Fender Deluxe Reverb loses Vibrato, Gains ODS Overdrive

[pkb]

I've had a wonderful opportunity to spend time inside a Deluxe Reverb with the AB763 circuit. I've tried a few different things to turn it into an amp with some rich singing overdrive and I've had limited, though worthwhile, success.

The amp came to me already heavily modified. A bunch of caps had their values reduced (PI coupling caps reduced from .1 to .033, for example), a cascade mod from the first half of normal channel tube to first half of vibrato channel tube on a switch, a FET device in the first tube socket with a switch to increase the bias of that tube socket for the FET, and a bright switch on a push-pull pot in place of the lamp on the front panel of the amp. These are a few of the mods.

I've removed some of them, changed others, and have decided that what will make this amp really interesting and useable is to remove the circuitry for the vibrato and replace it with the overdrive circuit from the Dumble ODS #124. I won't have to drill any new holes and the faceplate is already labeled so that I can sort of explain, more metaphorically, what those knobs do in terms of distortion (speed for gain, intensity for volume).

I'm also going to use the gaping hole where the lamp and then the bright switch was to implement a master volume knob similar in function to the ODS. Rather than having the switching be on a relay (OD or Clean), I'll use a DPDT switch on the master volume control to dictate whether the normal channel goes through the OD circuit or straight to the PI tube.

This setup will allow me to leave the vibrato channel relatively untouched, through its namesake will be removed. It will function as it did from the factory except for removing the possibility of using vibrato. It will still have functional reverb and go directly to the PI tube and maintain relative isolation from the other construction going on inside the amp.

Of further note is that I've separated the cathode bias/bypass on the second halves of the norm and vib preamp tubes which were previously wired together. There was some strange feedback between channels wherein the normal channel, when turned up past 5, would bleed through to the vibrato channel as a fuzzy flubby bass signal. This was obvious when I had the normal channel disconnected from the PI tube but still had signal passing from the Vibrato channel to the PI. Isolating the cathode grounding solved this.

The scheme is to have the normal channel wired up as if it were the clean channel of the Dumble ODS. I don't yet have plans to revoice it to be more Dumbleesque. It sounds great and I'll reconsider if it doesn't sound so great through the OD circuit.

I won't be making any changes or adjustments to the Dumble OD circuit except to try a fixed resistor in place of the OD trim pot and to use a push-pull DPDT pot/switch in place of the ODS switching relay. I'll pull out all of the vibrato channel's components to make some space for the new OD circuit stuff. I've photoshopped a very rough outline of this and am convinced that there are enough solder points and real-estate to complete the job.

My concern is my power supply voltage and capacitance. If I'm reading the schematic correctly (AB763 Deluxe Reverb http://www.ampwares.com/schematics/delu ... _ab763.pdf), the vibrato tube receives voltage from the B stage of the power supply circuit from a 16uf capacitor. This is a few resistors before the later stages of the amp and I'm wondering if it will be too difficult to achieve the prescribed 200v at the plates of the OD tube. Is the solution as simple as changing the 100k plate resistors to something a bit beefier/resistier?

Also, Dumble mysteriously used two 10uf caps in parallel to power the OD circuit. I don't understand, if not for space or component availability reasons, why he didn't use a single 20uf/500v capacitor rather than two 10uf/500v capacitors. From what I understand, they would act identically except for the possibility of the 2x10uf being of a tighter tolerance.

If I just go with the 16uf @475 volt cap that's in the amp and add some resistance to get down to 200v, will I compromise whatever small chance I have at getting a nice Dumble OD tone?

Any other advice for attempting this mod? Once I have a more workable draft of my layout plan, I'll post it for critique.

Sorry to write so much about the project without including pictures. I hope y'all survive the read.

PKB

[Cliff Schecht]

Two smaller caps in parallel will have a lower ESR than one bigger one, but I doubt HAD was thinking about this.. Probably was just using what he had on hand.

