Choke/R switch

[roberto]

the difference in choked or non-choked amp is audible also at low volumes. A different low mid colour and more transparency on mid-highs, plus a little bit more punch.
Choke + R in series makes the amp sound even different, with a little more sag but with more punch. It all depends on the values. You know mesa dynawatt tech works on something similar.

[Zippy]

the difference in choked or non-choked amp is audible also at low volumes. A different low mid colour and more transparency on mid-highs, plus a little bit more punch.

To which, choke or resistor, are you attributing these characteristics?

[jjman]

I would think the preamp would not draw any extra current when driven if it's stages are biased half way between cutoff and saturation. Even if not biased as such the extra current in the preamp when driven hard I'm thinking is tiny peanuts to the big resistor.

Same for the PI but a more normal sized peanut?

The screens for the output tubes would be the only place you would see any significant increase in PS current use, when driven hard, that goes thru the big resistor. This could drop the voltage in the preamp and PI causing the sag. However, it would also be dropping the voltage on the screens of the output which would also cause sag. Which is more audible? This assumes the choke/resistor feeds the screens which is common.

Another consideration is that using the resistor instead of the choke removes a huge amount of filtering. So you need to have some “surplus” in your filtering on hand to take up the slack when in resistor mode. Otherwise Mr. 120HZ will make noise.

[roberto]

choked amps

[Zippy]

The screens for the output tubes would be the only place you would see any significant increase in PS current use, when driven hard, that goes thru the big resistor. This could drop the voltage in the preamp and PI causing the sag. However, it would also be dropping the voltage on the screens of the output which would also cause sag. Which is more audible? This assumes the choke/resistor feeds the screens which is common.

Yes - on all counts. That brings to my mind the interesting contrasts in power supply strings - that of the "classic" series dropping string vs. that of the parallel-series dropping string of Matchless (for instance). The Matchless scheme seems to take the extra measures to ensure constant voltages in the preamp, independent of the current draw in the power stage.

[klingo]

FWIW i once measure screen voltage while playing(dangerous) on an SS recto 18watter with 32µf/plates then 1K5-22µf/screens(with 100R unbypassed resistor) and the voltage go from ~300v down to ~240v on big power chords...
a choke and then a switchable resistor with a filter cap in between is a nice solution imho
i have this set up in one of my amps BUT before the plates and without the in between cap so it's not fully tested...

[roberto]

klingo, same configuration as yours, but I usually have lower values resistors. I use also a XXkohm 2W resistor bypassyng the stand-by. This helps charging the caps a little before switching the st-by and lessens the PT stress.

[iknowjohnny]

I A/B's some resistors and the choke tonite. Funny how comparing them like that can turn your opinion 180 degrees, but now i think the choke does sound best. I like certain things about both, but i think the choke gives the best quality high end. it gets a tad harsh with a resistor. i'm thinking i might have to try the choke AND resistor as stated in an above post. I assume in series, but i wonder what would happen if you put them in parallel...anyone?

[roberto]

Chokes usually have a Rdc from 100ohm to 220ohm. A resistor in parallel is useless. Putting it in series, it helps to find a tonal balance. You can also try two different switchable resistors in series (one à la dynawatt, one smaller).

[iknowjohnny]

My whole issue here is trying to be 100% sure what i prefer and whether any of the other options are desirable enough to warrant drilling another hole in the chassis. Right now i want to somehow make it switchable w/o drilling just to be able to A/B it quickly. I was thinking more in terms of a choke and R and a switch to cut the R in and out while the choke is there all the time.

I'm trying to picture this in my mind, but it seems to me like doing this (choke and r in series) should be no different than simply adding a bit more resistance to the screen resistors. I mean, a few hundred ohms is not going to make any difference to the PI and pre nodes. But it could make a notable difference to the screens since they are only 1k as is. So to my way of thinking, tho it is the thoughts of a relitive noob, this should make very little difference if any over a choke only. Any thoughts on that?

[roberto]

One resistor is easy simpler and cheaper to switch than more.

[iknowjohnny]

It's not that, it's the hole itself. i just don't want to drill a hole and then find i prefer using it one way all the time and end up with a unused hole. so i need to be sure whether i want more than one setting before i drill.

[Colossal]

My whole issue here is trying to be 100% sure what i prefer and whether any of the other options are desirable enough to warrant drilling another hole in the chassis. Right now i want to somehow make it switchable w/o drilling just to be able to A/B it quickly. I was thinking more in terms of a choke and R and a switch to cut the R in and out while the choke is there all the time.

Hey bro. Having just completed my first amp build about a month ago, I too went through a very long period of indecision prior to settling on what I wanted. As a noob, I wanted the best of the best, whatever it took to squeeze more mojo out (that's called perfectionist's procrastination ). In reality, I had no basis for comparison except what I had gathered by hundreds of hours of reading on great forums like these, asking questions, and listening to clips. In the end, the decision is up to you. You can always just mount the whole contraption on a board (switch and all) and insert it between your B+1 and B+2 with a couple of wires. That way, when your amp is built and still on your bench, you can just test it and then make up your mind and mount whatever you like best. Just have to leave room in your chassis layout to include the choke if you decide that's what you like best. I included a choke and resistor because my schematic called for it, but I also learned that chokes can help reduce stress on a PT. I don't know how big of a deal that is and I know there are different ideas on that which we won't get into here.

Anyway, FWIW, I have a 6V6 Plexi with a 10H choke and a fat PT and it sounds insane. I did not test it without a choke so can't help you out with how it might sound otherwise.

It's not that, it's the hole itself. i just don't want to drill a hole and then find i prefer using it one way all the time and end up with a unused hole. so i need to be sure whether i want more than one setting before i drill.

I also added a bunch of mod switches to my first build. I did this specifically to be able to try out a lot of ideas and how changing the values of certain components and the various combinations thereof would affect the tone. I've got switches and pots all over it. Yeah, it's cool and useful if you want to experiment but for my next build, it's going to be a one trick pony. I've been told by guys who I respect like Mark Huss (and many, many others here), that the best amps are voiced correctly the first time. If you're going to make your amp more of a showpiece, then yeah, it's probably good to make up your mind what you want so you know where to lay out your permanent switches what your facelplates say. If you are going to mod it and change your mind, then who cares, drill it up! I made mine a showpiece when I probably just should have stuck with a blank chassis To do it again knowing what I know now, I would have done away with almost every mod/switch as I know what sounds good to me.

Not really answering your technical question but hope that helps you if you are struggling with the whole indecision thing. I'm not sure that a choke vs. no choke is going to yield you such a vast difference in tone as to warrant a switch. My plexi gets some great warmth but is quite punchy with the choke.

Regards,
Colossal

[klingo]

FWIW i played a bit with the unbypassed screen grid resistor on the 18watter.
just put a 2k2 pot between screen and the supply node (1K5/22µf) and played with it. i ended with a pot value around 500R, so i put back these 1K unbypassed resistors i had before (500R/both>1K/each).
imo unbypassed screen resistors have more effect on the distorted tone/compression because they are...unbypassed.
this apply to an 18watter and may not behave the same in another amp
(this has been done after the unbypassed 100R common resistor measurement i stated above)
from my experience it's necessary to have each tries A/B'able from a switch or dialable from a pot if you can without getting shocked

be careful

[roberto]

iknowjohnny, I've never suggested you to make any hole, I just said you that switching one resistor is simpler than switching more. You said you want to do an AB test with Choke+R in series, then this is the simplest way.