#124 FAQ for evaluation...

[ic-racer]

Another one of the files I came up with that I want to post with the schematic in the Files section...


***************#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: (link)
The schematic has been reviewed by many amp enthusiasts and it is very, accurate.

7) Are de-gooped pictures available?
Yes, here: (link)

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 series) across the Send/Return jacks that comes into play 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.

[jaysg]

F) There is a resistor/capacitor network (220k/250pf in series) across the input/output jacks that comes into play when these jacks are active.

Send/Return

Thanks for doing all this work, including the various schematics you've put out the last few days.

[bluesfendermanblues]

IC-racer

My respect to you!

A couple a weeks ago I questioned your quest for recreating the #124 in detalil - down to getting the same brand of components for your own #124 clone.

After reading almost all the postss in this forum.........
Well, I done a lot of experiments - preamp types, preamp V! V2 voltages, various brands of capacitors Sprague ODs, Xircom, carbon comp plates vs Dale RN65, Lead dress - AND I have to give in and agree with you and other dedicated builders on this capable forum, that YES the little details has a (sometimes) major impact on the tone of a Dumble amp.

I've been building amps for 15 years, and during the last couple of weeks, I've begun to question the wisdom, that I thought I'd gained from building and tweaking Fenders, Vox'es, Soldano's and boogies. It's a whole new journey getting big sounds from small changes.

Respect!

[ic-racer]

YES the little details has a (sometimes) major impact on the tone of a Dumble amp.!

You may not have read between the lines on my Fender Super Twin conversion thread, but I intentionally did just about every thing 'wrong' to see if a lot of this stuff is nonsense. Like I used all the 'wrong' resistors (1/2 w RadioShack CF and re-used many old CC), and I re-used almost all the old coupling caps, supplemented with additional RadioShack components. At one point I even used a 6C10 instead of 12Ax7 just to see. And the lead dress and component layout resemble a rats nest. I put the pre-amp switches way over at the end of the tone stack and used any wire type I had laying around.

I'm playing a little 'leap-frog' with the amps and the Super Twin was sounding pretty good, so I went and re-did the ODS, making it strictly like #124. Well, now the ODS sounds much better than the Super Twin.

There is a thread out there on HRM vs NON-HRM and it was suggested that the non-HRM has a smoother singing quality. Well, my #124 certainly has this, whereas the Super Twin does not, even though the OD circuit schematic are identical. My impression is that some of that component-type and lead dress nonsense is producing the refined tone of my #124.

So, I'm getting ready to open up the Super Twin again and match the component TYPE and lead dress of #124 and see how it changes it...

[bluesfendermanblues]

Are the preamp tubes "the same" in realtion to brand/tone between the SuperTwin and the ODS?

[ic-racer]

Are the preamp tubes "the same" in realtion to brand/tone between the SuperTwin and the ODS?

The stock Super Twin came with 6C10 as the 'distortion' tube. (this tube has 3 triodes vs the 12AX7's two triodes).

Something about the internal construction of the tube made one of the sections always feedback after rewired as a Dumble type overdrive circuit.

I did then change the Super Twin chassis over to accept 12AX7 and eliminated the third triode section by converting the reverb from a three-tube reverb to a two-tube reverb.

So, now both my Super Twin and #124 have use 12AX7 as the first two tubes. I think both amps sound better with RCA tubes, but I don't have enough to go around so I use some chinese tubes also, or swap back and forth as needed.

[hellrot]

Does anyone have the answer to Q20?

[dorrisant]

Biasing to 70% seems to be the accepted method. I don't think the power tube bias was as much a part of the "magic" as the small tweaks and attention to parts type and lead dress. The power amp seems to just reproduce the magic that is happening in the preamp. Notice the over-speced speaker wattage... this translates to no speaker distortion. A clean power amp seems to be the theme here.

My #124 was a very good amp until I changed to OD trimmer to a 500k value with a 1M2 resistor strapped across it for a value of around 350k. This made quite a bit if difference. I can hear the amp flip phase when cranked up on the OD channel now... Going back and tweaking all the trimmers the correct way and dialing in the FET bias will help to take the #124 circuit from a good pile of parts to a noteworthy, very memorable playing experience. All of this without changing the 70% bias setting.

Note: For anyone who knows better, please feel free to correct me.

[JD0x0]

Biasing to 70% seems to be the accepted method.

I would go a hair cooler than 70% personally. Blackface amps were apparently biased around 60% and still had their 'magic' and hotter isn't necessarily better tonally, especially in a preamp distortion amp. I'd suggest biasing by scope and ear and seeing what you like. I think 62-68% is a good goal to aim for.