Adding a presence control to a Princeton Reverb build

[Ibanezgrind]

Hi everyone,

I have been trying to find my solution to adding a presence control to my home build Princeton reverb. It sounds wonderful! I am using JAN Philips 6L6wgb and have a fixed bias/cathode bias switch. Man, those tubes cook in the best way in cathode bias mode! Sweet tones. I also have switchable neg. feedback with 3 different values (1.5K/47, 2.7K/47, and 6.8K/47). My need is to add some highs when I want to make the amp that much more versatle. A bright cap is only effecitve when the volume isn't maxed out. I have come across some good ideas but noting that really stands out as "YES, thats exactly what I need!"

I found a couple things in my research. The first from Tubeswell (I will attach below) points me in the correct direction. He changes the bypass cap to alter the high frequencies and adds an additional resistor in series with it, at least in a few of the examples. I know this is getting me really close!

Tweaking the neg. feedback resistor(s) (2.7k/47) is fine with me as long as I can get a presence control there. I know that the less negative feedback I have, the less effective the presence control will be so if it comes to it, I can ditch the 6.8K resistor in my switch. 

The other example I found is for a Tweed deluxe which I know isn't the same as a Princeton reverb's but it seemed like it could work with a few changes.

I have a 100ohm and 1K linear pot lying around and a variety of resistors and caps to choose from. I've read the standard .1uF presence cap won't work and to increase it to the 5uF range.. much larger!

And this is as far as I have gotten with this subject. I would appreciate any advice on what would work best. Thanks!

[vampwizzard]

https://robrobinette.com/AB763_Modifications.htm#Presence_Control

[Ibanezgrind]

Thanks Vampwizzard for sharing that link. This link points to a presence control for a LTP PI. I have a Princeton reverb cathodyne PI. I think I’m going to rig a switch that switches between stock (1.5K/25uF) cathode bias and the mod I attached - The boosted mid\high freq fixed presence.

[sluckey]

Just copy the circuit of the 5F4...

     https://el34world.com/charts/Schematics/files/Fender/Fender_super_5f4.pdf

[Ibanezgrind]

Thanks for replying Sluckey. I was tempted earlier to copy the same schematic but read some other fourms on the subject that mislead me and then started to think, "maybe it's not that easy." I should have come here first to confirm what I was thinking.

Now, I use the same 56K fb and 1500K tail and the 5K presence pot is tapped between the two. Is the 5F4's nfb ratio close enough to that of the AA1164? I know you mentioned copying the 5F4 exactly, just making sure. I remove the 25uF cathode bypass cap around the 1.5K as well? Is this an important part of the AA1164 sound or can I live without it?

I appreciate your wisdom! Thank you.

 

[sluckey]

Make it look just like the 5F4. It's new territory for me. Just try it and see. Let us know what you think.

[Ibanezgrind]

Will do!

I'll try it out tonight and report back tomorrow. Thanks again.

[jjasilli]

Sluckey gave you good topology; but you may have to adjust component values in your presence circuit.

Can't seem to find the output impedance of 5F4; but with 2 speakers I suspect it was 4 Ohms - compared to Princeton 8 Ohms.  So there's a difference in the NFB sourced output impedance.


Also the signal drive voltage from PI > Power tubes in the 5F4 needs to be higher to drive 6L6's, than in the Princeton's 6V6's.  This would affect the amount of NFB voltage required to produce the same effect. 

[66Strat]

The output impedance is 4 ohms for the 5F4 Super as JJ indicated. The 5F4-A Super is the 6V6 version and it also has a 56K feedback resistor. The 5E4-A Pro has an 8 ohm secondary like the Princeton Reverb. The Pro has a 100K feedback resistor. The 5K Presence control in conjunction with the .1 uf capacitor and the 1.5K cathode resistor will have the most impact on frequencies greater than about 1,000 hz. The value of the feedback resistor will primarily control the frequencies less than 1,000 hz. If you have a 100K pot, you might want to wire it in temporarily to find the resistance value that sounds best to you.

