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Thread: Taming Fender Red Knob Dual Showman

  1. #1
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    Taming Fender Red Knob Dual Showman

    Hello everybody,

    I am currently servicing the above-mentioned Amp of a friend. The amplifier is also identical with The Twin, sometimes called The Evil Twin, just without reverb and as a amp head.
    These amplifiers are better than their reputation and have some quite usable features. Besides the enormous weight, however, the sheer volume of this unit is a real shortcoming. Although the amp runs in Lo mode and I have already pulled a pair of power tubes, it is still much too loud and hardly manageable. This is partly due to the fact that the 250k Vol Poti of Channel 2 has a linear taper. Since I had no logarithmic taper pot at hand, I helped myself with a 39k parallel resistor between wiper and ground, and this seems to work. But I'm not sure how this affects the overall behaviour of the amp, since this lowers the load impedance.
    On the other hand, I'd like to reduce the parallel resistance even further, because this would dent the taper even more (better resolution at lower rotation angle). But maybe it's better to use a logarithmic potentiometer, possibly 500k and add a parallel resistor (about 250k) additionally.
    As this affects only Channel 2 I thought about something else.
    In front of the driving tube of the insert, the outputs of Channel 1 + 2 are summed via the mixing resistors R101, R102, R103 (470k each). Decreasing R102 or increasing R101 and R103 attenuates the signal. For now, I just soldered another 470k resistor parallel to R102. This seems to work fine, too, but I'm asking myself, and you, how far can R102 be lowered without producing other problems. Maybe it is better to increase R101 and R102.


    I am open for any suggestions and hints
    Sascha

  2. #2
    Senior Member Enzo's Avatar
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    I think you answered yourself. You said you already tried a parallel resistor and it helped. Instead of worrying about impedances within the circuit, just listen to the amp. Does it sound compromised in any way? You can tack other resistances in as experiments if you like. That takes just seconds, and your ears will be a better judge then us out here hypothesizing. Seriously.

    Same thing with series resistances, you already paralleled the one. Just try whatever else you think might be good. It won't hurt anything, and if it upsets the signal path, you will hear it.

    I don't know about your shop, but mine had drawers full of small trimmer pots. But a full size pot works too if you keep wires short. Instead of tacking a bunch of parallel resistors, I just tack in like a 1 meg trimmer. I can them dial it up and down and see what I think. Find a spot you like, pull teh trimmer and measure the resistance you chose.
    Education is what you're left with after you have forgotten what you have learned.

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    Meanwhile we determined the values of the resistors in question. It turned out that they could be lowered even more without any shortcomings to occur. We ended up with 18k for the parallel resistor of the channel 2 volume pot and 220k for R102.
    We also added a 820k parallel resistor to the volume pot of channel 1 to get a better control at low volumes. This works fine as far as I can tell.

    Furthermore I did some mods inspired by an articel in premier guitar by Jeff Bober, and I really recommend checking them out:
    -removing C17 for a more useable bass boost function
    -adding a 22F/25V cap parallel to R118 (cathode resistor of V3A) for better over all bass response
    -increasing R119 from 1k2 to 10k (less negative feedback), which gives a more natural feel

    But there was also a damage in the amp, which I first thought I fixed.
    R128 (2k7, 10W) and R129 (30k, 10W) didn't look healthy when I opened the chassis and later I realised that R129 had infinte resistance, although the amp was still performing well. Therefore I decided to replace all three wire-wound resistors to prevent the amp from future failure. I thought it was kind of a mechanical issue, but during my tinkering around I found out that R128 and R129 are getting very hot only after 2-3 minutes of operation. This means it is more likely to be an electrical issue and at least R129 will die sooner or later.
    The surrounding components (R130,131 and electrolytic capacitors C110,111,112) seem to be in good condition.
    The measured voltages are a little high but still in tolerance. I refer this to the 220V mains setting, whereas we have nominal 230V in Germany. I chose 220V just because of better results for bias and heater voltages. I also replaced R214 10k with 8k2 to get more negative grid voltage.

    Has anybody any idea?
    What can I do to prevent R129 from almost sure heat death?
    Sascha

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    g1
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    That resistor runs at about 5W. That is why it gets so hot. That is also why they use a 10W resistor there. R128 only runs around 2W so it doesn't get as hot.
    About the only thing to do would be to use a chassis mount type resistor. You would bolt it somewhere inside the chassis and run wires to the circuit board. It would use the chassis as a heat sink.

    m065217p01wl.jpg
    Last edited by g1; 09-14-2017 at 09:05 PM.
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    Ok, I try to make things clear, since I'm not that experienced and skilled I'd like to be.

    Simple Ohm's law says P=400V*400V/30kOhm=5,3333W, fine. But does that mean it's normal operation condition to produce such an amount of heat? Please remember, original R129 was already damaged, when I opened the chassis, but the amp still worked fine.
    This leads me to another question. What is it good for?
    R128 reduces plate supply voltage for the preamp tubes, i think.

