Point-to-point conversion Fender Blues jr.

[klooon]

I have a Fender Blues jr (I believe a version II) and I did some mods, basically changing the caps for better quality part and some different values (followed advice by BillM and some other sources). I was not so happy with working on the PCB, but managed to do the tricks.

One thing that is really bothering me that there is some kind of buzz/ugly noise present (it's increasing with volume). I am not sure this is because I messed up something while working on the PCB or that it was already present before... I checked a friend's unmodded Blues jr and I hear a similar annoying noise.

A good excuse to do some serious work on the amp. As such the amp sounds really fine to me, it's handy to toss around with, so why not...

Here is an example of the noise:
Blues Jr Noise 001.m4a

I tried to poke around in the amp, wire strips moving up and down, etc... nothing seems to help... Maybe somebody could confirm this?

Anyway, in this thread I like to take you with me through the conversion process and as usual your comments are extremely welcome!

I couldn't find any other examples on the internet, so if you see them please provide me the link!

[Chuck H]

Those amps are prone to oscillations and instability if the preamp decoupling is compromised because there are like phase signals which meet at the decoupling node for the plate loads. I could guess that if you hear a little of the problem in a stock amp that whatever you changed just accentuated it. It may be a minor instability caused by the same design flaw. It might be worth a try to add an additional node at the end of the B+ rail and use it for the HV on both triodes of the first 12ax7. Any minor instability caused by like phase signals on the HV rail should be minimized. That MAY include the noise you're hearing. And if not, at least it's a design improvement.

P.S. Your link doesn't work for me. It only allows me the option to save the clip as an HTML doc which appears as symbols and such.

[klooon]

Those amps are prone to oscillations and instability if the preamp decoupling is compromised because there are like phase signals which meet at the decoupling node for the plate loads. I could guess that if you hear a little of the problem in a stock amp that whatever you changed just accentuated it. It may be a minor instability caused by the same design flaw. It might be worth a try to add an additional node at the end of the B+ rail and use it for the HV on both triodes of the first 12ax7. Any minor instability caused by like phase signals on the HV rail should be minimized. That MAY include the noise you're hearing. And if not, at least it's a design improvement.

Thanks for your comments! Do you mean the following? If so, are these the right values or should I try different cap and resistor values instead?

[klooon]

P.S. Your link doesn't work for me. It only allows me the option to save the clip as an HTML doc which appears as symbols and such.

Could you please try this link?
https://www.dropbox.com/s/thfs3cofm5...20001.m4a?dl=0

[Chuck H]

Thanks for your comments! Do you mean the following? If so, are these the right values or should I try different cap and resistor values instead?

[ATTACH=CONFIG]31559[/ATTACH]

Yup, or something like that. I was actually thinking of keeping the B+ rail in series and dropping the preamp rail resistors to 5k. But considering it, a parallel node should work fine and is likely easier to implement.

[klooon]

Yup, or something like that. I was actually thinking of keeping the B+ rail in series and dropping the preamp rail resistors to 5k. But considering it, a parallel node should work fine and is likely easier to implement.

I compared my Blues jr. to my friend's once again, but it's really the same type and level of noise. So I assume it's in the design of the Blues jr (version II). BTW Is the new link with the sound clip working now?

I tried the extra node but it didn't change things. The noise is still there. So I start to believe it is a "combination of things" in the design. Hopefully the lead dress and the actual location where the components are placed physically in the circuit... So let's try to change these things for the better with a new layout on an eyeletboard.

I looked carefully at the grounding scheme during my Princeton Reverb build (link here of my forum posts).

I will use the following Blues jr. schematic as a baseline: Fender Blues jr. Schematic Rev.D

To be continued...

[Jazz P Bass]

You could try a 12AU7 in V2's spot.
It helps 'tame' the amp.

[klooon]

You could try a 12AU7 in V2's spot.
It helps 'tame' the amp.

Also "tame" the noise??

[klooon]

I see that in the current design, the negative bias voltage supply and the reverb voltage supply come from the same circuit (regulated by the IC). Could I expect some (noise) problems from the fact that the signals are injected in different parts of the circuit (i.e. reverb and power section) but they are filtered with respect to the same ground node?

