Simple Bass Preamp for Review

[Tage]

I have designed this preamp to only use 2 tubes

Any feedback or suggestions welcome and appreciated



I know it's hard to see, I even had the contrast jacked up...

Also any good programs for drawing schematics?

[loudthud]

The first stage with the cathode follower won't have much headroom. The plate voltage of the gain stage is too high and the cathode resistor of the follower is too low. If you don't have a scope to look at the signal, stick with the values from the 5F6A.

[Tage]

Ok I was kinda looking at the SVT input for that part, although that uses a 12DW7, with a 220K load on the ax7 side, centre-ish biased and the au7 side has a 47k cathode resistor. Would I be better off to just go with that set-up, but with a 100k cathode resister on the follower, so I can use a 12ax7? I know that this will have some compression, and that is totaly fine with me, a little preamp grind is ok, at the power amp will be fairly clean

[Chuck H]

I don't see any voltages indicated!?! It must be REALLY hard to read. I can't say about the headroom of the cathode follower but by the looks of it you'll never see over 12VAC on the grid. And because of the small bypass cap the voltage gain at lower frequencies is much lower than that. A VOX AC50 has considerably more gain potential before it's cathode follower since it's after the second gain stage and it uses a 56k cathode with 420Vp. AFAIK they're not known for being dirty amps. You can always try it as it's drawn and change it if it doesn't work out.

[Tage]

Yeah I had actually been looking at the normal channel of the ac100, which is pretty much the exact same. I was intending to run the preamp at around 300 volts just to have max headroom. What program do you use to edit those schematics? just paint or something?

the one issue with that preamp is it requires 3 preamp tubes to do even the normal channel. I suppose I could go, 12au7 gain stage-> 12ax7 DC CF-> Tone stack-> 12au7 Cathodyne PI-> Power amp

The only concern would be if there was enough gain left for the PI, I've always seen it preceded by a gainstage, as tat is were the PI gain is set, because Cathodyne PI have no positive gain. Although I'm sure it would work, as the tone stack would help prevent the PI being overdriven and help reduce that nipple distortion...

[Tage]

I also stubled on this when looking through Vox suff


http://www.voxamps.com/downloads/circuits/v125lead.jpg

I am mostly interested in the 5 band EQ, voiced a little lower, I have the values for the bass model, this was not a model I was familiar with at all.

Although the center tapped pots might be hard to find...

[Chuck H]

With something over 100V on the cathode I don't imagine the approximate 12VAC on the grid will clip the tube. It'll just be converted to a low impedance AC signal (which is the original intention of the cathode follower). As to the 5 band EQ... It's been my experience that inductors of appropriate rating and value are hard to come by if you don't special order them 10,000 units at a time. Also, inductors are typically noisy inside a chassis without extra measures to shield them. A lot can be done with a typical three band EQ and some "cut" and "boost" switches for a lot less bother. You could include an effects loop easier, and plug in a twenty band EQ if you like.

JM2C

[Steve Conner]

Indeed, or if you're not afraid of silicooties, you can replace the EQ inductors with gyrators.

If you had a pickup winding machine (or a friend who has one) then you could easily make your own inductors using RM or pot type ferrite cores. You don't even need a pickup winding machine, a variable speed drill will do.

But a big advantage of the gyrator is that you can adjust the effective inductance by replacing one of the resistors with a trimpot. This allows you to voice the EQ to your taste.

[Tage]

Yeah once I looked into sourcing the inductors I relized it's tough to get them in the values I needed, even with massive amounts of capacitance. Something tells me it would not be worth the hastle. I like the standard FMV tone stack with a mid sweep control, that allows the mid range to sweep from upper mids to low mids. I think a variable mid cut switch might be useful as well, I've always found myself cutting mids whenever I've played bass, which is not very often...

[Zozobra]

Indeed, or if you're not afraid of silicooties, you can replace the EQ inductors with gyrators.

If you had a pickup winding machine (or a friend who has one) then you could easily make your own inductors using RM or pot type ferrite cores. You don't even need a pickup winding machine, a variable speed drill will do.

But a big advantage of the gyrator is that you can adjust the effective inductance by replacing one of the resistors with a trimpot. This allows you to voice the EQ to your taste.

This is intriguing. I might have to look into this!

I have designed this preamp to only use 2 tubes

Any feedback or suggestions welcome and appreciated

[ATTACH=CONFIG]17931[/ATTACH]

I know it's hard to see, I even had the contrast jacked up...

Also any good programs for drawing schematics?

I use expresssch
ExpressPCB - Free PCB layout software - Low cost circuit boards - Top quality PCB manufacturing

Its pretty good. It doesn't have some parts you need as standard though (such as tubes) but they're easy enough to make. If you have difficulty I could send you the ones I've made.
The schems look pretty good to me. Heres one I made earlier:
http://dl.dropbox.com/u/8802910/poweramp.png

[Tage]

Alright I actually have express schematic, i'll draw up some tube parts...

[Intergalactic Tourister]

Why is V1a plate connected to V1b grid? And why is the 50uf/450v cap connected to V1b plate? Just curious as to whats happening there and the results you're getting from that.

I have a similar amp with 2 12ax7 preamp. I think it was originally a bass amp modded into a gainy guitar amp- I think.

V2 is kinda like your schem. I'm curious about how you're going to approach this. -Thanks



That is V2 above with .1uf coupling caps going to 2 6L6's.

[Tage]

well V1-2 is set up as a DC coupled cathode follower, it is a pretty common arrangement used in fender bassmans and almost all marshals as well as the vox topboost preamp. It is used to make a low output impedance stage, which helps drive tone stacks better without as much signal loss. The 50uf cap is a filter capacitor for the B+ supply. those are the scary caps that can kill us...

Not exactly sure what is going on in the schem you posted there, both those triodes have no grid referances and the second one has it's cathode resistor connected to the 100k, and not to ground, it would be awfully cold biased. Unless this is a poorly drawn phase inverter, which it could be as you said it is coupled to the power tubes.

If you want some good reading on gain stages and such I would direct you to the Valve Wizard and Aiken Amplification web sites

I will be posting a better version of my updated schematic this week.

[Intergalactic Tourister]

The 50uf cap is a filter capacitor for the B+ supply.

Just wondering why no plate resistor on V1b.

But this is the first Bassman style amp I've owned- And there appears to be no method to the re-wiring job.

Not exactly sure what is going on in the schem you posted there, both those triodes have no grid referances and the second one has it's cathode resistor connected to the 100k, and not to ground, it would be awfully cold biased.

Me either. This is exactly how I received the amp. It does sound cold biased but it really doesn't sound as bad as the schematic looks. It also has a bias trim pot.

Thanks