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More gain in a BF DR

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4/6/2000 7:57 PM

More gain in a BF DR

Hello to ya all Ampagers!
I’m the (happy) owner of a BF DR. The DR is modified (yes, its not a museum piece…) and carries 2 Svetlana 6L6 biased to 36 mA (the PT seems to be able to handle this change OK). The filter section is changed to SR specs. This was done in order to increase voltage and headroom in the preamp section (I’m hope I’m right about this ;-)

;-)

. In order to reduce flatulence the CBC in the first gain stage (V2) was changed from 22 uF to 1 uF and in the second gain stage to 10 uF, with a very nice result. Then the coupling cap by the PI was changed from 0.001 uF to 0.01 uF. To get some more gain the NF resistor was changed from 820R to 47K (I think) and V1 was pulled out. The tone stack was changed to 0.022 and 0.047 uF. The speaker (Oxford) really sucked and I had two old CTS 10´ speakers from the SR that I wanted to try. I made a new baffle and installed the CTS pair. I was unable to wire the speakers to get the 8 ohms required and settled for 4 ohms instead. This mismatch really made a BIG tonal difference – it did “open up” the amp and I believe increased wattage as well.
When reading posts here on Ampage there has been some threads mentioning changing the value of the slope resistor and the cathode resistor (V1), and other resistors as well, in order to increase gain. I love the sound of this amp but I would like to get just a little bit more gain (maybe also some more presence). Any suggestions?

Bjorn

4/6/2000 9:43 PM
bill	pull the tremelo tube for more gain at lower volumes. bill

4/6/2000 10:51 PM
John Stokes	Bill, what do you mean? Pulling the tremolo tube in a DR will not effect the tone at all. Where did you get that idea? Those amps have the roach type tremolo, so the circuit loading is all in the trem level control circuit. You'd have to disconnect the 50k RA pot to get the boost. The tube just drives the neon bulb, nothing more.

4/6/2000 11:59 PM
nek	Easier still, with the tremolo switched off, turn the intensity up to 10. Not as much gain as the disconnect. Will give an idea how the mod will sound without opening up the box, first.

4/7/2000 3:41 AM

Nek, take a look at the way the trem intensity pot works in this amp and you'll see that, where the trem pot's wiper is set should have very little effect on the gain of the amp compared to lifting the pot from ground.
The max it can be, whether the wiper is dimed or not, is still 50K and it's the 50k that's shunting the signal to ground, unless the LDR is shunting the rest off through the wiper.
Maybe I've always had it backwards and the LDR is low resistance when the oscillator is off.
I don't know what you are confusing this pot with, if anything, but can you explain what you ment or maybe what you've experienced?
Maybe you had an amp with a bad pot?

Bruce

4/10/2000 6:45 AM
nek	Bruce, I took a look at the schematics and I think you got it right. Something's probably broke in my amp. Symptom is the amp gets louder if the tremolo is switched off, when the intensity knob is turned up. Its a complete mystery to me.

4/7/2000 11:00 PM
bill	ooops! i will now remove foot from mouth, or try to clarify difference between ass and elbow. my suggestion was not based on any knowledge of my own (obviously) but on suggestions re BF amps in weber's first book. i'll have to check there to see what i meant. bill (red in the face)

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