Choke/R switch

[iknowjohnny]

When i tried this the bias changed a good bit ! I assume it's because the screens are after the resistor/choke. I can't recall whether a lower value changed it much, but i tried from 100R to 1k. Maybe only the higher ones like the 1k changed it much, i just don't remember.

[dehughes]

switchable..... as either/or ......... or both parallel?
I think you mean putting B+ tap after the whole assembly so it functions as a
standby....... that would make sence as a sag option ... if your not relying on
the choke for its smoothing action of ripple in the supply then paralleling the
resistor and choke makes sence too......

measure the DC resistance of the choke and choose....... so that you know
what resistances your inserting before the powersupply filter.....

if the option is going to be used inpart for its smoothing action or as part of
the supply filter I think a parallel choke/resistor would increase the ripple present
to a degree...... and the value of resistance might need to be larger than you
think if ripple is a major worry... anyway you choose.... mind the bias point
so you can compensate if the bias jumps past maximum dissipation...

Thanks Andy. I meant switchable as in either/or. My hope is that I can find a resistor value that will let it be switched in as an option to the standard Blonde Bassman choke yet still keep the tubes running close to that bias setting. I'm looking to have the option of the characteristics of a resistor or a choke with the flip of a switch, but without having to rebias...hence my inquiry as to the proper resistor value in the 6G6-B circuit. In seeing people do this with a 6L6 quad I find that the resistor value chosen seems to be nearly twice what the choke's DC resistance is, so I'm trying to extrapolate all that and translate that into my 6G6-B build. If the 5881s I'm using were to be cathode biased, I'd not worry so much, but I'm going to run them fixed bias so as to be able to "get home" to the typical 6G6-B feel/tonality while still being able to depart a bit. Does that make sense?

[iknowjohnny]

For the record, if you DID want to use a higher value r you still can. Just use a DPDT switch and use one side to bypass a series resistor in the bias circuit. You just have to determine the value.

[iknowjohnny]

For the record, if you DID want to use a higher value r you still can. Just use a DPDT switch and use one side to bypass a series resistor in the bias circuit. You just have to determine the value.

[dehughes]

For the record, if you DID want to use a higher value r you still can. Just use a DPDT switch and use one side to bypass a series resistor in the bias circuit. You just have to determine the value.

Yeah, that's an idea...not bad. I'd like to keep it simple though and find a resistor that is "close enough" to a choke in how it integrates into the power supply so as to make the choke/resistor switch relatively unobtrusive. That's the ultimate goal, and where I need the most advice.

[dehughes]

Okay, the DC resistance of the Bassman choke I'm using is 80 ohms. So, where does that leave me? Put in an 80 ohm resistor, or go higher...or....? What do you all recommend?

Thanks so much.

[Andy Le Blanc]

round it up to 100 or 150 ohms...... a more common value.....
50 sounds to low...

[dehughes]

round it up to 100 or 150 ohms...... a more common value.....
50 sounds to low...

Cool. Thanks. I know a choke and an R will function differently, even if both have the same DC resistance, so I'm looking to keep the tubes biased properly while having some of the characteristics of the R in place. I think 150 ohms sounds good....half of the Robben Ford mod value (nearly), which makes sense being as the voltages in my amp are kinda close to that idea, and I have half the tubes.

[Tonegeek]

I built an amp using a standby/choke/resistor switch as shown in the top drawing using a DPDT center off switch. It pops slightly when engaged (no more so than a standard Fender standby switch). Bottom drawing represents another way to do it that lessens the load on the switch and diodes. In this drawing main caps are charged when On switch is engaged. B+ comes off the other side of the switch. This was discussed in the Dumble forum extensively. Only problem I forsee is that since there is no load on the main caps when in standby, the voltage will be higher than when there is a load on. Just need to have the proper rating on the caps for this scenario. Also, it is a little more dangerous as the main are charged even in standby.

[dehughes]

Cool. What amp did you install this in...running what tubes?

[Zippy]

I think 150 ohms sounds good....half of the Robben Ford mod value (nearly), which makes sense being as the voltages in my amp are kinda close to that idea, and I have half the tubes.

If you are looking at the sag due to voltage drop across the resistor (Vsag = RI), you need to go the other way since half the tubes will result in less current across that resistor.

[Tonegeek]

Cool. What amp did you install this in...running what tubes?

It was a 50W Dumble HRM running El-34s. There is a subtle difference in clarity/compression between the settings. Choke has more authority, less compression.

[dehughes]

Cool. What amp did you install this in...running what tubes?

It was a 50W Dumble HRM running El-34s. There is a subtle difference in clarity/compression between the settings. Choke has more authority, less compression.

Cool. Thanks!

[dehughes]

I think 150 ohms sounds good....half of the Robben Ford mod value (nearly), which makes sense being as the voltages in my amp are kinda close to that idea, and I have half the tubes.

If you are looking at the sag due to voltage drop across the resistor (Vsag = RI), you need to go the other way since half the tubes will result in less current across that resistor.

Ah, right. Well, I'd like the resistor to not drop too much voltage at idle (i.e., closer to what a choke would be, and not so much as to totally screw with the fixed bias setting....), but I would like it to create some "sag" when the amp is up and cooking....thus differentiating it from the choke in terms of feel.

Any recommendations? Say, 500 ohms or so? I suppose what I need to do is calculate the voltage drop across the resistor given the voltage specs for a 6G6-B circuit, huh. Math....ech... My thought was that if the choke has 80 ohms of resistance (which it does...), then I ought to have an 80 ohm resistor to keep voltages "correct" for the bias setting, the rest of the tubes, etc... However, if I understand things correctly (which I probably don't) then my instincts say that the voltage drop difference from an 80 ohm and a 500 ohm resistor won't be that much. But again, I have to do that math....which will require more reading on my part...

[Noel Grassy]

What kind of current handling does the choke need to be rated for?

I found an old Triad 12Henry, 75Ma choke in my treasure collection. Is this adequate for a pair of EL84 in PP? How about a single EL84 and a single 12AX7 format?

I hope I'm not too far off topic.