Bogen amp project on the way

[Teleblooz]

My next project just dropped into my lap. About 3-4 years ago, a buddy of mine and I were talking. His college-age son was starting up a band and was looking for advice on getting a cheap PA. I had an old Bogen Challenger sitting in storage and gave it to him to get started up.

About a year or so later, somebody mentioned (might have been on this forum) about a guy who converts old Bogen tube amps to guitar amps. I thought that was a really cool idea, so I asked my friend if his son was still using that amp. He told me the thing had broken, but he'd look and see if it was still around. It didn't turn up, best guess being that they'd tossed it out at some point.

Friday I ran into him out in the parking lot at work, and he said, "Guess what? I was cleaning out my garage a couple of days ago - you won't believe what I found. Do you still want that thing? The power cord looks like the dog chewed it off, but other than that it looks like it's all still there."

He's bringing it in for me next Thursday (he works the weekend shift). So now I get to educate myself on the ins & outs of tube amps. With luck, I may be able to get a functional tube head to add to my gear roster. I'm psyched!!!!

[newey]

A guy named Joe Jasniewski literally wrote the book on converting tube PA amps into guitar heads:

www.rru.com/~meo/Guitar/Amps/PA2Guitar/index.html

There's a ton of additional info out on the net, but this seems to be a good starting place.

[Teleblooz]

Mar 14, 2010 17:16:04 GMT -5 newey said:

A guy named Joe Jasniewski literally wrote the book on converting tube PA amps into guitar heads:

www.rru.com/~meo/Guitar/Amps/PA2Guitar/index.html

There's a ton of additional info out on the net, but this seems to be a good starting place.

As it happens, I found this same page while surfing around earlier today. Maybe it's not as obscure a project as I'd feared.

Whether my bench chops are up to it is another question altogether though...

[newey]

It's apparently a popular project. I see lots of already-converted tube PA amps like Bogens and DuKanes on Ebay.

[ashcatlt]

I haven't read the above link. It seems to be swimming in some viscous (and likely flammable) liquid this evening... I'll try it tomorrow...

Replace the power cable! Should be obvious, I suppose, since the dog ate it. Many of these old tube PAs have those 2-pronged devil tails. Sometimes it takes some fairly extensive mods to replace this with a proper safety ground. I hope (for your sake) that this'll be relatively easy. Either way, though, it's the first thing you do. Don't plug it into the wall till you're done.

Then, make sure the PS caps are empty of charge. There's a sticky at the top of this Forum with instructions.

There's two real differences between a PA and a guitar amp:

1) Input Impedance. Many guitar amps sit around 500KΩ (or, you know, like 470KΩ). Most pedals are at around 1MΩ. The PA input could easily be 1/100th of this. That means major tone suck.

A buffer fixes this issue. Just about any mass-market pedal (even in bypass mode) will do, as long as it doesn't say "True Bypass" on it. A DI box would probably work too. I'd probably use my Boss Hyper Fuzz in boost mode, but we'll get to that...

2) The frequency range of the speaker. ChrisK used to say that a typical guitar speaker had trouble reproducing anything much more than "a warm fart". Guitar speakers (and the cabs in which they live) are not designed to reproduce the higher frequencies. Plugging the thing into a properly rated (both W and Ω-wise) guitar cab fixes this.

Some corollaries:

a) There also may be some gain issues. Passive guitars are (generally) louder than microphones and quieter than line-level boxes. You'd ought to be able to get more than enough gain out of a mic input, but might need some kind of boost to get any crunch out of a line input. (Thus, the booster)

b) It's quite likely that the tone stack will be voiced sub-optimally for guitar. Many products set their "treble" EQ around 10KHz. No passive guitar puts out anything you want to hear in the 10KHz range. There's a whole other story involved w/guitar amp tone stacks.

c) While it may seem tempting to mod this amp into something which will stand on its own as a guitar amp, a buffer/booster and/or EQ will either be an object you have in your possession already, or be useful in other situations. That is, you might be better off buying or building your "mods" in their own boxes.

