50's Bogen DB10 need to reduce gain -Negative feedback? - diyAudio
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Old 21st November 2012, 10:51 PM   #1
Bob_F is offline Bob_F  United States
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Default 50's Bogen DB10 need to reduce gain -Negative feedback?

Hi, I'm converting this DB10 to a Guitar/Harp amp. I've re-Caped it and it sounds great!
The help I need is when the guitar is connected to the second stage 6SC7 it not quite loud enough. When connected through both 6SC7 its way way to much gain. I need to calm it down a bit. I am an EE .... Computers, Transistors...just not knowledgeable enough with tubes. I have a few circuit diagrams of feed back for tubes but it shows it connected to the cathode which is connected to ground.
Thanks
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Old 21st November 2012, 11:11 PM   #2
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If you decrease value of R29 (feedback resistor) you may not like the sound when overdriven, also probably you will need to deal with compensation (you are EE so you know what happens when you increase feedback). It's a little bit trickier than with SS because here feedback goes through an output transformer.

Also you may decrease value of grid stopper of the 2'nd preamp triode, and add resistor in cathode for bias.

Also you can increase value of R4 and connect your guitar to the "Hi output mag" input.
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Old 21st November 2012, 11:20 PM   #3
Bob_F is offline Bob_F  United States
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I was thinking that I need to control the gain of the first input stage possibly ruining the output of the V2 back to the grid of V1?
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Old 21st November 2012, 11:31 PM   #4
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Moved.. Per forum rules all threads about musical instrument amplifiers belong in Instruments and Amps.
See quoted forum subheaders below..

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Tubes / Valves All about our sweet vacuum tubes Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum
And

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Instruments and Amps Everything that makes music, Especially including instrument amps.
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Old 21st November 2012, 11:35 PM   #5
kevinkr is offline kevinkr  United States
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Try the "amp" input first - this may be all the gain you need.

The first and second stage as designed are not appropriate for MI use, take a look at the input stages of typical Fender, Marshall and Vox amps for ideas on what to do here.
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Old 21st November 2012, 11:44 PM   #6
Bob_F is offline Bob_F  United States
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Ive found lots of old fenders with 6SC7's but that would reqire a total rebuild.
One stage to little
Two stages to much
Need to reduce overall gain of the two stages hopefully adding only a few resistors and maybe a cap or two.
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Old 22nd November 2012, 12:46 AM   #7
Bob_F is offline Bob_F  United States
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Feedback options?
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Old 22nd November 2012, 06:08 AM   #8
dgta is offline dgta  United States
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Every stage of that amp uses high mu tubes, so you can reduce the gain of ANY stage by simply going to a 6SN7. Of course you have to re-bias the stage appropriately.
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Old 22nd November 2012, 08:34 PM   #9
Bob_F is offline Bob_F  United States
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Default Problem Solved

I like this Idea as the amp stays original
Grove Tube adapter $70 is to expensive
Ebay (use a 12AX7 in a 6SC7 scocket)

I can use any of these:

gain of 100 12AX7 - ECC83, 7025, ECC803, E83CC, 6681
gain of 70 5751
gain of 60 12AT7 - ECC81, 6201, 6679
gain of 45 12AY7 - 6072
gain of 41 12AV7 - 5965
gain of 19 12AU7 - ECC82, 5963, 5814, 6189
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Old 22nd November 2012, 10:50 PM   #10
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Quote:
Originally Posted by dgta View Post
Every stage of that amp uses high mu tubes, so you can reduce the gain of ANY stage by simply going to a 6SN7. Of course you have to re-bias the stage appropriately.
That would be the easyist way out, here's cheep ones to try it out with. could problly just stick em in an try it.

6SN7GT - Triode, Dual, Medium MU, Russian | Antique Electronic Supply LLC

I haven't read the schematic but the 2 valves are real close to each other only the amplification factor is different will gives les mic feed back when playin harp too.
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