1973 pro reverb - weird wiring?

[Warrenghall]

Good day

for those knowledgeable in fender amp schematics I think I’ve got a weird one. I recently purchased a stock 1973 pro reverb. It currently play well and sounds good - although occasionally the volume fluctuates. I figure the first step is recap as it is till has all originals. When I opened it up I found a few things I found weird - and could not related back to the schematic1. In the filter cap can there should be 2 220 k resistors. In mine I seem to have 1 220 k and the other seems to be 300 ohms. Haven’t been able to find any schematics where bothe resistors aren’t 220 k’. Should I change them to be both 220 k as part of the recap?



2 when I look at the main board there are typically three double 25/25 capacitors in the middle of the board, which are replaced by 6 25/50 during the recap. I don’t have these. The first on the left has 2 original capcitors, while all the rest of the spots just have a single 25/25. Again - have never seen this on a schematic. Also - as these are not paper caps do I still need to change them or should I let well enough Be?

[phantomman]

Are you certain that the two power-rail resistors are original (they both should be Allen-Bradley carbon-comp types)? I would return these to standard schematic spec values (metal oxide resistors will provide enhanced longevity).

As for the dual 25/25 caps, these provide cathode bypass for the pre-amp tubes. They are electrolytic types and should be replaced at routine, regular intervals (usually when the electrolytics of the filter array are changed). The 25/50 values are suitable for this purpose.

[phantomman]

Addendum:

I just checked my Pro Reverb (an unmolested example from 1968, AA165 circuit revision) and the power-rail resistors are 1.0 KΩ and 4.7 KΩ. Thus, it appears that the schematic specs are an anomaly.

[Warrenghall]

Thanks for the quick response. I had hoped to post pictures which would have made my diatribe more understandable. So would you suggest leaving the two existing resistors in the array with their current values? Also my concern for the cathode caps are
1. mine are not the typical brown paper ekectrolytic but rather the hard white cylindrical type and
2. They are typically in pairs coming off in a triangle formation. In my case several of the preamps have a single cap not two? Very weird.

if you provide an email I’ll shoot you the pictures


Warren

[phantomman]

Generally I subscribe to the "if it ain't broke, don't 'fix' it" philosophy. If the amp is otherwise performing properly I wouldn't tamper with it. I would recommend inspecting the caps in the filter array and bias supply though. If any geriatric wear is apparent (leaking fluid, bulging hulls, etc) then I would replace those components since failure of any could cause catastrophic damage. I recommend F & T caps as the best electrolytics available.

[Michael Smith]

If they are balancing resistors in the "doghouse" they should be the same value. Dropping resistors can have different values. I am NOT an expert with respect to the Pro Reverb. OK I just looked at the schematics for Revision AA270 and AA1009, and they both have 220K Ohm 1 Watt balancing resistors across the reservoir caps which are wired in series (the big 70uF at 350 volt caps). These balancing resistors may not be visible until you disconnect one end of the reservoir caps and lift them up, as they are often mounted underneath. You can go with 2 Watt resistors here. Make SURE you observe the polarity of the reservoir caps and wire in the new ones exactly the same as the old ones. (Some smaller Fender amps like the Deluxe Reverb do NOT have the 2 reservoir caps wired in series.

Then both schematics show a 1K Ohm 1 Watt dropping resistor for the next filter stage, and a 4700 Ohm I Watt dropping resistor between the 2nd and final filter stage. Your 1973 Pro Reverb most likely uses one of these 2 circuit revisions, probably the AA270. I haven't studied the 2 schematics enough to determine the differences between these two circuit revisions, and the copy of the AA270 I found was very poor and hard to read.

[Michael Smith]

Addendum:

I just checked my Pro Reverb (an unmolested example from 1968, AA165 circuit revision) and the power-rail resistors are 1.0 KΩ and 4.7 KΩ. Thus, it appears that the schematic specs are an anomaly.

Those are the dropping resistors. Although looking at an earlier schematic the dropping resistors are 4.7K and 10K Ohms. The balancing resistors should be 220K ohms, per my review of the schematics.

As I noted in my reply to the OP, I am NOT an expert in the Pro Reverb.

[Warrenghall]

These are definitely the balancing resistors in the dog house. IÂ’m thinking of just installing 2 220k 3 watt resistors - can I use metal oxide? - and being done with it. Really wish I could post a picture to help me out here!

[Michael Smith]

These are definitely the balancing resistors in the dog house. IÂ’m thinking of just installing 2 220k 3 watt resistors - can I use metal oxide? - and being done with it. Really wish I could post a picture to help me out here!

Here is a picture of the doghouse of my Bassman amp, after I recapped it. You can see the 220K Ohm balancing resistors next to the reservoir caps (which are wired in series) Note the polarity. The dropping resistors are along the "bottom" of the eyelet board (away from the tube sockets).

The purpose of the balancing resistors is to make sure that the same voltage goes to each of the reservoir caps. You can post a link to a picture here. You just have to load the photo to an image hosting site like Imgur.