Would it be a better idea to make an additional bias supply circuit, taken e.g. from the HV? Like e.g. the Princeton Reverb (and others)??

[km6xz]

That amplifier is well regarded for value but there are some distinct design compromises. No bias adjustment is one of them but also the pc board layout, and ribbon interboard connections is poor. The output tube board is poor but it IS cheap;>) Since you want to get into it and clean it up. Doug Hoffman has recently created a line or replacement boards using turrets for the Blues Jr, and they are reasonably priced. The prototypes were just completed last week.
https://www.facebook.com/Hoffmanamps...about.overview
or Tube amp parts, Guitar Tube amplifier parts, Tube amp information
His boards are really nicely done and a real improvement over the stock boards, and includes the much needed bias pot.

[klooon]

That amplifier is well regarded for value but there are some distinct design compromises. No bias adjustment is one of them but also the pc board layout, and ribbon interboard connections is poor. The output tube board is poor but it IS cheap;>) Since you want to get into it and clean it up. Doug Hoffman has recently created a line or replacement boards using turrets for the Blues Jr, and they are reasonably priced. The prototypes were just completed last week.
https://www.facebook.com/Hoffmanamps...about.overview
or Tube amp parts, Guitar Tube amplifier parts, Tube amp information
His boards are really nicely done and a real improvement over the stock boards, and includes the much needed bias pot.

Thanks for the info! Nice. Unfortunately he leaves out the reverb (not enough space). I would like to try to keep it in though (for the moment). I will probably not work with turrets, but with eyelets and I expect that there is some space if I use a board with a larger height than he uses.

I don't like the idea of the solid state reverb though. Maybe I could add a one tube reverb. I found this on the forum:
One tube reverb link tubenit. This requires adding one extra tube. Then I still have one half tube left for maybe a tremolo...

This also separates the grounding and filtering of the bias and the reverb circuits. Would this be a good idea?

[Chuck H]

I'd keep the SS reverb. I'd just build the stock circuit on a piece of perf board and jam it in there wherever it makes the least noise. I'm a tube guy too. And I've never built a SS reverb into an amp, but if I were refurbing an amp that already had a SS reverb I surely would keep it stock. The "tube advantage" for tone is in the signal distortions and the compressions from tubes and their power supplies. There's just not enough advantage to those characteristics on a reverb. Save yourself the trouble. JM2C on that.

[klooon]

I'd keep the SS reverb. I'd just build the stock circuit on a piece of perf board and jam it in there wherever it makes the least noise. I'm a tube guy too. And I've never built a SS reverb into an amp, but if I were refurbing an amp that already had a SS reverb I surely would keep it stock. The "tube advantage" for tone is in the signal distortions and the compressions from tubes and their power supplies. There's just not enough advantage to those characteristics on a reverb. Save yourself the trouble. JM2C on that.

Thanks. Good point. The SS reverb is fine for me, it adds just a bit of 'coolness' to the amp which I like. I could put the perf board on top of the eyelet board, right?

I would, though, try to separate the power/voltage supplies from the bias ("C-") and the reverb (pos. and neg. 15V). Then I could also pull out the reverb altogether from the circuit if needed because of the noise.

I could do this by tapping the signal before the B+ rectification, just like in other fixed bias Fender amps, with a small negative voltage bias circuit. Further, I could add a tremolo circuit using one half that is currently not used of the V2 tube (like the one I used in the Princeton build, see link and which I like soooo much). What do you think?

[klooon]

OK. First I want to tap the bias from the 140VAC PT secondary tap. Similar to e.g. a Princeton reverb circuit (etc.), I could use the following:



Is there a (simple) way of estimating/calculating the values of the two resistors (dropping and bias resistor)? Or could tell somebody from experience what I should take?

Thanks.

[loudthud]

First, that bias supply won't work. The winding never swings more than .7V below ground.

If you want to know the voltages in the B+ decoupling string you need to know the current at each node, then use Duncans PSUD2 to calculate the voltages. Since the current changes when the voltage changes, it's more accurate to model the preamp nodes as a resistance instead of a constant current load.