A Collection of #124 Information (Schematics, Layout, etc.)

Dumble schematics, designs, pictures. Only members may post files here.

Moderators: pompeiisneaks, Colossal

Post Reply
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

A Collection of #124 Information (Schematics, Layout, etc.)

Post by ic-racer »

I have collected a lot of stuff on #124 and have organized it so that I could understand the amp and build an exact replica.

All the info comes from the net, so this is nothing new. Mostly I have just organized and re-arranged information.

First I will post the Schematic of #124. This is really a 'community project' and basically it is traced back to pictures of #124, and a pencil scetch by 'Billy,' along with commentary by Gill. Then the first real schematic was produced by ampdoc1. I then added a few things to the schematic, and odourboy took over and did the first 'complete' schematic, showing the FET input and all.

Using odourboy's work as a foundation, I added in numerous fine details that were available from the pictures.

The second schematic is a guess at what #124 may have looked like prior to modification in 1988. The schematic can serve as a generic early 1980s amp.

This fantastic layout was the creation of talbany, martin manning and the Amp Garage community.

Note: The footpedal jack should be Swtichcraft C5F, not what is indicated on the layout.
You do not have the required permissions to view the files attached to this post.
Last edited by ic-racer on Thu Nov 29, 2012 7:36 pm, edited 9 times in total.
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Overdrive Special 'User's Manual'

Post by ic-racer »

Well, a 'User's Manual' is maybe not much use if you build the amp yourself (you should know how it works!), but for those who have not built one, or are in the process of building, or just for the curious, this tells how it is supposed to work.


*************************************
Quick user's Guide to the Overdrive Special.

***********Front Panel:****************

FET: This input is for an acoustic guitar pickup.
NOR: This input is for an electric guitar

Volume: Sets the amount of gain for the preamp section. Use the "Master" to set overall amp volume. Higher levels of the Volume will distort the 'Overdrive' channel more.

Bright Switch: Adds a treble boost to both channels
Mid Boost: (similar to the bright switch) Functions in both channels; adds some extra kick, especially in the overdrive channel.
Jazz/Rock: The Jazz setting is a quieter, lower gain setting. Three position switch. In the middle position is produces PAB without the need for the foot pedal (info added 4/09).

Treble/Mid/Bass: These self-explanitory controls function in both channels. They are bypassed with the "PAB" (preamp-boost)

Drive/Ratio: These function only in the "overdrive" channel. The Drive, works in conjuction wth the "Volume" to control the overall gain or distortion. The Ratio is a seperate master volume. To set the Ratio control, go to the clean channel and set the overal clean loudness of the amp with the Master control. Then engage the "overdrive" channel and adjust the Ratio control to get an appropriate loudness of the overdrive channel.

Master: sets the overall loudness of the amp. Raises and lowers the loudness of both channels together.
Presence: Controls the amount of negative feedback. Higher settings reduce negative feedback and give more distortion. Lower settings increase the negative feedback on the power supply and reduce distortion.


********Back Panel ************

Ground Switch: Places a capacitor between one of leads from the mains to the ground. May eliminate hum in certain situations.

ON/OFF; Controls power to the amplifier. Make sure the standby is "off" before switching on. If the tubes get fed full plate voltage (about 450v) without being warmed up first, there can be damage to the cathode of the tubes.

Standby: controls the high voltage power to the tubes. Has the effect of silencing the amp. Let the tubes heat up a few moments before disengaging the standby when first powering up.

Fuse: Should be a slow blow 3A. Used to protect the circuitry from shorts.

Effects sent and return: This is a passive effects loop, There may be gain mismatch between the input of this jack and that of an effects pedal. Try it and see how it works. If you are serious about having an active loop, you can buy or build a Dumbelator which will plug in here and will have its own input/output into which the effects will plug.

There is a filter incorporated into the send/return jacks that is activated when a cord is plugged in. One can produce an overdrive smoothing effect by placing a patch cord in the send/return jacks.

OVERDRIVE switches on back panel: With the foot pedal unplugged, this switches control the Overdrive channel (which adds gain and distortion to the clean channel). So, it would be "ON" when set to "Manual". When the foot pedal switch is plugged in, keep this switches on "Pedal" for correct function with the foot pedal.

