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Hum pot implementation on 2a3
Hum pot implementation on 2a3
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Old 24th March 2020, 09:05 PM   #11
RPMac is offline RPMac  United States
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Kmaier, I was thinking of making a virtual center tap with two 100Rs for a fixed DC balance and using the humpot for AC balance. Should padding resisters be approximately one half of each leg of humpot(100R pot>22R each padding resisters)?

Emk2, I wish I could read the article...looks very interesting.
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Old 24th March 2020, 11:24 PM   #12
kokoriantz is offline kokoriantz  Lebanon
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My way. The bootstrap not only adjusts the hum but also reduces the cathode feedback .
Attached Images
File Type: jpg hum.JPG (35.7 KB, 82 views)

Last edited by kokoriantz; 24th March 2020 at 11:40 PM.
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Old 25th March 2020, 03:14 AM   #13
kmaier is offline kmaier  United States
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Quote:
Originally Posted by RPMac View Post
Kmaier, I was thinking of making a virtual center tap with two 100Rs for a fixed DC balance and using the humpot for AC balance. Should padding resisters be approximately one half of each leg of humpot(100R pot>22R each padding resisters)?

Emk2, I wish I could read the article...looks very interesting.
Using a pair of resistors for a virtual center tap and using it for a fixed DC balance is fine, but also realize that the resistor pair does two things: 1- They become part of the self-bias resistor value. 2- They also present an additional load to the filament current. Granted, the added current will likely be small, but if you use a pair of 100-ohm resistors, they effectively act as a 50-ohm bias resistor and should be accounted for in your overall bias voltage/current.

For the AC balance pot, I've always used a 100-ohm 2-watt linear pot. In general, a pair of 12-ohm resistors for padding is what I use. Also note that this setup presents an additional current load to the filament current and should be accounted for. Also note there are two reasons to pad the balance pot. 1- To allow a finer adjustment for the hum level, i.e., not a knife-point 2- provide a lower effective resistance in series with the bypass capacitor, which allows better nulling of the AC hum level.
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Old 25th March 2020, 08:43 AM   #14
6A3sUMMER is online now 6A3sUMMER  United States
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kmair,

I originally said:
"1. The schematic on the left has finer Resolution (easier to find the spot of lowest hum), but has less Range of adjustment versus using the 50 Ohm pot by itself. Use that schematic if you have enough range to find the sweet spot."

A 50 Ohm pot does not have as fine a resolution in the center, as the same 50 Ohm pot has when you add a pair of 15 Ohm resistors to that pot.

Also as has been stated in many of the postings above, unless the pot is far off center, then the majority of the current passes through the 15 Ohm resistors.

The left schematic has a 50 Ohm pot.

The right schematic has a 100 Ohm pot, yes of course a 100 Ohm pot has more resolution with the 12 Ohm resistors. But as already noted, the schematic wiring either needs to be changed, or list the exact model number of the "2A3" or "45" tube (that has a center tap).
I am sure someone on the forum will try and build to the right side schematic without reading the text below, and then wonder why it does not work with a real 2A3 or 45.

Last edited by 6A3sUMMER; 25th March 2020 at 08:53 AM.
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Old 25th March 2020, 09:52 AM   #15
emk2 is offline emk2
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Quote:
I wish I could read the article...looks very interesting.
I've tried with online OCR software then translate in english, unfortunately it doesn't work well at all..

Quote:
My way. The bootstrap not only adjusts the hum but also reduces the cathode feedback .
Is this a way to partially reinject noise in opposite phase through the grid?
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Old 25th March 2020, 11:40 PM   #16
6A3sUMMER is online now 6A3sUMMER  United States
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I used to have 45, 2A3, and 300B Direct Heated Triodes (DHT) amplifiers, single ended and some push pull.
Some of them used AC filaments, some of them used DC filaments.

