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how low can you go.....??

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Posted on January 1, 2017 at 08:27:21
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013



And I don't mean the limbo mon. I couldn't get JC Morrison's remark in his article in SP magazine about running the 845 "low and hot" (low volts hot current) out of my mind. I mean what works for the 845 might work for an 813 right? I first went to 325 vdc for the plate volts with 65 mA plate current, which I have already reported I liked, but this morning I was in my shop wondering what other tweaks I might try while waiting for the other switching transformers, and I was wondering just how low and hot I could get away with.
I went ahead and changed out the cathode resistors from 360 to about 160 ohms and ended up with about 265vdc plate volts and about 93 mA for the plate current on average. Now according to everything I have read you would not expect the 813 to perform well at those operating points, but it is the best sounding set up I have managed so far. It's like somebody strapped a set of brass cajones on them and sent them to a golds gym for a couple months of hard core body building and steroids! The weight and authority are really good. They are almost palpable. My question is why? Is the current the controlling and most important factor here?
I have attached a copy of the plate curves for the 813 in triode connection mode, and the thing I am wondering is how does it effect things to have the operating point that close to 0 for the bias? I would love to hear any explanations anyone has for why it would sound better this way, because I am really curious. Unfortunately JC never said WHY we should run the 845 low and hot, but it seems to work for the 813 as well. Enjoying the heck out of it so far.

 

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RE: how low can you go.....??, posted on January 1, 2017 at 09:09:29
Chip647
Audiophile

Posts: 1653
Location: The South
Joined: December 24, 2012
A couple of things. Once you get to close to 0 in grid signal voltage, you run into A2 conditions. The tube must be designed for A2 and the driver must be able to put out serious watts. The operating conditions change with the voltage, the tube requires a different impedance from the OPT at different voltages.

 

RE: how low can you go.....??, posted on January 1, 2017 at 10:29:35
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
So how close is too close? I don't think any of those things are in play right now or it wouldn't sound as good as it does.

 

RE: how low can you go.....??, posted on January 1, 2017 at 10:49:10
Triode_Kingdom
Audiophile

Posts: 5805
Location: Texas
Joined: September 24, 2006
It can be difficult to precisely identify amplifier characteristics by ear. Some people have really well-trained hearing, somewhat akin to musicians with perfect pitch, but the majority of us aren't that blessed. There's a good possibility you would find the underlying cause for what you hear if you were to measure response and distortion. We've had this discussion before though, and I realize you're only interested in tuning the amp to suit your hearing. In that regard, it sounds like you're on the right path.





--------------------------
Buy Chinese. Bury freedom.

 

RE: how low can you go.....??, posted on January 1, 2017 at 11:29:42
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
I understand what you are saying, but I was wondering if there was some general theory that might offer a possible explanation for this, you know like raise the current and you will get clearer sound or something like that.

 

RE: how low can you go.....??, posted on January 1, 2017 at 12:36:06
Tre'
Industry Professional

Posts: 12020
Location: So. Cal.
Joined: February 9, 2002



From what I can tell your amp produces less than 1/10 of a watt before reaching grid current (A2).

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: how low can you go.....??, posted on January 1, 2017 at 12:59:34
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
And that means what exactly?

 

RE: how low can you go.....??, posted on January 1, 2017 at 13:16:58
Tre'
Industry Professional

Posts: 12020
Location: So. Cal.
Joined: February 9, 2002
It means that if you play your amp louder than 1/10 of a watt the output tube does reach A2 (grid current) and the driver stage will have to deliver current to the grid and if it can't deliver the needed current the distortion will go up very quickly.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: how low can you go.....??, posted on January 1, 2017 at 13:48:34
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
Well, all I can say is if it is indeed doing that it must be a good brand of distortion because the amp sounds great. And it sounds good with any type of music I play, jazz, blues, classical and pop, so I know it isn't a certain recording that is making it sound good. But then aren't SET amps famous for their good distortion anyway?

 

what a hoot!, posted on January 2, 2017 at 06:57:28
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
Been playing with different combos of plate volts and current on the 813's in my bread boarded 26/813/813 amp. That is one of the things I love about SET amps, just a few parts changes can make a big difference in the sound. I first ran it up to 270 vdc & 93mA current, but this morning I dropped it back to 305 vac & 75mA. That's about half way back to the original values of 62mA & 325 vdc I started with. It is really interesting to hear the differences in the sound of the amp these changes produce. The highest current had the gutsiest sound, but the 75ma has a nice balance between that and the clean highs and pure tones of the 63mA point. Right now it looks like 75mA is what I will use, but I may load up on a bunch of different value resistors to use for the cathode resistors and see how much I can fine tune it to my liking. Never a dull moment with these toob thingeys....

 

RE: how low can you go.....??, posted on January 2, 2017 at 07:52:44
used-hifi
Audiophile

Posts: 472
Location: portage IN
Joined: March 18, 2003
If you have a decent enough sound card, just download arta or right art software it's free!

This way you can quantify what your hearing to basic Mesuring of FR and distortion and it's fun! And you will learn some things too.


Happy new year!

Lawrence

 

RE: how low can you go.....??, posted on January 2, 2017 at 08:55:44
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
Unfortunately I can't get wifi in my shop. I would really like to have a computer out there for many reasons. But I would have to get a separate ground line for it. I guess I could take my old laptop out htere after I loaded it in the house though....... thanks.

 

RE: what a hoot!, posted on January 2, 2017 at 08:59:23
dave slagle
Manufacturer

Posts: 4752
Location: NYC
Joined: April 27, 2001
It seems you like the higher currents but when you increase the current and keep the plate dissipation about the same you lose maximum A1 output power quickly as your bias approaches 0v. Since it is a 100w tube why not keep your original 325v and juice the current to 100ma?

Dave

 

RE: what a hoot!, posted on January 2, 2017 at 10:55:45
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
I would if I could, but I am trying to re-purpose the power supply from my old amp, and with four 813's the 325 vdc is the most I can get at 63 mA. If I go for higher current it drags the B+ down. Got any ideas besides a new power tranny?

 

RE: how low can you go.....??, posted on January 3, 2017 at 07:16:49
PakProtector
Audiophile

Posts: 11268
Joined: May 14, 2002
Am running 600V/110 mA into 5k( it is PP, 10k a-a). Works pretty well at 20% U-L. Going to try straight pentode in a few. That is if the urge to go back to 4E27's doesn't bite me first.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: how low can you go.....??, posted on January 3, 2017 at 08:43:21
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
I am going to try raising the plate current as soon as I get a trans hooked up that can pull it off.

 

RE: how low can you go.....??, posted on January 4, 2017 at 11:39:34
morricab
Reviewer

Posts: 8427
Location: switzerland
Joined: April 1, 2005
Interesting. The Aries Cerat Diana is running with 90 mA current with an 813 in triode but I do not know what voltage. It has incredible kick and drive. Of course the massive 18kg output iron doesn't hurt.

 

RE: how low can you go.....??, posted on January 4, 2017 at 14:53:32
vinnie2
Audiophile

Posts: 2518
Joined: September 28, 2013
If you can find the plate volts please post them.

 

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