Boris_The_Blade said:
I use them to power headphones -- good stuff.
Boris_The_Blade said:
Brian Beck said:
Subjectively, the 6C45 has similar specs, but sound wise is much inferior -- sounding shriller and edgier. I have an amp that can take either interchangeably, and the 7788 is much preferred.
The 7788 has a little brother, the 6688, that does have a Russian equivalent: http://www.diyaudio.com/forums/showthread.php?s=&threadid=110616 I've not seen a 7788 equivalent, though. For all but the most demanding applications, the 6688 is a really good tube.
7788
The tube was probably running about 1/2 hour before I took the picture. I'm not sure why you asked this - the getter is always working. When testing tubes that aren't grossly gassy, I've found that once the cathode is up to temperature (about 1 minute on small tubes) there is virtually no change in characteristics no matter how long I have them running on the curve tracer.
- John
The tube was probably running about 1/2 hour before I took the picture. I'm not sure why you asked this - the getter is always working. When testing tubes that aren't grossly gassy, I've found that once the cathode is up to temperature (about 1 minute on small tubes) there is virtually no change in characteristics no matter how long I have them running on the curve tracer.
- John
The half-hour warmup may have served the same purpose. The idea is that because of the relatively low particle velocity at room temperature, there can be a small amount of gas buildup. Elevating the temperature without drawing cathode to plate current causes the particle velocities to increase, which in turn causes more impacts with the getter, clearing out the residual gas without poisoning the cathode. Or so go the claims- I haven't done any before-and-after myself, but there was an article in Electronics World a few years ago about it, and Morgan Jones mentioned this issue in Valve Amplifiers.
Yes E810F/7788 does make a super triode; the main problem is that one never knows the true Ca-g of pentodes when triode-strapped, so it's important to bear in mind that substantial Miller capacitance is likely and will have to be driven.
The only example I can think of where a maker quoted Ca-g for a triode-strapped pentode is D3A. This one has an American number (7722 I think). It offers gm of 40 and mu of 85 with quoted Ca-g of 2.7pF. Quite a good choice for a MC phono stage I would guess.
7N7
The only example I can think of where a maker quoted Ca-g for a triode-strapped pentode is D3A. This one has an American number (7722 I think). It offers gm of 40 and mu of 85 with quoted Ca-g of 2.7pF. Quite a good choice for a MC phono stage I would guess.
7N7
I've seen somewhere else mention made of the 6Ж9П-E (6Sh9P) as a possible equivalent version. Haven't confirmed this for myself via a datasheet though (so consider this a possible pointer only).Boris_The_Blade said:
[Edit: I think the above tube might have been more in reference to the E180F/6688, so apologies]
7N7 said:
Yes I use triode connected D3A in the front end of my phono stage because they are so quiet. Sounds very good and is quiet enough for use with low output mc cartridges although I don't really have enough gain at 50dB for anything below 0.8mV - 1mV or so. (My line stage has about 4dB of voltage gain.)
arnoldc said:
The easy way is to get your hands on a good valve tester: I do not know about the American ones, but the British AVO VCM163 measures gm directly; I used to have one of these and apart from testing valves I used to use it to determine the current I must run to get the gm and hence anode resistance I wanted.
Failing this you must build yourself a little test rig. You will need a power supply (with milli-ammeter) to deliver your design voltage and an accurate source of negative voltage for the grid that can be measured of course. You can assume the manufacturer's claimed mu. Then just vary the -grid voltage in suitable steps measuring the change in current as you go along. This will enable you to calculate the gm and then of course the anode resistance: ra=mu/gm
The final method as described in Morgan Jones's Valve Amplifiers is to prepare or obtain (if they are available) a set of triode curves for your valve. Locate your operating point and draw an exact tangent to the curve at that point, or take an average between the two adjacent curves, that reaches from the top to the bottom of the graph. Then measure the difference in voltage between the upper and lower ends of the tangent line and divide it by the total current displayed on the vertical axis of the graph.
7N7
plate resistance
One way is to jig up a CCS-loaded common cathode circuit. Hold the grid voltage constant, then slightly vary the current. For each variant, measure the plate voltage. The plate resistance is then the difference in the two plate voltages divided by the difference in the two plate currents. To get valid numbers, keep the current differences small.
It should also be possible to measure it by jigging up the same circuit, then trying to drive a signal back through the plate from a known source resistance. The rp can be derived from a simple divider equation. I haven't actually done it this way, but I'm sure others have.
And finally, you can run a signal through the tube in common-cathode, monitor the output, then load it down with a known resistance. From the voltage drop, it's easy to get rp.
One way is to jig up a CCS-loaded common cathode circuit. Hold the grid voltage constant, then slightly vary the current. For each variant, measure the plate voltage. The plate resistance is then the difference in the two plate voltages divided by the difference in the two plate currents. To get valid numbers, keep the current differences small.
It should also be possible to measure it by jigging up the same circuit, then trying to drive a signal back through the plate from a known source resistance. The rp can be derived from a simple divider equation. I haven't actually done it this way, but I'm sure others have.
And finally, you can run a signal through the tube in common-cathode, monitor the output, then load it down with a known resistance. From the voltage drop, it's easy to get rp.
Ok, I got me a better 7788 sample.
This one seems to match up very well with the data sheet. For rp, just draw a line tangent to the curve at any operating point. Then it is just dV/di, like 7n7 said. For this sample I get roughly 1.5k, which is what we had expected.
The VACUTRACE will read out 1/rp (or gp) directly on its digital meter. At 200V, 20mA, it gave 0.48mA/V, or 2.1k. Not sure why the discrepancy, except for the 50V modulation on the plate might be a bit too much for this high gain tube. The test line is not straight and curves at the bottom, making the resistance appear higher than it really is.
jh
This one seems to match up very well with the data sheet. For rp, just draw a line tangent to the curve at any operating point. Then it is just dV/di, like 7n7 said. For this sample I get roughly 1.5k, which is what we had expected.
The VACUTRACE will read out 1/rp (or gp) directly on its digital meter. At 200V, 20mA, it gave 0.48mA/V, or 2.1k. Not sure why the discrepancy, except for the 50V modulation on the plate might be a bit too much for this high gain tube. The test line is not straight and curves at the bottom, making the resistance appear higher than it really is.
jh
hagtech said:Ok, I got me a better 7788 sample.
This one seems to match up very well with the data sheet. For rp, just draw a line tangent to the curve at any operating point. Then it is just dV/di, like 7n7 said. For this sample I get roughly 1.5k, which is what we had expected.
The VACUTRACE will read out 1/rp (or gp) directly on its digital meter. At 200V, 20mA, it gave 0.48mA/V, or 2.1k. Not sure why the discrepancy, except for the 50V modulation on the plate might be a bit too much for this high gain tube. The test line is not straight and curves at the bottom, making the resistance appear higher than it really is.
jh
Hello.
Nice curves, but I dont understand the reference to 0.48mA/V. On your curves at 200V, between -3 and -4V I read 15mA/V.
The 7788/E810F can do much better as the curves show. AT 40mA and say 150V you should get 50mA/V
7N7
Interesting. This reminded me I have 2 pairs of 6688 in store (too bad it's the little brother) and a 190-0-190 tranny. Anyone can suggest a nice schematic for 6688 linestage? A simple cathode follower with triode strap? What kind of values to use for grid and screen stopper? I'm probably going to use this to drive a small Tripath amp which drives a pair of full rangers.
thanks!
thanks!
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