Blues Jr Conversion Really Loud

[Tbone55]

Treble
Mid
Bass

It was too obvious. 🤣

[Latole]

I did not read all the answer .
 A very loud volume in new built could be due to erroneous values of voltage divider resistors at tubes grids

[Tbone55]

Treble
Mid
Bass

That was just to obvious. 😆

[Tbone55]

Thanks. That wasn't too obvious. 😂

[AlNewman]

The tonestack mod is easy to try, you can just put a jumper between the leg of the mid pot that connects to the bass pot and the wiper of the mid pot.
To me, it provides better definition between the 3 tone pots.
If you turn your whole tone stack to 0 you get no volume.
You can turn up any one of the pots and get volume in different frequencies.

[Bieworm]

When you have an amp where the volume goes from zero to really loud with a very little sweep in between, you could try a 12AY7 in V1 socket. Still a very musical tube, but a useable sweep on the vol pot.

[Latole]

From what I understand of his volume problem, and having used 12AY7s, I doubt it will make any difference.
 In my opinion, there's a building error.

[shooter]

there's a building error.

i'm in the; there's way to much pre gain for el-84's camp
wouldn't be surprised if you scoped it n found 2X max drive signal, n el-84's don't stay clean long with that much drive signal.

[Latole]

To drop the volume , a smaller pot value may do the job.

[Tbone55]

Treble
Mid
Bass

Jeez, that was too obvious for me. 🤣

[Tbone55]

Treble
Mid
Bass

Geez, that wasn't too obvious at all. 🤣

[acheld]

I have three Blues Juniors, one regular black BJ II Mexican build circa I dunno, one BJ III tweed reissue circa 2017, and one I built originally from Hoffman's kit -- but has now been converted to a BJ4.

Each has its own flavor.  The stock BJ II has lots of chime, breaks up early, and eats EL84s like candy.

The BJ III is very sweet with its improved Jensen speaker, and also ate EL84s until I lowered the bias with an add-on variable bias board.   It sounds nice.

My favorite, however, is an interpretation of Hoffman's BJ but converting it to a BJ IV format -- eg, self biased power section as opposed to the fixed (and ridiculously high Fender) bias scheme in previous versions.   To be clear, the variable fixed bias in Hoffman's design also fixes the problem; I wanted to see what the difference is with self biasing when I built it (frankly, not much).

My long winded point is that biasing plays a crucial role in these amps, and you can -- to a certain extent -- dial in how much early breakup your power section has by biasing cooler/hotter as you wish.  Many builders just dial in the 70% Pdiss as a biasing solution.  That's fine, and it works, but it's worthwhile to experiment with biasing cooler, or hotter, to see what works best for you.   We do know what hotter sounds like (Fender stock BJ II, III) and of course that results in shorter tube life.   Cooler can work well, and I'll often run my EL84s between 50-60% with lovely tone.   It has been fun to see how low I can bias EL84s and still sound good.

I like the discussion regarding the use of a 5751 as V1 and/or reducing the plate resistor value -- that will also give you some clean "space" to play with.

[Latole]

Try a 5751. Really nice tubes 👍

/Max

Between 12AX7 at 100 % gain and 5751 at 70% gain you wont hear any difference .

[Esquirefreak]

You sure will.

[EL34]

I can see replies up up from the last few minutes

[Esquirefreak]

I've also found that there's a difference in the dynamic response between the two tube types.

/Max

[Tbone55]

I can see replies up up from the last few minutes

Thanks. I'm seeing them to now.

[Tbone55]

Thanks everyone for your suggestions. There seems to be a number of things I could do to help fix the issue. It seems these Blues Jrs were always running hot and were prone to breakup very early. My amp is a Blues Jr III Special Edition although I don't know what's so special about it. My Hoffman conversion has the biasing resistor in it and I'm currently running the EL84s around -23mv.

The suggested fixes, as I see them appear to be as follows.

1. Replace V1 with a lower value (5751, 12AYX)
2. Replace V1 grid resistors with a higher value (47k, 68k)
3. Replace V2 and V3 with higher grid resistor values as well.
4. Mid tone pot modification/re wire
5. Lower bias voltage for EL84s to 50% or 60%
6. Replace volume pot with lower value. 500k?

The easiest to try seems to be to lower the bias voltage first. I'll try this first and post back the result. 

Thanks again for all your suggestions. Cheers!

[Esquirefreak]

Don't forget about the plate load resistors. To lower the gain, you decrease the values 👍

/Max

[dwinstonwood]

Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.

