Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on March 29, 2012, 03:59:54 pm
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Any suggestions to make this type of trem work " well " in a Tweed Champ ?
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I don't see why you can't diddle the cathode voltage of the second triode in the 5F1 just like the Bronco (or a Vibro Champ). Connecting your diddle voltage directly to the negative feedback might be "bad" in some way, but I don't know why it would be any different.
It seems that the "EC Vibro Champ" uses power tube bias vary tremolo. Now that might be interesting! Instead of grounding the 220K grid return resistor for the 6V6, connect it to the Intensity pot of the Bronco/VibroChamp circuit. However, I doubt the voltage swing from the Blackface tremolo circuits would be appropriate for diddling the 6V6 grid. Some combination of changing the value of the Intensity pot and modifying the bias of the jiggling tremolo triode (left in the schematic) by changing the 470K plate resistor and/or the 4.7K cathode resistor. The Blackface Princeton Reverb tremolo circuit might be a better starting point, except wire the Intensity pot with one end to ground (like Bronco) and the other end to the 220K grid return resistor.
Note: that last paragraph may be nonsense. Hopefully someone else can chime in to help.
Cheers,
Chip
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Notice how the 5F1 Champ has the 22k feedback resistor connect to the junction of the 1.5k resistor and cathode, while the Bronco/VibroChamp uses a 2.7k resistor running to the bottom of the 1.5k resistor, on top of a 47 ohm resistor?
The later amp's feedback loop is accomplishing the same thing, but scales the parts values from 22k/1.5k to 2.7k/47 ohm. The 1.5k cathode resistor no longer does double-duty.
So you can add the VibroChamp trem to the 5F1. Swap the series feedback resistor value from 22k to 2.7k, and add your new 47 ohm resistor. Copy the wiring of that tube section's cathode to ground (and feedback loop) from the VibroChamp. Add the VibroChamp trem ocillator/buffer circuit. You'll likely need to partially-copy the layout of the the VibroChamp eyelet board.
You don't need to copy the later amp's tone controls to make the trem work as-intended. Everything else, is essentially-same between the two amps. The added trem will work fine. If this is an already-built 5F1, you may be very tight on space and/or need a new eyelet board or other creative way to mount the parts.
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Thanks HBP..that's what I needed.
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Okay.. back to this. Transplanting the Vibro-Champ trem circuit doesn't sound good at all in the 5F1. The negative feedback resistor values need to stay the same in the 5F1 for the best tone. Back to what Fresh Start was suggesting , how can I add an output tube bias-vari trem to this single-ended amp ?
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1. IMHO the bronco/vibrochamp circuit has to be able to function in the 5F1. Maybe post an accurate schematic of what exactly you actually tried. And state specifically what issues you encountered. (E.g.: the 5F1 has no tonestack and may be feeding a stronger voltage signal to the 2nd preamp stage; perhaps a stronger trem voltage is required???)
2. Still, there's nothing wrong with trying power tube trem. For power tube trem in a cathode biased power amp, checkout the schematic for the Silvertone 1484 for general ideas. It uses a different trem tube, but trem voltage is the key. Measure the (+)voltage on your power tube cathode. This is the equivalent of (-)bias voltage on the G1 signal grid for a fixed bias tube. Probably the trem voltage should swing to (+/-) 1/2 of that: e.g.: for +15V on the cathode; full trem intensity would be, say -7V - 0- +7V, as a starting point. Clearly this deprives the power tube of bias on the (+)voltage trem cycle, but only briefly. So watch for redplating over time, say 2 hours. Also search the Forum and the web for trem > cathode biased power tubes.
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Okay.. back to this. Transplanting the Vibro-Champ trem circuit doesn't sound good at all in the 5F1. The negative feedback resistor values need to stay the same in the 5F1 for the best tone.
So you added the trem and it didn't sound good? What dies it sound like? You did
Assuming a 4w output across a 4 ohm speaker, there will be 4v RMS present at the speaker terminals. The feedback network in the 5F1 consists of the 22k series resistor and 1.5k shunt resistor, which define a voltage divider. These apply a reduced speaker voltage sample to the 2nd stage cathode. That reduction is 0.064:1 of the total speaker voltage, or 0.256v RMS.
The Bronco/VibroChamp feedback resistors are 2.7k and 47 ohms. These define a reduction of 0.017:1, yielding a feedback of 0.068v RMS. If anything, the new values have less feedback than the original values.
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Right ... it had less feedback. The tone became bright, louder, abrasive,with harsh overdrive, ( to MY taste ). I'd like to add trem , only if I can keep the same negative feedback values. The trem worked, ( that's not my complaint ). I had to move V2 plate connection at the power rail to the B+ . ( before the 22K to get enough voltage ) and remove the V1B 10@25Vcathode resistor to reduce the huge gain boost that the 5F1 doesn"t need.
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Just a suggestion... Use Doug's bias vary trem from his AB763 amp design. The trem circuit is very much like the Vibro Champ circuit, but should work well with your SE 6V6. Take a look...
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Per Hotblue's prior post, you should be able to reduce the size of the NFB series resistor to get more NFB. Temporary use of a pot might be helpful. Or: i) conjunctive filter after the power tube; ii) local NFB at the power tube -- note the additional 330pF cap in the Bronco AB764 schematic.
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Thanks everyone... I tried Dougs' trem today... hardly any depth, even with the mods suggested.
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Something's wrong, sluckeys helped everybody and their brother fix or modify their trem/vib.
Where did you take the B+ for the trem tube from?
Most Fender bias mod trems I seem to remember take it from the screen, 2'nd highest B+ in the amp.
Brad :think1:
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If 2.7k and 47 ohm doesn't get it for you, try a 820 ohm in place of the 2.7k. That would result in a level of feedback similar to what you had originally.
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Steve - you could use that same approach for a push-pull cathode biased amp too, right?
BTW someone here had a nice looking tremolo circuit that used a MOSFET in place of the cathode follower/buffer triode. Just in case someone only has 1 triode for the tremolo.
Cheers,
Chip
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BTW someone here had a nice looking tremolo circuit that used a MOSFET in place of the cathode follower/buffer triode.
IIRC, that was tubeswell and he got it from R.G. Keens mosfet follies?
Brad :icon_biggrin:
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Steve - you could use that same approach for a push-pull cathode biased amp too, right?
This is the same question I have... I am in the middle of trying to engineer a build for my local tube amp club(fender-style preamp, Valco-style PI, p-p cathode bias 6v6, no nfb), and I tried dropping a Bronco/Vibrochamp circuit right in, and it sounds good most of the time, but at higher levels of volume on the amp it goes from tremolo to motorboating. Sounds sweet at lower volumes though...
I also had no luck finding a source for 25KRA pots, so I sub'd in a 50kRA, hoping it was suitable. Anyone know if it is or isn't?