First tube amp build: Fender Deluxe AB763 style amp (no reverb).

[bob p]

When I build a Deluxe type circuit I use 40/20/20/(20). There's certainly nothing magic about 16/16/16 and if I had the extra caps on the board I'd use two on the first node for 32uF.

IMO 40uF or 32uF sounds better than 16uF without making the amp sound stiff, and it won't place the rectifier at risk. Since he's already got the caps he has the option to go with 32uF, or to rigorously follow the schematic.

Sometimes building exactly to the schematic isn't all that great.

[mikepukmel]

How do you Get 5 filter caps? I see four, the reservoir cap before the standby switch, and one each for the screens, PI and preamp nodes.

Yep, Randall, you're right! I ended up unintentionally building a bit of a Frankenstien here. The chassis, and such is from a deluxe reverb (cheapest chassis I could find by a factor of 1/2 about from other sites). The main board is a non-reverb. I read through all of the schematics from AB763 up through the 70's (1172?) (thanks to Rob Robinette for his great web site pages on the evolution of the Deluxe) . The schematics I have for the reverb model has 2 x 16's in (correction) parallel (thanks Randall). I have a bunch of photos of period era cap boards and it has 2 of those huge cardboard tube electrolytics near the rectifier tube.

I asked around and looked through data sheets and the GZ34's made today should handle 32uf without any problem. Far enough from rated 60uf max.

So, back and forth, reading various comments on various sites about what would happen if I used larger capacitance right off the rectifier, for a non reverb, I decided to build as close to a Deluxe Amp AB763 as possible, with a few minor changes: 1) 2x16's right off the rectifier, 2) no vibrato, 3) move the grid stop resistors to the grids on the first 12AX7 for each channel, then run shielded wire from the tie point on the input tube to the input jacks. Everything else is about the same within the constraints of finding new components that match values for the old schematic.

[mikepukmel]

When I build a Deluxe type circuit I use 40/20/20/(20). There's certainly nothing magic about 16/16/16 and if I had the extra caps on the board I'd use two on the first node for 32uF.

IMO 40uF or 32uF sounds better than 16uF without making the amp sound stiff, and it won't place the rectifier at risk. Since he's already got the caps he has the option to go with 32uF, or to rigorously follow the schematic.

Sometimes building exactly to the schematic isn't all that great.

Thanks for the tips and info, Bob. Yep, nothing magic, but I just don't know enough EE theory to know how big I can go without sacrificing reliability of something. (Wish I did!!). That's always my personal dilemma: I decided a long time ago to build as close a replica as possible. Then I get reading all you guys awesome comments about this mod or that mod, and which parts to use, and i start tinkering. I looked at what I came up with, decided, just before I ordered all of the parts, to put a few simple mod's in and this one seemed to be on the lower risk end. Hopefully.

[Justin Thomas]

Caps in series are like resistors in parallel. Caps in parallel are resistors in series. So your two 16s in series will add up to 8. The caps in theDR are in parallel.

Justin

[mikepukmel]

My recollection is that the DR uses 32/16/16/16 as the 4 nodes, so he's going to need five 16uF caps to get there, tying two 16 together for the first node. To do that he'll need to move the resistors one position to the left in his mockup before he solders them in, putting a jumper in the rightmost position.

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Those F&T are rated at 475V so I wouldn't worry too much about working voltage on them. that's a nice board. Where did you get it?

Hi Bob, All the boards in this frankenstien are from Hoffman.

Eyelet Board Order page!

Very well built, thick fiberglass. he has them in black and 'natural' 'glass.

[mikepukmel]

Caps in series are like resistors in parallel. Caps in parallel are resistors in series. So your two 16s in series will add up to 8. The caps in theDR are in parallel.

Justin

Thanks, yep, the 2 x 16's nearest the rectifier tube will be wired parallel, like the DR. (type'o in my original post)

[mikepukmel]

Randall makes a good point... the Deluxe does only have four filter caps. Are you using five to put the first two in series or parallel? If you are putting two in series for a margin of safety regarding voltage, be aware that your resulting cap will only be 8uF; you may want to use a pair of 32uF or 47uF in series to get closer to the original spec, and a little bit more never hurt. If you're putting two in parallel, I'm not sure if that boosts the VOLTAGE handling, even if it ups the capacitance. But I'm willing to get schooled on that...

Personally, I'd use two 32s in series, because I don't trust caps with only a 30V margin, especially if you need to do some testing with tubes out for a while... I think you'll be okay for the time being, but might be worth upgrading in the future.

Justin

Thanks Justin and Randall! Oh man, major brain strain involved, for me, on this part of the amp. So, I get the blackface AB763 schematic and it has 2 x 16uf, 450v caps, in parallel, right off the rectifier tube. (actually, they're all 16uf 450v). The transformer is marked as 330v going into the GZ34. Asking questions from various sources, most say that 475's are OK for this amp. But for, say, the twin, that has ss rectifiers, you are going to get very close to 1.414 *vac in (with small adjustments for the drop across the 3 series rectifiers), which is why they had to use two series 70uf 350v. Roughly double the voltage handling at about 35uf.

