Restoring a 1974 Deluxe Reverb - Seeking Advice

[martin manning]

Looks like Fender soldered a solder lug to the chassis and then the wires to the lug. If so, that's easier to work with.

[bcmatt]

Looks like Fender soldered a solder lug to the chassis and then the wires to the lug. If so, that's easier to work with.

Yes, that's what it looks like... but the lug is covered in solder so you can't really see it.

[bcmatt]

I noticed in a group someone talking about using thermistors to drop the AC voltage before the Power transformer like this:

Thermisters.jpg

I have no idea how much voltage, but they said a CL-80 or CL-90, and trying one on the hot wire after the power switch, and then a second on the neutral wire if needed.
Have you heard of this idea before or do you have any thoughts on this?

[bcmatt]

I see that probably my biggest problem is that I am seeing a wall voltage of 123.4 VAC. I should measure it more often.
Today I swapped in the JJs from my Tweedlish 5e3.
372VAC 5U4 plates -> 456VDC 6V6 plates -> 8.2 watts dissipation.

What do you think of the different 5R4 variants?
63 volt drop compared to the 50V drop of the 5u4.

FB_IMG_1707100643819.jpg

What do they refer to when talking about max input capacitance?

[martin manning]

Maximum input capacitance refers to the reservoir capacitor the rectifier cathodes are connected to.
Reducing the input voltage to the PT is an option. NTC thermistors are used to limit inrush current, but they do have some voltage drop even after they are up to temperature. Something else you might try is a bucking transformer, in the form of a 12.6 VAC filament transformer with 2-3 amps current rating. That would reduce all voltages by ~10%, and get you down around 410 on the plates.

[bcmatt]

Well, for now, I dropped the voltage to the schematic plate voltages with a 250 ohm resister right after the rectifier (4 x 1k 5 watt in parallel).

I also finished a head cab that I'll use for now.

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The death cap switch that's been replaced with different nfb options actually seems unnecessary. The stock value keeps the amp most balanced.

I think this amp actually needs a ppimv instead...

[martin manning]

How does it sound and feel with that extra resistance?

[bcmatt]

How does it sound and feel with that extra resistance?

Still very loud, bold and clean. I'm not noticing any sag, if that's what you are referring to.

Or, is it possible that the more resistance is also cleaning up the preamp too much? I guess I should check voltage readings all on down the line...

The problem is that I didn't get to play it much before trying to solve this "problem". Just a little in my home office.

We finally got it on stage last night now that I finished a cab for it yesterday. My friend played it during rehearsal while I mixed FOH.
Being a non-mv amp with no VVR or anything, it has to be run with the volume down about 3, which is still pretty spanky clean...

[martin manning]

Still very loud, bold and clean. I'm not noticing any sag, if that's what you are referring to.
...it has to be run with the volume down about 3, which is still pretty spanky clean...

That's what I was wondering. Running at low volume the current demand will be low, so that's probably why it's not noticeable. I think maybe a PPIMV would be a good idea.

[bcmatt]

Has anyone experimented with ppimv types in old Deluxes?

I would assume type 2. I've used them several times in different circuits.

I could see the type 3 one that switches between cut or mv also being useful... but really it would always be on mv setting so it's more about which sounds best...probably type 2?

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[martin manning]

The post-PI has the issue of upsetting the NFB loop, and for an amp like yours I think it would be best to avoid that and implement a pre-PI MV ala Marshall 2204. Simpler, too.

[bcmatt]

The post-PI has the issue of upsetting the NFB loop, and for an amp like yours I think it would be best to avoid that and implement a pre-PI MV ala Marshall 2204. Simpler, too.

So, what are the results of upsetting the feedback loop? Does it start to get fizzy?

What if I kept the NFB switch in that allows to switch between the stock 820R,a 1.5K and no NFB? I was already thinking I would put the MV pot in the extension speaker jack and leaving the NFB switch in... because I was curious if NFB setting preferences would change as the MV was introduced anyways.
I suspected that the slight preference for the stock 820R value yesterday was due to the fact that anything to tame the over-all volume on stage was helpful...

nfb2.jpg

[martin manning]

Post-PI MV lowers the loop gain, so the NFB ratio decreases. With the MV at the PI input it stays the same (it's outside the loop).

[didit]

Hello --

Kept meaning to drop in here with some advice from experience. A few years back I kicked off a short thread that's somewhat on-topic:: analysis of a pair of BFDRs. The detail from that I'd recommend is adding a pair of resistors on the rectifier anodes; i.e., the input side. Measure your transformer's secondary resistance and at the very least get the summed resistance (secondary to centre-tap plus add-on) on each side to at least 120R. As noted in the thread, I wound up installing 68R 2W. Its been stable and cool while running regularly for about 5 years now. Depending on your objectives targeting roughly 400V for initial B+ seems a good starting point.

Best ..

[bcmatt]

Hello --

Kept meaning to drop in here with some advice from experience. A few years back I kicked off a short thread that's somewhat on-topic:: analysis of a pair of BFDRs. The detail from that I'd recommend is adding a pair of resistors on the rectifier anodes; i.e., the input side. Measure your transformer's secondary resistance and at the very least get the summed resistance (secondary to centre-tap plus add-on) on each side to at least 120R. As noted in the thread, I wound up installing 68R 2W. Its been stable and cool while running regularly for about 5 years now. Depending on your objectives targeting roughly 400V for initial B+ seems a good starting point.

Best ..

Thanks! I'll definitely try that. So, you want a total of 120R per side including what the resistance the secondaries already measure? I'll check out your thread