    Unfortunately I wasn't allowed to open Attachment 44894 and therefore do not know what is hidden behind it. I also wanted to open a schematic (mine is of poor quality) from another thread and only got a black page. What's the trick about it?

    Would it had been better to start a new thread in Maintenance, Troubleshooting & Repair?

    Thanks for your help
    Sascha

  6. #6
    g1
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    Yes, 5W is a lot of heat, so that is normal. R129 helps reduce the voltage by adding a load to the supply. It will still work without it, but your voltage at points A,B, and C would be higher without it.
    The attachment 44894 was just a picture of an aluminum chassis mount resistor, I think it should be showing now (post #4).
    The schematic for the dual showman head I have is also not very good quality. But it is just a head version of the red knob "The Twin" combo.
    Attached Files Attached Files
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    Thank you once again!
    Your The Twin schematic is better than mine. But there are some minor differences, D202/203 for example are missing, I'll check if they are physically there.
    And am I right in assuming that the connection of R8+R9 in channel 1 tonestack is an error?
    The picture of the aluminum resistor is now visible, too.

    Now I'm thinking about replacing R129 with a resistor of higher wattage and/or value, or simply ignoring the heat problem. What do you recommend?

    Sascha

  8. #8
    Old Timer Tom Phillips's Avatar
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    Concur with g1. Those 10W resistors run hot. The voltage divider method of reducing the B+ for the pre-amp supply is a brute force approach. The resistors feel hot to us but the heat is OK for the resistor itself. However, the heat can be a problem for the solder joint connections and the PCB itself. Especially if the resistors are mounted with short leads and end up close to the PCB. Over time the PCB discolors and I have found deteriorated solder joints. One time when I needed to replace the 10 W resistors I relocated them off the PCB. Photo of mod attached.
    Attached Thumbnails Attached Thumbnails twin-pwr-r-mod.jpg  

  9. #9
    g1
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    I like Tom's solution for the resistors. You mentioned changing the value of R129, you should not do that, but you can go to a higher wattage.
    Quote Originally Posted by ruepselrum View Post
    am I right in assuming that the connection of R8+R9 in channel 1 tonestack is an error?
    Yes, it is an error.

    Quote Originally Posted by ruepselrum View Post
    there are some minor differences, D202/203 for example are missing
    If it does not have them, they should be installed. Just add them in parallel with the resistors R215 and R216. You can put them on the foil side of the board.
    The original design did not have them. Sometimes when those resistors burnt (due to power tube faults) they burnt that rear bias pot board. So they were retrofitted to any units being serviced under warranty, and for production, the rear bias board was revised to include them.
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    Quote Originally Posted by g1 View Post
    I like Tom's solution for the resistors.
    I like this solution, too. I think about setting at least the ground side off-board, although the PCB still looks pretty good, after nearly 30 years. Is it sufficient to screw on a solder tag to the chassis?


    Quote Originally Posted by g1 View Post
    You mentioned changing the value of R129, you should not do that, but you can go to a higher wattage.
    Why do you think I shouldn't change the value of R129? I think an increased value would decrease current and thus dissipation wattage. Don't get me wrong, I'm not questioning your valuation, I just just want to learn.


    Quote Originally Posted by g1 View Post
    If it does not have them, they should be installed. Just add them in parallel with the resistors R215 and R216. You can put them on the foil side of the board.
    The original design did not have them. Sometimes when those resistors burnt (due to power tube faults) they burnt that rear bias pot board. So they were retrofitted to any units being serviced under warranty, and for production, the rear bias board was revised to include them.
    That makes perfect sense to me, I'll install them.
    Would my Marshall JCM800 and a Hiwatt Custom100 of a friend, which I retrofitted with 1Ohm cathode resistors for bias adustment, benefit from this diodes, too?

    Good night
    Sascha

  11. #11
    g1
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    You probably used higher wattage resistors, and I doubt they are mounted where they would do any damage if they burn?
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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by ruepselrum View Post
    ...Is it sufficient to screw on a solder tag to the chassis?...
    I used terminal strips fastened to existing screws on the chassis. One side is a tube socket screw and the other side is one of the transformer mounting screws. That eliminates the need to drill any extra hols in the chassis. I think that is good practice even though "The Twin" is not considered a high value vintage amp.


    Quote Originally Posted by ruepselrum View Post
    ...Why do you think I shouldn't change the value of R129? I think an increased value would decrease current and thus dissipation wattage...
    Increasing the value will decrease the total dissipation wattage but it will also change the voltage divider such that the voltage at power supply nodes A, B and C will rise. In other words, R129 won't be working quite as hard but it also won't be doing it's intended job either.
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    Quote Originally Posted by g1 View Post
    You probably used higher wattage resistors, and I doubt they are mounted where they would do any damage if they burn?
    I used 0,6W resistors for the JCM800 and 1W for the Hiwatt. I installed one resistor per power tube and they are connected straight from pin 8 of the tube sockets to the chassis, as far as I remember to the socket screws. So I don't think they'd break other components in the damage case.