[sumgai]

I too have not yet read newey's found link, but I have to "note an exception" to ash's statements just above.

I find that nearly all of these conversions that are done by pros, or by good techs that follow the instructions, take care of 95% of all the things ash mentions.... tone controls, buffering for input impedance, that kind of thing.

However, these mods are really meant for one thing - to get that tube-and-transformer output stage. Most often, but not universally, that means either 2 or 4 6L6's, plus a transformer that's either too small, and thus distorts, or too large, meaning you can use it for a door-stop at the Fort Knox vault. Either way, one might get exciting tone, or one might not. But transformers, while not cheap, can be swapped out. And that's still a far cry from building one's own tube-style power output stage for a home-built amp.

TB, I say go for it, but first make sure you have the correct schematic on hand - it wouldn't do to find a part in there that was replaced by some yo-yo earlier in its life, only to find out that when you're finished with your mods, this bogus part suddenly takes everything out to lunch. Before you start your modding adventures, first make sure the amp comports with the schematic, at least for the stuff you're leaving in.

That's the best I can offer you at this time. But don't hesitate to ask, if something doesn't appear to be copacetic.

HTH




sumgai

[Teleblooz]

Thanks, guys. I've done a little exploring and have found schematics in abundance, but that won't do me any good until I get the thing back and get the model number.

I haven't seen the amp in years, so the details are more than a little fuzzy in my memory. I do remember changing out the 3-prong mic input and the screw-terminal speaker out for 1/4" jacks, but other than that I believe it to be dead stock.

[Teleblooz]

Got it today. It's a CHB-100, which means 100 Watts(!), On first blush, everything (except the power cord of course) looks intact, like it came out of my basement yesterday.


It's got a couple years of garage grunge on it, but the insides look pristine, and there's no visual or olfactory evidence of any toasted components (a scent never forgotten, once experienced).


As Ash guessed, it has a 2-prong "Devil Tail", which I'd naturally want to replace with a grounded cable.

I've downloaded a schematic, which I'll compare to the actual components forthwith, and a manual, which tells me very little beyond "turn the volume knob clockwise to make it louder". Duh.

It has 2 high-impedance mic inputs, one of which can be switched to a mag phono input, and two line-level AUX inputs, controlled by a cross-fader pot. In its earlier time with me, I got rid of the screw-type coax mic connectors and wired in 1/4" jacks. There's a "Tape/Booster" out, presumably a preamp out to drive a slave amp. There's also a jack labeled "WMT-1 HI-Z". Not sure about this one, but I don't think I really care at this point.

The speaker connections are screw terminals for 4, 8, and 16 ohms. When I had it before, I'd removed the 3-prong speaker out jacks and replaced them with 1/4" jacks, hard-wired to the 8-ohm lug.

In the research I've been doing I've come across a couple of references to pulling out two of the four 7868 power tubes to effectively cut the amp's power in half. Is that all there is to it, or does it involve some sort of wiring mod? And if so, which tubes? Seems to me like it would make a difference, but what do I know?

[sumgai]

TB,

Give us a link to the correct schematic for you unit, and I'll be better able to answer your questions. But without further knowledge, I can say that in general, pulling two power tubes will cut the output power, probably a bit more than half. But I need to see the schematic to be sure of that.

(Not to mention, it can't be just any two tubes, there's a balance to be maintained, so it has to be the correct two tubes. Thus again, the documentation is needed.)

HTH




sumgai

[Teleblooz]

Here's the schematic:
www.schematicheaven.com/hifiamps/bogen_chb100.pdf

And your comment about cutting the power by more than half is borne out by what I've found out about the CHB-35. It appears this is essentially the same amp as the CHB-100, but with two 7868's instead of four.

For what it's worth, here's the manual as well:
www.bogen.com/support/discontinued/pdfs/CHB100m.PDF

[Teleblooz]

OK, I couldn't help myself. After I pulled all the tubes, installed the new power cord & cleaned everything up...


