NAD Quad Reverb and restoration thread

[Wyatt]

@Wyatt - curious if you would share your thoughts on:

1. Replacing the bias balance with a fixed (adjustable) bias? I could keep the bias balance and just add an adjustable pot to adjust overall bias, leaving the silverface bias circuit.

Many people leave the balance circuit and add the adjustable bias circuit to have both. Not sure if there is a point to a balance circuit really works in a 4x6L6 amp...where is one going to fine two pairs of tubes that have the same mismatch?

2. Changing the PI configuration to a blackface (ab763) circuit?

No opinion. I like SF PI's in Deluxe Reverbs and such, amps that could use a little more headroom (but I also like my Fenders to have a certain steel guitar quality to their clean tone) . I'm usually a get it working stock, then make changes once you've established a stable starting point. That way you can compare the changes before and after.

 

[shoeless]

@Wyatt - curious if you would share your thoughts on:

1. Replacing the bias balance with a fixed (adjustable) bias? I could keep the bias balance and just add an adjustable pot to adjust overall bias, leaving the silverface bias circuit.

That's been on my ToDo list for my silverface Twin for a while now. Put a overall level pot in series with the balance pot, then you have the best of both worlds. It's an easy mod really, I've just been too lazy to do it. Let us know how it works out for you.

Like my Twin, you'll get a good workout moving that Quad around. Does it have casters at least?

 

[wagdog]

Many people leave the balance circuit and add the adjustable bias circuit to have both. Not sure if there is a point to a balance circuit really works in a 4x6L6 amp...where is one going to fine two pairs of tubes that have the same mismatch?



No opinion. I like SF PI's in Deluxe Reverbs and such, amps that could use a little more headroom (but I also like my Fenders to have a certain steel guitar quality to their clean tone) . I'm usually a get it working stock, then make changes once you've established a stable starting point. That way you can compare the changes before and after.

Yeah, I was going to leave the balance pot in because it's less of a radical change - just add the bias pot by replacing the 15k resistor to ground with a 10k pot and a 10k resistor (to start with).

I like what you said about just getting it to work stock - that's the plan. I'll change out the filter caps and add in the bias pot first and see where I'm at.

There is a 220k and .01 cap that comes off one of the legs of the bias and into the NFB (just before the 100 ohm resistor -> ground in the NFB). My reading on that says it's a way that fender used to help with unwanted oscillations in the SF amps. I'll try lifting one leg of that cap and seeing what it sounds like.

I'll leave the PI alone and see how I like it. I do generally like earlier SF fenders, so it may just get left stock.

That's been on my ToDo list for my silverface Twin for a while now. Put a overall level pot in series with the balance pot, then you have the best of both worlds. It's an easy mod really, I've just been too lazy to do it. Let us know how it works out for you.

Like my Twin, you'll get a good workout moving that Quad around. Does it have casters at least?

It looks like the quad cab isn't going to get restored. The top two speakers are pooched and I really don't see a need to replace them so I can have a 4x12 cab. I'll start by mounting it in my old twin project amp/cab (it's a gutted super twin reverb) 2x12.

The 4x12 does have casters, and a side handle and side feet. If for some reason I feel a 2x12 100w fender amp isn't enough, I may restore the 4x12... doubtful though.

 

[wagdog]

Parts came in!

The filter caps are Illinois brand - not my first choice, but they were in stock. As you can see, they are quite a bit smaller than the originals.



Filter caps removed (I mark the originals with numbers and +/- on the ends before removal and set them aside for reference).



Two caps replaced


New caps installed. I also use a hot glue gun to hold them on the board (not shown).



Original bias supply caps



New bias supply caps


Testing with a light bulb limiter and no tubes installed


Installed tubes. Amp sounds great.

As an experiment, I pulled one leg of the 220k/.01uf that comes of one of the bias legs into the NFB loop. Amp didn't oscillate at all and it has a nice shimerry feel to it. Leaving it off.

I checked the bias and it was really cold, 428v B+ and each 6L6 pulling about 25ma of current. That's dissipating 10.7w at idle. 70% dissipation for a 6L6 is 21w - however this isn't a magic number, if the amp sounds good at a lower bias, why burn up your tubes?

I left the bias balance pot and pulled the 15k resistor, substituting a 5.1k and a 10k pot. Using the pot, I got the bias warmed up a bit to just over 15w dissipation at idle. I couldn't hear a difference running it hotter, so I stayed there.

It sounds really good. I'm not going to rewire the PI, no need. I'll play the amp more tomorrow and then hopefully house it in the twin cab I have.

At some point, if I happen to come across a good deal on 4x 12" speakers, I may rebuild the quad cab.

 

[shoeless]

Parts came in!

The filter caps are Illinois brand - not my first choice, but they were in stock. As you can see, they are quite a bit smaller than the originals.



Filter caps removed (I mark the originals with numbers and +/- on the ends before removal and set them aside for reference).



Two caps replaced


New caps installed. I also use a hot glue gun to hold them on the board (not shown).



Original bias supply caps



New bias supply caps


Testing with a light bulb limiter and no tubes installed


Installed tubes. Amp sounds great.

As an experiment, I pulled one leg of the 220k/.01uf that comes of one of the bias legs into the NFB loop. Amp didn't oscillate at all and it has a nice shimerry feel to it. Leaving it off.