[David Root]

Sounds like a great project. I would say only don't discount the relay and the footswitch for clean/OD selection. It's not hard to do and believe me you WILL enjoy it!

[bcook]

Leave the roach wired up, except connect the variable resistor leads between the treble and bass pot. Your vibrato foot pedal becomes a PAB.

[Structo]

There was a Deluxe Reverb ODS on Youtube before, probably still there but it sounded great.

So I definitely think it is possible to get great Dumble tone out of that amp.

Dumble was known to use surplus parts in a lot of his builds back in the day when those places were around.
I think the internet probably doomed those places one by one.

I take it you will keep the 6V6 power tubes?

If you keep the tube rectifier you will be limited to what the power tube filter cap can be but maybe 40uF?

You could probably get away with 16uF on the other nodes.

[pkb]

There was a Deluxe Reverb ODS on Youtube before, probably still there but it sounded great.

So I definitely think it is possible to get great Dumble tone out of that amp.

Dumble was known to use surplus parts in a lot of his builds back in the day when those places were around.
I think the internet probably doomed those places one by one.

I take it you will keep the 6V6 power tubes?

If you keep the tube rectifier you will be limited to what the power tube filter cap can be but maybe 40uF?

You could probably get away with 16uF on the other nodes.

I'm not a confident schematic reader yet, but it looks like the first two filter caps are a pair of 16uf 450v wired in parallel. Wouldn't that be closer to the desired 40uf? It looks like the node directly feeding the power tubes is a 16uf cap, but the rub is that it's also feeding the vibrato circuit. I'm planning on using that node to power the OD circuit as well.

I don't yet understand how power supply circuits work well enough to modify them by, say, adding another filter cap of an appropriate value to power the OD circuit while replacing the 16uf cap powering the power tubes with a ~40uf job... is that realistic?

I just bout new 6v6's, so I'll keep those for a while. Is there work to be done if I want to go with 6L6's besides just plugging them in?

Thanks for the excellent suggestions. I'll put together some "before" photos and sound clips in the next few days.

[Structo]

Part of a tube rectifier's specification is how big the first filter cap is after it.

It is usually listed on the data sheet for the tube.

It's because a uncharged large cap appears as a dead short circuit for a split second until it can charge up.

The larger the cap, the longer it appears as a short or low resistance.

So too big of a first stage filter can harm or destroy a tube rectifier.

When capacitors are wired in parallel you add the values.
So two 20uF caps in parallel equals 40uF.

The formula is the exact opposite of the resistors rules.
When in series you use the formula you would use for two resistors in parallel.
So two equal value caps is half of one.
Two 20uF caps in series equals 10uF but the voltage rating adds so if the two caps are 350v caps you now have 700v worth of caps.

6L6's demand more current than 6V6 so it probably isn't a good idea to plug 6L6 into a 6V6 amp.
Also the bias would be completely different.

Here is a handy program to use.
It's the Tube Data Sheet Locator.
You enter in the tube number and it gives the pinout, substitutes, ratings and some data.
On the right hand side it has internet links to actual data sheets for that tube.

So when you run this program you click on the Search button then enter your tube.
Such as a 6V6GT.
It's important you enter the exact type tube it is or it will give you erroneous data.
For example a 6L6 and a 6L6GC are not the same tube data wise.

http://www.duncanamps.com/tdslpe/

It's important to note that even if you have been told so, there is high voltages and current inside a tube amp.
Many times the capacitors will hold a charge even after the amp is turned off or the plug pulled from the wall.
If you touch the wrong thing inside you could electrocute yourself.
So the first thing to do is learn how to safely discharge the caps before working inside an amp.
This does not mean you take a screw driver and short the + end of a cap to ground.
That is not safe and can harm you and the amp.

Anyway have fun.

