Blues JR OCTAL Conversion w/ 6V6 (BIAS PROBLEMS?)

[aestoval]

Hi,

I just finished converting my blues jr over to octal sockets and was trying to bias them. I put my DMM in mA and measured from the red center tap on the OT to pin 3 on the octal sockets. This should be right... but I'm getting some really high readings in the 70 to 80 mA range. I'm using JJ 6V6-S tubes. I replaced the bias resistor with a 50k linear trimpot and it seems like that should have been sufficient. Where am I going wrong? Do I need to alter the bias circuit any more than I already have? If anybody could help I would greatly appreciate it.

Thanks

Atticus

[Blue Whale]

Howdy, Excellent mod to be carrying out but the bias will be an issue.

I have had a quick look at the schematic and basically the bias is way out. The Blues Junior biases the EL84s at -10.7Vdc. Those 6V6GTs want to be biased at -31Vdc.

The bias in the Blues Junior is taken from the 15Vac+/- supply for the ICs. Those windings look to be supplying 20.5Vac with one side grounded. There is a take off point for -26.7Vdc but still not enough I fear.

So the three options that come mind are:

i - rework the 15Vdc+/-, this would require a lot; lifting the earth so that you get a swing of 41Vac and then taking matters from there. Seems too much bother to me.

ii- if you're keen to go with a fixed bias let me know I'll work out a -31Vdc from the HT supply much easier than the above.

iii- an easy option that'll get you going today; go with cathode bias. This will take two simple mods:

To do this disconnect the bias supply and ground the link point between the pair of 220K resistors.

Now lift the earth on pin8 of both octals; link both pin8s and between this and ground you will need to place in parallel a 250 or 270ohm 5W resistor and a 22uF or 25uF 50V or 100V electrolytic capacitor. Install the bias cap -ve to ground.

That's it you have a Blues Junior Tweed Deluxe. That's what I'd try.

Let me know what you think and how you get on.

[celeste]

Originally Posted by Blue Whale

[COLOR="Blue"] Install the bias cap -ve to ground.

This a negative supply, so ground is more positive, so shouldn't +ve of the cap go to ground

[ThermionicScott]

aestoval, what is your plate voltage with the 6V6's? 80mA @ 329V would give a dissipation of about 13 watts for each tube, which is under the 14W limit for 6V6's.

At 329V, you won't need quite as much bias as a Deluxe, so that's helpful. Changing the values of R37 and R31 will free up more of that -26.7V supply BW talked about. I bet it'll be plenty for your application. (Try a 470-ohm or 1K resistor instead of 33K for R37.)

- Scott

[Blue Whale]

Originally Posted by celeste

This a negative supply, so ground is more positive, so shouldn't +ve of the cap go to ground

No.

Changing to cathode bias you want to see about +20Vdc at Pins8 on the octals, now that you have grounded the junction of the 200K resistors

What we are considering here is the difference in voltage between the cathode and the grid.

Originally Posted by Scott S

... 80mA @ 329V would give a dissipation of about 13 watts for each tube, which is under the 14W limit for 6V6's.

You appear to have divide by two not allowing for 80ma across each half the OT as he is measuring here, thus each valve is dealing with 26Watts. Now that's pretty hot even for those modern JJs and the OT won't last long at that.

Originally Posted by Scott S

At 329V, you won't need quite as much bias as a Deluxe, ...

I run a 5E3 type in fixed bias at -32V with 335Vdc on the anodes with NFB, thus a Princeton.

Originally Posted by Scott S

Changing the values of R37 and R31 will free up more of that -26.7V supply BW talked about. I bet it'll be plenty for your application. (Try a 470-ohm or 1K resistor instead of 33K for R37.)

You can't change what the transformer is supplying, thus the max bias can be from this BJ set-up is -26.7Vdc . I fear that won't be enough. Still you could take the full -26.7Vdc very easily and supply the bias point as is and quickly see what you get the mA down to before going any further.

Let us know how you get on.

[aestoval]

OK, For those interested.

Plate voltage---305V

http://www.webervst.com/tubes1/calcbias.htm

here I found 70% of that should be 27.5 mA

I think this might cut it, right?

If it doesn't work I'll give some other options a shot. Thanks for the input.

Atticus

PS: I had more to say, actually a nice long post, but it didn't take and it all got erased, bummer.

[Blue Whale]

Your post is a tad unclear; you are not confusing the suggested 27.5mA with the bias voltage are you?

You want to be below 32mA across each half of the OT, just remember to be careful here as the voltages can kill, still you've managed to survive thus far #:o)

Perhaps less would be safer as I am unsure what OT spec Fender are using here. I like the sound of 29mA, and no lower than say 24mA and you'll have a great sounding amp.

Are you going to try the full -26.7Vdc as a fixed bias or configure for cathode bias?

What are you thinking?

[aestoval]

BW, sorry for the lack of clarity.

My plate voltage is 305V

My tubes are pulling 32.4 mA per tube now that I changed the other bias resistor (r51) to 470 ohms.

The bias voltage is now -26.7 V

any other info you need?

