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Old 8th October 2005, 07:01 AM   #11
rdf is offline rdf  Canada
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It's a confusing post. The schematic shows 24 kohm connected to the 4 ohm tap of a transformer with no 8 ohm tap, Danko then writes that he changed it to 12 kohm because his transformer had a 15 ohm and no 8 ohm tap. If his link is to the original schematic I would think Rf should remain unchanged.
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Old 10th October 2005, 04:56 PM   #12
kevinkr is offline kevinkr  United States
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No, the fellow who indicated the feedback resistance needed to be increased was correct. For the 16 ohm tap 47.5K to 49.9K would be right.

Transformer coupled tube amplifiers have differing voltage gains depending on the output tap being used regardless of whether global feedback is being used or not.

If say the open loop gain on the 4 ohm tap was 30dB, then the gain on the 16 ohm tap would be 36dB or 6dB higher because effectively the turns ratio between the 16 ohm and 4 ohm taps relative to each other is 2:1. (Note that the primary to secondary turns ratio is 1/2 as much on the 16 ohm tap as compared to the 4 ohm tap.) Scale the feedback resistance by the same ratio and the feedback margin will remain the same.

Kevin

Edited for clarity
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Old 10th October 2005, 05:35 PM   #13
dhaen is offline dhaen  Europe
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Thanks Kevin, that's a clear explanation of the logic I used. Hopefully the Danko will be confident to start here - he may still have to switch phase.
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Old 10th October 2005, 06:42 PM   #14
rdf is offline rdf  Canada
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Quote:
Originally posted by kevinkr
Transformer coupled tube amplifiers have differing voltage gains depending on the output tap being used regardless of whether global feedback is being used or not.

Thanks Kevin, I understand this relationship. What I don't completely grasp is Danko's question. The original circuit schematic doesn't show the 16 ohm tap used, not for feedback or for driving a load. Here's the circuit link in context (not Danko's home page):

http://www.jogis-roehrenbude.de/Lese...ox/Wandbox.htm

The original designer used a transformer without an 8 ohm tap and returned 24 kohm from the 4 ohm tap. When he says "The output transformer is not 4 and 8 ohm..", that appears to be a description of the original circuit. If Danko did the same - connected speaker and returned feedback from the 4 ohm tap - there would be no change since any additional taps are effectively out of the circuit. Is Danko returning feedback from the 8 ohm tap of his transformer? If so then neither doubling Rf or leaving it the same will result in an equivalent amount of feedback being returned to the first stage. It's a stumbling block of phrasing, not theory.
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Old 11th October 2005, 04:28 PM   #15
kevinkr is offline kevinkr  United States
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Just took a look at the schematic, and while somewhat unconventional in design, changing the feedback resistor to twice its original value will maintain the feedback margin at the original design value. Other issues may arise if the design was marginally stable and the replacement transformer introduces significantly greater phase shift than the original, otherwise there should not be a problem.

If infact he were to use the 8 ohm tap the relationship still applies - in this case it is still the square of the windings ratio which in this case would be 1.414 times that of the 4 ohm tap which would be roughly 34K..



Kevin
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Old 11th October 2005, 11:53 PM   #16
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This is a "to whom it may concern" type of entry. I have mentioned elsewhere (so apology) that:

One needs to be careful of output transformers with a "4-8-16 ohm" designation on the output (I mean the transformer itself, not the diagram). If this is a 2-winding (secondary) type of affair with 16 ohm from the 2 windings in serie and an 8 ohm tap halfway up one winding, phase conditions at high frequencies may differ notably depending on whether one winding (4 ohm) or 2 windings (16 ohm) are used (loaded). The leakage reactance to the primaries MAY differ depending on where the secondaries are situated physically and how they are loaded. While the feedback connected to a fixed tap with a certain R/C may give optimum rise time on, say, 16 ohm, it could border on instability using a 4 ohm load off only one winding. That is why in the better transformers all secondaries are always used: 2 in parallel for 4 ohm or in series for 16 ohm. An 8 ohm tap is usually somewhat of a compromise.

I have encountered this in practice and one needs to be careful if you cannot see the winding layout. The 220pF phase compensating capacitor may not be optimal for all connections even if the feedback stays connected to the same tap. Again a square wave generator and scope is really very well worth employing just to make sure.
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Old 12th October 2005, 03:27 AM   #17
rdf is offline rdf  Canada
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Quote:
Originally posted by kevinkr
If infact he were to use the 8 ohm tap the relationship still applies - in this case it is still the square of the windings ratio which in this case would be 1.414 times that of the 4 ohm tap which would be roughly 34K..



Kevin

No disagreement at all. In fact, I think this is the textbook correct answer to the question Danko asked (with further elaboration from Johan Potgieter.) Again, the schematic linked isn't Danko's. His appears to be different in a not clearly specified way, that difference probably being the use of 8 ohm taps for both output and feedback. That was the gist of my comments, Rf increases but not doubles. Whew, now back to attempting to remove the tiny oscillations from my no feedback el84 SE!
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Old 12th October 2005, 03:25 PM   #18
kevinkr is offline kevinkr  United States
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Hi rdf,
It's an interesting circuit and I also wonder how much his implementation differs from the one shown. It utilizes a parafeed driver circuit with what appears to me to be positive feedback via the cathode connections of the ECC85, presumably to boost the gain somewhat, if excessive this might result in instability and I would probably remove it.

On a different subject (your EL84SE) sometimes oscillating EL84s can be tamed by adding a 1K grid stopper resistor right at the socket, and in many cases whether triode or UL connected a small resistor in series with the screen grid can be a great help. I would recommend something in the 220 ohm range. Keep path lengths short.. (I am assuming that the EL84 is the source of oscillation.)

I use EL84 frequently in voltage regulator circuits and early on I had a lot of problems with VHF oscillation which manifested itself at dc and within the 20kHz audio bandwidth. The screen grid in this instance needed a high quality bypass cap right at the socket.

Hope this helps.

Kevin
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Old 13th October 2005, 02:54 AM   #19
rdf is offline rdf  Canada
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Thanks Kevin, I tried everything in the standard toolkit: plate stoppers, screen stopper to 2.2 kohm (one v high freq oscillation killed at 100), grid stopper to 10 kohm, cathode cap bypasses, B+ cap bypasses, litz wire, different brands of tube, sacrificed a chicken and burned candles. The layout's simple, clean, straightforward and based on a groundplane. The only thing that seemed to work until tonite was shorting the el84 grid to ground. Then I noticed the ground was oscillating too. Oscilloscope artifact, gotta love it.
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Old 13th October 2005, 04:38 AM   #20
kevinkr is offline kevinkr  United States
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Hi rdf,
I run into these sorts of things fairly frequently, it is a very noisy world we live in these days. Broadcast television, not to mention am radio, cell phones and the like product a lot of rf energy - the origin of which is not always obvious.

If the amplifier was turned off and unplugged and you still observe the waveform with the probe shorted and connected/disconnected from your amplifier ground plane then the amplifier itself is exonerated, if not then you might have some slething left to do.. LOL

Sounds to me like it is external emi pickup, the bain of the civilized world..

Kevin
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