the powered off resistance is 510K, so thanks for that check. I appreciate your efforts, I am now pretty sure the replacement coupling caps I have ordered will do the trick, should be here tomorrow. R

Stephe, no problem, you are very busy. Update, got new caps in, and powered it up, and the cap blew in a few seconds. HMM. So, removed the blown cap, and re-checked voltages. Well, at 80VAC input, the cathode of the good side was at 16vdc, the bad side was already at 35vdc. Bad, So, I checked around, the grid of the bad tube is at 16vdc!!!! So how did it get that high. I suspect the coupling cap, but it shows infinite resistance, and the proper capacitance, if you can believe my DVM. Anyway, how would you go about checking if it was the coupling cap? Roger

Haha, you must not check your website very often! Which is a good thing as your youtube must be taking a substantial amount of time. Coiuld be, I WAS poking around after I changed out the tube, so there may have been some movement of components that caused a short. I have replacement caps on order, I have neatened up a bit of the wiring so that when I replace the caps, it will be less likely to fail due to shorts or cold solder joints etc. Thanks for the timely responnse! (LOL) Roger

got a cheap NOS on the way, Wasn't expensive at all. Hope it lasts. I think I got a failing tube. Glad it was the tube, as troubleshooting what else was wrong would have been tough. Although being independent of volume setting indicated it was in the output section. Anyway, Keep on keeping on. I really enjoy the videos you produce, I have learned soooo much in the last 6 months it is crazy. My next project is to put a phono stage in an identical 10"x6"x2"bllack chassis. I will put an el cheapo phono stage, as well as a power supply, and 3 way selector switch. I can upgrade to a better SS phono stage later. The EAR is just too much $$ right now. I have most of the accessory parts needed for this project, so it is sneaking in at under $50. My TT is used and of unknown condiution, though it does turn at correct speed. and has a P mount cartridge. Thanks again. Roger

Thanks much. I appreciate your efforts. Roger

actually, I found it,and bought one with my other stuff. for some reason it is beefy resistor, and I thought it was the 120R 5w. in my very best Gilda Radner....."never mind"

The 560R got the dissapation n to 6.9 watts! very happy!

I have 51R and 560R resistors on order. My first try will be the 560R. Thanks for the feedback. Trying to figure this out has helped me understand the system a bit more. Roger

Ok, went to the tube bias calculator you showed. I guess I can use a ~550R 3W resister in the cathode output. The bias calculator says that will get me about 7 watts. It does not however, indicate how the bias voltage and current will change. I guess I could iterate using ohms law. I also checked the bypass cap, and it is good. Thanks, these problems sometimes fix themselves, just talkiing through even if it is to myself! LOL. Roger

Watts is V*A correct? The voltage drop across the anode to cathode is about 237Vdc. How do I calculate the current? the cathode voltage to ground is 17.6Vdc across 470R, so 17.6/470 = 37Ma, the other tube's plate current is 40mA. So, voltage drop of 218 times .04A = 8.72 watts (using the voltage drop, should I use the anode voltage?). That would be even worse, 236*.04 = 9 watts! The anode max power is 7 watts at under 250V plate voltage. Since the current is controlled by the cathode resistor, instead of 470R, how do I calculate the grid voltage if I say change the cathode resistor to say 500R? Won't that change BOTH the cathode voltage as well as the current, and perhaps the grid bias as well? I guess I got this far just using ohm's law. I am not quite confident that I could calvulate the new required resistance to bring the plate dissapation down to 7 watts. Any feedback, or help is appreciated. I fully intend to use this amp for my desktop music amp, speakers yet to be determined. The 6LU8 will be paired with some other speakers as well. I have some 89db sensitive Polk Monitor 1s, I am hoping I can use them with the 6LU8, but am not counting on it. I have other speakers (Advent TM 6" woofer, acoustic suspension) I can try as an alternate, and then there is always a purchase or build. I have built speakers before (1969, and 1970!!!). Roger

what size chassis did you go with on the 6lu8? Looks good and roomy! Roger

I get that the tubes are not identical. on further inspection I found that the voltage on pin 9 is vastly different. 217vdc on one and 125vdc on the other, and the diffference moves with the tubes. I suspect that I have a bad tube. I DID drop these tubes onto the floor, so although they might have been bad to begin with, I suspect I damaged one of them. Thanks for all the help, I will be replacing these tubes. Roger

The tubes were installed, dummies on the output, no input. I switched to using the 125v taps and now I am getting 220v on my B+. Now for something else. After switching the tap to 125v, checking voltages around the amp, I note that where you have indicated 220vjust after the 4.7k ohm and above the 33uf cap, the voltage on one side is 218v and the other channel is 213v. I checked the resistance of the 4.7k ohm resistors and they are both 4.7k. Either I have more curent going through that channel, or the cap is leaking? I DID have an issue with wiring up the 125v tap and accidentally hooked up the black and greay wires (I know, I know!) for a brief time at about 30vac. Nothing I saw was effected, but I worry. What reasons could there be for the discrepancy between the channels? Roger

Thanks Dan. The tab at the phono jack is isolated from the chassis, and the shields are only connected to the ground on the potentiometer and the star ground point. When I first listen to the amp, if there is any hum, that is the first thing I will look at. And I guess I agree with grounding the speaker output commons. Unfortunately, when I went to plug in my tubes, one pin hole was plugged with solder! I am awaiting the arrival of it's replacement.' Roger

One last item. I hace not found the BOM and final schematic for this amp anywhere. The project does not show up under your 'projects' tab. Roger

thank you. I just went back to review, and found my error. I will probably build this when I complete my 6BM8 build, which is awaiting a replacement socket due to my sloppy soldering job that plugged a pin hold. Roger

Thank you. I surmised from your intro to the project that you wanted to build an amp that required a preamp. Or was that another project? Roger

Thanks. I suspected they would 'work', but wondered if the specs were different enough to change the sound or distortion. Any way, is this typical? Different 'substitutions, or even 'equivalents' having that big a difference in specifications? Something to keep an eye out for in the future? Roger