... I just HAD to plug the thing in. To my delighted surprise, it worked!


I let it warm up for about 20 minutes and plugged in my guitar. I dimed the input gain and slowly brought up the master volume to see how it would react. A little weak on the top end, although that might be because I was plugged into a 1x15 cab. Decent overdrive, very responsive to the guitar's volume control. Having no reverb felt really odd though. But I can work with this.

Finally I just said "what the hey" and dimed the master too. This thing is LOUD! Nice satisfying Jimi Hendrix feedback, and the tone overall is definitely workable.

Of course, right about that time the wife came running out to the garage to tell me to turn it down - grandbaby napping & all - I understand. Tomorrow is another day.

If I can pull two tubes to get the power to a manageable level, I could gig with this beast just the way it sits.

[sumgai]

tb,

OK, I've seen what I need to see! ;D

Looking at the schematic reveals a very strange item, the resistor between the output transformer and the plate (anode) of two of the four tubes. IOW, one pair gets full voltage, the other pair gets a slightly reduced voltage. How they can claim that this will keep things balanced is beyond me, but then again, I wasn't consulted during the design phase, so I'm outta the loop here.

Anyway, you'll want to pull either the pair to the left or to the right, not the pair facing to the front or to the back. Pulling the left pair should give you slightly more output power than pulling the right pair.

Now, bear in mind that lots of things are all in cahoots with each other in there. For that reason, you are now upsetting the output transformer's ability to... err..... transform. In short, pulling two tubes increases the load seen by the transformer on the primary side, and thus the same will happen on the secondary side (the actual output) - it will go up too. If you were using the 8Ω output (presumably with an 8Ω cab), you'll need to switch to the 4Ω output in order to get the same output (both power and tone). Not making that switch probably won't harm anything, but you'll likely notice a difference, and it'll likely not be a difference you'll appreciate.

Why did I say it goes up, and yet tell you to go down? Think - the actual ohmage has gone up, which means that the labels have moved in the opposite direction. To maintain the proper ohmage for your speaker(s), you need to go to the connection that now has what you want. (In my example, that was 8Ω, which is now found at the 4Ω tap.)

But you are free to experiment with whichever tap you might wish to try. I think you'll find that what I've just said works and sounds best, but go ahead and try the others - you just might like one of them better.

HTH



sumgai

[Teleblooz]

Thanks, Sumgai.

So if I'm getting your point correctly, I can pull out either V5/V7 or V4/V6...

... and pulling V4/V6 (on the right in the above pic) will give me a little more output power. Then I connect my 8Ω speaker load to the 4Ω connection.

The question occurs to me - can I make this (realistically) switchable? In other words, rather than pull the two tubes physically out of the chassis, can I just switch them off, thereby getting a "convertible"?

Finally an observation: When I was playing it, I noticed that the tubes closest to the transformer (V4/V5) glowed noticeably brighter than the other pair, and that the light level changed with the amplitude of the signal. Would this be the result of the voltage disparity caused by the resistor you mentioned (is that the 3.3KΩ on the schematic?)?

[sumgai]

tb,

Mar 21, 2010 23:31:20 GMT -5 Teleblooz said:

So if I'm getting your point correctly, I can pull out either V5/V7 or V4/V6...

Yes, you've identified the correct pair numberings. (Note that the schematic shows no such numbers!)

The question occurs to me - can I make this (realistically) switchable? In other words, rather than pull the two tubes physically out of the chassis, can I just switch them off, thereby getting a "convertible"?

Yes, it can be done, but there's a bit more to it than just cutting off the B+ (the high voltage). Better to simply cut the signal coming in on the grid(s) of one pair, and to let the tubes idle with no signal. This way, the tubes' reflected impedance is still seen by the output transformer, and thus the output jack (nee strip) retains the ohms ratings as labeled.

This trick might introduce a bit of distortion, due to loading of the grids that are in use, but that should be controllable.