But this is all descriptive for me, I don't know enough to do the complete calculations with any confidence.

The other issue is component availability. These are the highest voltage 16uf caps I could find. There are 20 or 22 axials, with higher voltage ratings and many were radial and not available in axial, so that was a whole other bump to get over. At first I thought Id get the less expensive, modern components, and just make a different board that would take radial caps, with the bodies glued down with some kind of goop. Decided since I have the awesome Hoffman board, just get components that I can get easily, that other people are using as well, so i ended up with 5 x 16uf 475s.

I hope it doesn't all go up in smoke 6 months after I have the thing running. That would be bad.

[mikepukmel]

Everything Ive read says not to run the amp with the rectifier tube plugged in, but with the output tubes removed. I can sorta, kinda, sorta understand why by looking at the voltage/current draw curves on the data sheets.

Do you guys find that its necessary for testing to do that? If so, what are you looking to diagnose or test?

[Enzo]

???

When only the rectifier tube is installed and no others, there is nothing drawing current from the rectifier other than the caps charging.

When I build an amp, the first thing I do is power up with no tubes, to check to see if fuses blow, then to see if heater voltage is getting to all the sockets.

Next I install the rectifier only and power up to see if the B+ supply works.

Once I know I have good high voltage, I install the signal tubes. Then proceed to check out the amp thoroughly.

[mikepukmel]

???

When only the rectifier tube is installed and no others, there is nothing drawing current from the rectifier other than the caps charging.

When I build an amp, the first thing I do is power up with no tubes, to check to see if fuses blow, then to see if heater voltage is getting to all the sockets.

Next I install the rectifier only and power up to see if the B+ supply works.

Once I know I have good high voltage, I install the signal tubes. Then proceed to check out the amp thoroughly.

Learning here, so please bear with me. The way it was described to me, if you have all of the tubes installed, even if you're not playing, the tubes are drawing some current. As you play, at various loudness levels, the tube rect sags and recovers. so, the output voltage swings over some range. I don't know how to calculate that range. Anyway, there were posts on one of the builder sites that said if you yank out all the tubes except for the rectifier, the voltage will increase because you're not drawing current. But since the caps are installed and operating, the voltage could peak close to or over the filter caps rated voltage.



Anyway, that is what Ive read in a few places, sounded reasonable, sounds like its wrong.

[g1]

The cap voltage rating should be high enough to allow for the amp running with the regulator unloaded (other tubes removed).
So what they are suggesting is a poor way of dealing with a design error.
However, in some vintage amps, the increase in modern line voltage could lead to the situation where the unloaded voltage is too high for the caps. In these cases it is wise to monitor the voltage on the caps while power tubes are removed.

[mikepukmel]

The cap voltage rating should be high enough to allow for the amp running with the regulator unloaded (other tubes removed).
So what they are suggesting is a poor way of dealing with a design error.
However, in some vintage amps, the increase in modern line voltage could lead to the situation where the unloaded voltage is too high for the caps. In these cases it is wise to monitor the voltage on the caps while power tubes are removed.

Thanks. So, if the originals were 450v, back in the "safe low line voltage" days, and we're using 475's, with today's line voltages and who know what else that affects cap life, are the 475v caps probably ok for the first two caps in this amp? I can't find where anyone has measured the B+ voltages on a similar construction (new construction) Deluxe type amp. Lots of repair folks working on originals, but not many on new ones at least that I ca find.

[Enzo]

Who sells 475v caps? I believe you if you say they have them, I just never seen them.

If it were mine, I'd just grab 500v caps from the bin and go on with the rest of my life. DOn't obsess over the tiny stuff.

[bob p]

F&T Germany.

[mikepukmel]

Who sells 475v caps? I believe you if you say they have them, I just never seen them.

If it were mine, I'd just grab 500v caps from the bin and go on with the rest of my life. DOn't obsess over the tiny stuff.

Hi Enzo, Bob beat me to it. The amp parts sites all have brands, F&T and Sprague 16uf 475v. The 500v are all 22uf. This was one of those painful learning curve things. Before I started working on this little project, I was deluded to think I could look at a 50 year old schematic, then go to half dozen electronics distributors and just buy the part. Little did I know that the Sprague Atom's are like 14.00 a piece (absolutely ridiculously overpriced for a 50 cent capacitor), They are 16uf 475v. So, after looking at lots of photos of what people are building, I found F&T's for about 5.00. Still overpriced, but they fit the board I have and are relatively easy to get.

Not any kind of obsession, just for the first amp build, looking for parts that match the schematic