Just a quick update..
I have replaced the voltage divider for the preamp DC heaters.
With a floating polypropylene cap value 0.1uF 1000V from heater negative rail to Gnd..
ie AKA Aikidio pre hum reduction.
Noise level lower than with divider...
Next step is to try the same thing with the 6C33C's heater rails...
Regards
M. Gregg
I have replaced the voltage divider for the preamp DC heaters.
With a floating polypropylene cap value 0.1uF 1000V from heater negative rail to Gnd..
ie AKA Aikidio pre hum reduction.
Noise level lower than with divider...
Next step is to try the same thing with the 6C33C's heater rails...
Regards
M. Gregg
Here is a strange one,,
Remember the increase in position of the volume control that gave more noise in the mid position...I thought must be pick up...well its gone completly...no pick up....no noise nothing..
Just by floating the heaters...with a cap..
Great but very strange...
There is no way this is pick up from the heater cables they are on the other side of the chassis...also if I open circuit the input there is no noise...
What makes it strange is that the heaters were already biased up to 40V with the divider..now no divider only the cap....
Regards
M. Gregg
Remember the increase in position of the volume control that gave more noise in the mid position...I thought must be pick up...well its gone completly...no pick up....no noise nothing..
Just by floating the heaters...with a cap..
Great but very strange...
There is no way this is pick up from the heater cables they are on the other side of the chassis...also if I open circuit the input there is no noise...
What makes it strange is that the heaters were already biased up to 40V with the divider..now no divider only the cap....
Regards
M. Gregg
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Gregg , in this 40V voltage divider before you displace it , you had any electrolytic capacitor on it to the ground . In the past in a preamp I use a 50V voltage divider with a 100nF capacitor to the ground and as I rise the voltage of the divider ( not more than 70 V ) the sound is getting more clear especialy in the high frequency , you can try it if you want .Here is a strange one,,
Remember the increase in position of the volume control that gave more noise in the mid position...I thought must be pick up...well its gone completly...no pick up....no noise nothing..
Just by floating the heaters...with a cap..
Great but very strange...
There is no way this is pick up from the heater cables they are on the other side of the chassis...also if I open circuit the input there is no noise...
What makes it strange is that the heaters were already biased up to 40V with the divider..now no divider only the cap....
Regards
M. Gregg
I will have a look at this and see what I find... do you have any explanation for the increase in clarity Vs voltage increase?
Regards
M. Gregg
Sure! You want to bias the filaments with something higher than the cathode voltage, as the filaments are capable of emitting electrons when there is B+ in the tube, just like the cathode does. If you bias them a littler higher, the phenomena is drastically reduced. The filamentary emission is noisy and non-linear; you get rid of it and the tube works better.
This seems to be more of a problem with signal tubes than output tubes; there is no further amplification after the power tubes
there is no further amplification after the power tubes
LOL you got that right.......
All joking aside I am surprised that an increase passed the cathode voltage gives any futher improvement..
Regards
M. Gregg
I guess,
the difference with a cap setting the bias it will follow any changes in leakage that may occur..
atmasphere,
What are your thoughts on 6C33c heater ... do you think they need offset voltage...most thoughts are on leaving them floating?
Have you experienced any hum related to the heaters?
The Tim Mellow design offsets the heaters to different supplies..
I would like to hear your thoughts on Ct supplies...(single or double rectifier) and use of the Ct as opposed to floating the centre point of the supplies..
Its a shame that we have only had limited thoughts from yourself and I would be very interested on any you may have..
Regards
M. Gregg
the difference with a cap setting the bias it will follow any changes in leakage that may occur..
atmasphere,
What are your thoughts on 6C33c heater ... do you think they need offset voltage...most thoughts are on leaving them floating?
Have you experienced any hum related to the heaters?
The Tim Mellow design offsets the heaters to different supplies..
I would like to hear your thoughts on Ct supplies...(single or double rectifier) and use of the Ct as opposed to floating the centre point of the supplies..
