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Built my first 300B amp - have lots of questions!
Built my first 300B amp - have lots of questions!
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Old 12th February 2019, 10:46 PM   #11
jmpsmash is offline jmpsmash
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some manufacturers like Monolith claims a really wide frequency range, from single digit to 200+kHz. is that trustable? others like the Japanese ones tends to be in the 20Hz up.

How would you describe the sound of interstage compared to coupling caps?

the supply for the 6C45P is at 400V right now. Eventually I will modifying the amp to go interstage and fixed bias, but with 400V and -70V or so at the grid that's 470V DC between the windings.

I think the non-bifilar ones can take that DC voltage, but the bifilar ones tends to be lower at 375VDC.

is that the distinction? but in general ppl seem to recommend bifilar in terms of sound.
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Old 12th February 2019, 10:50 PM   #12
jmpsmash is offline jmpsmash
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oh wait. but is having transformer coupled means that the output voltage swing is higher than the supply (like in output transformer), so I don't need such high supply voltage?
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Old 12th February 2019, 11:13 PM   #13
kevinkr is offline kevinkr  United States
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Cascode CCS and LED based bias on the cathode is my current recipe. Works quite well.

I'd be surprised if you can get more than 80Vpp out of a 6C45 which is good for a 45 or 2A3 but short of what's needed for a 300B. The D3A in triode connection will do >180Vpp comfortably on 150V plate voltage with CCS (250 - 300V supply needed) or choke loading.

The quality of the cathode bypass capacitor on the 300B is pretty critical. I use fixed bias with typically about -70 to -80V on grid depending on the plate supply voltage - eliminates that bypass capacitor.

The D3A, 6C45 and other high transconductance types are very prone to oscillation and need careful layout. (short wiring, solid grounding at RF, ferrite beads, etc.)
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Old 12th February 2019, 11:15 PM   #14
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by jmpsmash View Post
oh wait. but is having transformer coupled means that the output voltage swing is higher than the supply (like in output transformer), so I don't need such high supply voltage?
I use MM amorphous core interstages in some of my amplifiers, they work very well. With triode connected D3A, 180V supply is more than sufficient.
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Old 12th February 2019, 11:15 PM   #15
RajkoM is offline RajkoM  Bosnia and Herzegovina
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Long before Damir had tested a lot of drivers for the 300B SET in another forum. These are his notes on the 6C45Pi driver:

AudioRoundTable.com: Group Build >> 300B SE Project, Part 7 - Common cathode 6C45Pi triode driver

You would also like to read about other drivers, especially C3g

Of course, you can fully believe what Damir wrote-

Best diying

Last edited by RajkoM; 12th February 2019 at 11:18 PM.
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Old 12th February 2019, 11:34 PM   #16
musical noise is offline musical noise  Switzerland
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Quote:
Originally Posted by jmpsmash View Post

the supply for the 6C45P is at 400V right now. Eventually I will modifying the amp to go interstage and fixed bias, but with 400V and -70V or so at the grid that's 470V DC between the windings.

I think the non-bifilar ones can take that DC voltage, but the bifilar ones tends to be lower at 375VDC.

is that the distinction? but in general ppl seem to recommend bifilar in terms of sound.

Your current operating point for the driver tube as calculated from the schematic you provided puts the plate to cathode voltage at around 165V or at about 168V plate-to-ground. IOW, there's 232 Volts dropped through the load resistor. The IT being a coil, presents a high impedance at AC while having very low DCR it drops only a handful of volts across its primary.

If you were to use an interstage in place of the RC coupling scheme while using the same operating point, you will need to drop the B+ voltage entering the IT down to around 168V ground referenced. That's because the IT presents a load that's close to a horizontal line and is also able to swing twice the B+ voltage. So, in this scenario the IT primary sees 168V while the secondary is grounded (assuming you retain the cathode bias scheme) and you will be well within the IT's voltage limits.

If you go fixed bias, then the -70V is added to the 168V for a total of ~240V across the windings - still well within margin.


PS. to drop to 168V from the B+ of 435V you will need a dropping resistor (R5) of 22.9KOhm. This is based on the calculated current through the tube of 11.8mA. Of course your measurement of this current will better determine the resistor to be used. Keep in mind that this tube exhibits high variability in its parameters between different samples, so unless these tubes are well matched, you might find that your measurements between the two drivers might differ quite a bit.

Last edited by musical noise; 12th February 2019 at 11:48 PM.
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Old 12th February 2019, 11:51 PM   #17
rongon is offline rongon  United States
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It's generally easier to achieve success with more parts doing less work than with less parts doing more work.

I was thinking it would be easier to achieve success with a 3-stage design than with a 2-stage.

Perhaps 6J5 input tube driving a smaller low-mu triode like 4P1L, 12B4, or triode-strapped EL86, driving the 300B?

If the 2nd stage had a plate choke and was DC-coupled to the 300B, that could help minimize blocking and grid current effects, etc.

Just an idea...
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Old 13th February 2019, 12:42 AM   #18
Hearinspace is offline Hearinspace  Canada
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With those 300Bs THE very first thing I'd do is go to a soft start dc filament supply like Rod Coleman's kits After that, Bob's your uncle.
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Old 13th February 2019, 02:20 AM   #19
kevinkr is offline kevinkr  United States
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I second that recommendation, better sound and the CCS limits inrush current to the operating value which is much kinder to the filaments.
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Old 13th February 2019, 04:51 AM   #20
grovergardner is offline grovergardner  United States
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Before you do anything else try replacing the 22uF Obbligatos with some standard 47uF electrolytics in series. Or even some cheap Solens--anything but those Obbligatos. I tried Obbligatos and found them unbearable in the highs. I was swapping cables, preamp, tubes, everything. Almost tore the whole project apart before I figured out they were the problem. Remove the 6.8uF Obbligato bypass cap as well.

For the external power supply, I'd lose the 1000uF. 100 is plenty. Heck, 47 is plenty. You might improve the bass that way. Also, are you really using a 200 ohm balance pot on the 300Bs? That's too high. 25-50 ohms is plenty.

In the end you might want to try a different driver tube. The Loesch Legacy 300B using an EL84 pentode driver would be an easy swap and it's very nice.

Nice looking amps, BTW!

Last edited by grovergardner; 13th February 2019 at 05:07 AM.
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