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Vintage threads from the first ten years

Looong post on DR mods

4/23/2000 9:25 PM
Bjorn	Looong post on DR mods After reading posts mainly by Bruce, Carl Z, Steve A and Jim S and after trying several mods (and several values on each mod!) this is how the –65 BF DR (AB 763) looks now. The changes were done one after the other, not all at once. I guess I “grew into” these changes over a period of two years. I’m primarily looking for a Freddie King sound, I love his tone (and BB’s and maybe also the early EC – but that was Marshall…). I’m mostly playing a Gibson with two humbuckers (PAF) and a –81 MIJ Strat (-62 RI) with replaced PU’s (Swedish RI -62) plus the Gorge L-Cable. The amp now has more volume and punch, more headroom, probably less distorsion (than 6V6) but a very nice 6L6 distorsion, much less farting with a nice low-end response and a very nice high end - without harshness or oscillation, probably more midrange as well. Some off the changes made on this amp were made on my SR as well. Although I can judge what tonal differences each mods does I do not pretend to have even a basic electronic understanding of what I have done . Mods: 1. The DR now carries 2 Svetlana 6L6, biased to approx. 37 mA (1 ohm method), instead of the 6V6 pair. 2. V1 was pulled out. 3. PI tube was changed to 12AX7 (12AT7 is nice too). 4. The filter section was changed to SR specs. 5. A 120 pF SM was added across the PI plates (V1) and Grid stoppers (6L6) were changed to 2K2. 6. The coupling cap by the PI was changed from 0.001 uF to 0.01 uF. 7. The NF resistor was changed from 820R to 1K2. 8. I kept the grounded 47R resistor (by the NF resistor) and bypassed it with a 1uF cap (with a push pull pot; Volume). 9. The tone stack was changed to 0.022 and 0.047 uF (middle, bass). I kept the treble cap (SM). 10. The speaker (Oxford) really sucked and two old CTS 10´ speakers from an SR were installed in a new baffle. I was unable to wire the speakers to get the 8 ohms required and settled for 4 ohms instead. 11. The cathodes were separated in the second gain stage in the preamp tubes. To reduce flatulence the CBC in the first gain stage (V2) was changed from 22 uF to 2.2 uF and in the second gain stage to 10 uF, with a very nice result. The coupling caps from the driver tube were changed from 0.1 to 0.047uF. The 220K grid leak resistors were replaced with 100K. 12. The Intensity pot was replaced with a 50K switch pot. 13. All caps are OD’s (715P) and SM’s and all electrolytes were replaced. 14. The middle cap (of three) in the vibrato LFO section was increased from 0.01 uF to 0.02 uF. Comments: 1-3. The tonal changes are obvious. The PT seems to be able to handle this change OK. I initially installed a preamp filament supply transformer (6.3 V, 2 A – I think!) because I thought the PT was running hot. However, this did not affect the heat problem. A small fan was installed instead to cool things down. 4. This was done in order to increase voltage and headroom in the preamp section (I hope that I’m right about this . 5. These prophylactic changes were done to in order reduce PO (I don’t have an O-scope). I also tried a 250 pF and a 500 pF but I believe these values stole some of the sparkles. 6. This little cap really affects the tone. I found 0.0047 to 0.01 uF to be the best value. 8. This ads a bit presence and highs. 9. Only a minor effect in tone, maybe a bit less Fender nasality. 10. This OT/speaker mismatch really made a BIG tonal difference – it did “open up” the amp and I believe increased wattage as well. 11. These changes cut the bass response and flatulence, when driving the amp hard, without affecting the amp tone. Very good mod. 12. This mod really increases gain and improves tone. 13. Changing the big non-electrolytes to OD’s really made a difference for the better. Exchanging the small ceramic ones to SM’s didn’t affect the sound very much (maybe a few more sparkles?). 14. This was done in order to slow down vibrato speed (3Hz to 1.5 Hz). Mods tried but not performed: I messed around with the slope resistor but found that the original value sounded the best. However, when playing a Strat maybe a decrease in the value sounds better. I also played with the little 10 pF cap, which is a very interesting cap. I think I’m not finished with this one… I reduced the value of the 68K resistors by the inputs by half and then removed them all together, but did not like the tone so I settled for stock values. The normal channel is still stock (but not for long… I think I will try some of Steve A’s mods). For the purists: All original parts were stored, no new holes were drilled. The amp is possible to restore to stock specs with original parts and tubes. The amp looks dead stock from the outside. Needles to say - I love this little amp! (Maybe now is the time to put soldering iron aside and learn how to play guitar…). Bjorn

