Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sonoflee on February 05, 2015, 11:01:03 am
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Hello,
I recently got my hands on an old Roberts reel-to-reel with dead tape mechanics, but a working single-ended tube amp (6AU6, 6AU6, 6X4, 6BQ5, 6AR5). I found a Lectrolab S400 schematic online that used almost the same complement (6AU6, 6AV6, 6CA4, 6BQ5), and decided to try my hand at creating a layout for it. I'm pretty comfortable with reading schematics & layouts when working on amps, but don't have any experience turning a schematic into a layout (if that makes sense), so I hoped I might get some feedback on the layout from some knowledgeable folks. Any major mistakes/problems? Suggestions on improving the circuit?
I'm posting the original schematic, parts list, & my layout below. Please let me know if you have any questions, and thanks in advance for your assistance!
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Note that the diagram shows a triode + double diode tube, likely a 6AV6, in V2 position rather than a 6AU6, which is a pentode. (I see you noted that correctly) It is the tremolo oscillator. It shakes the input grid of V1. I can say from personal experience, when you're cranked into distortion, the trem will not be so much loud-soft as it is distorted-very distorted. I much prefer the trem that shakes the output tube grid.
It is an interesting schematic, I'm curious how it will sound.
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Note that the diagram shows a triode + double diode tube, likely a 6AV6, in V2 position rather than a 6AU6, which is a pentode. (I see you noted that correctly) It is the tremolo oscillator. It shakes the input grid of V1. I can say from personal experience, when you're cranked into distortion, the trem will not be so much loud-soft as it is distorted-very distorted. I much prefer the trem that shakes the output tube grid.
It is an interesting schematic, I'm curious how it will sound.
Thanks for replying, and I think see what you mean about the tremolo. Am I correct in assuming that the Silvertone 1482 trem is of output grid-shaking variety (see attached schem)? I really dig that one, and I'm wondering if I could adapt it to this single-ended amp... opinions/advice/ideas would be welcome. Thanks!
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That is correct, the 1482 trem shakes the output grids, and is the one I prefer. Note it uses the 6AU6 pentode, however I have made that trem work with a 6AV6 also. I can't see why it wouldnt work with a single ended amp. Some say this "bias vsary" trem doesn't work as well with cathode biased outputs, but I've never had a problem with it. My 6av6 oscillator shakes so hard that I can't turn depth/intensity up very much. I have a diode on oscillator cathode and that really holds the gain up on the oscillator.
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Thanks again! The 6AU6 is actually easier, that's what was in the amp to begin with!
I'm attaching a frankensteined schematic, with the 1482 tremolo just pasted in place in the previous circuit - is it really as easy as this?
Just trying to get an idea on whether it will work as is, or if I need an additional ground at the power tube grid, different pot values in the trem section or anything?
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On your playe voltage supply, the C12 you have as placed would prevent any DC from getting to the plate, so best to get that power from the filtered screen node like the silvertone does. On the output, there needs to be a resistor between the shaker and grid of power tube, likely 220K-470k You may end up with a voltage divider there with another resistor that goes to ground. Reasoning is that the grids of various tubes are harder to shake, and if yours is easy to shake, the output from the trem would be too much. try to understand the way the shaker signal is tied to the grids of the Push pull types, and get closer to duplicating that setup.
Unlikely you would need to change values of the pots, but possibly a lower value on the intensity pot. You can learn this experimentally once operational. Try it, and if the trem is too deep, try clipping a resistor across the pot to learn which value provides the most useful range of intensity.