Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dave on June 08, 2020, 05:49:14 pm
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Hi guys,
So I've built my version of the last amp i'll ever need... but I'm being obsessive. So, its time to get some outside thoughts.
Its a 5F6-A (bright side) and a 6G6-B (bass channel) for a two channel amp.
The documented 6G6-B power amp and the power amp from 5F6-A (that you can actually see examples of) only differ
by the value of the cathode resistor of the PI. And that is only different by a couple of hundred ohms. So, I went with 6G6-B. So, this way I get a tweed Bassman that is almost stock and a 6G6-B that is perfectly stock.
My goal was, to tweak for minimal effect on the tone.
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Any chance you have a schematic of the entire amp in it's current wiring that you'd be willing to share? It would sure help in understanding what you're trying to convey?
with respect, Tubenit
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Hey Tubenit,
I sure wish I did, but I never made one. It really is, however, several existing circuits squashed together. I'm not that great with computers, but in a few days, I will see if I can make one.
The good part is, its only the oscillator that is not working right, so hopefully someone will have run up on something similar.
Dave
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I can't believe you don't have a schematic for this complicated project!
My Revibe uses the same vibrato circuit you are using. I made several mods to the oscillator. Take a look at my schematic. It has dc voltages, osc. signal voltages, and several notes that may/may not be useful to you. There is more than enough osc. signal to drive the stock modulator. Maybe some of the mods you made to the modulator require a much larger osc signal.
Could be useful to return the whole vibrator circuit to stock, get it working properly, then try some of your wild ideas.
Here's my schematic... http://sluckeyamps.com/revibe/revibe.pdf
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I can't believe you don't have a schematic for this complicated project!
Yeah, well. I got inspired and just went nuts. What can I say?
Thanks for the link to your schematic. I will definitely take a look and maybe even rip it out and start over as you suggest. Truth be told, I actually have it buttoned up right now because I called it done. I mean everything works beautifully including the trem/vib at moderate settings. But... the more I play it, the more irritated I get that it flakes out. It would be so nice if everything worked perfectly.
Dave
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You’ve seen how Vox switch the vib to trem? https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf
Not sure when in the trem mode, if the mute cap c22 decouples the audio signal and the modulation, or just the audio?
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You’ve seen how Vox switch the vib to trem? https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf (https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf)
Spent a little time studying this schematic and came to the conclusion that it is very similar to the Fender scheme, but... the LFO high pass filter looks like it may be just what the doctor ordered. Apparently from the looks of the Vox Scheme, they were having the same problem I am and were force to integrate a big honking filter at the end of the trem/vib circuit to keep the helicopter noises at bay. I am going to try to stuff something similar into my amp and see if that doesn't help me.
Dave
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So, PDF64...
You scoundrel, you got me thinking about ripping out the Fender circuit and replacing it with the Vox one to see if it would work better. Well, thankfully I didn't. I did however incorporate a very similar filter at the end of the Vibrato circuit to the one used on the Vox and... Voila. Sonic paradise! Problem completely solved.
Between looking at the Vox filter and thoroughly ready this, https://ozvalveamps.org/lfofilter.htm (https://ozvalveamps.org/lfofilter.htm), I incorporated the high pass filter as Roly lined out. The horrible noise went away immediately and the effect is so much more pronounced.
I apologize for not providing a schematic when I originally posted the question. Truth be told, I still haven't made one, however, this forum did what it always does. It got me on the right track to solving my problem. Thanks to everyone who chimed in and special thanks to PDF64 for pointing out that Vox circuit.
By the way, although I still haven't made a schematic, if anyone wants to build something like this, I'll be happy to either bite the bullet and make one, or at least provide whatever information you need to get your amp going.
Dave