Blues Jr. EL84 to 6L6 tube mod

[Middleman]

Thinking about making this mod over the weekend but thought I would check with any conversion experts first. Here are two Tube charts, the top is the 6L6 and the bottom is the EL84. Couple of questions.

1. Once I map the pin connections I assume that routing the wires from the PCB board will just be a matter of connecting the proper pins on the 6L6 that correspond to the EL84 as follows yes/no?

EL84 pin = 6L6 pin
1 N/C........1 N/C
2..............5
3..............8
4..............7
5..............2
6 N/C........6 N/C
7..............3
8 NC..........
9..............4

2. Once I swap the tube connector for the 6L6, wire it to the above pins, plug in the tube, is that it? Are there any other changes that need to be made i.e. resistors in line to pins or other connections?

I could not find other data regarding this mod, so I welcome any comments. (And I know how to drain the caps prior to the mod so don't need that reminder.

[BiggerJohn]

Hold on there, Hoss!

You is about to have a room fulla smoke. The 6L6 requires quite a bit more negative grid bias than an EL84, and a lot more drive voltage as well. Plus the optimal ploate to plate load impedance is way different. Lastly, the 6L6 sockets are bigger in diameter so you'd have to do a fair amount of metal cutting on the chassis. Where are all them metal flakes and chips gonna go? Into the guts of the amp?

You know about opinions, they're like, well, you know, everybody has one. But in my opinion, and with all due respect, if you don't know basic stuff like that, you really shouldn't be doing major surgery on an amp. Just trying to keep you safe.

You'd be much better off (again, a cheapo opinion o mine) getting a blank chassis from one of the usual suspects and trying to build from scratch, as opposed to chopping up a working amp. Think twice before you start what you are asking.

Hope I didn't ruffle no feathers.

[marshman]

Check out BillMs mad skills--he just converted a BJr to octals (5881/6V6GT) and said it works pretty well. I think he used stock trannies, but applied quite a few of his other basic hop-ups to get the tone where he likes it. Check it here:

http://billmaudio.com/wp/?page_id=353

--I stand corrected: I read the write-up for the monster BJr again and DID see that he upgraded the OT to a heyboer...my bad.

[JohnnyCrash]

EL84's are similar to 6V6's (still need octal sockets though), but as John said 6L6s require much more than just cutting/punching the chassis for octal sockets and rewiring.

You could leave the power transformer alone and run the plate voltages low - but you'd still likely not have enough current - so you'd need to change the power transformer afterall. You could change the board for more negative grid bias, but you'd still not have an impedance match with your output transformer.

This is messy... this wouldn't be a mod, it'd be a near rebuild.

More power, with 6L6's, for more bottom and volume? You're much better off leaving the BJr alone and building/buying an amp for this.

[Middleman]

Thanks for the warnings but that is not exactly what I was looking for (This is the DIY forum isn't it?)

If I was dropping metal shavings into the amp I would expect smoke....

The amp is already dissembled laying in wait. I am reviewing the Deluxe and a couple of other 6V6, 6L6 schematics to see what the the tubes are expecting in terms of performance.

Good catch on the output transformer at Bill's site. I will look into upgrading the output transformer.

Also, no warranties are being voided because I bought the amp used. So if we can refrain from nay saying and get to some recommendations, I would appreciate any comments or articles, links to important data. If anything significant fries this will become a Deluxe build by the way.

[JohnPurdy]

Middleman, please keep me posted on this mod. I'm very interested in hearing how it comes out. Maybe you could do some before and after sound clips? I'd like to hear the difference. John

[Cleeve]

If it were me I'd tie the 6L6 cathodes together and let them share a common resistor to ground for cathode biasing. The bias voltage for the EL84 circuit is going to be wrong anyway, so you might want to experiment with the glorious simplicity of cathode bias. Or use a combination of the existing fixed bias and cathode bias, which may be even easier.
To get more negative voltage for a proper 6L6 direct bias supply would involve a voltage doubler etc..

The 6L6 pair with cathode bias might not be as clean as a direct biased pair would be, but the output transformer should be the limiting factor here, so having the tubes distort sooner might be preferred. But mostly it's easy!

Heck go full boat and put in a triode-pentode switch too!

I repaired an old Alamo amp that used a pair of 6L6 as the output using a shared cathode resistor with no bypass cap and it was pure soul, despite a simple transistor preamp. Not many watts, but it had a poo speaker anyway, sounded great!

[JohnnyCrash]

Sure this is the DIY forum, but not every technical concept is possible or probable. Not trying to rain on your parade - but there is a great deal of rebuilding that will be required in addition to replacing both transformers (and then rewiring the entire power section). Sometimes if you ask for advice, you won't get the answer you want... and while this is not completely impossible, it is a huge technical mess and new transformers will be expensive. I really wasn't trying to be negative or discourage you - but this is a headache on many levels (technical, logistical, physical).

If you're still intent on doing this:

Don't worry about metal shavings. That's not big deal. You may want to invest in a die punch. 1" for octal sockets. Here's a set:

http://www.harborfreight.com/cpi/cta...emnumber=91201

No metal shavings, and an easy way to punch the tube sockets.

