• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Heater Wiring - the Good the Bad and the Ugly

Those kit amps from them and Knight I don't consider commercial products. Looking at the Dynaco original schematic, that is crazy.

The Dynaco Stereo 70 was a huge commercial success, and was better and cheaper than its competition.
Dynaco's products were available as either kit or wired, as the customer preferred.

The Stereo 70 was also ranked #2 on "The Twelve Most Significant Amplifiers of All Time" list by the Absolute Sound.
https://www.theabsolutesound.com/articles/the-12-most-significant-power-amplifiers-of-all-time/
 
There is an emerging use of output transformer secondaries as primary-side cathode-coupled windings - to enhance local feedback in the output stage. That can work quite well, but requires the relevant secondary windings to be well balanced at HF.

I've been building amps like that for a long time, and it works well as long as there is enough drive available.
 
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And, in practical amplifiers, it's only a coupla dB anyway. Worth doing if possible, but most output valves have low enough gain x OPT step-down (divided in half) to make it a final touch rather than a critical design element. EL84s benefit well (triode mu about 18 or 19, OPT ratio maybe 24 or 25 divided by 2).

All good fortune,
Chris
 
There is an emerging use of output transformer secondaries as primary-side cathode-coupled windings - to enhance local feedback in the output stage. That can work quite well, but requires the relevant secondary windings to be well balanced at HF.
People been doing that for years. One popular one is the Pulse Pultec EQ that Klark Teknik made a really bad clone of a few years back. Not because of this transformer, but because the way they altered the circuit design to make it cheaper.
 
My OP kinda got highjacked… maybe I can still use it:


Between trips to central TN to build a house (I’ve spent 3 out of the last four months there doing landscaping, electrical work, tongue and groove interior paneling, tiling, framing my future audio/man cave and more, oh, so much more…)

Came home a few days ago, fired up the Tektron 300b (it’s a TK2/50S 2A3 model switchable for 2A3, 211 (?) and 300b tubes) and that hum finally got me motivated to throw the amp on my “bench” (coffee table) while listening to Kenny Dorham, Shelly Manne, Miles through a Willsenton R8 tube amp with all the Skunkworks modifications I could justify and…

I couldn’t figure out months ago what was causing the hum. worse in left than right side tried different tubes (original Sophia 2.5V, EMLs, NEW Electro harmonix gold grid something or others), all hum. No hum when I listened to the amp at the guy’s house. I forget the speakers he used, but they were four figures more than I’ll ever afford.

The only schematic I have from Tektron is a general design from that era, some things are different, but the only ripple I could find measures in the microvolt range, like 0.8 mVAC. unhooked several capacitor leads (yes, I used a chicken stick to discharge them down from 450 volts first) and measured ESR’s in the sub-ohmic range on the 300b bypass caps. I poked around a bit, rerouted some wires, resoldered a couple of connections, fired it up and HMMMMMMMMM

Still.

In a moment’s frustration, I twiddled the voltage dial down to 2.5 V.

NO HUM.

Now THAT is interesting.

The amp is designed for 115 VAC. At 120 VAC wall voltage I get something like 5.9VAC filament voltage at the 300b tubes, so I run a variac between wall and power filter/conditioner to cool things down. Actual voltage from the 2A3 setting now is really closer to 3 VAC, and NO HUM.

Sure, I CAN run the 300b tubes on that voltage, but can someone throw me a bone here, advise on what most likely culprit may be and where to apply my - limited - time?

I have ALL the components in one large box ready to begin building the Skunkie Designs 300b amp once I’ve cobbled together the chassis - maple sides, steel top - which I’m also working on.

Thanks in advance.
 
The amp is designed for 115 VAC. At 120 VAC wall voltage I get something like 5.9VAC filament voltage at the 300b tubes, so I run a variac between wall and power filter/conditioner to cool things down. Actual voltage from the 2A3 setting now is really closer to 3 VAC, and NO HUM.
nice.
What I do in some instances is put an AC capacitor in series in the primary after the fuse to drop a little voltage. But it has to be sized correctly. I do this to old 110V radios back a long time ago, never had issues doing it either.

