Help me modify my diy 6L6 power amp

[Hybrid_Child]

Hi, I have a simple class A power amp that I build some years ago. It basically has a 12ax7 buffer stage and the 6L6 power stage. [Schematic and pictures below]

I thought it would be cool to modify it to be a simple pedal platform amp where I can plug in a distortion pedal directly and not have it sound all fizzy and harsh. I'm not that experienced with designing amps however, and I'm not even sure it's a feasible goal with only one preamp tube. So any advice is much appreciated.

Thanks.

[VMS]

Maybe bigger grid stopper on power tube and put it on the other side of the 470k resistor.


You could also parallel the preamp tube and maybe add a hint of negative feedback.


These come first in mind, i'm sure guys with more experience with 6L6 SE amps will chime in soon.

[VMS]

...you could also swap that 12ax7 tube for a pentode tube, i know tubenit here likes the 5879 tube.

[sluckey]

I would modify it into a Fender Champ 5E1. Only new a very few cheap components.

http://el34world.com/charts/Schematics/files/fender/Fender_Champ_5E1.pdf

[Hybrid_Child]

Thanks for the inputs. I think I'll stick with the 12ax7 just because I have a bunch of them lying around. :)

Wouldn't it make most sense to cascade the two triodes of the 12ax7 to allow for the most tone shaping?

Wow, yeah the Champ 5E1 circuit looks interesting. Thanks. :)

[pdf64]

A 160 ohm cathode bias resistor for a 6L6 seems low? eg perhaps not centre biased. Is the HT also rather low?

[Hybrid_Child]

Oh. Sorry I forgot to note the voltages. The voltage after the choke is 290V and after the 10k resistor it's 250V.

[Hybrid_Child]

I went through the relevant chapters in Merlin Blencowe's Designing Tube Preamps for Guitar and Bass today and I think I've got an idea of what I want to do now.

So allegedly the Fender Deluxe Reverb takes pedals well, so I snatched the input stage from that one and added a neat little Bass/Treble tilt control that was described in the Tone Stack chapter of the book. I'm not sure what to do with the 2nd gain stage though but I guess I can experiment there. Would it make sense to try adding a Bright cap to the Volume control?

I moved the Grid resistor for the 6L6 to the other side of the 470k resistor as VMS suggested. I've seen several single ended designs though where it sits before the 470k resistor which I find puzzling. 

Any comments or ideas?

Here's a schematics draft.

[shooter]

your schematic didn't take

are you powered up, or tore down? 
If it's still working, could you measure volts DC at the cathode of the 6L6, also plate.  those numbers will help you "tune" the preamp to drive the 6L6.  the value of Vk (cathode) is ~~ your bias.  this gives you an Idea how much signal you need to drive the tube, if your pre puts out 50VAC and your bias is 20VDC, you might wind up with a bad tube screamer!

here's the math for idle DC operation of the 6L6
cathode vdc /cathode resistor = tube current, (forgetting the screen current for now)
plate volt DC - Cathode Volts DC * tube current will tell you plate dissipation which you can compare against the tube data sheet and see if the tubes yawning or frying.  SE typically gets the most bang for buck around 90-100% plate dissipation

[HotBluePlates]

... a simple pedal platform amp where I can plug in a distortion pedal directly and not have it sound all fizzy and harsh. ...

... So allegedly the Fender Deluxe Reverb takes pedals well ... Would it make sense to try adding a Bright cap to the Volume control? ...

I would avoid adding a bright cap to the Volume control if you want to keep distortion pedals from being fizzy & harsh.


I have an original 1964 Deluxe Reverb, and have to wrestle with the bright cap to keep even clean sounds from being too bright/harsh.  I've avoided removing it altogether up until now (by keeping the amp Volume up around 8 and running my guitar volume very low, like 3).  However, I think I'm going to give in and do the simple, reversible change of taking the bright cap out.

I went through the relevant chapters in Merlin Blencowe's Designing Tube Preamps for Guitar and Bass today and I think I've got an idea of what I want to do now.

... Fender Deluxe Reverb ... I snatched the input stage from that one and added a neat little Bass/Treble tilt control ... I'm not sure what to do with the 2nd gain stage ...

A simple 100kΩ plate load & 1.5kΩ cathode resistor (maybe bypassed with a cap) seems like as good a starting point as any.

