How to convert and 18w EL84 into a dual 6L6

[paintballnsk]

Hey guys,

So a few years back, you all helped me get into amp modding. I've modded this wonderful little 18w into something pretty awesome.

Now I'd like to learn how to convert it to a 6L6. Would someone please point me in the right direction on how to do that?

Here's a link to my latest recording with it. I've attached the original schematics and the mods I made as well.

https://soundcloud.com/crossfirerecording/stormofparticles

Thanks!

[tubeswell]

That amp sounds great already.


Why not leave it as is, and build another amp for 6L6s?

[paintballnsk]

It's as much a learning experience as anything. The amp sounds good recorded because, well it just does. R121 SM57 on a splawn cab. I recorded with a TS808 in front of it. I had to do almost nothing to the guitars in the mix to get them to sing.

But, it still doesn't FEEL great to play. It doesn't have that nice plucky attack that a 6L6 amp has. Like for instance, my two favorite amps are the Splawn Quickrod and the Peavey 6505. There's something special about their FEEL that's really important to me. I have a Mark IV too, and though I can get it to sound great, it feels like crap on anything other than leads. Does that make any sense? I want to learn how to capture that feel with high gain, but I don't think it can happen with EL84's.

Also, I can't seem to find a 6L6 tube amp kit I like. If you have a suggestion, I'd love to hear it. Most people building Fender clones aren't looking for metal. And I also dont know how to convert a 50w marshall kit to 6L6. If I could find a Soldano Hot Rod 50 kit or a 6505 kit, I'd be all over it.

[tubeswell]

Well, there's the PT VA rating to consider (more power handling is required for a 6L6 PT):

• an EL84 filament uses 760mA, a 6L6 filament uses 900mA
• an EL84 likes a plate voltage between about 250 and 320V, a 6L6 likes a B+ between about 390 and 500V
• The High Tension winding for a pair of EL84s wants a current rating about 100mA, a pair of 6L6s wants a current rating about 200mA
• The required range of bias voltages is also different - higher in a 6L6, and so it requires more driving voltage to get the 6L6 grid to clip and drive a pair of 6L6s (for Class AB1) - which comes down to a higher supply voltage for the PI, and higher driving current  - blackface and silverface Fender amps with 6L6s typically have a 12AT7 in the PI slot

Then there's the OT (more power handling is required for a 6L6 OT), and the reflected load is different - an EL84 likes between 7k to 9k reflected load, a 6L6 likes about 4k to 5k

[Latole]

Short answer ; 6L6 will work and give clean tone 'cause 6L6 won't work very hard.
If heater voltage is 6.1 to 6.3 volts, PT is enough powerfull

[pdf64]

Well, there's the PT VA rating to consider (more power handling is required for a 6L6 PT)...The High Tension winding for a pair of EL84s wants a current rating about 100mA, a pair of 6L6s wants a current rating about 200mA ...Then there's the OT (more power handling is required for a 6L6 OT), and the reflected load is different - an EL84 likes between 7k to 9k reflected load, a 6L6 likes about 4k to 5k

Please can we avoid using terminology such as 'tube type x likes...' some or other HT voltage or load impedance, as it might lead to confusion and muddled thinking?
Rather circuits are designed according to the goals we wish to accomplish and from that, the limits of the tubes selected.
Given suitable heater current, bias and sockets, 6L6 will work just fine in the circuit below.
With cathode bias and the same conduction angle, they would put out a bit less power (due to the necessarily increased cathode voltage reducing the HT available for signal swing), but as yet, the desired power output has not been advised on.

[tubenit]

Like Tubeswell stated, the PT rating would be a VERY significant concern in my thinking switching to 6L6's. 

Can you post the rating for your PT, please?  Without you providing that information, it's difficult to give you a solid answer.

Have you considered switching to JJ 6V6's?  They are "sort of" 6L6-like tone wise, IMO.  And since your PT can handle EL84's, it would be able to handle the JJ 6V6's.

With respect, Tubenit

[paintballnsk]

Like Tubeswell stated, the PT rating would be a VERY significant concern in my thinking switching to 6L6's. 

Can you post the rating for your PT, please?  Without you providing that information, it's difficult to give you a solid answer.

Have you considered switching to JJ 6V6's?  They are "sort of" 6L6-like tone wise, IMO.  And since your PT can handle EL84's, it would be able to handle the JJ 6V6's.

With respect, Tubenit

Maybe 6V6 is a good starting point if I can use the same power transformer.

[HotBluePlates]

... circuits are designed according to the goals we wish to accomplish and from that, the limits of the tubes selected.
Given suitable heater current, bias and sockets, 6L6 will work just fine in the circuit ... they would put out a bit less power ...

... Maybe 6V6 is a good starting point if I can use the same power transformer.

EL84 draws 760mA of heater current per tube; 6L6 draws 900mA of heater current per tube.  As long as your power transformer can supply an extra 280mA on the 6.3v winding, you can use 6L6s in your amp (obviously with octal sockets).

What pdf64 is saying is if you make no other changes (aside from cathode resistor value, to obtain the correct bias), you will get "EL84 performance" from your pair of 6L6s.

We hope the B+ doesn't sag too much from extra idle current, though nothing says you actually have to idle the 6L6s at a current higher than you had with EL84s.  That is, you don't need to idle the 6L6s at 100% of their dissipation rating (and therefore higher plate current than the EL84s).  Also, the existing OT primary impedance will tend to prevent the 6L6s from drawing much more dynamic plate current than the EL84s did.

A drawback will be the larger bias voltage needed for the 6L6s, which also implies they will need a larger drive signal for same-power/same-distortion.  Without really investigating, I would assume the long-tail inverter has a high enough B+ voltage to be able to deliver the larger output (though that assumption should be tested).

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If you want "6L6 performance" from the amp, you're really looking to get a larger power transformer, larger output transformer (with a lower primary impedance to allow higher dynamic plate current), and possibly some tweaking to your B+ voltage and phase inverter (including the amount of feedback).

These changes could lead you to want some other preamp changes as well, but that's best figured out after the power supply & output sections are in their final form.