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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 8:34 pm
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Shimmy, is this the STATIC DC resistance of the two halves of the primary (each end to CT)? A slight variance would be expected --- more with a tranny that has smaller gauge primary wires --- as the length of each winding is longer for the second half of the windings, than the first half of the windings (to the CT).

In other words, each revolution or turn gets longer and longer, with each subsequent turn.

The measured ACTIVE AC impedance of each half should be the same, if you apply a sine wave onto the secondary and measure the voltage from each primary end to CT. There is a formula, that escapes me at the moment, for calculating the primary impedance from voltage reading. Then, you can figure out the turn ratio and what the secondary impedance (or speaker load) will be.

I'll try to look up the formulas. And get back to you. If this is what you mean by unequal resistance in the two halves to CT.


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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 8:54 pm
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Yes, DC resistance, but more than just a slight imbalance (82 and 118 ohms). The BDlx, BDvl, BJr and other similar amps all have very balanced OT primary DC resistance (ie, BJr - 99 and 100 ohms, BDlx - 53 and 54 ohms). The Impedance of the OT primary halves in the HRDlx will also be imbalanced, as the DC component is part of the AC impedance. Has to be more turns on one half to have the imbalance, but as I said it's by design in this amp only AFAIK.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 9:04 pm
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Shimmy, this is a dead give-away that those trannies have very small gauge primary windings. Wire with higher resistance per length. I've seen this in some low-end Peavey and Crate amps that have blown the entire output section. Take the tranny apart & you'll be amazed at the gauge used in some recent amps. Makes the windings in old Princetons look huge.

Very common in off-shore iron. Even in "high-end" hi-fi amps. It's one reason new products don't last too long. These trannies seemed to be more susceptible to amp instabilities than the old school heavy duty irons.

Be interesting to take one of those Blues Jr OPT apart. :mrgreen:


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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 9:30 pm
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Same amount of imbalance in the US made replacement that I bought. Again, only in the HRDlx, not the case with the other similar amps mentioned, and all of their transformers are from the same source, foreign made. It's not the wire size, and it's not the slight imbalance that you would expect from a little longer length of wire. It is designed this way for the HRDlx, as The Triode Store pointed out. To get 36 more ohms on one half, that would have to be hundreds, or thousands of extra feet, indicating many more windings on one half. BTW, I have not heard of any of these amps losing an OT often, if ever. If nothing else, they all seem to have decent iron.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 9:36 pm
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BMW2002Ti wrote:
Bluesky, I was told that the use of 82K-ohm on one triode and 100K-ohm on the other is a product of imbalance due to long-tailed PI topology. Best summarized in Valve Wizard's site:

" Balance: Because this circuit does not use a constant-current sink in the tail, the two outputs will not be perfectly balanced if both triodes have the same value anode resistors (although balance will be pretty good all the same). The difference in gain between the two outputs is given by:


A1/A2 = 1 + [(Ra+ra)/(Rk(mu+1))]
Where Rk is the total tail resistance Rb+Rt.
In this case we can see from the characteristics graph that ra = 40k, mu=45:
A1/A2 = 1 + [(82k+40k)/(47k(45+1))]
= 1.06

This calculation is for a 12AY7 Long-tailed PI. The difference in a 12AT7 or 12AX7 is greater.

So the inverting output will be 6% higher than the non-inverting output. This could be corrected by making Ra1 6% smaller in value, but in practice it is not necessary; a slightly unbalanced phase inverter is often quite benificial to guitar tone, due to the additional 2nd harmonic it introduces. Nevertheless, this is why the 'traditional' version of the circuit also uses mis-matched anode resistors. "


http://www.valvewizard.co.uk/ACltp2.jpg


That is true as shown by the first example using 100K ohm resistors for both plate resistors. The 82K/100K is still slightly unbalanced, just not as much as with 100K/100K. The calculator I referenced can be found here:

http://ampbooks.com/home/amplifier-calc ... iled-pair/

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 9:47 pm
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shimmilou wrote:
Interesting stuff. Noting that the HRDlx uses 82K and 100K for the two PI Plate resistors, resulting in a balanced PI output, but the OT primary is imbalanced, approx 82 ohms and 118 ohms. Maybe this OT primary imbalance also adds the second order harmonics, the same as an imbalanced PI output? I have not seen any other similar Fender amps that have an imbalanced OT primary. It is designed this way, I have even purchased a new OT from The Triode Store for comparison. Thoughts?


