6G6B design for lower voltage preamp with good OD?

[mountainhick]

This is for a cheapie. I am retasking an old radio, and have some other parts to integrate. I've been chewing on what kind of preamp to build for a 6AK6 SE 1 watter from an old radio.

I'd prefer to just leave the power tube config as is as starting point, but beef up a preamp to feed it. Integral to this radio I'd like to use: Existing chassis, the Power transformer, 6AK6, 12AX7 and 6X4 (I also have a solid state 6x4 to play with)

The radio as configured had 1.8A of 6.3 filament draw, so plenty amperage to run two or even three 12A*7' plus the 6AK6.

With the low voltage PS, I don't want to leave it as a clear only amp, but for it to have enough gain to get a sweet overdrive lead tone as well.

I'm considering a bassman style preamp based on looking at the 6G6B which looks to have lower voltages, but enough gain stages to both use a full tone stack and dial up the OD. I don't however have any personal experience with the 6G6B. A couple youtube vids seem to demonstrate it has good OD capability.

Good idea? Or what alternatives?

image is of the radio's PS. untapped subsequent RC stepping to lower voltages isn't appropriate since the voltages sent to the rest of the tubes is 115V and lower. I am not sure what's up with the 1000pf cap in the line... perhaps a high frequency bleeder?

I'm also not quite sure how to interpret the 6.3V PT output circuit. Is it OK to just reconfigure with an artificial CT and parallel to the tubes?

EDIT: and of course will add modcons to the incoming wall supplied AC including a fuse and grounded chassis!

 

[andrewRneumann]

I'm also not quite sure how to interpret the 6.3V PT output circuit. Is it OK to just reconfigure with an artificial CT and parallel to the tubes?

If you are measuring 6.3vac from that upper leg to ground, I don't see the problem with removing that choke(?) and reconfiguring for center tap / lamp. The way you labelled the drawing, it looks like 6.3vac from the upper leg to the middle leg, not ground.

That secondary winding has two wires coming out of the PT right?

 

[mountainhick]

If you are measuring 6.3vac from that upper leg to ground, I don't see the problem with removing that choke(?) and reconfiguring for center tap / lamp. The way you labelled the drawing, it looks like 6.3vac from the upper leg to the middle leg, not ground.

Schematic is identical to the radio's published schematic, I just re-drew it for clarity because the reprint is really hard to read. The only difference I may have gotten wrong is the B+1 voltage at the 40mf cap. That supply goes only to the OT, then schematic shows 185V at the 6AK6 plate. B+2 goes to the 6AK6 screen, then onwards to all other tubes with more drops/steps down.

Filament output from the tranny is 2 wire, no CT. Disconnected from rectifier and filtering it is 7.1V, may need dropping resistors. Or i could put the whole thing on a bucking TF.

There is no substantial choke, but there was a very small coil of the sort made by winding insulated wire around a pencil.

That secondary winding has two wires coming out of the PT right?

Secondary 6.3V as above.

Secondary HV has two plus CT. Voltage from my wall is closer to 123VAC and actual TF output is 377 hot to hot, 186V each side of center.

 

[tele_savales]

Really great blistering, fat tones when dimed. Not high gain at all, more like the breakup you'd get when cranking a Marshall non-MV plexi or a JTM 45. But more Fender-y. Very loud.

 

[andrewRneumann]

You could probably square-wave that 6AK6 with a single twin triode 12AX7 if you wanted to, even at those low voltages. A quick check yielded about 11v (peak) to reach overdrive.

I’m not familiar with those circuits you mentioned but I would guess you should stick to low-mu triodes in the preamp and try to build up the cascading gain gradually. With low voltages you’ll be overdriving the preamp really easy (I.e. very small amount of clean headroom.) Low-mu triodes should give you more headroom to play with and build up that nice cascading gain preamp sound I think you are shooting for.

Just my 2 cents.

EDIT: After further research, a better suggestion would be to look for triodes with a mu less than the 12AX7 provides (<100). This might still be a high-mu triode.

 

[mountainhick]

Really great blistering, fat tones when dimed. Not high gain at all, more like the breakup you'd get when cranking a Marshall non-MV plexi or a JTM 45. But more Fender-y. Very loud.

That sounds pretty good to me. I am not after super saturated heavy metal type gain, just robust OD.

