Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jonas on February 04, 2024, 02:21:35 pm
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I'm attempting to add 5879 to a Single Ended amp using similar values to the Gibson GA-40 (CH 2) set up. I have 328V at the node but only 32V at the plate and 41V at the screen?
Do I need to adjust the power supply or am I mixing the single back in incorrectly?
I've tried a couple different 5879's with same voltages.
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but only 32V at the plate
this is the "V1" position? (I'm not a pentode knower so that might be low)??
If so
the "goal" of V1 is to bring the input milli-volt signal to a useable Volt or 2, keeping the "noise floor" as low as possible. as long as it's "center biased" enough to swing clean ~5VAC I never worry much about vdc at plate.
I use AU7's typically, they hover around 80vdc.
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> only 32V at the plate and 41V at the screen?
Is that actually bad?
Only 3 of your 8 examples have voltage. The ratios of resistors and the tube's ratio of Ia to Is set the plate voltage. For this tube, ratio Rs/Ra of 6 or 8 make roughly center bias. Ratio like 3 makes low plate voltage. You are trying ratio near 2. No wonder your plate is laying low.
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Thanks PRR - I changed the values to #6; much improvement. Can you explain center bias?
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since sleep evades me here's my take;
you're trying to bias the tube so that you can swing the input AC signal, clean in both directions, amplifying the AC without distorting.
the Cathode VDC, "controls" the negative 1/2, while the plate, the positive half.
get the cathode to close to ground, it starts clipping
get the plate to close to B+ it starts compressing
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Centre-bias is the middle of the load line with respect to where the (signal) grid voltage curves cross the load line. To provide a hypothetical example for a typical preamp tube, take curve Vg0 as the left hand end of where these curves cross the loadline and (say) curve Vg-4 as the right hand end of where the curves cross the loadline. Center-bias would be Vg-2. I.e., the signal grid voltage can swing 2V either side of the idle point (Vg-2) without being clipped. (And, if your aim is a clean signal, the tube should be set up so that) Centre-bias also (simultaneously) happens to provide the point at which the plate voltage can swing either side of the tube’s plate voltage idle point (I.e., 141V in example #6) without being clipped.
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thanks tubeswell, explanation makes sense I'm looking at a few different TDS for the 5879 and trying to follow along and identity the load lines.
I see the RCA TDS recommends grounding pins 2 and 6 in all applications to shield grid 1 and plate from heater when using AC heaters. Then RCA TDS calls out N/C on these pins on the pinout diagram. My 5879 is not RCA brand so I'm leaving these pins ungrounded
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thanks tubeswell, explanation makes sense I'm looking at a few different TDS for the 5879 and trying to follow along and identity the load lines.
I see the RCA TDS recommends grounding pins 2 and 6 in all applications to shield grid 1 and plate from heater when using AC heaters. Then RCA TDS calls out N/C on these pins on the pinout diagram. My 5879 is not RCA brand so I'm leaving these pins ungrounded
Even if 2 and 6 are NC inside the tube, grounding the socket pins can still help reduce AC induction between pins. So feel free to experiment with it
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> Then RCA TDS calls out N/C on these pins
A shield is normally connected only one end, thus IS "NC" by pin-out logic.
I'd ground them.