SE 300B Custom Amp Build

[Curious_George]

2 hours ago, henry4841 said:

Excellent, professional looking. Looks like orange drop coupling caps, my favorite. What's the transistors for if they are TO-202, TO-204 devices. Hard to tell just from picture. First thoughts for bias as George at tubelabs does or possible a regulated voltage supply.  

The T0-3 devices are LT1085 regulators for the 300B filament circuit. Amp is very quiet. 
 

Yep.. Orange Drops. Last forever and sound great. 

[CWelsh]

13 hours ago, Curious_George said:

Finally done! Testing and playing music.

You make it look so simple. I'm in awe!

[Curious_George]

 

 

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[Curious_George]

Depending on the output tube used, you can get betwen 6.5 ~ 8 Watts out. 

[Curious_George]

 

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[Curious_George]

On 12/15/2021 at 9:36 AM, CWelsh said:

You make it look so simple. I'm in awe!

 

Thank you. I've been building stuff for a long time now. Visiting factories in Asia for 20 years also helped. You get a lot of ideas from manufacturing and talking to the engineers.

[henry4841]

I notice you used 5K output transformers instead of 3K ones. What is your reasoning for choosing that value? All the old designs I have seen used 3K. I ask for I am sure you had good reasons.  

[Curious_George]

On 12/17/2021 at 7:10 AM, henry4841 said:

 

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[henry4841]

2 minutes ago, Curious_George said:

 

 

5K is a somewhat universal impedance for many single-ended amps for DIYers.  

 

 

I honestly do not know how many tube amplifiers I built that I have right now and you are right most are built with 5K OPT's. Just curious is why I asked because like I said most designs I have seen use 3K. I am using One Electron's and the one they offer for the SE 300b is 3K UBT-3. Not sure they are even in business anymore. Antique Electronic supply no longer show them available. Makers of OPT's are getting fewer and fewer. 

[Curious_George]

On 12/17/2021 at 4:09 PM, henry4841 said:

I honestly do not know how many tube amplifiers I built that I have right now and you are right most are built with 5K OPT's. Just curious is why I asked because like I said most designs I have seen use 3K. I am using One Electron's and the one they offer for the SE 300b is 3K UBT-3. Not sure they are even in business anymore. Antique Electronic supply no longer show them available. Makers of OPT's are getting fewer and fewer. 

 

3K ~ 3.5K will give "maximum" power if that is a design goal. I prefer less distortion and extra drive signal is easy to come by if your design is good. 

 

In addition, if your transformer only has one secondary tap (lets say 6 Ohm), 3.5K seems to be a good compromise between 4 and 8 Ohm speakers. A 4 Ohm speaker will present a lower load than 3.5K and an 8 Ohm speaker will present a higher than 3.5K load. 

[captainbeefheart]

Nice Job!!!

 

Load will depend on B+ and what kind of power vs distortion you want. For voltages around 200v-250v you can use 2k-2.5k load impedance. When you start to get up to 400v you will have a higher load like 5k or even 6k. There is no rule, you can use what you want.

 

For example:

 

400v 3k load gives you almost 12 watts of power but distortion will be higher of course, if you choose 5k you only get 8 watts but distortion is -6db lower. Granted if you measured at the same output power, say both the 12 watt amp and the 8 watt amp measuring at 8 watts output they will be close for distortion so most designers tend to just go ahead and use the lower load impedance to get maximum power say capable of 12 watts but still only rate the amp at 8 watts output because that's where it is at 5% THD which is considered to be about the maximum you want with a SET amplifier. They get away with that much because it is second harmonic dominant and fairly benign.

 

I notice you are -1.5db  down at 20kHz, have you found where this is happening? Before or after the output transformer?

[Curious_George]

13 hours ago, captainbeefheart said:

Nice Job!!!

 

Load will depend on B+ and what kind of power vs distortion you want. For voltages around 200v-250v you can use 2k-2.5k load impedance. When you start to get up to 400v you will have a higher load like 5k or even 6k. There is no rule, you can use what you want.

 

For example:

 

400v 3k load gives you almost 12 watts of power but distortion will be higher of course, if you choose 5k you only get 8 watts but distortion is -6db lower. Granted if you measured at the same output power, say both the 12 watt amp and the 8 watt amp measuring at 8 watts output they will be close for distortion so most designers tend to just go ahead and use the lower load impedance to get maximum power say capable of 12 watts but still only rate the amp at 8 watts output because that's where it is at 5% THD which is considered to be about the maximum you want with a SET amplifier. They get away with that much because it is second harmonic dominant and fairly benign.

