Planar
I like them a lot.
I use the BI Planar resistors in various places :
Buy Through Hole Fixed Resistors | RS Components
I first got very keen on them when testing a crossover for a horn/onken speaker set-up.
I compared a 2W carbon film against one of the planar resistors, in series with the compression driver, and the thick-film was noticeably cleaner and had more tonal colour and top-end detail .
I use them for cathodes on output valves too .
Just bought a few more packets last week, to add to the 'stores' .
I like them a lot.
I use the BI Planar resistors in various places :
Buy Through Hole Fixed Resistors | RS Components
I first got very keen on them when testing a crossover for a horn/onken speaker set-up.
I compared a 2W carbon film against one of the planar resistors, in series with the compression driver, and the thick-film was noticeably cleaner and had more tonal colour and top-end detail .
I use them for cathodes on output valves too .
Just bought a few more packets last week, to add to the 'stores' .
Rich -
That's an interesting observation.
Can you just clarify the scenario you're talking about here , ie. "measuring cathode currents" ?
Is this a concern over short-term pulse handling capability when the resistor is being used near its dissipation limit ?
I'm just surprised , as I've never observed any problem with bias drift or value drift in the application of these resistors to cathode bias resistors in 300B ( XLS ) and 6B4G output stages. For instance I've used a pair of BI planar 470R 10W (in series) on cathodes of KR 300B XLS for 6 or 7 years . Before that I've used Caddock 930's with a suitable heatsink again for several years on a PP 300B amp .
As normal I always spec the resistor wattage so they're running at no more than 40% of full spec, to keep temperatures down .
That's an interesting observation.
Can you just clarify the scenario you're talking about here , ie. "measuring cathode currents" ?
Is this a concern over short-term pulse handling capability when the resistor is being used near its dissipation limit ?
I'm just surprised , as I've never observed any problem with bias drift or value drift in the application of these resistors to cathode bias resistors in 300B ( XLS ) and 6B4G output stages. For instance I've used a pair of BI planar 470R 10W (in series) on cathodes of KR 300B XLS for 6 or 7 years . Before that I've used Caddock 930's with a suitable heatsink again for several years on a PP 300B amp .
As normal I always spec the resistor wattage so they're running at no more than 40% of full spec, to keep temperatures down .
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I came across this when faultfinding a collegues KT88 50W quitar amp which was sounding queer; the builder put 10 ohm 0.6W 1% metal oxide film resistors in each cathode...then current versus bias easily measured. Simple math shows the wattage quite suitable, but the output stage develops considerable harmonics, and if a squarewave is punched through the amp then the fast rising current spike from the leakage inductance will be reflected in this cathode resistor. Metal film types and others have poor transient handling capability...eventually the ohmage drifted high. The solution for rough output stages is to use wirewounds.
The tricky issue about using M/O and film types is there is often no colouration change when they go faulty.
Richy
If that is the case, wouldn't that mean thick film resistors would not be suitable for use in speaker crossovers at high-ish power? All they see in that application is impulse currents, no?
Or does the above only apply to metal oxide types, and not to thick film types?
Or am I misunderstanding this?
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http://www.eetasia.com/ARTICLES/2000DEC/2000DEC01_AMD_PD_TA.PDF
some info. esp last section of article " thick film resistor performance"
Remember, if one checks the amp with a squarewave, the waveform one sees on the output transformer primaries, often isn't the same as the one on the secondary. Depending on the transformer build quality, the leakage cap/inductance components get lost as wasted energy transients on the primary as the winding coupling cannot be perfect. Miller effect also wastes, and the cathode sees this.
My cue is really to avoid metal oxide/film types in high dv/dt situations and that includes UL screen snubber taps. Also the rating of these resistors typ at 70°c is also in conjunction with a reduced voltage rating, often overlooked. As Kevinkr points out...choose the proper type.
Richy
some info. esp last section of article " thick film resistor performance"
Remember, if one checks the amp with a squarewave, the waveform one sees on the output transformer primaries, often isn't the same as the one on the secondary. Depending on the transformer build quality, the leakage cap/inductance components get lost as wasted energy transients on the primary as the winding coupling cannot be perfect. Miller effect also wastes, and the cathode sees this.