[Tbone55]

Don't forget about the plate load resistors. To lower the gain, you decrease the values 👍

/Max

Thank You for reminding me of that. I knew I'd forget something. 🤗

[Tbone55]

Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.

Thanks for the additional tip. How does that work to lower the gain from V1?

[shooter]

5 minutes, you'll understand simply by playing.
rif for 3-5 minutes, unsolder cap, repeat rif

[AlNewman]

Do you know what your bias is yet?

[Tbone55]

Do you know what your bias is yet?

Thanks for providing the video. I've seen some of Uncle videos but not that one. So my amp is cathode biased and I have the variable resistor that allows me to adjust the tubes bias. I've currently got it so my bias is around -23mv on one tube and -21.8mv on the other. I've got it set a little bit cooler than -25mv that seems to recommended to give 70% plate dissipation. Since the bias is the easiest thing to change I'm going to try and set it a bit colder and see how that works. If I'm getting anything wrong please let me know.

[AlNewman]

Could you just make a voltage chart of your power tubes at all the pins?

As well as all your power nodes.

And all the pins of your little tubes...?

And maybe an as built schematic.

[tubeswell]

Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.

Thanks for the additional tip. How does that work to lower the gain from V1?

The cathode bypass cap works to boost the gain of a cathode-biased tube by shunting to ground, any signal that otherwise would appear on the cathode if there was no bypass cap. The effect of the cap is to ‘fix’ the cathode at a steady voltage, thus effectively maximising the plate-to-cathode voltage under signal conditions. (More plate-to-cathode voltage = more gain; all other things being equal). If you remove the cathode bypass cap, you reintroduce the voltage swing at the cathode, which then introduces cathode current feedback back into the stage, thereby reducing the gain. An unbypassed cathode on a 12AX7 triode has about 1/2 of the gain compared to a fully bypassed stage.

[Tbone55]

Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.

Thanks for the additional tip. How does that work to lower the gain from V1?

The cathode bypass cap works to boost the gain of a cathode-biased tube by shunting to ground, any signal that otherwise would appear on the cathode if there was no bypass cap. The effect of the cap is to ‘fix’ the cathode at a steady voltage, thus effectively maximising the plate-to-cathode voltage under signal conditions. (More plate-to-cathode voltage = more gain; all other things being equal). If you remove the cathode bypass cap, you reintroduce the voltage swing at the cathode, which then introduces cathode current feedback back into the stage, thereby reducing the gain. An unbypassed cathode on a 12AX7 triode has about 1/2 of the gain compared to a fully bypassed stage.

Thank You for providing that explanation.  I appreciate it. 🤗

[Tbone55]

Could you just make a voltage chart of your power tubes at all the pins?

As well as all your power nodes.

And all the pins of your little tubes...?

And maybe an as built schematic.

Yes, I can. I followed the layout provided by Doug to build the amp. Would that be good enough?

[acheld]

So my amp is cathode biased and I have the variable resistor that allows me to adjust the tubes bias. I've currently got it so my bias is around -23mv on one tube and -21.8mv on the other.

I think you've done well!  And you're getting some great advice here. 

Just to help you with terminology:   A BJ III is "fixed" bias -- a confusing term that simply states that a fixed negative DC bias voltage is applied to the power tube grid (along with AC signal from your PI).   Confusing because we vary the bias voltage to suit our desired power tube state.

A BJ IV, however, is "self biased" or "cathode biased."   In this biasing scheme, the bias is achieved by varying the cathode resistor value to achieve the desired state.   All pre-amp tubes are self biased.    Dealer's choice on the power tubes.  For a good discussion on this, on why one vs the other scheme, check out https://www.valvewizard.co.uk/pp.html.

Your reference to mV as biasing doesn't have meaning by itself.  We all assume you mean plate current, but really what you want to specify is power (P=V*I). 

Uncle Doug's early instructional videos, while maddening to some, are generally excellent sources on the basics.

And over and over again, read Merlin Blencowe.  And then again.   When you think you have it, then Rich Kuehnel.

[Tbone55]

Thanks kindly acheld,

I am getting a much better understanding of how the biasing and also how the pre-amp section works. Sometimes the terminology is a bit confusing to me about biasing.

I was just watching one of Uncle Doug's video where he shows how to measure and calculate the bias for a 6V6 tube as well as what you would do to change the bias if needed. What I found really interesting was the the bias was set to provide the maximum power dissipation of the tube, 12W, and this was necessarry in order to provide enough headroom to avoid distortion. In the bias is set too cold the amp will distort a lot easier.

The complaint I've always heard up to the Blues Jr III has been that the biasing is too hot which causes early tube failure. Would this then mean that the plate dissipation would be 100% or 12w It seems the recommended PD is around 70% or approx 8.4w. It seems to me that I need to make the bias voltage more negative to clean up the sound a bit.

