Common anode mixer and Brown Fender trem operating points

[cdemike]

I'm working out an upcoming build that'll be a small two-channel Brown Fender style amp for my wife. She really likes the 6G2 Princeton circuit, so that'll be one channel, and I wanted to add a second channel borrowing from one of the bigger Brown Fenders (originally planned on the bass channel from a 6G4 or 6G4B Bassman, but am realizing it's likely more practical to aim for the tremolo-side preamp from the 6G4B Super/6G5B Pro/6G6B Bandmaster). The plan is to retain the first gain stage from each of the original preamp designs and to combine the signals at a common anode mixer since a resistive mixer would leave a lot of interaction between the channels given the 6G2's tone control arrangement. From there it'll drive an ECC-99 power section set up for push-pull cathode biased operation. I think I have most of the issues worked out, but I'm hoping to clarify two things.

- There are a few reports on here and on the Hoffman amps forum of common anode mixers producing a fuzz-like distortion characteristic. From what I can gather, it seems like the electrical point of divergence driving the difference between an overdriven common anode mixer and a plain triode gain stage at a comparable operating point is the non-linear response of the unused anode. My original thought was to leave the cathode un-bypassed in order to increase anode resistance but it seems like that may actually exacerbate the issue in terms of non-linearity. That led me to consider a split plate resistor arrangement so that it essentially works like both a resistive and common anode mixer, and I'm thinking the shared cathode arrangement might reduce the unused triode effect by sort of having the stage work like a long tail phase inverter, i.e. with signal reaching the second triode via the shared cathodes. I've attached a schematic -- does this seem reasonable?

- I'm struggling to get my head around the operating point of the splitter/mixer stage from the Brown harmonic tremolo circuit. If I re-conceptualize that stage's shared 4.7k/2uF shared cathode as a split cathode arrangement, I think it'd have 9.4k cathode resistance with a 1uF bypass capacitor. That'd create a somewhat similar operating point to a JCM800's cold clipper, which seems to run counter to Fender's general effort to maximize headroom. The cathode bypass capacitor would be an important difference between it and a cold clipper, but the stage would remain biased pretty cold, reducing headroom, right? If that's the case, is there a significant downside to center-biasing with an 820R cathode resistor?

I apologize for re-posing questions from the Fender section, but it seems like this might be a more appropriate place to ask in any event.

Edit: realized I should probably have posted a schematic of the harmonic tremolo splitter/mixer... https://schematicheaven.net/fenderamps/ ... _schem.pdf

[maxkracht]

- I'm struggling to get my head around the operating point of the splitter/mixer stage from the Brown harmonic tremolo circuit. If I re-conceptualize that stage's shared 4.7k/2uF shared cathode as a split cathode arrangement, I think it'd have 9.4k cathode resistance with a 1uF bypass capacitor. That'd create a somewhat similar operating point to a JCM800's cold clipper, which seems to run counter to Fender's general effort to maximize headroom. The cathode bypass capacitor would be an important difference between it and a cold clipper, but the stage would remain biased pretty cold, reducing headroom, right? If that's the case, is there a significant downside to center-biasing with an 820R cathode resistor?

Another forum post was just talking about harmonic term, got me to dig out my revibe and see if I wanted to make any changes from the last time I messed with it a few years ago. Mine is running 2k7/2uf for the modulator tube. Adding a 1k in parallel, to get closer to your 820, slightly sped up the tremolo. No idea why. It also added some tremolo thump on certain settings. I need to look at things on the scope to get a better idea of what is going on but probably won't have time for a few days. I don't think the added gain is helpful and might mess with it further in the opposite direction.

It's not exactly harmonic trem, but it made me think of the OG Dorf vibrato article. Interesting reading/advertisements. Article starts on page 52. https://www.worldradiohistory.com/Archi ... 4-04-R.pdf

[cdemike]

- I'm struggling to get my head around the operating point of the splitter/mixer stage from the Brown harmonic tremolo circuit. If I re-conceptualize that stage's shared 4.7k/2uF shared cathode as a split cathode arrangement, I think it'd have 9.4k cathode resistance with a 1uF bypass capacitor. That'd create a somewhat similar operating point to a JCM800's cold clipper, which seems to run counter to Fender's general effort to maximize headroom. The cathode bypass capacitor would be an important difference between it and a cold clipper, but the stage would remain biased pretty cold, reducing headroom, right? If that's the case, is there a significant downside to center-biasing with an 820R cathode resistor?

