Bogen amp project on the way

[cynical1]

TB-

That turned out pretty well. One question...so what does it sound like? ....sound samples...nudge, nudge, wink, wink...

+1 for a very respectable save.

HTC1

[Teleblooz]

Pretty decent, I gotta say, for what it is. With the way I rewired the gain stage, it has no problem getting a nice fat overdrive from the preamp (which it couldn't do stock). In fact it takes a little work to get a clean sound. There's room for improvement there, but I can live with it for now. At some point I'd like to wire in a presence control to get a little more sparkle into the high end. One of the schematics i scrounged up has a Fender-style tone stack that the guy built into one of these. Reportedly it sounds pretty good.

It'll never be a recording amp I'm sure. There's some background white noise, especially when you take the master volume up past about 12 o'clock. But since at that setting you're making dogs bark blocks away, even with two power tubes removed, I don't see that as a huge problem.

I expect I'll be tinkering with this thing for a long time to fine-tune it. It was most important to me to have a functioning, self-contained unit as a baseline, and I accomplished that. I'd feel perfectly comfortable taking this to a jam the way it sits.

And I haven't even tried it with pedals yet...

I'll try to get around to documenting my mods in schematic form this weekend, so those among you who can read these friggin' hieroglyphics can critique and (please) suggest improvements.

[cynical1]

Hey, my hat's off to you...last time I tried to fix and old amp it was a pyrotechnics show...

HTC1

[Teleblooz]

This one almost was. At one point I tried to wire in a switch to disable the grids on the second pair of output tubes. I must have misinterpreted the instructions, because after about 15 seconds, one of those tubes started to glow an intense orange and a loud hum was coming from the speaker. Thank goodness the fuse blew and saved it from self-destruct (I guess that's kinda what it's there for, eh?). So I set aside the question of that switch for another time while I concentrated on just getting the beast running.

[sumgai]

tb,

From your description, you attemtped to disconnect the bias supply from the grid(s). That would be highly catastrophic.

I believe I said that you can disconnect the signal input from these grid connections, and that would cause the tube to be "undriven", thus rendering a lower signal level to be driven into the OT.

Now bear in mind that in one respect, this works just like pulling two tubes - only at or near full volume. At anything much less, it's not really gonna affect anything, tone- or level-wise.

Wyzat?

Well, when you think about it, it starts to make sense. Consider: The power tubes are actually amplifying current, not just voltage. By reducing the drive (pulling half the tubes, or somehow cutting off the signal from them), you are actually lowering the current level of the output tubes, not the voltage level - after all, the power supply is still delivering the same voltage as before, but it's not be called upon to deliver as much current. Now, the current available, and being used, is still the same as before (meaning, with all 4 tubes working as normal), and the voltage is still up to snuff. So what happens at the speaker end of things?

Things remain the same - IOW, until the speakers demand more power, the apparent sound level remains about the same as before. Only when you demand more volume does the speaker respond by demanding more power, and since the voltage from the PS remains the same, then it comes down to more current being demanded. And now we see, and hopefully understand, if there's less "amplifying capability", then there's gonna be less current delivered to those demanding speakers..... and now you experience a difference in tonality and level.

That's it, really. That's the whole secret to why we try to get the tone at a lower level, and usually only partially succeed.

Those amps that truly deliver half-power at any level do it via other means, such as changing the bias level (as well as cutting off half the output tubes) to compensate for the fewer tubes doing work. There are other tricks too, but some of them are still proprietary and covered by patents or NSA's.

Still, it's a good learning experience, and sometimes things come together to trump what I just said. In spite of all the engineering denials ("But it can't do that!"), sometimes serendipity still jumps up and smacks us in the fratza, and that's why we remain Nutz, in the everlasting search for the that oh-so-elusive "Magic Mojo"! ;D

HTH


sumgai

[Teleblooz]

Well, I finally got around to documenting my mods in schematic form.


It basically amounts to:
1. removing input #2 jacks for mic & phono
2. wiring the output of input #1 into the now open half of the 6EU7 (V1B)
3. moving the Master Volume control to after the tone stack
4. removing the wiring supporting the remote control

In the amp's previous life with me, I had removed the 3-conductor speaker out jacks, replacing them with 2 standard 1/4" jacks in parallel. I refined that a little this time around, hard-wiring one of the jacks to the 8-ohm terminal, the other to the 4-ohm. The combo cab I built has an 8-ohm speaker. If I run at "half-power" with two of the 7868s removed I can just switch the speaker plug from the 8 jack to the 4. No harm, no foul.