[Hybrid_Child]

your schematic didn't take

...are you powered up, or tore down?...

Oh, that's strange. It's a public link from my dropbox. I attached it to this message. Hope that works better.

I didn't rip it apart yet. :)
Thanks for advice with the math. I'll measure those voltages when I'm home from work today.

...I would avoid adding a bright cap to the Volume control if you want to keep distortion pedals from being fizzy & harsh....


...I have an original 1964 Deluxe Reverb, and have to wrestle with the bright cap to keep even clean sounds from being too bright/harsh.  I've avoided removing it altogether up until now (by keeping the amp Volume up around 8 and running my guitar volume very low, like 3).  However, I think I'm going to give in and do the simple, reversible change of taking the bright cap out....

I just bought a Distortion pedal yesterday and tried running directly to the amp (I've been running a Line6 POD HD into it up until now). To my surprise the sound was actually rather bass heavy, which is why the bright cap came to mind. Of course the new circuit will sound different so maybe there is no need for it.

Btw just rip that thing out of your Fender. But yeah, making it reversible is probably a good idea. :)

[pdf64]

...I have an original 1964 Deluxe Reverb, and have to wrestle with the bright cap to keep even clean sounds from being too bright/harsh.  I've avoided removing it altogether up until now (by keeping the amp Volume up around 8 and running my guitar volume very low, like 3).  However, I think I'm going to give in and do the simple, reversible change of taking the bright cap out...

Is this on the reverb channel? If so, have you tried lifting the 10pF cap (which bypasses the 3M3 dry mix resistor at high frequencies)?
I hate the 'tizzy' effect that cap has (though I love a bright cap on the vol control); on every Fender where I've tried lifting it, the owner has agreed it's an improvement.

[pdf64]

...I attached it to this message. Hope that works better...

I think it would work better with the vol control moved to between the 1st and 2nd stages (in place of the 1M grid leak); as it is, the 1st stage is likely to overdrive the 2nd, such that you won't be able to get a properly clean tone.

[Hybrid_Child]

...I attached it to this message. Hope that works better...

I think it would work better with the vol control moved to between the 1st and 2nd stages (in place of the 1M grid leak); as it is, the 1st stage is likely to overdrive the 2nd, such that you won't be able to get a properly clean tone.

But wouldn't the volume control then affect the character of the tone control? Couldn't I just choose a value for the R7 that would set an appropriate level for the 2nd gain stage?

EDIT: Ah, I guess not since the resistance to ground would be the same no matter what position a volume pot in that position would be in.

[PRR]

> vol control moved to between the 1st and 2nd stages

+1; or at least, _not_ between power tube and its driver. The driver has a hard enough job without a heap of loss along the way.

Get that right (or happy) before you worry where the tonestack goes.

[HotBluePlates]

... Is this on the reverb channel? If so, have you tried lifting the 10pF cap (which bypasses the 3M3 dry mix resistor at high frequencies)?
I hate the 'tizzy' effect that cap has (though I love a bright cap on the vol control); on every Fender where I've tried lifting it, the owner has agreed it's an improvement.

No, I haven't tried that.  However, while I'm okay with the bright caps in my Pro Reverb and Super Reverb, the 47pF in the Deluxe Reverb is overkill.

I don't know if I'd ever remove the 10pF cap; I've always been fine with it in place in a great many other Fender amps.

... I just bought a Distortion pedal yesterday and tried running directly to the amp (I've been running a Line6 POD HD into it up until now). To my surprise the sound was actually rather bass heavy, which is why the bright cap came to mind. ...

bright caps on volume controls operate in the same kind of range as Presence controls: extreme high end.  And their impact changes when you change the Volume setting.  If your pedal is too bass-heavy you're much better off using the pedal's (or amp's) tone control(s) to reduce bass.  Otherwise, mod the amp to trim bass rather than add highs.

[92Volts]

22uf bypass caps on the 12AX7 are big enough to boost everything. You could remove them for less gain or use smaller values to boost treble (i.e. cut bass). Either steal values from other designs or play with this calculator: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

If you want something more complex than the 5E1 the newer Champs added tone controls: http://el34world.com/charts/Schematics/files/fender/Fender_Champ-AA764-Schematic.pdf

Tone controls are a big part of the sound, if you want it to sound like an old Fender I wouldn't mess with it. Especially with the Champ and normal channel of the Deluxe-- the tonestack is nearly all that's happening in those preamps except straightforward gain from the 12AX7 stages.

