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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

4/25/2000 8:58 AM

Rick Erickson

quote:
"C’mon, no replies? Even a: I hate what you have done to your DR would be OK."

Bjorn

Hey whatever works for you is o.k. with me. I personally took a much simpler approach to getting more headroom, better tone and no mushiness from my DR - simply replacing the speaker with a re-coned Altec 417-8H. I have had many players want to know exactly what I did to this amp (actually I have two, a '64 & a '68) and other than re-building them to stock specs this is the only mod. Strangely enough I repaired a very early Mesa Mk-I amp today which had an Altec 417-8H speaker in it. The amp sounded very warm to me which led me to believe the Altec wasn't a standard factory version. The cone looks factory but the speaker does not have the high-end harshness that most Altecs seem to have. I ran the Boogie into my Deluxe speaker and the Deluxe into the Boogie. Both speakers have nearly the same response. Apparently Boogie had the Altecs made for them without the extended high end that a stock 417 has. This is the first speaker other than the ones I've had reconed that sounded this good.
I have heard other recones that sound awful, lacking the punch & headroom that an Altec should have. They were probably done with the earlier non edge-wound voice coils from Waldom.
I use these amps on stage with an 8 piece r&b/rock combo. For most gigs I only use one but if the stage will allow it I'll run them both. I sold my SR's after putting this setup together. They were collecting dust. I would love to try a pair of these in a Twin Reverb.
Actually I just saw your post now, I wasn't holding out or anything. Thought you might be interested in a different approach though.
RE

4/25/2000 12:06 PM
anonymous	Rick, thanks replying. I too think that the speaker replacement was the mod that made the biggest difference. (However at 4 ohms instead of 8). I will try get hold of a good 12” speaker. The mods performed otherwise was done in order to reduce the bass response and to ad a bit gain. I do prefer the sound of the 6L6 before the 6V6 though. Bjorn

4/25/2000 3:36 PM
Eric H	I have heard other recones that sound awful, lacking the punch & headroom that an Altec should have. They were probably done with the earlier non edge-wound voice coils from Waldom. Rick, Where do you have your reconing done? I'm in San Diego (CA). I have a 418 (15") that I bought re-coned in around 1971 --It has the same upper harshness you describe. I put a bandaid on it by covering the aluminum dust-cap with cloth (thanks, Doc, or Carl --sorry, can't remember) but I'd be interested in a re-cone. I'm convinced there's a transcendent tone lurking in that speaker (maybe I just love the way it looks!). Thanks, -Eric

4/25/2000 3:36 PM

Eric H

I have heard other recones that sound awful, lacking the punch & headroom that an Altec should have. They were probably done with the earlier non edge-wound voice coils from Waldom.

Rick,
Where do you have your reconing done? I'm in San Diego (CA). I have a 418 (15") that I bought re-coned in around 1971 --It has the same upper harshness you describe. I put a bandaid on it by covering the aluminum dust-cap with cloth (thanks, Doc, or Carl --sorry, can't remember) but I'd be interested in a re-cone. I'm convinced there's a transcendent tone lurking in that speaker (maybe I just love the way it looks!).

Thanks,

-Eric

4/25/2000 4:42 PM
Jethro Tull Bodine	Hate to barge in.. but BOY HOWDY!!!All you fellers in the San Diego area must be underground!!Maybe we could get together now and again and have our own Homebrew Jam Fest..swap ideas and recipes so to speak..Love to hear from other SD area brewmeisters!! Jethro

4/25/2000 9:38 AM
Giacomo	Re: Looong post on DR mods I will ask Santa Claus an AB763 next Xmas! It's one of the best amp and I shouldn't modify it heavily. Regarding the mods you did I am not skilled enough to comment but suggest another mod that IMHO gives great control over the tone: add a mid pot. Perhaps it does the work of many cap replacings. I made it without drilling holes putting mini-pots in place of input #2. gt

4/25/2000 9:38 AM

Giacomo

Re: Looong post on DR mods

I will ask Santa Claus an AB763 next Xmas!
It's one of the best amp and I shouldn't modify it heavily.
Regarding the mods you did I am not skilled enough to comment but suggest another mod that IMHO gives great control over the tone: add a mid pot.
Perhaps it does the work of many cap replacings. I made it without drilling holes putting mini-pots in place of input #2.

