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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 13th February 2019, 05:25 AM   #21
jmpsmash is offline jmpsmash
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Join Date: Aug 2002
Location: SF Bay Area, US
Quote:
Originally Posted by Hearinspace View Post
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.
Quote:
Originally Posted by kevinkr View Post
I second that recommendation, better sound and the CCS limits inrush current to the operating value which is much kinder to the filaments.
Oh. I looked at the Rod Coleman's kit. Looks really nice. Right now I have a 5VAC transformer for each of the 300B. I will have to throw them away though. Need to think about it really hard first!

Quote:
Originally Posted by grovergardner View Post
Before you do anything else try replacing the 22uF Obbligatos with some standard 47uF electrolytics in series. <snip>
Thanks! And thanks for that really useful info. If you didn't tell me, changing the Obbligatos would be the last thing I'd try! I got those Obbligatos coz someone was getting rid of them, practically unused, at huge discount. I thought I found a good deal. ah well. Let me try swapping in some Mundorf Myltic as I have some lying around.

I just checked, those are 100R pots, not 200R as I have on the schematic. I have problem finding them at 2W ratings. I found them in Amazon actually and they are the smallest value. Where would be a good source?

As for different driver tubes, I do plan to keep trying different drivers and different designs. I am really enjoying this build and have learned a bunch already. I want to gain even more experience as I go.

Quote:
Originally Posted by kevinkr View Post
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.
I was afraid you would say that. They are very pricey. Might have to give Yves a mail. Which ones have you used? ITA-01?

If you take a look, I have left a space at the chassis for the eventual IT. Well... eventually.

Quote:
Originally Posted by kevinkr View Post
Cascode CCS and LED based bias on the cathode is my current recipe. Works quite well. <snip>
I briefly played around with LED bias. I had started the project with the popular dual stage DC coupled 6SN7 topology and used LED biased for the first stage. It worked but when I was probing around the scope showed some very noisy voltage oscillation at the LED anode. So i went back to resistor only bias (I didn't care for the extra gain as the 2x stage already have way too much gain). I might play around with it again, maybe with different brand's LEDs.

Right now, I will fix the shouty high first and then will try CCS next.

Quote:
Originally Posted by rongon View Post
It's generally easier to achieve success with more parts doing less work than with less parts doing more work. <snip>
How does DC coupling work here? It will need to be negative DC, no?

I had plan to try some form of 6L6 tube 2nd stage driving 300B. Too many things to play with.

Quote:
Originally Posted by RajkoM View Post
Long before Damir had tested a lot of drivers for the 300B SET in another forum. <snip>
great info! More stuff to play with.

Quote:
Originally Posted by musical noise View Post
Your current operating point for the driver tube as calculated from the schematic you provided <snip>
yes. that makes a lot of sense. really appreciate the explanation.

I always wonder about the use of a dropping resistors. Isn't there also some sort of feedback like an unbypassed cathode resistor? As the signal through the tube stage go up and down it draws varying current through the dropping resistor like a RC power supply ripple. And the larger the R and larger the ripple. Am I missing something?
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Old 13th February 2019, 06:35 AM   #22
grovergardner is offline grovergardner  United States
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Join Date: Apr 2010
Quote:
Originally Posted by jmpsmash View Post
Thanks! And thanks for that really useful info. If you didn't tell me, changing the Obbligatos would be the last thing I'd try! <snip>
Yes, try the Mundorfs instead of the Obbligatos. Better yet, try some Cornell-Dubiliers. ;-) I'll be curious to know if the Obbligatos were part of the problem. I was really shocked at how bad they sounded.

The 100 ohm pots are okay, but that's still a lot for a balance pot. I use these and grind a slot in the shaft with a Dremel for a screwdriver--or you can add a knob: 5 X Potentiometers - 25 Ohm - 5 Watt Wirewound Linear Potentiometers | eBay

I've never tried a CCS on the filaments, I've always preferred AC. Check the voltage--if it's a little high you can add a small resistor in series which will help tame the turn-on current. But maybe the boards would work well, and those are pretty pricey 300Bs. ;-)

Last edited by grovergardner; 13th February 2019 at 06:38 AM.
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Old 13th February 2019, 08:15 AM   #23
schiirrn is offline schiirrn  Germany
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Join Date: Nov 2018
Quote:
Originally Posted by jmpsmash View Post
As the signal through the tube stage go up and down it draws varying current through the dropping resistor like a RC power supply ripple. And the larger the R and larger the ripple. Am I missing something?
What resistor do you mean? The anode load resistor? The effect you describe is what makes a tube amplify signal. Small swing in larger swing out.