PAB: This switch is not fully functional. If fully wired up, the function would be redundant to the middle position of the Jazz/Rock switch when the foot pedal is not plugged in. (Info added 4/09)

**********Inner Potentiometers (trimmers)***************

FET INPUT TRIMMER: located on the board near the FET input. Adjust this to change the gain of your acoustic guitar plugged into the FET input.

OD TRimmer: Located closest to the guitar inputs on the rectangular board. Adjust for the lowest setting that will produce a good overdrive tone.

Phase input trimmer. Located closest to the power tubes. Used to balance the two sides of the phase inverter tube.
Last edited by ic-racer on Wed Apr 15, 2009 10:23 pm, edited 1 time in total.
User avatar
jaysg
Posts: 1211
Joined: Tue Feb 07, 2006 11:16 pm
Location: Sandy Eggo

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by jaysg »

How different is 124_schematic_880[1].pdf from the previous one: 124_schematic_200[1].pdf ?
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by ic-racer »

Here are some key notes on #124 that are often forgotten or mis-quoted (even by me :oops: )
The photo below [picture of #124] is an amp I played and it happens to be probably my favorite amp ever. Originally built in 1983, I think, the amp has a choke and the same cosmetics as RF's old amp (i.e., script logo). Note that the power supply 1st cap bank is two (you can only see one) 300uF Spragues. I've also seen an 1982 amp that looked identical to this one, and that belongs to a friend who ordered his Dumble after Robben got his in 1982. This other guy's amp also has 300uF caps, so I am inclined to believe that's the way Robben's amp started, since they are all from the same era.

This amp was "updated" by Dumble in 1986 and given the Skyliner preamp -- you can see the cap across the bass pot, and that the middle switch on the front panel has been converted to a MID switch (the panel reads "Deep," however), for example. Unlike the 90s amps, however, this amp still has the V1B feedback and the same dropping resistors as the older Dumbles. A 15pF brightness cap was added to the overall master volume.

I am personally not a fan of the Skyliner EQ (as implemented by me, in my amps) because I think it sounds too boomy, and some of you think two 220uF caps sound stiff. Interestingly, what I liked about this particular amp is that it was perfectly balanced... not bassy, not boomy, and very "unstiff" sounding. It gave up the goods very easily.

This is an example of one of the few amps that hasn't been messed with by folks other than Dumble. For the record, all that was done to this amp when I took the pictures was put new tubes in it.


Cheers,
Ayan Gill
These are probably the only things that are not noted on the diagrams and that cannot be made out from the pictures:

1. 500pf cap bypassing V1B's grid stopper.
2. The "Level" pot is 250K, and although the taper is not shown on the notes, it acts like it's linear since the amps gets very gainy very soon in overdrive mode.
3. The pre OD network is simply a 220K resistor in series with a 500K trimmer. There are some pictures of the trimmer removed from the board, and although labeled 500K, you can see that it actually measures just shy of 350K. It was set at approximately 100K to ground, and it appears as though it may have been taken apart and put back together. Billy and I wondered if maybe the builder had determined he needed that custom value and went through the hassle of taking apart the pot and shaving off a portion the carbon trace? We probably will never know.
4. B+ is 440VDC on the 6L6 plates, which seems low. The remaining plate voltages are: V1A and V1B approximately 190VDC each; V2A and V2B 10 volts higher; on the order of 280VDC for the non-feedback side of the phase inverter plate, and about 10 volts higher for the feedback side.
5. Other than the Dale plate load resistors for V1 and V2 and the precision metal films for their respective cathodes, all other resistors used seem to be carbon film.
6. The only stranded wire used seems to be for the filaments, the rest is solid core and looks like 20 AWG, high quality PVC (not teflon)

Ayan Gill
A couple of comments on the amp: it was purchased in 1984 and was modified by the builder in 1988 to bring it to Skyliner specifications. Some of the electrolytic caps are starting to bloat and will therefore be replaced, and I suspect the voltages will go up noticeably after the recap job... I will be curious to hear the amp again then to see how the sound has changes. As of now, only the caps on the bias and relay boards have been replaced, which can be seen in the pictures. The chassis is made of 1/8" aluminum, built very solidly, is heavy and uses weld nuts to mount to the cabinet.

