Triple the uF power tube caps value in AB763 Deluxe Reverb?

[Willabe]

A friend want's me to do a full cap job on his '67 Deluxe Reverb, caps are all original. 

Has anybody here ever tripled the power tube uF cap value in a AB763 Deluxe Reverb that has a GZ34 rectifier tube?

Stock is 2x16uF=32uF caps (in ||), F&T makes a 47uF/500v electrolytic cap. So, 2x47uF=94uF (in ||).
Will the GZ34 take it? Those 2 caps are before the stand by switch, so that will limit the in rush current (some) to just those 2 - 1st caps.
 

[sluckey]

It's pretty common to replace the 16s with 22s (total of 44µF). But 94µF will stress the rectifier and probably kill it in a short time. When the rectifier shorts it's will often take out some more expensive stuff too.

I don't recommend doing this, especially on a '67 DR.

[Willabe]

Yeah, I thought that's too much. (Gerald Weber book, says he triples the power tube nodes.)

Think 2@22uF's || is about the limit?

F&T makes a 30uF/500vdc.

I just want to up the power tube OT CT node and the screen, not the other nodes.
 

[sluckey]

The tube manuals say 60µF is the max for cap input filters.

     http://tdsl.duncanamps.com/link.php?target=002455FA

I've seen a couple amps with 60s but don't recall which ones.

I just want to up the power tube OT CT node and the screen, not the other nodes.

Why? 16s work very well in that amp.

[Willabe]

The guy says it's on the muddy side, he plays it on 7 to 10.

He needs a full cap job so as long as I'm in there I thought I'd up the B+ filter caps a little. Gonna change the CC B+ dropping R's and screen R's to metal oxide. And I'll add 1 ohm K bias R's on the 6V6's.

Wont hurt upping the filter caps, as long as I don't over do it. I thought G. Weber was pushing it too far. So I wanted to ask about it.

I'm going to put in smaller valued K bypass caps on the preamp tubes too. Probably 4.7uF? That'll help too.

And he want's me to take some of the mid range dip in the TS. So, I'll put in a 15K to 25K mid range R.(I'd put a mid pot in the 2nd speaker jack but he uses a 2nd speaker all the time.)

And he want's to be able to use both channels in ||.

I'll be in there a while, but I can use the $$. And I like working on Fenders, easy to get around inside of them.

This amp is clean as a whistle inside, outsides pretty clean too. I'll post some pics when I get some time.

Thanks Sluckey   

[jjasilli]

An alternative is to add a big filter stage after the exiting 1st stage.  The existing 1st stage then becomes a "pre-filter" stage, to keep the rectifier tube happy.

Rectifier > 16's in parallel > small value dropping R > 100uF if you want > power tubes. 


***
Another alternative: put 4X 32uF caps in series-parallel.  Effectively, it's more filtering without changing the uF value.

***
That said, I agree with sluckey "Why? 16s work very well in that amp"  Maybe the problem is tubes or speaker.  How does the amp sound to you?

[Willabe]

Haven't played it yet.

I just asked him some question about how he thinks it sounds now, how loud he plays and what kind of music he plays.

[SILVERGUN]

And he want's me to take some of the mid range dip in the TS. So, I'll put in a 15K to 25K mid range R.(I'd put a mid pot in the 2nd speaker jack but he uses a 2nd speaker all the time.)

And he want's to be able to use both channels in ||.

I have found a very simple, easily reversible way to eliminate the mid scoop is to add a cap inline with the wire going to the bass control from the mid cap.
You're basically lowering the value of the mid cap by adding a lower value in series.
You have to get down to about .002 to have it flatten out the scoop, so adding a .002 in series will get you close enough for rock and roll.

I haven't tried it with the channels running parallel, but I think that might be pretty cool, although I would expect the channel with the mid cap mod to dominate. I personally would do the mod to the Normal channel.

[SILVERGUN]

Thinking more about this...I'm pretty sure the channels on DR are out of phase, so even though you "could" jump them, it may not have a great result

[jjasilli]

Haven't played it yet.
I think we should wait till we get your evaluation of the amp.

[Willabe]

Haven't played it yet.
I think we should wait till we get your evaluation of the amp.

Agree.

He's my age, played all his life, still gigging regularly. I heard him play a little, he's very good.

He does have a nice Tubby Tone 12"/25w/8ohm alnico hemp cone in it. And all NOS tubes, RCA 6V6's, Mullard GZ34, Telefunken, Mullard and 1 RCA small bottle tubes. 

Thinking more about this...I'm pretty sure the channels on DR are out of phase, so even though you "could" jump them, it may not have a great result.

Yes, the BF reverbs 2 channels are out of phase, but that can be rewired. Guys have done that here before. He wants to use an A/B/Y box in the front.   

[SILVERGUN]

Thinking more about this...I'm pretty sure the channels on DR are out of phase, so even though you "could" jump them, it may not have a great result.

Yes, the BF reverbs 2 channels are out of phase, but that can be rewired. Guys have done that here before. He wants to use an A/B/Y box in the front.   

