How to mod to flip between EL34 and 6L6 power tubes?

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Che_Guitarra

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I'm just about ready to buy a Ceriatone 2204 chassis and slowly build up an amp in my spare time.  It's based on the Marshall JCM800 50W.

Here's my likely starting schematic.

JCM800_2204Ceriatone.jpg




I have no intention of installing the 1/2 power switch in the rear of the chassis, so this leaves me with a hole I could farm out to another switching mod... leading me to the point of this thread:


Just curious what schematic mods I would need to employ to make it switchable between EL34s and 6L6s? 
 
You could put a switch there that changes the bias voltage, like mesa boogie does. You'd still probably need to check tube quiescent current every time you change tubes, unless you're using tubes you know will bias correctly.
 
Che_Guitarra said:
Here's my likely starting schematic.
That's not a schematic. That's a layout. There's not a lot of guitar amp discussion here but you might get more responses if you post a proper schem.
 
Why do you need a switch? There isn't a huge difference. In practice, maybe more difference between EL34s that between some EL34 and some 6L6.

Put in different bottles. Use existing Bias trim. Find some other solution to your holy problem.
 
I have EL34's in a '60's Fender Bassman that I totally rebuilt in 1997,  added a 500R/1K grid2 switch for changing between 6L6 and EL34.  Don't remember why, I think the Gerald Weber book at the time suggested this.  I didn't know much back then.  Not sure I do now either.  It appears to be of little value to do so. 


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Just watch out for the suppressor grid which is internally connected to the cathode in a 6L6. So theres nothing attached to pin one on a 6L6. On an EL34 the suppressor is brought out to pin one on the tube and is usually tied to pin 8 (cathode) on the tube socket.  So if you plan on using both types of tube, just make sure that pin 1 - pin 8 connection is made on the socket. 

Then turn out the lights and look at the tube plates. If they are not glowing red your good to go. In fact that's how I use to bias my amps, just make the plates glow a little red in a pitch black room, then back off a bit. ;)
 
On a Marshall 50w bias is around the -31 volts mark  for a plate voltage of 420 volts with EL34's ,on a Fender with 6L6's your down to around -45 with a similar plate voltage , one other thing to bear in mind on the older fenders is that some of them use the bias pot to balance the standing current in the output stage , where other models allow you to adjust the bias globally on both tubes. Theres a resistor to ground after the bias supply and smoothing caps usually 15 or 27k in the standard fender setup , looking at yours the layout its slightly different  , you could put in a switched resistor between  point D on the bias pot and ground,value AOT(adjusted on test) to switch between  the correct bias voltage for 6L6 or EL34 operation . Screen grid resistor  of 1k should be fine for both kinds of valve.
 
> I have EL34's in a '60's Fender Bassman

Marshall started by building the Bassman in the UK, using UK parts, including EL34 because it was cheaper there than 6L6/KT66.

You find various bias voltages, and a few commercial amps with a 34/L6 switch. But that Weber layout has a true bias pot, and convenient test jacks. Just change the bottles and check bias.
 
this is a common mod to guitar amps, main considerations are pins 1-8 as mentioned above, and heater current, EL34's will drag more juice,


what about the output transformer?  no problems, plate to plate resistance and current is about the same,

Fender preamps into EL34's sound better than a Marshall to my ears, but that will depend on your speakers and guitar and what type of music you play.

main worry is having bias set to EL34 tubes with 6L6 tubes in the sockets because the sw got flipped by accident,  so maybe some cathode resistors combined with fixed bias just in case,

just worked on an Ibanez tube amp that had the sw for EL to 6L6.

 
Thanks for the replies guys  8)

Funny that a 6L6ed Bassman should rear its head in this thread.  I was all set on building a 6G6B Blonde Bassman as my next project - wonderful cleans, with more than enough gritty gain on the far side of the dials for a rootsy rocker like me.  Also influenced by the fact I cut my teeth on Setzer songs and surf guitar riffs.

But a few weeks ago I plugged into a friend's JCM800, dialled back the gain, and it was a revelation - it was the perfect grit tone.  And as a package, a JCM800 clone is probably a better match for my playing scope and guitar collection.  I'm not sure how well the older 6G6B circuit will marry with my superstrats?  That is a 'must'.  Hmmm.

