Update: 1967 Super Restoration.

Discussion in 'Amp Tech Center' started by Bowpickins, Sep 19, 2021.

  1. Bowpickins

    Bowpickins Tele-Meister

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    Hi, everybody.

    Giving an update now that I have everything I need to start working on the '67 Super Reverb I got in July.
    https://www.tdpri.com/threads/new-amp-restoration-attempt-67-super-reverb.1077038/

    After my last thread, I got a gold multimeter, a Klein tools MM700, a soldering station, and slowly started ordering the parts for a basic re-cap attempt. While waiting for the parts to show up, I did lots of reading and watched many different videos to understand what I was getting into, and the proper safety measures for this type of work.

    After a failed attempt to get my own discharge device made for my multimeter probes, I went with a Weber Snufferstick.

    I got the amp discharged yesterday, and managed to identify and safely test resistors on the tube sockets, and throughout the board. So far, nearly all my 100k Ω resistors have drifted significantly, ranging from 117k to 139k. I realize that, like speakers, a 100k resistor will not measure exactly 100k, but my current readings have me wanting to play it safe and change them.


    My 50v 100mfd bias cap has drifted from its 100mfd rating to the 150 range. I will replace this with a 100v 100mfd cap I bought after some of you recommended this to me in my last thread.

    20210718_153415.jpg


    Of course, I'm replacing the electrolytic caps on the board, as they are original to the amp and this amp has a crackle that my twin reverb had prior to being re-capped. Also replacing the Illinois Caps on the underside, as I didn't get any service history from the family of the previous owner.

    20210718_150438~2.jpg

    20210919_130302.jpg


    The 470Ω screen resistors have drifted as well, though not as bad. Unfortunately, I need to clip them out and replace, as they don't have enough lead wire to move them out of the heat rising from my power tubes. The 1.5k grid resistors are measuring 2.5k, so I'm replacing them and moving the new ones so they don't face the immediate heat as well. 20210919_125845.jpg

    With the amp safely discharged, I'm also replacing the power cord with a 3-prong grounded cord, and eliminating the "death cap".

    Will post more updates as I start with the re-cap.

    Joseph.
     
    Last edited: Sep 19, 2021
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Nice you’re being both careful and deliberate.

    Don’t quote me, but do I vaguely recall some typical upward resistor drift (not way too much) is felt to create old-amp mojo — for perfectly objective reasons — in some few locations?

    OTOH I don’t believe in old-electrolytic mojo, and you’re definitely on track replacing resistors that get cooked, like on the sockets.
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Fwiw and not to discourage your safe practices of discharging, the SR has draining resistors. Those are the two 220k resistors connected to the two caps that make up the first stage of filtering. One will almost always find that these resistors have drained the filter caps, but it is still a good practice to check the voltage before proceeding with work. One watts the standby switch in the operational mode. You can then check any point on the power rail to ascertain whether or not there is voltage in the circuit. I go to pin 1 of V1….if there is no voltage there, the filter caps are drained. Not all amps have these drain resistors, and one should always be safe and make the voltage checks….and use a static drain wire from say 0pin 1, V1 to maintain the drain. Electrolytics have been known to recharge…if there are no draining resistors.
     
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good points, Wally.

    BTW I’m sure you’ll know. Is there such a thing as leaving some old resistors that have drifted up a bit in value? If so, where? I’m trying to remember something I read once…
     
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  5. Timbresmith1

    Timbresmith1 Tele-Afflicted

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    I usually turn the amp off while not in standby. The pilot will gradually dim as the caps drain. Not foolproof, but...
     
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  6. Bowpickins

    Bowpickins Tele-Meister

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    Thanks for the information, Wally. I discharged the amp by running my Weber Snufferstick down every joint on the board and along the tube sockets, as I would be replacing a few caps along the main board. I made sure to set my discharge by any cable leading to the filter caps in the cap can.

    After I went though with the Weber stick, I set my multimeter to DC Volts and carefully checked each joint 3 times. My meter had to adjust to Millivolts in order to detect anything, so I knew it was safe to go in, with periodic breaks to check voltage levels again to make sure we weren't charging back up.

    Also, thanks for clarifying about those two resistors. It's good to know this amp has those bleed resistors, but I'll still always check before going in.

    Joseph. :)
     
  7. Bowpickins

    Bowpickins Tele-Meister

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    I managed to do part of the re-cap this evening. Finished replacing the caps in the cap can and will do the ones on the top board tomorrow when I get better/natural lighting from my window.
    Feeling pretty excited. :D