This doesn't mean you'll make a bad amp by putting together tone controls and gain stages from Valvewizard, but if you want it to sound like a particular amp, cloning that amp is your best bet.

[shooter]

when you get a chance could you get me your 6L6 cathode volts dc

thanks

[Hybrid_Child]

bright caps on volume controls operate in the same kind of range as Presence controls: extreme high end.  And their impact changes when you change the Volume setting.  If your pedal is too bass-heavy you're much better off using the pedal's (or amp's) tone control(s) to reduce bass.  Otherwise, mod the amp to trim bass rather than add highs.

Good point. Thanks. :)

22uf bypass caps on the 12AX7 are big enough to boost everything. You could remove them for less gain or use smaller values to boost treble (i.e. cut bass). Either steal values from other designs or play with this calculator: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

If you want something more complex than the 5E1 the newer Champs added tone controls: http://el34world.com/charts/Schematics/files/fender/Fender_Champ-AA764-Schematic.pdf

Tone controls are a big part of the sound, if you want it to sound like an old Fender I wouldn't mess with it. Especially with the Champ and normal channel of the Deluxe-- the tonestack is nearly all that's happening in those preamps except straightforward gain from the 12AX7 stages.

This doesn't mean you'll make a bad amp by putting together tone controls and gain stages from Valvewizard, but if you want it to sound like a particular amp, cloning that amp is your best bet.

I think I like the idea of experimenting better. Besides I have no idea what an old Fender Champ is supposed to sound like. :D
I was actually experimenting yesterday with simulating different tone control circuits before and after the 2nd gain stage, and I think I have a concept that might work pretty well. I'll see if I have time to present my idea with some frequency curves later today.

when you get a chance could you get me your 6L6 cathode volts dc

thanks

Oh sorry I forgot to post the measurements. The anode is at 270V and the cathode is at 12V. The output transformer is 4k ohm on the primary btw.

[92Volts]

I think I like the idea of experimenting better. Besides I have no idea what an old Fender Champ is supposed to sound like. :D
I was actually experimenting yesterday with simulating different tone control circuits before and after the 2nd gain stage, and I think I have a concept that might work pretty well. I'll see if I have time to present my idea with some frequency curves later today.

Tone controls after the 2nd gain stage would be sensitive to the power tube. The tube's grid behaves like a capacitor to cathode/ground (and its own plate to some extent). That's why master volume controls directly feeding the poweramp reduce treble, you put resistance in series with a load that demands more current at treble frequencies than bass.

This effect is smaller in preamp tubes than output tubes. Not only treble loss but also variations between each tube you use would be less, if you keep it between the 1st and 2nd stage, not after the 2nd.

I built the AX84 Lead 2 preamp (usually feeds the phase inverter of a push-pull amp) with single-ended output. It has way too much treble with a 6V6... too little with a 6L6. I love the tone with a single, vintage 6CA7 I happen to have. Good thing I'm not pushing it hard because I won't be able to afford (or even find) an exact replacement for that tube!

People talk about differences in sound between tubes but I'm not even pushing its limits, this is mostly from interaction of the tone/volume controls with the tube. And the input capacitance and gain (Miller capacitance) behavior of these tubes don't even look different on paper, it's just hooked up in a way that the effect is significant so differences are also significant.

All of this is interesting, but a tone control earlier in the circuit will be more predictable and swapping tubes isn't the cheapest way to fine-tune your sound!

[shooter]

cathode is at 12V

thanks, looks like you're getting all you can from the PA short of re-doing the HT.  should be easy to drive/overdrive the 6L6

[Hybrid_Child]

Okay so here's my idea at this point.

Moved the volume control in front of the 2nd gain stage which I guess makes it as much of a gain control really. I tried simulating the whole thing with NI MultiSim and it showed clipping already when the pot was at 10%.

I also added a mid cut circuit after the 2nd stage, which gave some cool possibilities of shaping the frequency response, it did also give a significant volume drop however. I'm not too worried about volume though since I will only be using the amp at home. But anyway if I decide to add the mid cut control I would probably make a switch to bypass it. The tilt control worked pretty well at least. :)

[sluckey]

Moved the volume control in front of the 2nd gain stage which I guess makes it as much of a gain control really. I tried simulating the whole thing with NI MultiSim and it showed clipping already when the pot was at 10%.

