2019, 05:19 PM   #45
JTCamp is offline JTCamp  United States
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I have now hooked a meter in series with one leg of the secondary ac, before any of the rectifier section. I am reading 0.45 ac amps. This transformer is rated for 200 mA on this winding.

Am I missing something? This transformer seems undersized.

Thanks to all.
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Old 6th January 2019, 07:58 PM   #46
JTCamp is offline JTCamp  United States
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Regarding the heat from the tubes... I found a calculator and did some adjustments. Calculator here Weber Bias Calculator


With the original 150 ohm common cathode resistors the calculator was saying 12W and 57 mA. That seems really high based on what I've read. I'm don't know what I doing though.

I found some resistor that in parallel give me 235 ohms of resistance. In the calculator it shows that to be 9 watts and 41 mA after making new voltage measurements. The tubes appear cooler but I'm pretty much in the dark on whether I'm going about this correctly.

Oh, and my hat is off to those who lay out a chassis with point to point wiring and make it look good and be functional. Really impressive.

Thanks,
John
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Old 8th January 2019, 02:58 AM   #47
JTCamp is offline JTCamp  United States
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I wanted to reply and apologize because this thread seems really confusing now that I've read it.

When this thread was merged recently I did not realize it was merged with a thread of mine from 8 years ago.

To that point, I created the most recent thread not remembering that I'd asked some similar questions the first time.

However, I still would like input if you can. Im concerned about the amperage on the secondary of the PT and the question of tube biasing for this amp.

Thank you
John
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Old 8th January 2019, 06:02 PM   #48
Alllensoncanon is offline Alllensoncanon  United States
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You seems to have two issues. One, the Edcor transformer you bought is under size for the K12G design. Second, you changed the output from Pentode to Ultra Linear without also correct the bias resistor. The good news is, the Edcor transformer should be able to support the push pull 10GV8 tubes if you pull back the power some.

I would increase the 235R output cathode (bias) resistor to limited the current to about 68mA per channel. Not having an UL 10GV8 curve to work with, my guess is, it should be around 320R or so. Use a volt meter and ohms law to find the right value. I = E/R
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Old 9th January 2019, 12:08 AM   #49
JTCamp is offline JTCamp  United States
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Thanks for replying. I'm going to get a selection of resistors and test the amp with them.

Can you explain how changing to ultra linear requires a bias change? Is it because of the changing of the plate voltage?

Also my reading of .45 amps on the PT seems really high. In my reading about this amp design, I have found at the most 300 mA. Is there a way to calculate the theoretically amperage? I assuming there is because you mentioned 68 mA per channel. Could changing the capacitor values on the power supply section help to give the transformer more head room? I have 4 UF4007 diodes taking the place of the original diode bridge but otherwise this section is unchanged.

So with more reading, I've discovered that the rating on the transformer is typically based on half wave rectification. In this amp's case there is full wave rectification. To that point, the AC amperage rating needs to be about 1.6 times that of the DC amperage load. Several of you have said that this amp would use about 150 mA. So I would need at least 250 mA of current available for this amp in stock form. Sad Face. Any creative recommendations for making this PT work?

John
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Old 14th March 2019, 04:42 PM   #50
Interested Observer is offline Interested Observer  United States
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@JTCamp, did you ever solve your transformer dilemma?

Jamie
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