Help on a valve circuit (sort of champ-ish) - tone stack, cathode follower, etc.

[midwayfair]

Not an amp forum, I know, but there's a heck of a lot more expertise here than in my house!

I got the urge to electrocute myself apparently and have decided I want to build a small amp. It's going to be based on the 5E1, except that I wanted to add a tone stack.

Here are my GOALS:
1. Edge-of-breakup with a bit of compression at a volume similar to the champ. I don't need high gain sounds, or even the sound where the champ is trying to explode. I am trying to avoid more than a little preamp distortion, so I don't mind not squeezing maximum possible gain out of the amplification stages unless noise is a concern. Think 4-5 on the volume dial of the original. Pedals can take care of the rest.
2. Tone stack that can be relatively flat if I want it to, but without using the BF (scooped) stack. The inspiration for this was the lead channel on my Tone King Imperial, which has a treble cut knob (as usual) and a "mid bite" that increases the mids as it's turned up at the expense of some bass. I was forced to conclude that this had to be some sort of capacitor bypassing a resistor, because I apparently can't use a variable resistor with a cathode bias cap. (thanks to Scruffie, for helping me not destroy something).
3. Specifically going for single-ended, 6v6 with a 12AX7 preamp. Ideally, I would only have the three champ tubes in there, but if I absolutely MUST have a cathode follower to drive the tone stack, then I'll use one. Any ideas what to do with an extra half a tube? I feel wasteful. Scruffie suggested using a high voltage MOSFET for the follower.

These are what I came up with so far:

With a cathode follower:


No cathode follower, mid bite moved to affect only the power amp:


A few extra notes:
1) I'm doing my best on the tone stack calculations, but I'm not totally sure they're right. The tone knob is a stupidly wonderful tone control with a range of 746Hz to ~8900Hz. The 100pF rolls in a treble bypass (boost) at 1748Hz, to help make up for the extra miller capacitance created by the 54.7K of the tone control and to make the tone control a little more versatile and boost the "good" frequencies. The tone stack as a whole still has the cut near the top of the presence range due to the fixed 4.7K, so I don't THINK it'll end up sounding harsh. Presumably, this is even more important if I were to have the full tone stack without the cathode follower, because that arrangement will be a little darker. It should also have a setting near halfway that sounds relatively flat. I tend to run my amps fairly bright but not maxed on the treble (6-8 on a typical Fender).
2) The "mid" control was calculated to get very close to boosting at the frequency of the G string on the guitar. I could use a 1nF there as well.
3) I used Merlin's modified cathode follower, but with the "stiffer" (is that right?) 100K cathode resistor. I'd prefer that the cathode follower had a minimal effect on the circuit -- I don't need more distortion, and I don't want to lose too much treble, but I was told to be worried about too much loading from the tone stack. If I can leave it out, that would probably make me happier. One less thing for me to do wrong.
4) Power and output will be the same as the champ.

And a few more general questions:
1) I'm having a hard time figuring out what some of the effective resistances should be, because I don't have any experience with valves and there's only so much I can understand from reading. For instance, is the volume pot effectively much smaller than 1M? The reason I ask is because the 22nF caps in the champ would have been overkill by a lot, and the didn't use a 1M at the input of the 6V6 when using the same capacitor value. Knowing this might change my mind about whether the cathode follower is truly necessary.
2) I haven't added a 22uF cathode bias cap to V1A. I didn't think it would help much with the tone stack loading, and I worried that it might contribute to distorting the first stage, but if it's a relatively harmless or beneficial change, I will certainly add it.
3) In the version without the cathode bias, I could still move the the mid bite to the same place as in the version with the cathode bias and with the midbite fully off, I would have a total of 157K dividing against the 1M, with a bit of extra high end creeping through. This doesn't look that bad to me (it's a 15% loss of gain compared with the champ, which I didn't want to run on 12 anyway), so I'm not sure if I'm missing something as it regards anode loading.

[Gus]

Have you used a real Champ or SE Princeton amp with a nice speaker cab?

I would build a stock Champ or Princeton first.  I would use a 12" speaker.  Then I would adjust the circuit.

In the 90's when you could still find tube PA for not much money I bought a 6x4, 12ax7, 6L6 SE tube PA with a nice output transformer with different output taps 4ohm ,8ohm etc.

I thought I would be clever and build an adjusted champ funny thing the stock champ wiring sounded the best with a nice 4x12 cab.

I am not telling you not to do what you are posting about but sometimes the stock circuits are very good and IMO are a good starting point to adjust the circuit from.  I did add a treble cut as a tone control.

Something that helps is elevating the heaters.

