Simple tonestack question

[firemedic]

I'm building a BF Pro reverb. I want to use a single 5879 stage for the normal channel & keep the Vibrato channel stock. Since this will be a one-stage preamp does anyone have any suggestions for a two pot tonestack w/ minimal insertion loss? I'm thinking a Skyline may do the trick but any comments are welcome.

[Iannone]

Perhaps a James.

[tubenit]

I would try this ONLY if you're willing to do the PAB and midboost switch also.

With respect, Tubenit

[jojokeo]

This is supposed to be nice for low insertion loss but I haven't built it to verify it yet.

[firemedic]

Tubenit you never do the same circuit twice....
I did try the skyline stack you showed me, on my TOS, & I liked it. but the PAB was different. 
How about a standard fender T/B stack but with 1M pots, & a 10k or 25k middle resistor? This IS a Pro Reverb after all.
 

[firemedic]

Or, I could safely increase the gain on the 5879 according to the datasheet. I would think that could offset some insertion loss at the tonestack?

[tubenit]

Having owned and played Fenders ....... the standard FTS is my least favorite.  Just a personal preference thing.  I don't care for the mid scooped tone. I prefer the mids to be accented.

Using the Duncan Tone Stack Calculator ...........

Plug in the James tone stack values and dial the treble and bass down to 3 and up to 7 & watch what happens

Plug in these values in the Fender tone stack and compare to original Fender values.

The low insertion model was used in the Soma84 by Kevin O'Connor & I used it in the original Carolina Blues Special (which was a Bogen conversion).

with respect, Tubenit

[imaradiostar]

How about the traynor/Dr Z style tone control? It's very low loss and excellent for a non-scooped tone.

See my vox+cascode design for an example- in the schematic section.

jamie

[firemedic]

Tubenit, the one on the bottom left looks intriguing. How exactly is the bass pot wired?
& I also like yours imaradiostar.... I'll have to flip a coin on this.
I just want to nail this tonestack situation down prior to drilling out my board. This is my 3rd build & so far all my amps look beautiful at first, then wind up with jumpers all over the place to accommodate little changes.
I guess I'l leave extra connection points on this section of the board for the inevitable tweaking.
Anyone had experience with maxing 5879 gain? I've tried 56k & 82k plate resistors but I can go much higher, has anyone done this & does it still sound good?
Thanks for the replies guys!

[imaradiostar]

Many tonestacks work best with a lower driving impedance. The "slope resistor" (33k on a marshall, 100k on a fender) is often tailored to the impedance of the preceding stage. The marshall/bassban cathode follower can drive a tone stack really hard. The Dr z style control works well even when driven by a pentode with high plate resistance.

The 5879 is a great little tube but I wouldn't go to higher plate resistances unless you need the gain. It's only going to make any noise or microphonics a bigger issue. Generally a pentode seems to sound better for guitar with lower plate resistances but that's just my opinion. Once you dial in a plate resistance that you like you can alter the cathode and screen grid resistances to get the screen compression and current draw that you want. You might want to read merlin's info on his website for a more in-depth description.

If you want a similar sound but lower resistances try a 6au6- it can operate with much lower plate resistances while still producing a lot of gain. It'll eat more current doing it but it's still way less than the output section.

jamie

[tubenit]

How exactly is the bass pot wired?

It's drawn oddly. It's just a typical bass pot wiring.

Look at the Princeton Reverb bass pot layout

http://www.el34world.com/charts/Schematics/files/fender/princeton_reverb_aa1164_schem.pdf


With respect, Tubenit

[firemedic]

Okay, thanks guys. I'll go for a 100k plate/1.5k cathode on the 5879 to increase the gain just a bit, and try both stacks. I'll post the results when it's built.

[firemedic]

Somehow I deluded myself into thinking I could just fire this amp up & start playing! No.

Note to all: do not use cheap imitation chinese mallorys!!!! They leak like crazy. I spent all day scrounging for some ODs to replace them ALL.

Ironically the 5879 channel has required the least troubleshooting. I tried the stack imaradiostar suggested, since it is designed for a single stage "pentode substitute" cascode, & it works fine. It's very rich, lots of midrange, not as much gain as the verb channel but that's ok. I used the bright switch to lift the stack 220k above ground, for a boost. I also had to drop the voltage a lot to that stage to keep it within 5879 limitations. The supply voltage is still @326vdc which is too much but the plate & screen voltages are good. I think. 100k/176v plate, 470k/109v screen. I may try a 330k there to squeeze out a little more gain.