Modded to add schematic.

[Ibanezgrind]

Great insight for me. thanks everyone! Let's see what happens.

[DummyLoad]

from 8 to 4 ohm load, we'd reduce R nfb from 100k to 68k. give or take.


--pete

[2deaf]

Note that 6L6WGB's are being used in this Princeton instead of 6V6GT's.  I'm calling the g_m of the 6L6 5000umhos and the g_m of the 6V6 3500umhos.  This has a significant effect on the calculations for the NFB resistors.

Attached are a couple of cheat-sheets that may be helpful here.  They are taken from solid-state theory and then applied to thermionic valves. 

[Ibanezgrind]

I did some experiementing last night and tried to do the 100KL pot as the fb resistor to find the optimal resistance and didn't have any. I stuck with the schematic values (2.7k/47) and used it's ratio and got as close as I could with a 68K/1500 combination. I left the bypass cap on just to see what would happen and as expected, the presence control didn't work. I took it off and tried again, success! There was a nice audible sweep from start to finish. However, removing that bypass cap around the 1.5K tail as per the 5F4, the wonderful tone the princeton had was gone. Although I liked what it did for my cathode bias setup for the 6L6wgb, cranked - it was glorious but it compromised my clean tone to which I would be playing clean most the time. The gain that cap provided was gone and so was the mod. Well, at least I tried it. Unless there is a way to keep the bypass cap then the presence control isn't for me. I did try out the mod in the first attachment I provided - the boosted mid-high frequencies mod (far right). I rigged up a switch to change between the stock 1.5K/25uf cathode arrangement and a 1K/1uf with a paralleled  .1uF cap. I think this will be what I will stick with since I get to keep both sounds.

Thanks everyone for the wealth of knowledge that you shared with me. 2deaf, I will ask my engineering buddies at work to help me decipher your attachments. Good info!

[jjasilli]

I left the bypass cap on just to see what would happen and as expected, the presence control didn't work. I took it off and tried again, success!

What bypass cap??? Do you mean the cathode bypass cap on the PI driver?  You need a split cathode resistance on that tube to do that with a partial bypass cap-- like in your drawings above.  Simply adding a cathode bypass cap to sluckey's topography will bleed all NFB to ground through that cap.

[Ibanezgrind]

Yes, I believe it's that cap. The V3B cathode in the AA1164 schematic. I'm sure thats exactly what happened and why it didn't work. Just testing is all 

[jjasilli]

The AA1164 schematic does show the correct way to inject NFB to the PI cathodyne driver tube when using a cathode bypass cap.  Note: the tail end of that cap is not connected directly to ground.

[2deaf]

What about that Sunstone/DvNator one?  You would use a 10K resistor instead of the 4.7K for R_f in order to have similar NFB.

[Ibanezgrind]

Oh! thanks for reminding me of that other attachment. I do have a 1KL pot available. What about the presence cap he has at 2.2uF? He adjusted that for frequency optimization over the standard .1uF. I am guessing I need a larger value than 2.2uF? I have no clue how to figure that out. I am all ears if anyone would care to explain this to me! Thanks in advance!

[2deaf]

The 180R/2.2uF combination is a good equivalent to the 4.7K/0.1uF combination.  There is this thing called a cut-off frequency that is useful for determining what will happen with frequency dependent circuits.  It's sort of like a breaking point.  For a resistor in parallel with a capacitor, the cut-off frequency (f_c) is 1 / (2 * pi * R * C).  For the 180R/2.2uF combination, f_c = 1 / (6.283 * 180 * .0000022) = 402Hz.  For the 4.7K/0.1uf combination, f_c = 339Hz.  Those frequencies are pretty close to each other in the logarithmic audio world. 

If you use 3.3uF and 180R, f_c = 268Hz.  If you use 1.0uF and 180R, f_c = 884Hz.  So 2.2uF is a good value among readily available capacitors.

