6L6 in Vibro Champ ?

[dude]

I noticed a 6L6 in a BF Vibro Champ I just bought...? I open up the amp and the cathode bias resistor was 1K ohms for the one 6L6 - was that the correct value for a single ended 6L6 cathode biased amp? I think it's a bit high.

Can't find any schematic for a single cathode biased 6L6. I have no idea if the original PT can handle the current draw of a 6L6. But the guy before me was using it for quite a while that way...?

Any info on using using a 6L6 in this all original 66 Vibro Champ? I know keeping it stock is the way to go but it can easily be changed back to a 6V6.  What would be the correct R value for a 6L6 and can the original PT handle the current draw?

I plan on putting the 6V6 back and changing the bias R to around 470 ohms, but wanted to try the 6L6 biased correctly, actually it was a 5881 which I think draws less current than a 6L6GC.

al

[sluckey]

I'm a firm believer in using proper power tubes in old amps. I might play around with preamp tubes but not power tubes.

[floyd]

I think that a 600-700 ohm cathode resistor should suffice, for a single-ended with a 6L6. I believe your PT is rated for 70 ma. Check the heater voltage with the 6L6... if it doesn't dip below 6.3V  AC , you should be fine.

[eleventeen]

Although we do not normally have precise specs for a "Fender Champ" power tranny, it is the same PT as used in a Princeton, 125P1B, which has (at minimum) 4 little tubes vs 1 little tube as in a Champ plus the second 6V6. So a genuine Champ has a massively overpowered tranny if you want to look at it that way. Those little tubes are 300 mils each (when run from 6 v), so there is no issue that a Champ can take the add'l heater current of a 6L6 with room to spare. 6L6 = 900 ma, 6V6 = 450 ma.


I agree with you that a 1K cathode resistor seems high. The stock R is 470 ohms (aka 500 ohms) My tendency would be go to a lower resistor value than stock.


A 6L6 requires more drive than a 6V6 so I do not know if you're going to be able to get "full 6L6 power" out of the circuit without add'l mods. But if that is your aim, to get a turbo-power Champ, then I think you're going to have to go lower on the cathode R so that the tube will pass more current. I can't give you a calculated value, but I'd probably go buy (or pull from junkbox) a 220 and a 270 ohm and a 330 ohm and 390 and patch them in on an experimental basis to see how those worked out.

[floyd]

I think you may "redplate" the 6L6 if you go below 470 ohms.. depending on the tube itself.

[dude]

I believe and feel the same way as Sluckey, you don't mod vintage amps. But since there was a 6L6 in this amp when I got it and I planned on putting it back to vintage specs (6V6 and 470 ohm bias) I figured I'd try the 6L6 properly biased before I went completely original.

It's a Vibro Champ, two 12Ax7's but it's good to know the PT can handle the current.

I found a Fender Champ 12 schematic (mid eighties) with a single cathode bias 6L6, the R was 390 ohms so makes sense what eleventeen says, thanks. I might just try a 390 and watch for red plating. I heard that Clapton used a tweed Champ modded with a 6L6 for a lot of his early stuff...?

Thanks,
al

[Willabe]

I heard that Clapton used a tweed Champ modded with a 6L6 for a lot of his early stuff...?

The story goes that Delaney Bramlett of Delaney and Bonnie gave EC, Dave Mason and George Harrison each, as a present, a Champ (I can't remember exactly but I thought it was a BF?) that had a 10" speaker (Altec/Lansing) installed with a 6L6.

Clapton used that Champ to record 80%+ of the Layla album.


                      Brad        

[dude]

Yeah, I remember now, that the speaker was a 10 not an 8 in Clapton's Champ, probably was a BF. So the story was probably true. I thought a 10 in the Champ (it will fit) might sound boxy and I'd never butcher a vintage amp. But I could just use an extension cab with a 6L6, two 12's at 8 ohms each in parallel. That ougth to be a pretty good tone. And just take out the 6L6 & 390R for a 6V6 & 470 ohm and your back to original without butchering the amp.

