Bogen K15 Buzz/Hum

[canterbury]

Boy am I glad I found this place. I am totally new to Guitars and Tube Amps. I actually just got a guitar for Christmas. Recently though a friend of mine gave me this old Bogen K15 and I was inspired by his CHB100 to convert it too use as a guitar amp. Someone else had already removed one of the MIC jacks and installed a 1/4in jack so most of the work was done. When I got it it didnt have any power tubes so I first tested the amp with a pair of 6l6gcs that my friend had laying around, at the time I was only able to test the amp with the phono input and all was well. I purchased a pair of Sovtek 6l6wxt+ and installed them in the amp. The main problem is that there is a pretty bad buzz/hum from the Mic channels if anything is plugged into them. The buzz gets louder as the volume is turned up, but if the volume is off there is no buzz. Like I said before though the Phono channel still sounds and functions GREAT. Something I did notice is the the 12ax7 on the preamp stage doesnt glow like the one in the AF Amp stage. The top of the tube has an orange glow but the bottom does not, I switched the tubes but the issue didnt move with the tube, like i said, Im new so that my not be a big deal.

Id be really grateful if you guys could help out. I have attached the Schematics for the K15. Im not an electronics guru but I can stumble my way through a schematic and I can use a multimeter. I do have access to an oscilloscope if thats necessary (and you guys can tell me how to use it).

Edit: I forgot to mention if i short the 1/4in jack out then the buzz goes away.

[Alex R]

If you plug a lead into an amp it will hum. Touch the exposed tip and it will hum a lot more. Plug it into a guitar & the hum should go away - does it?

The amplification factor is so big that the jack tip turns into a kind of aerial and what you hear is mains hum induced from the air. Radio Moscow comes in some days. Touching the tip makes your whole body the aerial. Touch the screen of the jack and you earth yourself - the hum goes away. When you plug in the tip is connected to the amp ground via the pickup windings, so you just hear what gets induced in those.

[canterbury]

The hum stays the same when a guitar is plugged in.

[hasserl]

Check to see that the input transformers for the mic input are not still connected. It has what's called grid leak bias input stages. The input jack tip should connect directly to the cap C4 or C5 depending on which channel you plug into, bypassing the transformer.

[canterbury]

Im almost certain that its still connected to the transformer. So the tip should be going to the cap c4/c5, what about the barrel?

btw, thanks for your help so far.

[canterbury]

Ok, so I connected the tip on mic2 to c5 and that has alleviated a LOT of the buzz. There is still some buzz present but Im still not sure where the barrel should be connected.

[hasserl]

The barrel or sleeve should be grounded. If I can find a schematic with a grid leak bias input I'll post a link to it so you can see how it should look.

[hasserl]

Here, take a look at this schem: https://taweber.powweb.com/store/5c5_schem.jpg

That's a four input amp, just follow the design for two of the inputs, say J1 and J4. YOu don't need to change the value of the stock cap / resistor in your amp, you just need to wire everything up in the same way. The grounded jacks will ground out the unused input to reduce noise from that side while nothing is plugged in.

[canterbury]

Awesome. Everything works great. I still have a little but of AC hum but the loud buzzing is all gone. Thanks for all your help.

[hasserl]

You're welcome, and congratulations. That looks like a very cool amp.

[canterbury]

Thought Id post a pic of my work in progress. The cabinet is 3/4 birch ply, nothing fancy. Ive got some Ampeg style grill cloth and some wine taurus tolex to cover it in. The speaker is a 12" Jensen Mod (35w).