fender 75

[bargoedboy]

bought this amp as a spares or repair job off ebay, amp looked ok in pics slightly tatty tolex etc and with ev speaker which worked ok.
got amp home and after further checking found amp had been broken at some time , repair carried out and tolex painted black
i know i should have been a bit more thorough in my checking but when you are buying an amp off a lady who seems to be a man as the voice got deeper and deeper i got amp quickly and left
now opening amp up i found the 470resistor and 1.5 resistor off of power valve base one was burnt out. i replaced them and while i was at it all electrolytics to correct spec, turned amp on hey presto working.
but (theres allways a but ) amp was a bit crackly and had a hum .
so stuck my TAD bias right unit on and it melted the wires from valve base to bias unit
switched amp off removed unit tried amp, working but not great, any ideas please

[Wally]

Drain caps. Inspect all of the work you did to make sure you didn't make a mistake....we are all human, after all. IF that looks good, then start checking the wiring to that particular tube socket. INspect the socket for cracks or arcing. IF that looks good, start measuring components that attach to that socket. IF that looks good, inspect everythign that leads to those components. At some point, if all looks good, you are going to need to start measuring voltages at various points and determining where things are going wrong. And.....one could be a skeptic and think that maybe a resistor or connection could have gone bad in the baising tool, right?

[bargoedboy]

thanks wally, just about to check all things over again.
socket seemed ok and good resistors fitted.
as i`m not experianced i am very carefull that i put caps in right way so removed one at a time to make sure i got the orientation right as well as value

[Wally]

OKay, your admission of inexperience moves me to say that you might want a tech to sort this one out. With that caveat and the required warning: Dangerous voltages are associated with that circuit!.....
Then I will ask....Are the tubes known to be good? IF not, that could be the root of those melted wires. IF you are going to do this work, then you are going to have to come to understand the voltages and how to take the measurements of them. THe first thing a tech wants to know when a cap job is done is if there are any shorts and if not what are the voltages. Did you use a current draw limiter to bring those caps up? This simple device will indicate shorts. IF you don't have a tube tester, then you are going to have to use known good tubes. YOu can't rely on what came in that amp, because the amp wasn't working properly, correct? Those tubes may or may not be of any use. A simple tester won't tell you everything, but it will tell you whether or not a tube should be thrown away. You are going to have to be able to read the schematic and/or the layout and understand how to measure those voltages....hopefully in a safe manner.
IN short, you are in the middle of a problem....the amp was purchased as unknown, it was problematic, you recapped and now things are still problematic. These are the situations that will will frustrate and/or instruct.
FWIW, if I had been in your place, I would have replaced the obviously burnt resistors, checked the tubes or installed known good ones, and taken assesment of sonics and voltages at that point...before doing any other work on the amp. IME, that amp is of an age whereby the electrolytics may well have been in good condition and not exhibiting signs of weakness....loss lows, lack of sparkle, etc. When one goes 'whole hog' and starts working 'blind', one doesn't have a firm grasp on what problems existed when and therefore what the resulting problems might be. NOte: it is not unknown for new caps to be bad....and therefore be passing AC and causing hum.
When you go back and inspect this amp, don't take for granted that your work was 'good'. We are all capable of making a mistake adn then being human overlooking that mistake because we accept that we did not make a mistake. IT is a trap that we humans are all too often guilty of. The error can be looking us in the face and we will miss it. INspect the amp as if you had never seen it before....fresh mind and skeptism, right? Take care and good luck with it.

[bargoedboy]

awesome advice and common sense, i am the first to admit if i make a mistake, so will definitely check and check again, let you know what i find.

[bargoedboy]

i believe the problem may be with the diodes on rectification area , so am checking further there , as at one time the amp had taken a bit of a knock and possibly the power switches etc were touching diodes

[BiggerJohn]

Careful in there!

Remember that the 75 is one of those so-called "dreaded" ultralinear circuits. Runs the 6L6s at real high voltages, so be sure to use good tubes. As i recall, this was Fender's attempt at a response to the early Mesas.

[bargoedboy]

thanks for thatbiggerjohn , will be carefull replaced the diodes as two were not reading at all on meter, am about to recheck out everything again, before i start to power up, fingers crossed.

[BiggerJohn]

Bargo,

You know since they do run the 6L6s at such high voltages, and since good ol USA tubes are no longer available, for safety's sake, I'd put 1k, 5W resistors in series with those 6L6 screen grids.

Best of luck to ya.

[bargoedboy]

i`m beginning to think i should try and find a good tech with regards to all this talk about how high they run the voltages, you`re all making me nervous.
better to be safe than sorry though.

[BiggerJohn]

Please my friend, do be safe.

[bargoedboy]

i will be safe, as i`ll stand and watch a tech do it.
seriously thanks for your replies , i know when i am out of my depth and i was getting a little worried and to me thats when you are likely to make a mistake , will let you all know what appens next.
the amp does seem to have a nice big tone when working apart from the hum , so looking forward to it being put right.