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    Quote Originally Posted by Tom Phillips View Post
    I used terminal strips fastened to existing screws on the chassis. One side is a tube socket screw and the other side is one of the transformer mounting screws. That eliminates the need to drill any extra hols in the chassis. I think that is good practice even though "The Twin" is not considered a high value vintage amp.
    Maybe The Twin/Dual Showman is no high value vintage amp, it's still quite a good amp, which benefits from improofments that are done with care. I appreciate that!


    Quote Originally Posted by Tom Phillips View Post
    Increasing the value will decrease the total dissipation wattage but it will also change the voltage divider such that the voltage at power supply nodes A, B and C will rise. In other words, R129 won't be working quite as hard but it also won't be doing it's intended job either.
    But wasn't it a solution if R128 would be also increased, to maintain the voltage divider's ratio?

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    Senior Member Enzo's Avatar
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    But then we are trying to save our solution rather than the amp itself. The amp has worked as stock for decades, no reason to change it now.

    Remember, that voltage divider also has the rest of the circuitry drawing off the node.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    The amp has worked as stock for decades, no reason to change it now.
    That's what I'd told, too, if anybody had asked me. But as the one who's questioning, I try to gain deeper insights. I hope I'm not too annoying.

    Sascha

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    g1
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    If you increase the value of R128 (to compensate for increasing value of R129), it will affect the way the voltages sag under dynamic conditions. (Larger R128 values will cause more sag at preamp supply nodes)
    I would like to think Fender tried it that way and did not like the sound. R129 is a fairly unconventional approach to setting preamp voltages and I would think they had a good reason to do it that way.
    It's easy enough to experiment with the value of those 2 resistors and hear for yourself.
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    I just added D202/203 and btw replaced R115/R116 with 1% types.

    It`s really hard to get this high wattage resistors.

    Thanks for your help and patience!
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    img_20171020_123824.jpg

    For the sake of completeness I'm adding a picture of how I solved the high wattage resistor issue. I couldn't get 30k nor 2k7 resistors with 10 Watt or higher. As mentioned before I wanted to raise wattage to cool things down a bit and get more reliability. I finally chose two 1k5/10W and two 15k/10W in series and also incorporated Tom's idea of moving resistors away from the PCB, at least partially.
    Unfortunately they are still getting VERY hot after a few minutes of operation and measured voltages are about 20V higher than stated values in the schematic at 220V mains setting. When I run the amp in 240V mains setting voltages are decreased as expected to about 10V below stated voltages.
    Which setting do you recommend? In Germany we have 230V mains supply and in my case 225V coming out of the wall.

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    g1
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    20V high where? On 500V that is not much difference, but on 30V it's a lot!
    Look at your heater voltage which should be 6.3VAC, which mains supply setting gets you closest to 6.3 ?
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    I'm referring to preamp voltages succeeding the voltage divider built of R128/129 measured across electrolytic caps, C110-112, node A to C, highest of them being 397V.
    Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.

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    Quote Originally Posted by ruepselrum View Post
    I'm referring to preamp voltages succeeding the voltage divider built of R128/129 measured across electrolytic caps, C110-112, node A to C, highest of them being 397V.
    Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.
    I need to purchase R129 (30k, 10W). Where can i get one??

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    Supporting Member Jazz P Bass's Avatar
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    Not too sure where to get the cement resistors.
    These are all wire wound: http://www.mouser.com/Passive-Compon...yzekizZ1z0wt4w

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    Quote Originally Posted by ruepselrum View Post
    Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.
    Then I would use the 240V mains setting.
    The 10V low in the preamp is only 2.5% of 400V, so well within tolerance. The tubes will be better off at the lower heater voltage.
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    Quote Originally Posted by ruepselrum View Post
    ...I wanted to raise wattage to cool things down a bit and get more reliability. I finally chose two 1k5/10W and two 15k/10W in series and also incorporated Tom's idea of moving resistors away from the PCB, at least partially. Unfortunately they are still getting VERY hot after a few minutes of operation...
    This is due to the laws of physics. When you use larger wattage resistors, whether it be a single higher wattage resistor or two resistors that comprise the same resistance of higher wattage, the total energy dissipated is the same as it was with the original resistor. Therefore, the total heat emitted into the surrounding area is also the same. The difference is that the surface temperature of the higher wattage resistor can be lower if the replacement resistor is physically larger. That could reduce the possibility of heat damage to surrounding parts and PCB.

    As you have observed, power resistors get really hot when they are doing their job. That's the way it works. The laws of physics again.
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