Adding an OT and tubes to make amp stereo

[Guitarist]

I'm sure someone has thought of this already. In this case I have a Hiwatt 50 with the MASSIVE power transformer which I think might handle two more output tubes and an extra driver tube. If the B+ voltage were to drop that would be OK with me as long as the heaters stay up. I could switch to 6V6 or 6L6 tubes if I HAVE to. If this power transformer was the same as the 100 Watt model it would be a go right? I think I've added about 700uF of first stage filtering prior.

Question: IF both output tubes get their B+ from the same point (center taps) would there be a lot if interaction between L and R channels? Or do the wings of the OT primary windings keep them seperate enough? Anything else to consider?

I am so happy today as I bought a 1982a Marshall cab with the vintage grill cloth used. Earlier bought a Lexicon MX400 reverb/delay unit as well.
I'm really hyped. So I've got a beautiful array of gear (OK also a midi patchbay for pedals) so if I can sync it all up....look out!

[imaradiostar]

I was searching for something else and thought I'd reply because I like your post. In about '98-99 I was really big into the Eric Johnson sound so I built a stereo Fenderish amp with a Fender Twin power transformer and a pair of new sensor universal 50 watt output transformers on one huge aluminum chassis stolen from a solid state Randall bass amp of unknown vintage. I built a solid pine cabinet with finger jointed corners and stained it dark brown. I use a gold door kick plate for the faceplate and gold fender style grill cloth from some internet vendor. I had two old speakers (Eminence, I think) from some old open back Crate cabinets. The speakers have HUGE magnets and the output transformers are almost as large as the power transformer so the amp weighs around 85-90 pounds and is taller and wider than a Fender Twin.

I used dual-gang pots and have knobs for volume, treble, mid, bass, verb drive, verb tone, verb level, and presence. I had a pair of small single-ended output TF's from some old hi-fi amp that I used as reverb drivers. I used two small tubes (6bm8?) with a pentode and a triode for reverb drive and recovery. When this turned out to not work well at all I added a single 12ax7 and used a triode per side to add reverb gain and lots of noise and hum. I also made little optoisolators using LDR's and LEDs wired into the reverb circuit to kill the reverb remotely using a signal from the dedicated home-made amp switcher I was using at the time. I was damn proud of myself.

All in all it's very loud and sounds mostly like a fender twin or overgrown deluxe (due to cap values- I prefer the softer upper midrange of a smaller and permanently wired bright cap). With the addition of my tc electronic chorus, a memory man delay and my strat and it ends up sounding a LOT like Eric Johnson's clean tone on Ah Via or Venus Isle.

Now that I'm older, wiser and lazier I really need to tear into it and change a few things. The output transformers have got to go- they're freaking ridiculously heavy. The speakers are tired but still sound OK, could probably go with some lighter weight vintage US style speakers. The power and grounding scheme is pretty poorly wired and really should be re-done in a more professional way. Last but not least I need to make the reverb sound better and make it quieter- I'd imagine this will envolve some sort of power mosfet driving a newish verb tank and a cleaner recovery circuit.

I'd say it was totally worth it but I definately wouldn't do this conversion to a Hiwatt! I have a friend with a really beat up old silverface twin that we converted into a stereo amp and used it to feed an old 4x12 with mojo vintage 30's. I just robbed the trem circuit tube (who uses trem?) and made it a phase inverter for the second channel, connected to the other preamp channel. It was a pretty simple conversion and other than adding a double bias scheme and dividing up the reverb drive 12at7 to feed two separate transformers and tanks it didn't require very much soldering. I think I may have modded the verb recovery so that it used less tubes and had more gain but it's all fuzzy now- need to open it up and check one day.

So...if you really want a stereo amp I'd start with a reissue twin or similar amp that already has 4 output tubes and two seperate preamp sections. A Super Reverb or other 50 watt Fender could be divided up to run a quad of high-power 6V6's instead of two 6L6's as long as there is room for adding two 8 pin sockets, tubes and deluxe type output transformers. I bet this would be easier to carry and sound prettier for most applications.

I also used to play through a small home stereo amp that I modded for guitar usage. I bought the amp with a blown power TF so I used what I had laying around. This put around 450 volts on the plates of 4 EL84's and I changed the cathode bias resistors so that the tubes wouldn't blow up or cook the output transformers. I used a few distortions, a delay and my TC chorus in front of it (as a buffer mostly) and really enjoyed the sound while I had it. I later took it apart after I built a real amp to play through. I do miss it though and I have plans to build something similar but more usable.

sorry for the long post, hope I'm not a buzz kill!

jamie