Quote:
Originally Posted by jmpsmash View Post
I briefly played around with LED bias. I had started the project with the popular dual stage DC coupled 6SN7 topology and used LED biased for the first stage. It worked but when I was probing around the scope showed some very noisy voltage oscillation at the LED anode. So i went back to resistor only bias (I didn't care for the extra gain as the 2x stage already have way too much gain). I might play around with it again, maybe with different brand's LEDs.
Was the oscillation gone when you switched back to a resistor?
Different brands are irrelevant. Check the curves of the LED in the datasheet and choose ones that are in their linear region with the current that will pass through them.
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Old 14th February 2019, 12:04 AM   #24
jmpsmash is offline jmpsmash
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Join Date: Aug 2002
Location: SF Bay Area, US
(Thanks to the mod who has been merging and cleaning up my replies. I will try to behave. )

Quote:
Originally Posted by schiirrn View Post
What resistor do you mean? The anode load resistor? The effect you describe is what makes a tube amplify signal. Small swing in larger swing out.
I mean the drop resistor. In the same principle, the varying current through the anode resistor creates voltage swing, the same current is also going through the drop resistor, that should produce voltage fluctuation too. (with the capacitor buffering the current a little)

Quote:
Was the oscillation gone when you switched back to a resistor?
Different brands are irrelevant. Check the curves of the LED in the datasheet and choose ones that are in their linear region with the current that will pass through them.
yes, the voltage at the top of the cathode resistor is stable and clean at quiescent. but when I swap in the LED, it is noisy. the LED is a cheapo one that I got lying around. Maybe it was over it's current capacity? there is no datasheet unfortunately. I will revisit it in the future with a LED with known characteristics.

Quote:
Originally Posted by grovergardner View Post
Yes, try the Mundorfs instead of the Obbligatos. Better yet, try some Cornell-Dubiliers. ;-) I'll be curious to know if the Obbligatos were part of the problem. I was really shocked at how bad they sounded.
I will try that this evening. Yesterday I swapped the 330 Ohm grid blocking with 1k and it tamed the high a little, not as piercing but still there. I will report back on how it sound with the Obbligatos swapped out. What's with the Cornell-Dubiliers? are they very good?
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Old 14th February 2019, 01:54 AM   #25
grovergardner is offline grovergardner  United States
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Join Date: Apr 2010
Quote:
Originally Posted by jmpsmash View Post
I will try that this evening. Yesterday I swapped the 330 Ohm grid blocking with 1k and it tamed the high a little, not as piercing but still there. I will report back on how it sound with the Obbligatos swapped out. What's with the Cornell-Dubiliers? are they very good?
They're just affordable well-made capacitors. You can get them from Mouser or Digikey. The 381LQ series is very good for power supplies.
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Old 20th February 2019, 04:58 PM   #26
jmpsmash is offline jmpsmash
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Join Date: Aug 2002
Location: SF Bay Area, US
Some updates in my attempt to make the amp less shouty at the highs.

I have made a few changes. Firstly changed the 330R grid block resistor into the 6C45P. Very minor difference if any. And then I removed the Obbligato across the 6C45P and replaced the Obbligato across the 300B with Mundorf Mylitic 45uF. I can hear some small difference but not big.

Lastly I replaced all the resistors with Takman REYs. They are supposed to be a bit softer and I do hear noticable difference. However, the amp is still a bit aggressive. Perhaps the 6C45P is what it is. I will try to use something difference and see how it goes.
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Old 20th February 2019, 05:46 PM   #27
grovergardner is offline grovergardner  United States
diyAudio Member
 
Join Date: Apr 2010
Quote:
Originally Posted by jmpsmash View Post
Some updates in my attempt to make the amp less shouty at the highs.

I have made a few changes. Firstly changed the 330R grid block resistor into the 6C45P. Very minor difference if any. And then I removed the Obbligato across the 6C45P and replaced the Obbligato across the 300B with Mundorf Mylitic 45uF. I can hear some small difference but not big.

Lastly I replaced all the resistors with Takman REYs. They are supposed to be a bit softer and I do hear noticable difference. However, the amp is still a bit aggressive. Perhaps the 6C45P is what it is. I will try to use something difference and see how it goes.
Sounds like you've covered the bases and find the driver tube not to your taste. I've never cared for these high-gm triodes myself. There are lots of options, I think.
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