Q./ How is a triode/pentode switch best implemented on a 6v6? A/. Easy- just use a SPDT switch. Connect the common to the screen, the NC contact to the normal screen supply, and the NO contact to the plate. Note: when you flip it the screen supply will be briefy cut, causing a pop in the speaker. To eliminate this, use a make-before-break switch. Klunk-klick every trip! Q/ Has anyone tried parallel feed (a.k.a. parafeed) output on a guitar amplifier? I have the appropriate choke and cap and I was planning to use a 70V line matching transformer as a versatile OPT or maybe just a small 5K to 8ohm SE OPT. A/ Parafeed outputs are almost never seen, because they require two pieces of iron instead of one, meaning more money. But there's absolutely no technical reason why you shouldn't. You have a wider choice of OPT- you don't have to use one made for SE because there is no DC on the primary. Q Would it be possible to run the filmanets to each tube (only 2 in this design) in parallel and employ a hum balance pit for each tube? Does this defeat the effect of a hum balance pot? A "They use a single hum-balance pot that balances the filament supply for all the tubes at once. The hum usually gets in through the preamp tubes, though. The signal in the power tubes is big enough to swamp it." Given that power supply mods are not a good hum removal solution in tweed type circuits, filament hum balance might be a really good thing jsn A/2 Unless you have multiple separate filament windings on your PT, you can only have one hum balance pot. I found that one pot works fine and gives a good null, even in an amp with 6 tubes. But, if it really bothers you, why not run your filaments off filtered DC? Having said that, though, I think you might be overestimating the role of hum from the heaters. It's really only an issue in high-gain rigs like Boogies, 5150s, and phono preamps. The Champ circuit is not high-gain, but it IS very sensitive to hum on the output stage B+, especially in triode mode. So I think your efforts would be better spent getting a well-filtered B+. Why not put a big choke in and take your output stage B+ off the second filter cap, after the choke. (Who said power supply mods were no good?) Steve C. Maybe disconnect the screen on the 6V6, unsolder the wire going to 6V6 pin 4, tape up the free end, and wire the 6V6 triode mode, 470 ohm resistor from pin 4 to pin 3 (plate). It'll cut power down to about 1 or 2 watts, I think. I'm putting in a switch to do this on a Champ clone I'm building right now. You could hard wire it and not have to drill or change anything. http://test.angela.com/catalog/guitar-amp-parts/fender_faceplates.html They have champ faceplates that were factory punched for the larger switches. Save the old one, and pick up one of these if you feel like modding. Cheers! Rob W. ;-) Hey Ted, you don't need a 5 watt 1K resistor there. You can use a 1 or 2 watt 1K resistor. The voltage is pretty low after the first dropping resistor so you don't really even need the thing anyhow. It will not hurt to intall it though. I'd stay away from a 5AR4 too unless you want to use one of those blue 6L6GCs later. If you have not removed the 25uf bypass cap on V1b's K and you converted back to the 22K FB resistor, you have NO AC FB! That cap is sucking the NFB voltage right to ground. I've not had a squeal problem like that but I'd start looking at a wire dressing issue. Also, those 7025STRs have more gain then quite afew other 12AX7s so try a different one and reduce the first cathode bypass cap down to about 2.2uF. Use a little 47pF to 68pF cap across the volume pot (top to wiper) to sweeten up the highs. Bruce Anyone used a Radio Shack 40-1242E speaker fo a guitar amp? They are extremely inexpensive and sound suprizingly good jumped into my Princeton clone and not mounted to the baffle. Of course it breaks up very early, like around 12 o-clock on the volume, but it should be very nice for 2W output. specs are 8ohm/2.5W-RMS/5W-max/6" diameter/pleated cloth surround/dimpled paper cone/ceramic magnet.