I left the AUX input in place for now. This is actually two line-level inputs wired into a sort of "cross-fader" arrangement. I saw no reason to remove it, and it seems like it may be useful in some as-yet-undetermined fashion. Besides, this way I don't have one more gaping hole in my control panel.

Where I'd like to go in the future with this thing (assuming it doesn't consume itself in a big fireball) is:
1. wire in a standby switch
2. find a way to get some more top end out of it - maybe redo the tone stack somehow? A presence control maybe?

[Teleblooz]

Sumgai - thanks for the additional info! Reversing the gain pot and adding the brightness cap were a noticeable improvement. And now I finally have something for the extra hole in the control panel - the bright switch. Found a nice little black-handle toggle switch that looks great there.

[sumgai]

TB refers to the fact that I visited with him in person on Saturday the 1^st of May.

Nice guy, nice guitars, very nice little practice room..... all around, I had a good time. Jammed for awhile on some oldies we both know, and I poured over his Bogen amp and the schematics thereto. I suggested a few changes, and I'm sure they'll get posted here soon, as it would seem they made a positive improvement.

TB, you also need to post pics of the finished project! In fact, I just checked the Gallery, and no one has yet posted an amp in there - go pluck that cherry!


sumgai

[Teleblooz]

Here's the amended schematic. As I said, I reversed the Input 1 volume pot, and added a switched 120pF cap to that portion of the circuit, creating a Brite switch. I've also drawn in the Standby switch, though I haven't gotten around to wiring it in yet.

[Teleblooz]

Oh, almost forgot - here's a pic of the revamped control panel:

[Teleblooz]

OK, I finally got to play the beast in a "full band" setting. My drummer buddy's finally back from his government-sponsored vacation in Iraq, and he was tapped to fill in for the house drummer at a local "jam night", so he called me up and invited me to come down and play a couple of tunes.

Although both my playing and the amp's performance were serviceable it became apparent that I still need to work on controlling my dynamics much more tightly with this amp than with any of my others. For whatever reason it's VERY sensitive to the guitar's volume control. While I see this as a good thing, it does mean I have to tighten up my technique some.

Anyway, I need some advice from someone knowledgeable with these circuits. What can I change to take down the overall input gain level just a bit? If you look closely at the pic of my control panel above, there's a little silver star sticker next to the GAIN knob (at about 9 o'clock) to mark my setting. This is really as high as I can set it, practically speaking, when I play with my standard Telecaster. Between that and 12 o'clock is mondo distorted; anything above 12 o'clock is completely unusable. Below 8 o'clock the volume drops off precipitously. So you can see I have a rather narrow window of usability there. If I'm playing a guitar with humbuckers or something else that's a little higher output, it's pretty much always crunch time.

Can I wire in a resistor or something to take the overall gain spectrum down a notch?

[sumgai]

May 28, 2010 3:35:00 GMT -5 Teleblooz said:

Can I wire in a resistor or something to take the overall gain spectrum down a notch?

Yes.

Go back and look at the schematic. See that very first resistor, the one in front of the first tube? It's a 1MΩ jobbie, right? Simply insert a 100KΩ resistor between the first capacitor and this resistor. That should knock the signal strength down about 10%, yet leave the bias voltage set at the proper point so that the tube still works as designed.

If that's not enough, then double that to 220KΩ (the next standard value) or so. If that's not enough of a drop, then we should consider other options.

Keep us posted on what happens.

HTH



sumgai

[ashcatlt]

Won't putting that big of a resistance in series with passive pickups produce a a loud (tone-)sucking noise?

[sumgai]

ash,

The risk is there, admittedly, but in point of fact, we're doing nothing more than imitating a volume control - we settle on one setting, then fix it with permanent resistors. Yes, that probably does have a small impact on "tone", but consider..... what's the tone like now, with all that unwanted distortion?

Experience tells me that the trade-off will be for the better, not the worse.

There are other steps to try, should this simple method not work to TB's liking.




sumgai

[Teleblooz]

I just put in a 220kΩ, since I conveniently had one left over from when I did the original mods. While there is a little bit of a difference, it's not much. No noticeable negative effect on tone.

I'd be interested in hearing more about some of these other options.

[ashcatlt]

The thing with the tone assumes that you're connecting directly to passive pickups. An active stage between guitar and amp will change the equation altogether.

