Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on June 06, 2014, 07:17:05 pm
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Looking for help on where to inject tremolo into the preamp. I’m thinking at V3b pin 6 or should I split the resistor into 2 resistors n put it at that new junction ? Or…..I did find, then lost, a schematic where it was injected at the grid, the resistor to ground was split in two and injected there, maybe V2b n make the 1Meg 2 470Ks? Anyway, comments or ideas?
Thanks again
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Find an amp that injects trem in the preamp. Copy what it does.
If you're adding trem to the channel with the 6SN7 gain stages, it is more likely to be added at V3a's cathode. But that will probably lead to some degree of pulsing background noise when the trem is engaged but you're not playing.
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I have 3 schematics, supro, Gibson n Rickenbacker, they each do it a little different, I worked on the Gibson n it did have some bleed-thru, the owner said "that was normal" n that was injected at the PI inter-stage tranny, no clue on the other two. Is bleed-thru "Normal" for cathode injected trem?
thanks for the help
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Is bleed-thru "Normal" for cathode injected trem?
What are you calling bleed-thru?
There are lots of cathode injected trem amps. Especially in low end amps. Look at the Marshall 18W or the Fender Vibrochamp.
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I have 3 schematics, supro, Gibson n Rickenbacker, they each do it a little different, I worked on the Gibson n it did have some bleed-thru, the owner said "that was normal" n that was injected at the PI inter-stage tranny, no clue on the other two. Is bleed-thru "Normal" for cathode injected trem?
Fender had a number of tremolo circuits, mainly because there are problems with every method of creating the effect. The right choice is mostly about balancing the negatives against what you gain.
A lot of older amps (and the cheaper models of newer amps) applied the trem signal to a preamp tube cathode. 2 "problems" with this approach: in the best cases, there tends to be a pulsing hiss when you're not playing (often this is a minor complaint); additionally, the trem oscillator (and buffer if present) needs to output a current which is comparable to idle current of the tube where trem is to be injected. That's because the trem oscillator or buffer will share a cathode resistor with the signal-carrying stage, and the trem circuit needs to pull a current through that resistor which can idle to signal tube closer to cutoff and back to ~double idle current (at maximum intensity). So you've got to worry about current to get a strong trem here, which makes relatively high-current 6SN7's seem like a poor choice; that's why I suggested copying a known-good plan (you probably won't realize all the subtle details).
Fender decided to improve this circuit and got what was probably his first patent (and maybe his only one on the tweed amps, I'm not sure). Leo made the Tremolux (http://el34world.com/charts/Schematics/files/fender/Fender_TREMOLUX_5E9A.pdf), where the trem buffer pulled current through the cathode resistor shared by the two section of a paraphase inverter. This was pretty slick, as the speaker pumps an output signal based on the differential output of the push-pull output stage, but the trem is applied common-mode. This creates a volume variation, but cancels the trem signal from reaching the speaker (under ideal conditions). Net result: trem without pumping.
But not every amp can use a paraphase inverter, so what do you do when you switch inverters? You can't apply trem the same way to a split-load or long-tail inverter. Fender moved to applying trem to the output tubes to vary the bias, as in the 5F11 Vibrolux (http://el34world.com/charts/Schematics/files/fender/Fender_VIBROLUX_5E11.pdf), then tinkered with pitch-bending vibrato (http://el34world.com/charts/Schematics/files/fender/Fender_VIBRASONIC_5G13.pdf) on its bigger amps.
The problems with these approaches are that the bias-vary scheme works best on amps that use 6V6's, with their smaller bias voltages (the trem doesn't have to be enormous to shake the output tubes' volume), and that not everyone plays with constant vibrato unless your name is Lonnie Mack. The vibarto circuit also takes a bunch of tubes and parts to make it work. So after a short run, Fender dropped that circuit, and the output tube bias vary reappeared on the small-ish Princeton Reverb (http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf).
