Hi All,
I have recently purchased 2 x 100 ohm Fender Humdinger Trim Pots Part# 002-8421-000 made by CTS.
I am trying to find out the wattage rating of them. The 'usual' CTS pots are 0.5 watt but I don't think that can be the case for the Humdinger as if it's wired across a 6.3v supply at the top end of it's 10% tolerance, it would be dissipating 0.48 watts which is a bit close for comfort. I have emailed Fender support and the tell me they have'nt got that information and to get in touch with CTS. I have tried looking for an email address for CTS but can only find on their official website a commercial 'professional' form to contact them, so far to no avail.
Does anyone know the answer please.
What is the wattage of the Fender 100 ohm Humdinger Trim Pot?
Roger
I have recently purchased 2 x 100 ohm Fender Humdinger Trim Pots Part# 002-8421-000 made by CTS.
I am trying to find out the wattage rating of them. The 'usual' CTS pots are 0.5 watt but I don't think that can be the case for the Humdinger as if it's wired across a 6.3v supply at the top end of it's 10% tolerance, it would be dissipating 0.48 watts which is a bit close for comfort. I have emailed Fender support and the tell me they have'nt got that information and to get in touch with CTS. I have tried looking for an email address for CTS but can only find on their official website a commercial 'professional' form to contact them, so far to no avail.
Does anyone know the answer please.
What is the wattage of the Fender 100 ohm Humdinger Trim Pot?
Roger
Roger, the pot itself would dissipate 0.4W nominal. It doesn't matter where the wiper is located, as negligible current should flow through the wiper in a typical installation. The wiper just shifts the DC level of the heater wires to allow the net heater voltage seen at a grid to be effectively zero, so that hum current flow is a minimum.
If you need to see that in a descriptive schematic then one reference is:
https://www.dalmura.com.au/static/Hum%20article.pdf
Ciao, Tim
If you need to see that in a descriptive schematic then one reference is:
https://www.dalmura.com.au/static/Hum%20article.pdf
Ciao, Tim
Tim,
I understand how the pot is supposed to work in this situation and I am aware that is doesn't really matter where the wiper of the pot is as the AC current to ground should be minimal. It is the overall power taken by the track end to end that slightly bothers me as this is a constant 0.48 watts if the 6.3v heater voltage is at it's maximum tolerance (6.93v). No-one seems to be able to tell me what the maximum wattage rating is for this particular fender (CTS) component. 'Normal' CTS pots are usually 0.5 watt but can't help but feel that because of this one's intended location it is probably rated higher. But I don't know and no-one else appears to know either.
Roger
I understand how the pot is supposed to work in this situation and I am aware that is doesn't really matter where the wiper of the pot is as the AC current to ground should be minimal. It is the overall power taken by the track end to end that slightly bothers me as this is a constant 0.48 watts if the 6.3v heater voltage is at it's maximum tolerance (6.93v). No-one seems to be able to tell me what the maximum wattage rating is for this particular fender (CTS) component. 'Normal' CTS pots are usually 0.5 watt but can't help but feel that because of this one's intended location it is probably rated higher. But I don't know and no-one else appears to know either.
Roger
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Here is a list of nineteen different 100 ohm potentiometers rated 1 watt or more, which are in stock and on the shelf at DigiKey. If you don't mind either attaching a knob, or cutting a slot in the shaft with a Dremel tool (to accommodate the blade of a screwdriver), I suspect any of them would work nicely. AND you get peace of mind that they are all have a big safety margin of RatedDissipation minus ActualDissipation.
Hi Mark and Ray,
Thanks for the list and I am aware of all the other pots our there fitted with spindles. The thing is, I have already purchased and fitted the Fender Humdinger pots in question assuming that they were suitable for the job (as they probably are). All I am asking is what power rating are they? I have also visited Technical Inquiry | CTS and filled in their 'professional/commercial' enquiry form but so far have had no reply. I would have thought that Fender 'Support' would have known as they market the product, but they don't. Incedendally, I have felt the pots after an hours use and they feel cool (as expected).
Thanks for the list and I am aware of all the other pots our there fitted with spindles. The thing is, I have already purchased and fitted the Fender Humdinger pots in question assuming that they were suitable for the job (as they probably are). All I am asking is what power rating are they? I have also visited Technical Inquiry | CTS and filled in their 'professional/commercial' enquiry form but so far have had no reply. I would have thought that Fender 'Support' would have known as they market the product, but they don't. Incedendally, I have felt the pots after an hours use and they feel cool (as expected).
