70s Fender Bassman 100 Head Sounds Harsh
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BonesMachine
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70s Fender Bassman 100 Head Sounds Harsh
So I've been struggling with this amp ever since I messed it up. I had turned it on without a speaker load hooked up and there was also an unused adapter in the speaker jack of the amp which didnt help. It definitely fried 1 tube maybe more stuff not sure? I put in new power tubes, filter caps and bias caps and also replaced the OT with a new Hammond. Still sounds tinny and harsh and has low output and breaks up really early. The amp is kinda loud but not really that loud with channel and master turned up to 10. sounds like a crappy distortion pedal is being used after 6. Unfortunately i barely got a chance to use it I messed it up pretty quick. I'm pretty sure it should be very loud and relatively clean cranked all the way!! I left the screen gird resistors alone because they measured in spec with a multimeter. I'm a novice so maybe out of my league at this point. Anything else easy that I can check?
Re: 70s Fender Bassman 100 Head Sounds Harsh
So what test equipment have you got access to, eg meter, scope, sig gen, dummy load?
I assume its the same for both channels?
What idle anode or cathode current is each output valve drawing at idle? Detail how you’re checking it.
Have you undertaken a voltage survey and compared results to those noted on the schematic?
Is this schematic correct https://el34world.com/charts/Schematics ... _schem.pdf
Has it had any mods?
I assume its the same for both channels?
What idle anode or cathode current is each output valve drawing at idle? Detail how you’re checking it.
Have you undertaken a voltage survey and compared results to those noted on the schematic?
Is this schematic correct https://el34world.com/charts/Schematics ... _schem.pdf
Has it had any mods?
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Stevem
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Re: 70s Fender Bassman 100 Head Sounds Harsh
Do you have at least 400 volts on pin 3 of each output tube and about that same voltage on pin 4 of each tube?
Do you have a way to generate a test tone to pump into the input of the amp.
Do you have preffibly a 4 ohm test load resistor, if not even a 8 ohm will do for now?
Do you have a way to generate a test tone to pump into the input of the amp.
Do you have preffibly a 4 ohm test load resistor, if not even a 8 ohm will do for now?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
I'll have to post later regarding any tests you mentioned... it is the same for both channels
As far as test equipment I currently have a Klein MM400 multimeter. I can get some basic equipment from Amazon if needed.
I'm thinking poor man setup:
- Android function generator app by Keuwlsoft. looks pretty handy can do sweeps (FREE)
- Hantek 6022BE pc usb oscilloscope ($65)
- Parts Express 8 Ohm 100W Non-Inductive Dummy Load Resistor($23) Looks like 8 ohm is easier to get than 4 ohm
anything else i might need while placing an order? how about some new 7025 and 12AT7 tubes? Current are original RCA from the 70's
i look forward to figuring this thing out!
As far as test equipment I currently have a Klein MM400 multimeter. I can get some basic equipment from Amazon if needed.
I'm thinking poor man setup:
- Android function generator app by Keuwlsoft. looks pretty handy can do sweeps (FREE)
- Hantek 6022BE pc usb oscilloscope ($65)
- Parts Express 8 Ohm 100W Non-Inductive Dummy Load Resistor($23) Looks like 8 ohm is easier to get than 4 ohm
anything else i might need while placing an order? how about some new 7025 and 12AT7 tubes? Current are original RCA from the 70's
i look forward to figuring this thing out!
Last edited by BonesMachine on Sat Jul 08, 2023 4:11 am, edited 1 time in total.
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
that schematic looks correct that is one i have for it. and no mods other than the work done i mentioned in my original post
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Stevem
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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 70s Fender Bassman 100 Head Sounds Harsh
The test I asked you to make have nothing to do with either channel, so please reread what I posted.
Those test only require your meter.
What you posted for gear is a good way to go, but get two of the 8 ohm load resistors so you can switch them into parallel to get a 4 ohm load when needed as when testing your amp.
Those test only require your meter.
What you posted for gear is a good way to go, but get two of the 8 ohm load resistors so you can switch them into parallel to get a 4 ohm load when needed as when testing your amp.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Andy Le Blanc
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- Location: central Maine
Re: 70s Fender Bassman 100 Head Sounds Harsh
If the rig was run with no load, your gonna have to start with the power supply and the power side of the amp. You can isolate the input of the phase invertor, grounded to check static conditions, or as a signal input to isolate the power side for testing, signal thru and O-scope analysis. Scrutinize the power tube sockets, parts and parts component board(s) for charring, PT and power rail. Good opportunity to review your work and confirm the circuits hook up starting from the power supply and output. Be patient, it is frustrating when you cooked it and have to dope out the results.
lazymaryamps
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BonesMachine
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Re: 70s Fender Bassman 100 Head Sounds Harsh
stevem yeah i understood your post the 1st read. your post mentioned "I assume its the same for both channels?". I was just letting you know your assumption was correct. thats it. I guess a little confusing how i worded it? Let me see what I can do with my multimeter and get back with some results
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 70s Fender Bassman 100 Head Sounds Harsh
Your not understanding something here because you keep bringing into the conversation that the amp has two channels when the simple test I am asking you to make only involves the four output tubes.
I think you maybe thinking that two of the 4 output tubes amplifies one channel and the other two output tubes amplify the other channel when that is not the case and you will then never correctly understand what is taking place in the amps driver/ PI section and in turn with the 4 output tubes.
