7-Octal: Rockster plus?

[Phil_S]

OK, not much enthusiasm for the last idea. We seem to have lost Matt H <shrug> maybe he'll return.

Here's something familiar with a twist or two. Remember, I'm working with 4 D-shaft 500K pots that I'd be wiling to replace if I could find a reasonable priced source. I'd like to keep the knobs. That gives me 4 pots and I've freed up the on-off rotary switch.

Without too many words, I think what we're looking at is a modified Rockster with a second input using a 6SJ7 with the same dress you see in the Gibson GA20. I'm not sure I've inserted the 6SJ7 appropriately.

For the tone stack, I attempted to modify it for the 500K pot and don't know if I've blown it or got it. I found I don't have the chops to make Duncan's TSC work for me. Oh well.

I see I didn't draw in the ground for the 220K grid leaks

Reactions and suggestions are invited and welcome and thanks for looking.

Phil

[matt h]

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[Phil_S]

Surprise! You're here! I'll take a good look at the DC30. I think you are saying I should use the switch to bypass the pentode...interesting. This may take a while to digest. Thanks.

[morcey2]

You need to get rid of the 22µ bypass cap on the cathode of the 6AQ7 serving as a cathode follower.

I've got a couple of organ chassis that have a bunch of octal sockets that I'm trying to decide what to do with. I've got a whole bunch of 6SN7, 6SC7, 6SL7, 6SJ7, 6J7 (top-cap version of the 6SJ7), and a few more I don't remember along with some 12 volt versions of a few of them.

My only $0.02 this late at night.

Matt

[overtone]

I like grid leak on 6SJ7s.
There is something odd about the cathode ground drawn tied to the "top" of the 0,05 g2 cap - surely that link should to be "after" the cap to make it of any use?

In the GA40 I have here there are two 6SJ7s before hitting the PI. These preamp coupling caps on the GA40 are lower (than the GA20) at 0,01 and 0,02.

[Phil_S]

You need to get rid of the 22µ bypass cap on the cathode of the 6AQ7 serving as a cathode follower.

I've got a couple of organ chassis that have a bunch of octal sockets that I'm trying to decide what to do with. I've got a whole bunch of 6SN7, 6SC7, 6SL7, 6SJ7, 6J7 (top-cap version of the 6SJ7), and a few more I don't remember along with some 12 volt versions of a few of them.

My only $0.02 this late at night.

Matt

Yes that cap on the c/f is an error made when copying the parts in the drawing program. Thanks for pointing it out.

Funny thing, this donor amp came with two metal 6J7 with the t/c intact. I clipped them out and they could be reused. I hadn't realized for some reason they are same as 6SJ7. I've also got several metal 6AC7 (not 6CA7/EL34) that came to me in an eBay lot several years ago that appear to be quite similar.

All those old tubes, if they still have life can be used to good ends. At some point, just dive in and build. If you want a simple project, use the 6SC7 in a tweed Bassman 5B6. It has surprisingly lots of clean tone.

I was looking at the Ampeg V4 schematic and noted that is uses a 12AU7 as a paraphase inverter. You could easily convert that design for a 6SN7 and run it at a much higher voltage.

Thanks for your 2 cents, Matt. All comments are appreciated at this brainstorming stage!

[Phil_S]

I like grid leak on 6SJ7s.
There is something odd about the cathode ground drawn tied to the "top" of the 0,05 g2 cap - surely that link should to be "after" the cap to make it of any use?

In the GA40 I have here there are two 6SJ7s before hitting the PI. These preamp coupling caps on the GA40 are lower (than the GA20) at 0,01 and 0,02.

Good catch on the .05 cap. It is a drawing error. Thanks.

I built a modified single channel GA20 a few years ago and what you see is how I ended up dressing it. It is a one trick pony, it is one great trick!

What is the affect of reducing the coupling cap from .05 to .02 or .01?

Also, I'm giving serious consideration to matt h suggestion to put the pentode in as a recovery stage after a c/f and I'm thinking that's where I employ the switch, as a bypass to the pentode. Thoughts on that?

