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47 Single Ended Pentode Amp/Headphone Amp
47 Single Ended Pentode Amp/Headphone Amp
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Old 20th May 2019, 01:57 AM   #1
Gungnir is offline Gungnir  United States
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Location: CA
Default 47 Single Ended Pentode Amp/Headphone Amp

So, having built a 6SN7 PP amp to power my HE-4xx planars (very power hungry), I wanted to build an amp for both headphones and effecient speakers with a little more power than the 1.5w 6SN7 PP.

I have decided on the 47 as my output tube which will more than likely be in Pentode or Ultralinear mode (maybe as switch two try both). I will be using a pair of Coleman regulators for the filament and cathode biasing I think I should be able to get close to 2w in pentode mode.

I am at the crossroads when it comes to the driver, however. I was originally going to use a 6C6 in Pentode with each screen regulated with a VR tube, but I started wondering if it would be difficult to keep the tube quiet (in pentode mode) for headphone use. I am not thinking of 7788 Triode strapped, with plate chokes and LED biasing, but I really wanted something that had that 1930s look to match the nice globe 47s.

I got a really nice pair of OPTs from Musical Power Supplies (Hifi version) with 4 and 32 ohm taps for under $100 US (cant beat that with a stick!)

I will be powder coating the chassis metallic gray and will probably coat the end bells Illusion Cherry, or Illusion Copper from Prismatic Powders.

Power supply will be CLC then each channel will branch off and have its own LC.

So, the big question is, how difficult would it be to get a 6C6 quiet enough in Pentode mode for headphone use? I have never used a Pentode driver (not triode strapped) before, so I dont have any experience with them.
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Old 20th May 2019, 08:02 PM   #2
FlaCharlie is offline FlaCharlie  United States
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Join Date: Feb 2004
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Quote:
Originally Posted by Gungnir View Post
So, having built a 6SN7 PP amp to power my HE-4xx planars (very power hungry), I wanted to build an amp for both headphones and effecient speakers with a little more power than the 1.5w 6SN7 PP.

I have decided on the 47 as my output tube which will more than likely be in Pentode or Ultralinear mode (maybe as switch two try both). I will be using a pair of Coleman regulators for the filament and cathode biasing I think I should be able to get close to 2w in pentode mode.

I am at the crossroads when it comes to the driver, however. I was originally going to use a 6C6 in Pentode with each screen regulated with a VR tube, but I started wondering if it would be difficult to keep the tube quiet (in pentode mode) for headphone use. I am not thinking of 7788 Triode strapped, with plate chokes and LED biasing, but I really wanted something that had that 1930s look to match the nice globe 47s.

I got a really nice pair of OPTs from Musical Power Supplies (Hifi version) with 4 and 32 ohm taps for under $100 US (cant beat that with a stick!)
I can't help you with your 6C6 question but I'm puzzled by your plan.

First, I can't imagine that there would be any appreciable power difference between a 1.5w amp and a 2w amp. With speakers that would result in only a 1db increase in output since doubling the wattage only gets you a 3db increase. A 1db increase is like moving about a foot closer to your speakers.

I'm not a headphone guy but I'm sure either one would be more than enough for headphones. And either one should be fine on really efficient speakers too.

But which highly efficient speakers are 4 ohms? Do you have something in mind?

I've got a bunch of 47s that I picked up a long while back after reading about a PP 47 design but I've never gotten around to doing anything with them.

Last year I modded one of the popular Magnavox SE amps into a true SET using 12BH7 output tubes. Here's a thread about it:

Flea Power SET - Magnavox 12BH7 Mod
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Old 20th May 2019, 09:23 PM   #3
Gungnir is offline Gungnir  United States
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It isn't so much the watts, just really want an excuse to make another amp ��

It just dawned on me that the 47 in Pentode has an amplification factor of 150! So the 6c6 in Triode has a mu of 20, which should be more than enough, hell I could run half a 6SN7.
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Old 20th May 2019, 10:08 PM   #4
6A3sUMMER is offline 6A3sUMMER  United States
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Oh, I like the flea power SET 12BH7 amp I saw before. The 47 amp has a lot more complexity in order to work:

The 47 has plate resistance, rp, 60k; and 2,500 micromhos transconductance.

