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Old 25th December 2012, 05:59 PM   #171
wicked1 is offline wicked1  United States
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Quote:
Originally Posted by tubelab.com View Post
Another possibility for parallel output tubes is the RLD approach. Since the grids are operated at the same DC potentials and the cathodes are paralleled SY employs a seperate screen adjustment for each output tube.
Any pro/con to doing it that way?
It's basically the same thing, right?
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Old 26th December 2012, 09:48 PM   #172
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Default PPP 807s

Hooked up parallel 807's (actually 807W/5933) on the breadboard.

Same driver as the KT88, with a little tweak to the bias circuit to lower the voltage. No effort to match the 807's, they aren't even the same brand. Tubes are directly paralleled (no separate bias).

Vp = 559V, Vg2 = 279V, Vg1 = -32V, Ik = 40mA/side or 20mA/tube, Pa = 11W per tube at idle.

1W THD = 0.03%, 12W THD = 0.20%, 5% THD @ 25.3V or 80W RMS into 8 ohms.

Square wave looks good but the amp oscillates with no load. May just need to put 100 ohms across the output, or maybe a screen bypass... needs investigation.

Hope everybody has a happy New Year!

Pete
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Old 27th December 2012, 05:03 PM   #173
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Played with the compensation to get the amp stable under no load/full load conditions. With no load, it was oscillating at 1MHz + 10Hz!

I needed to add another RC (1k & 470p, C9 & R27) on the first stage, change the feedback comp cap from 47p to 68p to kill the HF oscillation and properly damp the square wave response. Also increased the coupling caps to 1uF (first stage) and 0.22uF (output) to avoid the LF oscillation. This also reduced or eliminated the LF and HF response peaks I saw earlier. Its pretty flat now from 5Hz to 50kHz, but still has a 1dB peak at 75kHz.

Also added a 150 ohm 5W resistor across the output. Seems pretty stable now, at least with a resistive load from 4 ohms to open. Zout is 0.57 ohms (DF = 14).

If I increase the idle current up to 63mA/side (still only 18W/tube), 1W THD drops to 0.015% with a small penalty in max output power (74W vs. 80W).

Pete
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Old 30th December 2012, 03:16 PM   #174
wicked1 is offline wicked1  United States
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Thanks a lot!! I'm very close to building this thing. Beginning to realize exactly what I've gotten myself in to.. (Transformers are huge compared to my previous projects)

Could you recommend to-220 diodes for the power supply? While searching I found the fancy new SiC rectifiers and thought I'd try those, but am confused.. (while it's rated for 2A, mouser has the continuous current listed as something like 50uA for those cree, which I don't understand). Even skipping the SiC models, I still don't know what to look for in a to-220 package.

Also, do I need to pay attention to voltage ratings on resistors? The KOA's are only rated up to 400v, the CC's I've got are rated to 350v. I was thinking I should add a 100ohm-ish resistor to the plate of each of the parallel output tubes, and would be exceeding the rating on any of these resistors.
I guess that's the only place in the amp that sees such a high voltage.

Oh, one more... Any speaker (driver) recommendations? My only speakers now are some high efficiency fostex.. I think they say they can take up to 80 watts, but something tells me they aren't going to sound great with a loud amp.

Thanks, and happy new year!

Last edited by wicked1; 30th December 2012 at 03:41 PM.
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Old 4th January 2013, 06:56 PM   #175
wicked1 is offline wicked1  United States
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The fun begins! Done w/ decisions and pouring over data sheets, etc.

I went w/ the Cree SiC diodes.. I have discovered Mouser has a LOT of typo's in their product specs. I'll know if they work or not soon.
I should have the kt88 version up in no time. Then I'll build my regulators and get the gu-29 version running.

Aww, I've got no 7 pin sockets! Where's that package.. Guess it won't be this weekend
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Last edited by wicked1; 4th January 2013 at 07:04 PM.
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Old 5th January 2013, 03:41 PM   #176
syyma is offline syyma  United States
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There are so many motor run caps, it will take up a lot of the space. What kind of the irons that you use for this new build? I'm thinking about it as a winter project as well. Happy new year.

Sidney
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Old 5th January 2013, 05:10 PM   #177
wicked1 is offline wicked1  United States
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Yes, I had 5 motor runs already... So, decided to go all motor run. (power supply caps, and then even the 4 that are doubled up on the PCB are being replaced by 2 motor runs) Then I'm also using old soviet capacitors for bypass, etc, which are also huge (again, I've already got boxes of them, so might as well use them).. You can see a few towards the mid-top of the photo. This will be a very large amplifier! I want the final design to sort of showcase all the large components, so need to come up w/ creative chassis design. I may have to put glass sides around the monoblocks... I'm still far from putting a lot of thought in to that, though.

For iron, I'm using what Pete recommends on page 14 of this thread. I couldn't make up my mind about what to build, so let him do it for me (Thank you Pete.. else it could be next winter and I'd still be sitting here debating w/ myself on transformers)

If anyone else is doing the pentode thing, let me know.. I've built regulators for the bias and screen supplies. Am curious what others are doing.
The sad thing is, this may not even work.. I'm wanting to use gu-29's/829's, and it's not exactly a good idea. There are active threads right now about people unable to get them biased properly, and Pete mentioned feedback instability. I was going to use George's idea of separate screen regulators for each section, but the two sections share a screen.. I'm prepared to scrap them, and go w/ 807's though.

I was just thinking, all my existing amps have DC on the filaments..
Do I need to worry about that w/ PP and indirectly heated tubes?
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Old 5th January 2013, 10:16 PM   #178
syyma is offline syyma  United States
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AC on the filaments just fine. I've the same dilemma too, I have a quad set Tung Sol reissue 6550 left from George's SSE build, also I want to try the PPP 807 too, just not enough time at this moment. Take your time and no rush.

Sidney
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Old 6th January 2013, 12:24 PM   #179
Ejam is offline Ejam  Australia
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Default What Output Transformer?

Pete

The picture of your test rig above shows a different output transformer from your Tango on the website. Can you tell us which model your using.
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Old 6th January 2013, 12:27 PM   #180
Ejam is offline Ejam  Australia
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Default Feedback Options

Pete

Sorry, forgot to ask which feedback options are you using (plate to plate, global or a combination)?
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