How does the amp sound? Absolutely terrific, and I think part of the reason is the old caps, otherwise I don't see how the thing could be so deliciously gainy. You can live on the clean channel alone as it sustains very nicely, and adding the PAB to it results in a wonderful singing tone with a tendency to feedback on just about any note at will. The overdrive channel adds more of the same. Even though I would have thought the amp should be bottom heavy based upon its design (Skyliner EQ and larger than usual cathode bypass caps), the amp sounds perfectly balanced. The best of its kind I've ever heard, absolutely no traces of harshness or otherwise "ugly" breakup to be found.
Ayan Gill
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by ic-racer »

jaysg wrote:How different is 124_schematic_880[1].pdf from the previous one: 124_schematic_200[1].pdf ?
I think I added the foot pedal wiring. I will always place the most up-to date version at the top of this thread. Version number in the lower right hand corner of the schematic.
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Transformers

Post by ic-racer »

Some notes on the output transformer of #124 by Brandon:

Dull silver round end bells is a Shumacher
Stock fender twin/showman/dual showman. Sound great! Paper interleaved and all that. From what I understand when Fender went to the schumachers from the Triads they did so to save money. Everyone I talk to that winds Trannies says that the Triads that were in tweed fenders were of MUCH better quality, but beauty is in the ear of the beholder
!
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

#124 FAQ

Post by ic-racer »

***************#124 FAQ******************

1) What is #124?
#124 it the one-hundred and 24th, presumably consecutively serial numbered amplifier build by Mr. D in California, USA.

2) What type of amp is #124?
Overdrive Special

3) When was #124 built?
1984

4) Was this amp ever modified?
Yes, in 1988 Mr Dumble did the following modifications to #124:
A) Converted the tone stack to his newer "Skyliner EQ"
B) Converted the "Deep" circuit to a "Mid Boost" circuit
C) May have had the 6L6 control grid resistors changed from 1.5k to 5.1k.
Also, in 2006 the amplifier was re-capped by an independent technician.

5) Does #124 have the HRM (hot rubber monkey/hot rodded Marshall) circuit?
No

6) Is there a schematic available? Is the schematic accurate?
Yes, here: (top of this thread)
The schematic has been reviewed by many amp enthusiasts and it is very, accurate.

7) Are de-gooped pictures available?
Yes, here: ( www.roblivesey.com/dumble )

8 ) What does #124 sound like?
"The best of its kind I've ever heard, absolutely no traces of harshness or otherwise "ugly" breakup to be found. "

9) How does #124 differ from other Overdrive Special amps from Mr. Dumble?

A) Overdrive Input network is 220k followed by a 350k trimmer. Many more amps have been identified with 220k followed by a 100k trimmer. The exact reason for the 350k trimmer is not know but there are many speculations.

B) The Level pot is 250k, whereas many other amps have a 100k Level pot. The front panel pots may have been switched around during the conversion to 'Skyliner' specifications.

C) The preamp plate resistors are 100k, similar to other early amps. However, many (most?) of the later amps with the Skyliner EQ have HIGHER value plate resistors on the preamp tubes.

D) Later Skyliner amps also frequently have a different power supply board. #124's power supply board has two 300uf capacitors and this was characteristic of the amps during that era.

E) The clean stage has 10uf bypass caps across the cathode resistors. Many of the other ODS amps had 5uf capacitors there.

F) There is a resistor/capacitor network (220k/250pf in parallel) across the Send/Return jacks that is in series with the main signal path when these jacks are active. The exact reason for these components is not know but there are many theories.

NB: When #124 was modified in 1988, the plate resistors, bypass capacitors and the power supply were left as they were by Mr. D.

10) What are the specifications of #124?
100 Watts, fixed bias class AB.2 channels (common controls)
Foot-switchable 'boost' and 'overdrive'
Controls are Volume, Bright, Mid Boost, Jazz/Rock, Treble, Mid, Bass, OD Level, OD Ratio, Master and Presence.

11) What kind of tubes does #124 use?
Preamp tube sockets are stamped 7025 x3
Power tube sockets are stamped 6L6GC x 4

12) What kind of transformers does #124 use?
Fender twin components. Numbered as follows:
OT: Schumacher manufacture Fender Twin component 022889
PT: 022756
CHOKE: 022699

13) What are the values for the input to the overdrive channel?
There is a 220K resistor followed by a 350k trimmer wired as a voltage divider.