It's unlikely that you will need the full trim range. Given that, you could add dropping resistors at the ends of the pot. For a 100 Ohm pot, putting 50 Ohm on each end reduces the power in the pot 4x, and only reduces trim range 50%.
Just another way to get it done. For what it's worth.
Just another way to get it done. For what it's worth.
Yes, that's a good idea. I will wait first to see if I can get a difinitive answer from CTS. After all, these pots were made specifically for this situation. I would expect these these pots to run warm or hot if they were near the edge of their power rating. So far, that doesn't seem to be the case.
The pot is specifically marketed by Fender as a replacement for the '65 Twin reverb' amplifier.
The pot is specifically marketed by Fender as a replacement for the '65 Twin reverb' amplifier.
Just to add that these Fender Humdinger pots are about twice the price of the standard CTS pot. This is what makes me think that they are a more beefier pot (higher wattage).
Use a 500 ohm pot. Works the same but less than 100mW dissipation so you don't have to worry about it.
As mentioned ealier, I have already installed the 'Humdinger' 100 ohm pots (#5) and they seem to be running okay (not warm or hot).
In the last 15 minutes, I have received an email from CTS who make the pots for Fender. Their technical guy says that they are 0.5 watt which rather puzzles me as they are specifically made as replacement for the Fender 65 twin Reverb amplifier whose heater circuit is 6.3 volts AC.
In the last 15 minutes, I have received an email from CTS who make the pots for Fender. Their technical guy says that they are 0.5 watt which rather puzzles me as they are specifically made as replacement for the Fender 65 twin Reverb amplifier whose heater circuit is 6.3 volts AC.
Her's a thought..... Do you think that the power rating of a potentiometer is between the track and the wiper, or the track end to end?.
It's always the track (so end to end). Note that a dissipation rating should also come with a max ambient temp for that rating, and possibly even a derating curve, and also be read along with the resistance tolerance.
So your concern about 6.3+10% being nearly at the 0.5W limit assumes the pot is not located in a hot position where the 0.5W should be derated. You may also get some rating margin from the resistance being over 100 ohm.
I'd be more concerned about the voltage at the valve socket terminals (you didn't clarify if the +10% measurement was with or without valves), as that may be a more worrying concern.
So your concern about 6.3+10% being nearly at the 0.5W limit assumes the pot is not located in a hot position where the 0.5W should be derated. You may also get some rating margin from the resistance being over 100 ohm.
I'd be more concerned about the voltage at the valve socket terminals (you didn't clarify if the +10% measurement was with or without valves), as that may be a more worrying concern.
So it's end to end as I suspected. CTS support sent me a highlighted entry off their spec. table which showed:-
100 ohms - LD - 20%
Rating 0.5 watt @ 55 degs C
It didn't show any derating curve.
The original Fender Pro Junior mains transformer had a (European) primary of 230VAC with no 240VAC tapping. In the UK we have 240VAC of which at my house is normally about 243VAC. This TX gave a heater voltage of 7.21VAC with valves fitted. I changed this TX for a 3rd party good quality 240VAC one which gave 6.95VAC. I've changed it yet again for a Fender Blues Junior 240VAC one and this gives 6.90VAC with vales fitted. So all three read high on load. 6.90VAC is right at the top end of the valve heater spec.
100 ohms - LD - 20%
Rating 0.5 watt @ 55 degs C
It didn't show any derating curve.
The original Fender Pro Junior mains transformer had a (European) primary of 230VAC with no 240VAC tapping. In the UK we have 240VAC of which at my house is normally about 243VAC. This TX gave a heater voltage of 7.21VAC with valves fitted. I changed this TX for a 3rd party good quality 240VAC one which gave 6.95VAC. I've changed it yet again for a Fender Blues Junior 240VAC one and this gives 6.90VAC with vales fitted. So all three read high on load. 6.90VAC is right at the top end of the valve heater spec.
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More info.
Both amplifiers have just been on a 2.5 hour soak test and the temperature of the pots taken with an infra red thermometer. Both amps were about the same.
Room temp: 25C
Chassis temp immediately in the vicinity of the pots: 32C
Pot body temp: 41C
Both amplifiers have just been on a 2.5 hour soak test and the temperature of the pots taken with an infra red thermometer. Both amps were about the same.
Room temp: 25C
Chassis temp immediately in the vicinity of the pots: 32C
Pot body temp: 41C