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Started by belleraphon88, July 02, 2013, 11:26:01 AM
Quote from: J M Fahey on July 02, 2013, 09:03:52 PM1) I'm not so sure he's asking for DC voltages.What *I* understood from that question is that "sent to" means "with how many volts (audio) it is driven"May be wrong, of course.*If* this is actually the question, it's not easy to answer because it's within an NFB loop, plus the first triode gets *heavily* loaded by said NFB network, but I think it's between 1 and 2 V RMS at mid frequencies.There's a bass boost below 300Hz (often known as "resonance" and a highs cut above 5KHz, probably to reduce shrillness.2) I'm not that comfortable with that "microphonism" either: Why does the OP say so?What are the symptoms?How does he know it's the tube?At first I "saw" that stage as 2 cascaded triodes, which means *a lot* of gain, that would make it very sensitive to microphonics, but then I saw how it's implemented and perceived gain is *MUCH* lower than expected, it shouldn't be more microphonic than a regular 12AT7 PI, to compare it to a full tube power stage.
Quote from: belleraphon88 on July 03, 2013, 03:31:11 AMThere is a level control on the gain channel, when i turn it up past 1 the microphonics just get worst. Seems like the valve gets pushed harder.
Quote from: phatt on July 03, 2013, 07:00:45 AMQuote from: belleraphon88 on July 03, 2013, 03:31:11 AMThere is a level control on the gain channel, when i turn it up past 1 the microphonics just get worst. Seems like the valve gets pushed harder. Hi, What exactly happens past one?In what way does it negitivly impact your sound?Does it feedback?Phil.
Quote from: belleraphon88the amount of volts that gets sent to the 12AT7 valve in these randall G3 amps
Quote from: belleraphon88would it be possible to modify the amp power amp so that i can get rid of the valve.
Quote from: Roly on July 03, 2013, 07:20:46 AMHi and welcome.Let's dispose of one thing at the outset - your overheating output FET's have nothing to do with your microphonic valve problem.Quote from: belleraphon88the amount of volts that gets sent to the 12AT7 valve in these randall G3 ampsThe supply voltage is given on the circuit as 375V. From looking at the RetroValve specs they specify 120-400V, so it would appear that they are intended to work in amps circuits just like this one, and if you get the correct RetroValve to replace a 12AT7 (gain wise) then I can see no obvious reason why it shouldn't work.I do however share JM's curiosity about your string of microphonic valves. All valves tend to be a bit microphonic to some small extent, even brand new, but it sounds like you have either had a run of bad luck, or this particular circuit is sensitive to slightly microphonic valves, 'tho I can't see any obvious reason why that should be.To be quite specific; a "microphonic" valve produces a glassy tinkling sound a bit like wind chimes when tapped with a chopstick or similar, not crackles or loud blurts, these normally being cause by poor socket contacts - are we addressing the right problem here? Apart from the character of the sound produced it should also be very localised to the valve. There are other components (and dodgy connections) that can also produce strange sounds, but they are normally of a different character and not localised to tapping (gently) on the valve glass.If indeed your problem is microphonic valves then a solid state replacement would seem to be a good idea to try, and this RetroValve product seems suitable.Quote from: belleraphon88would it be possible to modify the amp power amp so that i can get rid of the valve.It is, but is a different order of complexity. Even as a tech I would be inclined to try the RetroValve path first as the simplest solution to your problem.The RetroValve page does make a point about their product changing the tonality of an amp, mainly because FET's don't have an identical transfer function to the valve they replace, however they are still similar, but it's something to keep in mind - it may not "crunch" or distort in quite the same way you have become used to with a real valve.If you do decide to try these out I'm sure we would all be interested in your report on how you find the replacement.Postscript - a "popping" sound isn't microphonics; sounds more like a base connection problem. As JM says, a sound sample of the problem would be helpful. Also, does wiggling the valve in its socket help, make the problem go away for a while?
Quote from: J M Fahey on July 03, 2013, 07:12:33 AMAgree, you are not describing what happens.Don't "diagnose" (as in "it's the 12AT7" .... ""it happens because the tube gets pushed" .... etc.) , just tell us: "I raise volume past 1 and then _________________________ "Fill in the blanks.An MP3 or a You Tube showing it *on yours* will help a lot.EDIT: we posted at the same time. Sorry but your description is confusing, post some MP3 or Video.You may have a problem, but I wouldn't describe it as Microphonics.
Quote from: Kaz Kylheku on July 03, 2013, 11:04:34 AMUnfortunately, the PDF doesn't give the schematics for the entire amp, just back end sections.It sounds like the signal is experiencing short volume dips or cutouts, which repeat at a low frequency rate of several times per second.This suggests itself to be a motorboating oscillation in some circuit that affects the bias of a stage.There is no outright reason to suspect that it is in the power amplifier. Even if it happens in every channel (which I suspect is not the case from a comment given), there are other common circuits that the signal goes through besides the power amp.Does the amp have a pre-amp out jack (or effects loop send) and can you hear the artifact in that output? If so, that tends to eliminate the power amp.Does the artifact occur only in one channel? That tends to blame that channel.I don't think you're hitting the tube hard just because you turn up the gain on the gain channel a little bit. What can hit the tubes hard is volume. The tube is just the differential summing stage for the amplifier's feedback, generating the error signal that is amplified. FET TR7 provides the voltage gain after that and drives the output stage. The gain of the amplifier is fixed, so the tube stage has to have reasonable clean headroom to pass through signal when the amplifier is played at the maximum volume. I.e. it is overall volume that will drive the tube, not gain on the gain channel.
Quote from: phatt on July 03, 2013, 10:05:31 AMYeah that is more like blocking distortion,, nothing to do with microphonics.The Amp works it's just got some issues so in time clever people here might nail down the offending part. Unlikely to be 3 bad Valves in a row so maybe a dying Capacitor or something in the design is prone to problems. Phil.