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Can someone explain this confusing schematic?
Can someone explain this confusing schematic?
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Old 12th September 2017, 09:28 PM   #31
smoking-amp is offline smoking-amp  United States
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"but it required readjustment almost every day"

that might be the clue.

There used to be center tapped plate inductors, called "balancing inductors", that would solve the problem nicely.
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Old 13th September 2017, 02:25 AM   #32
Tubelab_com is offline Tubelab_com  United States
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Quote:
Using a DC bias for the second LTP grid means that it has to track any variations in the DC level of the output from the previous stage applied to the first grid.
Not sure exactly what you are asking here, but using the schematic shown previously, I adjusted the offset pot for equal plate voltages on the second stage tube, and let the plates of the first stage fall where they may. Usually this was within a volt or so. If not I swapped tubes. After the amp was operational and burned in for a few days I tweaked the three pots (offset, first stage CCS current, and second stage CCS current) for best distortion profile.

The design evolved from the amp created here:

6L6GC AB2 Amp

I have used this driver board for everything from DHP's (307A's) to triode wired KT88's to big sweep tube amps. Once set up they don't require any more maintenance than any other tube amp. I am planning a triode wired 7403 amp with these boards sometime in the future.

Quote:
center tapped plate inductors, called "balancing inductors",
Forcing the plates of the driver tube to a fixed DC voltage via a tapped inductor should help, but this was an experiment to see what would happen with 3 directly coupled stages. It showed me that it was a possibility, but it was running from my collection of old Ebay sourced power supplies of various age and condition. No real useful stability measurements were made to determine where the errors came from.

I could use a set of these for another project. I am experimenting with a push pull cathode follower output stage. This needs a big bunch of drive voltage which is best created with a typical push pull amp and an inductor like you describe. I have been using a P-P OPT with a very light load to minimize ringing. Note that the little blue and black open frame Edcor's will short out in a spectacular manner when attempting to make 1000 V P-P of drive from a 600 volt supply.
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Old 13th September 2017, 04:39 AM   #33
smoking-amp is offline smoking-amp  United States
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"Note that the little blue and black open frame Edcor's will short out in a spectacular manner when attempting to make 1000 V P-P of drive from a 600 volt supply."

You mean those little XSM 2.5 Watters?
W = VxV/R, 2.5 = VxV/15000, Vpk = 1.4 x 193.65 = 273 V
although the specifications readout seems to be stuck at 50V

We need to have Edcor wind some GXPP15 types with a CT'd primary only, for balancing inductors. (twice as much primary winding could fit on them without the secondary) Hopefully balanced resistances, or a couple of interleaves to equalize the resistances. Or just ask for 4 stacked (insulated) equal turn windings, can configure them externally.

Last edited by smoking-amp; 13th September 2017 at 04:42 AM.
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Old 13th September 2017, 12:01 PM   #34
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Hyperman75 View Post
These equations are about the Loyez phase inverter.
Obviously, you don't understand how it works. <snip>
Let me explain how i see it works.With illustations !
An out of balance concertina driving an out of balance LTP, compensating eachother.
You can adjust the balance with the feedback resistor but that changes the input level too.Adjustement are best made with Rc.
The bewitch amp can function with one triode less and an R-C coupling.
Mona
Attached Images
File Type: jpg Loyade.jpg (95.0 KB, 73 views)
File Type: jpg BEWITCHnoVshift.jpg (20.3 KB, 64 views)
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Old 13th September 2017, 12:27 PM   #35
Hyperman75 is offline Hyperman75  France
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Mona
In the Loyez phase inverter, you have to consider that the return of the feedback is on the cathode of V1a (the lower triode).
Triode V1b is in grill commun mode, the input is the cathode.
It makes a negative feedback on V1a and a positive feedback on V1b.
So amplitude at the output of V1a at the plate is reduced and output amplitude of V1b at the plate is increased.
Level of feedback is correct when both amplitudes are balanced.
As already said (not by me but by Loyez) V2 triodes are not involved in the process.
Unfortunately the patent is in french and may be difficult to read for some people, I agree.

Jacques
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Old 13th September 2017, 12:51 PM   #36
Tubelab_com is offline Tubelab_com  United States
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Quote:
You mean those little XSM 2.5 Watters?
No, the OPT that I fried was originally bought for an experiment that was done along the lines of Pete Millett's Engineers Amp, but several years earlier.

