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!
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.
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.
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.
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.
great info! More stuff to play with.
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?
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. ;-)
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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.
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.
(Thanks to the mod who has been merging and cleaning up my replies. I will try to behave. )
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)
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.
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?
)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)
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.
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.
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.
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.
Hmm, thats enough to drive something like a GM70 or 845 to about 20 watts or more? What is the mu of D3a in triode? I like the 6e5p in triode because its very nuetral but doesnt have that kind of gain.
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As I've never built a 300B amp my input is not based on direct experience but from some listening to other DIY amps and reading etc.
I think as your first amp it looks great, really professional looking. I'm not sure I would recommend jumping off into fancy capacitors, different coupling techniques etc. There's no reason why this amp should not sound wonderful. And 90dB speakers are plenty fine in my opinion. Do you think you would learn some valuable insights of your own by sticking with this design and tweaking it to get the sound you like and to notice how the sound depends on the tweaks you make ???
Some suggestions:
Some speakers can be the problem - piercing highs and so it's not always the amplifier that's the issue. For 'bright' or 'forward' speakers you can sometimes pair them with amplifiers that reduce this but it never quite fixes it. I assume that you have been happy with your speakers otherwise.
The 6C45P is a wonderful tube but I've read varying reports on the sound and some of them point towards a steely sound. I don't know if oscillations is always the cause, some people say high Gm tubes can be this way (high gm also makes them more susceptible to oscillations) and some folk suggest that reducing the idle current on this tube sweetens up the sound somewhat (it will lower the gm). Why not try increasing R3 ?
Many modern sources provide a strong signal so unless you know your source has a lowish output you may be fine with the gain you have for the time being. Nevertheless, more gain could be pleasing. There are many ways to achieve that but why not get this version of the amp sounding right before jumping into the dark again ?
Output transformers are important - maybe I missed it, but which one do you have ? - large transformers tend to have better bass.
Power supply is very important - needs enough capacitance.
Tweak idea - might be interesting to try the so-called WE connection for C3. Instead of connecting the lower end of C3 to ground, find a high voltage capacitor that will then allow you to take the lower end of it up to the B+ supply. Remember to use one with sufficient voltage rating and connect it the right way around.
I think as your first amp it looks great, really professional looking. I'm not sure I would recommend jumping off into fancy capacitors, different coupling techniques etc. There's no reason why this amp should not sound wonderful. And 90dB speakers are plenty fine in my opinion. Do you think you would learn some valuable insights of your own by sticking with this design and tweaking it to get the sound you like and to notice how the sound depends on the tweaks you make ???
Some suggestions:
Some speakers can be the problem - piercing highs and so it's not always the amplifier that's the issue. For 'bright' or 'forward' speakers you can sometimes pair them with amplifiers that reduce this but it never quite fixes it. I assume that you have been happy with your speakers otherwise.
The 6C45P is a wonderful tube but I've read varying reports on the sound and some of them point towards a steely sound. I don't know if oscillations is always the cause, some people say high Gm tubes can be this way (high gm also makes them more susceptible to oscillations) and some folk suggest that reducing the idle current on this tube sweetens up the sound somewhat (it will lower the gm). Why not try increasing R3 ?
Many modern sources provide a strong signal so unless you know your source has a lowish output you may be fine with the gain you have for the time being. Nevertheless, more gain could be pleasing. There are many ways to achieve that but why not get this version of the amp sounding right before jumping into the dark again ?
Output transformers are important - maybe I missed it, but which one do you have ? - large transformers tend to have better bass.
Power supply is very important - needs enough capacitance.
Tweak idea - might be interesting to try the so-called WE connection for C3. Instead of connecting the lower end of C3 to ground, find a high voltage capacitor that will then allow you to take the lower end of it up to the B+ supply. Remember to use one with sufficient voltage rating and connect it the right way around.
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