7-Octal: Rockster plus?

[Phil_S]

I'm going to change my name to Matt. LOL. For your further amusement, my sister is married to a Phil. His father was also Phil (RIP, a really nice gentleman, was 90.) Picture the 3 of us in one house...actually, I'm Philip to my family, so it wasn't so bad, but, still, someone would call, "Phil" and there'd be all sorts of silliness for a moment. My wife also has a cousin Phil...and I went to summer camp in the same bunk for years with yet another Phil!

So, I've made some adjustments, some just housekeeping:
-Changed the cathode bias resistor to 100V.
-2nd tube, properly labeled V2.
-V1A (2nd stage) removed Rk. I can always add one.
-V2, bootstrapped the C/F, used a .047u cap...not sure if this is a good selection -- just guessed at it because I don't feel ready to calc it without actual voltages.
-PI, changes both Ra from 82K to 100K. Remember, this is a 6SL7, not a 12AX7 and internal plate resistance is less.
-PI Rtail changed to 15K. I'll probably experiment and might start with 33K.
-PPIMV changed to 250KA dual gang pot. That one goes in an extra hole on the rear so I don't need the D shaft type!
-Pentode: removed the grid leak; switched one pole to ground; guessed at Rk between 1K and 2K -- when I have known voltages it can be tweaked, but got to start somewhere; rewired the pentode volume pot as suggested. Agreed that I've got time to fiddle with the pentode at a later stage of the build, but I'd like to get the basic parts installed anyway. I need a relatively small switch, not one of those monster size ceramic wafer selector switches. Something like this? http://www.mouser.com/ProductDetail/Ape ... fpooEHk%3d

Regarding the fixed resistors in the tone stack, did I reason incorrectly that the 500K pot forces me to double everything? Or do I double the caps and half the fixed resistors? BTW, I may have a 50K D shaft pot on the bone pile and I may be able to fit this to the front panel for a mid control. Is the mid pot a waste of that turf and is 50K too much R? (Remember, there are 5 holes in the front.)

Regarding the scoop in the tone stack, this is something not in my wheelhouse. I don't really understand it because I don't understand the frequency bands. So, I'll ask, if it was yours, where would you want to see the scoop and where might you fix the R/C values in the tone stack? I can run this through Duncan's TSC but it might as well be Greek because I don't know what to look for. Is there any chance of a brief discussion of what to scoop and what it might sound like? I do mean brief.

Thanks everyone!

Edit: some good news on pots. Mouser has Alpha 24mm solder lug pots with D shaft 1MA and 25KB (mid control) and round shaft 250KA dual for reasonable prices; $1.55, $1.65, $2.73 respectively. I need some parts. I'm not fond of ordering from Mouser only because it is so daunting to use their catalog online, but I expect I can find *everything* I could possibly want there.

[matt h]

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[Phil_S]

I'm going to place an order with Mouser, so I get to use 1MA d-shaft pots and return the t/s to standard values. Though it was intended, I see that I failed to halve the resistors in the most recent drawing...guess I'm a slow learner ;-{ but it wasn't for lack of trying.

Can you elaborate a little on the choice of pot for the PPIMV? I did some on-line reading (who knows how reliable) and that is where I got the sense that 250K is a good choice, but I really have no idea. (See below, only one chance to get the right part.)

OTOH, resistors are cheap and I don't mind swapping out for one thing or another as long as I've got a few of them.

I dislike ordering from Mouser...a very difficult on-line parts finder that misses stuff on a regular basis. I need to try to think of everything I might want, because I'm only paying that $10 delivery charge one time! It's one thing to order a little extra, but it's really hard for an amateur who can't keep a big stock of anything and certainly shouldn't order a big pile of things like e-caps.

[matt h]

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[Phil_S]

220K || 250K = 120K. As you know, in parallel, the result is always less than the smallest value in the mix! I wasn't exactly sure this is a situation of parallel resistors because of the ground at the junction of the 2x 220K grid leaks.

Here's what I'm thinking...

It seems to me, if this is the case, no matter what pot you choose, the result is always going to be less than 220K. For example 1M || 220K = 180K as the maximum value. The exception is when the pot is at 0 because then it is not in the circuit (not sure about this) and so it magically jumps back up the 220K or whatever value grid leak.

This leads me to think the value of the grid leaks needs to be increased. A 500K pot at max || 470K grid leaks = 220K. As the pot is rotated towards 0 the parallel sum gets smaller. I am thinking we actually need a 1M pot and 1M grid leaks so we can start at 500K and allow it all to go down from there. But that works badly when the pot rotation is set for a low value.

It also occurs to me that the value of the grid leaks will affect the grittiness of the tone. At 500K (total) we'll get lots of grit and when the pot goes down below 220K, the total gets below what you say is desirable.

Here's what I'm concluding...

