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Old 24th March 2013, 08:15 PM   #21
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It wont make any difference , the grid doesn't draw any current so there in no voltage drop across the 47 k resistor .It is there as a grid stopper , I always use a 1k ohm metal film in its place , some old carbon comp resistors are noisey .
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Old 25th March 2013, 03:04 AM   #22
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You use a 1k grid stopper?

I was a little mistaken on how I described the connections. There was a 470k from signal to ground and 47k from signal to tube. I ended up removing the 47k and using the 470k as the grid stopper. I then added a .1uf cap from signal to grid. This eliminated one carbon resistor from the audio path and also decouples the input.

Should there be a resistor from input to ground to provide some impedance for the line-level source?

Last edited by lazzer408; 25th March 2013 at 03:09 AM.
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Old 25th March 2013, 03:56 PM   #23
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Quote:
Originally Posted by lazzer408 View Post
You use a 1k grid stopper?

I was a little mistaken on how I described the connections. There was a 470k from signal to ground and 47k from signal to tube. I ended up removing the 47k and using the 470k as the grid stopper. I then added a .1uf cap from signal to grid. This eliminated one carbon resistor from the audio path and also decouples the input.

Should there be a resistor from input to ground to provide some impedance for the line-level source?
Such a large grid stopper is not needed and normally is undesirable. 1-10k is usually fine. YES, you NEED a resistance from grid to ground so that the grid has a ground reference. The maximum allowable value will be stated in the tube datasheet. Also, the grid stopper resistor should be mounted AS CLOSE AS PHYSICALLY POSSIBLE to the grid pin. HTH
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Old 25th March 2013, 05:13 PM   #24
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I understand I need the grid resistor. As I mentioned, I decoupled the input with a .1uf cap and I used a 470k grid stopper. I asked about a resistor from INPUT to ground. Meaning source input before anything else.

Here's a schematic with the proposed resistor marked with an asterisk. If you say 1k is an acceptable input impedance (when not using a capacitor) then 1k may be suitable here. If I lower the grid stopper to 1k I'll need to increase the capacitor as well.

You'll have to pardon my not knowing something that may seem obvious. In most of my other tube amps I use a 1meg grid stopper but those are instrument amplifiers.

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Old 25th March 2013, 06:25 PM   #25
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Unless your preamp or signal source puts out DC the input cap really isn't necessary .
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Old 25th March 2013, 07:25 PM   #26
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I'm measuring a small DC voltage coming from my computers sound card or maybe a ground loop? I figured the cap would eliminate any risk of DC.

I removed the single 100ohm output cathode resistor and installed two 200ohm 5w resistors (one per channel) along with a bypass cap but I have 13v on the 6BQ5 output tube cathodes. Is that ok or too much? Schematic calls for 11v.

Also, can anyone tell me what amps are in a Magnavox model 1RP625?

Last edited by lazzer408; 25th March 2013 at 07:27 PM.
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Old 25th March 2013, 10:46 PM   #27
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The more I mod this amp the more I wish I left it alone. It seems to be loosing the "fullness" it had with the 45yr old components in it. It still sounds great but it seems like the treble is getting harsh yet very "twinkly", the mid-range is thin, but the base is much better then when I started. I used all poly-film capacitors with .1 and .047 between stages as the mod schematic recommended. Could the 13v bias on the outputs be causing this? I even replaced the crossover caps in the speakers just in case they were the cause. No change.

A closed hi-hat almost hurts at high volume. But the bass... THE BASS!! It's so smooth and buttery.

Last edited by lazzer408; 25th March 2013 at 10:51 PM.
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Old 25th March 2013, 11:09 PM   #28
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There seems to be some confusion with terminology. The grid stopper resistor is the one connected in series with the grid (47k on the original circuit, 1k suggested by others), and is there to stop HF oscillation, it should be soldered directly to the tube socket. The 470k resistor from grid to ground is the grid leak resistor. If you want the input capacitor, I don't think it is necessary to have the resistor with the star in your diagram. So you would have input to 0.1 uF capacitor, then 470k resistor to ground, then 1k resistor in series to grid.
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Old 25th March 2013, 11:12 PM   #29
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Quote:
Originally Posted by lazzer408 View Post
I understand I need the grid resistor. As I mentioned, I decoupled the input with a .1uf cap and I used a 470k grid stopper. I asked about a resistor from INPUT to ground. Meaning source input before anything else.

Here's a schematic with the proposed resistor marked with an asterisk. If you say 1k is an acceptable input impedance (when not using a capacitor) then 1k may be suitable here. If I lower the grid stopper to 1k I'll need to increase the capacitor as well.

You'll have to pardon my not knowing something that may seem obvious. In most of my other tube amps I use a 1meg grid stopper but those are instrument amplifiers.

Click the image to open in full size.
Ok, terminology is the problem here... in your drawing the 470k resistor from GRID TO GROUND is NOT a grid "stopper" resistor. A grid stopper resistor is a resistor is in series with the grid and prevents oscillation by forming a low pass filter. Your 470k resistor is often called a "grid resistor" and provides a ground reference for the tubes grid.

Refer please to the original schematic where a 47k resistor was in SERIES with the grid and a 470k resistor was from GRID TO GROUND. A capacitor in SERIES should not be needed.

To clarify; in your drawing there does not need to be a resistor to ground before the cap, there needs to be a resistor IN SERIES with the grid "a grid stopper" to "stop oscillations or the chance thereof, and a resistor to ground ( known as a grid resistor) to provide a dc ground reference for the tubes grid. A cap should not be needed. The grid to ground resistor comes BEFORE the "grid stopper" resistor which is the last thing in line with the tubes grid. I can refer you to a few schematics if that is not clear... HTH
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Old 25th March 2013, 11:13 PM   #30
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too slow... tikiroo beat me to it..;
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