I might just try it, might even be able to hang a switch somewhere to go back and forth, of course with drilling any holes in anything.

I loved the tone Clapton had when he was with with Bonnie & Delaney.

Thanks all,
al

[HotBluePlates]

I noticed a 6L6 in a BF Vibro Champ I just bought...? I open up the amp and the cathode bias resistor was 1K ohms for the one 6L6 - was that the correct value for a single ended 6L6 cathode biased amp?

Most cathode biased 6L6 amps ran 2x 6L6's with the same 250Ω cathode resistor you'd see in a 2x 6V6 cathode biased amp. Essentially the same cathode resistor works for both tube types (the 6L6 draws more plate current, through the same resistor, yielding a larger voltage across the resistor and therefore a larger bias voltage).

Whoever put a 6L6 in may have noticed it running hotter than they'd prefer, or they noticed they didn't get a lot more clean like they expected, and raising the cathode resistor value would raise the bias voltage and appear to address both problems. Really, they wouldn't get any more output power because the PT and OT haven't been changed to enable it, but the larger bias voltage on the bigger tube requires a larger drive signal before distortion, and makes the little amp break up at a higher number on the volume knob and appear to have more headroom. A SPL meter would likely dispel the myth that more actual output power or volume was attained, though...

I open up the amp and the cathode bias resistor was 1K ohms for the one 6L6 ... Any info on using using a 6L6 in this all original 66 Vibro Champ? I know keeping it stock is the way to go but it can easily be changed back to a 6V6.

6L6 and changed cathode resistor isn't "all original" in my eyes, but I think you mean there were no major changes to the amp aside from the cathode resistor swap.

Measure the heater voltage; if it's still ~6.3vac (or 3.15vac from one heater pin to ground), then the amp is holding up fine using a 6L6. I don't think you could hurt the PT, because the OT primary impedance would keep you from pulling too much current while you're playing it. So if the heater voltage hasn't sagged (implying you're stressing the heater winding with the 6L6's double-6V6 heater current), and you like the sound, leave it as-is.

[jjasilli]

This is what I did.  Schematic below.

[kagliostro]

Ciao JJasilli

which impedance did you selected, 5k ?

Thanks

Franco

[jjasilli]

Yes, 5000 Ohms. 


I.e., the Secondary wires > impedance SW > speaker were selected to reflect a 5000 Ohm Primary Load Impedance.  The 16 Ohm, 10" Emi Speaker, which I happened to have on hand, can be unplugged if a ext'n cab is used.

[kagliostro]

Thanks

Franco

[dude]

This is what I did.  Schematic below.

Can't open the file what program did you use to create it?

al

[Willabe]

Download link for ExpressPCB/ExpressSch;

http://www.expresspcb.com/expresspcbhtm/Download.htm


              Brad   

[jjasilli]

Here's a jpg

[alerich]

Although we do not normally have precise specs for a "Fender Champ" power tranny, it is the same PT as used in a Princeton, 125P1B

I know you are correct. I agree with you. Just wanted to say that I said the very same thing here in a thread many years ago and was chastised vehemently by one of the regulars. I believe the term he used referred to equine defecation.

Anyway, in regard to the 6V6 / 6L6 Champ thing, I have a seventies SF VC. Very close cousin to the BF VC. I have run it with 6V6/470R, 6L6/1K and even 6V6/1K. I don't notice much (if any difference) in tone or volume across any of the combinations. I use a Weber ceramic speaker. Sweating over the dissipation (hotter versus colder) in that little amp with that unremarkable OT is needlessly splitting hairs, in my opinon.  It's hardly a boutique amp and that little OT is what it is. After it was all said and done I ran mine with a 6V6 and a 1K cathode resistor. It was a little easier on the output tube and sounded functionally the same. If I wanted to swap in a 6L6 I was good to go.

