Members ksl Posted October 25, 2021 Members Share Posted October 25, 2021 Hi all,, I'll soon be taking possession of a dual EL84, 6CA4 recto & a single 6EU7 preamp tube. The only experience I've had w/the 6EU7 is with some older Gibsons. It's a broad question, w/many variables, but- what tube might offer the best overdrive, if the 6EU7 falls, 1short; a 12AX7, or perhaps a EF86/6267 tube for some early Vox-like breakup? I really NY tech would need to pull off some rewiring of the front end, but hopefully with good OD results, & a worthwhile venture? Thnx in advance for any suggestions~ Quote Link to comment Share on other sites More sharing options...
Moderators daddymack Posted October 25, 2021 Moderators Share Posted October 25, 2021 It is a PA head. Not intended to overdrive from the preamp; in truth, it is designed to stay as clean and un-distorted as possible. So trying to make it do what it wasn't designed to do is not a great idea. I had to think about this...what you have there is maybe the makings of a good bass head. Probably well suited to upright. Quote Link to comment Share on other sites More sharing options...
Members Mr.Grumpy Posted October 29, 2021 Members Share Posted October 29, 2021 I really think this is a bad idea...<insert standard warning about "tube amps contain lethal voltages" blah blah blah...> I think you'd be better off buying and assembling one of those tweed Fender clones, several 'small shops' make them. Having said that.... I've highlighted some components that could be changed to potentially increase the gain and bass response of the preamp. Remember this amp is built without bleed resistors on the power filter caps, so YES, it could have 300 - 400 volts DC on the B+ supply even when it's turned off and unplugged. Both of the capacitors I've highlighted have one side tied to the B+ line. Don't attempt this unless you own and know how to use a multimeter, full stop. 1. 0.01 mfd coupling capacitor between 1st and 2nd preamp stage. This could be changed out (or paralleled) with a larger value cap which will pass more signal and more bass frequencies from the V1a to the V1b stage. Maybe try 0.02. 0.05, 0.10, whatever. Too large a value may make the distorted sound farty or flubby. 2. Similar to #1, but it's the coupling cap between the V1b stage and the power amp. Again, a larger value cap will pass more signal and more bass on to the next amplifier stage. On both #1 & #2, you need to pay attention to the voltage ratings of these caps. The B+ voltage should be measured and any caps used should be rated for that plus at least 10% more for safety and reliability. 3. 330 K Ohm coupling resistor. This could be swapped for a lower value, it won't change the bias point because the grid is at DC potential. Similar to changing the cap in #1, changing it will (may?) drive the second stage harder. 200K ohm? 100 K? 50 K? See what happens! 4. 430 Ohm cathode resistor on V1a, changing this to a lower value may increase the output of the V1a stage. It could be as low as 100 ohms as it is on V1b, but I'd try something higher, 330 or 220 ohms. This WILL affect the bias of V1a, so don't do this unless you know how to read the plate current by measuring the voltage drop across the 220 K Ohm plate resistor. Also due to the current, this resistor should be rated for 1 watt. 5. Never mind, leave this one alone. The neon light is at an interesting place in the circuit...I suspect it may be more than a simple pilot lamp. Neon lamps have a fixed breakover voltage, so that neon lamp may be acting like a zener diode to regulate the voltage on V2 grid. Quote Link to comment Share on other sites More sharing options...
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