THD picovalve

[Chuck H]

I was considering how to make that amp ACTUALLY more Champ like. The schematic below should do that pretty effectively. Replace the master volume pot with a push/pull pot for the switch in this circuit (EDIT: this is also convenient since these pots have a DPDT switch that we may end up needing). When in Champ mode the master volume will be deactivated. I know you mentioned wanting it, but a master volume doesn't really work in a Champ circuit because most of the distortion comes from power tube clipping. You can get your dirt by just switching back to Picovalve mode which will still have the master volume or use the triode switch to reduce volume a little and crank the Champ mode whenever you can. The tone stack is deactivated in Champ mode also. Because Champs don't have a tone stack and that's part of their sound. You should be able to do these mods on the existing PCB without much trouble.

[donluca]

Thanks man, that's really lots of food for thought.

When I'll get the Picovalve I think I'm going to start slowly, by first trying putting lower gain tubes in there and see how it goes. I also really want to try the switch mod you proposed first to see what it will sound like.

The "Champ mode" is super intriguing! Although without a master volume I'm afraid that I'll be able to play it only during the day and when I know my neighbours (which had children recently) won't be home.

Again, thank you so much for your time and efforts in this, I'll report back when I'll finally have my hands on the Picovalve.

[Chuck H]

The "Champ mode" is super intriguing! Although without a master volume I'm afraid that I'll be able to play it only during the day and when I know my neighbours (which had children recently) won't be home.

I feel your pain on that one. I too live in a circumstance where it's rare to be able to crank even a five watt amp (Jeez! You'd think we lived in the lobby of the municipal library!). But as I said, a Champ isn't really a Champ, nor will it sound like one if a master volume is in use. Look at it this way, if you're cranking up a Champ it's to get dirt out of it. With the above mods you can switch to Picovalve mode and use the Master volume. Surely the gain on the Picovalve can be dialed down to "some dirt". But there's still some light I can shine. If it MUST be a distorting Champ, build an attenuator. That's what I did. Sounds WAY better than a master volume for amps that derive much of their tone by pushing the power tube/s. I have an easy and relatively inexpensive design that I've shared here before. I'll repost it here if/when you're interested.

[donluca]

I feel your pain on that one. I too live in a circumstance where it's rare to be able to crank even a five watt amp (Jeez! You'd think we lived in the lobby of the municipal library!).

I know, right? I had to sell my solid state/modeler Champion 40 (40W, 12" speaker) because with the volume lower than 2 it wouldn't produce sound and at anything higher my neighbours would complain and had to roll down my guitar's volume, which compromised the tone even more and kind of defeated having a SS amp in the first place.

I've long debated going for a roland cube 40XL which is a SS modeler with a Power Squeezer option which would drop down the 40W down to 2W, but then, I never tried a tube amp and with all the possibilities of keeping the volume output in check (pentode/triode, master volume, etc.) I thought I might give it a try.

Unfortunately headphones are not an option, they give me a sense of claustrophobia, even with the big over the ears open back ones.

Maybe I should just play acoustic :P

[donluca]

If it MUST be a distorting Champ, build an attenuator. That's what I did. Sounds WAY better than a master volume for amps that derive much of their tone by pushing the power tube/s. I have an easy and relatively inexpensive design that I've shared here before. I'll repost it here if/when you're interested.

I've done some research and came up with some solutions and I'd love to hear yours as well.

First, it's what people refer to as "Power Scaling". Now, correct me if I'm wrong, but this looks like a simple voltage divider put after the output transformer which can work because we're dealing with 5W max, so we can use small 10W resistors to dissipate the power into heat, provided they are properly cooled (ie: with a heatsink).

Here's an example taken from the VOX AC4TV:




Second, VVR (Variable Voltage Regulator). If I understood correctly, what this does is it affects the B+ voltage and it seems to make the most sense when applied just to the power section of the amp.

Here's an example on the N5X kit amp:



Finally, there are attenuators, but so far I've completely failed to understand how they are built. My limited understanding is that they should behave more like a speaker load (which is not purely resistive, of course) but I'm absolutely clueless about what's currently inside one.

I'm not aware of any other ways to reduce the output while still driving the tubes properly other than using low efficiency speakers.

[Enzo]

First off watts is not loudness. If a table radio can be too loud for your neighbors, well that is only a couple watts.

I worked for years in radio, and I wore headphones. But I agree, it was like I was stuck inside my head. SO I do what many do, I put one can over my left ear, and park the other can BEHIND my other ear,. Now I listen to the feed with the one ear, the other ear gets to hear the acoustic sound in the room. Try it.

