Help with 6L6 ultra linear with ef86

[walkman]

Hi

I'm modifying a cathode biased 6l6 UL PA to be more guitar friendly.

So far I have reduced the coupling caps to the 6l6's to 0.022 from 0.1, changed the PI from a DC coupled long-pair being driven by an ef86 to a Ac30 style PI. It runs off 353v with 100k droppig resistors and about 244v on the plate with a 12ax7. Can anyone suggest a suitable grid resistor, currently 330k?

Also I removed a dropping resistor prior to the first cap can to raise the B+ to the UL OT to 430v. The Cathodes of the power tubes are at approx 33v , with a 470R cathode resistor on each tube,by passed with 50uf. Is this bias ok , will it also suit el34's? What will be the output power?

There is a remaining ef86 running off 275v as the preamp ,and a spare spot for a tube, where the second ef86 was. I'd like to ad a 12ax7 as either a second channel, with single, parallel and series options, and / or use a triode as a gain stage to drive the ef86 into overdrive like the Matchless Clubman.

It has 4 input jacks , so i'd like to go from clean to warm to crunch to lead using each jack (and a A/B/C/D pedal or relays if not too tricky )

Currently the tone controls are a passive Baxendall i'm planning to change it to a route 66 stack to give it a more useful range.

Any suggestions of which way to go or experiences you'd like to share.

this is the original schematic, without the modifications for the Linear Concord 30

[Steve Conner]

Well, if you want my $.02 worth: Plug guitar into it, see how it sounds, modify to taste.

You don't really need multiple channels, you can control the amount of crunch with the volume control on your guitar.

If it was a Linear Conchord, it was probably based on the Mullard 5-20 circuit, and should have had EL34s in it originally. So yeah, it will suit EL34s.

Mullard 5-20 Audio Hi-Fi Amplifier

The 5-20 has loads of negative feedback, and for guitar you want to get rid of most of that. Changing the PI to AC30 style was probably the right thing to do, you could also try changing from UL back to pentode mode.

[walkman]

Hi Steve, its badged as a Northcourt 30 the same as the concord.

I looked up the mullard , a leak tl25Plus and the eico hf 20 which seems the most similar for comparison

I removed the negative feedback prior to doing the other changes and it did overdrive low on the dial.

Now without the second ef86 it is clean all the way round the dial with single coils unless i really dig in. The clean sounds nice, though i would like to get a little more chime out of it.

[Steve Conner]

Ok... I don't think the EF86 overdrives as well as the double triode type tubes. The screen bypass cap causes all kinds of weird pumping effects.

In one amp I built, I used the EF86 as a first stage, wired as a treble booster type circuit. Then I fed that, via a gain pot, into the input of an ordinary double triode front end like you'd find in a Fender.

So the EF86 itself isn't overdriving, it just acts as a built-in "boost pedal". There's a switch to convert the treble boost to a full-range boost, but that gets out of control pretty quick with humbuckers. I guess overall it's similar to the AC30 Top Boost channel, or a JTM45 with a treble boost pedal.

If you want to overdrive an EF86, probably best to triode connect it.

[jrfrond]

I gotta tell you the truth, I like the way it looks on paper the way it is. The only REAL issues I see with dual EF86's might be noise and reliability, but I am a freak for EF86 preamps and my own designs are around an EF86.

If you want to get that baby to crunch, try installing a post-PI MV control, using a dual 250KA pot to replace the 330K bias divider resistors. Keep the reduced value coupling caps you already installed. Of course, this is all subjective, but if it were in MY hands, that's where I would go first. One of the reasons is that you have a whole lot of gain up-front, and you can get the PI to overdrive nicely.

[chocopower]

Hi!

Casuality, i´m working in somethig really similar.

Amp is working ans stable. I just need to test the EF86 "clean/overdrive" swicht. each resistor cobination you get with the swicht are tested by it own, but not in 2 position swicht. As soon i did it i´ll back with more information.

GAŅAN II