If new transformers won't fit you may have to find a clean way to cut the chassis PT cutout.

After this physical work is done, you'll may have to mess with the negative bias supply. I would NOT go with a cathode bias scheme on this with 6L6's. I figure the whole point of your rebuild is to get the power and bottom end of the 6L6 tube.

The phase inverter circuit will likely be fine as is - though depending on how the amp sounds after the transformers are replaced, the power section is rewired, the bias is configured for 6L6's, and the right OT is used - you may have too much bottom end. You could tweak the input capacitor value on the PI or play with the preamp circuit.

I thoroughly disliked my BJr and sold it - they have some things that are drawbacks. The solid state driven reverb is one (nasty reverb). It is likely impossible to upgrade for a tube driven reverb (even with adding another noval socket and upgraded PT). The circuit will require room for its parts, the recovery circuit my need overhauling, and there are a few other huge speedbumps there.

Also, many of the coupling caps are very small values (likely to keep the overdrive from sounding tubby). This may limit the bottom end for 6L6's... I suspect it was one of the reasons I hated the amp.

While you're in there you may consider dumping the Master Volume control and installing a dual-pot Post Phase Inverter Master Volume. They usually sound better (depends on the amp/circuit). This might also allow for replacing those small coupling caps for bigger ones to improve bottom end and midrange fatness (if the small ones were to control overdriven woof).

--

BE SAFE

Also, looking at your first post it looks like you're either new to this, or have never been around the inside of a tube amp (you can't simply rewire tube sockets for a conversion).

If not...

There are dangerous voltages stored in the amp (via its filter capacitors) that are still there even when the amp is turned off and/or unplugged.

Be safe. Read up on amp safety, discharge the capacitors, and don't poke around in the amp until the caps have all been discharged.

Hopefully this helped better than my original response.

[Middleman]

JohnnyCrash thanks for the tips.

JohnPurdy will keep this thread updated.

Here is what I have found so far..

The ouput network between the 12AX7s and the 6L6s is very much the same across several Fender Amp schematics like the Blues Junior, despite the different tubes. There are variations but I did find the exact same resistor values and capacitor values and the same connections to tube pins. For example.

Between the 12AX7A and the EL84 on the Blues Junior you have this.

[Middleman]

Here is another schematic which is the Blues Deluxe and uses the 6L6GC.

[Middleman]

Although there are some structure similarities, the values for some of the caps and resistors are different the closer you get to the 12AX7As on the left. But the connections immediately around the 6L6 and the EL84 are exactly the same cap and resistor values. The pin connections as pointed out above are different as well but once you have these mapped there seems to be little need for changes immediately around the tubes.

Other differences are the 30.7VAC coming out of the 6L6 and the 41.3VAC coming out of the EL84 in the BJ. Additionally the the voltages on the BJ appear to be coming in at a lower VAC on the BJ vs. the BD. What does this all mean? No clue yet.

I do know that the 6L6s are going to need a higher output transformer from the Billm site but I need to do some more investigation to see if the Power transformer also needs an upgrade. The Billm site seems to indicate this is not necessary but I want to make sure. I am hoping this is only going to be the tube socket swap, new tubes and possibly no cap or resistor value changes but with a definite OT swap. That may be too optimistic however. Later.

[JohnnyCrash]

What you're showing there is the phase Inverter Circuit (V3 A and B is the PI tube). C- is the negative bias feed supply from the power transformer... this coincidentally is were I'd put in the dual 250k PPIMV control (replacing the 220k resistors with 2meg and putting them across the dual 250k pot).

The 1.5k (1,500 ohm) grid resistors will be fine on either EL84's or 6L6's. The PI circuit will likely be fine as well.

Lower plate voltages on any tube generally mean less output and more warmth/overdrive. If you swap out a bigger/better power transformer (not just the output transformer) you can increase the plate voltages to the 6L6's plates and increase bottom end and headroom. As mentioned earlier, you may have to tweak the "C-" negative bias circuit to bias up the 6L6's properly... unfortunately increasing the PT may mean replacing all of your B+ voltage rail resistors to give the preamp tubes their proper voltages.

A PT swap "necessary?" Not technically, but you should 100% do it. If you went to 6V6's I'd probably leave the stock PT - but going to 6L6's I'd aim for what the "mod" is meant for... 6L6 bottom, headroom, and volume.

In the end the PI and power section may resemble a 30-40 watt BF style Fender, but the BJr preamp is still a weak link in the chain. If you like the overall character of the BJr, then maybe the preamp isn't such a weak link for your tastes, but I see a few areas I'd improve... after all is said and done (6L6 power section and output, overhauling the preamp) you might be better off building a BF Super, Vibrolux, or Pro kit... again, not trying to discourage HAHA :)

Get a calculator and brush up on OHM's law for refiguring what resistor values to use in your preamp's B+ rail when you swap out the PT.