[donluca]

First off watts is not loudness.

Not necessarily as there are many other variables which vary the sound pressure you feel in your ears (gain, speaker sensitivity, distance from the speaker, room dimension and shape, etc.) , but it is definitely part of the equation.

I worked for years in radio, and I wore headphones. But I agree, it was like I was stuck inside my head. SO I do what many do, I put one can over my left ear, and park the other can BEHIND my other ear,. Now I listen to the feed with the one ear, the other ear gets to hear the acoustic sound in the room. Try it.

That's a very interesting solution! I still have my reference headphones lying around when I have to do some short critical listening (AKG Reference K601).
Something I'll definitely try out. Thanks for sharing!

[Chuck H]

The Vox system there is just a resistive attenuator type circuit designed to divide voltage to the speaker while maintaining a proper load to the amp. This is basically what ALL attenuators do.

Power scaling is very cool. It has the disadvantages of the bias being a little skewed off ideal, not a big deal usually, and it's best to have some way of limiting grid drive to the power tube/s with voltage reduction for best performance. I've never built one. Probably because I already have an attenuator On that note...

The attenuator is easier to implement because you don't have to retrofit it into an existing design. And it can be used with any amp you'll ever own at will without the need to build it into each one of them. JM2C on that.

If you build this in an aluminum box with those chassis mount, aluminum finned power resistors and mount the components to the chassis with thermal grease this design is good for up to a fifty watt amp and is adjustable from -2db down to ??? I'm not sure, but using my 20 watter cranked into it I can speak over it at full attenuation. The one I use has a capacitive/inductive dummy load but a few people here have built the strictly resistive version here and like it just fine. I also have a customer with the resistive version built into his amp and even at lower settings it sounds good. The one shown is for 8 ohms.

[donluca]

Right now I'm really, really liking the N5X approach, it looks like it was built with my precise needs in mind and it has everything you could possibly want.

And I'm really intrigued by the power scaling approach. I've found one used and working for not much, let's see if the deal comes through...

I'll keep in mind the strictly resistive attenuator. I was under the impression that it needed some kind of strange wizardry to emulate how a speaker works/reacts.
It's really straightforward and, most importantly, perfectly re-usable for any amp. I didn't think of using a rheostat, that's awesome because I gives me the option to really fine tune the output to my liking (hopefully) without altering the tone. I was wondering if a bright cap might be needed to avoid treble bleed when turning it down.

[Chuck H]

clean tones and slightly saturated bluesy tones at bedroom volume levels.

Have you heard the *outube demos of the N5X builds? Really a crispy and gritty thing. Which some people interpret as "British" overdrive sound. It's not a "saturated bluesy" sound. Cleans seem ok. A little cold sounding.

But then Champs were never the little blues machines people think they are either. Most of the ones I've come across were quite gritty when overdriven. So maybe that's what you're after?

If you haven't seen the N5X demos then I think you should just in case it's not what you're after.

EDIT: And BTW, my favorite design quirk on the N5X isn't the power scaling, it's the way the parallel power tubes are biased dissimilar to differentiate their clipping characteristics Neat idea.

[g1]

Finally, there are attenuators, but so far I've completely failed to understand how they are built. My limited understanding is that they should behave more like a speaker load (which is not purely resistive, of course) but I'm absolutely clueless about what's currently inside one.

I just want to clear up a bit of terminology. Perhaps evident from Chuck's posts, but he didn't explicitly state it, so:
'power scaling' is usually referring to VVR type approach.
'power attenuation' is in more simple forms like the Vox circuit you posted, and also Chuck's examples. It's most often in an external box but in low power amps like the Vox above, sometimes built in.
They can get very sophisticated to more mimic an actual speaker, but they are still power attenuators.

So not really 3 solutions, just 2 basic approaches with simple or complex variations.

[Chuck H]

And it could also be said that most of the preferred attenuators (BadCat Leash, Matchless/Dr.Z Air Brake, Ho/Ultimate** and even the famous Hot Plate) are (almost*) purely resistive loads. I have to admit though that since I have heard my design many times both with the active and resistive loads that the active load does sound better. But it's not a huge difference or a deal breaker. I could use either happily.

*Sometimes a "bright" switch or circuit is employed that includes a capacitor, but this in no way makes it an active load.

**I'm told that the current Ultimate attenuator uses an active load. But that is a recent development. I have it on good authority that for most of their run since the late 80's the load was just a resistor.