Looks like it's close, but not right on. Also, no worries about me being foolish around voltages. I used to do some electrical work and know the feel of a nice shock. I don't plan on getting to know that feeling any time soon so I take pretty extreme caution.

Thanks again. I'll be right by the computer checking for updates.

Atticus

[Blue Whale]

32.4mAs is great. I'd be happy at 32mA. Using the easily accessible -26.7Vdc as a fixed bias seems the easiest option all round. And those modern JJs will take I bit of pushing so no worries there.

As a finally check perhaps just check that the -15Vdc and +15Vdc are sweet, then enjoy the "new" amp.

Have fun, and all the best with your music.

PS: They'll all be doing this mod now to their B-Jnrs haha #:o)

[ThermionicScott]

Awesome! At about 10 watts per tube, they should live a long happy life. How does it sound?

- Scott

[aestoval]

Well, it's all back together now. There is very little problem with heat with these tubes. They run much cooler than el-84s. I'll say it was worth the $35-40 for sockets, tubes, and tube holders. The bias is still a little high (higher than before) at 34mA so I might still be open to suggestions, but it sounds amazing! If you got some soldering skills and you like 6V6s then this might be for you. I'll try to post some clips to give people an idea of how this sounds. I searched all over the internet for a clip of a blues jr w/ 6V6s and found nothing, so I guess it's my turn to help out everyone else who wants to know. I'll record tomorrow and post asap. Thanks again for all of the great help!

Atticus

[ThermionicScott]

10.4W is still pretty cool... don't get too fixated on hitting a 60% or 70% target exactly.

FWIW, I had to crank up my JJ 6V6s to about 20 watts per tube in order to get the faintest glow on the plates, so they can take it!

- Scott

[Blue Whale]

Personally I think your 32.4mA draw is fine.

Just a quick question before you go make a racket and get lost in the wonderful world of 6V6GTs.

When you say the "plate" is seeing 305V; do you mean the Anode Pins 3 or the Grid/Screen Pins 5 on those Octals?

Looking at the Fender schematic you should be seeing around 305Vdc on Pins 5 and 325Vdc on Pins 3. I am just curious, as I expect to see 325Vdc on the Anode. And at that; 32.4mAs would give you a nice 21watt amp, which is great for 2x6V6GTs. If the anodes are seeing only 305Vdc then you are a little under 20watts as Scott has stated.

You just need to be a little wary of the spec on that OT at that may be, as Fender are selling the B-Jnr at 15watts. May be check out what spec it is, if as I suspect you have 21watts and the OT's OK at say 22watts, then leave well be and enjoy it.

[aestoval]

BW, I'm referring to the voltage on pin 3. This should be ok, right? I think I'm about to lose power in all this snow! It's all over the place flickering and such. So, I'll post back asap, whenever that is... Thanks again. I'm playin' the amp right now and maaannn you gotta love the sound.

Atticus

[aestoval]

Here are a few clips I just did.

Just noodlin' around... These are all done with a Tele plugged directly in. In the super wet reverb clip, I got it that wet by adding a dwell control (dwell is maxed). All of the controls are at 12 o'clock except master at 12 and volume at 2.

http://www.soundclick.com/atticusstovall

just go here and they should all be listed...

Again, I have done absolutely nothing to these clips. They would sound a little better with some EQ, but I tried to be as honest as possible with the sound.

Thanks again, let me know what you think.

Atticus

[Blue Whale]

Great for taking the time to add some sound clips. To truly enjoy 6V6s you need to get them in to the OD zone, then the heavens open. Still I am sure others will be doing this B-Jnr mod now.

Regarding the OT; I am thinking that you'll be OK; at 305Vdc 42.5mA you putting out damn near 20watts full power.

I doubt Fender will be very much use re asking about spec, but may be. Otherwise someone like Allen Amps might know. http://www.allenamps.com/parts.php

I'd just keep a watch on how hot it gets for any indication that it's being stressed. It will get hot but not too hot.

Anyway if it fails, then you can upgrade #:o)

Enjoy it, and all the best with your music.

PS; The snow looks pretty heavy on the news, so keep close to a warm amp! as long as the power stays on.

[emtjr928]

I have been following this thread with keen interest. I have already done a number of mods to my cream board and have been curious about changing the power tubes as well. Hope you will post more feedback and observations about the change as you move forward.

[aestoval]

Yep, this it one of those mods I could never come back from. The dirty sound is soooo fat sounding. Listen to this clip I just put up on the soundclick page labeled "dirty 6v6". I really am amazed with the bass response this amp now has.

http://www.soundclick.com/atticusstovall

this clip is with the volume at about 7, master all the way up, tone all at noon except bass at 8. I added a touch of reverb after the recording. I hope you like. I listened to it on my stereo and the bass is just right for me...

Thanks again to BW and Scott for the help, the bias is just a tad hot, which makes this amp just sing cranked.

Atticus

Oh yeah, there is nothing in between the tele and blues jr, just a 15 ft cable.

[ASC67]

sounds sweet !

[ThermionicScott]

Indeed, it sounds great! Glad the conversion went well.