I had purchased some little PC boards from a fellow diyAudio member that had 7 pin sockets connected to the green screw terminals that I use on my amp PC boards. I wired up 4 of these with a pair of 6AU6's and a pair of 6AQ5's, all configured as pentodes in a manner similar to the cascaded LTP's discussed here. The original intent was for a driver board, but I got a cheap OPT from Edcor to test it as a stand alone amp. It worked far better than I expected, and I decided to try it with better parts, but it never happened. The experiment got lost in the shuffle and I couldn't find the breadboard when I wanted to test it further.

Now, about 10 years and two house moves totaling 1200 miles, I open a box of junk, and I find it.......So I stole it's OPT for some small guitar amp experiments.

That's the OPT I fried a few weeks back. It is obviously an Edcor but has no stickers or markings, and my guess is probably an XPP10-8K or XPP10-10K, based on what I bought it for.

There could have been some damage to the OPT from living in a box of junk for years, but it did work fine in a 4 watt guitar amp on 170 volts, and as a stand alone amp running 4 small $1 tubes at the 15 watt level.....an experiment that will see further development.

OPT autopsy was inconclusive considering the fireball that ate it, but it looks like an arc from primary to secondary along the outer edge of the winding near one of the terminals for the primary. This might not have happened if I didn't have the secondary grounded, but I was experimenting with GNFB.

The only power supply that I have capable of producing 600 volts has a 1.7 Amp output capability and 1000 uF of capacitance across its output. This power supply makes little mistakes into BIG fireworks!
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Old 13th September 2017, 01:59 PM   #37
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Hyperman75 View Post
Mona
In the Loyez phase inverter, you have to consider that the return of the feedback is on the cathode of V1a (the lower triode).
Triode V1b is in grill commun mode, the input is the cathode.
It makes a negative feedback on V1a and a positive feedback on V1b.
So amplitude at the output of V1a at the plate is reduced and output amplitude of V1b at the plate is increased.
Jacques
No, the feedback comes inverted by V1a on one side of the LTP and via V1b, not inverted, on the other side of the LTP.Making it NFB in both cases.
Mona
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Old 13th September 2017, 02:14 PM   #38
DF96 is offline DF96  England
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The feedback to the cathodes appears uninverted at both anodes of V1. However, as the two anodes have opposite signal polarities this is negative feedback for V1a and positive feedback for V1b.

Another way of looking at it is that the feedback has the same polarity as the signal already present at V1a cathode. It thus reduces the signal voltage across V1a g-k - so is negative feedback. It boost the signal voltage at V1a cathode, and so boosts the signal voltage at V1b cathode, thus boosting the signal voltage across V1b g-k - so is positive feedback. The net effect is negative feedback, because the reduction in V1A signal is greater than the boost in V1b signal.

Like most cathode feedback arrangements, this relies on common-mode distortion in V1 being small as the feedback cannot remove C-M distortion.
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Old 13th September 2017, 02:58 PM   #39
Ketje is offline Ketje  Belgium
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Oops DF96 is right, the FB goes to both cathodes, making one positive. But the extra voltage from the FB has to attenuated (smaller Rc) to restore the balance for the LTP, making the feedback to V1b is very small.
Makes no big change in the overall circuit.
Mona
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Old 13th September 2017, 03:08 PM   #40
smoking-amp is offline smoking-amp  United States
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I think the grid signals to the second stage have to be equal or those outputs won't be equal. (180 deg. phase inputs. And equal value plate R's on the second stage)

V1a has very little effective input signal left after the neg. Fdbk at its cathode. So not much signal needed at V1b cathode to get the same (180 deg.) result.

----

"Like most cathode feedback arrangements, this relies on common-mode distortion in V1 being small as the feedback cannot remove C-M distortion."

I assume the CM dist. is just from the inherent tube dist. (and its loading) So a differential front end would be better in that respect.

-----

Needing a DC balance pot on the input cathodes (as posted earlier by jfetter, from the patent I think), that needs adjusting every month, does not put this circuit into favorable status.
Attached Images
File Type: jpg BEWITCH-6550-Schematic.jpg (235.9 KB, 49 views)
File Type: gif IMG_0495.GIF (28.0 KB, 46 views)

Last edited by smoking-amp; 13th September 2017 at 03:35 PM.
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