    Before:
    20210919_130302.jpg

    After:
    20210919_200642.jpg
     
  8. Wally

    Wally Telefied Ad Free Member

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    Be careful of what you are sure, King Fan……because I surely do not know about…..mojo!
    Mojo…. I believe in resistances that are close to the designer’s choice….unless I want to change something! LOL…
    The most drastic example of drift in resistances ime was a 1952 5b5 Pro. The amp came to me in some non-original white vinyl. No it was not one of those rare white tolex amps done in the early tweed era. That much was easily discerned. It had a F15N..field coil…that had to be reconed. So, this was a big job. Cab went to St. Louis. Speaker went to Weber. I recapped the amp. During that, I did replace every tone and coupling cap because they were all paper-in-oil. Those do not stand up over time….they get replaced.
    So, when the cab and speaker was done, I put the amp back together. I fired it up and it sounded terrible. What could be wrong? well….resistors?? That was all that was left. I started checking them. 250K plate resistors measured in the 470K range. Every resistor on the board showed this type of severe drift….the amp had been played a LOT! The only resistors that had not drifted were the input gain stage 5meg grid leak bias resistors…they all measured at original values….there is not much voltage passing through them, right?
    All of the resistors on the board had had a lot of voltage passed through them, and their MOJO was BAD MOJO.
    After replacing all of the board resistors, I fired the amp up. Oh??? THAT was what a new 5B5 Pro sounded like back in the day…strong amp…great low end, beautiful highs, warm, articulate…beautiful break up when pushed. I had owned two 1959 5E7 Bandmasters and a 1956 5E8A Twin at the same time many years before I rebuilt that 5B5. This was before I did my own tech work. I had a great tech with great experience. All three of these amps had been well used during their long lives. None of these amps were 100% original in one way or another…so I was not worried about devaluing them. I told the tech that I wanted everything on the boards to be made new. He hesitated, but he accepted that the value was not going to be hurt and that I wanted them to be correct and as bulletproof as they could be. These three amps were glorious…just like the 5B5 I did many years later.
    So, I believe in maintaining originality as much as possible, but I also believe in having correct values…for my purposes. Could I believe that there are extremely knowledgable and experienced techs who know that in a specific application they might want plate resistors in an AB763 type of preamp to be 92.5K or 115k rather than 100K??? Yes…I am sure that there are those techs. Maybe that is why aDumble or a Trainwreck is worth so much???? Would I change a resistor in an amp that sounded good just because it was 10% high? No….after all, the resistor might be a 10% tolerance resistor and may have been that when it was new. If I were really into the sound the amp made, would I record all of the deviations from the schematic values?? maybe one should….but I started late in this work and have not done that. I note voltages…if they are proper, I proceed.
    Now…where might I think Mojo lives in these amps? My crude guess? In old amps, I hold that the output transformer might be the part that cannot be reproduced and that there was probably more variation back then than in today’s production due to better…or should I say…more accurate..machinery that does the work. One unit is more like another today in modern production, I would think. I would think. In guitars, the same is true for pickups….the winding is everything. The Gibson PAFS from the late 1950s can be worth thousands of dollars. And yet even the proponents of said pickups admit that there are great variances in the sound of individual pickups….even if they have the same magnets. I could be wrong about the OT being the heart and soul of an amp, but That is where the mojo is for me…as long as the circuit has been made correct. When all of the voltages, tubes, speakers, and biasing are the same and yet two amps sound different, I give props to the OT in the better amp. That is also why IF I were going to spend big money on the transformers in an amp, the first money would be spent on the OT. But…I don’t know mojo….who can know what one cannot see?
     
  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks, Wally. *Whatever* I may have read once was no doubt just someone’s opinion. If I really want an opinion, I want it from someone who bases it on a ton of experience, and measurement, and logic they can explain. And qualifies it with the clear statement it is an opinion. That’s why I asked you. :)
     
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  10. Paul G.

    Paul G. Friend of Leo's

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    Speakers are classified by impedance, not resistance -- this is why they never measure at their rating because impedance (AC) is not the same thing as resistance (DC). Your resistors should measure at, or close to, their rated value.

    Sometimes you may get what looks like a questionably low reading, it is because the resistor you're measuring is in parallel with some other parts of the circuit (i.e. input grid stoppers, channel mixing resistors). To absolutely verify them, you need to lift one end of the resistor from the circuit.

    Plate resistors are very critical as they are converting the AC current coming from the plates of tube into AC voltage required for input into the next stage. Values far from the specification have a big effect on the gain and tone of your amp.

    If you want your amp to sound like it did when new, it should be filled with components at, or near, the design specifications. In my opinion, of course.
     
  11. schmee

    schmee Doctor of Teleocity Ad Free Member

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    The 100k plate resistors are almost always off by quite a bit. If I change any, I will likely change them all with metal oxide replacements. The 100k's are often ones that cause noise in the old circuits. Mostly hissing. I have carbon comp new 100k here that are off as much as the originals often are. I dont use CC for anything unless that's all I have.

    OTOH, in an old amp, if it's quiet or collectible, I will leave them in even if off up to 30%.
    You indicate the amp crackles some. If it's a loud crackle, try wiggling the green heaters wires near each socket while the amp is on. You may find a cold solder joint on the tube solder tabs.
     
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  12. Bowpickins

    Bowpickins Tele-Meister

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    Alright! Finished re-capping the main board on Tuesday. Also installed my new bias cap with an F&T 100v, 100uf.

    20210923_160215.jpg

    20210923_160223.jpg



    While touching up a suspicious joint to address the phase inverter noise, I found that one of the coupling caps had its legs cut too short to fit inside the eyelet, so it was soldered to a cable leading from my phase inverter tube to the board. The one next to it doesn't look much better with a basic joint connecting its lead to the board; don't like the look of that connection.
    The amp was turned off and discharged so I could work on it, so no damage was done.
    Anyone have a recommendation for good replacement caps for this spot? Probably want a cap that can handle higher voltage, right?

    Edit: the caps are rated at .1uf at 400vdc, but I was getting 469 or 470v when initially reading voltage in this area.

    20210923_160235.jpg
     

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    Last edited: Sep 23, 2021
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  13. Wally

    Wally Telefied Ad Free Member

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    The voltage feeding the 82k and 100k resistors there should be around 450vdc according to the schematic. However, on the other end of those resistors, you should be reading lower voltages….230vdc by the schematic. So, your voltage supply is good, but those power supply resistors should be reducing that voltage.
    Somewhere around here I have a bag of those vintage caps. I have no problem personally with using Orange Drops there. Some of the most valuable and respected amps in the world use ODs. There are alternatives…some of them much higher priced. The value is probably more important than the price or brand.
     
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