My first post. I'll do my best to not ask hasty questions. I am not very experienced at all. Any help is sincerely appreciated.
I have a Bassman style circuit I've been working on - no choke, no tube rectifier, no standby switch. When I power it down, I can still here my guitar in the speaker for quite some time, like I can here the filter caps discharging?
Four of the filter caps (all 100uf/400v) have a 150k bleed resistor across them. After discharged and power cord disconnected, I put my meter across a bleed resistor and I can see the cap charging, but after the cap is charged and the reading stops changing, the resistance reads about 100k, not 150k (they are 150k resistors). Is that normal?
The power tube input grid bias supply (negative voltage) does not have its own tap, it comes right off of one of the main PT secondary leads. All of my B+ voltages are spot on.
I'm a little confused. Thanks.
(I'll be gone for a while, but I will be back, I'm not a troll.)
Newb Here - Question?
Moderators: pompeiisneaks, Colossal
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raveonstevie
- Posts: 48
- Joined: Sat Jan 15, 2011 5:22 am
- Location: Southern Indiana
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martin manning
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Re: Newb Here - Question?
Welcome!
If you are talking about a 5F6-A Bassman, those are hugh values for the filters. Why so big? Your amp is running on the energy stored in them after you turn it off, like batteries, in spite of the 150k bleeders... the RC time constant for 100uF and 150K is 15 seconds.
In most cases you can't measure the resistance of components in-circuit, as they are connected to other components and you are getting some combination of two or more. If one of them is a big capacitor, you meter is charging it and the measured resistance value will change continuously until the cap is fully charged.
If you are talking about a 5F6-A Bassman, those are hugh values for the filters. Why so big? Your amp is running on the energy stored in them after you turn it off, like batteries, in spite of the 150k bleeders... the RC time constant for 100uF and 150K is 15 seconds.
In most cases you can't measure the resistance of components in-circuit, as they are connected to other components and you are getting some combination of two or more. If one of them is a big capacitor, you meter is charging it and the measured resistance value will change continuously until the cap is fully charged.
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raveonstevie
- Posts: 48
- Joined: Sat Jan 15, 2011 5:22 am
- Location: Southern Indiana
Re: Newb Here - Question?
Thanks, Martin!
So that's normal. This isn't my first tube amp, LOL. It jsut seemed like a really long time.
I should have known I couldn't read the resistance in the circuit. After those caps charge, no telling what I'm reading, going through ground and whatever else. I'll save that for later.
Check this out. I install a new OT and connect each of the outside leads to the wrong tubes (backwards). So I power up and get the loud, uncontrollable squeal.
I power it down immediately. When I trun it off a hear a pop (not too loud, but bigger than a normal pop) and then several smaller pops until the caps are discharged.
What happened there? I thought I ruined my caps.
So that's normal. This isn't my first tube amp, LOL. It jsut seemed like a really long time.
I should have known I couldn't read the resistance in the circuit. After those caps charge, no telling what I'm reading, going through ground and whatever else. I'll save that for later.
Check this out. I install a new OT and connect each of the outside leads to the wrong tubes (backwards). So I power up and get the loud, uncontrollable squeal.
I power it down immediately. When I trun it off a hear a pop (not too loud, but bigger than a normal pop) and then several smaller pops until the caps are discharged.
What happened there? I thought I ruined my caps.
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Re: Newb Here - Question?
That squeal is oscillation caused by positive feedback because you connected the OT wrong. Just reverse the OT to plate connections.
It should not damage the caps. They are simply too big for the application. Assuming they are two series pairs in parallel, that's 100uF when 50 would be plenty. If they are all in parallel (400 uF) that is WAYYY over the top!
It should not damage the caps. They are simply too big for the application. Assuming they are two series pairs in parallel, that's 100uF when 50 would be plenty. If they are all in parallel (400 uF) that is WAYYY over the top!
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raveonstevie
- Posts: 48
- Joined: Sat Jan 15, 2011 5:22 am
- Location: Southern Indiana
Re: Newb Here - Question?
Thanks, David!
Yes, I switched the wires and the amp is up and running fine. It's just strange that I don't remember my other amps taking that long for the caps to drain.
This isn't an actual Bassman, just a "Bassman-esque" circuit, but the schematic shows 100uf/ 400v caps, and that is what was in there originally (I have replaced them).
Two in series, each bypassed with a 150k, in the first two "rails"(?), and then one 100uf in each of the last three rails. I guess they just used all the same value to cut costs by buying in bulk?
Yes, I switched the wires and the amp is up and running fine. It's just strange that I don't remember my other amps taking that long for the caps to drain.
This isn't an actual Bassman, just a "Bassman-esque" circuit, but the schematic shows 100uf/ 400v caps, and that is what was in there originally (I have replaced them).
Two in series, each bypassed with a 150k, in the first two "rails"(?), and then one 100uf in each of the last three rails. I guess they just used all the same value to cut costs by buying in bulk?