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Dumb-Bell Preamp Clone almost finished

4/25/2000 7:49 AM
Dominik	Dumb-Bell Preamp Clone almost finished Hi there, the last two weeks saw me assembling my newest project: a preamp with two seperate channels, one modeled after the Dumb-Bell Skyliner and the other after the 70s Dumb-Bell OD. I built the Dumb_bell-ator as well. I used a 19", 2 HE chassis , and had somebody make me an L-shaped brass thingy that the ceramic tube sockets are bolted on. I hopefully will be able to take some pics and post them on my homepage soon. I used a 80 mA tranny with 250 - 0 - 250 secondary and 6,3 - 0 V 2,7 A. I used two 1N4007 to rectify the B+ and three (two in series, one to ground) for the filament DC supply. I wanted to copy the DC-filament supply as posted by Randall Aiken, but my tranny did not have 2 3.15 V taps (with CT), but only one 6,3 V tap and 0 tap. I still used the 1000 uF cap to smooth the DC, but I guess it still has some ripple (don´t have a scope unfortunately). This DC supply also feeds the three relays (6 V). As in Mr Aiken´s plan, I tried to seperate the ground of the 6,3 V DC supply from the rest of the grounds (lifted ground so to say) but found that that leads to a bad hum. No wonder actually. I then assembled the Dumb-Bell-ator and tested it. Next were the clean sections of both channels. They sound really nice, and I have to admit, that the tone stack surely is very active in this design. The OD section came next, but after installing the channel switching and putting every thing together I witnessed a loud and nasty hum in the OD sections and some serious "bleeding" between channels. The power supply is pretty much standard with diodes- cap (33 uF) to ground - 1,25 kOhms (4 W)- 4,5 kOhms (1 W)-cap to ground (22 uF) - 4,25 kOhms (1 Watt) cap to ground (22uF) - 1,5 kOhms - cap to ground ( 22uF) and 1,0 kOHMs - cap to ground (22 uF). I witnessed bleeding into the Dumb-Bell-ator even so all signal wires of the two channels were unsoldered. I could still hear the OD signal. Channel II (70s OD) is very close to the Dumb-Bell-ator parts, so I changed the layout a bit. I have yet to see wether or not that did anything good. BTW I ran all the anode wires (wires to anodes) as one bundle back to the pS. I changed that as well, could that have been the problem?? BTW the schematics state a 100 log pot for the Drive control in the 70s OD section. I found that this is incorrect, since there is almost no effect on the content only in the very beginning of the sweeping of the wiper. That is all for now, I hope I did not bore yáll to death, Regards, Dominik

4/25/2000 7:49 AM

Dominik

Dumb-Bell Preamp Clone almost finished

Hi there,
the last two weeks saw me assembling my newest project: a preamp with two seperate channels, one modeled after the Dumb-Bell Skyliner and the other after the 70s Dumb-Bell OD. I built the Dumb_bell-ator as well. I used a 19", 2 HE chassis , and had somebody make me an L-shaped brass thingy that the ceramic tube sockets are bolted on. I hopefully will be able to take some pics and post them on my homepage soon. I used a 80 mA tranny with 250 - 0 - 250 secondary and 6,3 - 0 V 2,7 A. I used two 1N4007 to rectify the B+ and three (two in series, one to ground) for the filament DC supply. I wanted to copy the DC-filament supply as posted by Randall Aiken, but my tranny did not have 2 3.15 V taps (with CT), but only one 6,3 V tap and 0 tap. I still used the 1000 uF cap to smooth the DC, but I guess it still has some ripple (don´t have a scope unfortunately). This DC supply also feeds the three relays (6 V). As in Mr Aiken´s plan, I tried to seperate the ground of the 6,3 V DC supply from the rest of the grounds (lifted ground so to say) but found that that leads to a bad hum. No wonder actually. I then assembled the Dumb-Bell-ator and tested it. Next were the clean sections of both channels. They sound really nice, and I have to admit, that the tone stack surely is very active in this design. The OD section came next, but after installing the channel switching and putting every thing together I witnessed a loud and nasty hum in the OD sections and some serious "bleeding" between channels. The power supply is pretty much standard with diodes- cap (33 uF) to ground - 1,25 kOhms (4 W)- 4,5 kOhms (1 W)-cap to ground (22 uF) - 4,25 kOhms (1 Watt) cap to ground (22uF) - 1,5 kOhms - cap to ground ( 22uF) and 1,0 kOHMs - cap to ground (22 uF). I witnessed bleeding into the Dumb-Bell-ator even so all signal wires of the two channels were unsoldered. I could still hear the OD signal. Channel II (70s OD) is very close to the Dumb-Bell-ator parts, so I changed the layout a bit. I have yet to see wether or not that did anything good. BTW I ran all the anode wires (wires to anodes) as one bundle back to the pS. I changed that as well, could that have been the problem?? BTW the schematics state a 100 log pot for the Drive control in the 70s OD section. I found that this is incorrect, since there is almost no effect on the content only in the very beginning of the sweeping of the wiper.

