Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mrm0to on October 27, 2011, 01:24:44 pm
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This amp has a neat tremolo circuit - it's a separate power amp that drives the field coil on the speaker. I have a couple of questions:
1) I'm wondering about the 0.5uf cap that's between the screen grid on the tremolo pentode and the center tap of the output transformer (schematic linked below). Is it there to hold the DC on the screen? Any reason not to just connect it to DC ground?
2) The plate load resistor on the pentode - what's the point of this? The field coil is already at 1.5k, so 47 ohms isn't going to change anything we'd notice. When I got the amp the plate and screen were tied together. I went ahead and installed a 47 ohm screen resistor, figuring it's an error and left the plate connected to the field coil.
Yes, the tremolo works. It doesn't shut off, but I'm guessing that's a bad pot or ground on the intensity control.
EDIT: Nevermind, the tremolo doesn't work. As the amp warms up you can hear a warble through the speakers but once it's full-on that goes away.
EDIT 2: The Legend of Nat Daniel's Gold - Bad capacitor(s) in the oscillator circuit. It's all good now. Still curious about the .5 uF cap.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/silvertone1344.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/bargainbin/silvertone1344.pdf)
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The trem power tube is triode-wired... a BIG triode. Apparently they did not need gain, and did not need maximum power, and triode-wired is simpler.
When you connect a high-gain (even sorta-high) tube to "unknown" loads (the field-coil was never designed to be a tube load), stray L and C can lead to parasitic oscillations, maybe at radio frequencies. In those cases, a small series resistor may kill the oscillation with minor effect on normal operation.
The trem signal should be sine or it will sound glitchy. The output from the trem oscillator is not perfect, neither is the trem power tube. As the last step, a heavy cap from plate to ground kills the treble in the trem signal. 0.5uFd against roughly 1K node impedance (fieldcoil||triode) slants-down everything above 333Hz. One could argue for 5uFd for 33Hz cutoff; but that costs more especially since the large circulating current would be bad for vintage electrolytics. Probably they had a high "tik" and 0.5uFd damped it well-enough.
Take it off and see. Odds are you'll like it better with the cap.... SilverTone didn't put excess parts in.
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I tried it w/ the cap off and it didn't seem to even make a difference <shrug>. Right now it's back in.
Wired as a pentode the trem has a good range of operation - from off to a cutting almost completely out on its pulse. No clicks. I'll try it as a triode - if nothing else, I don't want to push too much into the field coil and ruin it.
Thanks.