As far as volume, a single 6AK6 won't compare to a pair of 5881s!

 

[mountainhick]

You could probably square-wave that 6AK6 with a single twin triode 12AX7 if you wanted to, even at those low voltages. A quick check yielded about 11v (peak) to reach overdrive.

I’m not familiar with those circuits you mentioned but I would guess you should stick to low-mu triodes in the preamp and try to build up the cascading gain gradually. With low voltages you’ll be overdriving the preamp really easy (I.e. very small amount of clean headroom.) Low-mu triodes should give you more headroom to play with and build up that nice cascading gain preamp sound I think you are shooting for.

Just my 2 cents.

The "bass" preamp section straight to SE power tube:

Would the cathode follower running on 180v do the trick?

I'd be really happy to crunch numbers if you can point me to any references or equations. While I've replaced parts and done small mods, this is my first foray into design (more like piecing together chunks from established designs).

 

[andrewRneumann]

The "bass" preamp section straight to SE power tube:

Yup. The negative feedback could go to the cathode of that last triode.
It may work just fine with 2 12AX7s. It’s easy enough to attenuate unwanted signal level before it hits the power tube. I’d be most concerned about disturbing the overdrive structure of the original design—which tube overdrives first, how far you can push it into overdrive, etc.

Would the cathode follower running on 180v do the trick?

I can’t see why it wouldn’t! But I’m not the smartest one around here.

I'd be really happy to crunch numbers if you can point me to any references or equations.

I think I would start by analyzing the schematic (6G6B). Where to start…
Ampbooks.com has some fantastic calculators for the most common tubes. You could analyze the 6G6B there and then try the same exercise with lower plate voltages to see how it compares.

If you are really just starting out in design, I highly recommend his (Kuehnel) books, especially the first two Basic Theory and System Design. He also walks through a Bassman preamp circuit on the website. That might be really helpful to see how at least one smart person thinks about it.

Rob Robinette probably has a micro Bassman build here. Higher voltages than yours, but probably useful none the less.

I have heard one advantage of the 5F6A circuit is that you can overdrive that cathode follower because it is later in the circuit. And that is supposed to sound really tasty. Just putting it out there as an option.

 

[andrewRneumann]

I was poking around looking for a good tube for your idea. I realized that recommending a "low-mu" triode was bad advice. What I should have said is, "Look for a twin triode with less mu than the 12AX7." And to make up for my mistake, I found a tube that I think might work well: the 5751. It is like a 12AX7, but the mu is 70 instead of 100. That's still considered "high-mu".

 

[andrewRneumann]

(I also have a solid state 6x4 to play with)

Go with the solid state rectifier. That will give you more voltage to play with. This is Class A, so not much power supply sag anyway. With the modern higher wall voltage on top of that, you may get more B+ to that preamp than you expect!

 

[mountainhick]

I was poking around looking for a good tube for your idea. I realized that recommending a "low-mu" triode was bad advice. What I should have said is, "Look for a twin triode with less mu than the 12AX7." And to make up for my mistake, I found a tube that I think might work well: the 5751. It is like a 12AX7, but the mu is 70 instead of 100. That's still considered "high-mu".

Thanks, no worries, I took it as what you meant, not what you literally said in your previous post. 5751 70mu, 12AT7 60mu yes?

Go with the solid state rectifier. That will give you more voltage to play with. This is Class A, so not much power supply sag anyway. With the modern higher wall voltage on top of that, you may get more B+ to that preamp than you expect!

The SS 6x4 is a tube 7 pin affair, so it's just a matter of plug and play rectifiers. I have the best of both worlds from the get go.

 

[andrewRneumann]

5751 70mu, 12AT7 60mu yes?

5751 yes, 12AT7 no. You’re making me look bad. Although the mu for the AT7 is in the right ball park, the low plate resistance makes it not a good choice for this application.

 

[mountainhick]

Can anyone share some insight as to why the treble control in the "bass" part of the preamp is separated from the rest of the tone stack? What are the trade offs compared to having it after the second triode/cathode follower?

EDIT: I ask because I have a possible scheme to use a lower voltage princeton type channel as the normal channel, but mix channels before the last preamp stage which would mean a tone stack with treble after the cathode follower in the "bass" channel.