Bias circuit loading?
Moderators: pompeiisneaks, Colossal
Re: Bias circuit loading?
this choke mounting and "shooting" in pi and out.tube is just asking for the "troubles", i dont know in which degree but...is it possible to transfer it in the middle down?
Re: Bias circuit loading?
grid stoppers for output tube are mounted too close with the fuse and main power lines?
grounding of the preamp.is grounded to the ouput tubes (worst point for this function)?
grounding of the preamp.is grounded to the ouput tubes (worst point for this function)?
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Stevem
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Re: Bias circuit loading?
Bepone, power tubes do not have enough gain for what your alluding to as being a issue to take place!
Have you ever looking at the position of the 4 output tube sockets in for instance in a Fender twin reverb or Showman and the location to those sockets of the fuse holder, on/ off switch and the accessory outlet in these and many other amps?
Have you ever looking at the position of the 4 output tube sockets in for instance in a Fender twin reverb or Showman and the location to those sockets of the fuse holder, on/ off switch and the accessory outlet in these and many other amps?
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!
Re: Bias circuit loading?
yes, you move one wire around the sockets in that amp and bass changes too 
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Stevem
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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Bias circuit loading?
In which amp , the OPs amp, or the Fenders I mentioned?
So let’s see here now, your comments have gone from the local A/C wiring being a issue, to moving around other local wires and doing such effecting Bass response.
At what frequency does this low end roll off take place as you must have the gear to measure that like, let’s say even just a NTI Minilyzer?
So let’s see here now, your comments have gone from the local A/C wiring being a issue, to moving around other local wires and doing such effecting Bass response.
At what frequency does this low end roll off take place as you must have the gear to measure that like, let’s say even just a NTI Minilyzer?
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!
Re: Bias circuit loading?
+ i said that nothing wrong here and need to be confirmed with currents in full tilt of the amp,
if case that current is big, recheck for the oscillations, and then reconstruction needed in all the points what i have mention and recheck again
p.s Rivera fender 75 , changing one power tube socket bass started to be usable (from too bassy -> to normal)
if case that current is big, recheck for the oscillations, and then reconstruction needed in all the points what i have mention and recheck again
p.s Rivera fender 75 , changing one power tube socket bass started to be usable (from too bassy -> to normal)
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Bias circuit loading?
Excess use of a laughing icon does not help to reinforce your opinion if your thinking such!
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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turbofeedus
- Posts: 205
- Joined: Wed Jan 30, 2019 5:37 am
Re: Bias circuit loading?
I appreciate both of your inputs, but let’s hit the brakes.
I’ve got a new scope coming tomorrow or Tuesday, I will return with images/video.
I have a suspicion I may have some issues in the preamp, I was briefly probing around (while my 2213 was still triggering) and the input into the DCCF was very badly asymmetrically clipped.
I’m new to looking at tube amps on a scope so maybe it’s nothing, we’ll see.
I’ve got a new scope coming tomorrow or Tuesday, I will return with images/video.
I have a suspicion I may have some issues in the preamp, I was briefly probing around (while my 2213 was still triggering) and the input into the DCCF was very badly asymmetrically clipped.
I’m new to looking at tube amps on a scope so maybe it’s nothing, we’ll see.
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Stevem
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Re: Bias circuit loading?
When your using a scope in any part of a circuit that has DC voltage on it you need to couple your probe thru a .01 @ 400 to 630 volt cap so as to not harm the scope if your probe has no built in cap, just resistance settings.
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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turbofeedus
- Posts: 205
- Joined: Wed Jan 30, 2019 5:37 am
Re: Bias circuit loading?
Correct me if I'm wrong, but don't most scopes have an AC coupling mode for the inputs, that does exactly what you're saying?
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Bias circuit loading?
Yes, but some old ones have gone shorted as I found out the hard way once.
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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turbofeedus
- Posts: 205
- Joined: Wed Jan 30, 2019 5:37 am
Re: Bias circuit loading?
Here are some scope pics of the preamp (preamp and master volume both at 5 for all pics).
Schematic again for reference: schematic
Input is a 1kHz sine wave at 500mV P-P:
V1, pin 2:
This is right after the coupling cap coming out of V1; pin 1, before the 470K/470K divider:
V2; pin 2; (why is it clipped so hard and asymmetric?):
V3; pin 2:
V3; pin 7 (big difference in amplitude?):
V4; pin 5 (150V P-P?):
V5: pin 5:
Schematic again for reference: schematic
Input is a 1kHz sine wave at 500mV P-P:
V1, pin 2:
This is right after the coupling cap coming out of V1; pin 1, before the 470K/470K divider:
V2; pin 2; (why is it clipped so hard and asymmetric?):
V3; pin 2:
V3; pin 7 (big difference in amplitude?):
V4; pin 5 (150V P-P?):
V5: pin 5:
Re: Bias circuit loading?
all normal, looks like amp is ready to play,
to make better reference , pls adjust as much as possible "clean" signal on resistor load (sinusoide but can be also bent this is marshall so acceptable), max output, before the clipping!
make sure your resistor can withstand the heat of 50W because in this test you can burn it easilly (you need 100W resistor on heatsink)
you are searching anomalities on sinus waveform (im using more complex signals to find this but ok), per example oscillations on 50kHz or 100kHz added to your signal, strange modulations, all what differs from sinus, it is easy to see..if they doesnt exist your amp is fine.
then during full power test you can measure currents and check bias Voltage dropping
to make better reference , pls adjust as much as possible "clean" signal on resistor load (sinusoide but can be also bent this is marshall so acceptable), max output, before the clipping!
make sure your resistor can withstand the heat of 50W because in this test you can burn it easilly (you need 100W resistor on heatsink)
you are searching anomalities on sinus waveform (im using more complex signals to find this but ok), per example oscillations on 50kHz or 100kHz added to your signal, strange modulations, all what differs from sinus, it is easy to see..if they doesnt exist your amp is fine.
then during full power test you can measure currents and check bias Voltage dropping
Re: Bias circuit loading?
Nice. Now move up to the new millennium. You don't need your camera like with your 2213. Just plug in a memory stick and push the "Save to USB" button.
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turbofeedus
- Posts: 205
- Joined: Wed Jan 30, 2019 5:37 am
Re: Bias circuit loading?
I knew there was a way! Thanks sluckey.
I stuck the scope on the load resistor, as expected there's very minimal output. I couldn't grab a pic, it was about 1.5VAC P-P before clipping, and very jagged and ugly.
Also the bias voltage seems low for the idle dissipation? I was reading -37V, and that was about 22mA of current @ 430VDC on the plates, so less than 50% max diss. I would have to make the bias voltage even more positive to get 60-70% dissipation.
For example, the schematic shows -57V bias for 468VDC plates. Hard to imagine the bias voltage being doubled for an extra 10% plate voltage.
I stuck the scope on the load resistor, as expected there's very minimal output. I couldn't grab a pic, it was about 1.5VAC P-P before clipping, and very jagged and ugly.
Also the bias voltage seems low for the idle dissipation? I was reading -37V, and that was about 22mA of current @ 430VDC on the plates, so less than 50% max diss. I would have to make the bias voltage even more positive to get 60-70% dissipation.
For example, the schematic shows -57V bias for 468VDC plates. Hard to imagine the bias voltage being doubled for an extra 10% plate voltage.