The impedance of the parallel resistor and capacitor drops off rapidly as frequencies increase above the cut-off frequency.  The decreasing impedance changes the NFB so that it becomes less effective as frequencies increase above the cut-off frequency.       

[Ibanezgrind]

2deaf, tried it out (10k/180r) with a 1kl pot with a 2.2uF and there’s life, but not much. I have to turn the volume up pretty high to really hear it. The effect is most notable at the end of the sweep and not gradual from beginning to end. The effect is there but really subdued. I’m happy we have a start. Any ideas where to go from here? I appreciate your help!

[jjasilli]

Did your original NFB work???  If, yes, a disconnect SW might suffice.  Or, add a SW'able treble bleed cap to ground in the NFB loop. 

[Ibanezgrind]

Well I didn't actually try the orinigal NFB values but used a 10K/180 combination suggested by 2deaf which is the same as 2.7k/47. I might have to copy what the Sunstone DvNator has, a 4.7K/180 as it brings more neg. feedback which will help the Presence be more present. I'll  keep experimenting.   

[jjasilli]

I don't agree with experimenting in the dark, especially if it doesn't produce quickly.  With the following info you can mathematically design a proper NFB circuit for your amp, subject to verification by empirical testing.  You can get help here with any of the steps.  Once you have a functioning NFB circuit, then you can complicate it with a presence circuit. 

I think you're using 6L6's in a Princeton circuit, but with what OT?  A schematic would be helpful.  Pick a known Amp schematic with NFB, which is very similar to your Power Amp, re Tube type. From that you can derive the power tube drive voltage, and verify that it matches your amp.

Knowing the output Wattage of the sample amp > its normal speaker load, AND the impedance of its OT secondary tap from which its NFB is sourced, you can compute its NFB voltage at its source (at full clean power).  Then look at its NFB circuit to see its voltage divider circuit.  The NFB series resistor forms a voltage divider with the cathode resistance of the tube which it feeds.  Now compute its net NFB voltage at the point of insertion.  That's presumably the net NFB voltage you need in your amp.  Until you have a good, working NFB circuit, IMHO it's premature to add a presence circuit.

[Ibanezgrind]

I appreciate this guidance. Seems easy enough to do! I will attach the OT schematic. It’s a normal Princeton reverb AA1164 with Jan Philips 6L6WGB, GZ34, 420V at the plates, using the 8ohm tap (16ohms if using 6v6s). I did increase the main B+ Filtering to 40uF. I think the closest power amp to mine is maybe the 5E5 tweed pro? Not sure exactly what variant of the pro. I know it uses 6L6G power tubes, 8ohm OT tap and 100K feedback / 5KL, presence. Please correct me if I’m wrong of if there is a better amp suited for my needs.

[2deaf]

Are you connecting an eight Ohm speaker to that eight ohm tap?  How were you switching between those three different NFB combinations? 

Assuming an eight Ohm load on the eight Ohm tap, I get an open loop gain of 37 and a closed loop gain of 22.7 for -4.2dB of NFB with the 47/2700 combination.  I get a 17.4 closed loop gain with the 47/1500 combination for -6.5dB of NFB.  As you have already pointed out, cancelling a good portion of -6.5dB will have a more noticeable effect than cancelling the equivalent portion of -4.2dB.

I have a couple of ideas that default to the stock NFB.

   

[Ibanezgrind]

Hi everyone, I wanted to post this before anyone gets a chance to continue looking into this subject for me. I decided not to go with this mod for this circuit. I think there are other ways to go about this. Instead of adding some high frequencies from the nfb circuit, I can tigten up the low end in the pre-amp section instead and make it switchable so I won't have to lose my Princeton clean tone that I love. There are plenty of resources availble for that and its more straightforward to me.

Thanks again for everyone's input and sharing their knowledge with me. I did learn some new things so it didn't all go up in flames 

Peace!