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Simplified pentode Loftin-White with plate-plate feedback
Simplified pentode Loftin-White with plate-plate feedback
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Old 26th July 2013, 07:56 AM   #1
homeskillet is offline homeskillet  United States
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Default Simplified pentode Loftin-White with plate-plate feedback

Hi. This is my first thread. I don't have any formal electronics training but I was raised around high voltage in the 70's and I would buy tube amps at garage sales and try to keep them going. I usually knew where to kick them...

I've been breadboarding amps for about 5 or 6 years now. My audio systems that I listen to are what I breadboard together. They are all bench designed and they, so far, are all direct connected since that's where I had my first success in amp making.

I've been reading with much interest the plate to plate feedback in recent threads since I've had no experience with feedback and it seemed like a relatively simple approach. It occurred to me to apply it to a Loftin White type design with a high gain pentode for V1 since I could never get those to behave in the slightest.

I wired this up tonight and it sounds rather nice to me. It puts out a nice sine wave on the scope at a strong volume. The response isn't linear on the scope but music sounds natural and nicely balanced through it. The speaker is a transmission line loaded 5" dayton full range pointing up with the center cap removed and a mushroom shaped reflector mounted in the pole piece. I set it in the corner to help the bass out.

Thanks for looking. -Fred
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Old 27th July 2013, 06:03 PM   #2
homeskillet is offline homeskillet  United States
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Default frequency sweep

I think I was incorrect about the lack of linearity. Unless I'm completely missing something. I did a sweep from 0 to 30 kHz with the oscilloscope on the speaker leads with the speaker as the load and there seems to be minimal change in the peak to peak voltage. There looks like a bit of a peak as the frequency climbs but not nearly as much as I'm used to. I posted a youtube video of it.

Simplified Loftin-White EF184-6550 with plate-plate feedback - frequency sweep - YouTube

Here it is playing some music.

Simplified Loftin-White EF184-6550 w/ plate-plate feedback - music sample - YouTube

Thank you. -Fred
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Old 27th July 2013, 06:32 PM   #3
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Looks good.

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Old 27th July 2013, 10:38 PM   #4
Paul Scearce is offline Paul Scearce  United States
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I'd be a bit concerned about the rating of the 4.5k resistor between the 6550 cathode and ground. If I'm reading the voltages right it is dissipating around 10 watts. If you would like to increase the linearity you could increase the current through the ef184. I really like direct coupled pentode and UL amps with plate-plate feedback.
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Old 27th July 2013, 10:48 PM   #5
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What is the current through the EF184. i like at least 5mA in this application - preferably about 10mA.

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Old 29th July 2013, 02:11 AM   #6
homeskillet is offline homeskillet  United States
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Hi. Ah yes. I miss labeled the schematic. There's a 25 watt rheostat under the cathode of the 6550. It gets pretty warm.

And yes, there was about 2 ma running through the EF184. I adjusted the resistance under the cathode of the EF184 and was able to bring it up to about 5.5 ma before the entire circuit went berzerk and started drawing tons of current. After a bit of tweaking and a 15k grid resistor on the EF184 it settled down. Running the EF184 hotter increased the gain, which is awesome. It doesn't seem to want to run any more than 5.5 ma through the EF184 without distorting at the moment.

Thank you guys so much for looking at it. I read your replies this morning at a friend's house. It really made my day! Thank you. -Fred
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Old 29th July 2013, 05:03 AM   #7
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curious about a few things.

how much power to the load?

how much power drawn by the 6550?

overall gain of the amplifier?
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Old 30th July 2013, 04:08 AM   #8
homeskillet is offline homeskillet  United States
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Made a few more changes: Removed the resistor between the cathode of the 6550 and the transformer which increased the voltage across the 6550; increased the B+ to 560 volts; decreased the EF184 cathode resistor to about 100 ohms which increased the current through it to about 8.5 ma.

I think that resistive divider that connected all the cathodes between B+ and ground on the original Loftin White was utilized for a sliding bias set up and a hum canceler. I finally got brave enough to just unhook it. It doesn't seem to suffer without it. It's pretty quiet and I don't mind the bias not changing with small and large signals.

The draw of the 6550:
There's 346 volts across the 6550 and 51.4 ma through it so I figure it's dissipating about 18 watts.

The power delivered to the speaker:
The peak to peak voltage at the speaker with no clipping is 20 volts @ 5 kHz. I used Volts^2 = Power x R with volts and power both RMS. Came up with 15 watts so I'm guesstimated it at between 10 and 12 watts since there must be some sort of phasing between voltage and current. Also judging just by ear how loud it is.

I don't quite know how to approach figuring out the gain. Would it be VA in/out or the voltage gain before the output transformer? I've seen gain on power amp knobs but I've never seen a unit given with it and it seems like you set the gain and measure the power delivered at the output.

Ran another sweep from 0 to 30 kHz and I didn't see any difference in the peak to peak voltage at the speaker leads.

It's stable now and sounds nice and stronger than before. It's really nice to listen to. Certainly the most polite sounding thing I've ever put together and I think I'm finally satisfied with the volume output. It sounds almost store bought at this point but it's not done yet I don't think.

I hope I answered everything. If I didn't or if I'm just completely off on something or lots of things please let me know. Thank you. -Fred

p.s. I hope this isn't a double post... here goes... close my eyes and push the button.
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Old 30th July 2013, 05:07 AM   #9
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20v p-p is 10v peak, so 0.707 of that is RMS, or 7.07vrms.

P = I^2 / R, 49 / 8 = 6 watts.

Gain would be the voltage in for a given voltage out converted to dB.
Not necessary to really know. Close enough to know how many volts at the input drives it to full output.

Hope the little amp is fun and you are enjoying it!

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Old 30th July 2013, 05:36 AM   #10
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Nice! That's good to know. I'm rocking out to Lady Gaga on pandora with 6 watts of raw thermionic power right now. Thank you. -Fred
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