Your thoughts?

[AlNewman]

I'm sorry, I didn't see your earlier posting with voltages and schematic.

I believe the Uncle Doug video I linked is a cathode biased amp, and yours is fixed bias. usually
The method of measuring via the transformer is still viable, but since you have 1R resistors on your cathode, that is just as good.

Cathode biased amps can be biased hotter than fixed bias sometimes to maximum wattage or even slightly higher.
Fixed bias amps should be biased cooler, the often recommended number is around 70%.
Yours will be  be safe at where you are biased, but you probably wouldn't want to go higher than 70 or 80%.
In fact for me it seems biasing it cooler can actually improve headroom, I usually bias at around 55%.

It doesn't sound to me like the distortion you are getting is a power tube thing though.  Tubeswell has a good suggestion of lifting the bypass cap and maybe that will be enough of a gain drop. 
 
Anyhow, I apologize for causing any confusion, I just skipped to page 2 and missed the posted voltages.

[AlNewman]

I was thinking about the volume and how it reacts with the grid leak R6 directly following (in parallel) to it.
Really, it limits your volume pot resistance at 120k, meaning it could explain why your volume seems to max out at 2-3 at the volume dial.

Most of the old fenders have the volume right after the tone stack, in which case the tone stack would replace that resistor, (sort of)?

It's possible fender put that value resistor there to limit oscillations in what is a high gain pre amp.

I'm not sure of suggestions in how to fix it easily,  besides perhaps trying a lower resistance pot for the volume control, like a 500k or something, and possibly increasing R6 to find the sweet spot.  Another suggestion may be to change the placement of your volume and control signal on the other stage with it's own voltage dividers.  Even a linear pot might work better in that situation, even though it seems counter intuitive.

Or, still thinking, maybe change the voltage divider going into the tube, IE use a 100k grid stopper and a 1M grid leak.

I'm sure if that could be an issue somebody else might have other suggestions.

[Tbone55]

Thanks kindly for you input and suggestion Al. I've been watching a few of Uncle Dougs other videos on biasing to get a better understanding. I'm a bit confused about my amp because I thought it was Cathode biased because of the resistors from the cathode to ground but what about the trimpot that's controlling the grid? Doesn't that make the tubes grid biased?

I'm in the process of taking new voltage readings from the tubes as I didn't measure some of the pins in my previous table. I was also wondering the same thing about that 120k resistor across the volume pot. Lots of things to think about.

[acheld]

I'm a bit confused about my amp because I thought it was Cathode biased because of the resistors from the cathode to ground but what about the trimpot that's controlling the grid? Doesn't that make the tubes grid biased?

Your Hoffman BJ conversion is a fixed bias amp.   The telltales are:

1. The presence of the biasing voltage circuit.   See: https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf, scroll down three pages and look for the C- next to the bias circuit and where it connects to the grids of the EL84s.

2.  The lack of "large" resistors from cathode to ground.  Most cathode biased amps also use a cathode bypass capacitor.  Your amp has neither. (The 1 ohm resistor from cathode to ground is present solely to allow you to estimate plate current.)

As mentioned before, a BJ IV is cathode biased, as opposed to earlier models (and your amp) which were all fixed bias.   An early schematic and PCB layout of the BJ IV are located here:  https://www.tdpri.com/threads/fender-blues-junior-iv-schematic.1104154/.   You will find a 270 ohm cathode resistor and 22uF cathode bypass cap from the cathode to ground. For learning purposes, would be really worth your time to compare Hoffman's beautifully simple schematic with the (necessarily) more complex Fender schematic to see the difference in the power tube biasing scheme. 

[Tbone55]

Thanks ac for the explanation. I see where the C- voltage is that feeds the grids of the ouput tubes which is adjusted by the trimpot. Appreciate the help.

Cheers!

[Tbone55]

I've completed going through and measuring the tube voltages at each pin and each node. V3 definitely has something funky going on.

[acheld]

V3 definitely has something funky going on.

Nope, looks good to me.  It's all in the art of biasing . . . our theme o' the day.

The "funky" thing you're seeing is that pins 3 and 8 are at much higher voltages than your pre-amp tubes.  This is expected.

Look at your schematic for V1a, and you'll see that the cathode resistor (Rk) to ground is 1.5k.   Now look at V3 for your long tail pair phase inverter,  and take a look at the funkiness in the "tail" -- in other words, trace the path to ground from your cathodes and note there is about 45k ohms in between both cathodes and ground (DC resistance only, keeping it simple, and also note that this is a shared resistance between both cathodes in the tube).

