Weird silverface Pro Reverb bias wiring (AB668)

[lincolnic]

Hi guys, first time posting here. I'm hoping you can help me out with an anomaly I noticed in my silverface Pro Reverb. I bought this amp a couple of years ago and it's in pretty good shape, but I'd like to convert it to the AA165 circuit. (It has an AA165 tube chart, but the circuit appears to be AB668. The transformers and pots date to mid-/late 1968, so that makes sense to me.) A little background about me: I've built several dozen pedals and one 5E3 -- I'm aware of proper safety procedures during amp work, but this'll be my first time working on an amp I didn't build from the ground up. Hopefully this is the right section of the forum to post in, I couldn't find any other threads about this specific issue.

I've had the necessary components to do the rewiring for a while now, but I'm just getting fired up to do the work. However, the wiring on the bias pot struck me as a bit unusual.



From what I understand, there should only be one wire on lug 3 of the bias pot, which goes to the rectifier board, but as you can see in the picture, I've got two on that lug. One of them does go to the rectifier board as expected, but I'm not sure where the other goes. (The picture I posted is from a while ago, before I realized it was possibly strange, and so far I've been too lazy to pull the chassis again and see.)

This second wire doesn't show up on the layout (and I may have missed it on the schematic -- I'm not the best at reading schematics yet). Have any of you guys seen something like this before? I want to get a better idea of what I'm looking at before I start soldering. I'm sure the real answer is to just open the thing up and see, but I'm hoping this is a known thing either way.

Here's the AB668 layout for reference: https://www.dropbox.com/s/s3jgy0rc9j4yiu5/Pro-Reverb-AB668-Layout.pdf?dl=0

My only thought is that the layout shows two wires going to the rectifier board (yellow and white), but I only see one here (yellow). Is it possible that this extra wire is the white wire I see in the layout? According to other pictures I took, it does seem to go along with the other bundle of wires to the immediate right of the optocoupler. Could they have chosen to solder that to the bias pot instead of the rectifier board? The more I look at it, the more I think this is the answer, but I figured this was the place to ask either way.

I have one other question: I need to replace the filter caps as well, and the two resistors in there are shot. I know the blackface and silverface circuits have different values for those resistors, can I replace them with the blackface values while I change the filter caps if I don't do any other blackface mods yet, or will that have a negative effect without the other changes? I was hoping that I could do the filter caps and those resistors at the same time, before I sorted out any other mods.

Thanks in advance for any help!

[Willabe]

Welcome!   

However, the wiring on the bias pot struck me as a bit unusual.

That amp has a bias balance circuit not a bias adjust circuit. If you going to BF that amp just redo the bias balance to be a bias adjust.

You posted a link for the layout drawing, the schematic is much easier to see the actual circuit. Look at that.

The layout drawing is to be able to locate parts in the chassis after looking at the actual schematic of the circuit(s) of the amp.

You want to see what kind of bias circuit is in the amp, so you look at the schematic to see what circuit they are using then you look at the layout drawing to find where the bias circuit and it's parts are in the chassis. 

I have one other question: I need to replace the filter caps as well, and the two resistors in there are shot. I know the blackface and silverface circuits have different values for those resistors, can I replace them with the blackface values while I change the filter caps if I don't do any other blackface mods yet,

The Fender standard value for resistors across the series connected filter caps is 220K@1w. Just go with that.

You won't do any harm. They are there to force voltage balancing of the 2 caps, so 1 cap doesn't get more voltage across it then the other cap and exceed it's voltage rating. (A bounce with those 2 R's is they will drain the B+ filter caps dc voltage when you turn the amp off.)


                      Brad     

[eleventeen]

I read the OP's s post talking about the "shot" resistors as the 1K & 4.7K "intranode" one-watters between power supply nodes as opposed to the 220K "balancing" resistors across the two series-connected e-caps connected at the highest B+ node.


Just copy the AA165 Pro Reverb as you've planned. I reco using 3 watt metal films for the 1K / 4.7K ones as sold by Doug or others. I usually replace the 220Ks while inside the "doghouse" though it's typically not necessary. 


