Sorry to bother you with another simple question. I am planning on DC heating some driver tubes with the Pete Millett regulated DC boards. If I want to elevate the circuit, do I tie the virtual center tap I'd create to the voltage divider or is there another way of accomplishing the elevation? I am not sure how elevating the center tap would effect the DC boards.
Millett Boards
Millett Boards
As long as the entire driver filament circuit is floating, and is only connected to the elevation voltage
at one point, you can connect it at any point that you like. You don't even need the virtual center tap,
just connect any node of the filament circuit to the (decoupled) elevation voltage. This won't affect
the DC boards, since no current flows because there is only a single point connection.
at one point, you can connect it at any point that you like. You don't even need the virtual center tap,
just connect any node of the filament circuit to the (decoupled) elevation voltage. This won't affect
the DC boards, since no current flows because there is only a single point connection.
Thanks! Is there a formula or general rule for the size of the decoupling capacitor? I was planning on 470K and 100K for the divider.
Then the filter impedance is 470k // 100k = 82k along with the decoupling C.
To decouple down to say 10Hz, use C = 1 / (2Pi x 82k x 10) = 0.22uF.
More C than that is fine, as long as the time constant is faster than the HV supply coming up.
Even 10uF would be ok and filter more.
To decouple down to say 10Hz, use C = 1 / (2Pi x 82k x 10) = 0.22uF.
More C than that is fine, as long as the time constant is faster than the HV supply coming up.
Even 10uF would be ok and filter more.
470K/100K may be on the high side. Many tubes recommend a cathode to heater resistance of no more than 20K. I typically elevate heaters to 25% of the HT and for this I use a 66K/22K network. You need to use 2W resistors and make the 66K out of two 333K resistors in series to manage the dissipation. I typically decouple with 100uF/160V.
Cheers
Ian
Cheers
Ian
Yes, that info is often missing from data sheets. The Telefunken ECC83 does have a mfr rating
of 150k maximum H-K resistance.
of 150k maximum H-K resistance.
The Telefunken datasheet is for new production. More often than not these new manufacturers just copy data from datasheets from the old days.
In the attached Philips datasheet, the value of 150K is said to be valid for phase inversion circuits (so LTP's and Concertina's), while for other applications 20K is the maximum value.
From my own experience it is safe to use 150K for dc elevation of the ECC83, the ECC40 and the E182CC. In a couple of my projects I use 68K with ECC88/PCC88. In one projects I have a PL36 and a PY81 elevated with 260K without them showing any signs of stress. All these resistors are paralleled by capacitance (mostly 10 uF, sometimes 22 uF).
There are many examples of amplifiers from the old days that show these kind of high resistor values for filament elevation. So I concluded some time ago that if the tube type is not too old, elevation with values like 100K will work fine, provided there's some capacitance parallel.
In the attached Philips datasheet, the value of 150K is said to be valid for phase inversion circuits (so LTP's and Concertina's), while for other applications 20K is the maximum value.
From my own experience it is safe to use 150K for dc elevation of the ECC83, the ECC40 and the E182CC. In a couple of my projects I use 68K with ECC88/PCC88. In one projects I have a PL36 and a PY81 elevated with 260K without them showing any signs of stress. All these resistors are paralleled by capacitance (mostly 10 uF, sometimes 22 uF).
There are many examples of amplifiers from the old days that show these kind of high resistor values for filament elevation. So I concluded some time ago that if the tube type is not too old, elevation with values like 100K will work fine, provided there's some capacitance parallel.
The tubes in question will be an ef86 with a 6s4a follower. I'll dig into the data sheets later today
An example of a preamplifier by Hiraga in which the filament supply of ECC81/12AT7 srpp and ECC82/12AU7 srpp is lifted with 82K with a capacitor of 1 uF parallel.
I would think that a zener would work fine but it can also be done with two resistors and a capacitor.
I would think that a zener would work fine but it can also be done with two resistors and a capacitor.
In a couple of Philips OTL amplifiers the filament supply for all the tubes is lifted with 47K (and 150K), paralleled by 0.47 uF, to about 52 V. These amplifiers contain EF86's
Note that in these amplifiers the cathodes of the EF86's are negative in respect to their heaters. Philips datasheets for the EF86 state 50 V as Vkf max. for this condition (and 100 V when the cathode is positive in respect to the heater) but the attached Mullard datasheet states 100 V and 150 V for these maximum values.
Note that in these amplifiers the cathodes of the EF86's are negative in respect to their heaters. Philips datasheets for the EF86 state 50 V as Vkf max. for this condition (and 100 V when the cathode is positive in respect to the heater) but the attached Mullard datasheet states 100 V and 150 V for these maximum values.
My post #9 was a reaction to a post which was later deleted (in it the question was if a zener could be used to lift the filament supply for 12AT7's in srpp configuration).
Thank you for answering the question I had. The information you provided is much appreciated.
From this thread I became worried about a Rkf of 20K for the ECC81 when using a zener.
I deleted the posting because I found a Philips datasheet for the ECC83 that mentions on page 4 Rkf 20K for "normal" use and Rkf 150K for phase reversing use. As such I was no longer worried about the Rkf of 20K (but may be mistaken).
Still pondering if I want to use a single tube per channel or if I shall use one tube elevated and the other at ground potential. The consideration is that in the latter case the elevated tube can be on 12 V DC but the other will be 6.3VAC and I like to stay away from AC in the driver. Thoughts?
EF86 is one of the tubes where 20K for Rkf is quoted except for use in a phase splitter.
Cheers
Ian
The expression 470k // 100k means in parallel, and which is what the C sees across its terminals for AC signals.
So 470k // 100k means 470k x 100k / (470k + 100k) = 82k
So 470k // 100k means 470k x 100k / (470k + 100k) = 82k
I have read a thorough research about how srpp works with good measurements and the outcome was that ecc81 and 82 are not the best suited tubes for SRPP.
There is no particular tube type best suited to an SRPP topology. It all depends on what you want the amp to do. Unfortunately myths abound about the SRPP.
Cheers
Ian
Which book is that? I cannot find it mentioned in this thread.
Cheers
Ian