300B Direct Couple SET Class A Design

[Hari Iyer]

For the first time, I will be designing and building my own 300B SET Class A tube amplifier. I have been given enough knowledge transfer by my mentor JM and he is confident that I can do this on my own. So attempting with some previous background and tips handed over. I have nothing to loose than some money.

This design will have the following primary goals,
- needs to be a direct coupled design
- only a 2 stage design
- AC heating for DHT 300B
- LSE power supply with single shunt

With this as primary design goal - iwill be using a 6SL7 as driver tube parallelled at around 4mA. The 300B will be at 43.5mA cathode bias. Will use dual 5u4gb as rectifier tubes.

I have completed one round of simulation with PSUD2 and all my design goals/ parameters are in perfect simulation. Now next step is to order parts and assemble.

Thanks for looking.

 

[HappyBear]

Reconsider using paralleled 6SL7 - JDM has repeatedly claimed that this sort of input stage results in a subjective skew of audio signals.

There are other high mu, low Rp driver tubes - try to find one.

HB

 

[HappyBear]

6N7.

6N7 - no, mu is 35 per data sheet. Jump factors will be insufficient.

12BZ7 - mu of 100, Rp is like a parallel 12AX7. That would be closer to parallel 6SL7 with only one triode section. No subjective skews.

There are probably others but you need high mu to maintain jump factors - that is critical.

HB

 

[Hari Iyer]

6N7 - no, mu is 35 per data sheet. Jump factors will be insufficient.

12BZ7 - mu of 100, Rp is like a parallel 12AX7. That would be closer to parallel 6SL7 with only one triode section. No subjective skews.

There are probably others but you need high mu to maintain jump factors - that is critical.

HB

Ideal jump factor will be 400 which thec12ax7 can provide. But the 12ax7 has very low plate current and cannot drive a 300B, it's good for a 2a3. The 6sl7 provides a jump factor of 280 ( less than 400) but drive current is good 2ma each section. The 6N7s has just a jump factor of 80 - easy to discard. The 12BZ7 is promising as they have both a good jump factor and good plate current and can drive the 300B at 4mA if paralleled . The cathode bias for the finals is a conservative 43.5mA for making the 300B last a life time. The plate dissipation calculated is around 10.3 watts for the finals and around 0.35watts for the driver tube.

 

[HappyBear]

EC91 is another one to consider with mu of 100. It has a lower Rp than 12BZ7.

 

[heliumflight]

For the first time, I will be designing and building my own 300B SET Class A tube amplifier. I have been given enough knowledge transfer by my mentor JM and he is confident that I can do this on my own. So attempting with some previous background and tips handed over. I have nothing to loose than some money.

This design will have the following primary goals,
- needs to be a direct coupled design
- only a 2 stage design
- AC heating for DHT 300B
- LSE power supply with single shunt

With this as primary design goal - iwill be using a 6SL7 as driver tube parallelled at around 4mA. The 300B will be at 43.5mA cathode bias. Will use dual 5u4gb as rectifier tubes.

I have completed one round of simulation with PSUD2 and all my design goals/ parameters are in perfect simulation. Now next step is to order parts and assemble.

Thanks for looking.

Great Hari ji. I am actively looking at a 300B and monitor this thread

 

[Hari Iyer]

Having a tough time past one month and am still struggling to source a good DC Choke locally. I understand that we can import them, then the cost will be very high. I am looking at keeping the cost down and import only essentials like the "tubes" and not unnecessarily increase cost.

 

[FlaCharlie]

Having a tough time past one month and am still struggling to source a good DC Choke locally. I understand that we can import them, then the cost will be very high. I am looking at keeping the cost down and import only essentials like the "tubes" and not unnecessarily increase cost.

Don't the Indian transformer manufacturers, who apparently make power and output transformers, also sell chokes?

 

[anilva]

Delta makes different types of chokes. Their catalog has all the models.

 

[Hari Iyer]

Don't the Indian transformer manufacturers, who apparently make power and output transformers, also sell chokes?

Yes they do. But they are not interested in selling just a pair.

 

[Hari Iyer]

Delta makes different types of chokes. Their catalog has all the models.

They don't sell for DIY. Have contacted them before. They don't even respond.

 

[anilva]

Yes they do. But they are not interested in selling just a pair.

I just bought a pair of chokes. They are quite responsive.

 

[Hari Iyer]

Finally was able to source the chokes from Pune. They shall arrive after a week and after that shall start assembly

 

[HappyBear]

Finally was able to source the chokes from Pune. They shall arrive after a week and after that shall start assembly

What driver stage tube was selected for your SE DC 300B project?

Best luck for a successful outcome.

HB

 

[Hari Iyer]

What driver stage tube was selected for your SE DC 300B project?

Best luck for a successful outcome.

HB

I am considering EC91/ 6aq4 for the driver stage. But my mentor is telling me not to build a 300B.

 

[HappyBear]

I am considering EC91/ 6aq4 for the driver stage. But my mentor is telling me not to build a 300B.

What final stage tube is being suggested by your mentor?

 

[Hari Iyer]

What final stage tube is being suggested by your mentor?

6005 - triple

 

[Hari Iyer]

Certainly better choic.

Will need to sell my current monoblocks. Any takers

 

[Hari Iyer]

6005 = 6AQ5. Right ?

Maybe

 

[HappyBear]

6005 - triple

In the past, JDM was opposed to using parallel tube sections in SE tube amplifiers due to a "subjective skew" he heard while using high efficiency horn speakers.

How would this design be an advantage over the SE KT88 mono amplifiers you have already built?

HB