4/23/2000 9:25 PM

Bjorn

Looong post on DR mods

After reading posts mainly by Bruce, Carl Z, Steve A and Jim S and after trying several mods (and several values on each mod!) this is how the –65 BF DR (AB 763) looks now. The changes were done one after the other, not all at once. I guess I “grew into” these changes over a period of two years. I’m primarily looking for a Freddie King sound, I love his tone (and BB’s and maybe also the early EC – but that was Marshall…). I’m mostly playing a Gibson with two humbuckers (PAF) and a –81 MIJ Strat (-62 RI) with replaced PU’s (Swedish RI -62) plus the Gorge L-Cable.
The amp now has more volume and punch, more headroom, probably less distorsion (than 6V6) but a very nice 6L6 distorsion, much less farting with a nice low-end response and a very nice high end - without harshness or oscillation, probably more midrange as well. Some off the changes made on this amp were made on my SR as well. Although I can judge what tonal differences each mods does I do not pretend to have even a basic electronic understanding of what I have done :-)

.

Mods:
1. The DR now carries 2 Svetlana 6L6, biased to approx. 37 mA (1 ohm method), instead of the 6V6 pair.
2. V1 was pulled out.
3. PI tube was changed to 12AX7 (12AT7 is nice too).
4. The filter section was changed to SR specs.
5. A 120 pF SM was added across the PI plates (V1) and Grid stoppers (6L6) were changed to 2K2.
6. The coupling cap by the PI was changed from 0.001 uF to 0.01 uF.
7. The NF resistor was changed from 820R to 1K2.
8. I kept the grounded 47R resistor (by the NF resistor) and bypassed it with a 1uF cap (with a push pull pot; Volume).
9. The tone stack was changed to 0.022 and 0.047 uF (middle, bass). I kept the treble cap (SM).
10. The speaker (Oxford) really sucked and two old CTS 10´ speakers from an SR were installed in a new baffle. I was unable to wire the speakers to get the 8 ohms required and settled for 4 ohms instead.
11. The cathodes were separated in the second gain stage in the preamp tubes. To reduce flatulence the CBC in the first gain stage (V2) was changed from 22 uF to 2.2 uF and in the second gain stage to 10 uF, with a very nice result. The coupling caps from the driver tube were changed from 0.1 to 0.047uF. The 220K grid leak resistors were replaced with 100K.
12. The Intensity pot was replaced with a 50K switch pot.
13. All caps are OD’s (715P) and SM’s and all electrolytes were replaced.
14. The middle cap (of three) in the vibrato LFO section was increased from 0.01 uF to 0.02 uF.

Comments:
1-3. The tonal changes are obvious. The PT seems to be able to handle this change OK. I initially installed a preamp filament supply transformer (6.3 V, 2 A – I think!) because I thought the PT was running hot. However, this did not affect the heat problem. A small fan was installed instead to cool things down.
4. This was done in order to increase voltage and headroom in the preamp section (I hope that I’m right about this ;-)

.
5. These prophylactic changes were done to in order reduce PO (I don’t have an O-scope). I also tried a 250 pF and a 500 pF but I believe these values stole some of the sparkles.
6. This little cap really affects the tone. I found 0.0047 to 0.01 uF to be the best value.
8. This ads a bit presence and highs.
9. Only a minor effect in tone, maybe a bit less Fender nasality.
10. This OT/speaker mismatch really made a BIG tonal difference – it did “open up” the amp and I believe increased wattage as well.
11. These changes cut the bass response and flatulence, when driving the amp hard, without affecting the amp tone. Very good mod.
12. This mod really increases gain and improves tone.
13. Changing the big non-electrolytes to OD’s really made a difference for the better. Exchanging the small ceramic ones to SM’s didn’t affect the sound very much (maybe a few more sparkles?).
14. This was done in order to slow down vibrato speed (3Hz to 1.5 Hz).

Mods tried but not performed:
I messed around with the slope resistor but found that the original value sounded the best. However, when playing a Strat maybe a decrease in the value sounds better.
I also played with the little 10 pF cap, which is a very interesting cap. I think I’m not finished with this one… I reduced the value of the 68K resistors by the inputs by half and then removed them all together, but did not like the tone so I settled for stock values. The normal channel is still stock (but not for long… I think I will try some of Steve A’s mods).

For the purists:
All original parts were stored, no new holes were drilled. The amp is possible to restore to stock specs with original parts and tubes. The amp looks dead stock from the outside. Needles to say - I love this little amp! (Maybe now is the time to put soldering iron aside and learn how to play guitar…).

Bjorn

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