Bogen K15 - What are your thoughts?

[Ottbucket]

I am new to this forum and a little new to tubes.

I just picked-up a Bogen K15. And wanted to get your folks thoughts.

It's a cathode biased 6L6PP with 12AX7/6AV6 pre and a 12AX7 LTP. (Schematic attached)

This is only my second conversion and my first PP amp - so, I thought cathode biased was a good choice.

Here is my plan once I receive the amp:
1. Visually inspect for (a) anything burnt/leaking, (b) verify wiring versus schematic.
2. Bring up on variac/dim bulb and check voltages v. schematic.
3. Anything funky, replace/repair. Maybe recap anyway.
4. Gut the mic transformer stuff and install 1/4 input jack. Install a speaker out jack.
5. See what it sounds like. Maybe change some of the values around the preamp tubes to be more traditional (68k input, 1M grid, 100k plate, 1k5 cathode) if it sounds bad.

Here are some initial questions since I am fairly new to this:
1. 300-0-300 secondaries seem kind of wimpy for 6L6, and the 6AX5 is a bit of an oddball recto (isn't it ?). So, do you guys see anything weird here? Anything you would change from the start? Otherwise, I was planning to leave any power section mods til last (like maybe setting up for 6V6s instead)
2. Any suggestions on the tone stack? Not very familiar with 2 knob ones. Maybe it'll work great??? dunno.
3. I know a 6AV6 is half of a 12AX7. Any value in changing sockets and installing a 12A_7 to give me another half a triode to play with? Maybe can have two channels each with a 12AX7 before PI. Or, cascade one channel thru? Or, cathode follower if tone stack sucks. Or, maybe leave the 6AV6 in there and set it up as a concertina to experiment with the sound of a LTP versus concertina (obviously not at the same time!). Or... what?
4. Any value in ditching the lower voltage B+ taps and only using the 300V for the entire preamp? What is the benefit of the original design where each previous stage gets less B+?

Anyway, what do you folks think are the potentials for this amp???

Rob

[g1]

Just for starters, be aware that the power output is limited by the size of the power transformer. You said something about the voltage being "wimpy" for 6l6's, but it's probably because of a "wimpy" PT. Thus the little rectifier tube.
What is the amp rated for as far as power output? Will that amount of power be sufficient for you? If not you will need to find a bigger power transformer and do a lot more work than you were expecting.

[Jazz P Bass]

There is a ton of info on the web about using the K15 as a guitar amp.
Stock, it's a 15 watt amplifier.

So it's not that it's 'wimpy', it is what it is.

[Ottbucket]

Thanks g-one.

It's rated at 15 watts - but I'm guessing that is conservative.

I am not looking for anything very loud. Don't actually plan to gig it at all.

I do this basically to learn cause I enjoy it.

My first build was converting a 1959 Admiral phono amp into a quasi AX84 P1 with some added switches for variable cathode caps, different feedback. It was fun, but in the end I got a decent little amp that really screams.

What I want from this is to see what I can learn about the PI, the 6AV6 and the lower voltages.

The lower voltages don't really stress me. It should make the 6L6 break-up lower (Yes?) and 330V on the plates make it a god candidate for 6V6, also if it is too loud.

What I am looking for here is a Fender-y amp.

So, what are your thoughts on what can be done with this?

[ron vogel]

with 330v to the 6L6, you might want to go with 6V6 instead. it'll bring up voltages in the circuit, and get you more headroom. you can easily get 15 watts out of it still. I usually see the 6L6 pull down 30-40 volts over a 6V6 in pairs.

[loudthud]

All "6L6"s are not created equally. The schematic specifies 6L6GB, not the more common 6L6GC that became avaialble in the early 1960s. The GB has a max plate voltage of 360V and plate dissipation of 19W. The GC has a max plate voltage of 500V and a 30W plate dissipation. The 6AX5GT is an odd ball rectifier tube. It's ratings are close to a 5Y3 but it doesn't require the floating 5V winding for the heater. It can use the same 6.3V as the rest of the tubes. Note that the center tap of the 6.3V is connected to the 6L6GB cathodes and the preamp tube has a 4.7 Ohm resistor in series with it's heater.

[Ottbucket]

Thanks Loudthud,

I hadn't noticed the GB or the heater wiring. I get the GB v GC comment (still thinking I might eventually go 6V6 - but that will be MUCH down the road).

I don't quite grasp the filament wiring. I see all tubes take the "W" & "X" taps except the first preamp tube. The first tube takes the "Y" & "Z" with the 4.7R in series. What does that do other than drop voltage?

Frankly, I haven't examined the schemo in detail yet. Just checked it for tube compliment and making sure it is cathode biased before buying amp. I'll get intimately familiar with the schemo once I trace it to the amp when I get the amp.