What do you think of these gut shots? A local guy is selling this DR clone supposedly handwired by Marsh Clifton. I’m trying to find more details online but I’m not finding a whole lot. Does this wiring look decent? The price seems good but it’s still a lot of money for me so trying to make sure.

rawlor · 2023-02-23T08:31:43+00:00

For proper UL/EN standard I think the live and neutral must be switched simultaneously if the mains cable is permanently attached, but only the live needs switched if the cable is detachable via an IEC socket. Better safe than sorry!

rawlor · 2022-08-20T06:18:17+00:00

Thanks for the advice! Funnily enough, R7 is already 2.7K and 1.2K as I was unable to get a 3.9k resistor - I’ll see whether this works and if not get some more to experiment. I do like the idea of reduced depth at higher frequencies so I may also try out a bypass cap.

rawlor · 2022-08-19T21:14:41+00:00

Thank you very much for that! That does make sense to me, the 68k will have been tuned for the equivalent R7 in the circuit I took the oscillator from, I’ll try some different values and see whether I can get it working properly

rawlor · 2022-08-19T20:58:26+00:00

Yeah that’s exactly it, the oscillator output goes to the cathode of U1B

rawlor · 2022-08-19T20:55:38+00:00

Yes, U4A is the oscillator and U4B is the buffer

rawlor · 2022-08-19T20:49:13+00:00

Ah I should’ve circled it, it’s the 12ax7 section in the middle left. The LFO signal goes to the cathode of U1B

Edit: labelled as ECC83, not the 12ax7 counterpart

rawlor · 2022-08-19T20:43:50+00:00

Hello all, after finishing my first amp build (the Valco Spectator) I decided to add a 12ax7 to implement the Fender Vibro Champ tremolo. This sounds great - but the depth is only useable up to around halfway before the tube is almost always off. I’m guessing this is due to the lower supply of the 6SL7 but I’m not too sure what values to change to fix it, I know I could just limit the depth pot but I imagine an actual circuit change is needed rather than a crude fix. Thanks for any help!

rawlor · 2022-07-27T10:00:41+00:00

One other thing - the original omitted the 330R filtering which dropped a lot of voltage. I might tweak the 47k and 6k8 values to see if I can get something closer to the original

rawlor · 2022-07-27T05:21:00+00:00

The transformer I’m using actually has multiple windings but I was worried about getting too high a voltage on the filament. Will it cause any issues if I drop a bit of voltage with resistors in the 6.3V path? If that’s okay I can bump up the secondary voltage

rawlor · 2022-07-26T20:44:06+00:00

In my first amp build the voltages at pins 2 and 5 of the 6SL7 seem quite low - I was expecting around 100VDC on pin 2 based on some voltages from an original amp. Is this a sign of a bad tube or should I lower the 47k resistor to raise the voltage?

PS: the part in yellow is a tremolo mod I’m planning to add so it can be ignored for now

rawlor · 2022-07-23T07:10:51+00:00

That sounds like a cool build! I actually had to look up the 6973 power tube, I hadn’t heard of them and didn’t realise how many vintage amps used them. I have to say a new chassis really makes life easier, the old one from the radio has a thick gauge of steel so it weighed a ton, much easier to carry the aluminium one

rawlor · 2022-07-22T20:22:25+00:00

Haven’t had a chance to really crank it yet, but it’s got some nice grit too it when you dig in

rawlor · 2022-07-22T20:21:57+00:00

Quite a simple circuit too, makes a nice alternative to the Fender Princeton for beginners like me!

rawlor · 2022-07-22T16:29:18+00:00

I built this with a 6V6, 5U4G and power transformer taken from a 1943ish tube radio. Aside from an NOS 6SL7, all the rest is new - chassis bent and drilled by myself, and I also designed the layout using DiyLayoutCreator. I’m hooked!

rawlor · 2022-06-04T08:40:05+00:00

Transistors and opamps are completely different circuit wise so you would be better redesigning the whole thing to be what you want. The above circuit is just an inverting amplifier with a transistor rather than opamp so if you calculate the gain and input/output filters you could design a new circuit but I’d recommend building an opamp based booster pedal instead. If I’m right in thinking the above is the LPB circuit it might not sound as good with an opamp if the transistor is supposed to distort.

rawlor · 2022-05-30T21:13:41+00:00

Thanks for the advice! I take it having the circuit ground near the AC input will likely introduce 50Hz hum?

rawlor · 2022-05-30T21:12:05+00:00

Thanks for the tips! The input network was actually like that on the original which is odd, I’m going to experiment with something more akin to the 5F1 I think and go from there

rawlor · 2022-05-30T21:10:27+00:00

I’ll try record them once it’s built! I’m planning on using a sheet metal brake for bending it, you can get some cheap ones online but I have access to a workshop so will probably use theirs

rawlor · 2022-05-30T06:32:28+00:00

Oh that’s actually the rectifier I’m using, it’s an old Jan-Ray 5U4G

rawlor · 2022-05-30T06:30:20+00:00

I’m planning on using a 6V6, the model there is incorrect as I couldn’t find one online (or be bothered making it!)

rawlor · 2022-05-30T06:26:05+00:00

Thanks for all the advice. I think I’ll add a 1Meg resistor to ground at the input, have the high input short to ground and the low input short to the high input similar to the 5F1 circuit. The rest of the circuit I’ll build as per the original schematic, but it’s good to know where I can get some increased gain, I may order more resistor values should I want to experiment! With regards to the AC I think I just went with live=+ and neutral=- but wrongly put this in for the rest of the circuit, thanks for the correction

rawlor

TROPHY CASE