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Single ended 45 constant current and bias voltage adjustments-help please
Single ended 45 constant current and bias voltage adjustments-help please
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Old 7th August 2019, 04:02 AM   #11
6A3sUMMER is offline 6A3sUMMER  United States
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Join Date: Jun 2016
kmaier,

I would not expect you to know this: If I remember kkstereo's earlier thread sometime ago, he is using the 5k winding of a 3.5 / 5k James transformer. the '3.' of the 3.5k is crossed out in the schematic. As he said, the circuit was originally a 2A3 amp.

I am glad for all those like you, others, and myself who have designed, built, and enjoyed listening to 45 amplifiers.

It is one thing to go to someone's house and listen, it is another to experience it in your home or at work (when everybody has already gone home).

Continued Happy 45 listening.

kkstereo,

Find the average voltage of your mains. Check weekdays, nights, and day and night on weekends. Then when you test changes to the amp, do that at the average mains voltage you found. You should be able to get a little closer to the plate to filament voltage you want, and the current that you want to run. You will have to adjust the 200 Ohm resistor individually (differently) for the two 45 tubes. Only a very large population of tubes allows you to match tubes.

You can see from kmaier's post that you could use a little more than 275V plate to filament, but at a little less current (he had 295V and 34mA). But he had to use a 1740 Ohm bias resistor (you can use a 120 or 150 Ohm resistor in series with the 1600 Ohm you have).

Happy listening, as I expect you already are.
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Old 7th August 2019, 04:17 AM   #12
kkstereo is offline kkstereo  United States
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Single ended 45 constant current and bias voltage adjustments-help please
6A3Summer, you was right after voltage go up it reading 52.5V & 54V on bias, so I will work with the resistors to get close to 275V plate to filament, 36mA, and 56V bias.

kmaier, which ac coupling value you recommend for the stages? . Speakers are 100-103db low freq to 30hz , should I consider NFB on this amp ?

I have a pair Cunningham 345 Globe that I plan use after all test. Now see that I will need get the adjustments with them from now. The max voltage are same ?
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Old 7th August 2019, 04:38 AM   #13
6A3sUMMER is offline 6A3sUMMER  United States
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kmaier,

Would you agree that reducing the 1.0 uF coupling cap that drives the 240k 45 grid resistor would be the first place to start adjusting the low frequency response.

It should also speed up the recovery time of any clipping due to drawing grid current.

kkstereo,

I would not run those old Cunningham 345 Globe tubes at 300V.

Sometimes even a shoulder 45 will bite the dust. I had a stereo pair operate for a wonderful listening session the night I completed the amp. The next morning, one of the tubes went gassy, glowed, and oscillated (relaxation oscillation, no feedback in the amp).

I had no other 45s, and only 2 days from a show. So I tore apart the amp circuitry, and built a 4-65 amp on the chassis before the show.
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Old 7th August 2019, 05:51 AM   #14
kmaier is offline kmaier  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
kmaier,

Would you agree that reducing the 1.0 uF coupling cap that drives the 240k
45 grid resistor would be the first place to start adjusting the low frequency response.

It should also speed up the recovery time of any clipping due to drawing grid current.
I took an approach to limit low frequency response starting at the first stage. My input/driver stage has an overall voltage gain of ~43.5 dB (150 V/V). The 45 grid requires twice the grid bias (59*2=118) for full drive (class A). Needless to say, allowing low frequencies to get into the first stage can also cause a bit of havoc as you go through.

So, my input stage uses a 3.48K bias resistor with a 6.8uF film cap bypass. This equates to a -3dB point of approximately 6.75 Hz. The driver stage has a 33.2K resistor (direct coupled from the input stage), so you want this to be a pretty low frequency. I use a 10.0uF film bypass. This equates to a -3dB point of 0.48 Hz.

The 45 output stage uses a 249K grid resistor with a 0.22uF coupling cap from the driver stage. This equates to a -3dB point at 2.9 Hz. Finally, the 45 uses a 1.74K resistor with a 24uF film bypass capacitor, which equates to a -3dB point at 3.8 Hz. Overall measurements of the amp at 1-watt shows a -3dB output at 25 Hz. With a driver stage bias of 2.8ma, frequency response is within 1dB out to 50KHz.