Finally an observation: When I was playing it, I noticed that the tubes closest to the transformer (V4/V5) glowed noticeably brighter than the other pair, and that the light level changed with the amplitude of the signal. Would this be the result of the voltage disparity caused by the resistor you mentioned (is that the 3.3KΩ on the schematic?)?

Yes, it's a possibility. I'll bet that the glow you see changing with the input signal is tinged with blue or even purple, am I right? That's a bad thing, it means that the vacuum is slowly depleting, and that air/gas is leaking in. Now, there are ways to "make" it show up, and one of them is to get the tube to operate in a non-linear fashion. Since only one pair is doing this, the first thing I'd do is swap them for the other pair - does the glow move too (stay with the tubes) or does it remain in the V4/V5 positions? If it moves, the tubes are just about at life's end - better start hunting up some replacements.

BTW, the "magic" resistor to which I referred is only 15Ω, and sits between the two tubes running in parallel with the chassis front panel. They have a red sleeve on the right side (as shown in your photos above) and are marked brown-green-black-silver. Doesn't seem like much, but at these voltages.......

Oh, and while I'm at it - that 3.3KΩ resistor, and the associated cap, is a snubber circuit. I think the primary purpose here is to absorb transients caused by inductive leakage coming off the transformer's primary. If left unabated, such spikes could easily damage any connected speaker(s). It's been a long time since I've seen this "device", it's not been a popular item since about the late 1940's.


HTH!




sumgai

[Teleblooz]

Mar 22, 2010 3:10:56 GMT -5 sumgai said:

Better to simply cut the signal coming in on the grid(s) of one pair, and to let the tubes idle with no signal. This way, the tubes' reflected impedance is still seen by the output transformer, and thus the output jack (nee strip) retains the ohms ratings as labeled.

This sounds like a piece o' cake. I'll likely also wire in a rotary switch for impedance selection.

Mar 22, 2010 3:10:56 GMT -5 sumgai said:

I'll bet that the glow you see changing with the input signal is tinged with blue or even purple, am I right? That's a bad thing, it means that the vacuum is slowly depleting, and that air/gas is leaking in.

One of them gives off a momentary bluish flash when the first high-amplitude signal goes though it, then behaves normally after that. I expect that one's on its last legs.

Mar 22, 2010 3:10:56 GMT -5 sumgai said:

Since only one pair is doing this, the first thing I'd do is swap them for the other pair - does the glow move too (stay with the tubes) or does it remain in the V4/V5 positions? If it moves, the tubes are just about at life's end - better start hunting up some replacements.

I swapped around, and the V4/5 positions are brighter regardless of which tubes are being used where. I actually kinda dig the "color organ" effect.

And I don't know whether my ears are experiencing "brain limiting", but the amp doesn't appear to be appreciably quieter with either pair removed.

Another idea that sounds intriguing is using the second half of V1 as an additional gain stage. This might help me get a more pleasing overdrive sound without having to max the output tubes. There's a guy who did it on a CHB-35A, but he changed the 6EU7 out for a second 12AX7. Would something along these lines work here?

vwtweaked.ca/images/Bogen%20BassFlex%2008feb10.jpg

[ashcatlt]

I'm not too surprised that it's not much quieter. It takes approx. 10 times the power to affect a doubling of the percieved volume.

Have you tried running in parallel into both mic ins? With 1M input-Z on each, you're probably safe using a passive split (y-cable).

[sumgai]

Mar 22, 2010 3:10:56 GMT -5 sumgai said:

Since only one pair is doing this, the first thing I'd do is swap them for the other pair - does the glow move too (stay with the tubes) or does it remain in the V4/V5 positions? If it moves, the tubes are just about at life's end - better start hunting up some replacements.

Mar 23, 2010 0:32:42 GMT -5 Teleblooz said:

One of the tubes gives off a momentary bluish flash when the first high-amplitude signal goes though it, then behaves normally after that. I expect that one's on its last legs.