The design of putting the MV pot in parallel with the grid leaks is flawed and we need a different design. Will this work?
-Connect the wiper to the PI as shown.
-One outer lug goes to the 1.5K grid stopper.
-The other outer lug goes to a 220K resistor.
-The two 220K resistors are joined and grounded at the junction.
This calls for a 250K pot or maybe even a 100K pot?

Edit: I found this schematic, which simply eliminates the grid leak resistors and now I remember, this is where I got the ideal that 250K is the choice for the MV. But...that is for a Rocket using EL84's. I am thinking the design works fine, but the pot value is wrong. Or the 250K pot is fine with a pair of 220K fixed resistors in series, on the inner legs, with the junction of the fixed resistors grounded. Is this making sense?

I won't have room for the cut control. That is simply eliminated or if essential, it has to be done with a trimpot or a fixed resistor.

[morcey2]

I think you're overthinking it a little bit. A PPIMV is a variable grid leak , among other things (voltage divider for the signal). You want to aim for a 220k-ish grid leak value. That value is when the PPIMV is dimed and you effectively have 2 resistors in parallel. (pot resistive track || grid-leak) So, if you pick a 500k pot for the PPIMV and 470k grid leak, you'll do fine. The reason you want the grid leaks in there along with the pot is in case of a pot failure. If the wiper somehow loses connection with the track, you'll have a floating control grid which can cause burn out a power tube very quickly.

Again, the 220k value is a maximum grid leak value. A 100k value would still work, but you'll lose a little bit of drive to the output tubes. With it being a cathode-biased amp, you could go as high as 500k and be fine(1M pot || 1M resistors). Don't get too wrapped-up in the actual value of the grid-leak other than making sure it's high enough under normal (cranked) conditions.

Matt

[matt h]

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[morcey2]

Just a +1 to what the other Matt said. The exact value isn't critical and having a fixed resistor to provide a solid grid-to-ground reference if a pot opens up is the important thing.

My worry about a 100ishK vs 200ishK (or more) was not just for drive reasons, but also thinking a bit about tonal consideration. It seems to me that the lower values (ignoring voltage dividing) are going rob a little bit of high end detail (is it called the miller capacitance that I'm thinking about for the output tubes that'll be in effect here?) Anyway, it probably isn't a huge difference, just a gut feeling that larger is a bit better in an application like this.

A cap between the signal lug and wiper lug on the MV pot could alleviate that somewhat.

You can leave it wired in the way you originally had it if you'd like, just up the values.

Just make sure that the MV pots are grounded with the grid leaks. That connection is missing in the last full schematic.

Matt

P.S. About 5 years ago, there were 35-ish people in my office. 7 of them were named Matt. 6 were named Dave. I was the original "Matt", so the rest of them had to use something else. We're up to 55 people and down to 4 "Matt"s.

[M Fowler]

I was working in the ER one weekend and someone said hey Mark and all five of us in the ER answered up, yes!

We had a good laugh about that after we all looked at each other and immediately understood why we had all answered up.

A lot of Marks out there in this big beautiful world and lots of marks yet to be made.

[Phil_S]

Matt(s): Thank you again.
Is this what should be done? Assuming C1 and C2 are .047, what should C3 and C4 be?

Hi Mark! Thanks for looking in.

Phil

[matt h]

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[Phil_S]

...then again, a coupla ceramic or mica caps aren't a lot of coin to tack onto an impending mouser order. so, get'em in the 120-250-470pF range.

Exactly what I needed to know and my sentiments exactly. Thanks.

[martin manning]

I'd do the MV like this (only the "upper" half of it is shown). If you have a dual 500k, then put 470k across the elements. If you're buying a new one make it a 250k and forget the 470k. The 2M2 is there in case the wiper fails.

[Phil_S]

Please help me with layout.

Slow progress on this one. Layout is proving to be a challenge. I built a cap board and wired the heaters. Power tubes are mostly done, so I'm working back from there.

The PPIMV is next and I realize I'm not quite sure about layout with the dual gang pot. Look at the schematic below. The pot, looking from the top, has 6 lugs in two rows (obviously.) For the sake of discussion lets label the lugs:
Front
A B C
Rear
D E F
where B and E are the wiper. What attaches to what? I am concerned that I will wire the MV pot so the the front and rear work in opposite directions

A upper .047
B (front wiper) 5K and 2.2M
C connects to D and to junction of pair 2.2M; single ground wire to appropriate ground spot
E (rear wiper) 5K and 2.2M
F lower .047

...or did I reverse C and F?

The 2.2M's go across the wiper and the inner lugs, and the 5K stopper goes between the tube pin and the wiper. (I will have to install a terminal strip because the jump is too long for the 5K.)

Should I put the .047's on the PI tube pins or on the pot lugs?

I will need to hang as much as I can on the pot and tube pins as it is crowded and limited space for a terminal strip.

Thanks.

[Zippy]

Regarding your tone stack - consider substituting a 25K pot for the 5K (or 10K?) resistor. That'll give you a mid range knob. At least sub a trimmer there. I don't know why people bother with treble and bass knobs and assume that a fixed value mid range is going to give them their happy place.