gt

4/25/2000 7:08 PM
Ray Ivers	Inductor-based Feedback Circuit Mods Bjorn, I'm answering this here in case anyone else might benefit from this stuff. If you connect a cap in series with an inductor across the 47 ohm feedback divider resistor in your BFDR, you will get a mid boost without affecting the frequency extremes much, if at all (your existing 1 uF cap will have to come out, though). Here are some values: 390 mH inductor, .47 uF cap = 370 Hz 390 mH (.390 H) .22 uF cap = 543 Hz 390 mH .1 uF cap = 800 Hz The formulae I used, from Boscorelli's 'The Stomp Box Cookbook', were (L = inductance): F (frequency) = 1 / (6.28 x [sq/rt of L x C) C (in farads) = (1 / [39.4 x F squared x L) L (in henries) = (1 / [39.4 x F squared x C) There's another issue, though, regarding the Q, or bandwidth, of this boost. With that 47 ohm resistor in parallel with the LC circuit, the boost is going to be relatively mild and spread out over a wide range of frequencies centered around the frequencies above. In order to make the gain boost larger and more focused, the feedback resistors are going to have to be changed. I would multiply them both by a factor of ten - this will give 470 ohms and 12K ohms for your current, non-stock configuration. If this doesn't give you enough, I think you might be able to double it again before you start affecting the bias of the phase inverter using the stock (22K) value of tail resistor. You're using a 12AX7 driver, though, so the whole feedback and bias situation changes - you could probably go up to a 2.2K / 56K feedback setup with no problems, and your feedback will be greater with the higher output stage gain. You're going to have to just plug this stuff in and try, though - you've got 6L6's in there and are running a 4 ohm speaker load, and the schematic is really only of limited help to me. Also, try putting back in your presence cap when you're done (you'll have to scale it for whatever value of bottom feedback resistor you end up with). I'm waiting for Boogie to get back to me about ordering inductors - I'll let you know what they say. I paid from 5 to 7 bucks each for them back in '94. Ray

4/25/2000 7:08 PM

Ray Ivers

Inductor-based Feedback Circuit Mods

Bjorn,

I'm answering this here in case anyone else might benefit from this stuff.

If you connect a cap in series with an inductor across the 47 ohm feedback divider resistor in your BFDR, you will get a mid boost without affecting the frequency extremes much, if at all (your existing 1 uF cap will have to come out, though). Here are some values:

390 mH inductor, .47 uF cap = 370 Hz
390 mH (.390 H) .22 uF cap = 543 Hz
390 mH .1 uF cap = 800 Hz

The formulae I used, from Boscorelli's 'The Stomp Box Cookbook', were (L = inductance):

F (frequency) = 1 / (6.28 x [sq/rt of L x C)
C (in farads) = (1 / [39.4 x F squared x L)
L (in henries) = (1 / [39.4 x F squared x C)

There's another issue, though, regarding the Q, or bandwidth, of this boost. With that 47 ohm resistor in parallel with the LC circuit, the boost is going to be relatively mild and spread out over a wide range of frequencies centered around the frequencies above. In order to make the gain boost larger and more focused, the feedback resistors are going to have to be changed. I would multiply them both by a factor of ten - this will give 470 ohms and 12K ohms for your current, non-stock configuration. If this doesn't give you enough, I think you might be able to double it again before you start affecting the bias of the phase inverter using the stock (22K) value of tail resistor. You're using a 12AX7 driver, though, so the whole feedback and bias situation changes - you could probably go up to a 2.2K / 56K feedback setup with no problems, and your feedback will be greater with the higher output stage gain. You're going to have to just plug this stuff in and try, though - you've got 6L6's in there and are running a 4 ohm speaker load, and the schematic is really only of limited help to me. Also, try putting back in your presence cap when you're done (you'll have to scale it for whatever value of bottom feedback resistor you end up with).

I'm waiting for Boogie to get back to me about ordering inductors - I'll let you know what they say. I paid from 5 to 7 bucks each for them back in '94.

Ray

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