Its a shame that we have only had limited thoughts from yourself and I would be very interested on any you may have..
Regards
M. Gregg
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I have always run AC filaments. I like to keep things simple as they tend to break less often....
I like to use a filament transformer that has dual outputs, so I can have one winding for each tube bank. I've not found that biasing the filaments of the power tubes has much effect, OTOH I have found that an electrostatic shield in the filament transformer is a good idea
Another thing that I have been careful about is the phasing of the filaments on the tube sockets. If you are running multiple tubes, you can reverse the phase of the filament on each tube in a given tube bank. Its not much, but can help with higher efficiency speakers. Otherwise you are entirely dependent on how well the tube banks are balanced against each other for hum cancellation.
Now the fact that this is the case does argue that biasing the filaments could help out the output section. But the primary concern here is hum, and if you can get the output section to be quiet otherwise then you have a lower parts count. I like to use the output tube filament supplies for the driver tubes as well, and that has worked out quite well- even on speakers of 107db you don't hear any hum!
Thats very interesting..
I notice on some Utube videos of your equipment..I do research..
The Hum always seems to be very low, It seem to me that the hum level on the output stage is a major concern for OTL.. I take it you do not use chokes on the +/- supplies?
Also the reproduction is very realistic...What are your thoughts on the frequency shift with the centre tap connected and disconnected?
I am going to try floating the 6C33c heaters with a cap..
Regards
M. Gregg
Why do you think that the link to the O/P heaters in the pre stage
Reduces the hum...Ah
Is it worth trying to reverse the heater supplies of the pre/driver stage..they should be in sync with the supply...?
Anti phase will cancel...
Thank's for your thoughts...
Regards
M. Gregg
The main reason we use the output section filament supply has to do with its electrostatic shielding. If you are going to run AC on the driver filaments, there had better be an adequate electrostatic shield. Wiring the driver filaments in anti-phase will be more or less effective depending on the topology of the driver design. We use entirely differential circuits so it does not affect our circuits so much.
If you are talking about a center tap of a filament supply transformer, a lot has to do with where that tap is connected. It can also be an issue if the tap is not perfectly centered. For that reason sometimes it is better to 'create' a center tap using a pair of resistors.
If a 12V CT transformer is employed for the output section, with half of the tubes running on each half of the winding, no matter to what potential the CT is connected, then you will have bias on the filaments that is different for each bank. This is why I prefer floating dual windings.
Yes , with higher voltage on the filament the phenomenon of secondary emmision is decreased , that means that electrons which emmited by the anode dosen't attracted again by the anode but by the cathode which now have high potentional voltage .
Just some more progress,
I have floated the heaters on the 6C33C's with two 0.1 uF polypropylene caps 1000V.
I have to put my ear into the speakers to hear any hum at this point..
so yes an improvement...Few more things to try...Very Very low at this point...Its interesting that the heater Tx for the 6C33C's is now mechanicaly quieter than before..
Regards
M. Gregg
I have floated the heaters on the 6C33C's with two 0.1 uF polypropylene caps 1000V.
I have to put my ear into the speakers to hear any hum at this point..
so yes an improvement...Few more things to try...Very Very low at this point...Its interesting that the heater Tx for the 6C33C's is now mechanicaly quieter than before..
Regards
M. Gregg
Alastair,
I am realy interested...how does the PL509 sound? Compared with the 6C33c...
What happened to Mr Murre?
Regards
M. Gregg
So far, it seems better than the 6C41C....
Bit early to tell, Will need some listening-time, but doubt the PL509 will better the bigger capacity '33C, although it May better half 6C33C/single-heater operation
--Just trying all sorts of valves, more for interest than anything else..
Ive wired them Triode, with G2 to Anode via 470 ohm 2W. I did originally just connected the G2 direct, but had hideous oscillation and a buzzing.--In Both channels strangely, even though the other still had the '41C
--According to the data, the G2 volts in relation to anode needs careful attention or oscillation can be an issue with the 509.
