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Double-Bubble! Place for discussing the 36W version...

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wiseowl
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Descisions descisions!

Post by wiseowl »

Sooo, my 36 watt is up and running, currently with a 12ax7 pre-amp using one side, the intention is to run both sides in parallel. This leaves me with a spare socket and I want to add a second channel, my current thoughts are either an EF86 channel or a cascaded 12AX7.

I don't have any experience of EF86 based amps, so what exactly does it bring to the table? I'm all for experimenting, but would like to have some idea before I go spending my hard earned.

WRT switching channels, I'd like to be able to use a foot switch so my intention is to use a relay. I'm thinking that if I connect each channel to either side of the PI, and then switch channels by grounding the other pre-amp output, before the coupling capacitor, that this will give a bumpless changeover. Am I correct in my assumption? If not what is the best way of switching channels?

Thanks

Martin
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rjgtr
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Post by rjgtr »

The EF86 is a little more complex sounding than a 12ax7 based channel and has a ton of drive. It's not distorted per se, but it will push the PI hard when turned up.

Your switching method will work. You can get a good relay circuit from Hoffman or go to my gallery and check out the SIII One. It doesn't switch channels, but the relay versions's circuit is reliable and easy to implement. Also check out some of the channel switching threads for other ideas like Richie's use of a SS relay for switching.
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wiseowl
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Post by wiseowl »

Thanks for the reply, I've ordered one today. What the hell it's a lot cheaper than buying an amp and if I don't like it I can always redesign that channel.

Has anyone tried using a 4016(quad analog switch)to do the switching?

Regards

Martin
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katopan
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Post by katopan »

Never used a 4016 in a valve amp, but extensively in a mixer I designed many years ago. Had digitally controlled channel switching with a couple of different versions of the CMOS analogue switching chips. From the datasheet the max input range is -0.5V to 0.5V above supply. With a signal referenced to ground that means you'll need a split +/- supply, and it can only go up to 15Vdc total (+/- 7.5Vdc). Set up like that, I'm pretty sure when you turn the active channel up so the signal is above 8V peak you'll blow the chip even though that CMOS switch is off. You'd be better using a relay or I'm a big fan of LDR's - they switch slower for pop free operation, have no switch contact bounce, use the same control circuitry and use less power. There's been a number of threads fairly recently on channel switching. If you get your switching supply from the heater secondary, just be careful not to earth things at more than one point in the circuit, and keep in mind that the switching control circuit is happy to float and doesn't have to be directly referenced to ground. All depends how your heater circuit is set up (CT to gnd, CT to gnd via 100r pair, or elevated).

My amp is an 18W with normal and EF86 channels. I'm going through a phase of loving the normal channel at the moment, but not long ago the EF86 was the favourite. They are very different. The EF86 has a different tone and drives the output valves harder (some say too hard but I disagree). Cranked I compare them as the normal is classic rock & blues, the EF86 is heavier rock. Clean to a bit dirty the normal is dark and smooth, the EF86 clear with tone set to the full range end and light and chimey at the bright end.
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wiseowl
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Post by wiseowl »

katopan wrote: Set up like that, I'm pretty sure when you turn the active channel up so the signal is above 8V peak you'll blow the chip even though that CMOS switch is off.
Good point, I was only considering the status of the "off" channel when the idea struck about using one of these.

Cheers.

Martin
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