300b Tube Amp Tuning /Schematic Deep Dive!

This video is a follow up to my 300B series on further tuning this amp, a deep dive into the schematic, explaining heater DC offset and hopefully help you understand how to "voice" a tube amplifier. I'm trying to explain this in non-technical terms so people will be comfortable playing around with these.
I know people are wanting a BOM/final but be patient. It sounds good but there is more in this amp. If you want to build this, go ahead and order the transformers, the chassis and get to work on the layout. None of those are going to change. I would hold off on the 300B DC heaters till I am sure what simply using AC sounds like!
Sorry I had to reupload, there was a problem with the first version rendering.

[@speakersr-lyefaudio6830]

I’m 20 and make speakers. I’ve been wanting to get into the electronics side of things. I found you yesterday, and I think you will be a great resource for learning. Thanks for putting this stuff up.

[@nuttyl283]

I want to go study tube schematic with you. So fun to learn from you. You elaborate everything so clearly. Please teach us how to draw a load line to tube graph to know how much OPT we going to use. Thx

[@MyIronHammer]

I have been assembling tube amplifiers for ten years, and on your channel I could learn something new about them. Subscribed.

[@SkunkieDesignsElectronics]

Welcome aboard!

[@donlamontagne7606]

Thank You for the video replay. It is starting to make sense now, can't wait for the next video. Great Work!

[@SkunkieDesignsElectronics]

Glad to hear it! And thanks for catching that!

[@horaciopascoli9398]

Excelente explicación, con mucha claridad y de gran utilidad... Muchas gracias desde Argentina...

[@thanapoll.849]

This schematic can be applied to 45 SE too by Change to 5K OPT. Even 2 watts but sound is very very good. CLCLCRC B+ about 320VDC bias -60VDC run about 34mA on anode.

[@SkunkieDesignsElectronics]

I'm using a 5K OT because a) It's what I had and b) lower distortion so it's possible to change a few things and use different DHT.

[@Warclock07]

Thank you, waiting to see your next video trying AC heater. I know some used AC heater without hum.

[@SkunkieDesignsElectronics]

More to come! It just takes time and I have other projects brewing as well :)

[@danielthomas3057]

I enjoy the detailed explanation of the schematic. Not sure if you had viewed Mark Blueglow's 807 amp build but Cascode seems similar to the SRPP circuit used in that build. Interesting stuff!

[@SkunkieDesignsElectronics]

I'll check it out in more detail. Mark does great videos too!

[@Pootycat8359]

Here's some info regarding AC vs DC filament voltage. First, I've heard that AC can introduce hum. I don't know if this is a common problem, but if it occurs, a hum balance circuit could be used to null it out. But DC can cause a problem. I've read about this, regarding the RF probe employed by the HP-410 B&C. These probes employ a little vacuum tube diode. DC, apparently causes "electron migration" in the filament, which shortens tube life. The "B" model uses AC, and the probe is always turned on. The "C" uses DC, but is on only when it's in actual use, which reduces, but probably doesn't completely eliminate the problem.

[@SkunkieDesignsElectronics]

One thing I did on this amp was I wired the heaters on the 300B tubes opposites of each other, i.e. the positive and negative of the heaters are the reverse of each other and I swap the tubes every few months to help avoid the DC shortening the tube life. If I was building this over and was using a different power transformer where I needed to buy filament transformers for the 300B tubes (I end up replacing this Edcor after it died after a fairly short, buzzy life..) I would probably try AC heaters.

[@Pootycat8359]

@@SkunkieDesignsElectronics Yes. I think the best approach is to go with AC. If there IS hum, it would be easy to null out, by applying a tiny opposite phase 60Hz signal to one of the grids.

[@michaeldoenninghaus5192]

Thanks Steph, verry interest. I love your cannel.

[@AndrewBPowers]

Great video again. Thank you!

[@SkunkieDesignsElectronics]

Glad you enjoyed it!

[@toddbevan4414]

Great video. Thank you

[@scaryperson27]

Killer video. Thanks!

