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 23rd February 2005, 09:12 AM #11 diyAudio Member   Join Date: Feb 2005 Location: Knoxville TN Yes, Philip, something is very badly out of whack here. If you put 1vAC into the 6j4, you ought to have less than 55 vAC out. With .088vAC in, you'd get less than 6.8vAC out. It is *impossible* to actuall get readings such as you have, no matter how many times you check them. Let's solve that problem first; you have something wrong with your test setup. How do you know that the source is putting out a sine wave? What are you using to check the voltages and what scale are you using for each reading (only* on the 6J4? Also, tell us again what the reading is at the cathode of the 6J4 (I'm in the reply screen and can't see the main thread). You would have a far better chance of working this out if you could find some way to get a good sine wave at 1vAC into the grid of the 6J4. Do you have a friend with a real signal generator? Don't even think about the output stage at this time. Cheers/Fader
 23rd February 2005, 02:31 PM #12 diyAudio Member     Join Date: Feb 2005 Location: Dallas, Texas John - thanks for your help. I think the input math is correct, my iPod is delivering 880mV (not 88mV) which by your math of 1V in = 55V out makes my reading of 34V out about right? I am measuring with the Radio Shack RMS multimeter in "auto" and have the Spectra-Foo FFT analyzer running on my Mac to confirm the sine wave. The cathodes on the 6J4s read 2.7VDC each. I am trying to get time on a scope & generator today and will hit the input with 1VAC. Thanks again. Philip
 23rd February 2005, 02:54 PM #13 diyAudio Member   Join Date: Feb 2005 Location: Knoxville TN OK, Philip, That last sounds all good. You don't need to sue a signal generator now that we've determned that you've got nearly a volt and that the sine wave is good. Better (moe convenient) test gear is nice, but now necessary if you've got *something* that yoiu can trust. So, you've got 34vAC out of the 6J4, so that's all good. Bias sounds good, and so did plate when we were looking at that. Now for the output: Bias sounds good as toes grid drive. E can't remember plate, but I think that was good. So, was the 10V or so output you mentioned initially the AC riding on the plate voltage? Or was it the voltage at the 8 Ohm speaker terminals? I suspect that it was the former. If so, and your tubes are good, we have a serious problem. Try this: With the signal applied to the input, put your RS DVM on the AC scale that suits the grid and tell it to do relative (dang, too bad that DVM dosn't have a dB scale!!!). Now move to the plate and report the relative reading, + or -. Though I don't believe that two 1626s would have gone bad in exactly the same way and value, we can't dismiss the possibility. Get another one, AAMOF get a pair, as you'll need them when you build your Parallel Darling :-) Let's see if the new ones are "similarly failed" ;-) Cheers/Fader clutching at straws :-)
 23rd February 2005, 08:24 PM #14 diyAudio Member     Join Date: Feb 2005 Location: Dallas, Texas OK - did a full bench test today using 1KHz at 1 V as a test source. All is fine until we reach the plates of the 1626s. At that point the gain disapears and the signal is heavily clipped. We took some current readings, measuring the Plate at 23mA at idle, 37mA with signal applied and the Cathode at 18mA under idle. This seems high, plus the 500R/5W resistor is getting pretty hot. My friend with the test gear is also suspecting the tubes so I will take your advice & order another pair & see what happens. In the meantime I might start my phono pre-amp, any suggestions? Thanks, your time is really appreciated. Philip
 23rd February 2005, 09:52 PM #15 diyAudio Member     Join Date: Aug 2002 Location: Melbourne Something to check - is the 1626 grid at 0VDC with no input signal? I'm thinking maybe leaky coupling caps could cause the problem. Maybe not but worth a look. I'd also measure the cold value of your cathode R just to make certain it hasn't gone weird. Although this probably isn't the problem, 880 mVrms input will try to drive the grids of the 1626 well positive relative to the cathode at your bias point. This will do weird things to the operation - bias voltage and current will shift, grid current will start - not good. If you want to ensure "normal" operation while testing, 250 mVrms input is more appropriate.
 23rd February 2005, 09:56 PM #16 diyAudio Member   Join Date: Feb 2005 Location: Knoxville TN Dang, Philip, I just spent 10 0minutes talking about phono and line stages and i hit some wrong key and it all went into the bit bin somewhere. I'll simplify a good bit here: Do you plan to scratch-build or buy kits? Do you plan to build a line stage? I have strong opinions on both these items, and will go on if you wish :-) Cheers/Fader
 23rd February 2005, 10:46 PM #17 diyAudio Member     Join Date: Feb 2005 Location: Dallas, Texas Actually, I started a new thread since this is moving OT, but anyway; 1) scratch build 2) yes, I will build a line stage but the plan is to initially drive my Darling (assuming she'll ever work) with the RIAA pre and then at least I'll be listening to vinyl through my (yet to be built) Fostex based speakers. The whole plan is to have built as much of a complete system as possible with my own fair hands! I'll leave the CD transport & DAC for later, likewise the turntable but hey it's a start. P
 23rd February 2005, 11:19 PM #18 diyAudio Member   Join Date: Feb 2005 Location: Knoxville TN Tood Show, Philip, I'll nove this over to the other thread, but will stand by on the Darling thread to await news and to offer help. Cheers/Fader
 25th February 2005, 06:28 AM #19 diyAudio Member   Join Date: Feb 2005 Location: Knoxville TN Philip, I just thought abut grid current and signed on to suggest coupling caps. Readingabove, I'll just second jeff's suggestions.
 25th February 2005, 01:12 PM #20 diyAudio Member     Join Date: Feb 2005 Location: Dallas, Texas Thanks guys! I'm waiting on some new tubes too. The coupling caps would be the 27uF films connecting the 6J4 plate to the 1626 grid I assume? Philip

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