[midwayfair]

Have you used a real Champ or SE Princeton amp with a nice speaker cab?

Yup! This is what gave me the itch to build one I've played through vintage units and a couple different reproductions and variations. My friend also has a champified Epi Valve Jr. that I've played through a few different cabs. I did like it better with the 1x12 than with the 1x6, but I still thought it got a little too gainy too quickly, had a little more bass for lead tones than I'd want, and needed a tone control when I ran any sort of pedal into it. The champ is, however, my favorite tiny amp overall that I've played (out of all the things like the AC4, Marshall Class 5, etc.).

The Princeton is a bit better, but I just really dislike its tone control. I've never run that one through a different cab, though, only played some vintage units.

Swart's SRT 12" is the closest I can buy. I think he's got something going on to manage the bass in his, because it doesn't sound nearly as tubby as the champ.


What did you use for your tone control? I wouldn't be opposed to dropping the midbite control in mine if it just makes this whole thing far easier, too.

[Gus]

A Epi Valve Jr has a EL84 output tube that has more gain than a 6v6.  I don't like the EL84 SE amps as much as the 6V6 amps

Part of the beauty of the Champ is how the three stages gain work together  You can change the EQ gain with the value of the 12AX7 cathode bypass caps if you use cathode bypass caps

I will look for my schematic for what I did.

[thelonious]

Looks like a cool design.

> Scruffie suggested using a high voltage MOSFET for the follower.

I did this on an amp where I had to fit a larger circuit into a smaller space, and it worked well. I used an IRF820 because that's what I had on hand, but after reading some of R.G.'s suggestions on geofex and stuff people wrote on AX84, I'd probably go with a LND150 if I did it again.

> I'm having a hard time figuring out what some of the effective resistances should be

I get confused by that, too. I tend to rely on calculators like http://www.ampbooks.com/home/amplifier-calculators/coupling-capacitor/ to be close enough for horseshoes and tube amps. I think you might find that if you're not boosting mids/highs with cathode bypass caps, you'll have quite a lot of bass and might need to use even smaller coupling caps---but maybe the midbite control would help offset that. Or maybe you're using a cab or guitar without a lot of bass, and you'll love it as is!

> I haven't added a 22uF cathode bias cap to V1A.

That's probably fine unless you 1) end up wanting more gain or 2) have noticeable heater hum without it. Apparently one of the reasons people fully bypass V1a's cathode is to cut down on heater noise. Keep in mind that you can put resistance in series with a cathode bypass cap in order to achieve just the amount of extra gain you want; it doesn't have to be either/or. Btw, Gus's advice about DC elevating the heaters is good. You just "ground" your virtual heater center tap (two 100R resistors) to the 6v6 cathode instead of to earth, and it will both cut down on heater noise and keep your cathode follower happier.

> This doesn't look that bad to me (it's a 15% loss of gain compared with the champ

It's at least worth a try without the cathode follower. If you don't like it, it's easy to add in the MOSFET source follower on a bit of turretboard later. LND150s are tiny.

Be careful about the value of R8. A cathode biased 6V6 will want <500k total grid resistance, which is grid leak resistor + grid stopper resistor.

[tca]

You could try a Garnet type tone control, there are some schems floating around. The Garnet's Mini Bass G90T has treble and a bass controls:



You would still gave the midbite.

[midwayfair]

Huh, that's a weird bass control. How does it work? It looks like it grounds the signal at the lowest setting.

No, wait ... it's missing a cap from lug 3 of the bass control to the junction with the .047.

It's a variable cathode bypass cap, though, which I haven't seen anywhere else. Does this mean that there ARE safe ways to do that? Because I'd much rather use something like that for the mid bite than the way I drew it as a bypass cap across a resistor.

Found this site with 1-knob tone controls, which is kind of cool ... There are some of these I haven't seen in stompboxes at all, too:
http://amps.zugster.net/articles/tone-stacks

Is a Hammond 1590D or X big enough for this?

[PRR]

IMHO--- you are re-inventing a wheel which does not need to be re-invented.

The AA Champ has a tone stack. Build that, loose, try. Then insert other tone-stacks of your choice. You do NOT finalize a personal guitar amp on paper (PC?), but with a hot iron.

The first stage (in AX7-6V6 form) "needs" the cathode cap or you won't get enough gain to rock-out. (Tho if you have a heap of pedals maybe you already have gobs of gain.)

The Six speaker can never be tubby. With a 4X12" and a deep guitar, I suppose it could be. Tweak the cathode caps down from 25uFd to 5uFd or 0.68uFd (the Marshall plan).