ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Dumble Mod for BF Normal channel (layout drawing)


 
9/6/1999 6:03 AM
Steve A.
email
Dumble Mod for BF Normal channel (layout drawing)
    I added in the basic Dumble ODS preamp (sans OD) to the Normal channel of my 65 Pro Reverb project amp. It sounds great so I drew up the mods on the layout drawing for an AB763 Twin (since it includes a Middle control on the Normal channel). I omitted the Rock/Jazz and Deep switches (is anybody using them these days?) but added in the tone stack defeat boost that has been discussed in some of the Dumble threads here.  
 
 
 
http://www.techaccessinc.com/blueguitar/dumbnorm.pdf  
 
 
 
Steve Ahola
 
9/6/1999 5:44 PM
Tom
email

Curious about the Eq cap values, in what amp did dumble use ,022/,047, ? (The Trojan Horse wrong Dumble Skyline mod schematic club)!!  
 
Tom
 
9/7/1999 8:13 AM
Steve A.
email

Tom:  
 
 
 
    Those caps happened to be what was already on the eyelet board and I thought that they sounded pretty damn good. I guess I should try out the 0.1uF/0.047uF caps spelled out on the schematic just to be scientific about it. (BTW it was just mentioned to me earlier tonight that with the downright weird mid control, you really do want the larger mid cap...)  
 
 
 
    The day before I "dumbled" my BF I had just taken out the mini-toggle switches that I had used to select between 0.047uf and 0.094uf for the bass cap and 0.022uF and 0.044uF for the mid cap (since with the standard BF Normal channel the 0.47uF/0.022uF combination really sparkles- so why bother with anything else?)  
 
 
 
    I would assume that everyone reading these posts can change out the tone caps on the Fender eyelet board with their eyes closed, but I made up the drawing to make it easier to see how to modify the wiring to the tone pots to accommodate the Dumble ODS schematic posted on my site (late 70's, from what I've heard).  
 
 
 
    BTW, I revised the Tone Stack Defeat switch to completely remove the stack from the audio signal path (although there is a slight switching noise not present with the original circuit). To balance the signal a bit when the boost is engaged, I added a 470k resistor to ground paralleled with a 56pF cap.  
 
 
 
Revised drawing with advanced Tone Stack Defeat boost:  
 
 
 
http://www.techaccessinc.com/blueguitar/dumbnorb.pdf  
 
 
 
Steve Ahola  
 
 
 
P.S. In the notes I just posted about a 1997 OD100, it sounded like there was a second tone stack with trimmer pots embedded in the epoxy. I suspect that the second tone stack is used to smooth out the sound of the OD section (perhaps the Skyline mod uses a similar idea). My guess is that this second tone stack is used after the two (?) stages of the OD circuit (assuming that it is similar in that regard to the late 70's models).  
 
 
 
    Other people building Dumble clones on this board have mentioned adding tone shaping components right after the OD section; I believe the next step to try would be to run it through a second tone stack and see what happens! (For starters, I'd try a Marshall/5F6A tone stack.)
 
9/8/1999 2:05 AM
Steve A.
email

To all:  
 
 
 
    Well I soldered those tone cap switches back in so that I could compare the preamp with the Dumble cap values against the "modern" values listed on my drawing. My own preference is for the 0.047/0.022uF combo since it seems to be a little bit smoother and with not as much bass to load down the sound. But I will leave the switches in there this time around because I also like the fuller bass from the 0.1uF/0.047uF combo...  
 
 
 
    It should be pointed out that the tone stack is downright weird, and unlike a regular tone stack in that controls seem to effect the overall volume much more than the typical BF stack. If you turn any of the controls down to "1" the signal is essentially muted. So you do not get as much control over the tone as you do with a Fender or Marshall stack. But the sounds that you do get are really nice sounding and at this time I like keeping the 3 tone pots "ganged" together (5-5-5, 6-6-6, 7-7-7, etc.) When set to 10-10-10 you get a really nice boosted sound which contrasts well with the tone stack defeat boost. (Trace- tell me more about Richie's TSD boost.)  
 
 
 
Steve Ahola  
 
 
 
 
9/8/1999 4:49 PM
Gil Ayan
email

quote:
"It should be pointed out that the tone stack is downright weird, and unlike a regular tone stack in that controls seem to effect the overall volume much more than the typical BF stack. If you turn any of the controls down to "1" the signal is essentially muted. So you do not get as much control over the tone as you do with a Fender or Marshall stack. But the sounds that you do get are really nice sounding and at this time I like keeping the 3 tone pots "ganged" together (5-5-5, 6-6-6, 7-7-7, etc.) When set to 10-10-10 you get a really nice boosted sound which contrasts well with the tone stack defeat boost."
 
 
 
 
Stevie, my man... I am kind of surprised to read the above statement. I agree that the tone stack is definitely on the weird side, but I'd say the main difference with the Fender style of tone stack is that the Dumble's seems to shift the dominant role away from the treble. That is, in a Blackface or Marshall TS, the treble is everything but not so on the Dumble. The treble can be turned up or down and it adds a little birghtness, but not "gain,"like the Fender treble does.  
 
 
 
A couple of notes on that Dumble thing: the "tail" on the bass control is indicated as 1.8K in the Dumble schematics. This leaves almost nothing for the middle control to be in paralle with when the bass is turned to zero, so in other words, to get some middle, you need to turn the bass up some. HOWEVER, note that on your own page in the "Notes on 1997 Dumble," your "source" mentions that the amp he saw has a 9.8K tail on the bass pot. This would allow for there to be more middle control even with the bass turned down real low.  
 
 
 
After you experiment with that, you may find that the Dumbletone stack is pretty much subtle, and does not offer as much variation as a Fender. For starters, you do get some sound with all controls turned to zero, like a Marshall if you would.  
 
 
 
Hope this helps,  
 
 
 
Gil
 
9/9/1999 1:57 AM
Steve A.
email

Gil:  
 
 
 
    I did some more testing and found that I do have an intermittent problem with my 250kA bass pot, which explained some of the unusual observations. However with the controls set to 5-5-5 you can essentially kill the signal by turning the mid pot or bass pot(but not the treble pot) down to 1.  
 
 
 
    I just wanted to reiterate that although I consider the tone stack to be "downright weird" compared to the usual suspects from Fender or Marshall, it is still a real work of art! It is designed with a guitarist in mind, not an electrical engineer. If someone were to run a simulation of the tone stack, I don't believe it would offer as much control over the signal as the Fender or Marshall stack, but you can really dial in a lot of great settings (unlike the F or M, which I usually just set to 7-4-7 99% of the time).  
 
 
 
    While I had originally given the tone stack the full credit for the great new sounds on my 65 Pro, further experimentation revealed that the 22Meg resistor/.047uF cap from the plate to the grid of the second stage seems impart some very magical qualities of its own. I tried adding it into the HRT OD channel preamp, and it cleaned up a particular harshness I had been trying to eliminate. After being pleased with the results from that, I then added a 250pF mica cap from the plate to the cathode of the first half of the dc-coupled pair, which smoothed out the signal even more. BTW I wonder if Dumble still uses that 22M/0.05uF combo in his newer amps (or maybe he came up with another trick).  
 
 
 
    Thanks for your explanations of the tone stack (I am still in awe of the preamp and at a loss for words in describing what it does...)  
 
 
 
Steve Ahola
 
9/10/1999 11:15 AM
GFR
email

There's a similar trick (a feedback resistor with a DC blocking cap on the 2nd stage) on the 6G6 Bassman.
 
   Page 1 of 3 Next> Last Page>>