Fender 68 Custom series Reverb changes

[g1]

Was working on one of these Custom silverface sort of re-issues from Fender. Supposed to be all the popular mods built in I guess. This one was the 68 Custom Deluxe.
It seemed to me the reverb was not as strong as what I am used to from standard blackface or silverface.
The only difference I could find in the tube circuits was as shown. I think they may have done it to get a more useable range out of the reverb pot, but it also seemed you could not get the full on surf sounds I am used to at the higher settings.





One other thing was when I measured the tank, I found the output resistance about 50 ohms, rather than the usual 200 ohms or so. Checking the schematic again, and the parts list, it was supposed to be the standard 4AB3C1B tank.
The tank did not have the number stamped in like the accutronics do, but had a label that was covered up by the stick on hook for the reverb cable. I pulled that off and found the number to be RRVL2AB1C1BV4. This is a Ruby branded Belton tank. A bit of digging tells me this is what Fender is using now in several amps such as Deluxe re-ish, this 68 Custom series, and others where the schematic shows the regular 4AB number.
So they are using a 'short' delay instead of 'long', and the output resistance is quite a bit lower. Magic parts calls the output impedance 150ohm but I don't think that is correct. More likely 1150 or 1500ohm impedance I think. (4AB output impedance is 2250 ohm).

So heads up in case you run into a Fender tank that measures 'wrong' resistance at the output end, it may be the new normal.
Not sure why Belton would think they can just change what 'B' means for output impedance, but they seem to have done it. https://shop.magicparts.com/ruby-rev.../RRVL2AB1C1BV4

[Jazz P Bass]

Nice post!

[Mick Bailey]

A reading of 50 Ohms would have had me thinking the output coil was partially shorted and led me down the wrong path, so thanks for the information.

[pdf64]

Doesn’t that model also have a change to the power amp NFB, giving a higher gain? Maybe the designer wanted to mitigate the risk of acoustic feedback from the tank, hence the increase in the value of R32.
I suppose that could be driving the tank output coil change too, reducing the demands on the tank’s suspension.

[g1]

A reading of 50 Ohms would have had me thinking the output coil was partially shorted and led me down the wrong path, so thanks for the information.

That was exactly what I thought, and replaced the tank (with 4AB type). I only found this stuff out later. I was chasing an intermittent fault and thought maybe it had finally failed to shorted turns full time. I expect I may see it back again.

[Helmholtz]

If turns were shorted there would be no reverb output.

An output DCR of 40R to 50R suggests an impedance of around 500 Ohm.
Typically the impedance is about 10 times larger than the DCR.
(see Belton tank numbering system here: https://www.amplifiedparts.com/tech-...d-and-compared)

Higher impedance is possible with a different transducer core, but is not likely.

Lower output impedance means lower output voltage.

Actual output impedance can be found from the inductance (L) as Z(1kHz) = 2000*pi*L.

[g1]

An output DCR of 40R to 50R suggests an impedance of around 500 Ohm.
Typically the impedance is about 10 times larger than the DCR.
(see Belton tank numbering system here: https://www.amplifiedparts.com/tech-...d-and-compared)

That is the link I was going by. Their Belton 'B' output impedance is spec'd at 2250 ohms, yet in the datasheet from the link in post #1, the part used OEM by Fender is spec'd at 150ohms impedance. So I was just guessing at a typo, but 500 ohm impedance makes much more sense by the other examples.

For the record, I checked with LCR meter. At 1Khz setting I measured the proper ~200ohm DCR coil at about 367mH, this ~50ohm DCR coil at 15mH.
From the formula you posted, their rating of 150ohm Z seems to be closest.

I agree that shorted turns should theoretically mean no output, but I try not to rule things as 100% impossible when I have to do hands on.

[Chuck H]

The suitability of the tank not withstanding, and FWIW...

The 1.5M series resistor for the reverb circuit increases the load value following the 3.3M dry mix series resistor changing the voltage division. This would be a modest increase in gain for the dry signal of about 2dB. I wonder if this is one of the "popular mods".?.

Seems like a clumsy effort if that's the case because it causes a roughly 8dB reduction in the reverb signal!?!

A better way...

You could change the reverb series and load resistors to 680k/270k (available in 10% standard) to get the reverb level back up to normal* and still get the gain increase of the "popular mod" on the dry signal. (NOTE*: I did leave the reverb signal reduced a little bit cautionary to other gain mods in that model)

Not much to do about the tank delay other than replacing the tank though.

I guess this amp is a repair or maintenance job so I don't suppose you'll make this change. Just pointing it out for anyone that wanted to raise the reverb level on these amps without losing the secret sauce.

[Mick Bailey]

If turns were shorted there would be no reverb output.

Agreed, but an occasional fault is a coil that reads much lower than usual DCR and gives low output. I rewind reverb transducers, though when stripping off the old turns have never seen anything suspicious. Perhaps moisture gets into the coil, or there's some physical movement that compromises the insulation. I rewind a full bobbin with #42g and then run water-thin superglue over it and a quick spray of Zip Kicker to cure it. To date, I've never had a repaired tray fail again and they always sound better than before. I've also found that a dab of epoxy on the input transducer improves the reverb - some of these are a really sloppy fit

[Helmholtz]

For the record, I checked with LCR meter. At 1Khz setting I measured the proper ~200ohm DCR coil at about 367mH, this ~50ohm DCR coil at 15mH.
From the formula you posted, their rating of 150ohm Z seems to be closest.

A 15mH inductor has an inductive impedance of 94 Ohm @1kHz, geometrically adding a DCR of 50R results in Z = 107 Ohm.
So that's your actual impedance.
This much lower than usual value is likely to result in lower reverb output, but could have been compensated e.g. by stronger rotor magnets.

The ~ 10:1 ratio is not a strict rule, but it's typical for most Accutronics and Belton transducers.
When the output transducer e.g. has a larger than usual airgap, inductance and impedance will drop without changing the DCR.
So the L/R ratio would be smaller.

Shorted turns, leakage or other loss effects will show in the Q value of the transducer inductance.
Assessment would require comparison with a good reference sample of same type.