Dumble Inspired 2x12 EL34 Combo

Phrygian77

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I've been holding off on starting this build thread until I had some momentum going. It's been in the works for a couple of years now, as an amp that I promised for a luthier friend who built one of my Teles. I was originally going to do a modded JTM45 with reverb and a cascade input, but I eventually settled on doing a Dumble-esque overdrive circuit following a Fender-esque preamp with reverb. There's a lot of Traynor YGL-3 and Dumble 183 that I'm trying to marry together. Ultimately, it's a prototype that I'll hopefully tweak until it's right, or I'll fail miserably trying.

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Phrygian77

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For simplicity, I used a Twin Reverb chassis and power transformer. I had originally planned to do this in a custom Matchless style aluminum chassis, a custom cabinet, and an Antek PT. The Twin sized PT is way more weight and VA than I need, but the voltages should be right where I want them and stiff. Together with the speakers, this thing is going to be heavy as heck.

The output transformer is a Heyboer 3.9k Marshall type from APD.

Input filter will be 50+50 and the screen filter will be 32+32. Caps for the preamp will go under the doghouse.
 

Phrygian77

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I used PEM nuts for mounting the transformers because some of those nuts would be obscured by eyelet boards.
 

Phrygian77

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Hopefully, I got this right the first time, and I can move on to a final version. Normal input, DMOS input, bright, volume, treble, middle, bass, reverb and overdrive LEDs, overdrive switch, OD level and volume, reverb level, master and presence.

Preview.png
 

Phrygian77

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Unfortunately, the Wurth cap on the relay supply board is too tall for the chassis, unless I were using 1/8" standoffs. To get it up on 1/4" standoffs, it had to go.

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Doug Hoffman's latest version of his relay power supply board won't take his turrets. The holes are too small. I meant to email him to ask about it. The pitch is 5mm, so I decided to try these Phoenix Contact terminal bocks (1715022). They fit perfectly.

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Test fitted the pad board, which will be used for an LND150 preamp.

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Phrygian77

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dang - pretty cool!

I hope so.

I didn't think through the whole relay switching thing as much as I should have. I wanted LEDs on the footswitch, but since I'm simply switching the relays with with 5Vdc on the ground side with just a stereo type 1/4" Cliff jack, I can't just insert an LED in between. So, indicator LEDs on the front panel, which unfortunately will be semi-permanently attached to the panel and soldered directly to the turrets on the relay boards. I'd rather have something that's more easily replaced if needed. It is what it is at this point. This is just a prototype after all.
 
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Phrygian77

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need to see circuit diagrams...

And give away my secrets?


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Preamp is similar to a YGL-3, except a 500k bass pot, and a 220pF bright cap. I'll be using the standard Fender 12AT7 reverb driver driven by the screen supply with a transformer. Reverb mixing will be a 1.8M resistor with an 8pF cap. I modeled this beforehand with Miller capacitance, and it should be pretty neutral, more so than the added brightness of an AB763. Reverb pot will be 100k wired backwards like a YGL-3. I may need to increase the divider resistor from the coupling cap of the reverb recovery since I'm dropping more dB in the mixing network than a YGL-3. That is, if the reverb is too strong. This is part of the problem here, extra dB gain from the reverb stage going into the Dumble-esque overdrive circuit. I'm still over by a few dB, but the trim pot should take care of that (then again, impedance / Miller capacitance difference).

Presence pot will be 1kL instead of 2kL. I'm using a 180 ohm resistor from the tail of the PI to ground with a 2.7uF cap for the presence (I think, need to check my notes). NFB resistor will be 1.8M.

The EL34 grid leaks will be 220k. 1M grid leaks on the PI.

Am I missing anything? Probably.

Edit: 1.8k for the NFB, not 1.8M.
 
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sds1

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I wanted LEDs on the footswitch, but since I'm simply switching the relays with with 5Vdc on the ground side with just a stereo type 1/4" Cliff jack, I can't just insert an LED in between.
You can though, check out how these footswitches are wired:
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When the LED is switched into circuit its Vf prevents the relay coil from engaging.

When the LED is out of circuit the relay coil will see the full 5VDC and engage.

LED ON = NC side of relay switch
LED OFF = NO side of relay switch

It's kinda backwards, but in a smart way.

These footswitch boxes are nice except for the cord which is guaranteed to fail quickly in my experience. If you decide to buy over build, I recommend removing the cord and installing a 1/4" jack. Some drilling required.
 
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Phrygian77

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@sds1 like I said, I didn't think through it very well. The reverb relay will shunt the recovery grid, since I wanted to avoid shunting at the level wiper because of the way it's wired and to avoid the impedance/tone changes that could cause (I may have to revisit this if it's noisy). I wanted the reverb always on without the footswitch and without a panel switch. Tip and ring shunts on the footswitch jack will connect to ground, although the one for the overdrive will interrupt the panel switch.
 

sds1

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@sds1 like I said, I didn't think through it very well.

Well that's OK, you said you wanted LED's on the footswitch, what I shared works with the low-side switching circuit you already have. Not sure if I made that clear, or if I'm misunderstanding something, but there you go.

Regarding the reverb relay, that is how I always do it too. No panel switch, always on unless the footswitch disables it. Shunt the recovery on the NO side of the relay switch.
 




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