I remember someone using an R-core type and asked me how to fix the hot operation. This is the same solution too.
 
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I tried a tip from another board expert who suggested adding 15 ohm resistors to bypass the wiper on the hum pots, so I did the left side. No change.
Trimmer is 102 ohms before doing this. He says a proper pot should be 50 ohms (25 ohms per side), but I don’t see any plans anywhere doing this.

All that aside, I was reading about power supply design (here: https://www.analogethos.com/post/power-supply) and he mentions 120 hz ripple from the earlier stages of the PS.

The 300b DHT filaments are fed from independent AC windings on the transformer, so it makes sense the 120 Hz hum should be coming from the driver section - but those are AC filaments too - or earlier, out of the power supply.

I have replacements for those and figure after 20 years (the amp is at least this old) maybe recapping the PS is in order.

I also have a chicken stick to drain the old ones; if the bleeder resistor is cooked or just doesn’t work.
 

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Hello.
I am new to tube testing.
I noticed when testing triodes that when setting 6.3 volt heater can vary some, I decided to vary current instead, since it provides the energy to tube, some require 6.5 volt etc to draw both heaters in parallel to draw 300 ma as stated in datasheet.
My question is , do I regulate bu voltage or current?
 
Okay I did some test.
This setup is telefunken ecc82, set up according to datasheet.
I started with adjusting to current, which yielded lover volt.
First photo 300ma
Second the actual quick test, I gave it 2 minutes warm up time.
Third picture, setting to 6.3 volt, with increased current.
Fourth. the test again , showing marginally better specs.
Ps. The rigol is tested and very close to spot on.
 

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I tried a tip from another board expert who suggested adding 15 ohm resistors to bypass the wiper on the hum pots, so I did the left side. No change.
Trimmer is 102 ohms before doing this. He says a proper pot should be 50 ohms (25 ohms per side), but I don’t see any plans anywhere doing this.

All that aside, I was reading about power supply design (here: https://www.analogethos.com/post/power-supply) and he mentions 120 hz ripple from the earlier stages of the PS.

The 300b DHT filaments are fed from independent AC windings on the transformer, so it makes sense the 120 Hz hum should be coming from the driver section - but those are AC filaments too - or earlier, out of the power supply.

I have replacements for those and figure after 20 years (the amp is at least this old) maybe recapping the PS is in order.

I also have a chicken stick to drain the old ones; if the bleeder resistor is cooked or just doesn’t work.

There IS no “bleeder resistor” in that circuit.

Replacing the PS caps didn’t take the 120 Hz hum off the amp output.

How the heck is it getting onto the 300b PTs anyway? Those things get AC from the trafo, not the PS. Even the 6SN7 heaters are AC off the trafo.

Thing is, I like the idea of having 2A3/300b/50 capability in a SET (if I can ever find a reasonably priced pair of 50’s), but that HUM.

Have people been putting up with 120 Hz hum out of set amps for 100 years now? Paying thousands (the Tektrons run upwards of $7k now, new) for these amps?

Puzzling.

And caffeine-deficient as I am, I don’t understand the point @amplidude is making regarding 300ma vs 6.3v.
 
Ah. Got it. I think.

So why would one to expect filaments in two different tubes to be spot-on resistance-wise? Vagaries of materials science, “doping” (alloying), coatings, cut-length would all affect impedance. running them in parallel would “buffer” that, averaging out the effect… but there’s some difference channel to channel then in how hot the filament actually gets and, subsequently, now hot the cathode, current flow, etc.

Interesting.

Matching tubes to those specs for effect (only those with the most vaunted 24 karat golden ears could even notice, using such a skill to sneer at the benighted plebes over on Audiogon, for example) would be a real bear.
 
But…

for the figures that really seem to matter, Gm and Mu, isn’t the effect of heater resistance irrelevant as it’s a component of the aforementioned?

It’s somewhat akin to saying a measurement taken at Niagara falls’ output is noticeable affected by damming up a leaky garden hose on a property 50 miles away.
 
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