Dumble inspired Fender Champ build

[Potato Cakes]

Hello, Everyone,

I wanted to share my latest build. It's a variation of a Champ with modifications that are attributed to the legendary amp builder Dumble. I added the extra challenge of cramming all of this into a Vibro Champ chassis, which was almost too small. I added a couple of things like selectable grid stopper resistors to V1a/b (56k and 22k), a switch to send the first input to both halves of V1, line level output, selectable negative feedback, and effects loop. I started out being meticulous with the wire routing but during the troubleshooting process my neat-ish work began to unravel. I also made the back panel to accommodate a number of features like selectable impedance, but I wound up abandoning trying to include every option possible on a small amplifier and the result was unused drill holes. I also opted to use a full wave diode rectifier instead of a tube as there wasn't enough real estate to safely fit another octal tube socket's foot print and wiring.

All that is left is to wrap the head cabinet in Tolex.

Thanks!

Paul

 

[ccaudle]

Have you fired it up to play yet?

 

[Potato Cakes]

I have been using it on a record that I am producing. It's very awesome. It responds very well to a wide range of input voltage from either an instrument directly to the amp or from effect pedals. I would recommend it for anyone experienced with tube guitar amp circuits and working with very high voltages. It just needs to be built into a bigger (at least taller) chassis as the Vibro Champ chassis makes access to wiring potentiometers and component layout unnecessarily difficult. I'll post all of my voltages for the above schematic when I remove it from the head cabinet again to wrap it in Tolex.


Thanks!

Paul

Edit: I wouldn't necessarily recommend this circuit. See below. It does sound awesome though.

 

[CJ]

Nice!

What is the ratio on that opt?

 

[Potato Cakes]

6k6 : 8R

This transformer here:

https://soursound.com/products/sst125a1a
The one thing I haven't worked out is the function for the switch you see labeled "Extra Sprinkles." This is meant to add a bit of gain to the input tube to make V1 do that thing tubes do when you apply an increase of voltage to the grid but not enough to make it obviously distort. At first I tried having selectable capacitance on the cathode of V1 but it didn't do anything. That is because I just am now realizing I had wired the capacitors to the switch in parallel and not series. I must have been thinking about resistors and not capacitors at the time. I'll have to revisit that when I take it apart to do the Tolex. The last thing I did was have selectable grid stoppers as I mentioned above but I forgot to change the impedance as well, so again nothing happens as far as input gain.

I think I'll do a couple more tweaks to this build like the capacitor value for the normal input tone is too dark for me. I think I'll reduce the value to 2n2 instead. I'm also going to change the values for the selectable negative feedback. That'll probably be the extent of any changes I would want to make. I still have to build a deluxe version (not Deluxe) of a 10W Vibro Champ, an Ampeg B15, and an output section only Spitfire (for stereo recording), so I don't want to spend too much more time making changes to amp that already works and sounds great.

Thanks!

Paul

 

[Potato Cakes]

Got the Tolex on last night. It was the first time I have attempted this and I wound up rushing to get it finished. It came out okay though the corners are not great. But it's done and moving forward I'm just going to buy cabinets that are already wrapped.

I did a bit more tinkering tonight to try to get some of the functions to work as I intended. For the "Extra Sprinkles" I wound up making the switch change the cathode caps and bias resistors for V1 between 25uF/2k7 to 680nF/820R. This does add a little bit more bite but there is an audible pop from changing the bias and the 30V drop at the V1 plate. So it's closer to what I wanted but it could be better executed. Maybe increasing the input impedance would be the answer but I can't spend another day tweaking this circuit.

The other thing I settled on was the negative feedback control. I brought signal back from the output transformer to pin 3 of the phase inverter through a switch that selects between 1k5, N/C, and 820R. This is probably incorrectly implemented for this circuit but the result is selectable low end roll off which I find pretty useful, so I'm going to keep it as is.

And now for some voltages:

Power Transformer Secondaries:
320-0-320

B+1 - 382
B+2 - 332
B+3 - 288
B+4 - 266

Preamp (I'm using a 12AT7 instead of 12AX7):
1 - 149
3 - 3.1
6 - 157
8 - 2.9

PI:
1- 190
3 - 1.4
6 - 238
8 - 44

Output:
3 - 371
8 - 21.5

I'm about 18V lower than what is listed on the schematic. I tried using a bridge rectifier but the initial jump in voltage is too high for the 450V rating on my capacitors. I could increase those to 630V and see if I can get back those 18V but I don't think it's worth it.

At some point I will add a 10% output as even though this is 15W, but for now I'm going to call this build done and move on to the next one.

Thanks!

Paul

 

[Chrisfromthepast]

This thing is really cute! I love the size and the color choices. Very cool.

 

[ct.waveform]

This thing is really cute! I love the size and the color choices. Very cool.

It's lovely, isn't it? Harks back to happier days when things were simpler and uncorrupted. Doing for amplifiers what Kath Kidston did for school bags. I love it.

(The point-to-point wiring is also sadly reminiscent of the Leak Stereo 20 I inexplicably gave away in my 20s.)

 

[GussyLoveridge]

Can you elaborate on this a little bit? Is it about tailoring frequency response?

I added a couple of things like selectable grid stopper resistors to V1a/b (56k and 22k)

 

[Potato Cakes]

Which details would you like elaboration?

