SE 300B Custom Amp Build

[Curious_George]

 

 

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[Curious_George]

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[captainbeefheart]

Awesome!!

 

I highly recommend a current source for the filaments instead of a voltage source, the 300b tubes will last so much longer this way. Rod Coleman makes nice boards if you don't have a board for this.

 

What are you driving the 300b with? Class A1 I presume?

[Curious_George]

I’ve used CC, VS and good ole AC on filaments and I can’t say which makes a tube last the longest. 
 

Sonically, CC are supposed to be all the rage, but I can’t tell any difference in sound between CC and VS either. 
 

Yes, Class A1, 6SL7GT & 300B. 

[captainbeefheart]

38 minutes ago, Curious_George said:

I’ve used CC, VS and good ole AC on filaments and I can’t say which makes a tube last the longest. 
 

Sonically, CC are supposed to be all the rage, but I can’t tell any difference in sound between CC and VS either. 
 

Yes, Class A1, 6SL7GT & 300B. 

 

I also cannot hear the difference between a low noise voltage source vs current source, as long as they are both well designed the sound will be good.

 

By far the majority of DHT tube failures I have seen were the filament going open. I find the most from AC powered filaments honestly. Most linear voltage source supplies will have enough impedance from the filter stages to somewhat limit the in-rush surge current from a cold filament. Big improvements all around vs AC powered filament supplies. The current source supply limits current the most and is the least stressful to the filament of all three. Some tubes are so rare and or expensive I try my best to let them have a long happy life.

 

6SL7GT is a great tube. They are high mu so I doubt you are cascading the two triodes for each channel, let me guess, "SRPP" driver stage? At first I saw the octal socket and though you may be doing the classic JC Morisson type 6SN7 cascading stages to drive the 300b which is very popular.

 

I currently have been wanting to get another 300b amp but really do not need any more amplifiers at the moment. You aren't helping building a 300b amp

[Curious_George]

I have used the SRPP as a driver for 2A3 and 300B in addition to the standard common cathode topology. I may use a 6AQ7GT in lieu of a 6SL7, which is 1/2 of a 6SL7 (plus a dual diode), so no paralleling needed there. 
 

This is just going to be an amp (versus integrated), so no extra gain by cascading stages will be implemented. 
 

I agree that AC would be the most likely to cause filament failure over CC or VS. I’ll be using a 317 for regulating duties and it can be configured for “slow turn-on”. 

[henry4841]

Would you care to show the schematic you are using? Along with the brand of 300b tubes. I used DC on my filaments of my build and Gold Lion 300b's. I used the J.C Morrison 300B schematic with some changes. Using regulated DC on the filaments was one of them. 

[Curious_George]

3 hours ago, henry4841 said:

I used the J.C Morrison 300B schematic with some changes. Using regulated DC on the filaments was one of them. 

 

I can post a schematic later today. I've got two pair of Chinese 300B's. One pair (Guiguang NGG) came with a 300B Music Angel amp I bought years ago and the other pair is Shuguang. Nothing special, but they have been troopers for all my testing and design work. 

 

I always liked JC's version (Micro 3.5) of the Loftin-White 2A3. Never built it, but may one day. 

[henry4841]

Lots of gain with the JC version. Rich has been a friend of mine for many years. Glad he found someone to build his amp for him. I only build for myself. Your build looks great so for. Rich said he is going with the same tubes I use the Gold Lions. 

[Curious_George]

I chose a chassis bigger than I needed in case Rich wants to upgrade the output transformers in the future. The circuit will be a standard textbook circuit (tried and true), but some tube rolling will be possible mainly for the rectifier and output tubes. The 6SL7GT does not have an equivalent (except a Loktal), so only different manufacturers of that tube will be possible. 

 

 

[rmlowz]

I really do appreciate what you are doing for me Nick.

