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Local feedback between grid-cathode
Local feedback between grid-cathode
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Old 28th July 2019, 09:38 AM   #81
JoeAlders is offline JoeAlders  Netherlands
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Originally Posted by Wavebourn View Post
Why you stuck on UL? What's the point?

Reading all those responses I fully agree with Wavebourn's
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Old 28th July 2019, 11:58 AM   #82
gabdx is offline gabdx  Canada
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I never built a pentode amp, this is my first. (well I built 2 CCS with pentodes with great success first try, just a few adjustments, pentodes are very easy to work with)

Looking at the pentode curves I assume that the gain mu is higher than in UL

As consequence I could run the amp with GNF(decent amount) + CBF, without overtaxing the poor little 6sn7 LTP.

At worst, I will disconnect the G2 supply which I aim to be around 400V, I never push the amp past 30 watts, and I will keep there the regulated supply for gain and splitter LTP.

I am just afraid that Pentode mode with GNF and CBF wont beat GNF + UL, audibly and in measurements. The bar is very high...
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Old 28th July 2019, 05:11 PM   #83
45 is offline 45  Italy
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So in the end it is basically a design flaw where the driver is not up to the task. Learning curve...no problem.
When using CFB this has to be taken into account before building anything. Usually CFB windings are in 5-10% ratio so it is not even remotely close to high demand of a cathode follower but it needs careful design nonetheless especially when the sensitivity and/or power level of the output tubes requires significant swing (without CFB already).
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Old 28th July 2019, 05:19 PM   #84
gabdx is offline gabdx  Canada
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45, I am just switching transformers in an old design to test all possibilities. I never intended this amplifier to have a CFB. I purchased the transformers last year for a new project, the mono blocks were built by me 15 years ago.

If there is something that I am learning here it is first, the pentode outputs, which I will see in a few days when I have my G2 fuse, second, that the output transformer quality counts a lot more than I thought.

It was overconfident in UL and GNF to resolve all audible flaws of output transformers. I though a hifi level transformer of good power was sufficient if non-linearities were addressed by global/local, UL, feedbacks. I knew that in SET the quality was paramount, I just didn't thought it would make such a big difference in a push-pull design.

I know that good designs do not use UL, for some reasons, it is always pentode with regulated G2 and regulated G1 bias.

I doubt that it is that good, I will see for myself in a few days, and I have big doubts because I find the sound in UL hard to improve upon, it is quite superb actually.

I am doing this in the perspective of the ultimate amplifier....

A direct zirconium contact input to a 6sn7 frosted, with a high pressure big carbon track mono volume pot, followed with a ecc99 with el822 LTP at around 15ma with over 450V of potential, to the kt120 in UL ( or pentode with CFB) to quads modified exotic super core transformers of around 150 watts, (they already blasted in THD and clearly audible differences my other OTs from 25 to over 100Watts).

The mono block will use a full wave, super high power soft diodes, with film input capacitor, then a 10H 200ma choke, and a 280uf 1000V capacitor to the HV B+.

The bias is regulated from a dedicated transformer with the uttermost quality trim-pots.

The pre sections are all film caps, super high power, regulated 100%. The anode resistors are super huge like 2 inches long, non-inductive super resistors, crazy
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Old 4th August 2019, 02:48 PM   #85
kodabmx is offline kodabmx  Canada
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Local feedback between grid-cathode
gabdx: Have you considered using one of these for a cheap and cheerful screen supply?

DC-DC 8-32V to +-45V-390V Step-up Module ZVS High Voltage Capacitor Charge Board | eBay

I have several, if you like I can send you one for 5$ plus postage. Probably the same price as ordering from China, but you'll get it in days, not weeks.
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Old 6th August 2019, 12:00 AM   #86
gabdx is offline gabdx  Canada
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Hi Kodabmx, I appreciate your offer.

I decline, I build my own regulator
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