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Old 2nd June 2013, 08:16 PM   #21
SandyG is offline SandyG  United States
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Thanks, working on it

Thanks for the pointers, the boards are in process now so can't make too much drastic changes.

Note some of the values are TBD still, so using a trimpot for testing the feedback is a good idea, as will be figuring out correct bias values for the input / driver of the amplifier.

1. Too late on this one, board are being made will see how it works. I have seen a few amps that use this and said what the heck, I'll give it a try. I have seen similar circuits driving an octet of KT88 and seemed like worth giving it a spin to see how it sounds.

2. The snubber circuit is a pcb stuffing option, I put it in the circuit as I had seen comments that in paralled tubed output stages it might be something that is needed. Will build the test unit without it and see if it is stable. Hopefully it will be, but have space on the board for the parts just in case

3. Yes, I think this is a good idea. Values are 'Get me going baseline' will be playing with these once things are up and running. I'll bug you for some pointers.

4. Good to know if I run into a problem!

5. Yes!

6. Those tube breadboard look very nice, but what would I do with the expensive PCB software! My idea with just doing the PCB is to design it with a circuit that is pretty generic and use good layout so I can hopefully have something close to working with just part value changes. I have been lucky on the last 2 amps (EZ10, EZ260). This amp is more complicated so hopefully I won't have to attack the completed PCB with the knife and dremel tool to make changes.

Thanks again for the pointers, I think these all will be useful as the first one gets built. I'll be posting progress as I get things rolling!!

Sandy
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Old 10th June 2013, 06:52 PM   #22
SandyG is offline SandyG  United States
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Christmas in June - PCB's are in hand so I can start doing some building...

Sandy
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Old 17th June 2013, 04:05 PM   #23
SandyG is offline SandyG  United States
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Completed a board. Here are the pics -

Sandy
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File Type: jpg Bottom_Board_Complete.jpg (147.7 KB, 331 views)
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Old 23rd June 2013, 06:26 PM   #24
SandyG is offline SandyG  United States
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Test amp is built and running. At first I thought it was unstable, and turned it off and went to sleep. Then the next day woke up and forgot that it had the same odd behavior as another amp I built but has the the screen/plates flip/flopped so it changed them around and it worked!

Amp is very clean and stable with no signs of waveform ugly across the audio spectrum. Begins to flattop at about 116 watts with solid state rectifiers and about 80 watts with a pair of old 5U4's.

This is with just guessing at some of the part values for the 6SN7 stages, and I'm sure their are some that would require changes between the voltage drop of the tube vs. the SS rectifier.

Need to get voltage readings and start working on the 6SN7's operating points...

Pics below of some of the build.
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File Type: jpg IMAG2045.jpg (112.8 KB, 130 views)
File Type: jpg IMAG2057.jpg (147.4 KB, 139 views)
File Type: jpg IMAG2060.jpg (155.7 KB, 172 views)
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Old 23rd June 2013, 06:38 PM   #25
kingneb is offline kingneb  United States
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The amp definitely needs some refinement.

On my PPP amp, I obtain around 130 watts RMS at 0.25% THD at 1k. Distortion rises to 0.65% at 10k, and higher at 20k. There is a similar pattern at lower frequencies with the transformer I have.

Are you looking for undistorted power at 120 watts? What is the power goal here?
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Old 23rd June 2013, 07:29 PM   #26
SandyG is offline SandyG  United States
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Yeah, I think it has a bit more to give. Would be nice to clip the 125 Watt mark But it makes what it makes!

Kingneb - What was the B+ to your KT88's? I'm seeing right about 505 at idle (with bias in the range of 40 ma). I have to change one of the resistors in the bias circuit as I can't move it over about the 40ma mark due to one of the dropping resistors being too small, but for safe testing it works but need a bit more adjustment.

Will get some voltage reading hopefully tonight.

One thing I did measure was the with the 10K soft start resistor that the relay cuts out after startup it looked pretty close in value. With about 11 seconds on the relay delay the voltage comes up to about 300 vdc with the solid state rectifiers. So the relay is switching 200vdc.

I could make the voltage come up faster but in looking at the voltage on the 450v 56uf cap that is on the cathode follower it briefly clicks about 469v then quickly settles to about 434v which is OK. I think if I change the resistor from 10K to a lesser value it doesn't give enough time for 'things' to start drawing power and keep the 450v caps in a safe area.

Optimally might just make the delay a bit longer and that will keep things well below limits. Didn't see any heating issues with the 10w 10k resistor either.

More to come...

Sandy
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Old 23rd June 2013, 07:43 PM   #27
kingneb is offline kingneb  United States
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- I am running 550 volts on my KT88/KT90 tubes.

- A KT88 should be biased optimally, at 500 volts B+, for 60mA for 30 watts static plate dissipation.

- Keep in mind the peak ripple voltage and cap ratings.

- For refinement, adjust driver B+ voltages and resistors for optimum linear operation.
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Old 23rd June 2013, 09:58 PM   #28
SandyG is offline SandyG  United States
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Thanks, will update the bias circuit and get some voltage reading hopefully tonight or tomorrow.

I think the ~50vdc lower difference in B+ will account for the lower power I'm seeing. What secondary voltage (AC) on your transformer are you running (425-430vac)?

I did find some 500v 68uf caps at Digikey and will likely use those for the that part of the circuit for a bit more voltage headroom on start up.

I'm not sure what you mean by "peak ripple voltage", but if it matters these caps are past the main filters (pair of series connected 450v) so it's pretty clean DC at that point, but again not sure I understand.

Thanks for the tips too!

Sandy
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Old 23rd June 2013, 10:10 PM   #29
kingneb is offline kingneb  United States
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For 120-130 watts I recommend you use 470uF of net capacitance (33uF before and the rest after the choke) since you are using optional 5U4s. 5U4s cannot take the inrush current beating of large caps. The large capacitance is so the power supply can recover from bass transients. There is a formula for the minimum power supply capacitance that can be found here:

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I also recommend you place two caps in series with bleeder resistors across them. This halves the effective capacity but doubles the working voltage. This allows for cheaper and bigger cap use.
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Old 23rd June 2013, 11:09 PM   #30
SandyG is offline SandyG  United States
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Thanks for the link.

I'm running 380uf total right now on the main B+, then each 6sn7 tube has a 56uf buffer. I have currently reduced the first caps off the rectifiers to a total of 50uf to support the 5u4's but with the solid state it would have been a total of 495uf .

Take a look at the schematics on the very first post (A few values have changed since the post), it will explain a lot of what I did on this amp and power supply. Note that it can be built in a few different configurations, ie, if you don't want to use the chokes, don't, solid state or tube for rectifiers, you pick and then build as desired. Tried to make it so I could try different configurations and see how I like the sound.

Sandy
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Last edited by SandyG; 23rd June 2013 at 11:18 PM. Reason: forgot something
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