Also #2, I've got two empty holes on the rear of the chassis. I'm wondering if I can move the output jacks towards the rectifier and use one hole for a MV pot (yay, I can use whatever I want) and the other hole for another switch of unknown use at this point. The question is a layout question, as I'm not sure how close I can come to the rectifier socket before it gets to be a problem. Pictures...I suppose I'd be willing to drill the back panel if there was a good reason. There is adequate room over near the power tubes. I'm not wedded to the two sockets I wired for the 6L6's. I can shift to the two closest to the OT and use the one near the edge for the PI. I'm thinking that may be an improvement if I want to mount a pot.

On the OT, terminals 4, 6, and 8 are the 4-8-16z secondaries, and terminal 2 is the common. Terminal 10 is the B+ CT and primaries are terminals 7 and 9. I can tuck the primaries in close to the winding and keep them on the chassis.

I'm looking for opinions on layout.

Thanks!

[Phil_S]

Hmmmm....lacking real original thought, but we all do that. This one has the DC30 preamp, inserting the 6SJ7 as a parallel gain stage directly to the PI using the on/off switch to lift it from the circuit.

Did I get Matt H's suggestions here? I'm not saying I'll build it just yet, but it's in the running.

I think I've got the 4x 500K pots and the switch on the front panel accounted for. The MV will go in an unused hole on the rear panel.

Did I insert, couple, and switch the 6SJ7 correctly? Anything else pop out?

Thanks for looking in!

[matt h]

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[Phil_S]

I realize this morning I've got 3 input jacks on the side of the chassis (original location) and only need 2. One hole gets a switch or a pot. It is an ideal location for yet one more switch to control the placement of the pentode if only I can manage the lead dress OK. I think I'm going to go forward with this. I will need a small parts order. Naturally, I don't have everything I need on-hand. Drat!

Look for a revised schematic. I believe good planning is the path to good results.

Thanks for all the help, Matt!

Phil

[morcey2]

Some minor suggested changes to the pentode setup:

Output from the plate was connected to the screen grid (orange circle)

Floating DC suppressor grid w/ cap connection. Suppressor should be connected to ground. (orange line)

I'd suggest running a standard cathode bias instead of a grid-leak bias, but that's just me. Grid leak bias is too dependent on the characteristics of individual tubes. That can be both good and bad. With the right tube, it sounds awesome, but as the tube ages or when it is replaced, the sound will change.

Matt

[Phil_S]

Matt M: Thank you. It was bad drawing, not bad intention

Regarding the grid leak bias, check the latest drawing in this post. I think I've got the space for a rotary switch of my choice. If I implement this, one setting is not connected to lift the pentode. When lifted the grid leak resistor keeps the grid from floating. Did I get this right? I suppose on the n/c connection, I can use that to connect a resistor from grid to ground, solving that problem. Is that right?

I'm not sure I can use the lift switch I show for the grid leak bias resistor. I figure that I'll need a cathode bias resistor when I lift the grid leak. This starts getting complicated...and I don't have room for unlimited switches.

I have Merlin's article on pentodes. I'll study up and see about cathode bias. Thanks for your input!

Phil

[matt h]

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[morcey2]

I think we need another Matt to start contributing. Two just isn't enough. It's starting to feel like the "Bruces" sketch from Monty Python.

+1 on the bootstrapped CF. I changed my Hi-Octane to have that and it's great!

As for the tone stack, playing with it in Duncan's Tone Stack Calculator it gives a significant scoop centered around 250 Hz when driven from a 1.3k impedance. Not what I'd want, but I know lots of guys who do.

Phil, you don't ever want the control grid and cathode fixed at the same voltage. What you end up with at that point is a large multi-element diode. I wouldn't worry too much about doing a switch to select the input for the pentode until after you have everything else working. You'll just add too many spots for noise to creep into the amp.

Anyway, it's late and my brain is desiring to do other things, but I'll look at it again in the morning.

Matt (the other one)

[matt h]

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