I checked the bias and it was really cold, 428v B+ and each 6L6 pulling about 25ma of current. That's dissipating 10.7w at idle. 70% dissipation for a 6L6 is 21w - however this isn't a magic number, if the amp sounds good at a lower bias, why burn up your tubes?

I left the bias balance pot and pulled the 15k resistor, substituting a 5.1k and a 10k pot. Using the pot, I got the bias warmed up a bit to just over 15w dissipation at idle. I couldn't hear a difference running it hotter, so I stayed there.

It sounds really good. I'm not going to rewire the PI, no need. I'll play the amp more tomorrow and then hopefully house it in the twin cab I have.

At some point, if I happen to come across a good deal on 4x 12" speakers, I may rebuild the quad cab.

Nice! It is surprising how much smaller today's electrolytic are compared to those old 70's Mallorys. They don't have that cool, retro cardboard cover either.

For the 10k pot, did you use a ceramic type trim pot, or just a standard carbon type?

 

[wagdog]

Nice! It is surprising how much smaller today's electrolytic are compared to those old 70's Mallorys. They don't have that cool, retro cardboard cover either.

For the 10k pot, did you use a ceramic type trim pot, or just a standard carbon type?

I used one of these: https://tubedepot.com/products/cts-linear-taper-pot-3-8-bushing

It's soldered to the top of the bias-balance pot. I should probably drill a hole for it so I can get to it without pulling the chassis, but it will do for now. I've got it mounted in the 2x12 cab from my old super twin reverb - works well.

I made a few more simple mods, which I'm going to post schematics for. Nothing extravagant, just pulled the wiring out for the master volume "pull boost". I lifted the leg of a resistor to disable that part of the circuit. I re-purposed the master vol pull to turn on/off the brightness cap that was installed permanently on it - sounds better with it off.

 

[wagdog]

Here are a couple schematics with the mods I made. These aren't that radical, most of the info on them can be found online. I didn't "blackface" the amp after all, it sounds really sweet right now. It has a singing quality that I love about silverface fenders. The bottom end is really strong as well - round, piano like lows with my strat.

WARNING: Do not attempt these modifications to your amp without fully understanding what you're doing.

This first mod is twofold:

1. Remove the connection from the bias supply to the negative feedback loop. I've read that fender put this in to prevent unwanted oscillations. The difference between with and without is very subtle, but without the connection it seems to have slightly more sparkle.

2. Added a bias pot by replacing the 15k resistor from the bias balance with a 5.1k and 10k pot in series. Adjusted the bias to run the 6L6s a little hotter.



The second mod removes the boost circuit that is on the master volume push/pull pot. I'm not sure what the boost is supposed to sound like, but it doesn't sound good. Since removing the boost leaves the push/pull switch open, I wired the permanent bright cap on the master volume to the switch, so it could be turned on and off. With the pot in, it's off, out it's on. I personally like it off, but can see where it might be useful.

There are two sets of wires coming from the master volume. One is a twisted pair, and one is a shielded wire. The shielded wire connects the shield to ground. One of the bias supply caps grounds itself through this connection. When the wires are removed, this cap must be ground elsewhere. Fortunately there is a chassis ground nearby.

3. Even with the boost off, this circuit loads the signal slightly. I couldn't hear much difference after lifting one leg of the 12k, but I feel better that it's out of the circuit.

4. The other set of wires (the twisted pair) connected to the reverb input jack and the reverb transformer. Removing both of these wires took it out of the circuit.

5. Use the push/pull a bright switch. You could use it for something else, but this seemed like a nice mod.

 

[wagdog]

I usually wait a week after buttoning up an amp project before it leaves the house. I'll play it a lot in the basement, and then take it to a practice session before gigging, just to see how it mixes with a full band.

That didn't happen this time.

I had a gig tonight w/my country band, and decided to bring this amp. I have a crate power block that rides in the truck, so if something catastrophic happened, I could finish the gig. I usually gig my MESA Express 5:25 - it's a two channel amp and I can get great clean and dirt tones from it. For this amp, I needed some kind of dirt/boost - so I brought along a tube screamer.

Wow, this amp sounded great in the mix. I have to be careful with the bottom end though so it doesn't walk over the bass player. What a pleasure it was to play this thing out tonight. It's mounted in an old super twin reverb cab that has 2x Jensen C12K speakers. It's very heavy, I'm guestimating 80 lbs. Worth it. Here's a pic of it in the cab:


The top end is really shimmery, especially in the 2/4 positions on my roadworn strat loaded with BG v60 pickups. It really cuts through the mix, but I had to be careful with the volume.

 

[GilmourD]

@Wyatt - curious if you would share your thoughts on:

1. Replacing the bias balance with a fixed (adjustable) bias? I could keep the bias balance and just add an adjustable pot to adjust overall bias, leaving the silverface bias circuit.

2. Changing the PI configuration to a blackface (ab763) circuit?

Both of these are easy enough to undo I suppose, but if neither is going to make a huge sonic difference I don't think I'll bother.

I did both of those mods to a AC568 Bassman and it really improved the feel of the amp.

 

[wagdog]

I did both of those mods to a AC568 Bassman and it really improved the feel of the amp.

I've got the bias pot put in (as well as existing balance pot).

I'm hesitant to mess with the PI circuit after playing it last night. I think "feel" is a good description of how playing the amp went; it just felt great, very responsive and alive. It's one of those things I find missing from solid-state amps. Hard to quantify, but it's a big part of the amp experience.