[@SkunkieDesignsElectronics]

Thanks, got one final one for this series.

[@oakmountainman7872]

I have always wanted a 300B amp that I could point to and say "I built that!". Do you think different 5v rectifier tubes would make a difference? No 5AR4 in my basement but I have a few 5R4G tubes available. Thank you for this series, your deep dives are very helpful.

[@SkunkieDesignsElectronics]

That rectifier will drop the B+ ~35 volts so choose a PT accordingly.

[@moodyga40]

1 k grid stopper helps on the input of the 6sn7

[@MisterBigwolf]

I think you are wrong, young lady. Maximum cathode-heater voltage for 6SN7 is +200V (total DC and peak). If you use AC heater for input stage, for minimum hum you may use DC bias for heater and make heater potential for the first tube higher than cathode potential. And will be better if you will increase input 1 milliohm resistor a little bit.

[@SkunkieDesignsElectronics]

It's 100V www.jj-electronic.com/images/stories/product/preamplifying_tubes/6sn7/6SN7.pdf On this amp is DC biased higher than the cathode potential of the first tube, but that was simply by the need to move the cathode/heater on the second section away from it's 100V limit. Honestly on my other amps I have just set the bias to ground and the amp is silent. This one is too, there is no hum. The 1M resistor is only there in case the volume pot fails, the actual grid leak is between the volume control wiper and ground. Hope this clears up your misunderstanding! :)

[@MisterBigwolf]

@@SkunkieDesignsElectronics Ok, I agree. What should be Input signal, or signal on the 300B grid for 8W output power?

[@pq9440]

Hi Skunkie, Thanks for the great video. I have a question about how to calculate the voltage for 6sn7 plate, the pt outputs 375v and after 5ar4, should be about 375*1.414 - 20 = 510v, and in the schematic, it's 430v, I'm a bit confused here. After 4.7kohm, it says 414v, I wonder how this value be calculated? My amp's pt is 480v ac(680v dc after 5u4), how can I adjust 4.7kohm to meet the requirement? Thanks.

[@SkunkieDesignsElectronics]

I use the PSUD2 software to sim these. No need to get into complex math for this, which IMHO people always seem to calculate wrong. www.duncanamps.com/psud2/

[@pq9440]

@@SkunkieDesignsElectronics Cool, thanks a lot.

[@pq9440]

Other question is how to calculate the resistance of load when using PSUD2? Thanks.

[@bondscalper]

You should sell tshirts with your Skunky logo. I would buy one.

[@SkunkieDesignsElectronics]

I plan to get some made up soon :)

[@阳光明媚-n7b]

I don't know where the milliammeter is connected in the circuit. Can you tell me. thank you.. A Physics teacher from China

[@SkunkieDesignsElectronics]

kzbin.info/www/bejne/gJeZnp2Kg8yid9U

[@阳光明媚-n7b]

@@SkunkieDesignsElectronics thank you .I found the video.

[@erwinflores6364]

Srpp will do good in the driver stage then use a 12AT7 driver tube

[@SkunkieDesignsElectronics]

That wouldn't even come close to having enough voltage amplification.

[@moodyga40]

27k I use 2 x 2.2 k in series with caps

[@阳光明媚-n7b]

How do I get the 300B circuit diagram?

[@SkunkieDesignsElectronics]

www.skunkiedesigns.com/300b You can download the BOM as an excel spreadsheet. Then open the image gallery, and you just right click and "Save image as" for the circuit diagrams.

[@阳光明媚-n7b]

@@SkunkieDesignsElectronics 谢谢您

[@阳光明媚-n7b]

how to adjust100Ω2W rheostat?

[@SkunkieDesignsElectronics]

By ear for least hum.

[@阳光明媚-n7b]

@@SkunkieDesignsElectronics thanks

[@阳光明媚-n7b]

how to adjust?

[@SkunkieDesignsElectronics]

Adjust what?

[@阳光明媚-n7b]

100Ω2W rheostat