After many false starts I got the reverb channel up too. It's not completely done but it does work, and I understand now why you guys like BF Fenders- man it sounds sweet. Once it's 100% I can't wait to bridge the channels.
I'll post a schematic of the modified normal channel ASAP.

[Tone Junkie]

Imaradiostar I could not for the life of me find that schematic you referenced so i could look at the tone stack. Thanks Bill

[firemedic]

I found it by searching "cascode" on the site engine. There's not a lot of posts about it so you should find it quick.
Update- I think imaradiostar (and others) were right about 5879 microphonics when increasing the plate resistance. It sounded fine on the bench but now that it's in the cab it's an issue. I'm messing with some other stuff now but I'll be reducing that value a little- prob 82k.

[jojokeo]

Update your findings on the microphonics issue w/ the 5879 and what values you use to get that taken care of.

[bobmegantz]

this may seem trivial, but keep in mind that the maximum boost from the stack is slightly less than the insertion loss.  That is, more control = more insertion loss.  You might want to consider a cut-only circuit, but that might not sound so good without some other shaping (which again infers insertion loss...).
Or, you could settle for 5-6 dB of boost/cut, which would decrease the loss.

[Tone Junkie]

Of course now it doesnt come up I seem to be a day late and a doller short again (LOL).
Bill

[firemedic]

Well I fixed the microphonics with a 500p shunt cap across the plate resistor. No more high pitched squeal. But it really took the life out of the sound.
So I'm throwing in the pink towel on this single stage preamp channel idea. I'll go with the regular "normal" channel, inserted w/ 2.2Mohm/10p in series at the grid of V4b. That should enable channel bridging. Sorry this thing didn't work out guys!

[Willabe]

Try a smaller cap, you might not need that much to kill it off. Try 250, if that still kills the squeal, then try maybe 180. You might be able to find a value that will stop the squeal and not kill the tone. Maybe 320 or so might work?


        Brad         

[firemedic]

Okay, it's worth a try. At least a lot less work than rewiring the tube socket.

[jojokeo]

Okay, it's worth a try. At least a lot less work than rewiring the tube socket.

Did you try Geezer/Tubenit's values of 56k w/ 470k screen & 470r cathode resistors? They worked great for me. Have you tried a different 5879 tube also? That would've been my first thing to do before messing w/ circuit values or voltages.

[Tone Junkie]

When I built my TOS amp I had to go through 3 = 5879 tubes before I found a good one, When I did it ended my problems and everything worked great. and it was with tubnit and geezers values obviously.
Thanks Bill

[firemedic]

Yeah, the 3rd tube I tried (Tung-sol) was noticeably less microphonic. That was all the tubes I had. All the sources I have read do indicate that maxing gain from little pentodes will cause microphonics/noise.
Since this is a single preamp stage going to the PI, thru a tonestack, I was trying to max the gain. From the datasheet I picked what I thought was a conservative set of resistor values (100k plate, 220k screen, 1.5k cathode) but I should have expected noise issues.
I've tried both 56k/470k/470 (geezer) and 82k/330k/820- IIRC (tubenit). They worked great & I figured I could squeeze just a little more out of them for this application. Maybe not.
So I'll go back in again and start with tubenit's values, which I'm using in my SE amp and my TOS. If need be I'll scrap the tonestack idea entirely, maybe using a coupling cap selector switch. Anybody know where to get one of those?

[Tone Junkie]

No chance of adding onother tube and going with there carolina OD type preamp.
Bill

[firemedic]

Nah, I'm keeping the reverb channel as close to stock Fender as possible.
Although......
I don't need lots of reverb.
Any thoughts on using one of the paralleled triodes on the reverb send tube-leaving the rest of the reverb circuit unchanged- to boost the 5879 stage post tone stack- then inserting that into the grid of V4b to keep the stages even? For channel bridging purposes.
That way, I could use geezers 5879 values, any tonestack at all (I happen to like the FMV), & have gain to throw away.
I know I can do that, but if anyone has tried it & found unforeseen problems please post. 

[firemedic]

Well I did those things. Unfortunately my little one is sleeping so I can only check voltages, not play. I'm pretty sure it's going to sound awesome, I made the 5879 channel almost just like my SE amp which is incredible. Maybe a little more tweaking.

Which brings me to my (hopefully) last question: what am I supposed to name this creature? I wanted to build a Pro Reverb AA165 but relied on the Hoffman AB763 for the bias-vary trem. But then there's the 5879 normal channel. Feeding a different grid. I've tried to find a BF Fender with this style trem but I guess they all used the optoroach coupler.
Frankenverb? Pentoverbrato? Suggestions please.