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Old 6th November 2018, 04:30 PM   #11
ciccio64 is offline ciccio64  Italy
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Ok: Big improvment... I had the heaters at 6.8V because the dedicate industrial trafo is a kind of beast (that is what i found available in my working place -)) So i lower it to 5.9 by means of a resistor. Well hum away!! I also put two good quality poly cap as by pass on the two electrolitics cap suplying the 6DJ8 anode.The difference is minimal but i think soundis litle more details and more tri dimensional. Now is really time to listen some music Thank you all for all your suggestion.
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Old 6th November 2018, 04:46 PM   #12
Alllensoncanon is offline Alllensoncanon  United States
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little help x newbe
Cheek your Parafeed schematic. The OPT primary is referenced to Anode and Screen as is.
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Old 6th November 2018, 05:17 PM   #13
ciccio64 is offline ciccio64  Italy
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Thank for pointing that out.That is an early schematic never corrected i end up wiring the parafeed as per attachment eccept i put the cap on the other side of the trafo
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File Type: png Parafeed%20Ultra-Empathic%20with%206DJ8.png (25.7 KB, 77 views)
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Old 6th November 2018, 08:26 PM   #14
6A3sUMMER is offline 6A3sUMMER  United States
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Your schematic in post # 13 is completely different.
It does not need the heaters to be elevated.
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Old 8th November 2018, 08:22 AM   #15
ciccio64 is offline ciccio64  Italy
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Quote:
Originally Posted by 6A3sUMMER View Post
Your schematic in post # 13 is completely different. It does not need the heaters to be elevated.
Hi the right schematic is the one I post first .Only the parafeed connection is made differently. Hence I post this second one to show how the parafeed is connected sorry if this created confusion. Ref the heater elevation .I am not able to have a technical discussion on it is above my level. However: The pre section is not my design it is basically a forewatt line pre amp by Bruce Heran. The original version use ECC802 but that was not sufficient to drive the KT88 so after consultation with Bruce I change to 6DJ8 with minimal modification Bruce recommend to elevate the heater (see below)

"The heaters must not be connected to the same ground as the signal, B+ or chassis. They need to float or tube failure may occur. The only connection of the heater circuit to the rest of the circuitry is a "bias" tap. This is developed from the main B+ supply and is about 1/3 the B+. No current flows through this connection (perhaps a tiny bit of leakage). It only establishes a reference for the heaters so that the difference between the heaters and any of the cathodes is less than 100 volts. One quirk of SRPP and other "totem pole" circuit configurations is that one of the cathodes is elevated quite a bit above ground.

In a SRPP the cathode on the upper triode has a voltage potential of roughly 1/2 the applied B+. So with 215-225 V on the anode the cathode is over 100 V. During start up this value is actually 1/2 the full B+ (of about 275 V). This value exceeds the tube rating for heater to cathode voltage and will result in tube failure. Perhaps this will not occur immediately, but it will as the tube ages. I have had this happen. Not a good thing. It usually results a huge spike in the output that could be catastrophic further down the audio chain."
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Old 8th November 2018, 10:13 AM   #16
6A3sUMMER is offline 6A3sUMMER  United States
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12AU7s were rated for +100 and -200V heater to cathode. ECC802 was approximately a 12AU7 equivalent, but +/- 100V heater to cathode.

6DJ8, 6922, and ECC88 were rated for 50V, 60V, 90V, 120V heater to cathode, depending on + or -, and the manufacturer of the tube. The early 6DJ8 had a higher heater to cathode voltage rating on One of the two triodes.

Changing from a 12AU7 or ECC802 to a 6DJ8 just to have more gain, does have some serious heater consequences.

There is a DC component of the heater to cathode voltage, and for the top triode in an SRPP, there is an AC component of the heater to cathode voltage. That AC component is the largest swing that is required to drive the next stage. Take a 300B for example, with 60, 70, 80, or even 90V bias. The SRPP will have to drive from +/- 60 to +/- 90Volts according to that bias.

If you use a 6DJ8, and have a Stereo amp on one chassis, you should use two 6DJ8s. A. One 6DJ8 for left and right Bottom triodes of the SRPP stages, and have the filaments at or near ground. B. Use another filament winding for a second 6DJ8 for the left and right Top triodes of the SRPP, and elevate that filament at or near to the cathode(s) voltage of the top tube triodes.

Now you know one of the reasons I do not use SRPP any more. I prefer using a triode that has a current source in the plate, i.e. a 900V IXYS part. It has low distortion and good signal swing, and eliminates the dual filament voltage supply problem.
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Old 8th November 2018, 01:38 PM   #17
ciccio64 is offline ciccio64  Italy
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Thanks for the detailed explanation.Alot to learn here. Googling around it appear that all you say is very correct and a proper SRPP should be done the way you say (do you have any schematic for such configuration?)On the other end to do that would require to re build the pre section... while the solution implemented already seems to be also popular (so i think not completely wrong or not?)

In any case if i leave like that what could happen, would that affect the sound or may lead to breack down?Bearing in mind the amp has been already working fine for almost one year (yes that the time that toke me to tune it as i like!!) also someone mentioned in previous posts a leak resistor what is that and were should be located.
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Old 8th November 2018, 07:19 PM   #18
6A3sUMMER is offline 6A3sUMMER  United States
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Many SRPP stages that exceed the filament to cathode voltage ratings, and many Concertina phase invertor stages that exceed the filament to cathode voltage ratings of the tubes work well for many years.

Some of those circuits do not work well, either in the beginning, or as time goes on.
Depends on the tube type, manufacturer, circuit voltages, etc.
What if you can no longer find a particular manufacturer's tube that is more robust?

Why gamble?
Stay within the tube maximum ratings. If that requires another 6.3V winding, and a resistive voltage divider, plus a bypass capacitor, and the voltage divided from the B+, in order to be within the tube's maximum ratings, I recommend doing that.

"You should make things as simple as possible, but no simpler" - Sometimes attributed to Albert Einstein, but the origin of the quote, and exact wording is questioned.

Last edited by 6A3sUMMER; 8th November 2018 at 07:22 PM.
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