• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Budget 300B Build Video Series

Jump to Latest
#102
After some thought and trying a few different things, I got this 6SN7 cascode working well. With 3Vpp in I am getting just shy of 140Vpp at the grid of the 300B with no clipping and it sounds fantastic. Also went to a 0.33uf coupling cap, which solved the weak bass and even running the 300B at 80% into a 5K OT it's putting out 6W of clean power. This is easily 3X the output of the original circuit and it sounds so much better.

I get this isn't a "Holy Grail" design, but it's a huge improvement on the "JC Morrison/Sun Audio" directed coupled driver setup and this could easily be used to mod amps built on that topology like the BoyuuRange A50.

300B Tube Amp: Final Analysis, Mods and Solutions for JC Morrison based Amplifiers. - YouTube
 

Attachments

  • 6SN7-300B-Final.png
    6SN7-300B-Final.png
    662.2 KB · Views: 269
#104
Question:

What pentode tube type can operate as a 6SN7 substitute in that cascode stage?
And have the same peak to peak swing, and be as linear as the 6SN7 (with the same B+, the same load RL, and the same plate current)?
(and no feedback allowed).

There probably are a few such pentodes, but . . .
I would like to see that.
 
#107
For a B+ of 400 VDC and 5mA (although 10mA would be better), several pentodes can perform in the weight class of even the best cascoded triodes. The really hot ones like the last generation of 45MHz IF amplifiers and crazy video baseband amps (from HE-double hockeysticks) have enough raw Gm to be used with unbypassed cathode resistors and still provide the 35dB gain needed.


400VDC B+ is even enough to incorporate the Blohbaum (mit der Umlaut) cascoded G2.


At some point stephe's explorations might verge on the "transconductance>transadmittance" school. Didn't work out for semi-cons, but appropriate for Class-A valves.


All good fortune,
Chris
 
#108
I have always wanted to build a 300 valve amp and you Video series is a real inspiration to do it !

Thank you Stephe, now the fun of finding suitable parts in the UK. If people could share good websites for parts that would be very helpful as I need to buy everything

Thanks in advance
 
#109
Hi SimonJ, there is no simple answer. You need to decide on your budget, then do a bit of research. The two critical parts of a 300B amp are the 300B power tubes, though there are good sources for them today that are not too expensive, and the output transformers.
Tube prices

Quality parts will reward you, but there is a law of diminishing returns, and there are snake oil salesmen.

Your choice of output transformers can be dictated by the overall presentation you are looking for in your build. Toroids are available, but are not to everyone's taste.

With a 300B amp, the sky is the limit from a spending perspective, so you need to decide what is your goal.
 
#114
The other decision is the impedance of the OT. 3.5K will give more power but take into consideration your speakers and the space you use it in. Many speakers are "rated" at 8ohms, but in reality are more like 6 ohms, which makes them harder to drive if you used a 3.5k with 8ohm tap OT as the load reflected to the 300B then ends up being under 3K. 5k OT will also give less distortion and if your speakers are pretty efficient, still gives plenty of dB of sound pressure in the room.

This decision on using 5K into 8ohm for me was easy because that is what I had on hand 🙂 One last thing to add, there are people selling the Shuguang 300B-Z tubes for a bit over $200 a pair right now. These sounds great and that is 1/2 of what they sold for a couple of years ago.
 
#115
@Stephe - I watched all your videos for this build today and SUPERB work THANK YOU!

Its clear you know your stuff and it has REALLY helped me. I know you said in the video you are working on a BOM which will be very useful to understand some details of the components as getting these wrong could be very bad and unsafe too.

Your tips with the variac and safety were a must so thank you again as i feel more confident to build this amp now 🙂
 
#116
stephe said:
The other decision is the impedance of the OT. 3.5K will give more power but take into consideration your speakers and the space you use it in. Many speakers are "rated" at 8ohms, but in reality are more like 6 ohms, which makes them harder to drive if you used a 3.5k with 8ohm tap OT as the load reflected to the 300B then ends up being under 3K. 5k OT will also give less distortion and if your speakers are pretty efficient, still gives plenty of dB of sound pressure in the room.

This decision on using 5K into 8ohm for me was easy because that is what I had on hand 🙂 One last thing to add, there are people selling the Shuguang 300B-Z tubes for a bit over $200 a pair right now. These sounds great and that is 1/2 of what they sold for a couple of years ago.
Click to expand...

Thanks for the tips and I have ordered those tubes today.

My speakers are 89dB efficient at 8 ohm with a min of 3.9 ohms so not the easiest load as I see yours are 96dB. My listening room is small and I am only 5 feet from the speakers

I am not sure what impedance to use in this case, thoughts?
 
#117
Keep in mind that it isn't uncommon for speaker manufactures to get a bit creative with their specs.

Stereophile's test of the Klipsch RP600M speakers had them in the 89 dB efficiency range and spending a fair amount of the frequency band around 4 ohms. So if your speakers specs are more accurate they could be closer than you think.
 
#118
invaderzim said:
Keep in mind that it isn't uncommon for speaker manufactures to get a bit creative with their specs.

Stereophile's test of the Klipsch RP600M speakers had them in the 89 dB efficiency range and spending a fair amount of the frequency band around 4 ohms. So if your speakers specs are more accurate they could be closer than you think.
Click to expand...

Right, I've seen that too and don't doubt it. I honestly think 5K in 8ohms makes sense for a lot of uses and would be better than getting a 3.5K trying to drive speakers that spend a lot of time between 4-6 ohms. Like I said, I had some 5K into 8ohm 25W OT already, so just went with those. If I were buying new, to use with my speakers, I probably would have ordered the 3.75K into 6ohms Edcor offers. At some point we are just splitting hairs unless you need 100% of the output.
 
Last edited:
#120
As Chris noted, UK transformer manufacturers offer excellent products. I have used the Sowter SA08 for over 20 years, and never think about replacing it. I have developed many ways to improve 300B-Se, and the Sowter has always been able to show the difference. They are not cheap, but a cheap OT is a mistake, IMHO. Good OTs don't wear out, and can give a lifetime of pleasure.

All the power supplies are critical in 300b amplifiers - more important than the valve quality. The driver stage design has a major influence on the outcome too. Pentode drivers are easy to build, using low-cost valves, and they outperform 1990s 2-stage triode drivers.
 
Last edited:
You must log in or register to reply here.
Top Bottom