Here's a little caption on clean preamp's and sustain from TUT. "For the maximum undistorted signal output, the anode voltage should be adjusted to half the supply voltage. For clean tones, this is relevant for good amp sustain." What I'm starting to figure out about good gain,sustain,drive......(whatever you want to call it) is that proper gain staging and the addition of the correct harmonics as the stages evolve is the trick. That is a much bigger challenge than just relying on signal clipping or hard distortion to bring out sustain. I'll let you know when I fully understand how to do it without a little luck on my side!!!:>) I don't know if this helps or is even relevant to your friends amp....but I tried. Benjamin P.S. What you mentioned from TUT is interesting because it suggests that you can get a cleaner sound with a higher valued plate resistor... That is contrary to much of the popular opinion here (namely you can get cleaner sounds with a 100k Rp and more distorted sounds with a 220k Rp). But it is line with a "taste test" posted here last year about plate resistors and being able to "clean up" a guitar sound by turning down the guitar pot. In any case, just replacing the plate resistors at random without adjusting the bias by replacing the cathode resistor (and output load, too, I believe) you can run into trouble. Maybe people here can post their suggestions for preamp tube bias (I think I heard that 0.75mA works pretty good). Steve Ahola P.S. I think it was Bruce Collins who mentioned a really easy way to measure the bias of a preamp tube: measure the voltage drop across the plate resistor and divide that by the value of the resistor... I'm sure that is just too easy for 3 digit accuracy, but it should get you into the right ballpark. Name: ken gilbert E-Mail: Date: 2/7/100 3:19 PM Subject: Re: Any recommendations on bias current for preamp tubes? P.S. What you mentioned from TUT is interesting because it suggests that you can get a cleaner sound with a higher valued plate resistor... That is contrary to much of the popular opinion here (namely you can get cleaner sounds with a 100k Rp and more distorted sounds with a 220k Rp). But it is line with a "taste test" posted here last year about plate resistors and being able to "clean up" a guitar sound by turning down the guitar pot. Steve, I think an important distinction to make here is that the higher value of plate load MAY give you more VOLTAGE GAIN, but it will also increase the voltage drop across itself. The fact that the gain may have increased is what will probably result in more clipping later on, on subsequent stages. The increased voltage drop across the plate load, however, is what allows a greater unclipped output voltage swing. So THAT stage may clip less, but later stages may clip more. In any case, just replacing the plate resistors at random without adjusting the bias by replacing the cathode resistor (and output load, too, I believe) you can run into trouble. Maybe people here can post their suggestions for preamp tube bias (I think I heard that 0.75mA works pretty good). Personally, I have always had great success by using fairly standard plate loads (100k and 220k) for 12ax7's, and then adjusting the operating point of the tube with a rheostat in the cathode circuit. It is amazing how much effect the grid bias and plate current has on the tone from the stage--probably because you are also simultaneously changing the plate voltage as well. For the out and out distortion, I've had luck with high value Rk's (say 10k) and 100k Rp's. This results in a very low plate current, generating perhaps a 10-20V drop across the load resistor. In this way, the positive excursion of the plate is limited by the B+ rail voltage, generating a nice clip. P.S. I think it was Bruce Collins who mentioned a really easy way to measure the bias of a preamp tube: measure the voltage drop across the plate resistor and divide that by the value of the resistor... I'm sure that is just too easy for 3 digit accuracy, but it should get you into the right ballpark. Steve, think about what you just said above... the current through the plate load resistor IS the plate current, and in preamp circuits, there ain't much grid current to concern yourself with! So it is indeed exact enough for most purposes... at least by 3 digits. Generally speaking, I like low plate current for distortion, high plate current for warmpth and dynamics. HTH, Ken Name: Scott Swartz E-Mail: nnswartz@inlink.com Date: 2/7/100 3:25 PM Subject: Re: Gain vs. distortion vs. sustain... any ideas? The below statement from TUT is close, but not exactly right. "For the maximum undistorted signal output, the anode voltage should be adjusted to half the supply voltage. For clean tones, this is relevant for good amp sustain." The reason why is the when the grid is at its most positive, the tube needs a finite plate to cathode voltage drop (remember the current through the tube is highest at that point). If you set the plate voltage at 1/2 B+, the signal will be clipped when the grid is going positive before the the signal is clipped when the grid is going negative, which will be the full 1/2 B+. If you want maximum headroom with equal clipping levels for positive and negative peaks, the plate voltage should be set at 60-65% of B+. If you have a scope, you can easily verify this. Also, running at 60% will be a higher current operating point where the plate curves are straighter, relatively speaking. Name: SteveR E-Mail: Date: 2/9/100 5:19 PM Subject: Re: And the man who pestered Steve speaks! I was just over at Randall's site and was thinking about your question. If you used a local NFB loop on one of the early stages (most likely first stage) wouldn't you get a limiting or compressing effect? It wouldn't behave like an active device but would limit the output swing so that you would have a more constant signal level, and you could drive the subsequent stages a bit harder. I think it would increase sustain without necessarily adding overdrive distortion. The triple giant preamp uses this in its clean circuit. BTW I've got a homebrew four 6V6 power amp that is SWEET. When clean it's deep and chimey (is that a word?). It compresses real nice before its starts to break up. I went with cathode bias and a concertina splitter. I may experiment with a long-tail or paraphase splitter, but "if it ain't broke..." good luck, SteveR .