So what about trem on the big 6L6 amps? Aside from the brief flirtation with the 6G16 Vibroverb (http://el34world.com/charts/Schematics/files/fender/Fender_vibroverb_6g16_schem.pdf), Fender moved on to grounding the signal ahead of the phase inverter with an optoisolator in its big 6L6 amps (http://el34world.com/charts/Schematics/files/fender/Fender_TWIN_REVERB_AB763.pdf). The oldest method still lived on in the student-model VibroChamp (http://el34world.com/charts/Schematics/files/fender/Fender_VIBRO_CHAMP_AA764.pdf). I think Fender targeted a price-point with the VibroChamp, gave the Princeton the best trem (as long as you're playing a relatively low volumes; it can get swamped when you crank the amp wide open), and gave the big amps a trem that works at all volumes (though maybe doesn't give you the sweetest response).
Of course, now you can get a pedal with tap tempo trem, a selection of waveforms, multiple speeds at the stomp of a switch or sweep of a foot pedal, and numerous different rhythms to the trem. So it really boils down to which circuit works best with the rest of the amp you're designing given the shortcomings of each approach. Or which pedal sounds best in front of the amp...
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Here a different approach to tremolo
(http://www.dougcircuits.com/AC4V3.jpg)
Here some explanations about the Relaxation Oscillator
http://www.magnatoneamps.com/neon.html (http://www.magnatoneamps.com/neon.html)
http://www.dougcircuits.com/neon.html (http://www.dougcircuits.com/neon.html)
K
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You should see that at its core, this is still a "ground/unground" method of injecting the trem. The unusual aspect is the type of oscillator that generates the signal controlling the optisolator.
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Thanks again for the help. Lots of choices n I do like the ldr/neon version, very clever! The kiss principle at work!
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Ok, here’s what I came up with for mixing the tremolo in;
I went to the valve wizards site, read-up on resistive mixer, put that together with a 6J6 cathode follower that was “discontinued” from this build. Now I’m re-wiring as a common cathode follower mixer?(V4).
I’m guessing I will get sound of some sort out, my question is about the resistor values at V4s grids, the 100k was in the original cathode follower that worked fine. The 270k, and 1Meg are the “typical” from the wizard. Are they close enough to solder them in for sound checking?
My other question is with the footSW, can I move it to the top of the 1M pot, .047 junction, like in the Revibe?
Thanks for hanging in there with me!
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If happens that you have a 6BE6 tube under hand .......
(http://www.ozvalveamps.org/playmaster/pm103cctamp_2joe.jpg)
K
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Ok, here’s what I came up with for mixing the tremolo in;
There are a couple mistakes on that drawing that will probably prevent the amp from working. I don't understand what you are trying to do with V4b.
Why won't you just copy a tried and true circuit rather than reinventing? Several of the Supro amps use an identical trem circuit and inject the trem signal into the cathode of the first or second preamp triode. Look in Hoffman's schematic library. Here are a couple. There are others too...
http://el34world.com/charts/Schematics/files/supro/supro_1624t_scematic_263.pdf (http://el34world.com/charts/Schematics/files/supro/supro_1624t_scematic_263.pdf)
http://el34world.com/charts/Schematics/files/supro/Supro_S6688.pdf (http://el34world.com/charts/Schematics/files/supro/Supro_S6688.pdf)
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The schematic to which you refers is this
(http://i.imgur.com/7kgkiMp.jpg)
I think the wrong thing is about thinking to "merge" the signal from the preamp with the signal generated by the oscillator
As far as I can know you don't want to mix the two signal, you want to use the signal from the oscillator as an automatic volume control
that lowers and rises the signal of the preamp to the following stage, the way you want to use the 6J6 tube seems will give you the frequency
generated by the oscillator united to the signal of the guitar .............. not a pleasant effect
if you are looking for unconventional way to rise and low the signal according with the speed of the oscillator, here is one other schematic
that uses a 12AU7 tube
(http://i.imgur.com/fGN4eO0.jpg)
K
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I think the wrong thing is about thinking to "merge" the signal from the preamp with the signal generated by the oscillator. As far as I can know you don't want to mix the two signal, you want to use the signal from the oscillator as an automatic volume control
that lowers and rises the signal of the preamp to the following stage, the way you want to use the 6J6 tube seems will give you the frequency generated by the oscillator united to the signal of the guitar .............. not a pleasant effect
They are the same thing (mixing guitar signal and trem signal, or using the trem as an automatic volume control). The tremolo frequency is too low to hear as "sound".
