6L6GCs in a blues jr voltage help

[mguitarworks]

Hey Guys, ive just put in  a set of 6L6GCs STRs in my modded bj. its an export 240v. ive added a filament transformer to help out and have a TO20 O/T....Sounds awsome the best its ever sounded...(its in a larger pine cab with a 15inch speaker and full size reverb tank)

Stats:

330v plate voltage

bias voltage -27v  @r32

biasmeter reading on tube 46.5ma

-5v voltage drop across transformer

does this all sound ok, will this set up work safely , reliably long term?

Thanks

[Fresh_Start]

I haven't done the math but would be surprised if the PT and OT handle the 6L6s over the long-term.  Don't forget that the heaters are on a separate secondary winding, so I'm not sure a separate filament transformer really helps all that much.

How hot does the inside of the chassis get with the 6L6s hanging upside down?

Cheers,

Chip

[mguitarworks]

Its in a bigger cab so theres plenty of air...

 the P/T doesnt get very hot at all which a suprise..the other 2 are cold...

does a under sized or oversized O/T have any effect on the P/T performance.current drawing  ect

Cheers

[tubeswell]

Set the 6L6s up to idle somewhere between 62-75%. If you've added extra filament current, that would help with the heaters. But the high-tension winding on the PT may still have difficultly if its less than about 150mA once you crank it. As Fresh Start said - how hot does the PT get?

As for the OT, it also needs to be robust enough to handle the power. Might be a good idea to put a HT fuse in (if there isn't one already).

[mguitarworks]

hi tubeswell..

why 67-75%,  i thought the colder the better ...ive got it at 46ma (bias meter) at 300 plate volts which is roughly 55%...which is the coldest i can get it...sounds good

how would bumping it up to 75% help the P/T ? wouldnt that stress the P/T and O/T more??

p.s P/T is not getting very hot at all...its a bit erratic though, i check it after everytime i use it and some times its bearly warm and other times its quiet warm no matter how long or loud i play..but overall not very hot and no smell

cheers

[mguitarworks]

hey tubes well..where/how can i check if the ht winding is less than 150ma on the BJ?

cheers

[Fresh_Start]

Please understand that I'm not trying to rain on your parade, but the stock transformers in a Blues Jr. really aren't up to the task of handling a pair of 6L6s.  6V6s - sure.  Heck, I'm not sure they're really up to the task of handling the stock EL84s all that well.  You should see how much bigger the Mercury Magnetics "upgrade" output transformer is compared to the Fender OT.

hey tubes well..where/how can i check if the ht winding is less than 150ma on the BJ?

cheers

Add up the current demands of 3 12AX7s and two EL-84s running hot, fixed bias.  Data sheets suggest approximately 40 ma per EL84 plus 3 ma per 12AX7 => 90.  If you've really got the 6L6s biased so cold they're idling at "only" 46ma, you're still way over the power demands of a pair of EL84s at max signal no matter how hot they're biased.

As far as the OT goes, a pair of 6L6s is looking for something like 4,000 ohms primary impedance versus roughly 8,000 ohms for a pair of EL84s.

http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf

As far as the "correct" bias point, it's all in the ear of the beholder but 65-70% of max plate dissipation is a typical starting point.  I don't know exactly what a pair of 6L6s biased at 55% of max would sound like, but "sterile" and "brittle" both come to mind.  In stock form, the Blues Jr. is actually biased pretty darn hot - like 80-85% IRRC - and they tend to burn through EL84s.  The first mod I did to my Blues Jr. was adjustable bias so I could dial it down a bit and stop torching tubes.

Respectfully,

Chip

[tubeswell]

hi tubeswell..

why 67-75%,  i thought the colder the better ...ive got it at 46ma (bias meter) at 300 plate volts which is roughly 55%...which is the coldest i can get it...sounds good

how would bumping it up to 75% help the P/T ? wouldnt that stress the P/T and O/T more??

What I meant was - the high-tension winding on the PT may still have difficultly if its (rated) less than about 150mA once you crank it.

If it holds up for a while, it 'may' hold up for quite a while

As far as the bias point goes, it ideally needs to be somewhere between bigger than 50% and smaller than say 80% for Class AB1 operation. If it runs okay cooler, then keep it cooler.

how can i check if the ht winding is less than 150ma on the BJ?

email fender

[PRR]

> the P/T doesnt get very hot at all

Then it's fine.

[mguitarworks]

Hi guys,

I think re the biasing is all the"rules" go out the window with my amp because its all so mismatched,impedances, undersized trannys ect...

The thing sounds great..i mean really great..this anit no tiny chipboard BJ its in a double size premium pine cab with a 15inch jensen and full size tank...its really got nothing to do with a BJ even with mods. it goes toe to toe with a hotrod deluxe fro some reason it does. (probably the 15.)