The HRD III and BDRI use the same OT, PN 5043800.

http://support.fender.com/schematics/gu ... _Rev-A.pdf

http://support.fender.com/schematics/gu ... _Rev-B.pdf

I would agree with what you said. The BDRI does not have as much preamp gain as the HRD, so it makes sense that Fender would use an unbalanced PI and OT to increase the amount of 2nd order harmonics. Since the preamp of the HRD III has higher gain and therefore generates more 2nd order harmonics, using an unbalanced PI in conjunction with an unbalanced OT may result in too much 2nd order harmonic distortion.

I don't know how balanced an original 59 Bassman OT is and I haven't measured the OT in my 5F6A clone.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 10:47 pm
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Bluesky & Shimmy, very interesting. I never heard of purposedly built imbalance OPTs. Seems a whole lot easier to imbalance the PI plates. Or simply run non-matched output tubes. (Kinda puts a big Q mark on paying more for matched tubes).

Thanks! For the answers. :D


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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 11:05 pm
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BMW2002Ti wrote:
Bluesky & Shimmy, very interesting. I never heard of purposedly built imbalance OPTs. Seems a whole lot easier to imbalance the PI plates. Or simply run non-matched output tubes. (Kinda puts a big Q mark on paying more for matched tubes).

Thanks! For the answers. :D


I've not measured the OT in my BDRI. Since the schematics show both the HRD III (as well as the II) and the BDRI share the same OT, I'm taking shimmilou at his word that they are unbalanced.

Interesting point about running unbalanced tubes (or PI). Since power amps are mostly odd order harmonic distortion, if the goal is more even order harmonic distortion, either one would do the job, I guess. But given that preamp distortion (even order) sounds very different from power amp distortion (mostly odd order), would increasing the even order distortion in the power amp make it sound too close to preamp distortion? Would that really be desirable?

I have never had the nerve to crank my BDRI up all the way to hear what the power tube distortion sounds like. I have cranked up my 5F6A clone to "12" (For about 2 minutes!), and it sounds righteous. :mrgreen:

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Tue Nov 27, 2012 11:35 pm
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In 6V6GT push-pull amp, like a Princeton Reverb --- I like the output stage being balanced (at least idle bias-wise). And like you, I like the front end (single-ended) OD'ing slightly before the power tubes start to OD'ing. Or a combo of both. With the tubes OD'ing with their natural tubey essence. And not overly done by setup or pedals.

Prolly just me...

Have a good one! :mrgreen:

EDIT: I remember that even the plate voltages may vary in amps that use undersized gauge windings in their trannies --- using perfectly matched output tubes. As in some early push-pulls (my Woodie Model 26, I remember having plate voltage difference of almost 10VDC). It's not because of something wrong with the output tubes, OPT, or output stage setup... but because of the resistance difference in the OPT windings to CT. However, I recall an old time tech telling me that this is okay, as the Impedance is what is important. Not the resistance.

This tech always used a signal generator and measured the AC voltage on a scope to test the worthiness of trannies. FWIW... :D


Last edited by BMW2002Ti on Wed Nov 28, 2012 8:25 am, edited 1 time in total.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 1:44 am
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I just want to thank the OP for taking the time out for this thread. The contributors have also added to my enjoyment. Just wanted to acknowledge the generosity.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 6:45 am
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modwiz wrote:
I just want to thank the OP for taking the time out for this thread. The contributors have also added to my enjoyment. Just wanted to acknowledge the generosity.


You're welcome. It's all part of what I enjoy about this forum. :D

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 6:37 pm
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bluesky636 wrote:
modwiz wrote:
I just want to thank the OP for taking the time out for this thread. The contributors have also added to my enjoyment. Just wanted to acknowledge the generosity.


You're welcome. It's all part of what I enjoy about this forum. :D


BTW. Your guitarfetish recommendation was another gift. Located some great pick-up that should help me clean up my Tele and the P-90's on my MP Jaguar without compromising the tone I wanted the guitars for in the first place.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 7:22 pm
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bluesky636,

If you get the chance, please measure the DC resistance of each half of your OT primary. I remember measuring the BDvl, HRDvl, BJr, and some other amps, and their OT primaries were all balanced. I thought that the BDlx was also balanced, and I thought that it had a different OT than the HRDlx, but maybe not. :?: I do get a balanced idle current and fairly balanced Plate voltages with new matched output tubes in the HRDlx though, but noticed that my original output tubes had imbalanced idle current before I recently changed them. If the BDlx has an imbalanced OT primary and imbalanced PI, that would account for some of the difference in tone compared to the HRDlx.

