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Old 1st October 2012, 10:40 PM   #51
kmaier is offline kmaier  United States
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A very good thread. As DHTs are discussed, I feel it's also important to consider the different filament topologies used as they will have differing affects against the (AC or DC) filament supply.

Consider the simplest arrangement, the 45 triode. A single "M" structure inside a single grid/plate assembly where the supply voltage (and hence cathode lines) are applied at the "ends" of the "M".

By contract, an RCA/Cunnigham single-plate 2A3 has a center-tapped filament. It's structure is multiple vertical wires tethered at top/bottom. However, it's divided into two sides; left/right, where you can electrically consider it an "M" like the 45, but the center of the "M" is one of the filament lines, and the two "ends" of the "M" are tied together for the other filament line. This creates a pair of filaments wired out of phase within the grid/plate assembly. The hum level with AC filaments is extremely high and can not be nulled out to anything near an acceptable level.

The WE 300B is also a center-tapped filament as above, but not as many wire runs to for each half. Still, an AC supply on these doesn't do well with hum level as above. Having a 5-volt vs a 2.5-volt supply also contributes to additional hum levels with AC.

Later 2A3 tubes have an equivalent structure to a pair of 45 triodes in parallel. including the filaments, so functionally it behaves likes a pair of 45's paralleled, but rarely do both sections (including filaments) match that well.

The 6A3/6B4G is a different beast... in most samples, it's like a dual triode 2A3 where the plates and grids are tied together internally, but the filaments are tied in series, so how does the logic of using either side as a cathode point work in this arrangement?

Beyond this, other transmitting tubes like the 3C24, 808 and others use a helical wire in a cylindrical grid/plate assembly and the ends (top/bottom) are the filament/cathode lines.

As there are different topologies used in different DHTs, and the above is not a complete list of all types, how do you determine the best approach for an effective filament supply, and effective cathode connection, against the type of filament structure? I have my own views/reasons on this but I'm not looking to hi-jack the thread.

Regards, KM
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Old 2nd October 2012, 06:08 AM   #52
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Default AC no hum

The problem with DIY and this thread, is that either amps were made from 1900 to 1925 and then from 1990 to now, and nothing exists in-between.

I have worked on, re-stored and modded about 500 or so tube amps, guitar amps and early vintage gear. I re-pair early rare guitar amps, for pro players that use them constantly.

Most of the tube guitar amps, have insane high gain, AC filaments, and no hum.

A ways back, I posted a pic of a old all triode Seeburg jukebox amp, that uses a 45 backwards to make -bias for the output 45s, pretty clever for 1935, and all AC filaments. There is a pot on the side of the chassis on these amps. This pot injects AC into the circuit, and you can control the phase, and null the hum. It has a touchy spot, but once set, I could not hear any hum, and these amps are very quiet when no signal.

Radio designers, back in the AM radio days, used AC filaments all over, built very large audio power amps for modulation, and mike preamps, mixers etc. and never had any hum problems.

Also, all the Hi-Fi gear in the 50s and 60s, most had AC filaments, although Fisher, Scott, Citation etc. had DC filament power for the phono preamp tubes.

Another common trick, used in two Scott 265 amps I restored last month, is to use the DC from the kathodes of the output tubes to drive the preamp tubes, two 12V tubes for 24 volts DC.

If you research out early tube gear circuits, you will find most has been done, and many triode amps ere built for big AM transmitters, using AC filaments, and I have never heard a AM station , and FCC rules back then would never have allowed any hum, distortion, and you had to have flat response from 35 to 15 Khz back then, for most gear made.
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Old 19th November 2012, 10:28 PM   #53
GordonW is offline GordonW  United States
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One other factor that has been ignored, in terms of effects of varying heater voltage, compared to specified voltage, is the amount of current being asked from the cathode of the tube, compared to its emission capacity at that heater voltage.

If a tube is only being asked to provide a small amount of current, then it is perfectly OK for the heater voltage to be lower than spec. As long as the total electron current from the cathode is less than the steady-state emission capability of the tube, at that heater voltage, cathode stripping will NOT occur. Tube life, accordingly, will not be affected in an adverse manner.