IF he's willing to have you mod it (being an original, and all), that sounds like a great idea. I saw some easy instructions over at the Rob Robinette site. https://robrobinette.com/AB763_Modifications.htm#robrob_Normal_Channel_Reverb_Mod
I think it would be pretty cool to be able to run the 2 channels together and mix the extra mids in as needed.

[Willabe]

IF he's willing to have you mod it (being an original, and all), that sounds like a great idea.

Yes, he is, he asked me about if I could do it.

It's reversible, so no harm done. He's a player, he doesn't want to sell, he uses it for gigs.

I think it would be pretty cool to be able to run the 2 channels together and mix the extra mids in as needed.

Yeah, that's why he wants to use an A/B/Y box in the front.

I saw some easy instructions over at the Rob Robinette site. https://robrobinette.com/AB763_Modifications.htm#robrob_Normal_Channel_Reverb_Mod

Thanks for the link SG. 

[Willabe]

The K bypass cap on the trem roach driver in this amp is a 25uf/25v. Looks to be the cap installed at Fender. 

Schemo shows 5uF/25v.

Which should I order?

Would a 5uF help strip out any unwanted thump or is it better to use the 25uF to better drive the roach?

I could order a 10uF/25v.

Or will it make little to no difference?

[sluckey]

All my schematics for AB763 DR show a 25/25 cap. Where do you see a 5/25 cap?

[Willabe]

My book only has AB868 schemo, it shows the 5uF cap. (Dave Funk tube amp workbook.)

I wonder why Fender changed the value?

[sluckey]

Why are you looking at that schematic? You say it's an AB763 so get the right schematic. Since the amp has a 25µF cap in it, replace with a 25µF.

The Fender roach trem circuit doesn't (usually) have any 'thump'. It is more likely to have a 'tick' due to the flash of the neon bulb.

[Willabe]

I'm looking at it because it's the biggest schemo I have, my eyes aren't what they used to be. 

I tried printing up the AB763 from Doug's library, but my printer finally up and died.

I saw the 5uF and wondered why Fender changed it?   

All other parts values are the same. (Except for the 5U4 rect. tube and the 6V6 grid bleed caps.)

I'll stay with the 25uF then.

Thanks Sluckey    

[Willabe]

My friend was just here and ordered all the parts from Doug. One stop shopping! 

He bought the amp from the original owner who bought it in '67, was sitting in a closet for 30 some years before my friend bought it from him. Looks like it's never been worked on or even opened until now, everything inside looks original, parts/solder joints/wire. I've never seen a cleaner chassis inside. (Not that I've been inside a lot of amps as some here have been.)

He decided on;

1. Full cap job, B+, 6V6 plate and screen caps will be upped to F&T 22uF/500v, the other 2, F&T 16uF/475v, K bypass, some will be 4.7uF to strip some bass end, he plays humbuck's, and -bias supply cap.

2.New metal oxcide for the B+ dropping R's and for the 6V6 screen R's. 

3. Replace the 2 prong power cord with a new 3 prong 12'er, cut death cap out.

4. Mod the 2 channels so their in phase so he can use an A/B/Y box with verb/trem on both. (Robrob mod) 

5. Re-voice normal channel to a lead channel voice. (Robrob mod)

6. Safety diods mounted on tube rectifier socket.

7. Install bear claw tube holders on the 3 - 8 pin octal sockets, their missing.

8. Fix non working trem.

9. Fix broken PI tail R.

10. And anything else I might find.

[sluckey]

SAD! 

[Willabe]

Why?

It's all reversible. Nothing drastic. I'm not throwing away any parts, I'll give him those. 

[sluckey]

Just my sentiment. Hate to see an old classic modified. Old '60s DRs are getting rare and fetch a good price on eBay. I know you'll do a good job.

You know the tremolo requires a footswitch (or shorting plug on the FS jack) to enable the oscillator?

[John]

Willabe, if it's feasible to do, I'd put in the new caps & power resistors (and the 3 prong cord) and then play it and see what it sounds like. I'm wondering if "the sound" won't be a lot closer to what he's looking for just with those new caps?

[Willabe]

Just my sentiment. Hate to see an old classic modified. Old '60s DRs are getting rare and fetch a good price on eBay. I know you'll do a good job.

Willabe, if it's feasible to do, I'd put in the new caps & power resistors (and the 3 prong cord) and then play it and see what it sounds like. I'm wondering if "the sound" won't be a lot closer to what he's looking for just with those new caps?

Yeah, I know, and that's very fair to say, but he plays this amp on at least 8 with humbuckers always. Leo didn't voice his amps for being played that loud and he used his PUP's to voice them.

I'm not drilling any holes in the chassis or adding any eyelets/turrets in the circuit board. (See below.) I'll try and leave as many of the original parts as I can.   

Not many use the normal channel, so it's just sitting there unused and it's only 4 parts,

1st gain stage;

1. Plate R, 100K to 220K,

2. K R, 1.5K to 2.7K

3. K bypass cap, 25uF to .68uF

2nd stage

4. Coupling cap, .047 to .0047

Then he'll use that channel and I'll try and be careful when I take the old parts out and give them to him. Should be able to reverse it back to original then.

