Reverse Engineering a Dabeck Fat 45

[dbishopbliss]

I have been wanting a JTM45 style amp for a while and I recently came across this amp in my local CL. The price of the amp was less than a kit and it has a number of premium parts that would cost even more if I tried to include them in my own build. The builder is a guy named David (Dave) Smith in Dallas who seems to be a well regarded amp tech. However, it seems that he is no longer in business and has not answered when I have reached out to the phone number included on the amp.


There are some notable differences between the amp and a JTM45 such as:

• Split cathode on the first gain stages of inputs one and two with different values
• Push/Pull pot that changes the gain structure (for lack of a better term, not sure what it is actually doing yet)
• Extra preamp tube - I think it is for the buffered effects loop
• Switchable rectifier - either tube or solid state
• Bias pot that supposedly allows for 6L6 (current tubes), KT66 or EL34s.

There is nothing wrong with the amp and it sound great. But I am curious to compare the circuit to a JTM45 plus it would be nice to have it documented for any future service. However, the way he installed many of the components it is hard to see the values. Also, there are some components that are under the board and its not apparent where all of the wires are going to because some of the connections are under the board.

Please share your advice on how to trace and document the circuit?




free picture upload

[sluckey]

I'd start by measuring voltages for PT secondaries, all filter caps, and tube pins. Record all, even the zero volt readings.

The 1st preamp, PI, power tubes and power supply should be pretty easy just using your eyes and occasionally an ohmmeter. The rest will take a lot of meter work to find where each wire goes. Don't use your continuity test function. Check actual resistance. (Zero ohms is continuity, 50Ω is not!) You'll probably need to take the knobs off and remove the pots and input jacks from the front panel so you can lift the board up to see under side wires and jumpers. Take your time. You know the basic structure of the amp. Work from that knowledge.

Sounds like a lot of work but it will probably go quickly. The only really unknown is the FX loop.

[shooter]

^^^what he said
take pics, they're cheap, lots n lots, all sides, everything. comes in real handy.

[pdf64]

It looks to me as if V1&2 have got their triodes wired in parallel.

So there is no fx loop valve, the loop is just a basic passive insert between treble wiper and power amp input.

Are you sure there’s components under the board? That would be weird 

[dbishopbliss]

Are you sure there’s components under the board? That would be weird 

There is at least one yellow cap I can see sticking out... maybe there is more?

[tubenit]

Dabeck Electronics - Custom Amp Repair by David Smith - Richardson, TX (webs.com)


Dabeck JTM 45 Clone | Reverb

[pdf64]

…
Dabeck JTM 45 Clone | Reverb

Just to note that the fx loop of the amp in that reverb listing has an extra pot and switch that this amp doesn’t.

[dbishopbliss]

…
Dabeck JTM 45 Clone | Reverb

Just to note that the fx loop of the amp in that reverb listing has an extra pot and switch that this amp doesn’t.

Plus it has extra switches for tone. Not really the same amp but certainly based off a JTM45.

[dbishopbliss]

Dabeck Electronics - Custom Amp Repair by David Smith - Richardson, TX (webs.com)

I'm not sure what you are seeing, but most of the links are dead on that site. As I mentioned, I have tried reaching out to the number and no reply. I read in a thread that was 10+ years old on another forum that Dave was an older guy. I hope he is doing well or rocking out in a better place.

[AlNewman]

Seems like a lot of work to reverse engineer a good sounding and properly working amp just in case it needs a service someday, but have fun.  Feel free to share the schematic here when you figure it out.

[sluckey]

Seems like a lot of work to reverse engineer a good sounding and properly working amp just in case it needs a service someday, but have fun.  Feel free to share the schematic here when you figure it out.

I do this kind of stuff for pure enjoyment. And to keep the noodles greased.   

[PRR]

Dabeck Electronics - Custom Amp Repair by David Smith - Richardson, TX (webs.com)

I'm not sure what you are seeing, but most of the links are dead on that site. ...

The site appears to have been partly moved to "webs.com" perhaps temporarily (or forever).

