What Does This Resistor Do?

[syscokid]

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?

HEAVY FILTERING CHAMP SCHEM.png

For reference:

CHAMP AA764 SCHEM.png

[martin manning]

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.

[bepone]

helping to remove humm from Ub+ which is heard on the speaker

[Stevem]

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.

[syscokid]

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:

HEAVY FILTERING CHAMP SCHEM.png

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?

[syscokid]

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:

AA Amp Related Photos 306.jpg

[sluckey]

Would you post the entire schematic for that Allen Amp Chihuahua?

[syscokid]

Would you post the entire schematic for that Allen Amp Chihuahua?

Sure!

Allen Amp Chihuahua Schem 2019.png

[martin manning]

I'm guessing in order to keep in line with the original schematic voltages, that perhaps the 10k dropper should be a 1k?

That's where I'd put my money.

[sluckey]

Thanks

[syscokid]

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???

[ViperDoc]

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???

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.

[WhopperPlate]

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???

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.

Yes

[syscokid]

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:

Allen%20Amp%20Chihuahua%20Schem%202019.jpeg

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!

[martin manning]

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.