But the tremolo signal generally has to be very much larger than a guitar signal in the preamp when they're mixed. Further, it generally requires unnecessary extra parts to combine the signals any way other than varying the bias of a preamp or output tube, or grounding/ungrounding a signal.
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Thanks HotBluePlates
I've the tendency to discard things that I've seen
this arrangement is very similar to what Shooter want to do
(http://i81.photobucket.com/albums/j207/merlinblencowe/Vibrotron/ValveVibrotron.jpg)
K
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No, that's about 20 times more complicated. However, the feature I think you're looking at is the shared plate load resistor for the mix stage.
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the feature I think you're looking at is the shared plate load resistor for the mix stage
Yes, I meant those part of the circuit
K
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"I don't understand what you are trying to do with V4b."
Since the chassis did have 2 6j6’s, now defunct, I was thinking I could use one to combine/mix the signal from preamp2(V4b) with the trem(V4a), use the pots(1meg) to adjust how much trem vs dry signal are mixed.
"But the tremolo signal generally has to be very much larger than a guitar signal in the preamp when they're mixed".
Ah!, I was thinking backwards, that if the trem was large it would add/subtract too much of the guitar signal, to much “swing”.
I think for now I’ll wire up the trem, get it working since it’s a copy of the skylarks it should work standalone. Then decide if I wanna wire up the 6j6 or just drill it out to a 9pin n be conventional. The preamps are working, only had 2 musicians test them but I did get, like it!
Thanks for all the inputs, they are great!
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You don't "mix" (linear addition) tremolo with signal.
You *modulate* the gain of an audio stage.
Oh, yeah, if you stuff a BIG trem signal into any amplifier, its gain will change. But it will also distort with huge trem. And the amplifier tends to amplify the trem, making big problems for later stages. Notably: the phase which decreases gain in (say) the 1st stage is such that it will DEcrease INcrease gain in the 2nd stage.
Agree with the idea to PLAGIARIZE. About 95% of all possible AND practical trem systems have been used in one amp or another. Steal!!
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PRR, artists I hang out with say "a good artist will borrow, a great artist will steal" anyway;
Well after some self injected problems, I got the trem osc working, and wired up the 6J6. It works! Before I hooked up the 6J6 I did some gain adjustments to better match the Vac-pp of ch 2 to that of channel 1. Unloaded both are now about 130v-pp without distortion. Once I hooked up the 6J6, the output from the 6J6 dropped to about 12v-pp. I have half a 12ax7, so I think I’ll add another gain stage to ch 2 to n get it back closer to ch 1.
Thanks to all for your insight n help
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I would like to see the new schematic
Thanks
K
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I attached the most recent one….But, I found the reason for so much signal loss. The 6j6 is only 48vdc at the cathode “tap”, (470ohm/22k junction). I didn’t have a 33k plate resistor so I just sub’d a 47k. Going back to the 33k and bumpin the 22k cathode resistor to 47-56k n see if that will give me enough room to develop the AC signal closer to Channel 1.
Also fighting an intermittent parasitic oscillation (9.6khz). I walked it back to V3a’s plate. I quit for now, I’ll go back n make changes to the 6J6, then pull tubes n verify it is V3. My gut says it goes away cuz v3 is just “picking up” another tubes problem, or amp builder installed problem!