Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ariel tube fanatic on November 11, 2024, 05:14:06 am
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Hey all,
I have a Fender 5e3 that's been acting up, eyelet board made.
these are the voltages i'm getting on the 12ax7s
B+1 = 371v
B+3 = 215v
v1a = 66.7v (after 100k plate load resistor)
shared cathode v1a and v1b = 2.4v
v1b = 67.2v (after 100k plate load resistor)
shared cathode v1a and v1b = 2.4v
v2a = 63.8v
cathode = 2.2v
v2b = 135v grid
stopped measuring there cause it seems something is wrong ? maybe something is drawing alot of current?
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Acting up how?
Voltages aren't too useful if you have a bad tube or tubes, so you need to know if those are good.
This is an actual Fender brand Custom Shop amp? How old? Warranty?
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yes, it is a genuine fender
volume drops and crackling noises (frying bacon), the weird thing is the amp sounds fine and only sometimes those problem appear
but the voltages are way too low from what i remember about the 5E3
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Pull V1 and see if the V2 voltages improve
Then vise versa.
Discard and replace the tube whose removal caused the voltages to improve.
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i tried switching between them and removing V1 both times
voltages only improved by 13v, so i guess that's normal for a tube to draw that much voltage, could it be that both tubes are bad and drawing alot of current ?
without the tubes i get 269v on V1a and V2a and same goes for V2b, but i guess that to be normal without anything drawing current.
i will have a spare set here tomorrow but it seems like both tubes are bad ? never seen 2 preamp tubes go together
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I didn't follow your response. Please pull the tubes one at a time and report the voltages identically to how reported them in your first post.
Using your numbers from the first post you're pulling 1.6mA at the cathode. Halving this and multiplying by 100k plate resistance puts you at about 135v at each plate.
However your numbers across the plate resistors at V1a&b indicate pulling 3ma combined through the two plate resistors. So either your measurements are wrong at the plate, or some current is vanishing somewhere.
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The symptoms you are describing wouldn't lead me to checking voltages first. The first thing I'd do is properly clean all the jack, pots and tube sockets. Then I would test the tubes. If no tube tester, then a one by one tube swap with KNOWN GOOD tubes might be useful.
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I did clean everything first, it is not that old, from 2016, I will buy new tubes today and test to see if it is just the preamp tubes
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An amp can be 6 months old, but kept in poor conditions that will cause dirt and oxidation to build up on contacts. Point being, age is not an indicator of whether or not the amp needs a cleaning. The first thing you do with just about any amp is PROPERLY clean the pots, jacks & sockets...and open contact switches if it has any.
The next most common problem with tube amps is the tubes. That's why you have to either test them, or simply swap them with known good tubes. Sometimes testing them doesn't show a problem, so always having a variety of known good tubes on hand is important.
When asking for help/advice, you have to let the forum know what steps you've already taken and how you took them. If you don't, you're just wasting the time and effort of those trying to help you by increasing the chance that something very simple was overlooked.
There was a thread on here several months ago where a guy was having problems with an amp and he got all kinds of advice and questions, but the problem persisted. He finally posted a little video, and it was noticed that he was doing all his testing with the amp plugged into a leak light, which was throwing everything off. Simply taking the leak light out of the circuit solved his problem. Clear and concise communication of your processes will help with solving your problems more quickly.
Good luck.
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leak light :l2:
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That's what my second year electronics repair teacher called a "light bulb limiter". He was quite a character: ee degree; long time electronics tech; rock n' roll guitar player with Tourette's Syndrome. At the test benches, we had two categories of mistake: an "uh oh" or an "oh sh!t". Uh ohs are usually easily solved/repaired. Oh Sh!ts usually involved smoke, fire and $$$ to fix. RIP Ed Merwin.
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turned out to be 2 bad JJ preamp tubes, one bad grid resistor (was crackling), one bad rectifier tube and i think also the power tubes needs a changing (i get crackling and lots of white noise even without preamp tubes in and only new rectifier and power tubes in)
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the quality of the amp on one hand is very good, sprague electros and good quality overall regarding the power amp wiring and lead dress, on the other hand we've got 1/4 watt carbon films in the preamp and the fender branded capacitors seem to have almost 1v dc after the grid/capacitor.