converting PA head to 1/4' guitar input - diyAudio
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Old 5th March 2006, 11:56 PM   #1
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Default converting PA head to 1/4' guitar input

I have a old Bogen K15 PA amp that I've partially converted for use as a guitar amp. I have a K10 that sounds awesome. I bought that was already done, and thought I could copy the mod. (It didn't work) I've installed the jack, used one side of the mic input, and have WAY too much level, and it's quite dark sounding. If anyone can tell me what I should do to convert this correctly, please help.
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Old 6th March 2006, 12:24 AM   #2
Optical is offline Optical  New Zealand
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Im not familiar with those amps, but the dark sound will most likely be caused by having the wrong input impeadance, a guitar needs ~1Mohm - a mic is much much less.

The gain can be reduced depending on the mic preamp.

if you can find a schematic it would help greatly.
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Old 6th March 2006, 04:19 PM   #3
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Thank you for the quick response to my posting. I have a schematic, but have recently moved and can't locate it. Are there certain (standard?) steps to follow when doing this? Would a couple of pics be of amy use?

Thanks.
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Old 6th March 2006, 09:19 PM   #4
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Does your amp have RCA jack inputs? Specifically, a mag-phono input? I recently got a Bogen M60A that I'm going to be doing a full conversion with, I was playing it through the amp simply by rigging an adapter to plug into the mag-phono input. If your amp has RCA jacks, try splicing into one of those instead of the mic and see what happens.

I'm not familiar with the K15. I know what they look like and I'm pretty sure thay were made in the late 1950s but that's about it.
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Old 7th March 2006, 01:57 AM   #5
Optical is offline Optical  New Zealand
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Quote:
Originally posted by tonemonster
Thank you for the quick response to my posting. I have a schematic, but have recently moved and can't locate it. Are there certain (standard?) steps to follow when doing this? Would a couple of pics be of amy use?

Thanks.

ok, ive found a schematic online, it looks to be hosted personally so i'd download a copy asap. http://www.flickerdown.com/forsale/k-15.pdf

im not that good at redesigning valve gear, but lot's of people on this board are. someone will point you in the right direction..

Actually as has been suggested, the phono input bypasses the input stage without additional filtering, you could try using that as your input first. the gain might be ok, but the input impedence is a a little too low, so you might notice slight a loss of highs again..
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Old 7th March 2006, 09:24 AM   #6
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From that circuit it looks dead simple to do, just remove the mike transformer, and connect the jack to C4 or C5 (depending which channel you use).
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Old 7th March 2006, 03:20 PM   #7
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Wow, that's set up way different than my Bogen. My phono inputs actually feed into the first preamp stage, and I have "aux" RCA jacks that are set up similar to that.

I bet you'd get some more people's opinons if you posted this in the tube section on this forum.

-Darren
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Old 7th March 2006, 03:39 PM   #8
Brion55 is offline Brion55  United States
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Make your input like the attached schematic. Best to have a shorting jack. Also, you could make "bright" and normal channels like many guitar amps by adding the cap shown to one of the volume controls.
Attached Images
File Type: gif bogen mod.gif (3.3 KB, 240 views)
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Old 8th March 2006, 02:21 AM   #9
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Thanks for the info! I'll be sure to let you know how it works out.
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Old 30th June 2006, 01:25 PM   #10
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Smile Me 2

I just hooked up the K10 I bought on ebay and wow, lot'sa gain for just one preamp before the PI.

I am also looking for a schematic, and any hints on drilling out the rivets to replace tube sockets, etc.

I'm good with schematics, wiring, testing, troubleshooting, a little week on metal shop stuff.

Peace,
Rico
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