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Vertical 6EM7 Portable Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5134
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Author:  john55 [ 14 Mar 2019, 02:01 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Suncalc wrote:
In designing my coupling circuits I am very careful about bias recovery time. Generally I design the signal chain so one stage will transition to grid conduction first. Then I manage that stage to limit bias excursion recovery time. Here is the passage from my design notebook concerning bias excursion.
Attachment:
Bias Excursion and recovery.jpg

Read these two passages carefully and it should become more clear what I am saying.


Thanks for the explanation Matt. I've read those two passages many times and I'm getting the gist of it. I think I have much more to learn before I fully understand it.

Author:  ILoveHiFi [ 16 Mar 2019, 22:08 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I only skimmed through those pages, but the main take away is that unless you want a amp that works well on overdrive.
Otherwise the effect dosen't occur untill you overdrive.

Smaller grid resistors for better bias recovery time only makes your amp perform better during overdrive.
Overdrive always means distortion anways regardless of how small you make the grid resistor, for dudes that want real low distoriton hifi is always better to have schematics that are less proune to overdrive.

Author:  Suncalc [ 17 Mar 2019, 11:44 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

ILoveHiFi wrote:
Otherwise the effect dosen't occur untill you overdrive.
The point being missed is this: At higher volume levels, due to the dynamic nature of musical levels (especially in some classical scores), there is always a risk of brief periods of overdrive. How the amp performs during these brief periods is of paramount importance to the overall listening experience. I just try to make sure that when I design an amp, I address not just the average performance, but all the corners of the operations envelope.

Author:  john55 [ 17 Mar 2019, 12:01 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I take it that when you design an amp you are looking for as much power as possible without compromising the quality, therefore you design each stage to run full out. As you say, there is always something in a piece of music that will push it into overdrive unless you design it to run at say three quarter power from the start. I understand all of that and why you need a fast recovery time when it's designed to it's limits. What I would like explained if possible, is the 400 bpm. Why that figure, and how does that relate to 50ms recovery time.

Author:  Suncalc [ 17 Mar 2019, 19:30 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
What I would like explained if possible, is the 400 bpm. Why that figure, and how does that relate to 50ms recovery time.
Well traditionally the Vivace tempo, which is the fast tempo in classical music, tops out at about 180 beats per minute. However, there are some Spanish guitar pieces that actually push the 250 beats per minute limit although few can play them. In modern music, there are some beats which are upward of 300 BPM. This is about 5 beats a second. The 400 bpm limit I use relates to about 6 and 2/3rds bpm or one beat every 150mS. Since recovery takes about 5 times the time constant, I set the maximum time constant at about 30mS. 30mS*5=150mS. 1/150mS = 6.667bps or 400 bpm.

Author:  john55 [ 18 Mar 2019, 02:57 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Thanks Matt. I've just found a post of yours on a thread called "coupling capacitor" viewtopic.php?f=9&t=5113

For anyone looking to understand bias excursion and bias recovery this is a MUST READ.

Author:  john55 [ 19 Mar 2019, 13:31 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Hi Matt, In your post on a thread called Coupling Capacitor, ( link mentioned in above post), there is a part schematic to make it clearer. It's a very clear explanation and I've understood everything. Silly question, but can you tell me what the abbreviation " Reg " stands for, as in " Reg = Ro + Rg2 ". I've been searching for hours to no avail.

Author:  Suncalc [ 19 Mar 2019, 17:27 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

john55 wrote:
I've been searching for hours to no avail.
I'M SORRY!

That's my sloppy handwriting. It should be "Req" as in "R equivalent". It's just my shorthand. I hope you didn't spend too much time on this. :blush:

Author:  john55 [ 21 Mar 2019, 12:47 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Please don't apologize. I've learnt more in the last three weeks than I ever thought possible. Mathematics is not my strong point, but a good explanation, with examples works wonders.

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