[donluca]

I just want to clear up a bit of terminology. Perhaps evident from Chuck's posts, but he didn't explicitly state it, so:
'power scaling' is usually referring to VVR type approach.
'power attenuation' is in more simple forms like the Vox circuit you posted, and also Chuck's examples. It's most often in an external box but in low power amps like the Vox above, sometimes built in.
They can get very sophisticated to more mimic an actual speaker, but they are still power attenuators.

So not really 3 solutions, just 2 basic approaches with simple or complex variations.

Thanks for clearing that up. When doing research on the internet there's so much confusion around those terms that ambiguity and doubts often arise.

@Chuck: I've heard some demoes on youtube but, as always, those are nearly worthless since people use pedals, different guitars and speakers with them, which means their tone is all over the place.

If you asked me what I refer as to clean tone (albeit with a slight hint of saturation), I'd say Clapton's Wonderful Tonight ( https://www.youtube.com/watch?v=xl7Hd2r0LOs )

The bluesy-creamy tone I refer to is perfectly embodied in Larry Carlton's "Emotion Wound Us So": https://www.youtube.com/watch?v=cOeB4oANTVg
To be pretty honest, if I could sound (and play) half as good as he does I'd be ok for the rest of my life.

I don't disdain some classic rock like Dire Straits (think Sultan of Swing, Down to the Waterline, Tunnel of Love, etc.)

I wouldn't go much beyond this, but I admit that sometimes I enjoy going pretty far into overdriven territory with Cream (Sunshine of your love), Pink Floyd (Time/Brain Damage/Eclipse), some Gary Moore (Still got the blues for you, Over the hills and far away).

Talking of bright switches, I know they don't make them active, they just avoid the treble roll off.

I'm also considering the VHT 6 Special Combo just for the sheer convenience of having a combo instead of separate head + cab.

Here's the schematic which vaguely reminds me of the Fender Champ (which, I swear, if you read what people write online, looks to be all about the cleans while to me it's pretty distortion oriented, no wonder Clapton used one cranked to the max for Layla).



I'd probably strip away or add a switch to bypass that tone stack, or maybe add pots instead of fixed resistors to have proper tone control.
And definitely add a power attenuator because this thing is gonna be loud as hell without it, even if I throw a very low efficiency speaker in there (thinking about the Jensen reissues one of them is 92dB).

[Chuck H]

Ok. To describe the amp above,..

It's basically a AA764 circuit black face Champ with the tone controls fixed at:
bass -2.5
treble-2
plus a little extra mid cut as the stock Champ uses a 15k resistance at the bottom of the tone stack (represents a fixed resistance for a mid pot) where the 6.8k is in the VHT. 6.8k, incidentally, is the standard value for all the larger black face Fender amps.

The push/pull/foot switch is a tone stack moot control. Though it doesn't moot the tone stack entirely, but nearly. It puts the signal near a condition of 50% voltage division off the first gain stage.

Then there's a tone control in the form of a simple variable low pass filter. I suspect this is typically used when cranking the amp to roll off the nasties.

The fixed tone stack condition seems very conservative. The shape of the EQ curve somewhat balances for HF and LF with a few dB dip in the mids around 1kHz. And this seems very consistent with what I've noticed as a preferred shape for a basic tone among many professional players from all genres. In tone stack mode this would have a very round, balanced clean sound.

With the tone stack mooted (my word, no one else can use it ) the EQ is near flat and signal is boosted about 10dB (that would be a lot). This is more akin to what you would find in a 5F1 Champ, but with the "tone" control to roll off treble. In my experience the 5F1 can get nasty treble in overdrive. Though most of this nasty is happening in the clipping power tube and the asymmetrical class A wave form the tone control earlier in the circuit probably helps a bit and I suspect that's why it's there.

That is my entirely immodest evaluation of the VHT circuit. And...

It seems like the most relevant choice for the tones you describe. You'd still need to quiet it down by using an attenuator or adding a VVR circuit or something. In my experience you won't get "saturated" type overdrive to the degree of that Larry Carlton sound from any single ended amp because they don't have any real sag compression. So you would probably need to use a little compression from a pedal AND get very good with your guitar volume knob and picking technique like Larry

Looking at the Picovalve, it's a more modern sound. A little thicker and tighter. More focused for the added articulation with higher gain levels. This could be a useful BandAid for masking less than perfect technique. Don't take that too hard. It's an actual criteria I build into my own amps. It could be versatile for what you want with the mods I posted. Albeit still loud without an attenuator (or VVR). The N5X is going to have a more chaotic sound. Very expressive in the HF and upper harmonics. Sort of "in your face". It has the VVR circuit, but I think that's about all it's offering for what what you want.

JM2C (or more like a whole nickle)

Phwew! That's all for now.

[Enzo]

I guess that makes it a mute point then...