Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on January 29, 2023, 04:02:56 pm
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I have been studying the Yorkville YBA-3A amp schematic (with great interest!) and wanted to ask if any of the folks here might could explain some things to me?
1. I am very curious about the "treble expander" and "bass expander" circuits that are built around V2 (7025) triode sections. Am I correct in saying these are adjustable negative feedback circuits being used to manipulate frequency response?
2. I am unfamilar with using such a low plate load resistor (22K ohm) and low cathode resistors (100 ohm and 150 ohm) as being used in V2 triode sections. Evidently the negative feedback is reducing current through the triodes?
3. Why does the first stage triode of V2 (for treble expander circuit) have a large cathode bypass capacitor whereas the second stage of V2 (bass expander) has no cathode bypass capacitor. Is this due to the treble expander stage having to pass through bass frequencies in addition to boosting the treble?
I am very curious to learn the particulars of this very interesting circuitry and also to hear opinions of the sound from users of this amplifier. Attached is schematic. Thanks to all.
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Hi, late post, but it‘s strange.
I searched several schematics of this amp and all the same values, which I cannot believe.
A 12AX7 as V2 consums with 22K anode-resistor about 11mA - too much.
May be a 12BH7, ECC81 or a ECC88 would be fine, may be the right value would be 220k.
But all schematics show these values, strange.
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Voltage across R10 is 300V-245V or 55V. 55V across 22k is 2.5mA. (I don't see where you see 11mA?)
This IS an extreme condition for 12AX7. But it should "work" as drawn. Gain about 40, peak output about 40V, distortion at that level well below 5%.
If it was 220k then no need for 1W part. (This does not need 1W unless the tube has an internal short, which is possible.)
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I wouldn't put too much faith in Traynor's schematics. I've been inside a few, and none have matched up to their posted drawings, although everything seemed original to factory. More of a guideline than a rule type thing.