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SE 300B Custom Amp Build


Curious_George
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1 minute ago, Curious_George said:

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

 

220k is a good value 👍

 

You don't have to worry about loading down your driver because it has a low output impedance so you could get away just fine with 100k FWIW.

 

Some folks using the 6SL7 differently than you are would want to try and keep the 300b grid leak as high as possible but you won't have any issues with the driver arrangement you are using.

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Nice job!

 

The 6SL7 is a really nice tube -- along with the 5687, one of my favorite input/drivers.  The 6SL7 is also great with both sections paralleled and direct-coupled to the final stage -- thus getting rid of those pesky but often necessary coupling caps. ;) 

 

Your chassis layout is particularly well-done, with the power supply physically well isolated.  Granted the rectifier's job is, depending on one's viewpoint, perhaps a bit more plebeian when compared to the signal-handling valves in the front and final stages, and is usually quite less expensive -- particularly compared those large power triodes -- but, I don't know, maybe it would have been kind of fun to have the rectifier a notch or two more visible.   That octal hole behind the PT is also a good place for a large chassis-mount single or multi-section electrolytic can.   Keeping 60Hz alternating current as far away from the music as possible is always paramount, and you've done a great job in that respect.

 

PS:  Tubes can also function as a pilot light!

 

Super-clean work - lucky new owner!

 

 

 

 

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3 hours ago, The Dude said:

Coming along nicely, I see you posted the schematic, out of curiosity, is there a BOM and how much $$ could one expect to spend on building one? 

Not cheap. I spent right at 8 or 9 hundred building mine and that is just parts alone. The tubes themselves are right pricey along with the iron. Worth every penny though. George is building this one for a customer so it is probably appropriate to contact him via private message. 

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11 hours ago, erik2A3 said:

Nice job!

 

The 6SL7 is a really nice tube -- along with the 5687, one of my favorite input/drivers.  The 6SL7 is also great with both sections paralleled and direct-coupled to the final stage -- thus getting rid of those pesky but often necessary coupling caps. ;) 

 

Your chassis layout is particularly well-done, with the power supply physically well isolated.  Granted the rectifier's job is, depending on one's viewpoint, perhaps a bit more plebeian when compared to the signal-handling valves in the front and final stages, and is usually quite less expensive -- particularly those large power triodes -- but, I don't know, maybe it would have been kind of fun to have the rectifier a notch or two more visible.   That octal hole behind the PT is also a good place for a large chassis-mount single or multi-section electrolytic can.   Keeping 60Hz alternating current as far away from the music as possible is always paramount, and you've done a great job in that respect.

 

PS:  Tubes can also function as a pilot light!

 

Super-clean work - lucky new owner!

 

 

 

 


Thank you for your kind comments. Lately, this layout seems to be a good aesthetic. I figure most people want to see the big power triodes and hiding the rectifier enhances the front 4 tubes.

 

I like to add a front pilot light because in daylight conditions, it is hard to tell if the amp is on. 

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On 11/22/2021 at 6:40 PM, captainbeefheart said:

 

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.


I have not fully investigated the phenomenon, but for example, if I use a large bypass cap on a 2A3 amp with a 750 ohm cathode resistor and let’s say 100uF cap, I’ll get a low frequency boost on the low end. This has happened on Lundahls, Hammonds, Edcor and James transformers. The “boost” is slightly different on each transformer. I discovered I can flatten the response by using a smaller cathode cap by trial and error. 

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10 hours ago, Curious_George said:


Thank you for your kind comments. Lately, this layout seems to be a good aesthetic. I figure most people want to see the big power triodes and hiding the rectifier enhances the front 4 tubes.

 

I like to add a front pilot light because in daylight conditions, it is hard to tell if the amp is on. 

Just one suggestion for aesthetics. Paint those covers on the transformers black! I built many just leaving the covers blue but the last 2 amplifiers I took the time to paint them black and like the look much better. 

P1030824.JPG

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7 hours ago, Curious_George said:


I have not fully investigated the phenomenon, but for example, if I use a large bypass cap on a 2A3 amp with a 750 ohm cathode resistor and let’s say 100uF cap, I’ll get a low frequency boost on the low end. This has happened on Lundahls, Hammonds, Edcor and James transformers. The “boost” is slightly different on each transformer. I discovered I can flatten the response by using a smaller cathode cap by trial and error. 

 

Sounds like low frequency instability where rolling the bass off with a smaller cathode bypass capacitor lowers the gain enough of the low frequencies to keep it flat.

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59 minutes ago, henry4841 said:

Just one suggestion for aesthetics. Paint those covers on the transformers black! I built many just leaving the covers blue but the last 2 amplifiers I took the time to paint them black and like the look much better. 

 

 

Nick asked me if I wanted them painted, I like the dark blue I am color blind and one of the colors I can see is blue

 

Rich

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I have built a couple versions of Electra-Print's DRD single-ended designs.  Some of you probably remember when the 300B DRD (Direct Reactance Drive) was first published in Vacuum Tube Valley.  I thought you might recall that article.  Welborne Labs (whom you may also remember) began selling kit versions of the DRD amps, and I built a pair of the 300Bs, as well as the 45, for other forum members here a long time ago., 

 

Not to get Curious George's great thread off track, but just thought to share it since some different input/driver variations have been discussed here.   Jack Eliano, who designed the DRD, also winds extremely well-made transformers and chokes......and they are definitely on the expensive side, but in my view well-worth the cost.

 

I also built a pair of 2A3 mono blocks based on a parallel feed output designed by another forum member (also a long time ago), and then rebuilt it using certain elements of Eliano's DRD.   I still regret having sold those amps.  They were absolutely stunning, with amazing transient response, air, detail (not too much), and very decent low end reproduction.  The output stage uses a very high impedance choke instead of the much, much more common grid-leak resistor; and, as also mentioned above, is significantly more expensive.  But gosh, what an amazing sound.

 

Here's the DRD schematic from the Electra-Print website - just in case you (or others) might be tempted to have a go at this very unique design.

 

https://www.electra-print.com/300bdrd.php

 

And the pilot light comment I made:   Of course you are right.  An actual lamp, as you said, is definitely much easier to see than glowing filaments.  As I re-read that part of my response, it sounds sort of nit-picky.   I didn't mean for it to come across that way.  If it did, apologies.

 

 

 

 

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