 

I notice you are -1.5db  down at 20kHz, have you found where this is happening? Before or after the output transformer?

It is the output transformers in this case. These particular Edcors are the GXSE line, which are right below their “premium line”. Typically, you can get to -3dB @ 40kHz with the GXSE’s but I guess it depends on who winds them on a particular day. 
 

The CXSE’s are spec’d for more bandwidth but they are 25 Watt beasts and the core is huge. Great bass of course but I think the smaller GXSE line sounds better, especially if the ones you get have the higher bandwidth. 
 

My wife said this new amp sounds better than my 2A3 that goes out to 40kHz. 

[captainbeefheart]

6 hours ago, Curious_George said:

It is the output transformers in this case. These particular Edcors are the GXSE line, which are right below their “premium line”. Typically, you can get to -3dB @ 40kHz with the GXSE’s but I guess it depends on who winds them on a particular day. 

 

Probably another reason many 300b amps use 3k load transformers, less turns will give less cross winding capacitance and better high frequency response compared to 5k.

[Curious_George]

1 hour ago, captainbeefheart said:

 

Probably another reason many 300b amps use 3k load transformers, less turns will give less cross winding capacitance and better high frequency response compared to 5k.

 

Probably, but it definately depends on the brand of transformer and winding technique. I have some Lundahls and I'm not too impressed with them. My favorite right now are a set of James JS6112HS's. 

 

My 16k 211 output's go out to 32KHz. Pretty good for that high of an impedance. 

[captainbeefheart]

45 minutes ago, Curious_George said:

 

Probably, but it definately depends on the brand of transformer and winding technique. I have some Lundahls and I'm not too impressed with them. My favorite right now are a set of James JS6112HS's. 

 

My 16k 211 output's go out to 32KHz. Pretty good for that high of an impedance. 

 

Very true!

 

I should have said the more turns (or higher impedance) the harder it is to wind a high quality transformer.

 

James is really good. I have a set of 3k Transcendar Gerry wound for me I am liking right now.

 

Honestly our tired old ears won't notice -1.5db at 20kHz anyway. For my age I still do ok on hearing tests because I have always stayed away from high volume anything and wear ear protection. I have always been extremely sensitive to poor quality high frequency sounds, ringing, dissonant distortions, etc.. it's not just fatigue it actually hurts my ears, it even makes my skin and teeth feel very weird I hate.

 

It's more of a bragging rights thing to have extreme bandwidth amplifiers, well extreme on the top end anyway bass is good. I don't see the need for a stereo amp to need to put out full power at 100kHz. I mean it is a good indication the amplifier is of good design but I don't really think there is much information past 40kHz that we will perceive. I say 40kHz because of harmonics and them being part of the bigger picture.

[Curious_George]

On 12/19/2021 at 5:01 PM, captainbeefheart said:

Honestly our tired old ears won't notice -1.5db at 20kHz anyway. For my age I still do ok on hearing tests because I have always stayed away from high volume anything and wear ear protection. I have always been extremely sensitive to poor quality high frequency sounds, ringing, dissonant distortions, etc.. it's not just fatigue it actually hurts my ears, it even makes my skin and teeth feel very weird I hate.

 

I have always protected my hearing due to extreme sensitivity to loud high frequency noise / sound as well. Even going to a loud bar or restaurant I will wear ear plugs because I will get a headache. Occassionally, I will get a strange look, but I don't care. 

[Curious_George]

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[mike stehr]

My 2A3 amp started out using those pseudo 300B solid plate 2.5-volt TJ voice of music balloon globes.

I suppose they were used so one could use direct heating; a bit easier to pull off than 5.0 volts AC.

 

The output transformers are Scholl if I recall, with a 3.6K primary using 4- and 8-ohm taps.

The amplifier outlived the TJ globes, so I swapped to 2A3.

 

I've used Sovtek 2A3 tubes for couple few years and still have them. Still have the TJs.

Used Sylvania 5930/2A3W. Ugly, but tough...

Currently I'm using GE 2A3s. H plate or some such.

 

It's a good sounding amplifier to my ears...

 

 

[Curious_George]

The 2A3 is my favorite tube as well. It is more forgiving if you just want a basic "no bones" amp that sounds fantastic. You can run the 2A3 filaments on AC and hardly have any trace of hum, even on horn speakers, making the 2A3 circuit very simple. 

 

You can run the 300B on AC too, but even with a hum bucking pot, you will have more hum than a 2A3. 

[Curious_George]

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