 

Rich

[captainbeefheart]

1 hour ago, Curious_George said:

I chose a chassis bigger than I needed in case Rich wants to upgrade the output transformers in the future. The circuit will be a standard textbook circuit (tried and true), but some tube rolling will be possible mainly for the rectifier and output tubes. The 6SL7GT does not have an equivalent (except a Loktal), so only different manufacturers of that tube will be possible. 

 

 

 

Too bad you already cut the holes for the Octal preamp tubes.

 

A nice driver with lots of rolling options is the 2C51.

 

That way you can roll 2C51, 5670, Western Electric 396a, or even the 6N3P. Bonus is all of these are not too expensive except for maybe the Western Electric 396a tubes. Another neat thing with these types of tubes is the pinout allows for a really nice mirror layout around the socket, the P2P ends up looking really neat and clean that way but it's easy because of the pinout.

[Curious_George]

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[Curious_George]

 

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[captainbeefheart]

1 hour ago, Curious_George said:

Here is a preliminary schematic. I say preliminary because the power transformer has not been wired up and tested in circuit yet. I have simulated it, so it should be close. If anything needs to change it will be the 150 ohm 10 watt resistor.

 

I may use another driver circuit. I have not decided yet. This one is working quite well in another amp and sounds good. I may continue to use it in this amp too.

 

The 15uF & 20uF cathode bypass caps may need to be changed as well to contour the final frequency response.  

 

 

The cathode impedance of the 300b is ~200ohms, with a 20uF cathode bypass capacitor your -3db frequency is 40Hz. I would increase it to at least 100uF to push the -3db point down to 7Hz that way there it is flat down to 20Hz.

 

As for the driver stage the cathode bypass capacitor doesn't effect gain, it will increase distortion so I usually leave it out but with or without that cathode capacitor you will always end up with a gain of mu/2.

[Curious_George]

I always tweak the output tube cathode bypass cap in circuit because too large of a cap can give your a "boost" in the low-end depending on output transformer parasitics, etc and then the frequency does not roll-off smoothly. I have also seen the driver tube bypass cap affect the final frequency response with a peak as well. 

 

I have an AP, so it is just the push of a button to measure frequency response out to 200kHz in about 10 seconds. 

[captainbeefheart]

4 minutes ago, Curious_George said:

I have also seen the driver tube bypass cap affect the final frequency response with a peak as well.

 

Yes of course with a normal common cathode gain stage but the driver circuit you have shown the cathode bypass capacitor will not change gain at any frequency, gain will always be, mu/2

[captainbeefheart]

43 minutes ago, Curious_George said:

too large of a cap can give your a "boost" in the low-end depending on output transformer parasitics

 

This had me thinking.

 

Which output transformer parasitics will create a low frequency resonance? I am fully aware of keeping the cross sectional winding capacitance low in order to keep the resonance higher in frequency, usually well past 20kHz for good transformers and up past 100kHz for great transformers. I cannot think of any C or L values that will give us a low enough frequency to boost bass?

 

One very common thing that is misunderstood is using too low value choke and filter caps creating a resonance in the bass frequency range. For single ended amps the signal passes directly through the power supply.

 

For example 5H choke and 47uF give a resonant frequency of 10Hz, good enough, I usually shoot for under 10Hz. The 5H and 220uF will be half that at 5Hz. I have seen many 5H or less chokes and 10uF or even lower putting a resonance right in the bass frequencies, no bueno.

 

Any information you have on the output transformer resonance at bass frequencies would be greatly appreciated, thanks.

[captainbeefheart]

I just noticed you have a 390k grid leak resistor for the 300b, maximum grid circuit resistance for cathode bias is 250k and fixed bias 50k.

 

For optimal stability 100k is a good choice as you will see in many designs. For fixed bias the good engineers start to use grid chokes for maximum stability because of their extremely low DC resistance but high AC impedance. Bonus is you also do not load down higher rp driver tubes which will give you more gain with less distortion.

 

 

[Curious_George]

I typically use 220k. I cut and pasted some of the circuitry from other amps. Thanks for the catch.