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I replaced the cap (death cap) tying the hot to neutral on the incoming power cord and the heavy hum disappeared. I am reading about the benefit of removing that altogether.
I have purchased about a dozen replacement caps and will start with those. The amp sounds great now, but those caps are original, they are over 60 years old.
The vibrato section is not functioning at this time.
Larry
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Well, the tremolo stopped working this evening. I changed the intensity pot (1meg, linear) and I lost the tremolo effect. I don't think it has anything to do with the pot change because it checks out. I thought I found a short yesterday, but apparently not. The footswitch grounds pin 6 (not pin 5 as stated in the previous message) of the 6SL7 through a 2K resistor. I will continue to search. I have two resistors that indicate 41k each on the 6SN7 tremolo tube, but they measure 52k and 72k. They are in series and provide a different voltage drop for each of the plates on 6SN7. I have ordered new resistors and another pot for the tremolo speed. I am still on the upslope of the learning curve for tube amps.
Larry
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Well, I found that I only had one bright glowing filament in the bottom of my 6SL7. The other was barely glowing, if at all....not sure. The two plates in the 6SL7 are supplied by the same value voltage drop resistors (270k), so I think I have a failing tube on the tremolo circuit. I ordered a new 6SL7 tube (Tung-Sol 6SL7GT).
Larry
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I calculated the power dissipation (PD) in the two 6V6 tubes as 12.5 and 14.2 watts, both are cathode biased through a single 300 ohm resistor and a .47uf cap. I think that is a little "hot" according to my spec sheet. So I am thinking of lowering the PD a little by upping the resistor value. Then I thought maybe I will balance the PD between the two tubes AND lower it a little.....maybe shoot for 10 Watts per tube. So installing a bridging pot and a pot to ground would give me both adjustments. OR maybe a fixed and a variable resistor on each cathode to ground, giving me a range of 250-400 ohms for adjusting cathode bias. Thinking......
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11-10-2017, 09:25 PM
(This post was last modified: 11-10-2017, 09:27 PM by journeyman.)
Thanks for posting. I wish there was more activity on this forum, but the info that you and others post is really good and helpful for the rest of us as we attempt to resolve issues with these old Amgegs. Keep 'em comin'.
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jjmt,
for each tube, following the process explained by Uncle Doug in his youtube video:
- (power off) measure the resistance between pin 8 of the 5Y3 and pin 3 of the 6V6. This is the impedance on half of the input side of the output transformer ®
- (power on) measure the voltage drop across that same region (V).
- calculate plate current as V/R (PC)
- measure plate voltage relative to ground (PV)
- calculate Power Dissipation (PD) PD = PC*PV = 10 watts
6V6 tube has a max rating of 12 watts. 10/12 = .83
I installed independent bias resistors (550ohms and 680 ohms) and capacitors for each 6V6 to make the outputs approximately equal at 10 Watts. The amp has plenty of volume, so you may be correct that I can reduce the output of the tubes further.
I did not chart the results after I replaced the cathode bias resistors and caps, I just assumed that the results would fall out of my plot at 10 watts per tube (red dots on plot I posted earlier). To get down to 70%, I would be at 8.4 watts. To do that I think I would need 680 and 1000 ohm bias resistors by interpolation for the outer tube and extrapolation for the inner tube.
I am happy to hear your thoughts. Am I on the right track in your mind?
Larry
I feel like I am broadcasting in the blind.