I will not build another DHT amplifier unless I use DC power for the filaments.
It solves several problems, and I have written about the problems with AC powered DHT in various threads in this forum.
And many others have written about that too, on this forum too.
AC is OK. DC is better.

Last edited by 6A3sUMMER; 25th March 2020 at 11:49 PM.
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Old 26th March 2020, 03:04 AM   #17
kmaier is offline kmaier  United States
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AC or DC... in general, I would tend to agree (on DC), but not in every case. I also wrote up a lot on this topic many years ago. I think it depends, mostly on the type of DHT being used.

For something like a good spec'd 45 DHT, I've been able to get hum levels below 90dB referenced to 1-watt output, using an AC filament supply. Granted, I don't have too many 45s that meet this criteria, but certainly enough to supply my personal needs for life.

For the 45 DHT, I've found that ALL of them have the identical "M" style filament. Using DC on this tube pushes one end of the internal structure at a higher bias voltage than the other, which can be a 5% change. I currently have numerous NOS pairs of 45s that can yield over 80dB S/N ratio referenced to 1-watt is more than respectable.

On a side note, I've read posts where folks have touted amazing sound from SET amps that are less than a few watts... superb low-level detail, etc., yet state that the hum and noise output from the amplifier is quite a bit over 1 millivolt, which has a S/N ratio less than 60-65dB.

Back to the topic however. I also have some original NOS single plate 2A3 DHTs and some Western Electric 300b DHTs. The filament (cathode) structure for these are very different from a 45. Effectively, they are a center-tapped filament. Meaning there's a left side and a right side. One filament pin goes to both sides where they meet in the center of the structure. The other filament pin goes to both outside ends of the two sides. In this configuration, an AC filament is going to be useless as the S/N level will be very poor indeed. A DC filament supply must be used for proper S/N ratio. Also, due to the center-tapped configuration, you don't have a situation where one side of the structure has a higher bias than the other.

I also have some NOS 808 triodes and 3C24 triodes. While these are typical transmitting triodes, they can be used in audio amps in Class A2. However, the helical filament does not balance well for hum with AC, so these also must be used with a DC filament supply.

So, for me... I have some lovely 45 DHT and 2A3 DHT (dual plate versions) amps that run AC filament supplies with fixed DC balance and adjustable AC balance that achieve over 80dB S/N referenced to 1-watt output. They are near dead silent with higher efficiency speakers as well. If I can avoid the added complexity of the DC filament supply and achieve the required specification, I'm fine with it... otherwise I'll employ a DC supply.
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Old 26th March 2020, 10:08 PM   #18
6A3sUMMER is online now 6A3sUMMER  United States
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kmair,

As you well know, there are at least 2 effects of using AC filaments on DHT tubes.

1. The AC current through the filaments creates time varying magnetic fields.
Those (multiple) filament wires are attracted to the steel plates at a 2 times line frequency rate (at the rate of Alternations). Any imperfection of the filament's placement between the steel plates on each side of the filament wires, will cause the filament to vibrate toward the closest side of the plate.
This modulates the DC current at a rate of 2 X line frequency.

2. When the AC voltage is instantaneously at 0 Volts, the distribution of DC current to the plate is relatively constant along the filament wires. Consider the transconductance at this state.
When the AC voltage is instantaneously at the crest (peak) of the sine wave, the distribution of DC current to the plate is Not relatively constant along the filament wires. Consider the Different transconductance at this state.
The issue is that the transconductance is changing at the rate of alternations of the AC (2 x the line frequency).
That causes intermodulation of 2x line frequency and a test tone, or musical note.

You have already noted that the transconductance changes when the DHT filaments are DC powered. That alludes to changing transconductance when AC (Time Varying DC) is applied.

Measurement test:
Apply a sine wave test tone to the amplifier (pick a frequency, use one at mid frequencies or high frequencies). Look at the output of the amp with a spectrum analyzer.
You will see sidebands below and above the test tone.
Example, 60 Hz power line frequency, 120 alternations; 1000 Hz test tone . . .
The spectrum analyzer sees: 880Hz sideband, 1000Hz, 1120Hz sideband.

