Fender Deluxe Reverb loses Vibrato, Gains ODS Overdrive

[NickC]

Whoa!

I was going to ask what you are discharging?

10,000 watt transmitters?

Did some shows up in the backwoods of Canada (north of Maine) years ago. The band was invited to an in studio interview and to play a few numbers live on-air. The FM station was literally a radio-shack, 20 feet away from the transmitter mast. They set us up in the shack, which was the broadcast booth. I had a strat and Fender tube-amp .......... which was utterly unusable from all the hash and EMF. The DJ cued up a tape, and walked me to the admin office up the driveway to call a friend of his and see about borrowing an acoustic guitar. While we did that, the engineer ran a 100' extension cord outside, up the driveway and plugged my amp in. He expected me to play from there. He had the most puzzled look on his face when I asked how I could hear what the rest of the band was playing if I was outside ... 100' away? True story.

[pkb]

All of the parts arrived, mostly from www.newoldsounds.com (they've got good prices, good selection, but have trouble shipping things quickly for the last couple weeks). I started by replacing the stock 16uf filter caps with fresh ones @475v. They all tested like 20uf caps, but I'm not upset. Maybe the Fender gods are.

I made a couple of recordings of some unfocused noodling guitar with the "before" mods and then plugged in the soldering iron and took them all out. I also completely removed the components for the vibrato circuit (just the vibrato circuit, the actual channel is still there without the tremolo functionality). The only component remaining is the tube socket.

Attached is my shamefully messy redraw of Weber's Fender AB763 layout from his amp kits sales site (https://taweber.powweb.com/store/kits_60a.htm#6A20H). I used Ted's layout rather than Fenders because it's easier to read. Other than some higher-voltage electrolytics, it seems to be dead-on to the Fender AB763 layout.

Also attached are a few before and during pictures of the mod in progress. The "before" pictures point out the cascading switch that puts signal from the normal channel after the volume control into the grid of the vibrato channel before the tone stack. The signal is also then routed through a master volume control with push/pull switch to add/remove the undistorted normal channel signal direct to the phase inverter tube.

[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3445.jpg[/img]


[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3446.jpg[/img]


[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3448.jpg[/img]


Below is an attempt at a "presence switch". It's got a 330pf cap coupled with a 470ohm resistor (half of the resistance of the stock neg. feedback ground resistor). It definitely adds presence and not much mud. Just a SPDT on/on switch.
[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3449.jpg[/img]




[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3456.jpg[/img]



[IMG:800:533]http://i315.photobucket.com/albums/ll45 ... G_3457.jpg[/img]



[IMG:800:641]http://i315.photobucket.com/albums/ll45 ... ODMod2.jpg[/img]



Can you see the #124 ODS overdrive channel? It's beginning to take shape using the space and leads vacated by the tremolo stuff.


Ignore most of the purple and yellow scribblings: I was trying to think through a lead dress and switch design that temporarily fell through because the DPDT/1M audio pot/switch I got wouldn't work for any of the available chassis holes.


I have some questions about the B+ voltage calculations. I don't know what to expect when the circuit is finished and I plug it in, but my guess is that I'll have a lot more voltage at the plates than #124 had (200v). The #124 schematic calls for 100k plate resistors. Since I'm getting the B+ from the first filter cap and that's also the cap feeding the power tubes, is there a way to calculate the size of the resistors I'll need to achieve the correct voltage? I'm hesitant to build the circuit and turn it on without knowing whether I'll get 220v or 470v and if the new 12ax7 will survive it or not.

I'll also work on putting the "before" sound clips somewhere. I'm not a guitarist, so don't expect the playing to sound any particular way.

Please pardon my photoshop skills. The layoud mod was done in a hurry but it is useful.

So, B+ management ideas? Anything else?

Thanks!

[pkb]

The circuit is fully installed and the amp doesn't catch fire when turned on.

Unfortunately, the B+ voltage is quite high. With 100k plate resistors, the voltage is 412v. With 1m resistors, it still measures ~350v.

Should I be taking the B+ from a different part of the amp?

I'm sure I don't understand how the power supply circuit works and how to calculate voltages based on the large number of factors going into that equation. Is there some basic info that might set me on the right path here?

Thanks

[pkb]

And after jumping the power supply that feeds the preamp tubes over to the OD circuit, I've achieved 285v. For some reason, the 100k resistors I installed make no difference to the voltage. Measured on either side of the resistors, the voltage is only 2 or 3 volts difference (but at least it's lower after it goes through the resistor). Measuring the difference in voltage before and after the 100k resistors feeding the first tube of the normal channel shows a ~100v difference and the B+ running at 193 or so. The pre-resistor voltage is the same at about 285v.

Also, the OD circuit doesn't make sound. I'll have fun re-thinking every solder joint tomorrow.

[Fischerman]

I don't know your level of experience pkb so pardon me if I write something obvious to you.

Ohm's law can go a long way. V=IR.

So if you have a resistance with a voltage across it...you have current flowing through that resistance. If you have a resistance that has no voltage across it, you have no current flowing through that resistance. So you don't appear to have any current flowing across those 100k plate load resistors. If you didn't have a tube installed that would do it...or if the cathode resistors were not grounded...anything that would prevent current from flowing.