That is all for now, I hope I did not bore yáll to death,
Regards,
Dominik

4/25/2000 6:23 PM
Gil Ayan	Dominik, obviously you chose to do a different layout so I have no idea why you had all of those cross-talk problems. Regarding the 100K log pot, a blanket statement like that is not always applicable, as per my exprience. It all depends upon the taper of the pot, for one; use an old style pot and it will have a 30% throw to it, so it is a lot closer to a linear pot than would be the more common 10% throw log pots... Then, you know that there is a trimpot preceeding the 1st OD stage in those amps. Set that low enough and sure, you will need to up the signal level between coming out of the 1st overdrive stage. You can use a 100KL pot there, as many Dumbles actually used. However, if you make the signal going into the 1st OD stage stronger, you will find that a 100KL pot between OD stages may be too much too soon/ It is a fine balancing act, and you may find that hitting the fisrt OD stage hard and the second soft gives you a more compressed sound that hitting the 1st OD stage softer. Gil

4/25/2000 6:23 PM

Gil Ayan

Dominik, obviously you chose to do a different layout so I have no idea why you had all of those cross-talk problems.

Regarding the 100K log pot, a blanket statement like that is not always applicable, as per my exprience. It all depends upon the taper of the pot, for one; use an old style pot and it will have a 30% throw to it, so it is a lot closer to a linear pot than would be the more common 10% throw log pots...

Then, you know that there is a trimpot preceeding the 1st OD stage in those amps. Set that low enough and sure, you will need to up the signal level between coming out of the 1st overdrive stage. You can use a 100KL pot there, as many Dumbles actually used.

However, if you make the signal going into the 1st OD stage stronger, you will find that a 100KL pot between OD stages may be too much too soon/ It is a fine balancing act, and you may find that hitting the fisrt OD stage hard and the second soft gives you a more compressed sound that hitting the 1st OD stage softer.

Gil

4/25/2000 6:35 PM
andy fuchs	Another thought on this circuit location (input trimmer)is to not completely ground the bottom of the pot. I like putting a few K-ohms on the (formerly) grounded end of the pot. This way you can reduce the overdrive input, but never completely shut it off. This could be done on the second pot as well, if you wish.

4/25/2000 8:08 PM

Gil Ayan

The chicken or the egg? - Re: Dumb-Bell Preamp Clone almost finished

quote:
"Another thought on this circuit location (input trimmer)is to not completely ground the bottom of the pot. I like putting a few K-ohms on the (formerly) grounded end of the pot. This way you can reduce the overdrive input, but never completely shut it off. This could be done on the second pot as well, if you wish.
"

Andy, so did you get this idea from Dumble, or did Dumble get it from you?

Gil

4/25/2000 8:19 PM
andy fuchs	Ouch ! (g) Actually, I (and a number of my customers) found that there are some real nice settings to be found on the low end of the pots. I personally found it a little annoying to have the pot shut things off completely. Choosing the taper of the pot helps too, but having a few 'k' at the bottom end (I can't tell you the value or I'd have to kill you), makes a pleasant difference.

4/26/2000 10:41 AM
Dominik	Re: Dumb-Bell Preamp Clone almost finished Hi Gil and Andy, thanks for your replies. I should have been a bit more precise with my sayings: Indeed I chose a different layout. I drew two different versions and tried to realize the second one. I used two long terminal strip boards (facing each other) and the parts are solder across the strips: The tube sockets are mounted on a single brass raket in front of the terminal strips socket-----socket---socket---socket---socket (CL1/2) (OD1/2) (Cl1/2) (OD1/2)(Dumbbellator) ---\|---\|---\|---\|---\|----\|----\|----\|---\|---\|----\|-- a b c d e f g h i j k ---\|---\|---\|---\|---\|----\|----\|----\|---\|---\|----\| front plate The tranny is located to the far right end side of the chassis. The powe supply is to the far right side in the middle (facing the amp). Maybe one reason for the bleeding of the channels is that I take the voltage for the FX-loop (350 V) after the first resistor of the PS. The OD1/2 of the first channel gets it´s voltage here as well, so no isolation between these two. Oddly enough, the other channel derives it´s voltage (306 V/ 303 V) further downstream (after three filter caps, each 22 uF and three resistors, like in a normal PS), but still the bleeding is there. As said the OD of channel one was very close to a plate cap of the FX-loop. I changed that. (Damn I wish I had some pics to show you). The clean channels sounded really nice upon first testing, altho I found that the tone-stack of the 70s generic needs some fine tuning yet. The thing about the drive pot: I swapped the 100 k log (new style, 10 %) with a 1 M lin pot I had at hand. I played around with the trimmers as you said, but found that the overall effect of the 100k log pot was not what I wanted. I think the 1 M pot will make the signal more compressed but that is not really a concern for me.. I used one common ground for each tube, where the grid resistor and 1 M input resistor are connected to. This ground is very close to the tube (terminal strip). I then ran every local ground back to the ground of all the filter caps and from there one wire to the common ground of the AC-line bolted to the chassis. BTW: the fx-loop really fatens up the signal. One can even dial in some different tones by setting the return level and send level and using the shift switches (cap across pot). I still have to get the 6,3 V DC for the filaments right (maybe thats were some of the hum is coming from)and the OD channels and Clean channels need some fine tuning. If I would do the porject again, I would only build one channel and the Fx-loop. Thanks for any input, Dominik