60k x 0.025A = 150. But in that case, the only Load is the tube's own plate resistance, rp.

The Gain in pentode mode is transconductance x load impedance. You have to have a real load if you are going to get power out of the amp.

The 47 data sheet also says 7000 Ohm load; 250V Plate and Screen, bias -16.5V.

2,500 micromhos is 2.5mA/V 0.0025A/V

0.0025 x 7000 = 17.5 (Gain)

The 47 Gain is 17.5, not 150.

17.5 x -16.5V = -288.75V plate swing, and+288.75V plate swing into 7000 Ohms. But the plate voltage is at 250V, so it can not swing down by 288.75V. So the data sheet also says 2.7 Watts out.

If the output transformer has a 0.5 dB loss, 2.7 Watts becomes 2.4 Watts. If the 7,000 Ohm primary DCR is 350 Ohms, and the 4 Ohm secondary is 0.2 Ohm, the transformer will loose 1 dB. 2.7 Watts becomes 2.14 Watts.

You will need negative feedback, or else the damping factor will be 7000/60,000 = 0.117,
Yes less than unity. And without negative feedback, the inductance of the output transformer will cause there to be no bass frequencies at the output.

Since the 47 is a directly heated filament/'cathode' you have to use DC filaments (or all you hear in headphones is hum). You can use the 47, but it takes some work.

Last edited by 6A3sUMMER; 20th May 2019 at 10:21 PM.
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Old 21st May 2019, 04:24 AM   #5
Gungnir is offline Gungnir  United States
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Join Date: Oct 2013
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Quote:
Originally Posted by 6A3sUMMER View Post
Oh, I like the flea power SET 12BH7 amp I saw before. The 47 amp has a lot more complexity in order to work:

The 47 has plate resistance, rp, 60k; and 2,500 micromhos transconductance.

60k x 0.025A = 150. But in that case, the only Load is the tube's own plate resistance, rp.

The Gain in pentode mode is transconductance x load impedance. You have to have a real load if you are going to get power out of the amp.

The 47 data sheet also says 7000 Ohm load; 250V Plate and Screen, bias -16.5V.

2,500 micromhos is 2.5mA/V 0.0025A/V

0.0025 x 7000 = 17.5 (Gain)

The 47 Gain is 17.5, not 150.

17.5 x -16.5V = -288.75V plate swing, and+288.75V plate swing into 7000 Ohms. But the plate voltage is at 250V, so it can not swing down by 288.75V. So the data sheet also says 2.7 Watts out.

If the output transformer has a 0.5 dB loss, 2.7 Watts becomes 2.4 Watts. If the 7,000 Ohm primary DCR is 350 Ohms, and the 4 Ohm secondary is 0.2 Ohm, the transformer will loose 1 dB. 2.7 Watts becomes 2.14 Watts.

You will need negative feedback, or else the damping factor will be 7000/60,000 = 0.117,
Yes less than unity. And without negative feedback, the inductance of the output transformer will cause there to be no bass frequencies at the output.

Since the 47 is a directly heated filament/'cathode' you have to use DC filaments (or all you hear in headphones is hum). You can use the 47, but it takes some work.
6A3summer, thank you for your help and advice,

I need to do some research on negative feedback so I can determine how to implement it. How do you feel about ultralinear mode?

I am really excited to build this amp as something different to my usual simple SET amps. I like the idea of using a directly heated pentode such as the 47 because prices of 45 (my favorite output tube) are just getting out of hand.
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Old 9th November 2019, 03:18 PM   #6
Gungnir is offline Gungnir  United States
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Default Best amp I have made!

Amp sounds sublime through headphones and speakers. Dead quiet through headphones. Driver tube is 6c6. Ultralinear mode sounds the best to me!
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Old 9th November 2019, 04:39 PM   #7
lexx21 is offline lexx21  United States
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47 Single Ended Pentode Amp/Headphone Amp
Would you be open to posting the schematic? That is a nice looking build.
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Old 10th November 2019, 08:13 AM   #8
6A3sUMMER is offline 6A3sUMMER  United States
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Gungnir,

Wow!
Looks beautiful.
It sounds like your research and hard work has given you something
great to listen too.

I "second" the motion for you to post a schematic.
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