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Rebuilding an old tube amp. - Page 2

post #16 of 25
What amp are you referring to? No "output caps" in the magnavox.
post #17 of 25
Thread Starter 
Today I cut out the tone controls but left the gain knob in. Going to attempt to clean it up without replacing it. Next I'm going to rewire the 6eu7 to a 12ax7 to try and reduce some of the noise. I also threw a few resistors on the output circuit to simulate a larger load, which helped with noise as well
Edited by wroachbarrette - 8/12/13 at 2:24pm
post #18 of 25
I'd question if there is any value in trading the 6eu7 for 12ax7. In my limited experience ecc40 (although rimlock socket) is my fav but I'd get some more recommendations.
Edited by macm75 - 8/13/13 at 4:58pm
post #19 of 25
Thread Starter 
Quote:
Originally Posted by macm75 View Post

I'd question if there is any value in trading the 6eu7 for 12ax7. In my limited experience ecc40 (although rimlock socket) is my fav but I'd get some more recommendations.

Im hoping that by switching to a lower mu tube it will reduce some of the noise that's left in the circuit. Though there's a chance tube socket also needs to be replaced as well. I am open to other suggestions however

post #20 of 25

While changing a tube that is a driver to a lower gain one . You lose drive . Yes it will be quieter   but the output for a given input will drop. And as the circuit is set up for a high gain tube you will lose small detail which is probably what you want if you want to reduce noise . But that is not how I would go about losing noise. Is that tube the input tube?. If so it plays a major part in the amount of noise injected into the  music signal.The first stage of any amp is the decider in how much noise an amp  has You should be checking the plate feed resistors and if they are carbon change for metal film. and if there is cathode bypass change the cap. and resistor thats there and then change the inter-tube coupling caps for high voltage poly film. There is a lot more including the power supply if it does not have a series choke but a large resistor.and rectifier tubes can be noisy and have low  emission  but that would be a start.  


Edited by duncan1 - 8/14/13 at 12:42pm
post #21 of 25
Quote:
Originally Posted by wroachbarrette View Post

Im hoping that by switching to a lower mu tube it will reduce some of the noise that's left in the circuit. Though there's a chance tube socket also needs to be replaced as well. I am open to other suggestions however
Both tubes, 6eu7 and 12ax7 are high mu. Both 100 amp factor
post #22 of 25

12 AX7--high mu?????-Not in this life time= 12AX7=Brimar= 1.6-ma/v-gm--Ferranti=1.6-ma/v-gm-Tungsram= 1.6 gm and many more manufactures including MULLARD. What you are probably mixed up with is that the E-series of twin triodes= ECC81/82/83.The 12AX7 is the DIRECT equivalent to the ECC83[mullard] which if you check with ANY reputable tube equivalent book is ==1.6 GM  Its the ECC81 which has the highest gain of those three=4.8 GM=12AT7. Checked on 5 internationally recognized tube equivalent books. 

post #23 of 25
Quote:
Originally Posted by duncan1 View Post

12 AX7--high mu?????-Not in this life time= 12AX7=Brimar= 1.6-ma/v-gm--Ferranti=1.6-ma/v-gm-Tungsram= 1.6 gm and many more manufactures including MULLARD. What you are probably mixed up with is that the E-series of twin triodes= ECC81/82/83.The 12AX7 is the DIRECT equivalent to the ECC83[mullard] which if you check with ANY reputable tube equivalent book is ==1.6 GM  Its the ECC81 which has the highest gain of those three=4.8 GM=12AT7. Checked on 5 internationally recognized tube equivalent books. 
Mu is amplification factor
post #24 of 25

Exactly! and its the ECC81 that has the highest.=12AT7-- 12AX7 =lowest gain. -of the three. Putting a low gain tube in place of a high gain changes the spec.  of the amp which is set up for a high gain tube.It all depends on how you measure gain on a tube tester and whether it tests it the correct way or not . but gm can be measured and shown on a tube tester.  


Edited by duncan1 - 8/15/13 at 6:19am
post #25 of 25

Mutual conductance=gm=mu times 1000 over Ra=MA/VOLT. Taken from =the Radio+ television Engineers Reference  Book.Advisory Editor=W.E.Pannett, A.M.I.E.E.quote=while mutual conductance is a direct indication of the  efficiency of the tube -amplification factor means little unless impedance is taken into account.unless the CIRCUIT conditions are such that a higher impedance tube can be used to advantage. The US use the term trans-conductance  but it gives the same result of efficiency the result being given in micromhos instead of MA/V I hope this clears this up.


Edited by duncan1 - 8/15/13 at 7:22am
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