If the crunch is as bad as you say, it's going to take some serious attenuation. This can be accomplished with a bigger resistsor in place of the 220K. This is a simple voltage divider, and can be calculated easily. I'm pretty sure you're going to run into serious tone issues before you get to something with which you can live.

You're likely going to need to do something about the overall gain of the unit. First and easiest thing I can think of is to reverse the mod you implemented where the first preamp stage feeds into the second.


Edit - used the word "pretty" 4 times in 3 sentences!

[Teleblooz]

Thanks Ash. First off, there's a couple of givens:
1) I never use active pickups. It's just one more place where a battery can (and undoubtedly will) go dead at the most inopportune time. Fewer gewgaws = more reliability.
2) Reversing the preamp to pre-mod configuration isn't an option. When it was set up that way, the only thing the amp provided was volume. While this is desirable for a PA, in this context it's useless.

I can live with what it sounds like now, and for the most part I really like it, so this gain reduction isn't a "gotta have" so much as it's a "nice to have". I'll play around with resistor values now that I know where in the circuit they need to be inserted.

[Teleblooz]

Y'know, I've been doing a little thinking about Ash's suggestion about reversing the mod (so much for sticking to my principles). There's a way to wire that in as an option without actually reversing it. I could just use a simple SPDT switch to bypass the second gain stage, go straight from the output of V1A into V2. Except for the lack of a master volume pot at that location, that's how it was wired up pre-mod.

What would be the effect of moving the 220k resistor that we just added, from the input of V1A to the same spot at the input of V1B? My logic is that this resistor is there specifically to attenuate the signal going into the first stage. If I were to wire up the aforementioned switch (taking out the second stage), the input would be even wimpier than before. But if the resistor were located at the input to the second stage, I'd at least have the pre-mod signal level available.

Your thoughts?

[Teleblooz]

Well, I tried wiring up the hi/lo gain switch and ended up screwing something up. When I powered up I got no signal in the low-gain setting, and after it had been on for a couple of minutes, the high-gain setting started to sputter out, finally going mute altogether. Ruh-roh!

So, putting my brilliant analytical noggin to work for a change, I figured I'd try to isolate where the snafu happened. The first thing I did was - you guessed it - reverse all the mods I'd done on the input, and run through just the half of V1, as originally configured. To my everlasting surprise, it actually worked. I'm guessing therefore that whatever went south had to do with the second half of V1 (the input formerly known as #2).

Knowing better than to try pushing my luck twice in any given 24-hour period, I closed 'er up and decided to just play for awhile. And I noticed something strange. When I turned the gain knob to about the 12 o'clock position I suddenly realized I had overdrive - not the wimpy-arsed sound I had when the amp was still stock, but actual guitar-ampy tube overdrive! Not huge like when both sides of V1 were engaged, but respectable and, more importantly, controllable.

So right now my best guess is that it's because I moved the Master Volume from where it was, just after V1, to after the tone stack. Whatever it is, I like it.

I'm sure the gift horse's oral exam will commence soon, but for now I'm just going to enjoy it.

[Teleblooz]

In the interest of punctuation, I thought I'd post a pic of the Bogen in its final (external) form:

Thanks go out to my friend Mike who graciously allowed me to dig into his box of car chrome.

[sumgai]

Ford's bottom of the line, at the bottom right, and their top of the line at the upper..... +1'able right here! ;D

[Teleblooz]

I am the Alpha and the Omega.

[sumgai]

May 17, 2011 11:12:03 GMT -5 Teleblooz said:

I am the Alpha and the Omega.

Now that's downright weird.... your wife told me almost the exact same thing. IIRC, it was along the lines that your starting and ending were at about the same time....





<insert Chuck Barris gong sound here>

[Teleblooz]

<GROAN>

[bogenboy]

I'd love to build a chb100 like yours! I just finish a Bogen CHB10A I call "Cherry Bomb"

[bogenboy]

By carlbess2 at 2011-05-31

[newey]

bogenboy-

Hello and Welcome to G-Nutz2!

That's a pretty amp build, but we'll be needing a gut shot . . .

[Teleblooz]

I actually gigged with it for the first time last weekend!
I really like the sound of this thing and don't miss having reverb at all, surprisingly. It's very responsive to my playing and takes my pedals well.


And bogenboy - that's a FINE looking cabinet!

[popsaka]

...Mama said ''son, you're gonna drive me ta drinkin' if ya don't quit drivin' that hot rod Lincoln''

[popsaka]

...pix of the chassis, please