Last year I converted the rectifier to solid state, added a preamp tube for a PI, converted the rectifier socket into another output tube socket, installed a P/P Deluxe OT and turned it into a little pseudo Princeton.

[jjasilli]

Bedsides the VibroChamp above I also hotrodded a SF Princeton with 6L6's,and a 5U4 rectifier.  The PT got very hot; too hot to touch.  In retrospect this may not have been a problem; after all it is metal.  But it bothered me at the time.  One solution, which I didn't think of, was to remove the bell caps for more ventilation.  What I did was add a mini 12VDC fan to blow on the PT, mounted off the PT mounting bolts with Erector Set parts, and powered off the heater supply.

[eleventeen]

My SF Princeton Reverb w/6L6's is a wonderful amp, one of the best amps I've ever owned. It has an Altec 417-8H speaker, trannies from a Dynaco 35-watt amp and SS rectifier.


Did your fan do the deed in terms of keeping things cool? I would expect it to be technically overloaded. I'd also expect the OT to be way small for what I want out of an amp. I would not do that mod today, I did it 35 years ago when Princeton Reverbs were things we laughed at.

[HotBluePlates]

Although we do not normally have precise specs for a "Fender Champ" power tranny, it is the same PT as used in a Princeton, 125P1B

I know you are correct. I agree with you. Just wanted to say that I said the very same thing here in a thread many years ago and was chastised vehemently by one of the regulars. I believe the term he used referred to equine defecation.

What did he chastise you about? There always a chance he's learned something since then, if it was years ago...

[jjasilli]

My SF Princeton Reverb w/6L6's is a wonderful amp, one of the best amps I've ever owned. It has an Altec 417-8H speaker, trannies from a Dynaco 35-watt amp and SS rectifier.


Did your fan do the deed in terms of keeping things cool? I would expect it to be technically overloaded. I'd also expect the OT to be way small for what I want out of an amp. I would not do that mod today, I did it 35 years ago when Princeton Reverbs were things we laughed at.

The PT is overloaded with heater current and B+ current too, due to the 6L6'S.  The fan draws even more heater current, but it did cool-off the PT. The stock OT was fine.  I did basically the same mods to the Princeton preamp as to the VibroChamp posted above.  Also installed a 12" Emi Red, White & Blue in the Princeton.  Both the SF VibroChamp & SF Princeton sounded anemic to me, stock.  After the mods they produced nice vintage Fender tone.  Didn't care much for the Princeton's overdrive tone. 


I still have the VibroChamp.  The Princeton was sold to Danny Kalb of the Blues Project.  It wasn't for sale, but he loved the amp and I couldn't say no.

[jojokeo]

JJ - Nice design mods  however it appears that the schematic needs a touch-up on the coupling cap coming from the plate of V1b to the power tube and not the B+ point.

Although we do not normally have precise specs for a "Fender Champ" power tranny, it is the same PT as used in a Princeton, 125P1B

I know you are correct. I agree with you. Just wanted to say that I said the very same thing here in a thread many years ago and was chastised vehemently by one of the regulars. I believe the term he used referred to equine defecation.

What did he chastise you about? There always a chance he's learned something since then, if it was years ago...

My guess = big daddy (who's long ago left thank goodness) he did a lot of Champ-inspired mods/designs because of his living in apartments w/ his mother & his handicaps.

[jjasilli]

JJ - Nice design mods   however it appears that the schematic needs a touch-up on the coupling cap coming from the plate of V1b to the power tube and not the B+ point.


Good catch; thanks!  I revised the schematic (V1a had the same mistake).

BTW:  I cannot take credit for the mods.  The preamp gain mods; diode in the B+ rail; and bypass cap on the last filter cap are from Dan Torres.  Don't remember where I got the ideas for these mods:  choke, but I love filter chokes for the screen stage;  6L6 mod with Hammond OT (maybe from Angela);  smaller value cathode bypass caps in the preamp; tremolo mods.  Also lots of inspiration form this Forum.  Anyway, none of these mods originated with me.  RAW mod is from Allen amps, but I learned about it on this Forum.

