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12HL7-triode SE Headphone Amp - I finally built something!
12HL7-triode SE Headphone Amp - I finally built something!
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Old 12th July 2018, 03:12 AM   #51
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
12HL7-triode SE Headphone Amp - I finally built something!
In case anyone is following this build...

I picked through my collection of 12HL7s and found two of identical construction that look like they're made by GE, but one is labeled GE, the other RCA. They have their getters on the top of the bottle, 'powder coated' grey plates and square 'windows' with shiny metal structural plates visible inside. I have these two in the amp and it sounds well balanced between left and right, with matching subjective sound quality. (I like the sound of this type of 12HL7 in this circuit.)

The Raytheon 12HL7 I was using before has a smooth plate of a shiny, dark brown-to-black metal. I found a Philips ECG with matching construction. I tried the two in the amp and they match in 'tone quality' but are a little mismatched in level (channel balance is off). I find the tone quality is somehow 'softer' and less 'present' than that of the powder-coat grey plate GE type. This type doesn't sound 'right' to me. I don't know what to make of that. Perhaps this type has a tendency to draw grid current at grid bias of -2V?

There are two other types of RCA 12HL7s which I haven't tried in the amp yet. One has a 'clear top' bottle with its getter on one side and a black 'carbon' plate material. The other has the getter on the top of the bottle, also has a black carbon plate, but has a large shiny metal plate on one entire side of the tube's construction. It's a pentode construction type I haven't seen before.

I'll try to listen to the clear-top RCAs and report on what those sound like in this amp.

It's interesting that different examples of this tube type 'sound' so different from each other in this circuit. I guess their characteristics vary widely for a tube.
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Last edited by rongon; 12th July 2018 at 03:15 AM.
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Old 13th July 2018, 04:42 AM   #52
stocktrader200 is offline stocktrader200  Canada
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you probably need another tube gain stage to balance the tube distortion.
I found a tube gain stage driving the volume pot sounds better then solid state driving it alone.
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Old 13th July 2018, 07:16 PM   #53
rongon is offline rongon  United States
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12HL7-triode SE Headphone Amp - I finally built something!
At this point I'm not so worried about the sound quality as I am about channel balance. It's pretty distracting if there's an imbalance left-to-right and you're listening in headphones.
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