|
Post by Dana {aka gp} on Oct 17, 2006 10:14:16 GMT -7
A few different threads in this forum have mentioned that a noticeable improvement in sound can be achieved by replacing EL84 (6BQ5) power tubes after as little as 30 hours of use.
This surprised me but I don't doubt that it's true, especially for new production tubes. What I do wonder, though, is what happens to the tubes to make them go bad?
Specifically, have they truly reached the end of their useful life or have they just drifted in performance so that pairs and quartets of power tubes are no longer matched? Given that new tubes vary so much in performance, it only seems natural that they might age at different rates.
Has anyone here tested sets of used power tubes to see how their absolute and relative performance has changed after 30 hours or so? If so, would you bo so kind as to share your results.
[somewhere in the cyberspace of LA and DC, the thermionic call goes out to Myles and Mike K...]
What about phase inverters? Myles has suggested that it might be a good idea to replace the phase inverter when a new set of power tubes are installed. Would a phase inverter truly go bad after just 30 hours, or would it just go out of balance (if ever was to begin with)? This is ask because an unbalanced dual triode can still be used in other preamp positions, right?
Thank you in advance for your help!
-Dana
|
|
|
Post by myles on Oct 18, 2006 9:56:10 GMT -7
A few different threads in this forum have mentioned that a noticeable improvement in sound can be achieved by replacing EL84 (6BQ5) power tubes after as little as 30 hours of use. This surprised me but I don't doubt that it's true, especially for new production tubes. What I do wonder, though, is what happens to the tubes to make them go bad? Specifically, have they truly reached the end of their useful life or have they just drifted in performance so that pairs and quartets of power tubes are no longer matched? Given that new tubes vary so much in performance, it only seems natural that they might age at different rates. Has anyone here tested sets of used power tubes to see how their absolute and relative performance has changed after 30 hours or so? If so, would you bo so kind as to share your results. [somewhere in the cyberspace of LA and DC, the thermionic call goes out to Myles and Mike K...] What about phase inverters? Myles has suggested that it might be a good idea to replace the phase inverter when a new set of power tubes are installed. Would a phase inverter truly go bad after just 30 hours, or would it just go out of balance (if ever was to begin with)? This is ask because an unbalanced dual triode can still be used in other preamp positions, right? Thank you in advance for your help! -Dana Dana, Cathode material burns up as the tube is active. As this is burned off, there are impurities generated from the cathode material and other parts in the tube that get hot. These impurities are attempted to be absorbed by the barium (the getter flash) in the tube. As the materials burn up and as the barium gets to a point where it is no longer effective the tube's performance drops. .... the phase inverter. Many folks never pay attention to the phase inverter which is the most hard worked preamp tube. Some amps have output sections that are more easily driven than others depending on circuit design and output tube type. Master volume amps have the master either pre or post phase inverter. How one has the amp set up can also really tax the phase inverter on some designs. No matter what the amp I generally change the phase inverter every other tube change if the amp is a grid biased 6L6, EL34, 6550 or KT88 amp. On the Z EL84 amps I change the phase inverter about every 300 hours or so but I generally measure the PI when it is installed and keep tabs on it. When it's transconductance or it's current drive ability drops to 20% below it's initial value I toss the tube. As far as PI balance .... on a good day 1 in 20 preamp tubes have matched triodes on a curve tracer. On a bad day it is less than 1 in 50. The reason GT has steep matching prices on PIs as these are not matched on a tube tester, they are matched on a vacuum tube curve tracer across the entire operating range of the tube with signal under load. Static matching is much faster and easier (and less pricy in the end) and does a much better job than hit or generally miss in hoping for a matched tube. Many folks will statically match a dual triode for $2 or so. Over at GT the cost to do this is $10. On the GT tubes, I am the one who does this and I keep a report on the tubes with the chart of it's curves, plate resistance, transconductance, gain, current output, leakage, and other factors. Folks that have had me blueprint their amp get documentation when the amp is done where the specs on every tube in the amp are known and recorded. Many times additional tubes are supplied for different applications for the amp. The amp's voltages are also doumented. This way I learn how the amp is used over time and can come up with a reliable service plan for the amp.
|
|
|
Post by Telemanic on Oct 22, 2006 15:08:43 GMT -7
Hi Myles, In regard to the Phase Inverter, there seems to be some differing opinions about how much or little they affect the overall tone of the amp. Some say it has no affect at all, while others say it has a big difference. Considering current capabilities and balancing to be equal what are your thoughts as to the PI having a bearing on purely the tone, per say? And in that regard, would a "curve balanced" Groove Tube, out perform in every case a nice NOS that happened to be balanced by as you decribe, the typical less exact methods most sellers would use?
Just curious if the PI is a tube that the guy who wants to run ALL NOS in his amp, should deviate from, and use a current production, Balanced GT, and get better results? Thanx!
|
|
|
Post by myles on Oct 24, 2006 10:13:45 GMT -7
Hi Myles, In regard to the Phase Inverter, there seems to be some differing opinions about how much or little they affect the overall tone of the amp. Some say it has no affect at all, while others say it has a big difference. Considering current capabilities and balancing to be equal what are your thoughts as to the PI having a bearing on purely the tone, per say? And in that regard, would a "curve balanced" Groove Tube, out perform in every case a nice NOS that happened to be balanced by as you decribe, the typical less exact methods most sellers would use? Just curious if the PI is a tube that the guy who wants to run ALL NOS in his amp, should deviate from, and use a current production, Balanced GT, and get better results? Thanx! I have a ton of writing on my GAB website on phase inverters.
|
|
|
Post by Telemanic on Oct 25, 2006 10:52:58 GMT -7
Thanx Myles, i recall that now that you mention it, you have a tremendous amount of resource there. I will check it out, Thanx!
|
|