4/26/2000 10:41 AM

Dominik

Re: Dumb-Bell Preamp Clone almost finished

Hi Gil and Andy,
thanks for your replies. I should have been a bit more precise with my sayings:
Indeed I chose a different layout. I drew two different versions and tried to realize the second one. I used two long terminal strip boards (facing each other) and the parts are solder across the strips:

The tube sockets are mounted on a single brass raket in front of the terminal strips

socket-----socket---socket---socket---socket

(CL1/2) (OD1/2) (Cl1/2) (OD1/2)(Dumbbellator)

---|---|---|---|---|----|----|----|---|---|----|--
a b c d e f g h i j k
---|---|---|---|---|----|----|----|---|---|----|

front plate

The tranny is located to the far right end side of the chassis. The powe supply is to the far right side in the middle (facing the amp).
Maybe one reason for the bleeding of the channels is that I take the voltage for the FX-loop (350 V) after the first resistor of the PS. The OD1/2 of the first channel gets it´s voltage here as well, so no isolation between these two. Oddly enough, the other channel derives it´s voltage (306 V/ 303 V) further downstream (after three filter caps, each 22 uF and three resistors, like in a normal PS), but still the bleeding is there. As said the OD of channel one was very close to a plate cap of the FX-loop. I changed that. (Damn I wish I had some pics to show you). The clean channels sounded really nice upon first testing, altho I found that the tone-stack of the 70s generic needs some fine tuning yet. The thing about the drive pot:

I swapped the 100 k log (new style, 10 %) with a 1 M lin pot I had at hand. I played around with the trimmers as you said, but found that the overall effect of the 100k log pot was not what I wanted.
I think the 1 M pot will make the signal more compressed but that is not really a concern for me..
I used one common ground for each tube, where the grid resistor and 1 M input resistor are connected to. This ground is very close to the tube (terminal strip). I then ran every local ground back to the ground of all the filter caps and from there one wire to the common ground of the AC-line bolted to the chassis.

BTW: the fx-loop really fatens up the signal. One can even dial in some different tones by setting the return level and send level and using the shift switches (cap across pot).
I still have to get the 6,3 V DC for the filaments right (maybe thats were some of the hum is coming from)and the OD channels and Clean channels need some fine tuning. If I would do the porject again, I would only build one channel and the Fx-loop.

Thanks for any input,
Dominik

4/26/2000 5:19 PM

Gil Ayan

Regarding your layout, yes, it could be that you are coupling something there (capacitively). But, as you mentioned, you went for a different amp all together by including the extra tube for the buffered FX loop. According to some, the power supply is half of the sound of the amp, and if you changed it to accommodate another stage, I am sure that will change things some --perhaps for the better, of course.

quote:
"I swapped the 100 k log (new style, 10 %) with a 1 M lin pot I had at hand. I played around with the trimmers as you said, but found that the overall effect of the 100k log pot was not what I wanted.
I think the 1 M pot will make the signal more compressed but that is not really a concern for me..
"

Well, the thing is that you raise the impedance of that portion of the circuitry as well. You will get more gain out of that stage (less loading on previous stage and higher resistor to ground in the voltage divider formed with the grid stopper resistor). Also, using a larger pot will lower the frequency break point of the first OD stage...

All of that put together would seem to me that your amp will be more prone to farting out that way.

quote:
"BTW: the fx-loop really fatens up the signal. One can even dial in some different tones by setting the return level and send level and using the shift switches (cap across pot).
"

Agreed 100%.

quote:
"I still have to get the 6,3 V DC for the filaments right (maybe thats were some of the hum is coming from)and the OD channels and Clean channels need some fine tuning. If I would do the porject again, I would only build one channel and the Fx-loop.
"

Sure, there are a number of ways you could go. Personally, though, you will never know what the real thing sounds like until you build it without any changes whatsoever -- even then, the sound may not (and probably WILL not) be "the same" as the real thing either. There is no "ultimate amp" out there, and even the best of stock amps could probably be tailored to suit a particular individual's needs. But, if I were you, I would give the real thing a listen if I got a chance, and THEN tweak it to suit your needs. If you start off with something already different, and you make changes on the fly to address the issues tha tcome up, chances are you will end up something completely different sounding after all is said and done. Again, that may was well be your ultimate amp though!

Gil

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