Oh, I just noticed you're back Don. Great, I was worried about you!
I was looking for a post you made that IIRC was about an unusually big power pentode in a compactron. What I'm actually looking for is a Fender-Champ-in-1-bottle, a power pentode in the same tube as a small signal pentode. There are a few I know, but IIRC you had found one with a 17W plate.
I was looking for a post you made that IIRC was about an unusually big power pentode in a compactron. What I'm actually looking for is a Fender-Champ-in-1-bottle, a power pentode in the same tube as a small signal pentode. There are a few I know, but IIRC you had found one with a 17W plate.
Same trick often used on guitar amps to reduce the risk of smoke and fire when someone cranks it full on without a load.
Diode from plate to ground, plate to B+ or plate to cathode.
The damper tube diode produces less sonic trash than silicon does when the amp is driven hard into clipping with a mismatched speaker load.
Note that many guitar speakers have their resonant impedance peak within the guitar's frequency range, so it's normal for a guitar amp to see 30+ ohms on it's 8 ohm tap.
I find that a big fat resistor in the 500 ohm to 2 or 3 K ohm range in series with the silicon intrudes even less, and offers about the same degree of smoke protection.
Ditto a TVVS diode and resistor combo across the speaker terminals.
Diode from plate to ground, plate to B+ or plate to cathode.
The damper tube diode produces less sonic trash than silicon does when the amp is driven hard into clipping with a mismatched speaker load.
Note that many guitar speakers have their resonant impedance peak within the guitar's frequency range, so it's normal for a guitar amp to see 30+ ohms on it's 8 ohm tap.
I find that a big fat resistor in the 500 ohm to 2 or 3 K ohm range in series with the silicon intrudes even less, and offers about the same degree of smoke protection.
Ditto a TVVS diode and resistor combo across the speaker terminals.
re: leadbelly
38HE8 has got a pentode in it that should be up-rate-able to 15 Watts if the damper diode is powered down. Most of the 38___ variety have the pentode heater alone on pins 10 and 12, at 21V at 0.45 mA. (12HE7 don't seem to have that option) The pentode when doubled up (paralleled) gives identical plate curves as the old style 42KN6, which had two pentodes paralleled inside (each 15 Watt, 30 Watt total).
The 38HE7 pentode is fairly similar to the 21HB5 too (which is 18 Watt and used to be $1), the 38/12HE7 GE datasheet is online at Franks tubedata.
Some other tubes on the $1 list with 12 Watt pentodes are the 6MF8 and 6KY8.
38HE8 has got a pentode in it that should be up-rate-able to 15 Watts if the damper diode is powered down. Most of the 38___ variety have the pentode heater alone on pins 10 and 12, at 21V at 0.45 mA. (12HE7 don't seem to have that option) The pentode when doubled up (paralleled) gives identical plate curves as the old style 42KN6, which had two pentodes paralleled inside (each 15 Watt, 30 Watt total).
The 38HE7 pentode is fairly similar to the 21HB5 too (which is 18 Watt and used to be $1), the 38/12HE7 GE datasheet is online at Franks tubedata.
Some other tubes on the $1 list with 12 Watt pentodes are the 6MF8 and 6KY8.
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If it has to be two pentodes, there is the 6GE8, but it's only 7 Watt and 1 Watt.
6BW11 gets you 4 W and 3 W pentodes.
The old standby 6LU8 and 6LR8 get you a 14 Watt P with a 2.5 W triode.
6MY8 16 W with 2.5 W triode (uprated 6LU8)
15MX8 14W with 1.5 W triode (uprated 6KY8)
832A 15W P + 15W P
6BW11 gets you 4 W and 3 W pentodes.
The old standby 6LU8 and 6LR8 get you a 14 Watt P with a 2.5 W triode.
6MY8 16 W with 2.5 W triode (uprated 6LU8)
15MX8 14W with 1.5 W triode (uprated 6KY8)
832A 15W P + 15W P
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George already explained. Damper diodes prevent high voltage spikes on the primary when the amp is clipping without load. A foolproof.
For those who understand Dutch:
Nederlandse Vereniging voor de Historie van de Radio
Lecture by Bernard Terlingen, now 80 years old, on how he used to collect television valves from waste bins, built high-power audio amplifiers with them and used those to play the Hallelujah from Handel's Messiah at the start of the new year at a high enough volume to overcome the sound of the fireworks. The turntable had to be placed at the other side of the house to prevent oscillations due to acoustic feedback.
Nederlandse Vereniging voor de Historie van de Radio
Lecture by Bernard Terlingen, now 80 years old, on how he used to collect television valves from waste bins, built high-power audio amplifiers with them and used those to play the Hallelujah from Handel's Messiah at the start of the new year at a high enough volume to overcome the sound of the fireworks. The turntable had to be placed at the other side of the house to prevent oscillations due to acoustic feedback.
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I found it a major frustration of this thread and the associated ones More Ruminations on Screen Drive/Crazy Drive and Show me your screen drive circuits that I could only ever find a single schematic of an actual amp (here and here).
Neither incarnation of that amp performed well at all in my simulations.
Does the lack of actual implementations suggest that crazy drive is a dead-end?
A year ago I put together a breadboard with tubelab's UDB driving a pair of 6HJ5s to do extensive testing to see if the whole crazy drive idea is viable or not. Unfortunately, projects for other people have prevented my actually doing any of that testing.