[ER]

You want to go off of "D" not "B" on your power supply, you should be seeing around 200V or less (originally170-180v with 110v at the wall). Check the plate voltage on V1, you want the same or just a tad more for your OD tube. Or better yet add one more CR filter stage to the power supply and adjust your dropping resistors to get the right voltages, so you would add an "E" to you power supply which would go to the clean stage and reserve "D" for the OD tube.

Let me know if that makes any sense...

For the time being, run it off "D" and see where that gets you.

Check all voltages to ground, not across a resistor. Clamp the negative lead to a good ground point and check voltages with the positive lead with one hand and keep the other one in your pocket. Good luck!

Oh yeah, and download this free program and play around with it;
http://www.duncanamps.com/psud2/index.html

Peace, ER

[pkb]

Than you very much for the advice. You've accurately guessed my level of experience. Two months ago, I didn't know how a resistor functioned in a circuit or what a capacitor might be doing. I also hadn't owned a guitar amp until a couple months ago. No offense taken at any assumption made about experience level. My two threads on ampgarage encompass most of what I've done with amps.

Is there any reason the power supplies to the OD and clean channels should be separated? Will there be interference or crosstalk via the power supply between the channels?

Would there be anything wrong with adding another power cap after D and without a dropping resistor that would supply just the OD tube? Additionally, is there any magic about a 100k plate resistor or can I adjust the plate resistor value to achieve the 200v present in a DumbleODS?

[ER]

That's how we learn, it takes a while for things to sink in. You never have all the answers, you just learn to ask the right questions.

"Is there any reason the power supplies to the OD and clean channels should be separated? Will there be interference or crosstalk via the power supply between the channels?"

Everything gets amplified as it's fed from one stage to another, you'll notice the increase in filtering corresponds to earlier and earlier stages of amplification in the amp. You want the earliest stages to have the most filtering because any AC ripple will get amplified over and over again with each gain stage.

"Would there be anything wrong with adding another power cap after D and without a dropping resistor that would supply just the OD tube? Additionally, is there any magic about a 100k plate resistor or can I adjust the plate resistor value to achieve the 200v present in a DumbleODS?"

There's no need to add another cap, the reason for having another dropping resistor is to fine tune the voltages for each tube for its particular job, and the extra cap gives it better filtering since you're adding extra stages of amplification to the signal with the OD.

You can dial in voltages with the plate resistor but this can effect the operating point of the tube, and it needs to be done in conjunction with the cathode resistor. Dumble stuck with a 66:1 ratio between the two to keep the tube in a good part of the curve. 100k:1.5k is what fender used and also what's in #124 (what folks call "low plate") There are others but for your application I'd stick with 100K:1.5K and work with the power supply to get what you need.

Check the plate voltage on your clean stage tube (pins 1 and 6) it should be right in the ball park voltage wise, you want the same for the OD tube.

Also double check your resistor to make sure it's really 100K or still 100K if you checked beforehand (unsolder one end and measure it).

Good luck, ER

[pkb]

The OD channel is functional! It took some chin-scratching and resistance measuring to figure out what the problem was. In fact, there were a few mistakes: One of the cathode resistors was 150k not 1.5k; the .005uf coupling cap was hooked up to the bias supply instead of the output (haha here) and once those two problems were fixed, it "worked". It still had an alarming parasitic oscillation that was resolved by grounding the coaxial shielding of the leads going to and from the pots as well as putting the resistors on the tube socket lugs instead of on the resistor lugs. I also removed the resistor that was standing in as the trim pot before the first side of the OD tube since I didn't feel like I was getting enough saturation.

The OD channel sounds great with the normal volume below 5. Lively, great bloom and sustain. With the normal channel above 5, it starts to get really thuddy and heavy sounding. I think this is because some signal is bleeding into the OD channel using a different path. I confirmed that something was up by unsoldering the lead that returns the signal from the "ratio" knob to the DPDT normal/OD switch but leaving the switch in the OD position. What was left was the bad parts of the sound from when the normal channel's volume was turned up beyond 5--the heavy, buzzy, thuddy, dull signal and none of the nice OD channel balanced gain and drive. It was quiet relative to the amp's potential volume so it's not as obvious when everything's maxed out, but at lower levels, it gets in the way.

Another interesting thing is that the AB763 circuit shows 1.5k cathode resistors. My amp shows that it uses the AB763 circuit. The cathode resistors for the normal/vibrato channel measure at 1.8k but do match up with the voltage (180v) on the schematic. I can't get better than 148v for the B+ plate using 100k plate/1.5kcathode resistors for my OD channel even when completely removing the dropping resistor between the C and D power caps.

I did end up adding a 500v 20uf cap to the doghouse for the OD channel after "C" and then putting the 16uf cap for the normal channel inside the chassis--thus adding a power cap to the existing 5. Could this account for voltage loss?

Another issues is that the master volume knob seems to throw a blanket over the sound. The amp only speaks clearly when the master volume is on 10 and the volume is regulated with the preamp volume controls. I did put a 15pf "bright" cap across the pot as suggested by #124's circuit but it seems like I'd need a whole lot more pf's to make up for the other high and mid-high losses.

Lastly, I don't understand why the 100k ratio pot was chosen as linear. It's useful between 0 and 3 and then immediatly overruns the normal channel's ability to keep up with it. I'll try replacing it with an audio taper pot to see if it's more agreeable to work with.


Sorry to cram so many different "issues" (I like to call them puzzles) into one post and not include any eye or ear candy.

Thanks for reading and considering...
Patrick