Ive wired the heaters in series (2 x 40V) and running them from one side of the +_ O/P stage secondary on AC, with 100 ohm to drop the volts to approx 80V...
Using the original 220 ohm cathode-resistor, I have 115mA cathode-current, at 150V anode. There is approx 2V drop across the 470 ohm G2 resistor
Bit early to tell, Will need some listening-time, but doubt the PL509 will better the bigger capacity '33C, although it May better half 6C33C/single-heater operation
--Just trying all sorts of valves, more for interest than anything else..
Ive wired them Triode, with G2 to Anode via 470 ohm 2W. I did originally just connected the G2 direct, but had hideous oscillation and a buzzing.--In Both channels strangely, even though the other still had the '41C
--According to the data, the G2 volts in relation to anode needs careful attention or oscillation can be an issue with the 509.
Ive wired the heaters in series (2 x 40V) and running them from one side of the +_ O/P stage secondary on AC, with 100 ohm to drop the volts to approx 80V...
Using the original 220 ohm cathode-resistor, I have 115mA cathode-current, at 150V anode. There is approx 2V drop across the 470 ohm G2 resistor
Good old grid current comes to mind...wonder why it affected both channels...
So your heaters are across the AC of B+ side of HT..I guess you could run the other side across the B- AC how have you got arround the heater cathode leakage issue with the bottom valve?
Divider resistors or floating centre Gnd point?
Floating Cap...
It will be interesting to hear your results....I guess the heat is way less than the 6c33c..
I wonder if you will find a valve that beats the 6C33c...
Regards
M. Gregg
Alastair,
Something that has been going through my mind is how small could I make an OTL....
Just thinking about it for fun...NO o/p Tx as you have seen <<great idea one less Tx series the pre maybe like the old TV's run the heaters from the B+AC....
I mean with about 5W per channel mico OTL..
Very low heat tubes...very small chassis..
Don't know if I would build it at the moment but to put it down on paper...
Physical size being the main factor..Ok and decent sound..put it under the Telly..or run in the kitchen...bedroom maybe...LOL
Then again..LOL
Regards
M. Gregg
Something that has been going through my mind is how small could I make an OTL....
Just thinking about it for fun...NO o/p Tx as you have seen <<great idea one less Tx series the pre maybe like the old TV's run the heaters from the B+AC....
I mean with about 5W per channel mico OTL..
Very low heat tubes...very small chassis..
Don't know if I would build it at the moment but to put it down on paper...
Physical size being the main factor..Ok and decent sound..put it under the Telly..or run in the kitchen...bedroom maybe...LOL
Then again..LOL
Regards
M. Gregg
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Ive decreased the cathode-resistors to 150 ohm, now have 155mA through each PL509.
Heat is Much less than the 6C33C!
For each PL509, we have 31 watts--or around about-- total inc. heater, heat dissipation....
Compare that to 37 watts --or thereabouts--Just for the heaters Only for the 6C33C!
A 'Tiny' OTL I think is easily possible, But would be governed by the size of the main smoothing caps and the toroid to supply the current needed for O/P stage...
I cant reference the AC heaters to anything, as they are using the Same Winding as the Positive half of the O/P supply.
The H/K Max allowable for a PL509 is 250V, so guess under that....!
BTW--There's No Hum, --guess you would ask that...!
Heat is Much less than the 6C33C!
For each PL509, we have 31 watts--or around about-- total inc. heater, heat dissipation....
Compare that to 37 watts --or thereabouts--Just for the heaters Only for the 6C33C!
A 'Tiny' OTL I think is easily possible, But would be governed by the size of the main smoothing caps and the toroid to supply the current needed for O/P stage...
I cant reference the AC heaters to anything, as they are using the Same Winding as the Positive half of the O/P supply.
The H/K Max allowable for a PL509 is 250V, so guess under that....!
BTW--There's No Hum, --guess you would ask that...!
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