14) What's the deal with the trimmer on the overdrive input?
The CTS trimmer is marked "500K" but was tested and reads about 345.5K. May have been 'hand selected' by Mr. D from a number of trimmers or modified.

15) Where was the trimmer set when it was examined?
It was set to about 100K from ground.

16) What is the Negative Feedback Resistor value? Was it measured or is this the marked value?
4.7k, marked value.

17) What is the slope resistor?
150K Metal Film

18 ) What type of resistors were used for the preamp platee and cathodes?Vishay/Dale RN65 metal film for Plate
Mepco metal film for Cathode
These mil.spec. components are underrated at 1/2 watt.

19) How about the other preamp resistors?
Carbon Film, mostly 1 watt.
6L6 Screen Grids look like 5 watt.
Power supply resistors look like 2 watt.

20) Does anyone know the value of the output tube cathode current (bias) as set by Mr Dumble?

??

21) In addition to the 10k potentiometer for the fixed bias, how many trimmers on on the various boards?
A) 5k phase inverter trimmer on the preamp board
B) 345,5k overdrive input trimmer on the preamp board
C) 10k FET input trimmer on the FET board

22) Were any voltages measured when the amp was examined?
Yes, as follows:
V1 190v
V2 200v
6L6 Plates 440v
PI plates 280v and 290v

23) Who owns the amp.
The owner has asked to remain anonymous.
Last edited by ic-racer on Mon Aug 25, 2008 7:11 pm, edited 1 time in total.
tubedogsmith
Posts: 597
Joined: Mon Jan 17, 2005 11:52 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by tubedogsmith »

Thanks for putting all that together, you must have been a historian in another life :D What's the deal with the series .01 caps off the R/J switch in the second schematic?
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by ic-racer »

tubedogsmith wrote:Thanks for putting all that together, you must have been a historian in another life :D What's the deal with the series .01 caps off the R/J switch in the second schematic?
It looked to me like two .01s held together with the black glue. Don't know why he used those two, but maybe he did not have any .005's when he built that amp. Here is the picture.

The series 10Ms on the local feedback loop may be similar situation, but I always wonder why he did not use the 22M he used in other parts of the amp. He may have thought 22M was sonically different from 20M.
You do not have the required permissions to view the files attached to this post.
tubedogsmith
Posts: 597
Joined: Mon Jan 17, 2005 11:52 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by tubedogsmith »

OK, I see what you're talking about. It looks more like parallel caps but there's only one lead on the switch and the ground buss.
skrumian
Posts: 1
Joined: Fri Oct 31, 2008 4:27 am

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by skrumian »

.
Last edited by skrumian on Tue Nov 04, 2008 6:21 am, edited 1 time in total.
llemtt
Posts: 323
Joined: Sat Feb 04, 2006 3:13 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by llemtt »

ic-racer wrote:... Don't know why he used those two, but maybe he did not have any .005's when he built that amp...
HAD is known for using whatever he has in his drawer...
ic-racer wrote:...He may have thought 22M was sonically different from 20M...
eventually running out of 22M, but anyway sonically different, there's a 10% difference and you can hear it

cheers
teo
RHGraham
Posts: 57
Joined: Fri Feb 08, 2008 6:36 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by RHGraham »

Hey, thanks for all this work, everyone involved, and for posting it.
Very cool.

8)
Randal
Blues Junkie
User avatar
Structo
Posts: 15446
Joined: Wed Oct 17, 2007 1:01 am
Location: Oregon

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by Structo »

What about the 50/100 slider switch with the red led?

Does that simply take the cathodes to ground disabling them?
Tom

Don't let that smoke out!
User avatar
ic-racer
Posts: 1310
Joined: Tue Dec 18, 2007 2:24 pm

Re: A Collection of #124 Information (Schematics, Layout, etc.)

Post by ic-racer »

Structo wrote:What about the 50/100 slider switch with the red led?

Does that simply take the cathodes to ground disabling them?
Instead of lifting the two cathodes totally from ground, there is still a little current flowing. The small amount of current through the LED is not enough the get the 2 tubes running, so they are essentially shut off.

Also interesting is that it looks like on Gregor's amp (#094) he still runs the two 'shut off' tubes through resistors to ground (again rather than lifting them totally), WITHOUT the LED. Not sure the significance of this need to keep a little trickle of current through the two 'off' tubes.
Post Reply