BTW, my HRDlx OT is stamped B-050438, and EIA606-740, Schumacher '97.
Image

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 10:38 pm
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shimmilou wrote:
bluesky636,

If you get the chance, please measure the DC resistance of each half of your OT primary. . . . BTW, my HRDlx OT is stamped B-050438, and EIA606-740, Schumacher '97.


I will try and do that the next time I have the amp open.

My JJs have all balanced within a couple of mA and plate voltage within a couple of volts.

That is the number that shows up on the BDRI, HRD II, and HRD III schematics and that I referenced above. The original Blues Deluxe shows 047269 on the schematic for the OT. The Devilles are a different OT, of course. I don't have any plans to pull the chassis and veryify the number on my OT, though.

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Post subject: Re: Modifications To The Fender Blues Deluxe Reissue Amp
Posted: Wed Nov 28, 2012 11:49 pm
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Back again for another round. Tonight we'll talk about negative feedback from the speaker to the phase inverter.

In simple terms, negative feedback is a voltage sourced from a later stage in an amplifier and applied to an earlier stage. It reduces the overall gain of the amplifier (closed loop gain) as compared to the forward (open loop gain) of the amp without negative feedback. The primary results of this decrease in amp gain is a reduction in distortion levels as well as increased stability in the amp. The calculator that I will be using later has an excellent description of the impact of negative feedback when an amp is played "clean" as well as the changes that occur when the amp is overdriven into distortion.

http://ampbooks.com/home/amplifier-calc ... /feedback/

The intent of this mod was to determine the loop gain of the BDRI in the stock configuration, compare it to the loop gain of the 5F6A Bassman, and change the level of feedback if necessary to bring the BDRI in line with the Bassman. The mod itself is fairly simple involving only one resistor.

CAUTION!!! Tube amps contain high voltages that can be lethal. Be sure to check the power supply caps with your DVM prior to working on your amp and drain the caps if necessary. Make sure your amp is unplugged and never stick both hands inside a live amp.

The NFB calculator allows you to enter and vary several parameters in order to calculate the optimum NFB resistor. In the case of the stock BDRI, the NFB resistor is 68K ohms. In this case, I wanted to find the loop gain as a percentage of the forward gain.

Entering the circuit values and the PI gain calculated from the other night, I set the output transformer feedback tap to 8 ohms. I then varied the closed loop gain slider until I got a calculated NFB resistor value of about 68K ohms. The closed loop gain turned out to be about 61% of the forward gain.

Image

I then calculated the closed loop gain for my 5F6A Bassman clone. Note that the presence resistor value in both the BDRI and my 5F6A clone is 3.9K ohms, a 4.7K ohm resistor in parallel with a 25K ohm pot set for minimum presence. In this case, the closed loop gain of my 5F6A Bassman was about 56% of the forward gain.

Image

I then recalculated the NFB resistor value for the BDRI necessary to give me a closed loop gain of about 56% of the forward gain. I found that a 56K ohm resistor gave a closed loop gain of about 57% of the forward gain. Close enough. :D

Image

The NFB resistor in the BDRI schematic is shown here:

Image

The resistor is located on the underside of the tube PCB near the PI tube. It is about in the middle of the board, but can be carefully removed without having to remove the tube PCB.

Image

The new resistor is soldered on top of the PCB.

Image

One component that I could have changed but didn't is the PI tail resistor, R56 in the BDRI. The 5F6A used a 10K ohm tail resistor, the BDRI a 6.8K ohm resistor (as does the 59 Bassman LTD Resissue). If you substitute that value in the calculator in place of the 6.8K Ohm resistor, you will see that the closed loop gain changes very little for the same 56K ohm NFB resistor value. It wasn't worth the effort.

I should also note that the original 5F6A Bassman used a 4.7K ohm presence pot. Using that value along with the stock 10K ohm tail resistor and 27K ohm NFB resistor results in a closed loop gain that is about 50% of the forward gain. To get that amount of gain in the BDRI would require a a 42K ohm resistor (47K ohm being the closest standard value that I could find).

Next we will talk about a few other mods to make the power output stage of the BDRI look just like the 5F6A Bassman. See you tomorrow night. :D

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