Edit: Yes, you can use metal oxide resistors.

[phantomman]

Those are the dropping resistors. Although looking at an earlier schematic the dropping resistors are 4.7K and 10K Ohms. The balancing resistors should be 220K ohms, per my review of the schematics.

Correct.

[Michael Smith]

Here is a layout diagram. I circled each of the 25/25 Cathode bypass capacitors. 6 in total (or 3 if you have the "double" caps). Also, there is a 5/25 in the tremelo circuit. Note pins 8 of the first 2 preamp tubes are tied together. I would see no reason to deviate from this layout (other than upping the values to 50 volts as mentioned above if you wish).

[Warrenghall]

With reference to the schematic above the first cathode cap,is a double, as it should be, but all the rest seem to be just one cap,instead of 2. https://imgur.com/mS6TLYf

now here’s the view of the doghouse showing the two resistors that should be 220k each - which clearly aren’t. https://imgur.com/HDbO7bL

again - if you look at the board, in my opinion, everything is as it came from the factory.

thoughts? Should I just replace the cathode caps with new ones of the same value or just leave them. I am pretty set on reconfiguring the doghouse resistors to 2 x 220k

thanks

[Michael Smith]

With reference to the schematic above the first cathode cap,is a double, as it should be, but all the rest seem to be just one cap,instead of 2. https://imgur.com/mS6TLYf

now here’s the view of the doghouse showing the two resistors that should be 220k each - which clearly aren’t. https://imgur.com/HDbO7bL

again - if you look at the board, in my opinion, everything is as it came from the factory.

thoughts? Should I just replace the cathode caps with new ones of the same value or just leave them. I am pretty set on reconfiguring the doghouse resistors to 2 x 220k

thanks

When you post your pictures to Imgur, select the image and then click on "BB Code" and hit copy. Then come back to the forum here and hit "Control and V". Then your pictures will actually appear in the forum, rather than having to open up an Imgur page to see them.

With respect to your doghouse picture, you can't actually see the balancing resistors. They are underneath the 2 largest capacitors. (the 2 on the far right in your picture). I just see the 2 dropping resistors, a 2.2K and a 1K. Those filter caps are clearly the originals based on the date code, 13th week of 1973.

I will have to study the cathode bypass caps picture more closely and compare it to the schematics from that era. Someone else may have some insight.

Edit: One more thing, the 2 largest caps in the doghouse are wired in series (positive of one connected to the negative of the other) Make sure you wire the replacements in exactly like the originals. Good thing you have a photo of the "before".

[phantomman]

Edit: One more thing, the 2 largest caps in the doghouse are wired in series (positive of one connected to the negative of the other) Make sure you wire the replacements in exactly like the originals. Good thing you have a photo of the "before".

+1000!

Always advisable when performing restoration/repair work.

[Warrenghall]

Michael and phantom man - thanks for your help. I’ve recapped a silver face twin in the past, as well as my Princeton, but never an amp with these anomalies.

michael. i thought these were the dividing resistors not the dropping ones, so my mistake there.

with respect to the cathode caps I would appreciate your insight as I seem to be short a few! This being said - should they be changed as part of the recap ( they are considered electrolytic) If I do, my local vendor only has 22 microfarad and 33 microfarad. In that the originals are 25 can I just use the 22’s?

Thanks again

[Michael Smith]

Fender did not always follow the schematics when building amps. If you are happy the way it sounds, and wish to replace the cathode bypass caps (yes they are electrolytics), then just replace what is there in the same configuration. 22uF should be OK. The fact that your dropping resistors differ from the schematic, 1K and 2.2K ohms vs 1K and 4.7K ohms per the schematic may be OK also. You should probably also replace the Bias cap and resistor. Most techs up the value of the Bias Cap to a 100uf/100volt, but a 50uF 70volt per the schematic would be OK. You may have to J-hook the new Bias cap in, depending on how long the original cap's leads are and whether your new cap's leads are long enough to reach where the old cap's leads connect underneath the Bias eyelet board.

[phantomman]

You may have to J-hook the new Bias cap in, depending on how long the original cap's leads are and whether your new cap's leads are long enough to reach where the old cap's leads connect underneath the Bias eyelet board.

+1

Be sure to note the polarity of the bias-supply cap -- the positive lead is tied to ground.

[Michael Smith]

I faced a similar dilemma when I got my 1977 Deluxe Reverb earlier this year. A "tech" had made some modifications that I didn't understand. I restored it to the nearest schematic (by year) that I could find, and it sounds great now. Before it was breaking up way too early.

[Michael Smith]

+1

Be sure to note the polarity of the bias-supply cap -- the positive lead is tied to ground.

Good advice Phantomman. We would not want to see a New Year's Eve fireworks display inside of an amp.

[Warrenghall]

Agreed on everything said above. Am putting my parts list together to order. Is it common to replace bios resistor when doing a recap? If I measure and it’s still within tolerance I imagine I can keep it?

[phantomman]

Is it common to replace bios resistor when doing a recap?