I swapped around, and the V4/5 positions are brighter regardless of which tubes are being used where. I actually kinda dig the "color organ" effect.

Well, if the blue shows up only momentarily, what colors are you seeing as it puts on the "color show"? And in what way are they "brighter"?

And I don't know whether my ears are experiencing "brain limiting", but the amp doesn't appear to be appreciably quieter with either pair removed.

Completely normal and common reaction. You've just been introduced to the MarketingSpeak class of how to bamboozle the public with numbers. I'll explain....

It takes "X" amount of watts to push a speaker to a given level of "Y"dB. This is extremely dependent on the speaker's efficiency factor. What that boils down to is that just because you double the output power (to the speaker), you don't necessarily get twice the output sound level (in dB). The take-away here is, speaker efficiency is not linear. Remember that, and life will get much simpler.

The same thing goes for the reverse direction. Cutting down the power might not reduce the output level by the same amount, as you may be operating at (or near) the speaker's point of maximum efficiency.

Suffice it to say, the rules are a slippery slope, and the road is always murky. If someone hands you a formula and claims it will answer all your questions, don't believe them.

Another idea that sounds intriguing is using the second half of V1 as an additional gain stage. This might help me get a more pleasing overdrive sound without having to max the output tubes. There's a guy who did it on a CHB-35A, but he changed the 6EU7 out for a second 12AX7. Would something along these lines work here?

Well, yes, in theory, it should work. However....

First, by changing to the 12AX7, the "inherent tone" of the 6EU is discarded. Along with whatever drive characteristics that may have contributed to the tone, among other things.

Not to mention, you're sacrificing the remaining inputs that will no longer be usable. So long as you're comfortable with that idea, then there's no reason not to do this particular mod. But I think I'd add a second gain control, just in case..... you understand. Your linked circuit leaves the gain wide open, and that's just begging for either blown components, speakers or ears.

HTH



sumgai

[Teleblooz]

Mar 23, 2010 12:22:16 GMT -5 ashcatlt said:

I'm not too surprised that it's not much quieter. It takes approx. 10 times the power to affect a doubling of the percieved volume.

Yeah, I figured it was something like that, but I did expect to hear some difference. I think maybe Sumgai's comment on speaker efficiency is the crux of the biscuit here.

Have you tried running in parallel into both mic ins? With 1M input-Z on each, you're probably safe using a passive split (y-cable).

My theory a little fuzzy, but wouldn't this result in proportionally less input signal into each? I'd rather overdrive the inputs than "underdrive" them.

[Teleblooz]

Mar 23, 2010 15:58:50 GMT -5 sumgai said:

Well, if the blue shows up only momentarily, what colors are you seeing as it puts on the "color show"? And in what way are they "brighter"?

I think maybe my "color organ" comment was a little misleading. There aren't any other colors per se, just that comforting tube-filament orange. And "brighter" is exactly that - the light emitted gets more intense.

Another idea that sounds intriguing is using the second half of V1 as an additional gain stage. This might help me get a more pleasing overdrive sound without having to max the output tubes. There's a guy who did it on a CHB-35A, but he changed the 6EU7 out for a second 12AX7. Would something along these lines work here?

Well, yes, in theory, it should work. However....

First, by changing to the 12AX7, the "inherent tone" of the 6EU is discarded. Along with whatever drive characteristics that may have contributed to the tone, among other things.

Not to mention, you're sacrificing the remaining inputs that will no longer be usable. So long as you're comfortable with that idea, then there's no reason not to do this particular mod. But I think I'd add a second gain control, just in case..... you understand. Your linked circuit leaves the gain wide open, and that's just begging for either blown components, speakers or ears.

Losing input #2 would be no loss to me at all. Is there a way to do something similar using the 6EU7 tube? Are the 6EU's similar enough in characteristics to the 12AX7 to wire up in the same manner?

[ashcatlt]

Mar 23, 2010 17:52:43 GMT -5 Teleblooz said:

Have you tried running in parallel into both mic ins? With 1M input-Z on each, you're probably safe using a passive split (y-cable).