I have been tweaking this more to try to get it sound how I think it should. The last thing I am wanting to "fix" is the tone section. Overall, the amp sounds very dark. Even when cranking the tone I can't seem to get either channel to produce the sparkle or chime (I apologize for subjective descriptions) that I would expect out of Fender type circuits. I am using a 12AT7 in the preamp section and I have changed the bias resistors on it to the recommended 2k7 but left the 25uF cathode bypass capacitor.

I have seen one other person post that they have also built this circuit and is experiencing the same issue. I am wondering if there is something amiss with this schematic.

Thanks!

Paul

 

[Matador]

That 4n7 with a 1M tone pot seems like a pretty low cutoff frequency if the pot is dialed fully 'on', meaning that it's (mostly) like an 'all pass' filter to at least guitar frequencies, which might be passing a lot of bass through and make the overall response 'dark'.

Did you try shrinking that value?

 

[ct.waveform]

Have you modelled (or measured) what the frequency response is, even just hypothetically as CR sections, and how they affect the frequency response? I haven't done much in the realm of PA (and mostly that's has been on the speaker side) but what people seem to be after for that Fender incision is a rising response to about 2400Hz and a rolloff thereafter. At least that's what Clapton has programmed into his setup and those Stratocasters that bear his name. It also happens to be just what those old 10" Fane and Celestion drivers used to do, which I'm sure is not a complete coincidence.

I didn't see any global feedback but if you put some in, just resistive and perhaps with a rolloff at the very top, then you can be sure what is coming out after that will be substantially flat. Even if you just use it as a temporary A/B comparison it might help point you in the right direction. Incidentally, a CR, rather than just a C across the gain feedback resistor is usually very much better, both theoretically and in practice.

 

[Potato Cakes]

I did. I changed it to 2n2 on the bottom side but didn't mess with the pot value as I don't have anything smaller that would work with the knobs I have, which is the most important part. And as I started typing this I changed it again to 1nF and it seemed to make the bright channel have more top end when the tone control is most of the way up but didn't hear any noticeable change on the normal channel. As you pointed out the 1M value is probably a bit much and would be better suited with something between 50k and 250 with the appropriate capacitor value.

I did go and put the 1k cathode resistors back at the V1 position where I am using a 12AT7. I also changed the plate resistors to get the voltage above 200V on both V1 and V2. I also changed the wiring of the negative feedback selector to where I originally had it (56k plus output transformer secondary/No OT secondary No V2a cathode bypass cap/V2a cathode bypass cap). The local feedback resistor was changed from 3M3 to 33M per a knowledgeable and well respected online guitar amp guy.

I did not notice much of a difference changing the phase inverter balance pot. I just left it at the halfway point.

After pulling this thing apart and changing component values and wiring schemes many times, I have come to a number of realizations. First, this circuit was meant to be a breakup/distortion amp. It sounds amazing (with the tone most of the way up mind you) but it's the only trick this pony will do. With the negative feedback switch you get at least two more tricks. I had also been trying to get this thing to have as much clean volume as the Matchless Spitfire as they both are 15W amps but I failed to realize the volume-tone control configuration and the phase inverter circuit differences would prevent me achieving this. I think I also would want the tone control to act as a tilt EQ instead of low pass filter. This would be much more useful to me.

I don't think I will build this amp again nor recommend it for most people. The way that the volume and tone controls interact is difficult to explain to most players and only really makes sense if you have a good understanding of the schematic. I also think if I was to build this idea again of a more powerful Fender style circuit it would be something of the 40W Bassman/Deluxe variety with the low/mid/high tone stack.

Thanks!

Paul

 

[Matador]

Also keep in mind the standard Fender arrangement of Gain -> Tone Stack -> Gain also has significant insertion loss in the 600-1kHz region (e.g. a high-mid cut), with all of the controls at noon. I think the mid cut followed by a 'bright' gain is part of where the signature Fender sound comes from. I'm also a big fan of switching all of the cathode bypass caps to 1uF (like Soldano uses) as this can really de-muddy an amps tone.

And totally agreed: the Dumble arrangement gives far more gain than you would see in a typical Fender, so you have to really love the 'pushed with a pedal' Fender tone to like the Dumble circuits.

 

[Potato Cakes]

I have thought about using 1-5uF caps like you mentioned, but I think I will save that for a new build. I have also had that same conversation with another member here regarding insertion loss of the full tone stack versus a single tone knob and have seen that same mid cut on an analyzer like you described.

On to the next one!

Thanks!

Paul

 

[Sorr]

I have found that those Vishay HT caps sound dark.
If you left the amp as is and just changed all four HT caps to F&T axial types. You will find it will be much brighter.

 

[Potato Cakes]

I will keep that in mind for when I build this next amp. Although I did not think DC filter caps would affect the audio spectrum unless they were faulty or were not of the correct value for a particular part in the circuit.

 

[Sorr]

"Although I did not think DC filter caps would affect the audio spectrum unless they were faulty or were not of the correct value for a particular part in the circuit."
Try it and see.

 

[Potato Cakes]

Changed the filter caps to F&T and didn't hear much of a difference.

 

[user 137017]

A nice looking amp. Im just starting on a build closely based on a KLD 18PVH. (waiting on the transformer tape at the moment as china is a bit distant from the UK) ill try to keep my build as tidy as yours.