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Old 19th August 2007, 01:36 PM   #1
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Default Floating heater supplies

Hi Everyone.
I have a preamp in srpp topology with a b+ OF 250v DC ON THE TOP ANODE (ok caps button off again..) According to morgan Jones this topology will strain the insulation between the heater and the cathode (6sn7) as the cathode voltage will be 1/2 of anode voltage.
I have a 6.3 v supply to the heater in question and basically need to know if I have to float the heater supply.
If so whats the best method?
Thanks
Nick
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Old 19th August 2007, 02:20 PM   #2
7N7 is offline 7N7  United Kingdom
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Default Re: Floating heater supplies

Quote:
Originally posted by duderduderini
Hi Everyone.
I have a preamp in srpp topology with a b+ OF 250v DC ON THE TOP ANODE (ok caps button off again..) According to morgan Jones this topology will strain the insulation between the heater and the cathode (6sn7) as the cathode voltage will be 1/2 of anode voltage.
I have a 6.3 v supply to the heater in question and basically need to know if I have to float the heater supply.
If so whats the best method?
Thanks
Nick
Yes with about 125V on the anode of the lower valve you will. I suppose you are using one valve for this SRPP? This is a problem because of course one cathode is at 125V and the other at something like 6 - 8V I expect. Your best solution is to float the supply at about +65V and hope for the best.

Alternatively if you have another SRPP - perhaps it's a stereo amplifier, then use one 6SN7 for the bottom half and another for the top half. then the top half valve can be floated at say 140V (always good to have the heater positive with respect to the cathode) and the lower one at say +20V.

There is a good circuit for a supply for this application in MJ's "Valve Amplifiers" - the third edition gives a full explanation.

It is called the "THINGY".

7N7
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Old 20th August 2007, 12:37 PM   #3
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Default I have regulator boards

Hi 7N7
I have regulated lt ps for the heaters based on the LM338k.
I just dont get how elevating the heater supply is implemented. with my setup i turn on the LT supply first then wait for warm up and then switch in B+
I suspect I am showing my ignorance here in not understanding the distance between elevated and floating etc. I suppose I could reduce the B= to 175V and since the cathode Volts will be around 1/2 of this it may be within the 100v diference specified for the 6sn7 .. I could also try a different topology.
Thanks
Nick
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Old 20th August 2007, 02:26 PM   #4
7N7 is offline 7N7  United Kingdom
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The normal method with a DC supply is to connect two e.g. 47 ohm resistors - one to each pole of the heater supply output. The other ends of the resistors are connected together. You then apply the appropriate voltage to the central point - i.e. where the resistors are joined.

Obviously your heater supply is not connected anywhere to chassis earth or HT 0V?

No current will be drawn from the HT supply; you are merely elevating the position of the heater supply relative to ground (0V).

The heater supply will of course only be elevated when you switch on the HT - which of course is when you need it.

7N7
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Old 27th August 2007, 05:02 AM   #5
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Default heaters are grounded

Hi 7N7
My heaters are grounded as i found this reduced the low level of hum to non existent.
What I did on the weekend though was to reduce the b+ of the top anode to 165V and I have achieved a heater to cathode voltage difference of 80 odd volts. I am happy with the sound.. in fact I am amazed at how much better it sounds with a reduced B+.
This effectively satisfies the manufacturers specifications.
Or does it?
Thanks again
Nick
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Old 27th August 2007, 07:42 AM   #6
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Hi,

Take a look at the voltage divider circuit for the floating heaters on my latest contraption:
http://bb.sicomm.us/diyAudio/6SN7_SR...nestage_v1.gif

This will give the heaters about a +50V bias (around +30V with a 300V supply) above cathode potential and does two things; 1) satisfies manufacturer's ratings and 2) lower any hum you may have by at least 20dB by reverse-biasing the hum causing junction.

It's good for most totem poles, except the ECC88.

Cheers!
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Old 27th August 2007, 08:05 AM   #7
7N7 is offline 7N7  United Kingdom
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Default Re: heaters are grounded

Quote:
Originally posted by duderduderini
Hi 7N7
My heaters are grounded as i found this reduced the low level of hum to non existent.
What I did on the weekend though was to reduce the b+ of the top anode to 165V and I have achieved a heater to cathode voltage difference of 80 odd volts. I am happy with the sound.. in fact I am amazed at how much better it sounds with a reduced B+.
This effectively satisfies the manufacturers specifications.
Or does it?
Thanks again
Nick
Of course I am pleased that you have improved the amplifier, but I still think that you are approaching the problem from the wrong end!

The heater element, insulation and cathode form a "diode" which should be turned off. The way to turn this off is by making the heater positive with respect to the cathode. Even if my cathodes are at bias potential only, I generally run the heater supply at a minimum of +20V - RCA recommended +40V in their applications book.

So if I were running say a cathode follower, mu follower etc., with say the cathode at +150V, I would arrange to sit the heater supply on +190V or thereabouts.

7N7
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Old 27th August 2007, 11:29 AM   #8
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Default ok what now

Hi Guys and thanks.
I wish I had of known re this before i built my pre amp!!!.
I am including schematics for the heater supply and the preamp.
B+ for the line stage is 165V (I will increase to 175V).
Now it seems that i need to rewire the preamp so that one tube is the lower half for each channel and one is the top anode. Otherwise we will have the same problem if the one dual tube is used for upper and lower halves of the preamp.
I can access 250V in the preamp( I have a seperate psu with umbilical chord).


Quote:
he normal method with a DC supply is to connect two e.g. 47 ohm resistors - one to each pole of the heater supply output. The other ends of the resistors are connected together. You then apply the appropriate voltage to the central point - i.e. where the resistors are joined.
The pramp I have also has the jim haggerman riaa phono stage (using 6sl7 and 6sn7) I assume i will have to provide an elevated voltage to the heaters there.
SO my question(s) are
1. Does using one tube for left and right channels of the top half of the srpp and the other tube for the bottom half affect seperation between channells
2.I will need a voltage divider for the phono stage heaters and then the srpp.. any hints there as to values
3.does the fact that a diode is created twixt the heater and cathode matter and if so how does it manifest?
4. is my setup with the reduced b+ to place the heater voltage pd to within specs going to work long term assuming i cant easily replumb the power supplies
I have attempted to include a hastily assembled schematic of my preamp depicting the 3 basic systems. The haggerman design is the property of Jim Hagerman. The srpp schematic is of unknown origin but i acknowledge it is someone elses work.
Thanks
Nick
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Old 27th August 2007, 01:25 PM   #9
oshifis is offline oshifis  Hungary
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I have been using a preamp with 3 SRPP stages per channel for 10+ years. One half of each tube is for the bottom and the other half is for the top of the SRPP stage. The B+ voltage is 350V and one end of the filament is grounded. The filament voltage is LM317-stabilized DC. I haven't experienced any problem so far; it is dead quiet.
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