What was your input level at C1 in the sim?

[Hybrid_Child]

Oops minor detail. 

500Hz sine wave
2 V peak-peak

[HotBluePlates]

... I tried simulating the whole thing with NI MultiSim and it showed clipping already when the pot was at 10%. ...

Clipping, or distortion?

2v peak-to-peak is about 71mv RMS, and a reasonable input signal.  I can imagine hearing (seeing) some distortion, but I wouldn't expect to see any tubes driven to cutoff/saturation.

[PRR]

> 2v peak-to-peak is about 71mv RMS, and a reasonable input signal.

You dropped a decimal (it's under your right foot).

710mV, which is generally a very hot signal for guitar. It should probably be possible to eat 700mV and stay clean, but for happy-distortion gain this may be with Vol way down.

[HotBluePlates]

> 2v peak-to-peak is about 71mv RMS, and a reasonable input signal.

You dropped a decimal (it's under your right foot).

710mV, which is generally a very hot signal for guitar. ...

What he said!   

The missing decimal is right next to a pile of guitar picks I dropped earlier...  So about 10x bigger than a typical non-boosted guitar signal.  Yep, clipping not surprising.

FWIW, Fender seems to show the Deluxe Reverb Reissue will make full output power (with 5% THD) with ~105mV at the grid of the input tube, when Volume & Tone controls are full clockwise.

[Hybrid_Child]

Ah okay well that's good to know. It might actually be an actual volume knob then and not just a gain knob. Although my intention with this amp is for it to be a pedal platform amp, so the input signal level might be higher than that of a raw guitar signal.

Anyway this is what I arrived at. So I dropped the mid cut control idea and went with a so called Passive Baxandall Tone Stack. This allows for both boosting and cutting the mids with just two knobs, so I dig it. :)
I was still not sure about values for the 2nd gain stage so I just copied the center biased input stage since my goal is to have as much headroom as possible.

I have board and turrets on the way in the mail now. Will be fun to see if it'll be any improvement from its current form. :)

[Hybrid_Child]

And it's done. 
Getting the parts and finding the time delayed me with the project. But here it is. Litterally just finished wiring it up. Now I just need to test the thing. 

[rake]

Have you considered making V1B switchable?
Bypass it for "clean" and engage it for "hi-gain"?

[Hybrid_Child]

Have you considered making V1B switchable?
Bypass it for "clean" and engage it for "hi-gain"?

I didn't consider that. Should be an easy mod though. :)

[dude]

Lower C2 to 4.7uf, this will take some of the bass out at the first stage. Takes the mud  out with out adding too much treble. I find the  standard 22uf, too bassy. This a common change in Fender amps, especially  the 5E3. Experiment with different values anything from .68 to 10uf.

[Hybrid_Child]

Lower C2 to 4.7uf, this will take some of the bass out at the first stage. Takes the mud  out with out adding too much treble. I find the  standard 22uf, too bassy. This a common change in Fender amps, especially  the 5E3. Experiment with different values anything from .68 to 10uf.

Cool. I'll try that if I end up finding it too bassy. :)

Just tested it and it seems to work just fine. The tone controls do what they're supposed to but they're not exactly radical in how they affect the tone. The volume/gain control does exactly that. The amp is clean below 12 o clock and after that it gradually distorts more and more. It's pretty loud at that point though so I'm not worried about not being able to keep it clean at the apartment level I'll be using it at. It does have a bit of 100Hz humm that I suspect might come from the wires going to the 6,3VAC transformer. They run just below the pots as you may be able to see in the photos. It's nothing I can't live with but I might see if I can do something about it at some point.

[dude]

Make a jumper for C2, use a 4.7uf with clips. Play the amp the way it is now and then just lift that cathode cap at either end, clip the 4.7uf across the Cathode R (neg to G) play, I "almost" guarantee you'll hear a difference. Takes a few minutes to solder two alligator clips, or maybe you can just use the clips without solder if your careful or maybe just tack it on the cathode R leads . You'll notice an overall tightness, less bass  and the distortion won't be as frizzy or heavy on the bass side with gain or vol up, hard to explain in words but well worth the try. Let me know what you think, I'd like your opinion. I think I read above someone else suggested lowering this cap. If you don't have a 4.7uf use a 10, or lower, Marshalls use a .68uf.


al