Deluxe Reverb replica build

[martin manning]

Thanks!

What values did you use for the PI plates? 100k 110k and a 10k trimmer?

100k, 91k, and 10k trimmer. 470R bias and 22k tail per the schematic.

[pfarrell]

HI Martin—great build! I recently completed a scratch-built Vibroverb (https://www.tdpri.com/threads/ab763-vib ... d.1088870/)—Man, what an amp—it's posted on TDPRI forum though—I'm new to ampgarage—mostly due to Dumble research. The only thing I'm messing with on the VV is making a speaker cab with a 15".

I wanted to know more about your pedal mod with the resistor—what value? I built a Princeton before I built the AB763, and did a lot of experimenting to get the reverb usable beyond 3 (was adding a lot of noise to an otherwise VERY quiet amp) and one of the issues was the aftermarket pedal I bought no shield on the reverb wire, and no ground on the housing... when I picked up the "official" Fender repro item—it cut the reverb noise by half! I'm eager to even improve upon that if possible—though based on reports not many go much past 3 on the reverb—and a lot of people seem to report that noise is inevitable—IDK—and mine is quiet up until around 7 (I have built a lot of hi-fi DIY gear and that's all about good grounding and quietness—guitar amps are a different animal in many ways and maybe my expectation is unfair). The new build—Vibroverb—is more noisy in the reverb than the PR, got better after a tube swap—it's quite usable though....

Also, what did you find with your EH gold pins and noise?
Thanks!

[martin manning]

I wanted to know more about your pedal mod with the resistor—what value?

I ended up with 180k.

...the aftermarket pedal I bought no shield on the reverb wire, and no ground on the housing... when I picked up the "official" Fender repro item—it cut the reverb noise by half! I'm eager to even improve upon that if possible—though based on reports not many go much past 3 on the reverb—and a lot of people seem to report that noise is inevitable—IDK—and mine is quiet up until around 7

The repro foot switch I bought had one shielded lead and one unshielded, but the switches were miss-wired or miss-marked. I just flipped the Rev/Vib sticker over. With the shielded side connected to the reverb and the housing grounded, it is as quiet as could be expected.

Also, what did you find with your EH gold pins and noise?

The amp went home before I got to try them. In my experience, finding quiet tubes for input stages is mostly a trial and error process.

[Reeltarded]

*makes note*

martin flips stickers..

[martin manning]

A small update on this project... I happened upon a good deal on a new production (Mexico) Fender vintage-type two-button foot switch, the internals of which are shown below. It has the same cable configuration as the knock-offs (one shielded, one not), but in this case the housing is grounded with the reverb cable shield. Switches are plastic-bodied, but they feel pretty smooth and solid. The Rev/Vib sticker is much more substantial (thick plastic, I believe) , and oriented correctly ;^) I'll be adding the 180k anti-pop resistor to the trem switch.

Regarding the EH gold pin 12AX7's, I traced the samples I have (two) and didn't find the sections to be particularly well matched but acceptable. The odd thing is that the heaters in both samples draw 170 mA each instead of the spec 150. Never seen that before. Plugging them into the V1 socket of my Express gives a good indication of noise and microphonics, and both seem quite good.

[gktamps]

Beautiful build, Martin, and nice first amp project with your son! Do you think he'll build any more or has he been able to avoid being bitten by the bug?

Question - why the extra bias shunt resistor from your ground test point to ground, since you already have 1 Ohm resistors from each red jack to the ground jack?

Cheers,

Greg

[martin manning]

Beautiful build, Martin, and nice first amp project with your son! Do you think he'll build any more or has he been able to avoid being bitten by the bug?

We have done a lot of pedals together, but this was a first opportunity for him to get comfortable working with high voltage, and measuring/testing the various parts of the circuit. There's a bit of everything in an AB763. He's pretty well fixed for amps, and can borrow mine, so no new projects are planned as yet. With the knowledge gained he should be in good shape if so inclined.

Question - why the extra bias shunt resistor from your ground test point to ground, since you already have 1 Ohm resistors from each red jack to the ground jack?

That is for balancing the power amp using the PI trimmer. Connecting a scope probe to the top of that extra resistor lets you see the current pulses on each side (both positive), on a single channel, and that test point (the common bias test jack) is accessible from outside the chassis for a quick check. The trimmer is adjusted so that all the peaks are at the same voltage.

[gktamps]

He's pretty well fixed for amps

Hmm. His restraint is insulting. How can one have enough amps?

Connecting a scope probe to the top of that extra resistor lets you see the current pulses on each side (both positive), on a single channel

Very interesting. So, tip of probe to red jack and ground of probe to top of the extra resistor?

[martin manning]

He's pretty well fixed for amps

Hmm. His restraint is insulting. How can one have enough amps?

Too many other irons in the fire, I think.

Connecting a scope probe to the top of that extra resistor lets you see the current pulses on each side (both positive), on a single channel

Very interesting. So, tip of probe to red jack and ground of probe to top of the extra resistor?

No, scope probe tip on the black bias test jack, scope ground to chassis/earth ground.

[gktamps]

Excellent - thank you Martin!
Greg

[martin manning]

Hoping this is the end of the foot switch saga. One of the plastic-bodied Carling switches in the Fender vintage foot switch failed, so maybe it was sold cheap for a good reason. Installed two actual vintage style Carling switches, with a 180k across the trem like before, and it's all good again. The first one I made up using a knock-off foot switch unit still works, so unless you really want the look of vintage style RCA plugs, those are probably the better deal.

[Oddvar R]

Great build, good looking. Is the grounding scheme compatible with the BAndmaster AB763 I wonder?

[martin manning]

It's basically the same, without the reverb, so yes it could easily be applied to the Bandmaster.

Note: a layout drawing has been added to the first post in this thread.

[angelodp]

Martin what cable did you use for the foot switch please?

A

[martin manning]

Martin what cable did you use for the foot switch please?

It came with the foot switch, Fender brand vintage style.