My theory a little fuzzy, but wouldn't this result in proportionally less input signal into each? I'd rather overdrive the inputs than "underdrive" them.

My theory a little fuzzy, too, but I don't think it really works that way. Sure does seem like it should be a voltage divider there... Maybe SG or one of our other EEs can shed some light.

Anyway, I think the added gain from the second tube overcomes this. The point is to punish the tubes in later stages a little bit more by hitting them with the output of both preamps. I have done this on a number of different two-channel tube guitar amps in the past. It works quite effectively to produce extra crunch in those models which don't invert the polarity on one of the channels.

Maybe we'll need to design a Series/Parallel switching arrangement for your preamps.

[sumgai]

tb,

Three things:


One, the "doubling the volume requires 10 times the power" bit is ripe for investigation. However, I'd rather not get into it just now, for various reasons. And the bit about speaker efficiency, while not a red herring, is also debatable amongst the engineering set. My mind is pretty much made up on that one, but I'm having the dickens of a time trying to get everyone else on board!


Two, splitting the signal into parallel inputs....

There were actually a couple of points/questions here, so I'll just hit the highlights. Dividing the incoming signal along parallel paths has interesting results. For one thing, each path's active device (tube or transistor) will probably remain "clean", i.e. little or no distortion, comparatively speaking. But combining these clean signals for the following stage can certainly be enough to "punish" the input of that next stage, as noted by ash.

The chances of blowing anything up while experimenting with this idea are pretty slim.


Three, tube conversions.

The 6EU7 works the same as a 12AX7, but the two tubes are not pin compatible. The big difference is that the 6EU7 shares the heater across both halves, whereas the 12AX7 has separate heaters for each half.

According to legend and lore, 6EU7's are cheaper than the 12AX7's, but the trade off is, there are no direct substitutes - if you want to swap differing tube types to get a sense of what kinds of tone you might get, you can't do this with the 6EU7. The 12AX7 has, of course, a whole family of types that are claimed to all sound differently when inserted into the same socket (and then tested, presumably under the same conditions).


Recommendations:

I'd stick with what you've got, for now. If you want to experiment with increasing the gain with an extra stage, use your linked schematic to the CHB-35, but use the pin numbers for the 6EU7 as suppled on the CHB-100's diagram. Once you've got that working, if you either don't like the tone, or are a glutton for punishment, then you can go ahead and change the socket over to accept a 12AX7. All the parts values should be the same, you shouldn't need to monkey around in that regard.

For more info, google for "6EU7", and you'll get an eyeful!

HTH




sumgai

[Teleblooz]

Mar 24, 2010 0:08:15 GMT -5 sumgai said:

Recommendations:

I'd stick with what you've got, for now. If you want to experiment with increasing the gain with an extra stage, use your linked schematic to the CHB-35, but use the pin numbers for the 6EU7 as supplied on the CHB-100's diagram. Once you've got that working, if you either don't like the tone, or are a glutton for punishment, then you can go ahead and change the socket over to accept a 12AX7. All the parts values should be the same, you shouldn't need to monkey around in that regard.

I was kinda hoping you'd say that. If I can, I'd rather stick with what's already inside the beastie rather than introduce more variables.

OK, next question - can I use the preamp out to drive a reverb tank and run it back into one of the line-level AUX inputs? Or, less desirably, an effects loop?

Let me say that I really appreciate the opportunity to get this kind of feedback from you guys - thanks muchly! My engineer friends at work may be wizards in the digital realm, but scratch their heads at this primordial technology.

[sumgai]

tb,

Depends. Are you talking about a Fender Reverb Unit, or a just a plain tank of springs?

Former case - yes, that should work fine.

Latter case - no, not even close to possible. See this schematic for a glimpse of what has to happen around a spring tank in order to make Dick Dale smile:

Fender Reverb Unit

HTH



sumgai

[Teleblooz]

Mar 24, 2010 17:26:58 GMT -5 sumgai said:

tb,

Depends. Are you talking about a Fender Reverb Unit, or a just a plain tank of springs?

