Presence control not working

[dude]

I used the chassis of a Crate Vintage club 20 and converted it to a 6V6 Plexi but with two 12Ax7's, schematic below. I used a 18 watt super lite bd. with a TMB layout but changed values for 6V6's. I used the values in the PI from Mark's Plexi.

The amps sounds ok but a little muddy sounding or like a blanket is over it. I used only one volume for the input similar to an 18 watt lite but with a gain pot, think 18 watt lite preamp.

The presence control doesn't seem to have any effect on the amp, wired exactly as the schematic below. I've checked the wiring several times all looks good.

I'm thinking the OT I used could be the problem, I used a MM 59 tweed OT for a Tweed Deluxe, what I had on hand at the time. I'm thinking of trying a different OT know to have more headroom and a bit more wattage. But with the Presence Control not working has me baffled.

I can't seem to see how an OT can effect the Presence control not working (maybe a little?) but with the negative feedback, I don't know.

So my question is can the Tweed Deluxe OT (very early break/up) cause the dark tone I'm hearing and weak Presence control response? This OT is several years old and I never liked the tone in my Deluxe, just too much early break/up.

Also, I'm thinking of changing the 5K6 grids on the 6V6s to 1K5 like the 5F4, Tweed Super, would this help with the dark, milky tone? Maybe even lower the .02, to .01 from the plates of the PI...?

Any thoughts?

Thanks, al

[dude]

I realize without the actual schematic I used, hard to reply.

I mixed them both, the 18 TMB Super lite and Marks 6V6 Plexi schematic and don't have an actual schematic, my first attempt to try to build something from my knowledge. Maybe I'll draw something up for this build.

I'm going to change out the OT, just to make sure it's not that but from my experience rarely has an OT been the problem, then I'll know it's something in the design.  Anyway, it's a good way to learn.

al

[dude]

The bigger OT helped some with the tone and the Presence Control works but not much change.

I can hear some difference from off to full on but I'd like it to give me more Presence or brightness. Amp just doesn't cut, not bad but a little milky sounding and not bright and clean. Gain up high gets distorted, not ready in a bad way but just trying for a cleaner tone before distortion.

Any suggestions on the cap value, .1uf  in the presence control or any other suggestion appreciated.

I drew this schematic of what I did. Any obvious faults?

al

[Willabe]

I had the same problem with the presence control on my Gibson GA-77, so did Sluckey on his 6V6 plexi.

With an 8ohm OT secondary tap and a 100K/5K pot there is not enough -FB to hear a change. The presence pot defeats the -FB but if there's too little -FB there's nothing to defeat so you don't hear any real change as you rotate the presence control.

You need to make the series 100K -FB resistor smaller. Or use a 100KL pot temporarily to dial in the R's value. When you get it where you want, take out the pot without moving the pot wiper, then measure the resistance of where it's set. Use a R close to that value.

I ended up with a 33.2K. (Standard Dale 1/2w value RN65D series.) Now my presence control works great.   

[sluckey]

I drew this schematic of what I did. Any obvious faults?

Yes. You forgot a very important cap on the bottom half of the LTP PI.

[DummyLoad]

I drew this schematic of what I did. Any obvious faults?

Yes. You forgot a very important cap on the bottom half of the LTP PI.

and shorted the tail resistor.


edit: based on how you drew the schematic, perhaps that was an oversight?

--pete

[dude]

I drew this schematic of what I did. Any obvious faults?

Yes. You forgot a very important cap on the bottom half of the LTP PI.

Where?  I'm a dummy, well I feel like a dummy tonight.

EDIT:  I had one 6V6 bad, smoked glass,  filaments not lighting.  Put in matched pair, brightness back but still the Presence knob not very noticeable.

Any advice.? Whats the cap on the LTP and LTP  stands for ?

With new tubes known good, sounds better, pretty good. New tubes, big difference. But no Presence....

al

[sluckey]

See pic for missing cap. I changed the 100K NFB resistor to 22K to increase NFB. This should also make the presence have more effect.

[dude]

Thanks, I used a .1uf cap in the PI there, I just forgot to draw it in the schematic.

