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300B PP Parallel amplifier
300B PP Parallel amplifier
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Old 3rd August 2018, 12:41 AM   #1
ArcheusLedoux is offline ArcheusLedoux  United States
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300B PP Parallel amplifier
Default 300B PP Parallel amplifier

A friend in Texas, "David" just finished a project I had asked for help with last year. A Parallel 300B PP amplifier, rated for 40 watts. The Amp was built with two amplifiers in mind, Namely the Samson PAM-29 and the Brooks 12A as the main references but with a few tweaks. I wanted the ability to have a transformer coupled circuit on board with a tube driver so you could switch back and forth. 7N7 Loctals were chosen as the drivers because of literature from the UTC engineers who preferred them over 6SN7.

Another point to make. The four octal sockets could be used for Beam Power tubes yes. But I knew that Emission Labs 300B-XLS Data sheet. made a version of the 300B tube with a center tapped heater. This "Special" 300B required an Octal socket. Emission Labs make some absolutely beautiful tubes. But they're a bit out of my budget. Especially needing a quad for just one channel.

Links to David's youtube videos
YouTube
YouTube
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Last edited by ArcheusLedoux; 3rd August 2018 at 05:16 AM.
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Old 3rd August 2018, 01:03 AM   #2
sumotan is offline sumotan  Indonesia
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Wow nice
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Old 3rd August 2018, 01:29 AM   #3
Alllensoncanon is offline Alllensoncanon  United States
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300B PP Parallel amplifier
Very Nice. Why just one (monophonic) ?
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Old 3rd August 2018, 01:53 AM   #4
azazello is offline azazello  Bulgaria
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IMO, SE 300B sounds better PPP 300B....
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Projects: SE GK71, SE 12BQ6, PP KT88, OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,...
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Old 3rd August 2018, 02:02 AM   #5
ArcheusLedoux is offline ArcheusLedoux  United States
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300B PP Parallel amplifier
Quote:
Originally Posted by Alllensoncanon View Post
Very Nice. Why just one (monophonic) ?
Limited parts. I would like to build a second once some other iron is found.
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Old 3rd August 2018, 02:09 AM   #6
TonyTecson is offline TonyTecson  Philippines
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300B PP Parallel amplifier
depends on the speakers you are driving the amp with...
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Old 4th August 2018, 04:24 AM   #7
ArcheusLedoux is offline ArcheusLedoux  United States
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300B PP Parallel amplifier
Here are the Brook 12A and PAM 29
One thing I've never been able to reconcile is that the Audio choke in the PAM29 circuit is on the Output tube grid side.
On the Brook 12A the audio choke is on the driver tube plate side.
Thoughts?
Attached Images
File Type: png pam29 - Copy.png (144.2 KB, 300 views)
File Type: png Brook12-A - Copy.png (235.2 KB, 303 views)

Last edited by ArcheusLedoux; 4th August 2018 at 04:47 AM.
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Old 5th August 2018, 09:22 AM   #8
45 is offline 45  Italy
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Quote:
Originally Posted by ArcheusLedoux View Post
Here are the Brook 12A and PAM 29
One thing I've never been able to reconcile is that the Audio choke in the PAM29 circuit is on the Output tube grid side.
On the Brook 12A the audio choke is on the driver tube plate side.
Thoughts?
The grid choke in theory would cause less bias shift (and so less distortion) when grid current kicks in. The anode choke allows you to get more headroom because you can have the same nominal plate load without the voltage drop of a resistor. However none of these chokes are cheap if their quality has to be good. So why don't get the best of both and simply use a decent interstage transformer? The Brook amplifier comes from an era when the typical bandwidth of interstage transformers was typically 100-10000 Hz! That's why the choke was a better option. A Lundahl LL1635 is not much expensive and has more than enough headroom for a 300B amp.

Same thing for the 300B's.

Quote:
Originally Posted by ArcheusLedoux View Post
Another point to make. The four octal sockets could be used for Beam Power tubes yes. But I knew that Emission Labs 300B-XLS Data sheet. made a version of the 300B tube with a center tapped heater. This "Special" 300B required an Octal socket. Emission Labs make some absolutely beautiful tubes. But they're a bit out of my budget. Especially needing a quad for just one channel.
Don't know if you have different options respect to me but Sophia E., if you buy directly from them grade A tubes, are expensive. They have the cheaper option with punched plate (that's no mesh by any means) at $425 a pair but these cannot take more than 26W for acceptable life service. For good life better to stay within 22-24W. The other options range from 600 to 1200 a pair! They are obviously made by Full Music.... Full Music made and branded aren't cheap either. Those sold by generic sellers who give no guarantee on the quality are 99% grade B weaker tubes. Not worth, IMHO.

EML XLS is rated 55W and at normal 300B bias point will last at least twice...possibly 3-4x respect to weaker Chinese types. In case don't waste money on the V4 version. Best performance comes with DC filament supply where the centre tap is useless.

If you really want to get good 300Bs and save money I would buy the JJ from a trusted seller. Even better the 2A3-40 because of the 2.5V filament.

Last edited by 45; 5th August 2018 at 09:36 AM.
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Old 5th August 2018, 09:32 AM   #9
TonyTecson is offline TonyTecson  Philippines
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300B PP Parallel amplifier
that is because the grid chokes can have much much lower dc resistance compared to grid leaks of say 100k..about 10x lesser or even better...
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Old 5th August 2018, 09:50 PM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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Grid chokes can have interleaved E's and I's. There is no DC present.

If you do not balance the two plate currents in a plate choke, it either has to have more laminations, or have an air gap (and more turns, or else it will have less inductance). Otherwise it will saturate too early. More turns might mean poorer high frequency response.
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