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Old 2nd June 2015, 02:56 PM   #1
Clelow is offline Clelow  United States
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Default Finalizing 300B Amps==Please Comment

Listmates,

My tech friend and I have been working on a pair of 300B monoblocs for the last few months. I insisted we take it slow so that I could ask questions and experiment with different components and try alternate designs. I am still playing with different cap values and types in an effort to refine the sound of the amps. However at this stage, I am very pleased with the amps. Voices are so real it seems as if the singer is in the same room. The amps are flat from 40Hz to 20khz @ 5 watts. Total output is app. 8 watts.

I am almost ready to have some chassis fabricated for them. Before doing so, I would like to make final revisions so I know exactly what I am working with. Consequently, I would appreciate your help. Do you see anything I should do differently or at least try to do differently prior to putting the amps in final form?

Please consider the interstage transformer, parafeed choke and output transformer as immutable. I am going to experiment with UltraPath as it will be easy to implement. I would like to hear from you and appreciate your comments and suggestions.
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File Type: pdf 300 B Parafeed Schematic.pdf (137.6 KB, 158 views)
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Old 2nd June 2015, 03:26 PM   #2
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Well, this kind of an amp has many possible variations so I will address just the most important issue I see: the hum pot on the 300B. OK, the 300B is not shown as a DHT, that's OK since we know what you intend, but what is important is that you have the entire current for the 300B going through the wiper of the hum pot and the hum pot appears to be 500 ohms. First, the 500r hum pot is dropping much more voltage than it should; second having the output tube current going through the hum pot wiper causes considerable noise and, in my experience, considerable hum. My recommendation is that you change the 500r hum pot to 2W 50r and parallel the wiper with two 10r to 15r resistors. The resistors carry the current and the hum pot fully nulls any hum.
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Old 2nd June 2015, 04:52 PM   #3
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The schem shows that the CT of the 6,3V is grounded. Is this a drawing error? With the CT grounded that would short the 300B filament to DC ground, not the proper bias voltage.
As for the parafeed cap, 20uF is very big. I use 4uF in my parafeed 300B.

Last edited by Parafeed813; 2nd June 2015 at 04:55 PM. Reason: auto correct typo of my phone
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Old 2nd June 2015, 08:01 PM   #4
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Quote:
Originally Posted by Parafeed813 View Post
The schem shows that the CT of the 6,3V is grounded. Is this a drawing error? With the CT grounded that would short the 300B filament to DC ground, not the proper bias voltage.
Yes, I didn't catch that: I thought there was a 5V transformer tap for the 300B. The amp definitely needs another 5V transformer or at least remove the grounded center tap.
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Old 3rd June 2015, 12:21 AM   #5
Clelow is offline Clelow  United States
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Gentlemen, you are correct. The 6.3v center tap is not grounded. My bad. It is simply taped off under the chassis. I have asked the Administrator to substitute the correct schematic.

Palustris, thanks for the information on the revised hum pot. I plan to implement the change as there is more hum than I like. I have been hesitant to go to a DC supply as I prefer AC on the filaments.

Parafeed813, thanks for the tip on the parafeed cap. There is lots of conflicting information on the internet as to what the value of this cap should be and how its value is calculated. I may have gone in the wrong direction on it.
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Old 3rd June 2015, 03:51 AM   #6
tomchr is offline tomchr  United States
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The administrators tend to be busy volunteers. I suggest just posting the corrected schematic.

I suggest trying a regulated B+ supply. It will greatly lower any risidual mains hum. B+ induced hum generates IMD products at multiples of the mains frequency around the signal frequency, so even if the hum is not obtrusive from the listening position, it'll add a layer of fuzz to the sound.

I also suggest using DC for the filament supply, at least on the 300B as that is another source of hum introduction.

I usually aim for below 1 mV RMS of hum on the output of the amp. I've only been able to get that low with B+ regulation and DC filament supplies in my 300B amps.

Finally, I suggest trying LED biasing on the input tube. You should be able to find an IR LED with a 1.2 V forward drop. Just make sure the LED can handle the peak current flowing through the input tube.
If you insist on using batteries, I'd move it to the grid so it's out of the signal path, and add a charging circuit that keeps the battery charged when the amp is powered off.
You may also want to try bypassed resistor biasing on the input tube. A Nichicon Muze KZ would be a nice bypass capacitor. They're available at Mouser and Percy Audio.

Just some ideas (you did ask ) I like the simplicity of your circuit overall. The 300B is a tough tube to drive but the results are very rewarding.

Tom
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Last edited by tomchr; 3rd June 2015 at 03:57 AM.
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Old 3rd June 2015, 03:51 PM   #7
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What about a (gas)zener to protect the last PSU cap?
It is rated at only 160V. If the rectifier heats up quicker than the 417A it will see 450V.
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Old 3rd June 2015, 05:23 PM   #8
Mooly is offline Mooly  United Kingdom
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Post #1 now has the correct file attached.
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Old 3rd June 2015, 07:01 PM   #9
DieterK is offline DieterK  Germany
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I think you will become problems with the 417a cause the gate current begins below -1V. Normaly it should be biased at -2.4V to -2.6V to be secure against gate current. A second Problem is that the 417A don't drive the 300B till clipping. But on the other side the combination sounds great (Sorry, my english is not the best. I hope you understand me)
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Old 3rd June 2015, 07:25 PM   #10
Clelow is offline Clelow  United States
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Tomchr, Thank you for the suggestions. I have a few comments and questions below.

Quote:
Originally Posted by tomchr View Post
The administrators tend to be busy volunteers. I suggest just posting the corrected schematic.

Quote:
I suggest trying a regulated B+ supply. It will greatly lower any risidual mains hum. B+ induced hum generates IMD products at multiples of the mains frequency around the signal frequency, so even if the hum is not obtrusive from the listening position, it'll add a layer of fuzz to the sound.
I went with a choke input power supply in an effort to avoid a regulated B+ supply. In the past a regulated B+ did not sound as good as an unregulated choke input supply. Perhaps I should re-visit this.

Quote:
I also suggest using DC for the filament supply, at least on the 300B as that is another source of hum introduction.
When I have tried DC on the filaments in the past, the amp did not sound as alive as with AC. I am hoping the suggestion by Palustris will provide a happy median by reducing hum but allowing for AC dynamics.

Quote:
I usually aim for below 1 mV RMS of hum on the output of the amp. I've only been able to get that low with B+ regulation and DC filament supplies in my 300B amps.

Finally, I suggest trying LED biasing on the input tube. You should be able to find an IR LED with a 1.2 V forward drop. Just make sure the LED can handle the peak current flowing through the input tube.
I considered LED's and probably should have tried them for the 417A bias. The consensus seemed to be that the battery was a better sounding approach. Perhaps I should revisit this issue.


Quote:
If you insist on using batteries, I'd move it to the grid so it's out of the signal path, and add a charging circuit that keeps the battery charged when the amp is powered off.
I wanted to avoid having to add another capacitor to block the upstream DC had I put the battery on the grid.

Quote:
You may also want to try bypassed resistor biasing on the input tube. A Nichicon Muze KZ would be a nice bypass capacitor. They're available at Mouser and Percy Audio
.

In fact I did do this. I liked the battery bias a little better.

Just some ideas (you did ask ) I like the simplicity of your circuit overall. The 300B is a tough tube to drive but the results are very rewarding.

Tom
Thank you sir!
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