So I figure the best of both worlds is to build a JCM800, and design it off the bat to be usable with 6L6s.  Maybe I might end up with a SLO100 with only a 1/3rd of the gain, and that would be perfect. I'd go the other way around (build a 6G6B and make it usable with EL34 bottles), but it just seems the JCM800 would make a better foundation for my experimentation.
 
I made up a guitar amp on an old tube 100w P.A chassis ,I used a three position two pole toggle to connect either or both sets of output tubes cathodes to ground , I used 6L6's and EL34's and made a resistive voltage dropper to bias the EL34's properly (they sounded poor with 6L6 bias voltages) . There was a Boogie topology known as the simulclass, you could run both 6L6's and EL34's together ,heres the schem.
Slight derailment from original topic ,but might be worth a look all the same,

In reference to PRR's post ,yep the original Marshall JTM45 is almost indentical to the Bassman ,initially Marshall used mil surplus 5881's then KT66's but soon after settled on the EL34.
 

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Hi... sorry for chiming in here

If it is Brian Setzers sound you are chasing then it is pretty easy but still a bit troublesome.

Brian plays his Fender Bassman (6G6B blonde) cranked very high... big scenes and large crowds. That is pretty hard if you don't play the same events he does. And the most significant addition to his sound is the Roland space echo which also gives some extra juice to the sound... I have tried this for many years myself and had to get a "smaller" amp to crank.

And finally it is his playing style and technique that really is the core of his sound... and probably the hardest thing to copy :)

So to summarize it a bit... crank the amp and get the space echo (expensive and hard to find) and dial in a lot of treble...!

I believe that what makes the blonde Bassman different from the older bassman's is that the 6G6B circuit is using SS rectifier instead of the rectifying tube as in the older models of Fender amps... it will give you a more pronounced bass response when the amp is cranked. You mod in older fender amp circuits by replacing the rectifying tube för SS rectification... I have tried this on several amp builds. Fender then got back to using rectifying tubes in their blackface series.

My experience is that it all starts with how loud you can play on your events... and as CJ mentioned... what kind of music do you play, the guitars you use and more are more important than the specific output tubes that are in the amp.


Best regards

/John
 
I think my ideal amp is something that can dance from Setzer to Hendrix to Van Halen style riffery.  That's probably my niche.  But I play everything from the Beach Boys to Chet Atkins, from Radiohead to Pantera, FYI.  That said, I have my own voice on a guitar - my main desire would be an amp that nails the territory as defined in sentence #1 of this post.

I play two main types of guitar: strats/teles, 80s-style superstrats.  I've built (and still own) a Brownface Princeton and a Deluxe Reverb (as heads) in the past... both nail a certain spectrum of what I play, but leave me wanting in other areas.  Specifically, I want an amp that does better justice to my superstrats, without being a too-modern gain monster.  I like a gritty, clear, and articulate rock tone.

Cabinets:  I've built two 1x12 cabinets loaded with Weber 12A125s, and a 2x12 closed-back cab loaded with a Celestion V30 and Lynchback.
 
PRR said:
Why do you need a switch? There isn't a huge difference. In practice, maybe more difference between EL34s that between some EL34 and some 6L6.
+1. If one wants to use the potential of changing the character, there are several things that need attention: transformer primary impedance, bias voltage, pin 1 connection and heater current. I still have a chassis lying around, with a transformer with a 6kohm primary with taps at 4kohm.
 
here is the Ibanez Thermion TN120 bias switch scheme, it goes from 6L6 to 6550, but you could mod it to suit the EL34's,

you should notice a difference in sound between the 11,000 and 5,200 trans mu figures for the EL34 and 6L6
 

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If you were going to use a dual pole switch ,you could add a second preset pot too so that each set of tubes could be set up optimally without need to sub in a resistor of the correct value. The mismatch impedance wise doesn't really make a huge difference when it comes to guitar amplification ,selecting a different tap on the secondary could help mitigate it to some degree though. Quite a difference in tone between a 6L6 and an EL34 ,especially when cranked,modern Sovteks with the hard vacuum tend be loud and clean and never really brown out in the way the EL34 does.
 

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