I think you maybe thinking that two of the 4 output tubes amplifies one channel and the other two output tubes amplify the other channel when that is not the case and you will then never correctly understand what is taking place in the amps driver/ PI section and in turn with the 4 output tubes.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
ok yeah you know whats confusing here is that user pdf64 asked "I assume its the same for both channels?"... ok so whatever i totally understand what you are saying we can move on now 
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
ok I have some test results:
- voltage plate/pin #3: ~454 on all sockets
- voltage screen/pin #4: ~455 on all sockets
I was unable to get the plate current because I might be seeing some weirdness? I tried 2 methods described at https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
I was using where the OT red wire connects to the board as the center tap
- 1st method tried: Measure Bias With the Output Transformer Resistance Method
I measured the voltage at the OT center tap and it was the same as plate 3 (454)? I was expecting a voltage drop here as described in the above link
Resistance OT center tap to plate: ~13ohm for all sockets
The last step is to divide the voltage drop by the resistance to get the bias current. But I am not seeing any voltage drop? So I dunno... any insight?
- 2nd method tried: Measure Bias With the Output Transformer Shunt Method (*** I was extra careful ***)
So I wasnt getting any kind of reading on my meter with 1 probe at the OT center tap and the other on plate pin #3. Amp and standby were on (obviously) and I made sure the red lead was in the correct meter socket and that it was in DC mA mode. meter was reading 0 nothing no activity on all sockets? I tried auto range and each individual range. So I dunno... any insight?
I still need to do a schematic voltage survey...
- voltage plate/pin #3: ~454 on all sockets
- voltage screen/pin #4: ~455 on all sockets
I was unable to get the plate current because I might be seeing some weirdness? I tried 2 methods described at https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
I was using where the OT red wire connects to the board as the center tap
- 1st method tried: Measure Bias With the Output Transformer Resistance Method
I measured the voltage at the OT center tap and it was the same as plate 3 (454)? I was expecting a voltage drop here as described in the above link
Resistance OT center tap to plate: ~13ohm for all sockets
The last step is to divide the voltage drop by the resistance to get the bias current. But I am not seeing any voltage drop? So I dunno... any insight?
- 2nd method tried: Measure Bias With the Output Transformer Shunt Method (*** I was extra careful ***)
So I wasnt getting any kind of reading on my meter with 1 probe at the OT center tap and the other on plate pin #3. Amp and standby were on (obviously) and I made sure the red lead was in the correct meter socket and that it was in DC mA mode. meter was reading 0 nothing no activity on all sockets? I tried auto range and each individual range. So I dunno... any insight?
I still need to do a schematic voltage survey...
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Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: 70s Fender Bassman 100 Head Sounds Harsh
ok, one of the problems with the 70s era Fender amps is that there is no bias adjustment pot, just a bias balance pot that adjust the bias between each pair of output tubes.
The first thing to do is to remove all 4 output tubes , turn it on and then measure the negative bias voltage on pin 5 of each output tube.
I would say that by adjusting that bias balance pot you should read -52 to -56 volts on all of those output tubes, with -56 being safer for now.
If not then do not reinstall the output tubes until we figure out why.
Also in terms of you not being able to get a current reading with your meter it may have a small fuse in that circuit.
If it does then you may have blown it.
The first thing to do is to remove all 4 output tubes , turn it on and then measure the negative bias voltage on pin 5 of each output tube.
I would say that by adjusting that bias balance pot you should read -52 to -56 volts on all of those output tubes, with -56 being safer for now.
If not then do not reinstall the output tubes until we figure out why.
Also in terms of you not being able to get a current reading with your meter it may have a small fuse in that circuit.
If it does then you may have blown it.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
Negative bias voltage on pin 5 is -57 for all sockets
Yup the small 500ma 600v fuse was blown in my meter... doh! Brand new meter wonder how that happened could that plate current measurement have blown it? Looks like the Klein replacement is unavailable on Amazon. There are some similar 500ma 5x20 fast blow fuses on Amazon but they are 250v instead of 600v. Would these work or do I need the 600v ones for this amp?
So whats the deal with there not being any voltage drop at the OT center tap?
Yup the small 500ma 600v fuse was blown in my meter... doh! Brand new meter wonder how that happened could that plate current measurement have blown it? Looks like the Klein replacement is unavailable on Amazon. There are some similar 500ma 5x20 fast blow fuses on Amazon but they are 250v instead of 600v. Would these work or do I need the 600v ones for this amp?
So whats the deal with there not being any voltage drop at the OT center tap?
Re: 70s Fender Bassman 100 Head Sounds Harsh
I see plenty of replacement fuses...BonesMachine wrote: ↑Fri Jul 14, 2023 10:47 pm Yup the small 500ma 600v fuse was blown in my meter... doh! Brand new meter wonder how that happened could that plate current measurement have blown it? Looks like the Klein replacement is unavailable on Amazon. There are some similar 500ma 5x20 fast blow fuses on Amazon but they are 250v instead of 600v. Would these work or do I need the 600v ones for this amp?
https://www.amazon.com/Fuses-Action-500 ... 0BHQ8PM2X/
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BonesMachine
- Posts: 35
- Joined: Fri Jan 28, 2022 2:28 am
Re: 70s Fender Bassman 100 Head Sounds Harsh
i need 5 x 20 fuses those are 6.25 x 32. maybe i could cram one of those in there?