This resistance keeps the cathode at a higher voltage than a much smaller (say, 1.5k) resistor would do, and that's how biasing is done.

For a great discussion of your AC coupled long tail pair PI, see:  https://www.valvewizard.co.uk/acltp.html.   I can't come close to Merlin's ability to understand and explain . . .

[Tbone55]

Thanks ac for explaining that. I see what you're saying about the resistors but am still puzzled about the voltage being that high. I'll check out the reference you provided and see if I can wrap my head around what's going on.

So it seems the amp is working as it should after all and I need to look at doing some of the things that were suggested now to tame this little beast. Thanks sincerely for your help.

[Tbone55]

For a great discussion of your AC coupled long tail pair PI, see:  https://www.valvewizard.co.uk/acltp.html.   I can't come close to Merlin's ability to understand and explain . . .
[/quote]

This was a great article. I had no idea thats how V3 worked. While I do find it hard to understand just how everything works it does explain why the voltage on the cathode is so much higher than V1 & V2. There's quite a big signal being fed to the power tubes.

One thing that I also found interesting is the use of either a 12AT7 or 12AY7 in place of 12AX7 in order to increase the headroom, maybe even a 5751. It might be worth a try to use one of them and reduce the signal level a little to help provide more headroom and less pre amp distortion. 

Any thoughts?

[acheld]

either a 12AT7 or 12AY7 in place of 12AX7 in order to increase the headroom, maybe even a 5751. It might be worth a try to use one of them and reduce the signal level a little to help provide more headroom and less pre amp distortion. 

Always worth a try to sub in different tubes.   My personal bias is that if one tames the pre-amp too much, the life goes out of the amp.   I once concocted a Princeton that would deliver 24 watts of undistorted 1kHz with the volume set at 90% -- this was for a jazz guitarist who told me that he wanted NO distortion at all.  However, the amp lacked life IMO, and the prospective customer was not impressed.  When I replaced all the 5751s in the pre-amp with standard 12AX7s, it came back to life.  Now it's a nice sounding amp with some soft distortion at noon. 

If I was going to sub in some tubes -- you could try a 12AY7 in V1, or maybe a 12AT7 in the PI.   I do like 5751s, but the difference (from a 12AX7) is subtle.   Lots of different opinions to be had on what tube works best where.  Easy enough to try it and see what happens.

You made a list of stuff to try earlier.  Don't forget about that.

[Tbone55]

Thanks for your reply ac. Changing the tube is just one of the things on  my list and it's easy enough to do. I'd really to be able to turn the volume up a bit more before getting breakup, maybe to the noon position. I'm going to work through my list one change at a time and see what happens. I'll post my findings as I go along.

Thanks again fir all your help and advice. Much appreciated.

Cheers!

[Tbone55]

One great mod is to change the stock tonestack into more of a vintage TMB style by changing the Mid pot to a variable resistor, and changing the values in the tone stack.  That gives you better control of what frequencies you'd like to push.

Hi Al,

What's the difference between a tone pot and a variable resistor? What values should I look at for the resistor values? Is there a schematic I should follow?

Thanks.

[Willabe]

You slide the metal band left or right to adjust.

 

[acheld]

In the context of a tone stack, they would be the the same thing.

A variable resistor would be set up by using the wiper contact and one of the fixed poles.   

Most often (but not always!) in a tone stack, the "tone pots" are set up as dividers, where signal arrives at one pole, and then is divided into the wiper and the second fixed pole.

More generally, a designer does need consider power dissipation and many other factors (see https://www.bourns.com/docs/product-datasheets/82.pdf as an example spec sheet) when employing any resistor.  Many commonly used pots are not as robust as you might think -- when used outside of their design parameters.

Willabe, of course, has got it right.   Check out https://www.bourns.com/docs/product-datasheets/PTA.pdf, one my personal favorites.  But also note the low power rating and the low maximum operating voltage.

[Tbone55]

Thank you for the explanation and the references. Much appreciated.

Cheers!

[AlNewman]

You just have to put a jumper between 2 lugs of your mid pot.

[Tbone55]

Thanks for providing that info AI.  Haven't tried that yet but I will.

Cheers!

[Tbone55]

I have a question about one of the Hoffman mods I did. It has to do with V2. In order to get a stronger signal push on V2 Doug suggests putting jumper wires from pin 1 to 6, pin 2 to 7, and pin 3 to 8. Could the issue I'm experiencing have anything to do with this mod? Maybe I'm getting a much stronger signal than what is needed?

[Tbone55]

2nd image for previous post.