You're already in there and it only takes another five minutes or so......

When a surgeon removes a couple of pre-cancerous lymph nodes from a patient's abdomen he usually takes the appendix out as well just as a prophylactic measure.

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[Michael Smith]

Correction to something I posted yesterday. When I looked at your photo of the "doghouse", I said I saw a 2.2K and a 1K dropping resistor. I should have said I saw a 2.2K and a 10K Ohm resistor.

That being said, when I was looking at the Fender Filter Cap Kits that are sold for various models, they use a variety of dropping resistor values.

Unless someone else has an objection, I would replace your dropping resistors with the same values that you currently have. In all of the kits and schematics I have seen, the balancing resistors are always 220K Ohms.

[Warrenghall]

To complicate things further I went in today to look at the bias bord and the resistor there seems to be 100k, and it’s reading at 105. Per spec it should be 180k. Replace with 180 or replace with 100? I’m concerned if I start putting part of the amp back to spec I’ll ruin the good tone it currently has. Am I maybe better to just change the electrolytics and resistors in the dog house and leave the rest?

[Michael Smith]

To complicate things further I went in today to look at the bias bord and the resistor there seems to be 100k, and it’s reading at 105. Per spec it should be 180k. Replace with 180 or replace with 100? I’m concerned if I start putting part of the amp back to spec I’ll ruin the good tone it currently has. Am I maybe better to just change the electrolytics and resistors in the dog house and leave the rest?

Don't you mean 1K vs 1.8K? The AA1069 schematic uses a 1.8K ohm resistor in the Bias circuit, while the later revision, AA270 uses a 1K ohm.

Both circuits use a 50uF/70Volt Bias cap, but in the kits they sell now, they often supply a 100uF/100Volt bias cap.

[Warrenghall]

Michael you are correct I did mean 1 k ohm. I’m thinking now I have the AA 270 version of the amp. And I will upgrade the bias cap to 100/100.

[Michael Smith]

Michael you are correct I did mean 1 k ohm. I’m thinking now I have the AA 270 version of the amp. And I will upgrade the bias cap to 100/100.

That would be my guess also (AA270 version). I think that was the last revision to the circuit made in Feb, 1970 before they went to the master volume circuit in the mid 70's. But as you probably know, Fender often made minor revisions to circuits that were never documented with an official Circuit Revision schematic. They could have also changed recommended component values in a Service Bulliten, although your amp doesn't look like it's had anything changed from original condition.

[phantomman]

Fender often made minor revisions to circuits that were never documented with an official Circuit Revision schematic. They could have also changed recommended component values in a Service Bulletin, although your amp doesn't look like it's had anything changed from original condition.

+1

This occurred infrequently but it's a factor to be considered when servicing these amps. As well, component value-drift often has a significant effect on a particular amp's tone and should be taken into account when maintenance is required.

[Michael Smith]

I've looked at your photo of the underside of your chassis more closely, and with respect to the placement of the electrolytic caps, it's the same as my Deluxe Reverb (after I fixed some wiring errors and added a missing cap). Total of Six 25uF/25Volt caps which I have numbered 1 thru 6. (You can't see #2 very well, but its there). Note that #6 is not on the eyelet board, but on one of the tube sockets, just like in your picture. And I have noted the position of the 5uF/25Volt electrolytic cap in the tremelo circuit.

Hope this helps.

[Warrenghall]

Final question - more of a confirmation. From looking at the AA270 schematic it seems this version of the pro reverb has adjustable bias rather than bias balance. Am I correct?

thanks guys

warren

[Warrenghall]

Okay. Answering my own question but, after looking at the schematic, I believe this is bias balance. I will install a 10 k pot and 10 k resistor in place of the current 15 k resistor to give me bias adjustability. Opinions?

[Michael Smith]

Okay. Answering my own question but, after looking at the schematic, I believe this is bias balance. I will install a 10 k pot and 10 k resistor in place of the current 15 k resistor to give me bias adjustability. Opinions?

I "think" your amp has a bias balance configuration. I did a google search for "Adding a bias adjust trimpot to a Fender bias balance" and found a pretty good article on a website called "Blueglow". A few other suggestions came up in the Google search, but I haven't read them all yet.

I would be interested in seeing how your modification goes.

Edit: Uncle Doug has an interesting video on bias adjustments called "The Effect of Output Tube Bias on Tone, Gain, and Headroom.....A Side-by-Side Comparison" It's on YT and I'm watching it now.

[Warrenghall]

I think a lot of this originated with some articles on Doug Hoffmann website about adding an adjustable bias pot to a Princeton. I did the mod about 20 years ago to both my Princeton and silver face twin and it worked out great. The problem is, when you only do these jobs every 20 years ( like recapping) you forget what the heck you did!

[Michael Smith]

I've only been messing around in the inside of tube amps for a year or so, so I still have a lot to learn. I just watched a couple more Uncle Doug videos on amp biasing. He does a thorough job of explaining the procedure, analyzing schematics, etc.