What Does This Resistor Do?
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What Does This Resistor Do?
I have an Allen Amp Chihuahua 1x10 combo kit that I assembled a few years ago. I was recently paying more attention to the difference between the main's filtering of the Chihuahua versus a stock Blackface Champ, which is what the Chihuahua is loosely based on. More robust-speced transformers and heavier filtering is part of David Allen's version of a BF Champ. What or how does the 100 ohm/5 watt resistor that's in between the A and B node affect the circuit?
For reference:
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Greg
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Re: What Does This Resistor Do?
It's the R in a CRC filter, which reduces the ripple on the B+ voltage. In single ended amps hum can be a problem because they lack the cancelling effect of a push-pull power stage.
Re: What Does This Resistor Do?
helping to remove humm from Ub+ which is heard on the speaker
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Re: What Does This Resistor Do?
Fender could have added that same extra stage of filtering if they used a 5AR4 recto in place of the 5Y3, but the fact that they wired one leg of the filiments to the chassis in all of there student level single ended amps made for a higher 60 hz noise floor then the 120 hz noise floor of having only that one stage of filtering on the output tube plates.
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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: What Does This Resistor Do?
Question #2:
Looking back at the partial Chihuahua schematic, you’ll notice the DC voltage from the B node (Plate) is documented at 362v. After the C node (Screen) the voltage is shown as 350v. But the voltage difference of 12v is impossible because there’s a 10k/3w dropper in between the B and C nodes. In reality, the voltage drop is approximately 90v. With the 6L6 engaged, my voltage readings are shown in red in the following updated Chihuahua schematic: The C node also filters the reverb tranny and its V2 12AT7 plates. Therefrore, all the DC voltages for V1, V2, and V3 (Reverb recovery and Driver), are in reality much lower than one might calculate judging by the original schematic. My question is: Is that 10k/3w dropper the correct resistor? Or was there an error in the original documented voltage? Or maybe it's something else?
I'm guessing in order to keep in line with the original schematic voltages, that perhaps the 10k dropper should be a 1k?
Looking back at the partial Chihuahua schematic, you’ll notice the DC voltage from the B node (Plate) is documented at 362v. After the C node (Screen) the voltage is shown as 350v. But the voltage difference of 12v is impossible because there’s a 10k/3w dropper in between the B and C nodes. In reality, the voltage drop is approximately 90v. With the 6L6 engaged, my voltage readings are shown in red in the following updated Chihuahua schematic: The C node also filters the reverb tranny and its V2 12AT7 plates. Therefrore, all the DC voltages for V1, V2, and V3 (Reverb recovery and Driver), are in reality much lower than one might calculate judging by the original schematic. My question is: Is that 10k/3w dropper the correct resistor? Or was there an error in the original documented voltage? Or maybe it's something else?
I'm guessing in order to keep in line with the original schematic voltages, that perhaps the 10k dropper should be a 1k?
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Greg
Re: What Does This Resistor Do?
Wow! I just found a photo of an Allen Amp Chihuahua built by David Allen himself, and to my eyes I clearly see a 2.2k dropper in between the B and C nodes:
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Greg
Re: What Does This Resistor Do?
Would you post the entire schematic for that Allen Amp Chihuahua?
Re: What Does This Resistor Do?
Sure!
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Greg
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Re: What Does This Resistor Do?
I do have a 1k/5w (Green Vishay Wirewound) that I could try and compare the new voltage results. I bet I end up with 2.2K in there.
So, this amp has been operating under these conditions for over six years. The power tube's screen is getting hit with about 25% less voltage than the tube's plate. I have no idea how this could affect the operation of this amp. BTW... The amp sounds fantastic with its master volume maxed, and the volume control at 6 to 7. Reducing the MV below 8-ish reduces the gain of the signal/tone. Maybe this is because of the discrepancy of voltage between the screen and the plate of the power tube and lower voltages to the preamp/reverb circuit???
So, this amp has been operating under these conditions for over six years. The power tube's screen is getting hit with about 25% less voltage than the tube's plate. I have no idea how this could affect the operation of this amp. BTW... The amp sounds fantastic with its master volume maxed, and the volume control at 6 to 7. Reducing the MV below 8-ish reduces the gain of the signal/tone. Maybe this is because of the discrepancy of voltage between the screen and the plate of the power tube and lower voltages to the preamp/reverb circuit???
Greg
Re: What Does This Resistor Do?
I once used a 2W 5K pot as a screen node drop tester in my Express build and found the greater the gap in PT plate vs screen voltage, the more I lost transients. So the lower the screen voltage, you get more compression. You also get longer-lasting tubes, depending on your plate voltage. I wonder if others have found the same thing.syscokid wrote: ↑Thu Jan 18, 2024 12:40 am I do have a 1k/5w (Green Vishay Wirewound) that I could try and compare the new voltage results. I bet I end up with 2.2K in there.
So, this amp has been operating under these conditions for over six years. The power tube's screen is getting hit with about 25% less voltage than the tube's plate. I have no idea how this could affect the operation of this amp. BTW... The amp sounds fantastic with its master volume maxed, and the volume control at 6 to 7. Reducing the MV below 8-ish reduces the gain of the signal/tone. Maybe this is because of the discrepancy of voltage between the screen and the plate of the power tube and lower voltages to the preamp/reverb circuit???
Just plug it in, man.
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Re: What Does This Resistor Do?
YesViperDoc wrote: ↑Thu Jan 18, 2024 1:05 amI once used a 2W 5K pot as a screen node drop tester in my Express build and found the greater the gap in PT plate vs screen voltage, the more I lost transients. So the lower the screen voltage, you get more compression. You also get longer-lasting tubes, depending on your plate voltage. I wonder if others have found the same thing.syscokid wrote: ↑Thu Jan 18, 2024 12:40 am I do have a 1k/5w (Green Vishay Wirewound) that I could try and compare the new voltage results. I bet I end up with 2.2K in there.
So, this amp has been operating under these conditions for over six years. The power tube's screen is getting hit with about 25% less voltage than the tube's plate. I have no idea how this could affect the operation of this amp. BTW... The amp sounds fantastic with its master volume maxed, and the volume control at 6 to 7. Reducing the MV below 8-ish reduces the gain of the signal/tone. Maybe this is because of the discrepancy of voltage between the screen and the plate of the power tube and lower voltages to the preamp/reverb circuit???
Charlie
Re: What Does This Resistor Do?
Ok… I thought it would be nice to chart and post the voltages after replacing the C-node dropping resistor of 10k with a 1k:
Amp is operating in 6L6 mode. Reverb circuit seems to be getting a more proper dose of DC. Amp feels more flowing, if you know what I mean!You do not have the required permissions to view the files attached to this post.
Greg
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Re: What Does This Resistor Do?
Much closer to the schematic values now. I would try increasing that dropping resistor, maybe go ahead and try a 2k2, as your screen voltage is still higher than the plate, and the preamp voltages are also high. Do you know what the OT primary impedance is? My guess would be it is 4k in 6L6 mode and 8k in 6V6 mode.