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Old 29th October 2010, 03:26 PM   #1
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Default Need advice on 300B circuit

Have anyone try this circuit? How about its sound?
I am find out 300B circuit for my new diy tube amp.
Any suggestion for another interesting 300B SE circuits?

Thank you in advance.
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Old 29th October 2010, 06:23 PM   #2
kmtang is offline kmtang  Canada
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Probably you found this schematic from Tubecad.com. I haven't try this Akido driver but I would say there are so many different things contribute to the final sound result, such as OPT, bypass caps, signal coupling cap, tube type, B+ voltage, etc etc.... Everything counts.


Johnny
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Old 29th October 2010, 06:52 PM   #3
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The schematics look good, but Johnny is right about all the passive components having their influence on the sound.
Anyway what is not looking that well is the very high primary dcr of the output transformer. Maybe the transformer is a small one so that many primary windings are necessary to reach enough inductance and also to prevent the core from saturation. Likely the secondary dc resistance will be high too (not good for damping factor).
It is my experience that especially in these types of amplifiers lower dc resistances have a major (beneficial) influence on sound quality. But many people seem obsessed by having a 300B amplifier; in this design I guess that the output transformer is the limiting factor.

Pieter
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Old 29th October 2010, 07:05 PM   #4
tomchr is offline tomchr  United States
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The schematic looks fine, though the circuit will suffer from blocking distortion (aka "farting out") when driven hard. A good transient like the beat of a snare drum will do this and it sounds awful. I'm not saying that the amp will sound awful under all conditions, but if you crank the volume and hit the blocking distortion it will. There's a good description of how blocking distortion occurs here.

For a better 300B amplifier, I suggest looking at the Tubelab SE. You can buy a kit from George for a resonable price and there's plenty of help in the tubelab forum should you get stuck.

You can also wait a week or so. I'll be posting a build thread on a 300B amplifier that I've decided to name The Engineer's 300B Amplifier. As the name implies, it's a little over-engineered. But I really like how it sounds.

In the schematic you posted, the 300B runs at 417 V, 75 mA. That's mighty hot... I wouldn't go past 400 V, 65~70 mA. I've heard stories of 300B's doing thermal run-away, though, with the cathode resistor in place, that really should not happen.

~Tom
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Old 29th October 2010, 07:16 PM   #5
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Don't quite agree Tom.
The Aikido driver stage has a pretty low output impedance; OK not as low as a Fet source follower a la Tubelab, but I don't think it will be so desastrous as you predict.
417V 75mA is 31 watts of plate dissipation. I have no problems using good quality 300B's like JJ with this kind of power, but there are many different 300B's around so that will depend on the particular 300B.

Pieter
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Old 29th October 2010, 07:30 PM   #6
MikeR is offline MikeR  United States
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Plate to cathode voltage is 416-70=346 volts. At 75 mA that equals 26 watts.
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Old 29th October 2010, 07:50 PM   #7
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I second the advice about tubelab.com

Fairly designed and built, without fancy whistles-n-bells.
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Old 29th October 2010, 07:59 PM   #8
45 is offline 45  Italy
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Quote:
Originally Posted by tomchr View Post
The schematic looks fine, though the circuit will suffer from blocking distortion (aka "farting out") when driven hard. A good transient like the beat of a snare drum will do this and it sounds awful. I'm not saying that the amp will sound awful under all conditions, but if you crank the volume and hit the blocking distortion it will.
Maintaining this circuit while getting rid of blocking distortion one could move the .22 uF decoupling cap at the input, use +/-150V supply for the Aikido and eliminate the 100K + 10K grid resistors for the 300B as well. The connection of the 300B would be at 0V and one doesn't need to modify its cathode bias. Of course the amp could only be very slightly class A2 although far better than this with RC coupling, IMHO.
I don't like that huge DCR of the OPT (this is even higher than that of a 14K Hashimoto Sansui SE OPT!!) and prefer the cathode (source) follower solution however.....

45

P.S.
The 10K 300B grid resistor should limit the blocking distortion but I don't think it works with this tube. This is a more a remedy for small devices like a medium-low power pentode.

Last edited by 45; 29th October 2010 at 08:04 PM.
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Old 29th October 2010, 08:26 PM   #9
45 is offline 45  Italy
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Originally Posted by 45 View Post
Maintaining this circuit while getting rid of blocking distortion one could move the .22 uF decoupling cap at the input, use +/-150V supply for the Aikido and eliminate the 100K + 10K grid resistors for the 300B as well. The connection of the 300B would be at 0V and one doesn't need to modify its cathode bias. Of course the amp could only be very slightly class A2 although far better than this with RC coupling, IMHO.

45
In order to have a smooth latch-up one can use solid state rectifiers for the +/-150V and tube rectifiers for the 300B. Best tube rectifiers are those like 12AX4GTB that already have a slow filament warm-up. In addition one could supply the rectifier filament with higher voltage and putting a (wirewound) resistor in series with the filament itself.
For example, one could use 2x12AX4GTB for a fullwave rectifier + 8R2 10W resistor between the two filaments (all in series) and a secondary voltage of 30V/0.6A instead of 25.2V/0.6. In conclusion one trades off 3VA for a nice 20 seconds natural delay for the 300B anode voltage.

45
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Old 29th October 2010, 08:44 PM   #10
cjkpkg is offline cjkpkg  United States
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I will be doing a 300b roughly based upon the JE Labs schematic. I may change around the front end a little. Although I am very tempted to use an aikido front end for it.

I currently use an aikido preamp. I just completed a moskido with an octal aikido front end to a MOSFET power stage. Next up is a 300b.

I think an octal version would be ideal...probably a single 6SN7 is what I will do. Broskie has published the CCDA. This topology is a single 6sn7 front end per channel wired a little differently than the JE Labs example.

I will agree, however, that the selection of OPT's, caps, and even resistors will all add up to the end result.

JE Labs 300b

Last edited by cjkpkg; 29th October 2010 at 08:54 PM.
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