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Old 29th January 2005, 01:10 PM   #1
garbage is offline garbage  Singapore
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Unhappy rca83 hash?

hi all

i am using rca83 fullwave mercury rectifier in my 300b amps.
the 300b is a drd design, with siemens c3g as driver.

am facing a hum problem. seems like psu hum. it does not increase with volume.

psu filtering is done by 8uF/10H/55uF/5H/55uF. ultrapath cap is 45uF.

psu ground and signal ground are separated, but meets at the same point. chassis is grounded to safety earth. does not seem to be a grounding issue as when in breadboard, on a piece of wood, the hum is also there. before this, i used one single ground for psu and signal.(separate grounds did not make much difference)

i have also added 100nF cap across rca83's anode and cathode, tuned the 5H choke with a 0.22uF capacitor, but that still does not seem to reduce the hum much.

any suggestions?

thanks.
garbage
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Old 29th January 2005, 05:48 PM   #2
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Er, a mercury type is supposed to use small hash chokes in series with the anodes. Less capacitance the better.

Indeed, you'd have much better luck with silicon diodes, which only have reverse recovery switching issues (easily controlled). Yes, this from a person who, unfortunately, hasn't seen a type 83 in use...

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Old 30th January 2005, 03:14 PM   #3
kmtang is offline kmtang  Canada
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I don't think the hum is caused by the PS circuit. The C-L-C-L-C 8uF/10H/55uF/5H/55uF filter circuit should provide you with excellent filtering.

The hum possibly from the filament circuitry of the 300B. Did you adjust the hum pot? If you want to further cut down the hum from the filament, it's better to use DC supply for it.

I had hum problem when using Russian made 6SN7. I need to raise the DC potential of the filament voltage to approx 80Vdc to kill the hum generated from the 6SN7 driver stage.

The 300B amp I built was based on the design from JE Lab.
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Old 30th January 2005, 03:52 PM   #4
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The hum is definitely not caused by the 83 - it may (i've never noticed) generate some HF hash but it certainly can't make hum.

It might be heater or grounding related or due to magnetic coupling between a power transformer and the c3g
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Old 30th January 2005, 05:36 PM   #5
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One of the posts in this thread might be helpful.

"My understanding is that mercury vapour rectifiers have a negative resistance at part of their IV characteristic and that this can combine with the leakage inductance and stray capacitance of the mains transformer to produce a circuit that can oscillate if excited. Since the system is excited at a 50Hz (or 60Hz) rate, this can lead to squegging at a 50Hz rate. Ferrite beads are usually touted as the cure. The RSGB and ARRL manuals are good places to look for information on using these rectifiers."

(don't know how to quote properly on here but the above was posted by EC8010)
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Old 30th January 2005, 07:20 PM   #6
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Quote:
squegging at a 50Hz rate
Never happened to me but if EC8010 says so it may well be possible. Easy to eliminate the 83 by substituting some sand in its place
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Old 30th January 2005, 11:50 PM   #7
garbage is offline garbage  Singapore
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hi all

thanks for the responses.

i've just changed my speakers from totem model 1 (87db) to an az2 (93db) in order to better match my amps. the hum is killing me now. i can still hear it from my listening position. previously with the totems, i am not able to hear it unless i go within a foot or so. will have to find some way to get rid of that hum...

Quote:
Originally posted by kmtang

The hum possibly from the filament circuitry of the 300B. Did you adjust the hum pot? If you want to further cut down the hum from the filament, it's better to use DC supply for it.

I had hum problem when using Russian made 6SN7. I need to raise the DC potential of the filament voltage to approx 80Vdc to kill the hum generated from the 6SN7 driver stage.
the 300b heaters are dc. no hum pot, just connected the cathode resistors to the - heater supply. i tried hum pot, but no difference.

i've read that there is a max voltage when raising filament voltages. what is a safe level in my case? an when raising the filament voltages, do i connect to the + or - of the filament supply?

my driver stage is a triode strapped pentode, on ac filaments though. all filaments are floating, ie, not connected to circuit ground.

Quote:
Originally posted by analog_sa
The hum is definitely not caused by the 83 - it may (i've never noticed) generate some HF hash but it certainly can't make hum.

It might be heater or grounding related or due to magnetic coupling between a power transformer and the c3g

...
Never happened to me but if EC8010 says so it may well be possible. Easy to eliminate the 83 by substituting some sand in its place
seems like most people think that it is the heater of the driver. ok. will look into that. probably go dc for it. i've tried different grounding arrangements, but still there is no audible difference in hum reduction. i'll put in sand as a last resort to isolate the problem, but would very much still like to use the 83.

Quote:
Originally posted by martinab2
One of the posts in this thread might be helpful.

"...Ferrite beads are usually touted as the cure. The RSGB and ARRL manuals are good places to look for information on using these rectifiers."

(don't know how to quote properly on here but the above was posted by EC8010)
i noticed that thread after i posted this. (sorry moderators... )
where should the ferrite beads be placed? along the heater supply of the 83?

ps. i think there is no method of cross-post quoting. you are not alone there.
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Old 31st January 2005, 01:26 AM   #8
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Ferrite beads or rf chokes would do the job - Insert them in the leads between the transformer and the plates of the 83. Scrap switchmode power supplies are always useful for sourcing the sort of chokes you will need
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Old 31st January 2005, 03:10 AM   #9
garbage is offline garbage  Singapore
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ok, i guess the ferrite beads should be placed nearest to the anode of the 83? would one on each anode do fine? or should i just put more?

thanks.
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Old 31st January 2005, 10:10 AM   #10
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Quote:
Originally posted by garbage
i've read that there is a max voltage when raising filament voltages. what is a safe level in my case? an when raising the filament voltages, do i connect to the + or - of the filament supply?
The heater must be within 120V of the cathode - or so I think... the datasheets are in German, and I don't read German

You connect the centre tap of the 6.3V winding to a potential divider to raise it 40V or so above the voltage at the cathode. If you have no centre tap, connect two 100 ohm resistors in series across the winding, and connect the centre of the two resistors to the potential divider.

Quote:
Originally posted by garbage
my driver stage is a triode strapped pentode, on ac filaments though. all filaments are floating, ie, not connected to circuit ground.?
Floating heater potentials are generally bad. You would prefer to determine it rather than let the other what-nots in the circuit and the valve determine it for you. It may also be the source of the hum.

Quote:
Originally posted by garbage
where should the ferrite beads be placed? along the heater supply of the 83?
As Martin (and Tim) has already said, in series with each of the anodes.

A question please, the title of this thread is "rca83 hash" - does this mean that you are getting a low frequency - 50Hz or 100Hz or so with few higher harmonics, or buzz/hash with low frequency stuff but also more audible (and more annoying ) higher frequency components?
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