Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fiat_cc on August 20, 2019, 06:11:00 pm
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Howdy y’all.
I have been tinkering with variations of the 18 watt tremolo preamp for a while, and whilst I love the sound of the voicing, and I love the effect of the tremolo, I am a bit tired of the finicky nature of this tremolo circuit. So, my solution is to simply insert the Fender VibroChamp tremolo circuit. Same preamp bias wiggle effect, but with improved reliability. Win win right (someone will probably tell me I’m wrong ...)?
My question here is (see attached image for reference), in the original, the plate load resistor for the tremolo driver and the first preamp gain stage is effectively split, with the tremolo taking its supply from after 100K load, whilst the gain stage has a further 220K. If I wanted to replicate this gain stage, but without the tremolo driver, am I right in thinking that a 330K would get me right in the ballpark, or is my thinking here flawed, and if so, why?
Cheers.
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Thinking more about this, both triodes draw current through the first 100K, so would a more correct value actually be around the 420K mark (is 430K the closest common value?)?
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Pin 6 will see; A – Vdrop of R11, so measure the VDC at the junction then match
Your bigger problem is pin 3 & 8 where V3a shares the 820 + int value for it’s bias point
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Pin 6 will see; A – Vdrop of R11, so measure the VDC at the junction then match
Your bigger problem is pin 3 & 8 where V3a shares the 820 + int value for it’s bias point
So you’re saying the plate load resistor remains at 220K, but with the B+ dropped by a whole lot? s far as the trios is concerned, wouldn’t it just be pulling current across both resistors though? It doesn’t ‘see’ the fact there are two separate resistors there.
As far as cathode resistors go, I was going to start with a 1.5K there and see if that got me in the ball park (I could take measurements and draw a load line, but I’m a better practical learner usually). 1.8K or 1.2K May be a better option too.
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I'm still missing "your vision" are you separating the 2 into individuals?
anyway, simply measure the VDC at the plate, start with the 220k and adjust till your new R matches your current conditions
as far as Rk goes, IF you're making the tubes 2 separate things you need to keep the VDC at the cathode ~ where it is now, again measure, so "voicing" stays close, or not
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I'm still missing "your vision" are you separating the 2 into individuals?
anyway, simply measure the VDC at the plate, start with the 220k and adjust till your new R matches your current conditions
as far as Rk goes, IF you're making the tubes 2 separate things you need to keep the VDC at the cathode ~ where it is now, again measure, so "voicing" stays close, or not
Yes, so I will be using the VibroChamp tremolo, which uses two triodes. Currently my design uses one tube reverb, the original tremolo channel gain stage and tremolo, and a phase inverter. The new design will use Fender style reverb, a whole tube for tremolo, the original gain stage (as close as I can) with the second triode being the reverb recovery, and the phase inverter.
I am basically trying to separate the gain stage from the tremolo to make it stand alone, as the second triode in that envelope will be used for an entirely different purpose.
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so I guessed correct;
I love the sound of the voicing
to keep that you need to replicate the conditions which currently are being shared. You want to know what they are now while their conjoined, so vdc is a good 1st step. I would even "play around" with the int pot Ignoring the trem :icon_biggrin:
the reason I believe the bias point for V3a changes with the intensity pot, you could "cheat" by making your new 1st gain stage have Rk be 1.5k fixed and 470 variable. NONE of this could amount to anything....but :dontknow:
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All of that gave me a headache...Build the 18 watt lite IIB. Buy a couple of pedals. Be happy.
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so I guessed correct;
I love the sound of the voicing
to keep that you need to replicate the conditions which currently are being shared. You want to know what they are now while their conjoined, so vdc is a good 1st step. I would even "play around" with the int pot Ignoring the trem :icon_biggrin:
the reason I believe the bias point for V3a changes with the intensity pot, you could "cheat" by making your new 1st gain stage have Rk be 1.5k fixed and 470 variable. NONE of this could amount to anything....but :dontknow:
The way the VibroChamp trem works, the intensity pot also effects bias. It’s really very similar, only the VibroChamp version has a cathode follower as a bugger after the oscillator, and starts more reliably.
I think I’ve got enough to go on now though.
All of that gave me a headache...Build the 18 watt lite IIB. Buy a couple of pedals. Be happy.
What I’m building isn’t really an 18 watt anymore. It uses the basis of the tremolo channel preamp, but with a Super Reverb power and output transformer set, a choke, and switchable cathode or fixed bias.
I just want to solve the unreliable tremolo issues.