What help im after is to set it up to run as reliably and safe as it as possibly can. i dont seem to be getting the answers to questions i ask re the O/T.

[mguitarworks]

Getting back to my original question...

Will running an Allen TO26 O/T in anyway help or hinder the P/T  vs Running the TO20 ive got in there now?

cheers

[sluckey]

Will running an Allen TO26 O/T in anyway help or hinder the P/T  vs Running the TO20 ive got in there now?

I don't think it will make much, if any, difference to the PT/

[mguitarworks]

Hi Steve..if it did would it help or make the P/T work even harder?

Thanks
Mark

[Fresh_Start]

A quick look at the Allen site indicates TO 20 has a slightly lower primary impedance of 6600 ohms when compared to the T0 26's 7000, Is this close enough?  One of Allen's competitors (Hammond) has a primary impedance at 6950 ohms the Blues Jr. OT replacement. 

Its hard to compare transformer specs when they don't exist.  See Kaglio's thread on how to read Hammond specs for info on what RCA once upon-a-time, suggested for OT's. 

Did I miss something on this thread, don't EL84's and 6L6 have different bases, so either an adapter is required, or the sockets are changed out.     Also, the 6L6 has a higher heater demand 0.9 amp vs 0.76 amp, since a filament transformer was provided, this would not be an issue. 

I do not have the skills at this time to evaluate the other impacts of change over.  I will spend some time with the circuit and do an evaluation.  I learn as I go. 

This isn't a Blues Jr. anymore.

Mark - if you like the way it sounds and the PT isn't getting too hot to touch, go for it.  Just don't bias those 6L6s much hotter or you may risk overwhelming the OT.  Allen's TO20 is designed for a pair of 6V6s or EL-84s.  IOW a 12-15 watt amp, more or less.  A pair of 6L6s is capable of producing 50 watts or more. 

Hoffman sells several big output transformers intended for a pair of 6L6s.  Is your 15" speaker rated at 8 ohms?  Hoffman's 018343 OT would be a better match for the 6L6s; however, your smaller OT with twice the typical primary impedance may be keeping your PT alive.  Your Frankenamp may sound great because you've mixed an unusual selection of power amp components.

Just for fun, do you have a pair of 6V6s you could throw in there and listen to?  Voltage, OT, and power demands would be "better suited" to those output tubes.

Respectfully,

Chip

[mguitarworks]

Hi guys,
I converted it to 6v6s roughly 2 years ago which it runs easily..ive just gone up to 6l6gcs and dont want to go back...
good comment "the TO20 might be keeping my P/T alive"... my feeling is putting in a bigger O/T will make my P/T work even harder than it is now.. but thats just my feeling.. i would like to know it technically from someone who knows for sure.

How much of an impact do you think my filament transformer has hepled the P/T?

cheers

[mguitarworks]

P.s Yes my 15 is rated at *ohms

[mguitarworks]

Hi ..the 6l6s are definently no replating or getting close..330v on the plates...my bias meter is showing 46.5ma (on the tube) the bias voltage is -27v ( at top of r32 resistor) and voltage drop across O/T is -5v.

are the 6l6 running pretty cool?

Thanks

[sluckey]

yes

[mguitarworks]

Hi Steve.. im checking the P/T after evertime i use it and many times its not even warm and thats playing flat out for an hour. yesterday i put some 6v6s in just to hear the difference. The next day when i turned on the amp with 6v6s in the fuse blew. Which it never did with the 6l6s or with the 6v6s previously.

I have got an Allen TO26 O/T on the way and a bias board as well...so we will see how it goes.

Thanks

[mguitarworks]

Hey guys...

Is there any "electrical" reason why when i play flat out full volume for full hour P/T doesnt even get warm....and when i play late at night low bedroom volume for 1/2 hour it does?

cheers

[sluckey]

It's like a VW Beetle. The faster it goes, the cooler it runs.   

[mguitarworks]

hi steve.. great answer, i makes a lot of sense...but i dont understand it technically.

cheers

[mguitarworks]

not talkin about the vw 

[mguitarworks]

Thanks drgonzonm.... but you have been no help what so ever...try not to waste peoples time

cheers

[mguitarworks]

Hi, Im in Australia 240v, power is pretty even here at all times of day peak or midnight  all the same never more than 5 v + or -..

when i was referring to "late night" it was more to describe how low volume i was playing at. if i play at the same low volume in the middle of the day it gets hot just the same.

Its to do with how hard im pushing the amp...when i push it hard it stays really cool...when i play at really low volumes it gets hot.

I would like to know WHY this happens "electricaly"


Thanks