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cap coupling to interstage transformer or direct coupling?
cap coupling to interstage transformer or direct coupling?
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Old 17th May 2014, 09:40 PM   #1
btran is offline btran  United States
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Default cap coupling to interstage transformer or direct coupling?

Hi all,

I have built a pair mono block 45 SE amp with cap coupling. The circuit is base on 2A3 Ranking Gordon SE amp. It sounds good and I just wonder there is way to improve the sound by mod it from cap coupling to IT or direct coupling? Thanking in advance for your input!

Below is the schematic of the amp that I build
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Old 17th May 2014, 11:56 PM   #2
directdriver is offline directdriver  United States
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That's the Bugle circuit. Rankin also designed a direct coupled 45 circuit using 6BM8 as driver and current source published in Sound Practices that you can try. But the B+ needs to be raised to 450Vdc though. If you're using choke loaded power supply per Bugle schematic, changing it to cap loaded will increase the B+. The 6BM8 pentode section really is acting as a slow warm up before the triode section starts to draw current so the 45 grid is not slammed with high DC voltage. If you want to try octal tubes, the 6EM7 will give you similar result with minor tweaks.

I have built similar direct coupled 45 circuits before and it can sound really good, provided the output transformers are quality items.
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Last edited by directdriver; 18th May 2014 at 12:12 AM.
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Old 18th May 2014, 12:15 AM   #3
btran is offline btran  United States
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What's circuit that you have build on direct coupled 45, and the output transformer that you can recommend?
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Old 18th May 2014, 12:50 AM   #4
directdriver is offline directdriver  United States
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I am currently not using any SE amps. I have built 45 circuits using Tango XE-20S and many PP irons on the cheap such as irons from Sansui 1000A. The Tango is transparent on the top but bass shy. Heard Nature Sound transformers with permalloy core that sounded very natural. I used 6DL4 as driver direct coupled to 45 and sounded nice. Also 6SL7 is more lush. All of these projects were done many years ago. I am sure other members are more qualified to comment here.


PS, In reply, it's best not to quote the entire post to avoid redundant images. A little editing will work. Sorry, I am OCD like that.
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Old 23rd April 2018, 04:11 AM   #5
anystereo is offline anystereo  United States
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Has any one out there built the gordon rankin constant current 45 amp schematic posted here? I am gonna breadboard it and see how it goes, I have the iron and tubes and caps. Wondering how the 20 turn pot plays into the design. Is it used to tune into the 180v at the plate? The meter's range is noted to be 100 VDC. I don't get it.
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Old 23rd April 2018, 08:42 AM   #6
analog_sa is offline analog_sa  Europe
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cap coupling to interstage transformer or direct coupling?
The meter is measuring the drop across the 1k, for a current of 35mA. The 30uF cap quality is critical.
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Old 23rd April 2018, 09:02 AM   #7
artosalo is offline artosalo  Finland
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Quote:
It sounds good and I just wonder there is way to improve the sound by mod it from cap coupling to IT or direct coupling?
With IT coupling the frequency response and transient response get worse and hum/interference pick-up will increase. But I don't know about the sound improvement.
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Old 23rd April 2018, 10:39 AM   #8
6A3sUMMER is offline 6A3sUMMER  United States
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The easiest way to simplify the circuit in post #1 is to remove the top triode section of the 12AT7 with an IXYS 900V or 450V current source. And a stereo amp will need only one 12AT7, and you do not have to worry about the filament to cathode voltage rating of the (now removed) top 12AT7. I expect the sound quality to be very close to each other between the 12AT7 top triode circuit, and IXYS circuit.

As to the Pentode constant current circuit: The 180V is approximate. Change the 45, and it will be different. Change the B+, and it will be different. As was stated in post #6, adjust the potentiometer for 35V across the 1k resistor, the 45 will have 35mA.

Actually, one of the 45 data sheets suggested operating points is 36mA, and 275V plate to filament. It is the highest power out in the data sheet, but I do not remember the required primary impedance of the output transformer, I think it is 4600 Ohms. That should give 2 Watts (if the output transformer is low loss).

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Old 23rd April 2018, 11:36 AM   #9
claudiomas is offline claudiomas  France
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Quote:
Originally Posted by artosalo View Post
With IT coupling the frequency response and transient response get worse and hum/interference pick-up will increase. But I don't know about the sound improvement.
with IT coupling the frequency response and transient response improve and hum/interference pick-up decrease the sound improvement its great DHT triodes was made to work with transformers
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Old 23rd April 2018, 12:09 PM   #10
artosalo is offline artosalo  Finland
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Show us a transformer that has better frequency- and transient response than a piece of wire.
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