[jojokeo]

What is gained by the B+ diode?

[tubeswell]

Re: the 6L6. To set it up optimally for single-ended centre-bias Class A operation, the calculation for optimum load resistance (Zout) is a product of the power dissipated by the driving tube (Pa) and the potential of the plate (Va) thus:


Zout = Va/(Pa/Va)


For a 5881, Pa = 19W-20W. What plate voltage are you running it at? (Once you have that figured out, you set the value of the cathode resistor so that the output tube idles at maximum, or close to maximum plate dissipation. This gives you the cleanest possible input signal). Watch your screen grid dissipation - you probably either want a 2-3W 470R screen grid resistor there, or set the screen voltage so that it is about 50V-60V below the plate voltage.


As the others have noted, if the PT doesn't look like it is being stressed by the current draw from the 5881, then there shouldn't be a problem with it. As long as the OT load resistance is in the ballpark, you're good to go.

[jojokeo]

Here is another way to dial in a cathode biased amp:
 
Plate dissipation calculation as follows:
1) measure the bias resistor (with the amp off & the resistor "cold") for ACTUAL resistance, write that value down.
2) turn the amp "on" & allow to warm up ~1 minute. NOTE: If this is the 1st startup, watch the plates of the tubes to be sure no redplating occurs.
3) measure the DC voltage from the plate to the cathode (one DMM lead on the plate, & the other on the cathode). Write the voltage down.
4) measure the voltage from the cathode to ground, write that # down.
5) Divide the cathode volts by the bias resistor value
6) Divide the result by the # of tubes
7) The result is the cathode current per tube
8) Multiply the cathode current by the plate to cathode volts

That is the plate dissipation in watts.
Adjust as necessary to get what you want appx ~70% in PP or ~90% for SE

[jjasilli]

What is gained by the B+ diode?

This is a Dan Torres mod, which he calls "rectifying" the preamp.  The concept is:  if you build the Power Amp to sag, it has to pull more current causing a voltage drop.  The diode in the B+ rail helps prevent a sagging power amp from pulling juice from the preamp, causing it to pull what it needs from the wall outlet and not the preamp.  (To my mind this is somewhat similar too VVR in the power amp only, and not the preamp.) 


That said, in Class A, the PA is pulling full current all the time (except for the screen).  So I'm not sure the diode makes much of a difference in this case.

[jojokeo]

Thanks JJ, I was thinking the same (no difference) in the present use. Had a feeling that it could only do as you described but still not sure how effective it would actually be?
 
Sometimes it seems that people's mods are just smoke and mirrors. Take that RAW control: just a fancy name for tone stack resistance giving a higher amount of signal in the same way a high value mid pot or a simple switch could do, or a simple tone stack lift. IMHO, it would be better to put a high resistance on a switch and use the pot & hole in the chassis for an actual MID pot which would actually be much more useful. The "tweed mod/raw control" doesn't do anything above a certain amount of "k-ohms" anyway. You get your gain boost and that's that.
 
Or, I'd rather the pot/chassis hole be put into the feedback loop controlling that instead of it's current configuration and then just still have the PAB on a switch or switched tone stack lift.

[jjasilli]

Checking the 6L6 tube charts, a single tube in Class A might pull about 10mA more current compared to idle.  Across the 1K B+ dropping resistor in my VibroChamp, that's a 10V drop in B+ to the preamp stages.  Certainly no effect on gain.  Maybe a some effect on loadline / tone???



Re RAW control, you convinced me.  A SW would be better than a pot, and easier to use on the fly.

[jojokeo]

Here's a point to point build with the tone stack parts soldered directly to the Treb & Bass pots. Everything works normally until you switch the Bass pot OFF. Then you get the lifting effect. Now you can create how much lift and also filter the non-stacked signal too to limit bass and tighten that up especially for higher volumes. This is suggested w/ any real to moderate power to keep it from being too muddy.