Now, apply DC to the filaments, and repeat the test.
Great, the 880Hz sideband, and the 1120Hz sideband are gone, and only the desired 1000Hz tone is left.

A third effect of AC DHT filaments is when the amp is at or near clipping due to the grid to DHT filament voltage at or near 0V.
I will not even discuss this one in this thread, because no Hi Fi amp should be driven to clipping for good musical playback.

Guitars amps are another animal. Clipping, often a great thing.
And somehow, guitar amps keep showing up on Tubes / Valves Threads, instead of on the Instruments & Amps Threads.
Just my observation.

Last edited by 6A3sUMMER; 26th March 2020 at 10:21 PM.
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Old 26th March 2020, 10:27 PM   #19
6A3sUMMER is online now 6A3sUMMER  United States
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kmair,

45 tubes sound great.
One thing I have noticed about most 45 tubes . . .
The grid wires extend both above and below the plate structure.
That eliminates or at least reduces the "diode effect" when the tube is near cut off.
Many other tubes do not do that.
Instead, the filament wires conduct directly to the plate where there is no intervening grid wires.

Do you think that is part of the intrinsic linearity of the 45 tube?

Kind of reminds me of the difference between overhung and underhung voice coils,
versus driving either one beyond its linear range.
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Old 27th March 2020, 06:12 AM   #20
kmaier is offline kmaier  United States
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6A3sUMMER,

All good points.... and valid. You can also do a similar test with DACs and see some interesting artifacts, etc. Studio grade convertors perform better, but cost a lot more. In these types of tests, you should also be looking at the signal level for those sideband signals. If they're down 80-90dB, then there's a good chance they won't be audible... but there's a lot of variables in a "system" which might highlight them or not.

The 45 amps I designed and built back in 2008 have respectable ratings:
- 25Hz - 50KHz within 1dB at 1-watt output
- 2.25 watts RMS before clipping
- 0.35% THD at 1-watt (1KHz - less at higher frequencies)
- Hum and noise output around 150 microvolts (16-ohm output) - ~88.5dB below 1-watt.

And these specs are with AC filaments and a zero-feedback design. Granted, hand-selected and tested tubes from a pretty large stock, but still, they perform very well. I guess in some ways, I just don't like the concept of a DC filament supply on the 45 triode. Most others, as noted above, simply won't yield acceptable results with an AC filament supply.

RE the post on the 45 triode itself. I have a large number of these and different manufacturers as well. They include: RCA, Tung-Sol, Ken-Rad, Zenith, Philco, Sylvania, GE, Cunningham and possibly others. Most are ST glass but I have half a dozen older globe style as well.

I've done extensive measurements on these... but in general, ALL have the same basic internal construction. The differences lie mainly in internal supports, i.e., mica discs, vibration decoupling to the glass envelope, robustness of the wire framing, etc. There's also a lot of rebranding done during those days. I actually have a pair of RCA 45s in original boxes with identical date codes. Yet, one is made by RCA and the other is made by Sylvania. IIRC, Most of Zenith's tubes were made by Sylvania and marked for them (Zenith).

In any case, my overall favorites are the Sylvania 45s, but with specific internals. The worst are always the Globes, period. As much as people have raved about them, they're mostly terrible in operation. As there's no supports in the envelope, they "sing" when mechanically excited, even low-level playback is apparent. Perhaps that inherent mechanical feedback (from the sound from the speakers) is what some people perceive as a wonderful tube sound... I don't.

One thing is certain however, the 45 (as long as they're good ones) have a very linear transfer function. Perhaps it is a side effect of the grid structure extending beyond the anode, but the filament also does this.

I've also noticed that pretty much everyone who has listened to a well done 45 SET amp likes them... who knew.
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