My cue is really to avoid metal oxide/film types in high dv/dt situations and that includes UL screen snubber taps. Also the rating of these resistors typ at 70°c is also in conjunction with a reduced voltage rating, often overlooked. As Kevinkr points out...choose the proper type.
Richy
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I have 3wt 1% 1ohmers on the cathodes of a few (guitar) amps as well as an audio amp. I don’t know if they are “thick.”
Are the risks failure, drift, reduced high frequencies, or something else?
Does using an overrated wattage value mitigate or eliminate the risks?
If I bypass them what should I see (or not see) on my scope?
Are the risks failure, drift, reduced high frequencies, or something else?
Does using an overrated wattage value mitigate or eliminate the risks?
If I bypass them what should I see (or not see) on my scope?
JJman
The type of resistors we are talking about are flat, like the ones linked to in my post on page 1, or the Caddock or Vishay 'thick film' or 'planar' resistors. They don't look like a typical metal oxide, carbon film or wirewound resistor.
I suspect over-rating the wattage would mitigate, and at some point eliminate the problem. Seems like from what Rich describes, it's more likely to occur in guitar amps, where things are typically pushed to the limit.
The effect is change of resistor value, hence drift of bias and potential for the output valve ( tube ) to over-dissipate and fail, if you don't spot it in time.
I guess closer examination of the tech. specifications would show you what you can and can't do .
I'll continue to recommend then in a HiFi context with normal de-rating , they've served me very well .
The type of resistors we are talking about are flat, like the ones linked to in my post on page 1, or the Caddock or Vishay 'thick film' or 'planar' resistors. They don't look like a typical metal oxide, carbon film or wirewound resistor.
I suspect over-rating the wattage would mitigate, and at some point eliminate the problem. Seems like from what Rich describes, it's more likely to occur in guitar amps, where things are typically pushed to the limit.
The effect is change of resistor value, hence drift of bias and potential for the output valve ( tube ) to over-dissipate and fail, if you don't spot it in time.
I guess closer examination of the tech. specifications would show you what you can and can't do .
I'll continue to recommend then in a HiFi context with normal de-rating , they've served me very well .
Agreed!
perhaps its flash over! Maybe, high voltage MF would have survived. However the sudden power dissipation may be an issue.
Regards
M. Gregg
For interest,
?Through thick and thin?film resistors of course? | EDN-
And the ceramic type in the Picture.
Regards
M. Gregg
?Through thick and thin?film resistors of course? | EDN-
And the ceramic type in the Picture.
Regards
M. Gregg
Attachments
I have a question regarding output tubes guitar quartet el84 cathode bias power resistor, concern is achieving that "wet" on-edge sound, called even order harmonics. the vc3112 schematic calls for a 60ohm 10watt power resistor, so i boosted current by replacing it with two paralleled radioshack 100ohm 10watter sandblocks. current increases thru the quartet bank but still i am not satisfied with the sought harmonics. am i venturing into the wrong avenue? in other words rich even order harmonics are groomed within the preamp stages ecc83's and not the el84's? i already upgraded from the stock 8ohm ot to mercury's 16ohm capable output tranny and it delivered an immense improvement, yet i still desire even more richness. is it the high tension rectifiers, common 1N4007 silicon rectifiers, that kill even order harmonics? if you view the internet schematic for preamp channel two (high gain), i augmented the ecc83's cathode bias resistor 4.75K with a polypropylene foil cap 474. this dramatically shifted this channel's signature esp at midway power but i don't know whether the design engineer intentionally sought omission of ANY bypass cap here for reasons of intentional negative feedback. channel two gets too thick (muddy) when master volume is elevated, say 2/3 of 30rms rated power more so with humbuckers driving the signal. but with single coils, tele, the signature is somewhat tolerable.
Thanks for that link, Fig 1 is very handy for applications with a possible very high pulse current on a long duty cycle. I think it's not strictly dv/dt that is the issue - rather peak current and its duration - even if the rms value stays within overall rating. Snubbers and dampers spring to mind as one likely candidate to have to think about - this might explain an odd case I encountered once where a surge limiting resistor blew repetitively despite being rated 10x theoretical rms dissipation.
But I think for normal signal applications, Fig 1 suggests its not an issue.
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