Note that the cutoff frequencies are not grouped together, which helps minimize phase problems as you go down towards the lower response of the amplifier. Hope this helps clarify what I'm doing and why.

Also, Gordon at Wavelength Audio authored an excellent article on a 45 SET design many moons ago... had a lot of good logic and design tips complete with calculations for slew rate, frequency response, etc. The amplifier was called a Bugle 45 and is a good read. You can get the PDF here:

http://www.wavelengthaudio.com/bugle.pdf

Note on OPT being used, i.e., the James transformer. I also have a pair of S-6113S.... nice iron for the price.

Regards, KM

Quote:
Originally Posted by kkstereo View Post
6A3Summer, you was right after voltage go up it reading 52.5V & 54V on bias, so I will work with the resistors to get close to 275V plate to filament, 36mA, and 56V bias.

kmaier, which ac coupling value you recommend for the stages? . Speakers are 100-103db low freq to 30hz , should I consider NFB on this amp ?

I have a pair Cunningham 345 Globe that I plan use after all test. Now see that I will need get the adjustments with them from now. The max voltage are same ?
The globe 45s (includes 245 and 345 tubes) are the original design and I wouldn't push them much beyond the 275 voltage rating and keep them below 8 watts dissipation. The early ones can be fragile... remember, those tubes were made almost a century ago. The newer ST-style 45s are more robust in construction and will typically be more reliable over time.

As for NFB, it's unclear what type of load your speakers present to the amplifier. High sensitivity is good, but what does the impedance curve look like? In general, a full-range driver (sans crossover) is usually pretty easy to drive, so you shouldn't need any NFB. But again, it really depends on what the impedance curve of the speakers are.

See my previous post for coupling and bypass frequencies. Hope this helps.

Regards, KM
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Old 7th August 2019, 09:59 PM   #15
kkstereo is offline kkstereo  United States
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Single ended 45 constant current and bias voltage adjustments-help please
2A3Summer,
I updated today the diaphragm (attached now) cause the 1.0uf/400v coupling cap was changed weeks ago to 0.1uf/400v after recommended in last thread, about the opt yes are hooked into 5k tap from Tango FW-20S (no James trans).Also the 220 ohms from B+ was installed yesterday due the original 150 ohms reached 340V B+ yesterday when wall voltage rise to 126V from wall.

About the old Cunningham 345 Globe tubes, I decided not used in this circuit due the risk. I prefer try keep the 45's to max 300V, now is sounding powerful even low frequencies.

Kmaier, about the NFB , is just curiosity how compare with and without it, I found having to much gain for my room right now , I didn't found speakers impedance curve - only found 8 ohms @ 400hz.
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Old 7th August 2019, 10:23 PM   #16
kkstereo is offline kkstereo  United States
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Single ended 45 constant current and bias voltage adjustments-help please
2A3Summer & Kmaier,
I need revise all your recommendations and numbers VERY SLOWOOLY ! in the way that I can get the idea on what me to do with that on my amp. 2A3Summer already know that took me days to digest the recommendations from last thread (I'm slow & low technically).
Thanks.

after tried a weak 45 tube that still had, it match better, so returned the 150 ohms B+ resistors, now having 52.6v and 52.1v on bias voltages.
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Old 8th August 2019, 03:29 AM   #17
kkstereo is offline kkstereo  United States
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Single ended 45 constant current and bias voltage adjustments-help please
so, 52.6 /1600 = 32.8mA bias
52.1 /1600 = 32.5 mA bias
269.4 x 32.8 = 8.836w dissipation
269.9 x 32.5 = 8.771w dissipation
Thanks 6a3Summer, I can do now the calculation, and the 45 is in good range now.
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Old 14th August 2019, 01:07 AM   #18
kkstereo is offline kkstereo  United States
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Single ended 45 constant current and bias voltage adjustments-help please
6A3sUMMER & Kmaier, the 150 ohms resistors in series with the 1.6k installed, and the magic of the 45s show up , it is just an involving detailed sound that I can not turn it off .

THANKSSSSS!
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