Former case - yes, that should work fine.

So I'm getting from this that I could, given a complete effects unit, use this configuration as an effects loop? Say if I picked up a stomp box reverb it would function correctly here?

[sumgai]

tb,

I didn't see an actual effects loop on that amp. Where are you planning on connecting the in's and out's to the looped effect?

But yes, any ordinary stompbox should be usable in a standard effects loop, even if it's normally seen in the guitar -> stompbox -> amp configuration. The big difference would most likely be, the amp's pre-amp stage buffers the stompboxes. In this way, the guitar's tone makes it all the way to the amp without harm (unless you're using a crappy cable ).

Usually a good idea, but it does entail some extra cabling.

HTH



sumgai

[Teleblooz]

Tabling the FX loop question for the time being.

Well, I started modding last night. Unfortunately, it seems the mod schematic was seemingly not the final story on this particular mod, and I've had to do some re-thinking. When I first plugged it in I got no signal from the input to the power amp at all.

One thing that's been gratifying about this process has been that what theory I used to know has been coming back to me from the long-unused dusty corners of my brain. When I first started collecting these schematics I had a little moment of panic - it seemed overwhelming. But a couple of nights ago I just sat down with the things and mentally tracing the signal path, and now it's making sense to me again. I understand why the guy ran the path where he did and I also think I've got a good handle on what he left out. Today I ran my half-baked ideas past one of my engineer friends who has some experience with tube circuits, and much to my amazement he agreed with my diagnosis and proposed remedy. Tonight I'll try it out. The first part involves reconstructing some circuitry I had initially pulled out. It also seems that the Master Volume, where it sits in the circuit, is essentially useless in this new context. After consulting my engineer buddy, I think I'm going to reposition it after the tone stack rather than before.

I've already repositioned the input jack, and the power tube switch and standby switch will be easy. Maybe I can wire in an enable/disable switch for that gain section and mount it in the space where the Input #2 volume pot was.

As to the cabinet, I just pulled the trigger on some Tolex, piping, and grille cloth. Tomorrow it's off to Home Depot for a sheet of 5/8" plywood. I was thinking about what to use for the vent grilles, and I noticed in my garage a section of wire-mesh rain gutter covering left over from doing my gutters last Fall. It's bent in an arc, but I think I can flatten it out enough to screw it down inside the vent holes (I love re-purposing random junk in my garage). It occurs to me that building the cabinet is the only place in this project where I'm actually spending any money! Well, maybe a switch or two...

[Teleblooz]

IT WORKS!!!!

Last night after work I did the "reconstructive surgery" to install the circuitry the other guy left out on his mod. Plugged it in this morning and the amp just screams (in a good way). Now that I'm back to functional I can start working on some of the other items - moving the master volume, installing the switch to cut the signal to the last 2 power tubes, and putting in a standby switch. I'll have to drill out the rivets holding the MAG/MIC selector slide switch. Unfortunately it's SPDT where I need DPDT to cut both the signal lines, but I do have a drawer full of DPDT slide switches that should drop in there quite nicely.

Once I get this amp to a more-or-less "finished" state (assuming nothing blows up in the meantime) I'll get in there and draw a schematic of the changed circuit & post it for the edification of posterity, but right now it's just a bunch of scribbling on the old Bogen schematic.

[Teleblooz]

I started to get "bench fatigue", so put the guts aside for a while in favor of the wooden bits. The box is starting to take shape:


I've been notified that the Tolex, grille cloth, and hardware are on their way to me.

[Teleblooz]

Update: The cab is almost done, and it looks GOOD!

In the process of fitting everything together I managed to break one of the 7868 tubes. Clumsy me. This of course removes any question of running at 100W at this point. No biggie.

[Teleblooz]

Ladies and Gentlemen...

The Bogen.