Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: davidb on March 20, 2022, 08:31:34 pm
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Just got a Fender PA-100 that I'm planning to recap and refurb, and do some modifications down the line. I'm new to this and still learning, so any advice is appreciated.
First, the good: all amp preamp controls seem to work fine, amp is pretty loud and clean.
The bad: reverb is shot, starts to squeal loudly when turned up above 2. Tried swapping tubes and turning amp volume low, neither fixed the issue - rules out a bad tube or resonance in the tank/amp.
The ugly: there is a slight burning smell after the amp is powered on, and there seems to be a little bit of wax that leaked from the power transformer at some point.
My questions:
- Is it worth debugging the reverb and burning smell before I replace all the polarized caps in the amp? I'm thinking I want to recap and check the bias first since the amp has never seen a tech.
- Are the blue and ceramic caps worth replacing?
- Should the screen resistors and power filtering resistors be replaced?
- Is the power cord wired in a safe way on these? I'm also not sure if I should remove the ground switch cap.
- What else should I look for in a never serviced Fender from the 70s?
I'll keep you all posted as I keep working on it. Thanks in advance for the help!
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Some answers
1- First : Remove all power tubes, amp ON ; smell burning or not ?
Big brown filter caps must be replaced.
2- No.
3- No if they read on specs or they don't have look toasted
4- Remove dead cap, there are a better and safety way to rewired.
AC power cord ; black wire to the Power Switch IN, Out to the fuse IN, Out to the one transformer primary wire
White to other transformer primary wire
Green : chassis
5- Replace small whites bypass 'lytics
Replace the white filter cap on bias circuit
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Schematic ; wich one is yours ?
https://el34world.com/charts/Schematics/files/Fender/Fender_pa100rev.pdf
https://el34world.com/charts/Schematics/files/Fender/Fender_pa100.pdf
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Circling back to this, it's been a busy time but I've been able to get back to modifying this amp. Thanks Latole for the response, my amp is definitely the second linked schematic.
So far I have replaced all electrolytic caps, repaired some noisy solder joints, added adjustable bias, and boosted the reverb signal by adding a cathode bypass cap. Cleaning the reverb jacks seemed to have solved the oscillation problem. I also put in a PPIMV that works really well, and the amp sounds good, though there is some noise to work out.
Next up is to replace the power cord and remove the death cap, which I should have done first... I think I will convert the ground switch to a negative feedback control, and maybe add a Vox Style tone cut to the power section. Then I'm looking at converting one channel to a fender tweed, and another to a blackface, and maybe the last one to a marshall.
I also found out that 2 power tubes are to blame for the burning smell. They are biased properly, but I think their heaters are just getting old and starting to cook. I tried swapping the tubes, and the problem stuck with the tube rather than the socket. I'm going to order some new tubes and replace the resistors on the sockets with metal film. Is that likely the best course of action? I've heard of some people using hot tubes til they die, but I don't want them to cause any damage to the components.
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metal film or metal oxide?
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I have metal film handy, is there any reason I should use metal oxide instead?
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i prefer flameproof metal oxide for b+ dropping and power tube screen resistors for how they fail. i'm sure a sufficiently sized (power rating) metal film is fine but 2-3-5w metal oxides are relatively cheap
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I have metal film handy, is there any reason I should use metal oxide instead?
I will use it. It is good enough
Fender always use carbon type resistors
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Reviving this thread since I’m finally able to work on it again…
I’m perplexed by this loud hum that is not affected by my Type 2 Lar Mar master volume that I’ve added. The amp was pretty quiet and played well, but then suddenly there was a pop when I plugged into an input jack and the noise appeared. When I remove V5 the noise goes away, and I replaced it with a new tube to no avail. V1-4 did not change the noise. If the noise is coming from around the V5 gain stages, how is it not affected by the master volume? The master volume still works since I can hear other noise increase when I turn it up.
I’ve also noticed that the bias voltage got unbalanced when this happened. I think it has something to do with the master volume, so I measured the voltages on it.
1a -47.4
2A -47.4
3A -38.4
1b -47.4
2b - 23.4
3b -20.7
Sure enough, some of the bias voltage seems to be changing. I measured resistances on the pot and they are similar on both gangs, so I don’t know where to go next here. I wont be turning the amp on until I get that bias voltage down.
Also FWIW, I took out the hum balance pot from the circuit since it messed with the master volume. I did add a bias pot which I don’t think is part of the problem.
Schem of original amp: https://el34world.com/charts/Schematics/files/Fender/Fender_pa100.pdf
Thanks,
David