What do do with this iron

[guitar-rocker]

Hello all, I recently bought a Wurlitzer 7020 amp off of fleabay, primarily just for the bumblebee caps in it. I does have some massive iron in it, that powered a pair of 6L6's, and was wondering if anyone had an idea of a good build with it. I was thinking of a standard 18w TMB preamp, but wasn't sure if that's the way to go. Any help would be appreciated. I've built A Hammond AO-35 conversion 18w, and a Hammond AO-43 conversion off a GDS S/C chassis that's ready to fire up this weekend, but the 6L6's are a new thing for me. Looks like the PT 18454, and the OT 18451 could easily be rated 35-40watts, which is why I posted here in this forum, instead of the 18w forum. Thanks. Terry

[guitar-rocker]

I realize that I could go the 5F6 route, but I really just wanted more of the Marshall 18W / 36w tone.

[CurtissRobin]

It's a cool find but given that the foundation of the 18 watt tone is in the output tubes you wouldn't get anything like that sound from 6L6's. Maybe the guys hanging out at ppwatt.com could give you some direction on that OT.

KennyO
who pulled two tubes from a Twin clone using Dual Showman iron and pretends it's a blackface Pro ... but that's another board and it doesn't get much use anyway since I've got my 18's

[guitar-rocker]

Thanks CR. I didn't literally expect to get 18w sound from this project, and have a total of 5, 18watters, already in stages of builds. I just wanted to check out the tones from the 6L6's, but without the squeeky clean front end of a F*nd*r type amp. I had a Carvin a while back that I had the local amp tech at "Uncle Alberts" modify for me with the Marshall type preamp wiring, and it was stellar, but sold it off. That was before I was brave enough to try amp building myself. I though since I had the surplus iron, I'd try that scheme myself. I do own a Carvin Belaire for when I want crystal clear tones, but that wasn't the idea here. But thank you for the tip.

[CurtissRobin]

The typical 18 watt preamp is one stage of voltage amplification ahead of the PI. There's not much gain or "gain" to be had there so the preamp contributes only a little to the characteristic 18 watt sound.*

Wiring two triodes in parallel as in a Lite IIb enhances (enriches, fattens, thickens) the sound but contributes precious little to either kind of gain.

Cascading two or more stages (as in the Plexi 18 watt or the TMB version) increases both gain and "gain" and contributes significantly to coloring the sound in the way I gather you want. Its sound would diverge bigtime from the 18 watt sound simply because it's shaped in the preamp stages instead of in an EL84 power amp stage. I'd say it'd be a very worthwhile project to put a Plexi preamp in front of a 6L6 power amp. (Plexi heads modded to take 6L6's are, or at least were, not rare.)

KennyO

* By gain I imply the voltage gain engineers refer to and by "gain" I imply the sound and sustain characteristics guitarists are looking for.

[zaphod_phil]

An 18W TMB amp, but with 6L6s in the power stage could be a pretty sweet amp. If you keep the voltages down at typically 18Wish levels (eg 350V) and stay with cathode bias, they will break up nicely too.

[guitar-rocker]

So... if i'm looking in the downloads at the 18w plexi for a cut and paste, is the AC output on the PT shownin the scematic around a 290-0-290? I have yet to install a new cord to power up the PT for voltage checks, as I'm still trying to trace some wiring. This amp came with a 5u4 rectifier, and I was wondering where to do a voltage reduction to get it down more in the range that Phil has suggested. Also, as I'm tracing the wiring out (came without tubes) it looks like the screen resistor to the 6L6's is a 5 watt 390 ohm paralled to pins 4. Does that sound right? Coming from the PI, it looks like the decoupling cap was maybe a 100nf, on through to a 1500 ohm resistor, if I'm reading the color bands right. Lastly to get more of the tone we are talking about, would I want to stay with the 18w cathode setup of a 150R/5W and a 100uf? Thanks for any input. Terry

[chabby]

I've worked with similar organ amps set up like that and my guess is you may not need to reduce voltage for 6L6's. Or, you could do it after the first filter cap with resistors like on every SF and earlier Fender amp.
But if you do need or wat to lower voltage, you can wire in a Zener diode or two between the recitifer and first cap, right after the first cap or as some might say the PT's CT.

But some theorize it's more protective to avoid doing it at the CT and just do it on the power rail. My first organ amp ended up with a couple Zeners so i could lower voltage for an EL84 18 watt which I later changed to a tweed deluxe type circuit with 6V6's it was more suited for(and junked the zeners). You could build a very cool Plexi type, Lead Master or JCM 800 with it too. I just finished converting my old 18 watt TMB to old early JCM800 Lead Master config with amazing results. Of course I tweaked it to have AC/DC balls and slightly less gain. But it's the coolest little amp I've ever had now and sounds exactly like its 50 Watt older brother, but not quite as loud and no EL34's.

[zaphod_phil]

You don't have to reduce the voltage. It just depends if you want an earlier breakup or not. Otherwise the problem with 6L6s is that they tend to stay very clean until pushed to the limit. 6V6s break up sooner, but you would need to trick the OT, by running an 8 ohm speaker load on the 4 ohm output or a 16 ohm load on the 8 ohm output, to get a more suitable load impedance on them. Very likely you would need to knock down some voltage as well, due to the decreased load of 6V6s vs 6L6s. EL34s would also work nicely in this kind of an amp and give a great Marshall vibe.