This is why amps such as Scott, Fisher and such, which routinely ran 12AX7s (nominally 12.6v heaters) at 11v, sometimes even lower, work as well as they do. If the tube that can deliver 1.25ma, is only being asked to deliver .3ma current... then there is NO PROBLEM. In many phono and input preamp sections, this is the case!

Don't believe me? Many of the experts of the past, including Morgan Jones, Norman Crowhurst and others, go into detail about just this relationship. Read up! Many of those books are available free, for online download...

Regards,
Gordon.
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Old 20th November 2012, 09:19 AM   #54
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Default 12AX7 and 6SL7 heater wiring

I'm trying to use two 12AX7's and a 6SL7 in a two channel preamp circuit and having heater hum problems. One 12AX7 is for CH#1. The 6SL7 is for CH#2 and the remaining 12AX7 is a line driver. The power trans has a 6.3 volt non-center taped winding, so Iím using a 200 Ohm hum balance pot. I have pin 9 of the 12AX7's going to pin 8 of the 6SL7 and pins 4&5 of the 12AX7's going to pin 7 of the 6SL7. Does the phase of the 6.3 volt AC need to be wired in reverse for the 6SL7 in my description above and will that make any difference? I've proven that the hum is comming from the heaters by making an octal to 9-ping socket converter and installing a 12AX7 in place of the 6SL7. What would be the proper way to wire the 12AX7 and 6SL7 heaters in this circuit?
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Old 20th November 2012, 09:42 AM   #55
SY is offline SY  United States
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If the hum is 60Hz, you probably have an issue with heater dress or grounding. Tightly twisted wires close to the chassis (run in the corners is ideal). Local bypassing of both sides to chassis with small ceramic caps. Ground reference at a "virtual" CT (take a 100 ohm resistor from each side of the 6.3V winding to ground or to a DC voltage which is well bypassed to ground at AC).
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Old 20th November 2012, 04:47 PM   #56
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The heater wire dress is good. I've built amps before, but not combining the 12AX7 center taped heater with a 6SL7 single heater. I'll try the small caps, but have determined that the 6SL7 heater is the issue. When the hum balance is set for the first channel, the second channel is humming and vice versa. Those older octal tubes have issues and that is why designers moved away from them to the 12AX7. (Microphonics and poor shielding) I'm open to more suggestions.
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Old 20th November 2012, 05:34 PM   #57
DF96 is offline DF96  England
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A humdinger assumes that hum only comes from one valve (usually the first in the circuit), or that the stray capacitive coupling is such that one setting does for two valve stages. Two identical parallel circuits might satisfy the second criterion, but two different circuits might not.
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Old 20th November 2012, 06:07 PM   #58
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I've already proven that the 60 Hz heater hum problem is in the second channel tube (6SL7) by replacing it with a 12AX7 on an octal socket. Iím wondering if anyone has had problems combining center tapped heaters (like the 12AX7) with single heater tubes (like the 6SL7) and if the heaters have to be wired in a specific phase. I will eventually try reversing the heater leads to the 6SL7 and see if the 6.3VAC phase makes a difference. I have another 120Hz power supply noise to deal with right now. This preamp is built like a Fender guitar amp with a brass grounding plate under the pots. There is a lot of amplification and a balanced line driver for +4dbm. Where the filter caps and other components are grounded to the chassis can make a huge difference in noise at very high gain levels.
Iíve built several high gain tube guitar amps that have zero hum, just hiss.
Ground loop and hum problems are the worst to diagnose.
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Old 11th December 2012, 10:49 AM   #59
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Default Shielded cable for heater wiring

Any thoughts on this cable to connect tube heaters the cores are stranded but they are twisted and more importantly foil wrapped with a drain wire Whitchurch can be grounded to keep the ac signal in
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Old 11th December 2012, 10:52 AM   #60
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Default Oops forgot the pic

It is screened instrumentation cable I use a lot of it at work
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