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Old 5th March 2014, 03:21 PM   #21
JPS is offline JPS  India
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Quote:
Originally Posted by 2bamp View Post
I tried a couple of 6201 and even a 12AX7. It seems like the driver just isn't pushing enough into the power tubes. Has anyone else run into this?
Try 12AT7 instead of 6201.
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Old 15th March 2014, 11:08 PM   #22
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2bamp-

I just tested my new RH84 with audio from my phone's headphone out first and the volume was underwhelming. Plugged into my computer, I get way more. I haven't plugged it into a proper 2V source yet, but I bet it will be pretty consistent with other 5W amps I've used. Get thee a preamp (or a 2V source)!

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Old 16th March 2014, 02:58 PM   #23
2bamp is offline 2bamp  Canada
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I have found the same thing. Using my netbook as a source gives way better results. In fact, I am pretty happy with it, and since I almost exclusively use my tablet, I am also glad to have a use for some more "osbsolete" electronics. Next one will get an additional gain stage.
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Old 30th March 2014, 08:16 PM   #24
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I have always found that it makes sense to build it as the designer originally intended and then modify after that if we can define a specific "problem". Mr. Kitic does answer emails promptly and is a big help when tackling the RH projects.

I just finished a RH307A and there is nothing that I think needs changing. An iPod is "enough" but the DAC from my "big" stereo has a lot more grunt.

According to Alex Kitic, 1V input normally drives his amps to full power. You can check your players spec to see what it needs. I would not change the feedback. Many MP3 players have limited output to prevent deafness.

The comment about it being "Schade" is not correct. They do have a similar feedback arrangement, but similar is not the same.

Changing the 12AT7 to a 12AX7 will not work. The same holds true for a 12AU7. Mr. Kitic explains why at his blog. I have no difference in gain with a 6201 or 12AT7.

My next amp is probably a RH84 to complement the RH307A Super in a triamp situation. It will drive horns.
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Old 30th March 2014, 09:11 PM   #25
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Quote:
The comment about it being "Schade" is not correct. They do have a similar feedback arrangement, but similar is not the same.
It is Schade feedback, just a rather poor example of it.

Shoog
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Old 31st March 2014, 05:13 AM   #26
rowuk is offline rowuk  Germany
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Default ???

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Originally Posted by Shoog View Post
It is Schade feedback, just a rather poor example of it.

Shoog
Considering the remarkable sound quality, maybe "poor" is a less applicable description.

I do not consider all anode/anode feedback to qualify as Schade. Granted, we do stand on the shoulders of giants........regardless of what we implement in tubes.
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Old 31st March 2014, 08:14 AM   #27
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I suggest you go and read the Tubecad article to see why it is Schade feedback.

Shoog
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Old 14th September 2014, 08:59 AM   #28
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http://www.tubecad.com/march2001/2001_03.pdf

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Old 14th September 2014, 05:23 PM   #29
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The difference you suggest is at DC and not at AC so I do not share your analysis of how it effects the AC signal. Current is the force into the relevant node (here the grid) and is dependant on the effective impedance of that node. If it is low impedance node then it will have a current character. This is the case in the Schade arrangement.

With a driver tube the function of it in AC is to present a rather fixed voltage divider feedback path to ground and the AC signal thus reduced is presented straight to the output tube grid. This is why it is a gross mistake to use a triode which has a variable impedance and hence varies the feedback applied.

I emphasis - for the AC signal the two arrangements you describe are effectively one in the same - which is what we are interested in here.

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Old 14th September 2014, 09:29 PM   #30
disco is offline disco  Netherlands
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So, this SW I recently saw might be a product of variable feedback?

Click the image to open in full size.

RH84, 4 ohm load.
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Last edited by disco; 14th September 2014 at 09:32 PM.
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