[Cleeve]

Um never mind.. I was just gonna suggest tacking on extension wires to octal sockets for bench testing before drilling etc..
but that could be a hazard with such voltages present.

[Middleman]

JohnnyCrash, thanks again. Your comments on the 6V6 vs 6L6 are something to think about. Not sure I want to get in to a major mod for the PT because, I agree, I could probably just move to a custom built. I have a 81 Vibrolux already and was not looking for that tone.

The goal was to move towards a Deluxe sound but in the smaller package of the BJ. That would take some more consideration because of that circuit here.

[JohnnyCrash]

Originally Posted by Middleman

JohnnyCrash, thanks again. Your comments on the 6V6 vs 6L6 are something to think about. Not sure I want to get in to a major mod for the PT because, I agree, I could probably just move to a custom built. I have a 81 Vibrolux already and was not looking for that tone.

The goal was to move towards a Deluxe sound but in the smaller package of the BJ. That would take some more consideration because of that circuit here.

That looks like a tweed Deluxe. Are you aiming for the tweed Deluxe sound or the Deluxe Reverb sound (2 completely different sounds)?

In any case, going with a tweed, cathode biased Deluxe power section won't get you a complete tweed Deluxe sound. The cathodyne phase inverter is much different than the BJr's long tailed pair PI circuit. Also, there is a great deal of tweed Deluxe character in the preamp that just will not be there using a BJr preamp. Most of your photos and ideas are focusing on the power section - but unfortunately, you have to view the preamp as part of the whole as well.

Running a cathode biased 6V6 power section with a BJr preamp in front may require preamp tweaking/reworking to "fix" some things that may pop up in the conversion (depending on your tastes). It won't sound like a 5E3 tweed Deluxe, but it may make it more "bluesy" - depending on your tastes, this could be a bad "bluesy." For instance, you will lose tightness in bass response. You will get a warmer, lower output grit though...

I guess it depends on what you're aiming for - I assumed you wanted the opposite of a loose, low output tweed Deluxe - I thought you wanted big clear 6L6 power.

[Middleman]

Work slowed down my effort this week but here is my next step. I am going to plug the following amps into the BJ cabinet tonight. This is to get an idea of how the single 12 inch speaker in the small cabinet responds. Fortunately I have the following amps at my disposal.

Vibrolux (6V6)
BDRI (6L6)
5E3 (6V6)

I want to get an idea of what exactly tonally I might be stepping into by the various tube sets. If the honking midrange tone of the BJ does not change all that much, I may move off this conversion. If I hear something I like, I will probably mod in that direction.

[Middleman]

After the testing above I came to the following conclusions.

1. The honkiness of the BJr is primarily its tone stack. Using the amps above, that midrange issue is, well, not an issue. (I do have a work around )

2. I really liked the 6L6 sound through the BJ cab & speaker but (and with a nod to BiggerJohn for pointing out the challenges to 6L6s) the modifications would be significant. It's a big blooming tight low end and it sounds really good through the small BJ cabinet.

3. The 6V6 had less bass and was cleaner overall. I could go either way on the 6L6 or 6V6 (actually I want them both) but in the end the 6V6, other than the socket modifications, looks straight forward with the precaution of adding the Billm bias mod first, to be able to adjust the bias. Also the output transformer will be an addition.

Blues Junior work around.... The thing the BJ Junior needs is volume. If you run the Middle on 0, the Bass on 7, Treble on 6 and keep the fat switch engaged. You can crank the master to 5 and the channel to 6 and come very close to the 6V6 sound. Mine has the twin stack mod and with these settings, it's pretty close. The bad rap this amp gets is due to the lower volume sound which is not pleasing and very boxy sounding. At higher volumes however, this amp holds up next most of the others above and the tone changes to be less boxy and more full.

Overall, this leads me to not want to mess with modding the power supply tubes. Still mulling this one over.

[jh45gun]

Johnny thanks for posting the link for the knock out punches what size would work for the 9 pin sockets

[JohnnyCrash]

Originally Posted by jh45gun

Johnny thanks for posting the link for the knock out punches what size would work for the 9 pin sockets

I'm not sure. That set in the link was slightly too large for MOST of the noval sockets. Now I just use a step-bit for preamp sockets - works great on aluminum or steel chassis and has no burs.

Come to think of it, if somebody can just find the one size for octals - that's probably all you'd need - an octal punch and a step-bit.

[Cleeve]

Originally Posted by JohnnyCrash

I'm not sure. That set in the link was slightly too large for MOST of the noval sockets. Now I just use a step-bit for preamp sockets - works great on aluminum or steel chassis and has no burs.

Come to think of it, if somebody can just find the one size for octals - that's probably all you'd need - an octal punch and a step-bit.

That's the rig I have, a step drill and one punch for the octals.
At Harbor Freight, I saw a step drill big enough to do octals too, also I saw one at Lowes, but it was pretty expensive, being a proper Greenlie brand or whatever. I may stick with using a punch for octal, since the forces on the drill and the steel get bad enough with the smaller 9 pin hole, the octal step drill would probably kill me if something got loose.