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Tube Rectifiers vs Sag Resistors

11/10/2003 8:13 PM
Man o'Blues Tube Rectifiers vs Sag Resistors
This topic has probably been kicked around here before, but I'd like to know if a low value power resistor (e.g. 100 ohm 20W) wired in series between the ss recto and the first filter cap node can effectively emulate the sag produced by tube rectifiers.  
Any feedback or caveats would be appreciated.
11/10/2003 9:37 PM
Ian Anderson

I know this is a round-about answer, but the Weber copper caps can be ordered with a specified amount a sag and also come in 'hard wire' versions if you don't have a tube recto socket.  
... Ian :)
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11/10/2003 9:59 PM
Man o'Blues
Yes, the copper caps would be an alternative but a 20W sag resistor, at 1/4 the cost of a copper cap, is looking pretty good right now.....
11/11/2003 1:18 AM
Don Symes

If I did the math right (unwind W = I^2*R to solve for I), that should be good for over 400mA. Give it a go.
11/11/2003 1:02 PM
Mark Abbott

I'm by no means an expert on this but, I remember when Kevin O'Connor had the 100ohm resistor (as a tube rectifier simuator) in one of his books a friend tried it out, and he hated it. He said the tone lost a lot of highs, and became very middly.  
I once did something similar to this, that being the light bulb current limiter (the [cheap] safe power up tool), and I found I had similar results.  
It's quite funny for a while, and could even be a cool trick as a different sound while recording, but I couldn't live with this.  
Should you want to go this way, I'd try it out (very briefly) with various 5 watt resistor values. Which are cheaper.  
Another option would be to use a 1K resistor instead of a choke, to get some power supply sag.  
I have read where people have tried increasing the size of the screen grid resistors, I haven't tried it myself. I think there is a thread saved in Blue guitar about this.  
Anyway, I hope this helps, and let us know how you go with the resistors.  
Yours Sincerely  
Mark Abbott.
11/11/2003 1:29 PM

A rectifier's sag is not linear like a resistor. I don't know whether or not the difference is audible. However, I've never had a tube rectifer kill off the high end or make an amp's tone all middly, like these other guys have experienced with sag resistors, so maybe that is the audible difference.  
11/11/2003 1:57 PM

My $.02. I have had very good results with using series resistance in both the B+ line before the first filter cap, and using it further downstream right before the CT of the OT primary. The first connection sags the whole amp and the second only the output stage.  
I have noticed zero effect on frequency response. The only effect is compression under load.  
To my ears, a properly chosen series resistance is indistinguishable from a tube rectifier. Others have verified this in side by side tests in amps I have doctored.  
I can't comment on others experiences, but have done this enough times with uniformly positive effect that I suggest you try it and see what you think.  
BTW, I understand the Copper Cap from Weber works well, too. It is just an $8 commercial solution delivering the same result as a $.22 wirewound resistor.  
Good luck,  
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