Before the move, from wayback machine: https://web.archive.org/web/20110629053836/http://www.dabeckelectronics.com/

MySpaces, seem dead, or I need a login:
www.myspace.com/dabeckslinger
www.myspace.com/dabeckcustomamps

Street address given as The Guitar Shop 1056 E. Beltline Rd. Richardson, TX 75081 

Google Streetview shows that this is now an insurance agency.
https://insuranceofdfw.com/
Wayback suggests they been here since 2021.
Note same concrete beams, window/door, wall-light, even same stick-on house numbers. New sign and OPEN light.

"David Smith in Texas" won't be a useful search.

[dbishopbliss]

It looks to me as if V1&2 have got their triodes wired in parallel.

I spent some time looking more closely at the amp's layout and you are correct. It looks as he used a separate tube with their triodes in parallel for channels 1 and 2. Channel 1 appears to be using the traditional JTM45 values while Channel 2 is using values associated with a Plexi amp. From what I understand he could have used separate cathode resistor/bypass caps for a single tube. After doing some reading it seems that running the triodes in parallel improves the signal to noise ratio. Is there another reason to do this? Or is that a good enough reason.


Still working my way through the rest of the amp. Gotta figure out what the push/pull pot does and how the loop is wired (appears not to be buffered in this amp).


Looks like the second gain stage, cathode follower, tone stack, phase inverter are pretty standard. There is some weirdness going on in the power supply that I'm not following, but I will post separately on that.

[pdf64]

… it seems that running the triodes in parallel improves the signal to noise ratio. Is there another reason to do this? Or is that a good enough reason.


Still working my way through the rest of the amp. Gotta figure out what the push/pull pot does and how the loop is wired (appears not to be buffered in this amp).
…

In a valve guitar amp like this one, I think any signal to noise improvement would be negligible, only existing in theory.

My guess is that he got a load of chassis punched for the extra valve, for his new bells and whistles version, as per tubenit’s reverb link.
But then he found an older circuit board, that couldn’t accommodate the extra parts etc for the buffered loop, that he wanted to make use of.
So the options were to use a blanking plate over the extra hole, or find some use for an additional valve using the old style board. And the parallel triodes ended up as the chosen option.

Perhaps I’m being cynical, but that way, he could sell a more basic inside amp on the back of the publicity for the new bells and whistles amp. Explain it as being an early version of the same thing.

Regarding the pull switch, my guess is a cathode bypass for the second stage.

[dbishopbliss]

Found this quote from another thread. Seems to explain the "Fat" moniker...

... Excerpt taken from this ampage discussion:http://archive.ampage.org/threads/1/gagd/011345/Parallel_preamp_tubes-1.html "....it's rare to find a 12AX7 that has both sections bearing identical characteristics. So when the two sections are paralleled, there are two slightly different output signals being blended at the common plate load and coupling capacitor. This can appear as a sort of "thickening" of the sound, which can help make a circuit sound "fatter". " ...

But where a dual-triode is used with shared plate and cathode resistors, you get something different from using the triodes individually.  You've effectively created a "composite tube" which has twice the transconductance (Gm), half the plate resistance, and (as a result) a bit more gain when using same/similar plate/cathode resistor values.  Tubenit & others have found that parallel triodes seem to yield about 30% more gain (with a 12AX7) than using a single triode.  The sonics are changed, and the composite tube sounds a bit fatter than the same triode used singly.  And lastly, the extra Gm helps slightly in lowering the noise floor.

[dbishopbliss]

I finally got a reply from the number on my amplifier... poor guy has had the number for 2 years and constantly gets calls.

[dbishopbliss]

Amp related question... The amplifier currently has 6L6 tubes in it. I know the early versions of the JTM45 had 6L6, but shortly afterwards they went to KT66. I know that KT66 draw more current and that all amplifiers/transformers aren't up for the task. How do you tell? Is there a way to determine who made the transformers?


BTW - Made good progress on the layout but its pretty crowded in the power supply. Going to take some time to trace everything.

[sluckey]

Are you drawing a layout and not a schematic?

[dbishopbliss]

Are you drawing a layout and not a schematic?

I'm going to do both. Starting with the layout since it is easier for me to see the parts and trace the wires. Then I will refer to the layout to draw the schematic.