![]() Mostly, you pay the for what they know, not for what they do. Remember, sometimes you pay someone to do the work. When you can demonstrate you are more prepared for this, I feel certain you'll get help. There is a good sticky thread at the top of the technical section on materials to read. I'm saying you've got to educate yourself a whole lot more before sticking your fingers into a dangerous place. I'm not saying you can't or shouldn't do this on your own. For that reason, I expect you may not get a direct answer to your questions. As noted, this work can be lethally dangerous. Knowing what to do is not the same as knowing how to do it. That gives you some more things that need to be done just from a maintenance standpoint and some possible hum solutions.Respectfully, the content of your post suggests you are not ready to do this work. Depending on the type of cathode bypass caps they might need checking or automatic replacement (I replace all dual-lead paper and "Whale" brand and check most others except white Mallorys - those rarely go bad in my experience and I only change them if specifically indicated. Same with moving filament aires away from everything.Ħ. sometimes simply moving grid and plate wwires so they cross at as close to 90 degrees as possible can audibly lower hum. It's surprising how much can be due to =lead dress that isn't apparent in "data". Has the lead dress been changed in any way? Are grid wires running parallel to plate leads? Were are the filament wires positioned and are they tightly twisted? Pictures of the chassis are really important wen trying to think of solutions to hum problems. Swap out V1 and next V2 with a new 12AX7 o check for both hum reduction and microphonics.ĥ. A weak or badly balanced tube here can cause hum.Ĥ. Replace the driver tube with a new 12AT7. What's the plate voltage, the power tube bias in ma, and what bias method did you use? Sometimes the bias setting can cause hum mismatched bias usually willģ. Also change the resistors in the cap "doghouse" to metal film - that's a normal safety and noise reduction upgrade.Ģ. Normal service life is 15 years - they may work longer but blow without warning and can cause hum. ![]() Change the other electrolytic filter caps and the bias cap. Here are some things to check/do (even though some appear to not be the source of hum in this amp)ġ. As there had to be changes here the lead dress may have been fouled. If the standard 1We 470 ohm resistors are in place replace them with 3W sand cast or wirewound. ![]() There should be 3-5W 1k (sometimes higher) installed with the E元4's, which are fine with 6L6's. They also require beefed up screen resistors with a higher value. Any suggestionsĮ元4's cannot be installed in AB763 circuits without rewiring the power tube sockets. The hum is not loud, but annoying enough to make the amp too noisy for recording. I can only assume it is the ripple on the supply applied to the power tubes that is causing the hum?Īs I understand it, however, 10V ripple is quite normal for this type of amp. ![]() Nor is it coming from supply noise further up the chain, as the ripple has changed to pretty much a sinewave after the next smoother. This tells me it’s not coming from the bias supply ( which is half-wave rectified and has ripple at 60-HZ). The hum on the speaker output is pretty close to a saw tooth shape at 120Hz. I replaced these with 100uf ( as 70uf is hard to find around here) HT ripple is now about 10V p-p I checked the smoothing caps, and the first two 70uf electrolytics were both bad on my capacitance meter. Looking at the HT on a scope, there was about 45V ripple P-P. Everything worked, except for a fairly bad hum on the output. I replaced the socket, the grid resistors on both tubes, and put a pair of new 6L6s in. On one tube the grid resistors had smoked. The problem appeared to be that someone had put E元4s in the power tube positions within any further mods. Ok,…I need some help, as my experience is mostly in transistor and IC based circuitry Ī friend brought me a Fender Bandmaster AB 763, (not working), that he had obtained very cheaply.
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