[NickC]

.................................. It's important to note that even if you have been told so, there is high voltages and current inside a tube amp.
Many times the capacitors will hold a charge even after the amp is turned off or the plug pulled from the wall.
If you touch the wrong thing inside you could electrocute yourself.
So the first thing to do is learn how to safely discharge the caps before working inside an amp.
This does not mean you take a screw driver and short the + end of a cap to ground.
That is not safe and can harm you and the amp.

Anyway have fun.

+1

I built this cap discharge tool with 2" diameter PVC pipe (and end-caps), heavy ground wire, heavy-duty alligator clips, and a 10k 10watt resistor in series inside the pipe:

[img1728]http://homepage.mac.com/swamptone/Let_It_Bleed.jpg[/img]

I clip it on and walk away for awhile (with 3-prong power cord plugged into wall, standby switch set to "operating/amp on" mode ......... allowing the charge to drain off slowly and safely. After a while I meter it, just to be on the safe side. If charge is drained, unplug from wall and resume working on the amp.

I got careless a month or two ago and got zapped. Once bitten, twice shy.

[talbany]

.................................. It's important to note that even if you have been told so, there is high voltages and current inside a tube amp.
Many times the capacitors will hold a charge even after the amp is turned off or the plug pulled from the wall.
If you touch the wrong thing inside you could electrocute yourself.
So the first thing to do is learn how to safely discharge the caps before working inside an amp.
This does not mean you take a screw driver and short the + end of a cap to ground.
That is not safe and can harm you and the amp.

Anyway have fun.

+1

I built this cap discharge tool with 2" diameter PVC pipe (and end-caps), heavy ground wire, heavy-duty alligator clips, and a 10k 10watt resistor in series inside the pipe:

[img1728]http://homepage.mac.com/swamptone/Let_It_Bleed.jpg[/img]

I clip it on and walk away for awhile (with 3-prong power cord plugged into wall, standby switch set to "operating/amp on" mode ......... allowing the charge to drain off slowly and safely. After a while I meter it, just to be on the safe side. If charge is drained, unplug from wall and resume working on the amp.

I got careless a month or two ago and got zapped. Once bitten, twice shy.

Nick
That's cool..You should market that thing..You could call it the Taliban cap bomb..
Warning: Do not go near an airplane with that thing..

Tony

[NickC]

........... That's cool..You should market that thing..You could call it the Taliban cap bomb..
Warning: Do not go near an airplane with that thing..

Tony

Funny you should mention that ............ I watched Hurt Locker over the weekend .......... man-o-man what a disturbing film! No .... I won't be going near any aircraft .... or anywhere else with it in tow.

One good thing about my homemade cap-discharger is it's size ...... makes it conspicuous ..... easy to spot on my cluttered work bench.

[Structo]

Whoa!

I was going to ask what you are discharging?

10,000 watt transmitters?

That is similar to what I have helped newbies with.

[img:509:466]https://lh5.googleusercontent.com/-6iMI ... 20Tool.JPG[/img]

[pkb]

I've been proceeding with the advice to unplug the amp in on/play and strum the guitar until until the amp doesn't make any more sound. I can't find any reading above 5v anywhere after doing that and have replaced resistors, capacitors, redone wiring, added, and removed--all without the slightest noticeable shock.

Is this not a good way to do it?

[M Fowler]

I used romex wire, large resistor, plastic tube, alligator clip/wire for my cap zapper.

[heisthl]

The beauty of the FET simulation resistor is that the typical D style build bleeds automatically if you just leave the standby switch in operate position when the amp power is removed. Takes less than a minute to bleed off to a level that won't bite.

[ChrisM]

I've been proceeding with the advice to unplug the amp in on/play and strum the guitar until until the amp doesn't make any more sound. I can't find any reading above 5v anywhere after doing that and have replaced resistors, capacitors, redone wiring, added, and removed--all without the slightest noticeable shock.

Is this not a good way to do it?

I do that.

Or
Short V1 plate to ground and turn the standby to operate. All voltage slowly bleeds through the plate resistor.