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Old 19th September 2021, 06:03 AM   #71
spiggs is offline spiggs  United States
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Had some time for testing today. Bias at 65mA and speakers to the 4 ohm taps to give me a 6K load and took a listen. An improvement. I felt it added a bit more sharpness or clarity in the lower range with the same punch as before and that SE sound throughout.

After this I swapped the PT for an Antek 3T275 to try out a lower B+ Adjusted everything and ended up with the following

B+ 376V
R115/R215 344V
cathode 61V
bias at 80mA

So that puts 283V across the tube and 22.6watts dissipation. At this lower voltage the pots seem slightly less sensitive so hitting the target values was a bit easier.

Speakers back on the 8 ohm tap and cranked up some music. I would say this sounded at least as good as the prior test. At this point I think my test speakers are at the limits of their sound quality.

This seems like a good setup so far. I'll continue to listen to this over the next week. I have some multiturn pots on order and a softstart board to finish up next. I am also working on a way to get some capability to measure distortion so hopefully I can dial in the amp further.
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Old 19th September 2021, 02:26 PM   #72
zintolo is offline zintolo  Italy
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What is the expected range of DF we can obtain? 2-4?

Originally Posted by Tubelab_com View Post
As Zintolo found in his simulations, changing the ratio between tube and fet can boost this to extremes in a computer sim, but real world limitations like mosfet SOA, limit things. For now I have been running 10 to 12% of my total dissipation in the fet. Further experiments are being done to find more options.
Indeed I think that higher feedback ratios are possible only in two ways: pentodes with smaller plate dissipation or parallel some p-mosfets to share the current (is it possible?). At the moment I'm simulating the circuit with GU50, as I have many of them (not macthed unfortunately) and seem a good candidate for the project.

Originally Posted by Tubelab_com View Post
Back in the early days of the CED I did experiment with the KT88. I will get to these experiments eventually.
SpreadSpectrum in his thread with similar feedback on KT88 ( If UNSET and the RCA50W Had a Baby ) reported to have started with 10% then raised to 20% and then 30% as optimal, but I've not asked further details about how he dissipated the heat.
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Old 19th September 2021, 05:18 PM   #73
Parafeed813 is offline Parafeed813  Netherlands
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Made some better measurements today.
Still not optimal, no THD so the output wattage is around visible clipping.
EL36 with the lab supply set at 350V and the 6k OPT's loaded with 4ohms to make it 3k.
Below 80mA the top clips first. At 90mA I got the most symmetrical clipping (14W output). 100 and higher gave bottom clipping first.
And yes, there was some redness on the plates. I didn't let them cook for a long time, but I was surprised to see how much abuse these little guys can take.
On to the bigger brother: E130L.
Lab supply maxed (395V). Very similar results as with the EL36. 14,5W at 100 mA. More current, bottom clipping.
On to 2 ohm load (1k5).
Now we're talking: 18W at 130mA, 22W at 140&150, 28W at 160&170mA. Yes, redness starts again.
Need more voltage!
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Old Yesterday, 07:31 AM   #74
spiggs is offline spiggs  United States
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Changed out the trimmer pots for these Bourns multiturns PV37W104C01B00 and adjusted everything again. Easier now to dial in and seems to stay stable around +/- 2% of target value. Also finished up a soft start board to make turn on a little gentler.

Starting to think about chassis layout. George you mentioned that oscillation can be a problem with this amp. How careful do I need to be in routing the input and output? Can I run both to the back panel keeping them as separate as possible or should I keep the inputs on the front and outputs on the rear? I am planning to mount the PT under the top plate and OPTs on top.
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File Type: jpg UNSET_multiturn.jpg (1,003.7 KB, 47 views)
File Type: png UNSET_layout.png (28.6 KB, 52 views)
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Old Today, 11:55 AM   #75
OldHector is offline OldHector  Sweden
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I used these trimmer resistors recently. They are a bit more pricey, but I like the idea I can mount them behind a hole drilled on the top plate.
Bourns trimmer
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File Type: png Screenshot 2021-09-23 135445.png (22.8 KB, 4 views)
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