As for the bias control, the "tail" resistor is a 27K on a AA165, you have a 15K. You could cut out one of those 10K's and place it in series with the 15K = close enough. Then, I note a difference between the 470 ohm resistor coming off the power transformer bias tap going to the backwards diode that rectifies the bias voltage. Your amp looks like it has a 2.7K R in that spot. You *may* need to reduce that R in your amp to get sufficiently negative volts for your bias. I like having MORE than enough negative volts at the pot because it allows better bias voltage range OR to run the amp cooler/cleaner (if that's your preference)


My usual approach is to 1: replace the caps (as you are doing) 2: Blackface the bias ckt 3: Replace the 6 qty 100K plate resistors (the ones in "V" formations) in the preamp section (drastically reduces hiss) and 3: replace the 470 ohm screen resistors on your 6L6 tubes.


So that would be my reco. Get your 2 qty 470 ohm screen R's and one extra for thie bias ckt. And half a dozen 100K's for the preamp plates. The 2 qty 200Ks if you like. All of these, 2 or 3 watt MF. It's just a few more resistors than you were already planning. $2 total.


 

[Willabe]

I read the OP's s post talking about the "shot" resistors as the 1K & 4.7K "intranode" one-watters between power supply nodes as opposed to the 220K "balancing" resistors across the two series-connected e-caps connected at the highest B+ node.

Probably right. That must be why he said different values for BF and SF.

You can change those R values from SF to BF without harming the amp.


               Brad   

[lincolnic]

Hi guys, thanks for the advice so far.

I read the OP's s post talking about the "shot" resistors as the 1K & 4.7K "intranode" one-watters between power supply nodes as opposed to the 220K "balancing" resistors across the two series-connected e-caps connected at the highest B+ node.

You're correct in guessing that I meant the intranode resistors -- apologies for not being more clear! Good to know that I can replace those with the BF values before I do anything else.

That amp has a bias balance circuit not a bias adjust circuit. If you going to BF that amp just redo the bias balance to be a bias adjust.

You posted a link for the layout drawing, the schematic is much easier to see the actual circuit. Look at that.

The layout drawing is to be able to locate parts in the chassis after looking at the actual schematic of the circuit(s) of the amp.

You want to see what kind of bias circuit is in the amp, so you look at the schematic to see what circuit they are using then you look at the layout drawing to find where the bias circuit and it's parts are in the chassis. 

Yep, I'm aware that the amp is currently set up for a bias balance circuit. I think you missed my actual question, though. I'm asking about the actual wiring of the bias pot that I took a picture of. Everything I can find about converting the bias balance to a bias adjust circuit only mentions one wire coming off of lug 3 of the bias pot. In my amp, there are two wires soldered to that lug. I was specifically asking for help identifying that second wire, as I haven't seen it mentioned anywhere or in any pictures I could find of other silverface amps. That's why I posted the layout, because that wire also doesn't appear there and I was guessing about what it might be. I don't want to start desoldering things until I know exactly what I'm doing.

Just copy the AA165 Pro Reverb as you've planned. I reco using 3 watt metal films for the 1K / 4.7K ones as sold by Doug or others. I usually replace the 220Ks while inside the "doghouse" though it's typically not necessary.

I bought components for this a while ago, and I got 1W 1k/4.7k resistors for those spots as 1W was the power rating in the schematic. Is this okay, or is there a good reason to go to 3W?

As for the bias control, the "tail" resistor is a 27K on a AA165, you have a 15K. You could cut out one of those 10K's and place it in series with the 15K = close enough. Then, I note a difference between the 470 ohm resistor coming off the power transformer bias tap going to the backwards diode that rectifies the bias voltage. Your amp looks like it has a 2.7K R in that spot. You *may* need to reduce that R in your amp to get sufficiently negative volts for your bias. I like having MORE than enough negative volts at the pot because it allows better bias voltage range OR to run the amp cooler/cleaner (if that's your preference)

Good catch on the 470 ohm/2.7k resistor difference. My amp does indeed have the 2.7k resistor, and I'm planning to reduce it to 470 ohms as well. And I'll be replacing the tail resistor on the bias pot as well.

[eleventeen]

If you look carefully at the schematic, you will see that indeed, 2 wires junction on what I will call terminal "C" on the above diagram. There is nothing wrong with that at all. One, fat black, comes from the rectifier board as you've said. The other wire (in blue) goes over to the trem circuit and when activated, cuts off the trem oscillator by feeding big negative volts to the grid of the osc tube (through the 2.2 M resistor) That connection exists in the AA165 circuit.