Changing the channel mixing point so both channels are in phase and both have verb/trem, is reversible too. I'm going to use a single turret stand off in place of 1 of the 9 pin tube socket screws. Heck I can even save him the original wire from under the eyelet board.

Really should change the e-caps since their are at least 50 years old. Changing the B+ e-caps could save the PT. Upping the values of the 1st 3 from 16uF to 22uf is not drastic. Changing the old cc B+ dropping R's and screen grid R's is just for safety. (I haven't measured them but they could have drifted anyway.)
Installing 2 ss diods on the rectifier socket could save the PT if the rectifier tube ever shorts.

And rebuilding the -b could save a set of power tubes and the OT. I ordered a new -bias filter cap, 100uF/100v, new R, 470/3w metal oxcide, and a new ss diode.

These things should help give the amp many more years of service.

Unless the amp is hissy I'll leave the original cc plate R's. And as long as the tube dcv's aren't off from a coupling cap leaking dcv, I'll leave them alone.

I'd rather not change all the cc R's to metal film and all the ceramic caps to silver mica. Some would do that.
   

You know the tremolo requires a footswitch (or shorting plug on the FS jack) to enable the oscillator?

Yes. He has the ft. sw., said he tried using the trem with it plugged in and stepped on it, but I'll check it out. I can jumper that RCA to ground with an alligator jumper cable and see if it works.

Thanks, Brad Willabe     

[tubeswell]

Making them all 47uF (especially at the smoothing/screen filter cap node) will stiffen the supply and add bass

[Willabe]

Making them all 47uF (especially at the smoothing/screen filter cap node) will stiffen the supply and add bass

Thanks Pete, but he plays on at least 8 and mostly on 10 and uses a second speaker, 15", with humbuckers. So he's got too much bottom end. I ordered 3@22uF and 2 @16uF.

Should be good.     

[jjasilli]

I agree with your approach, but would implement more conservatively.  Personally, I like gain stage 1 modded to 220K plate; 2.7K K; 1uF by-pass cap.  This really un-muddies an amp and gives it more drive.  It may be the only deviation from stock values needed to achieve the customer's purpose.  Then I would do similar to downstream gain stages.  Only then modify the PS. 

[Willabe]

I agree with your approach, but would implement more conservatively.  Personally, I like gain stage 1 modded to 220K plate; 2.7K K; 1uF by-pass cap.  This really un-muddies an amp and gives it more drive.  It may be the only deviation from stock values needed to achieve the customer's purpose.  Then I would do similar to downstream gain stages.  Only then modify the PS.

See reply #23.

[sluckey]

There is another popular mod that reduces a 'muddy' tone. Haven't seen it mentioned in this thread...

Change the .1µF coupling caps between the PI and PA to .047µF or even .022µF.

[Willabe]

I'm almost done with the amp.

Here's a few before pics. You can see how bright the cloth wire colors are.

[Willabe]

.....

[SILVERGUN]

Nice!...I have a '68 silverface that's waiting for a similar job.
It's in "player condition" so I'm not worried about resale since someone already changed out the baffle and grill cloth.

I'm lookin forward to hearing your feedback on this one.

[dunner84]

FWIW, you can eliminate the need for rewiring the channels to be in phase. The radial aby box has a phase switch, and is dirt cheap....

[Willabe]

Thanks, but that wont give him verb/trem on the normal channel.

It is a good option though. 

[Willabe]

If my friend runs a 16ohm speaker in the ext. jack with the cabs 8ohm, that will be a 5.3ohm load, yes?

Should be fine, (at full volume) in the BF Deluxe Reverb?

 

[jjasilli]

Yes, the amp will see that load.  But the speakers may not be happy.  It's generally not a good idea to mix speakers of different impedances. See:  https://geoffthegreygeek.com/calculator-speakers-in-parallel/


The lower impedance speaker will draw an unfair amount of current.  At the very least, this will throw off the balance of their volume. 

[Willabe]

Thanks for the link jj. 

The speaker he has in the cab will handle the amps full power by itself.

So he'll have to try it with both speakers and see if he likes it. 

[Willabe]

Finished the amp, he loved it! 

He said the amp is a lot louder now, probably the cap job. And the sustain on the lead channel is really good without being gritty or too distorted, very smooth, even without any verb. I was playing slide thru the lead channel, volume set on 4, volume on 6 was even better, still smooth sounding, really nice! It really sings now. That surprised me. I didn't think I'd like the lead channel mods sound. I was way wrong! We ran it thru my WGS alnico 15", I love that speaker. 

Both channels together with the Y setting on an A/B/Y box is nice too. 

I didn't have time to take after pics, he has to bring it back so I can reinstall the reverb tank. One of the verb cables RCA jack was broken.

And I need to change a cap to slow down the trem. The verb and trem foot switches work fine.

He liked it so much he took it to use on a gig he has this weekend. Wouldn't leave it until I fixed the verb cable and slowed down the trem for him. 

If I build a Deluxe Reverb amp someday, I would not hesitate to build it exactly the same.

When he brings it back I'll take some after pics to post.

Thanks, Brad Willabe.