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Old 22nd July 2011, 04:18 PM   #1
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Default Just designed a 300b amplifier....

HT going into the 300b is 500volts to yield a 87% or so dissipation for the 300b.

Its been carefully designed and optimised, did this all on my own, please ignore the values of the transformers. its not quite complete but took me an age to draw as I;m not used to the drawing software.

use a 400volts mains transformer, gz34 rectifier

ideally, a hammond 300bx and a 2k5-3k5 output transformer, hammond 1627 would be great.

I wouldn't go lower than 2k5 which will give around 10 watts, or above a 5k ohm output tx which will be
much more linear (less distortion) but will reduce power output to around 6 watts.

The operating points have been carefully designed and should be much more linear than the jelabs design.

feel free to build it and report how it sounds, I reckon its better than the jelabs, but I would, wounld't I?

needs an extra bypass cap after the 4k7 HT dropper in the psu, and of course all the extra bits like earthing and heaters.

a grid choke and we connection on the output stage plus....
if you wish to push the boat out, use an extra mains tx for the driver stages, choke filtered of course.

...will be the cherry on top of the cake and should improve things no end.

I will build it myself one day

I have a few lovely preamp designs to match, but not drawn them yet, all my own work

questions please ask....
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Last edited by lt cdr data; 22nd July 2011 at 04:46 PM.
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Old 22nd July 2011, 04:53 PM   #2
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its been designed with the aid of tubecad, se tube cad, and duncan's psu, perhaps the psu could do with altering the values a little, but it will work perfectly as is, ie lower input cap before the choke and higher one after, typcially a 1:10 ratio is best, and perhaps even going to choke input.

an 82 ohm power resistor may be required before the first cap on the psu to limit inrush to protect the rectifier, and if the mains tx doesn't have enough winding resistance, add some resistors on the anodes of the rectifier, that's not too widely known. 100r or so should do the trick.

the driver stage can amply supply the volts to drive the 300b to full power, and there's plenty of slew current to charge the capacitance of the 300b output stage, all good design points have been taken care of


So you can be sure its well optimised to be linear and should sound great,


feedback may be added using a resistance decade box to tune the values, be prepared to ditch it if that ruins the sound, and of course grid stoppers may be needed.

Last edited by lt cdr data; 22nd July 2011 at 05:05 PM.
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Old 22nd July 2011, 06:24 PM   #3
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Hmm...

Odd op point for the 6SN7....IMO....

Very low plate-loads, and you have cathode-de-coupling as well....

100N and 100K coupling and grid-resistors are going IMO to cut the LF response pretty badly....

Personally for the 6SN7 stages you have, I would use say, 47K/560 ohm and 33K/470 ohm for the gain-stages, with 1M grid-resistors for it too..
--Doing this, you can get rid of the cathode-caps, as you have a higher gain and reduce the distortion a little

For the 300B I would use a 470K grid and 220N coupling....

Just my thoughts...
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Old 22nd July 2011, 07:26 PM   #4
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here's a lovely preamp circuit to match that will partner it beautifully, all designed by me again
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Old 22nd July 2011, 09:18 PM   #5
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Have you built and tested in the real world, any of these designs?

Simulation isnt all its cracked up to be!
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Old 22nd July 2011, 09:37 PM   #6
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hi Alaistair, no I havne't yet, however, it isn't a "simulation", as such, apart from the psu designer, tube cad which I used, is more a mathematical design aid, an utterly invaluable one

using real load lines, and modelled on equations that designers use in practise, so its essentially similar to a paper design as any tube expert would do, so I am confidant that it will work as well as any paper design would, from graphs.

its just a lot quicker and probably more accurate, too accurate for 10% rules of engineers and tube/cap. tolerances

the low loads and low value bypass caps were used deliberately, however, they are still outside the audio band of 20 hz, around 5-10 hz, people make mistakes of using far too higher cap. values where they don't need to, 5hz is plenty enough, as used here. won't cause any probls in use. caps used here are plenty enough values.

best wishes.

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Old 22nd July 2011, 10:53 PM   #7
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Err....

Reactance of 0.1uF cap is 79.6K At 20Hz....

Therefore, you have a potential divider at 79.6K and your 100K grid-resistor...

So, you're well over 3dB down--Just on One Stage with 0.1uF and 100K At 20Hz....

You've done this Three Times over three stages.

The reactance of 0.1 at 5 Hz is nearly 320K.....

Its too late at night to work out how much attenuation you're gonna have, But its going to be much higher than you Think Commander!
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Old 23rd July 2011, 12:03 AM   #8
kevinkr is offline kevinkr  United States
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There is a fatal assumption in this design process and that is that real tubes behave exactly like the ones described in the characteristic curves of this tool, and they actually don't - at best they are an approximation so the concerns expressed here about whether or not these designs work as well as predicted are completely valid.

I'm not trying to be mean, but the designs discussed are at best derivative, and there are far better designs if you are willing to look and ask. (I am not a fan of the original JE labs designs or any of its derivatives, mediocre sound forever.) The plate load resistors are too small IMHO given the rp of the 6SN7 as are the coupling capacitors. I prefer to optimize each stage prior to the output for the best possible linearity under those conditions, and live with the performance of the output stage which I usually run hot..

I've done a lot of designs (commercial and hobby) with the 300B and 10W into 3K is possible with fixed bias, 400V @ 80mA on the plate and > 6% thd - which makes for much unpleasantness sonically speaking. Honestly 5 - 6W at a couple of % is a more honest rating target unless you like the sound of compression and distortion which you get in spades above this power level..

LF poles should be staggered (particularly if you use global NFB - I don't in most SE amps) and the -3dB cutoff point of the circuit should be a couple of octaves below the lowest freguency of interest if you wish to maintain reasonable phase shift - exception made if the OPT cannot handle very low frequencies gracefully. Take a look at designs based on the 7788, D3A/7721, 6688, or the C3G - any of these will drive a 300B either triode or pentode connected as you prefer..

I use spice in my design process and spend a lot of time on the bench where I discover just how close my predicted performance is to what I modeled - nearly 30yrs of experience and I still get nasty surprises..

I like the 6SN7 and 6J5 too, but there are other viable choices.

One other thing I would say as someone with oodles of design experience is it takes a lot chutzpah to offer an unproven design to others on a forum such as this with the stated assurances that it is a significant improvement over the original - given that you have not even built it. I am very hesitant to share my unproven designs with others here, and do so only having stated that it/they could be/or are utter rubbish..
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Old 23rd July 2011, 01:17 AM   #9
GTHICM is offline GTHICM  Viet Nam
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Default JE Labs Dlx 300B

your comments are interesting, Kevin. I have been living with some JE Labs 300B amps for awhile and their performance is, well, underwhelming. I have tried no less than 10 different 6SN7s to get reasonable sound. Only one brand seems to give reasonable, but not spectacular results. This seems consistent with your observations about these designs.

No harm, as next year I wil embark on a rebuild to a better design, once back in the US.
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Old 23rd July 2011, 03:43 AM   #10
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If you want to do 6sn7 to 300b, this thread is worth a read: Driver valve discussion
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