18watt+18watt

[toyota_tim]

I have been considering building an amp with a single preamp and two 18watt power amps. This would make stereo effect possible in an effects loop. It seems that the main difference between that and a 36watt would be two OTs instead of one but it looks like someone is aready running a 36watt with two OTs. Many of you are already running 2x12 cabinets. Wouldn't it be nice to run them in stereo? Couldn't the outputs be bridged if you did want to run 36watts to one speaker or mono cab? Can anyone see any major disadvantages to this idea? I have an old knight stereo amp that I am planning to convert in this manner. I also have a pair of matching stromberg-carlson pa heads that I would like to do the same with.

[toyota_tim]

I started working on the old knight stereo amp. So far I have rewired one channel into a "lite" version just to see how it works. I left the ouput tubes wired the way they were and stripped out everything from the phase splitter back. It sounds great!! Now I am ready to wire up the second channel and I was thinking of the possibilities. Originally this amp had 6-12ax7s, 2 pairs of el84s and an ez81. My plan is to leave the ouput tubes for each channel wired the same with a switch in front of each one selecting which phase splitter it is driven from. This would let me run both 18watt channels off one phase splitter for 36w mono output, or each channel separate for stereo 18w. Each phase splitter will be different. I could do a similar switching after each preamp channel to determine which phase splitter it gets sent too.
Does anyone see any problems with this arrangement I might not have thought of or have any ideas on possible configurations?
I still haven't decided the best way to feed and return stereo effects. I am hoping that I can get only dry in one channel and only effect in the other channel for effects like delays and stereo chorus but be able to get a mix of dry and stereo effect in both channels for effects like reverb. I guess some of that will depend on my processor. What I don't want is the dry signal to ever have to pass through the processor.

[loverocker]

Hi there Tim Well, I like the wet/dry approach, too...

...but my guess is the overall success is going to depend on the type of FX you put in the loop. The thing with the 18Wer is that the amp's distortion happens in the PI and EL84. So in your 'wet' channel you'll be distorting the effected sound, which in my experience almost always sounds poor (esp for time-based FX like chorus, echo, reverb).

I think that only with a simple effect in the loop - single echoes, ADT, or non-time-based FX - would it sound 'right'. Of course if you stick to cleanish tones, the problem is way less severe.

Don't want to rain on your parade...

BTW: One EZ81 for all those valves!

[toyota_tim]

Hi there Tim Well, I like the wet/dry approach, too...

...but my guess is the overall success is going to depend on the type of FX you put in the loop. The thing with the 18Wer is that the amp's distortion happens in the PI and EL84. So in your 'wet' channel you'll be distorting the effected sound, which in my experience almost always sounds poor (esp for time-based FX like chorus, echo, reverb).

I think that only with a simple effect in the loop - single echoes, ADT, or non-time-based FX - would it sound 'right'. Of course if you stick to cleanish tones, the problem is way less severe.

Actually that is part of the reason for a stereo amp. Each amp channel will only have to distort one signal, dry or the echo, dry or the chorused. I think it is when you get both signals being distorted at the same time by the same PI and output tubes that strange things happen. That is why Brian May always used three amps when he played live. Only one got the dry signal. Each of the other 2 received one delay tap. This prevented the intermodulation like you get when you play certain cords with distortion. The sound was more like three guitars playing in harmony instead of one guitar playing three notes.

BTW: One EZ81 for all those valves!

Hey, thats the way the original amp was designed. I didn't touch the power supply or the output tubes. Actually I have two of these amps now. The one I modded was an as is one I just picked up off ebay for cheap. I'm still using the other to listen to music.

[loverocker]

I think it is when you get both signals being distorted at the same time by the same PI and output tubes that strange things happen. That is why Brian May always used three amps when he played live. Only one got the dry signal. Each of the other 2 received one delay tap. This prevented the intermodulation like you get when you play certain cords with distortion. The sound was more like three guitars playing in harmony instead of one guitar playing three notes.

Yeah, if the May approach feeds what is simply a once-delayed/octaved version of the dry signal into the other amps, it would work perfectly. What doesn't (in my experience) work is when you feed a 100% wet repeating echo or reverb signal into an amp that then distorts. It's that level of complexity that messes things up. For example, the distortion's compression alters the way that the echoes decay.

It's worth experimenting with, but I did that for ages before working out why FX loops were devised

But actually, there's another approach... instead of feeding the FX unit with the preamp signal, you can use a voltage divider to tap off part of the signal from the dry side's speaker output instead (like the Line Out on a Fender Super Champ). Then feed this (via a level pot) into your FX - set 100% wet - and feed the output of the FX to the PI input of your wet channel. Bearing in mind what I said above, IMHO you'd have to operate the wet side power amp in the clean domain, but that should work. It's a sort of stereo development of the way my Guytron is designed.