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A Loftin-White style SE with EL34
A Loftin-White style SE with EL34
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Old 22nd January 2019, 03:25 AM   #1
BJosephs is offline BJosephs  United States
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Default A Loftin-White style SE with EL34

I'm investigating the possibility of doing a LW style amp with an EL34 for the output stage. I have a pair of Transcendar 5k:8/4 7W OTs that are somewhat under utilized in their current project as well as an Antek AS-1T200 for power. The OT's can only handle 75mA, and 5K is a bit high for the 375v voltage in most datasheets (For SE triode), but going up to 400v looks like a good fit at 5k for about 7W. With the primaries in series the Antek should do about 565 @ 150mA. I'm thinking I drop about 140v across the cathode circuit to power a single triode stage (either ECC99 or E88CC) with a choke load. TubeCad has an article I pillaged all the basics from:

Loftin-White Amplifier

Attached is a screen shot of my current plan. About 60ish mA of bias current (max), a 3k cathode resistor (Dissipates a hefty 6W) and a 1k 2W bias pot (currently set to 350 ohms in the screen shot). The idea being that the voltage drop across the pot (trim-able) plus the drop across the DCR of the choke (fixed with bias current of the preamp stage) sets the -Vgk at the grid. The EL34 cathode current minus whatever gets diverted for the preamp stage sets the drop across the 3K, which becomes the preamp stage's supply.

I'm still shopping for a choke. I suspect I'll end up with a Lundahl from the LL1668 series. There's an outside chance I try a CCS version but for now I like the idea of trying a choke.

I didn't start out planning a DC coupled amp, it's just that the parts I have seem to work out just right. Thoughts?
Attached Images
File Type: png Screen Shot 2019-01-21 at 10.53.52 PM.png (73.2 KB, 730 views)
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Old 22nd January 2019, 04:24 AM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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The idea looks good.

I will not take the time to check your DC levels.
That circuit should work if you get the DC right.
. . . But I did notice one thing:

You put R13 on the wrong side of R10. Grid stopper resistors belong connected directly to the grid tab of the tube socket, with Nothing else connected to the grid tab (otherwise the grid stopper can not do its work).
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Old 22nd January 2019, 10:16 AM   #3
BJosephs is offline BJosephs  United States
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Fixed, thanks
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Old 22nd January 2019, 10:30 AM   #4
BJosephs is offline BJosephs  United States
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Here are some further insights from simulation.

One thing that worried me was burning up the potentiometer under some combinations of settings. While a pot that can handle some power is necessary it looks like a 2W should be OK. The first two attachments show dissipation when the pot is varied between 100 and 900 ohms with 0, 680, and 1500 ohms of choke DCR. The DCR matters because the choke will drop some of the bias voltage - a magical 0 ohm choke would put the greatest burden on the potentiometer.

The 4th screen shot is anode current over the same spread of values assuming a 680 ohm choke. The range looks good.

Last are the DC levels at the grid and cathode along with Anode current.
Attached Images
File Type: png Potentiometer dissipation with zero choke DCR.png (80.6 KB, 701 views)
File Type: png Potentiometer dissipation with modest choke DCR.png (87.1 KB, 631 views)
File Type: png Potentiometer dissipation with high choke DCR.png (77.3 KB, 609 views)
File Type: png Anode Current.png (154.9 KB, 601 views)
File Type: png DC Levels.png (133.1 KB, 64 views)
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Old 22nd January 2019, 11:38 PM   #5
kokoriantz is online now kokoriantz  Lebanon
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This is not Loftin& White . You have invented a new circuit . I love it .
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Old 22nd January 2019, 11:57 PM   #6
ArcticBrew is online now ArcticBrew  United States
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It looks like the first "Loftin-White Amplifier with Choke Load" schematic in the tubecad link.

Steve
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Old 23rd January 2019, 12:16 AM   #7
BJosephs is offline BJosephs  United States
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Quote:
Originally Posted by kokoriantz View Post
This is not Loftin& White . You have invented a new circuit . I love it .
I wish! Check out the TubeCad link and read up on improvements to the LW topology.
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Old 28th January 2019, 01:35 AM   #8
BJosephs is offline BJosephs  United States
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Just a quick check in: I wired up my PT as a test, and I’m glad I did because with no load my B+ came in a bit lower than expected. Under load it will be even lower than that so I think I’ll be starting from about 520v rather than 560. I’ve ordered a few different sizes of cathode resistor so I can tune a bit. I also placed an order for 100H inductors. The dcr is a bit higher than I’d hoped but it really only matters in the preamp tube fails or is removed and I expect it would only change the bias by about 20%. I’m going to try and get a prototype wired up (sans chokes) in the next week or so.
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Old 28th January 2019, 08:45 AM   #9
6A3sUMMER is offline 6A3sUMMER  United States
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The "new" amplifier configuration in post #1 is very close to the Electra Print "DRD" amplifiers.
Electra Print gives credit to Loftin-White amplifier circuit, but Electra Print adds a 100H choke as the input tube plate load.
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Old 28th January 2019, 10:30 AM   #10
BJosephs is offline BJosephs  United States
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Electraprint’s schematic is dated 2007. There is a TubeCad article linked inside of the one I linked, published in May 2000, that shows this configuration. Regardless, it seems unlikely that no one dropped a choke in a LW amp before either of them.

The Tube Cad Journal: Design Idea-A safe Loftin-White amplifier

Last edited by BJosephs; 28th January 2019 at 10:58 AM.
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