Another forum post was just talking about harmonic term, got me to dig out my revibe and see if I wanted to make any changes from the last time I messed with it a few years ago. Mine is running 2k7/2uf for the modulator tube. Adding a 1k in parallel, to get closer to your 820, slightly sped up the tremolo. No idea why. It also added some tremolo thump on certain settings. I need to look at things on the scope to get a better idea of what is going on but probably won't have time for a few days. I don't think the added gain is helpful and might mess with it further in the opposite direction.

It's not exactly harmonic trem, but it made me think of the OG Dorf vibrato article. Interesting reading/advertisements. Article starts on page 52. https://www.worldradiohistory.com/Archi ... 4-04-R.pdf

Thanks! This article is great. I remember reading once that Leo Fender (or George Fullerton or whoever was responsible for designing the Brown harmonic trem read an article in a trade magazine -- seems like this might be the article in question!). really bizarre how the mixer tube could impact the trem speed in your revibe. I almost wonder if it's a speed of onset thing in terms of the phasing effect, i.e., an EQ difference that makes it seem faster? I'll definitely be interested in knowing what the scope shows.

[maxkracht]

really bizarre how the mixer tube could impact the trem speed in your revibe. I almost wonder if it's a speed of onset thing in terms of the phasing effect, i.e., an EQ difference that makes it seem faster?

Yes, my money is on a perception thing opposed to reality. Was just jumpering in the resistor, so there's a click when it's connected which messes with time perception, then it gets louder, which might also mess with perception. Reminds me of how a clock goes "tick tock", but in reality it just goes "tick tick". No change in speed or pitch but our brains create a beat.

[maxkracht]

I think this was partly real partly in my head. After looking at the scope, I found some ringing on the lfo signal. I vaguely remember having some sort of noise problem with the lfo/inverter tube which went away after swapping tubes/moving wires. JJ ecc83s ring, just about everything else doesn't. Poked around a bit more and got the inaudible ring to turn into an audible pulsing buzz that happened every second or so. Move the plate wire away from the grid on the inverter and all the bad noises go away. Now I can't replicate the original high frequency ringing thing, but I think it was only on the hi pass side and might have been causing some sort of phasing thing that shifted my speed perception when I changed the cathode resistor?

[cdemike]

Truly, truly appreciate the follow up on this! So it seems like the cold bias on that stage could be based on stability concerns then, which would make sense to me. Given that I'm planning on a miniature output section anyway, the lower gain doesn't bother me a ton, but I did want to ensure I wasn't losing out on headroom since I'm trying to get a clean-ish sounding amp out of this process.

Good to know this is so sensitive in terms of lead dress. I'm trying to be especially diligent regarding lead dress given the amount of complexity I'm trying to jam into this amp, but the added wrinkle in terms of sensitivity around this stage is definitely good to know going into planning the board's layout. Does your revibe use the Weber layout?

[maxkracht]

Not sure if cooler bias has anything to do with stability, I think you're just supposed to know not to put those wires together. Here's a pic of a vibrasonic I worked on a few years ago, notice how the grid and plate wires are routed. It had been worked on previously (not my mods), so maybe someone moved those wires around, but probably not. I don't think headroom is an issue, but you shouldn't hurt anything by playing with cathode resistor values. Easy to start cooler and add resistors in parallel until you get it where you want.

IMG_3254.jpg

I think my revibe is a Hoffman layout. I built it the first time maybe 15 years ago, didn't get it working properly and rebuilt it some time since then. Don't remember many details from the first iteration. I was probably 20 and didn't have any idea what I was doing. Still don't, but at least now I am aware...

[cdemike]

That makes sense in terms of the lead dress piece, though it's interesting to me that there'd be such a high degree of sensitivity even though those signals are out-of-phase. I was thinking the cooler bias would correspond with lower gain, especially given how much a cold clipper reduces gain in a JCM800, but again the bypassed cathode changes things significantly. I appreciate the tip for starting high and adding parallel resistors -- I can definitely see myself putting a bunch of resistors in and ending with a pile of parts with sad, zig-zagging leads.

It sounds like the anode mixer arrangement might be something I also need to experiment with. If that's the case, I'll report back when the amp gets underway (probably not until later this month given how my schedule has been lately).