I have a Fender 60 Watt Hot Rod DeVille with 2 12 Eminence Speakers and I recapped it not long ago and I changed one cap C1 to 22 uf to add some more tone I remember, Other than that one mod it is stock. 2002 Model Year. I had a nec and ge in it for about a year now and I was testing out some RCA 6L6 GC tubes in it today and individually They were 98% and 100% Emissions with 50 mV and 46.5 mV each. @ 489 plate, 487 grid and -057 bias. Cold Idle I set the bias to 63.5 and when they get hot it goes up to 102-104 mV at idle two tubes, it's Max idle @ 85 % . The tubes sound really good there Hot and I don't have any negative bias adjustment left. I turned the lights off and I was looking for red plates but I didn't find any but I noticed a little bit of blue haze in the tube near the getter and I tested them for shorts and grid leakage with my tube tester. The Amplifier has never sounded better than it does Now. I tried a pair of RCAs in it when I first got it and I didn't like the sound super bassey At 60 through 70 mV idle so I put what I had in it The NEC and GE instead At 63.5 mV idle no signal. It was okay but not really what I was looking for. I have noticed that blue haze before in 6L6 GC and other vintage 6L6 tubes. I was told Year's ago to just ignore it if they sound good. I played it for a few hours and it plays good. Am I worrying about nothing or should I be concerned ? I've heard some people say that 85% is fine with 6L6 GC Vintage tubes. I have 3 other pairs of The RCAs matching, I used to have a Fisher Ambassador that I converted to use 6L6 GC and One of the OTs went bad so I pulled the tubes out and I saved them. I forgot about them and I found them in a bag together the other day and they do match but I had another 5 of them also. I have two 97% @ 30/30 mV , two 97% @ 27/27 mV and two 100% @ 67/67 mV. They don't have any shorts or grid leakage. I'm A little bit afraid of using the two that Are even higher At 67 mV with The limited negative bias voltage and I don't really want to Use the other four because it's a Quad of working good RCA BP tubes. The Amplifier is actually quieter now At 104 idle than it was with the old tubes at 63.5 mV idle. I can run the settings down at 3 now and I have a good balanced room volume, they were up at 5 to 7 before on the clean channel. I don't want to burn up my 1200 dollar amplifier but I don't want to have a crappy sound either. I've got some really good vintage pre amp tubes in it. I know that I could play it safe with two of The other tubes but they probably are not going to sound near as good with 20 less mV idle. I've never really tried pushing the limits before with Guitar Amplifier tubes but it seems to me like the best sounding player's definitely do ! . I remember an old Sovtek MIG 100 lead I had that had 580 volts plate and EL 34 Vintage Mullards it was loud and clear and it would just about strip the paint off the wall. I want something like that but A little less loud. The 60 Watt DeVille seems just about right to me. I can still get good tone but At A lower volume. I play At home now and I just want to have a lot of extra power or headroom in Case I play out with A live drummer sometime. It really sucks when you can't play through the mix because you have a weakling Amplifier. And I tried the Hi power transistor stuff and up loud they didn't cut it for me. I had a custom built Crate Amplifier once that did but it broke down in short order. I'm done with those things. I guess that I'm just looking for opinions about The 6L6 GC RCA black plates and their limitations. The Old RCA tube Data is not really up to par with modern guitar amps with the Big voltage drops between plates and grids of 50 and 100 volts. I don't know exactly how to make sense of it for my application.
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If you want a short life foryour valves,go ahead.
The factory set the bias at 60mA meaning 30mA per valve.
Between +B and Z should be around a 50volt drop.
Too much current and the output transformer will swamp, causing over current peaks, not enough and there will be crossover distortion.
The factory set the bias at 60mA meaning 30mA per valve.
Between +B and Z should be around a 50volt drop.
Too much current and the output transformer will swamp, causing over current peaks, not enough and there will be crossover distortion.
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What exactly is considered short tube life at 85 % compared to normal tube life at 70% ? . Is it going to be the 15% difference between the two ?
Maybe I could change a resistor to add a little more negative bias and bring it down a little to say 80% ?
I just watched a video about the amplifier and He said that the screen current and the resistor on the test point together would lower the idle about 4 to 6 mV. , I'm not sure if he is saying for both tubes or for each tube but that would help bringing it closer to 80 % , 80% is 49.08 each. He is basically saying that you need to know how to read it properly.
I'm thinking that I might be just chasing a ghost At This point after watching the video and being At 80% instead of what I thought it was At 85% because I wasn't reading it properly. I don't think that 10% over the recommend 70% is going to hurt anything. It's 5% for each tube. It's not good to ignore the screen and TP resistor when you bias a amplifier because you could end up too cold down in the 55 range if you used the minimum 60 Fender setting. I'm starting to understand now Why I have been having trouble trying to get good tone from the tubes before I put this set in this amplifier. At the low safe Fender 60 setting It was only around 55% even though the meter said 60% .
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You want 60% max. Need more negative voltage to lower the current.
And ya, 1K resistors on the screens.
And ya, 1K resistors on the screens.
No, an exponential law applies.
E.G. to estimate aactual life expectance from accellerated tests, Arrhenius' equation is used not only for chemical reactions (its original purpose) but also to assess wear in mechanical and electronic components or systems. Be it ball bearings or solid / hollow state ... or whatever.
Accelleration factors and activation energy for the formula have to be determined experimentally for a particular component, but the result is usually that a
+ 10 Kelvin increase in temperature causes a factor of between 2 and 4 decrease in useful lifespan.
This does not cover manufacturing defects or similar shortcomings. just wear and tear from increased stress.
I decided to play it safe and I took the 30 mV pair of RCAs out of the Quad that I have and I set them At 67 idle . The other pair at 50 mV each was working okay but I don't want to take a chance on something going wrong. The RCAs sound too good to put the ge and nec back in there. I tried to but after playing The RCA All day The sound difference was just too much for me to overcome and I'm going to have to stick with The RCAs.
I learned a few things about it and to consider the TP resistor and the Screen current when biasing , it's not that much just 4 to 6 but it needs to be considered to get a warmer sound. I don't know about putting more resistance on the screens weather it is worth it or not. I know that they did do it in the old days.
old tubes had more robust screens. new amps aren't built with the idea that new production tubes have more delicate screens, so they push the tubes too hard and the screens fail prematurely. best thing to do is lower the screen voltage with a 1K resistor at the g2 pin.