I'll lower the neg feedback resister, Willable has a good idea about the pot to find the right value.

The whole time one 6V6 wasn't working, no wonder the amp didn't sound right. I went to check the bias and one 6V6 had no readings across the 1 ohm R. A new pair and the amp came alive.

Friday night here in Philadelphia and a few too many beers last night, 

Just when you think you're getting the building down, you have to never stop looking for the simple stuff like amp not plugged in or bad tube.

As soon as I can move a little better I'll fix the Neg FB loop.

Do you think lowering the 5K6 grid resisters on the 6V6s to 1K5 like the 5F4 Super would help with more head room in this circuit, maybe cut a little better?

Some amps have 1K5 some 5K6 grid R...?


Thanks,
al

[sluckey]

Do you think lowering the 5K6 grid resisters on the 6V6s to 1K5 like the 5F4 Super would help with more head room in this circuit, maybe cut a little better?

I doubt you'll hear a difference. Fender liked 1.5K for the 6V6 and 6L6 based amps. Marshall liked 5.6K or 8.2K for their EL84 and EL34 based amps. I used 1.5K grid stoppers and 470Ω screen resistors in my 6V6 Plexi.

[dude]

Do you think lowering the 5K6 grid resisters on the 6V6s to 1K5 like the 5F4 Super would help with more head room in this circuit, maybe cut a little better?

I doubt you'll hear a difference. Fender liked 1.5K for the 6V6 and 6L6 based amps. Marshall liked 5.6K or 8.2K for their EL84 and EL34 based amps. I used 1.5K grid stoppers and 470Ω screen resistors in my 6V6 Plexi.

I can try them, only takes a few minutes to hang a few 1K5s, thanks.

The schematic shows 100 ohm to the screen grids, that's wrong I used 1K,  5watt for each 6V6, thought I might try 6L6s, I think the PT can handle it, but I might have to change the bias range R if I want to try 6L6s.

BUT:

I just noticed the Presence pot is scratchy in the middle range, and touching the legs on the 5K presence pot made a scratchy noise like there was "voltage there". I checked and there is 5vdc on the wire coming from PI to the pot.

I don't think this is normal maybe one of the caps in the PI is leaking dc voltage, is this normal to have low voltage on that pot leg? Coming from the 10K tail R...?  which cap might be leaking, if this voltage isn't normal?

al

[sluckey]

That's very normal for the presence circuit on your schematic. It's not a problem. Usually a presence pot doesn't get twiddled very much. Just set it and forget it. If you don't want dc voltage on the pot causing a scratchy sound when you turn it, then modify the presence circuit like this...

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm800_lead_50w_1987.pdf

Do you have more presence control with a smaller NFB resistor?

[dude]

That's very normal for the presence circuit on your schematic. It's not a problem. Usually a presence pot doesn't get twiddled very much. Just set it and forget it. If you don't want dc voltage on the pot causing a scratchy sound when you turn it, then modify the presence circuit like this...

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm800_lead_50w_1987.pdf

Do you have more presence control with a smaller NFB resistor?

Rather than change the Presence circuit I'll live with slight noise in the pot.

Yes, I tried a 27K for the -FB, works much better now. I'll hook up a temporary pot to check for the best value.

Funny, I converted this amp several months ago after trying several times to get the original Crate VC20 to sound more like an amp than a buzz pedal. I gave up and on the suggestion from HBP gutted the Crate. I had to plug holes, add tin over the holes, fasten and cut socket holes in the side of the chassis, relocate almost everything. I crossed the 18 watt Super lite with Marks Plexi and the new build never did sound like it should have. I just let it sit for months, then started on the problem again but there was no problem except the wrong presence pot R value.  The problem was I had a weak 6V6 that was a NOS 6v6, "new out of the box".  It was getting worse and the tube was the culprit all the time.

So lesson learned, don't forget the basics. The more technical you get, the more you'll likely forget to check the simple things first.
I'm happy. Now I can tweak a few cap and resistor values if I want to dial in the tone I like. Sounds pretty good like it is, though.

Thanks as always,
al