You just leave that wire right where it is. In fact, you leave both wires where they are. Per your pix, you completely cut out the LOWER 10K resistor. The other 10K, you cut the "non-pot" end where it appears to hit the main parts board. Leave the other end soldered to the pot log as it is. Then, cut the "lug" connection on the existing 15K from the bias pot LEAVE THE GROUND END and place the loose end of the upper 10K in series with the 15K. Now, that upper lug has 10K + 15K ohms to ground and that is our (AA165) 27K "tail" resistor. 


Now you take a wire from the wiper of the pot, connect it to the junction of 2 qty 220K ohm R's (that junction goes nowhere else) and the non-junction ends of the 220Ks connect to the 1500 ohm R's (grid stoppers) on the individual 6L6 tubes. Because the point where those 220Ks junction is only fed by the single wire from the bias pot wiper = it goes nowhere where there is a terminal --- you have to decide whether it's OK for that point to float in the air or if you want something different. Me, two resistors with short leads soldered in a "V" is OK with the junction point hanging in the air. YMMV.


Left over from AB668-ville will be 2 qty 100K resistors. These in effect get replaced by the 220Ks. The 100Ks you just cut out. One you show in your pix. Cut it away. The other one is at the end of the yellow wire that comes off the wiper of the bias pot. That yellow wire goes to the jct of the 2 qty 220K's, up in the air unless there is a vacant terminal nearby. (There *could* be if the fiber board is leftover from AA165-ville) Otherwise, that feed junction can hang. You're done.

When I say "you're done" you may want to neaten things up for example maybe you hate the 2 resistors (10K + 15K) in series. 

If you are going to replace the 2.7 K dropping resistor coming off the bias tap....meaning, you now have to go buy the part....then if it were me, I would buy 2-3 watt MF intranode resistors because they are a lot quieter and do the 100K preamp plate resistors as well. Those 100Ks----there is nothing so cheap you can do to a Fender that can make as big an improvement in hiss and background noise. On an old amp (and a 1968 is only a few years newer than a real blackface) the difference can be jaw dropping. It's far beyond worth it.

[lincolnic]

Thanks very much for all the explanation. What I was really asking (which I didn't quite know how to phrase until I read the explanation, if that makes sense) is whether the white wire in my amp is the blue connection you outlined in the schematic there. I suspected it was, and I'll confirm it when I do the filter caps (as I'll do those before I do anything else). As I mentioned, I'm not great with schematics yet, so your outline was very helpful!

If you are going to replace the 2.7 K dropping resistor coming off the bias tap....meaning, you now have to go buy the part....then if it were me, I would buy 2-3 watt MF intranode resistors because they are a lot quieter and do the 100K preamp plate resistors as well. Those 100Ks----there is nothing so cheap you can do to a Fender that can make as big an improvement in hiss and background noise. On an old amp (and a 1968 is only a few years newer than a real blackface) the difference can be jaw dropping. It's far beyond worth it.

Believe it or not, I actually don't have any issues with hiss or background noise on this amp -- it's pretty quiet. Since it ain't broke, I'm not going to fix it just yet, but I do appreciate the advice and if it does become noisy that's the first thing I'll do. As it stands, I've already bought the parts for the mods I intend to do, so I'm going to see how far that gets me. I matched the values on the schematics, and if it turns out I need to go higher wattage, I'll do that next time I make an order.

Again, thanks for the help. All my questions have been answered and I've learned a few things!

[HotBluePlates]

... What I was really asking (which I didn't quite know how to phrase until I read the explanation, if that makes sense) is whether the white wire in my amp is the blue connection you outlined in the schematic there.

Look at the layout you linked originally.

At the diode on the bias board, there are 2 wires (yellow, white) which go to the bias pot and to the tremolo circuit.

Your amp has the yellow wire from the diode to the bias pot. But it has the white wire running from the bias pot to the tremolo circuit. Because the white wire connects to the bias pot lug which has a wire running over to the bias diode, your amp's wiring is electrically the same as what the layout shows.

Fender probably realized they could save a few inches of wire by hooking things up the way they did in your amp versus they way they drew it originally. It changes nothing in how the amp functions, but might matter to them in a production environment where they're saving that few inches in hundreds (or thousands) of amps.