Help a newbie please.
Here are two schematics, my Super Twin head and a Studio Bass. Besides some EQ centers and some limiting of the treble on the studio bass, the difference is mostly that the Super Twin has a really horrible 'distortion' knob and associated tube stage, both of which I intend to repurpose, because it's a horrible clipping noise that doesn't change, so it's not emotive or expressive at all.
First, I don't really understand the existing 'distortion' circuit well enough, so that's what I'm asking for help with first.
I see the footswitch, and above that the knob switch that clicks open when rotate counterclockwise to the stop. So does do these swtiches:
1) turn the distortion on & off by altering the cathode voltage to turn the V3B stage off & on
-or-
2) V3B is always "on" or somewhat "active" in the circuit but the switches change the diode labeled 035604 from having a varying DC signal across it (so the diode does not do anything and does not add distortion) to removing that DC component and having AC applied to it (in which case the diode does switch, and adds distortion)...
I'm guessing number 2)?
If anyone can explain the details of the stock distortion circuit I'd appreciate it.
Here are two schematics, my Super Twin head and a Studio Bass. Besides some EQ centers and some limiting of the treble on the studio bass, the difference is mostly that the Super Twin has a really horrible 'distortion' knob and associated tube stage, both of which I intend to repurpose, because it's a horrible clipping noise that doesn't change, so it's not emotive or expressive at all.
First, I don't really understand the existing 'distortion' circuit well enough, so that's what I'm asking for help with first.
I see the footswitch, and above that the knob switch that clicks open when rotate counterclockwise to the stop. So does do these swtiches:
1) turn the distortion on & off by altering the cathode voltage to turn the V3B stage off & on
-or-
2) V3B is always "on" or somewhat "active" in the circuit but the switches change the diode labeled 035604 from having a varying DC signal across it (so the diode does not do anything and does not add distortion) to removing that DC component and having AC applied to it (in which case the diode does switch, and adds distortion)...
I'm guessing number 2)?
If anyone can explain the details of the stock distortion circuit I'd appreciate it.
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Currently V2AB (both halves in parallel) for the EQ section gets it signal from the typical Fender preamp output of V1B, and applies its tone-altered signal to V3A.
I want to repurpose V3B for additional gain, and put it in the circuit between V1B and V2AB. I'll probably have a fine-adjustment "drive level" pot across a fixed resistor on the chassis between V1B and V3B, and the existing front-panel "distortion" pot will be an attenuator between the "relocated" V3B and V2AB.
I want to repurpose V3B for additional gain, and put it in the circuit between V1B and V2AB. I'll probably have a fine-adjustment "drive level" pot across a fixed resistor on the chassis between V1B and V3B, and the existing front-panel "distortion" pot will be an attenuator between the "relocated" V3B and V2AB.
It's a bit of a confused circuit, I'm not surprised you don't like it.
The switch is used to control the bias of the diode labelled O35604 by feeding some negative current to (or rather from, the circuit).
The diode is fed some premenant bias current through that 470k resistor from B+. When the distortion is "on", the diode is biased on, or on the verge of on. This means that when the cathode follower is amplifying a signal, the diode clamps the 3.3k resistor to ground for upgoing signals, and becomes open circuit for downgoing signals. The load on the cathode follower is therefore constantly changing between 3.3k and about 23.3k, causing distortion.
In the 'off' mode the diode's anode is pulled negative so that it should remain off all the time, so the cathode follower works normally into a 23.3k load.
It's a bizarelly overcomplicated circuit. Thanks for posting it!
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My personal favorite guitar tone starts with an Alembic preamp with one channel's output jacked into the other channel's input, putting both channels in series instead of parallel. It had the most wonderful shimmery treble-boosted highs when running "clean". Running "dirty" that treble pre-emphasis via the 'bright' switch and tone controls of the first "channel" made for rich harmonics, then the subsequent treble-cut in the second 'channel' removed the irritating 'fizzy' sharp corners off the waveform, leaving you with all kinds of rich beats without all the irritation. The 3 volume controls had a wonderful ability to match the guitar player's signal level and set the level where that clean/dirty line was. The only thing it didn't have was the ability to vary the ratio of clean vs dirt like a Dumble or the parallel channels of a Marshall with the inputs double-jacked in parallel.
So, that Alembic was:
ch1 gain stage 1, tone & volume controls, ch1 gain stage 2
ch2 gain stage 1, tone & volume controls, ch2 gain stage 2
power amp's master volume.
I later added an adjustable "drive" volume pot between channels, but it wasn't really necessary.
This one will be:
(V1A) gain stage, tone & volume controls, (V1B) gain stage,
(repurposed V3B) new gain stage, (repurposed "distortion" pot) new DRIVE control,
(parallel V2AB) tone drive stage, rotary graphic EQ, (V3B) tone recovery stage, "output level" master volume, cascaded long-tail pair, 6L6s.
One question I have is whether I can use the SPST switch in the "distortion" pot for anything constructive, like eliminating that repurposed V3B new gain stage from the signal path, returning the topology to stock. I can't just bypass that new gain stage by connecting input to output or I imagine it will be shorted or oscillate despite being out of the signal path?? Maybe it could switch a cathode resistor to reduce the gain of V3B and leave it in the signal path.
So, that Alembic was:
ch1 gain stage 1, tone & volume controls, ch1 gain stage 2
ch2 gain stage 1, tone & volume controls, ch2 gain stage 2
power amp's master volume.
I later added an adjustable "drive" volume pot between channels, but it wasn't really necessary.
This one will be:
(V1A) gain stage, tone & volume controls, (V1B) gain stage,
(repurposed V3B) new gain stage, (repurposed "distortion" pot) new DRIVE control,
(parallel V2AB) tone drive stage, rotary graphic EQ, (V3B) tone recovery stage, "output level" master volume, cascaded long-tail pair, 6L6s.
One question I have is whether I can use the SPST switch in the "distortion" pot for anything constructive, like eliminating that repurposed V3B new gain stage from the signal path, returning the topology to stock. I can't just bypass that new gain stage by connecting input to output or I imagine it will be shorted or oscillate despite being out of the signal path?? Maybe it could switch a cathode resistor to reduce the gain of V3B and leave it in the signal path.
You have to show more of your proposed schematic to be sure of that.
Maybe you could use the switch simply to switch in an attenuator, so you can quickly access a very clean tone, despite having three gain stages?
Well now I did it. I bought another SuperTwin. Fender Super Twin Amplifier | eBay
The price is so right if a little more work gets them to sound better. But I think getting them quiet as a modern boutique amp may involve a lot of work: DC heaters raised in respect to the cathode via a voltage divider to the B supply, replacing some carbon-compound resistors with modern quieter metal film ones, better power supply filtering, and changing the fixed-bias supply to full-wave rectifier / filter / regulator / final filter. Individual bias pots. Ought to make great clean amps, and have potential for more. I paid $379 which is a cheap price for just the transformers and speakers!
The price is so right if a little more work gets them to sound better. But I think getting them quiet as a modern boutique amp may involve a lot of work: DC heaters raised in respect to the cathode via a voltage divider to the B supply, replacing some carbon-compound resistors with modern quieter metal film ones, better power supply filtering, and changing the fixed-bias supply to full-wave rectifier / filter / regulator / final filter. Individual bias pots. Ought to make great clean amps, and have potential for more. I paid $379 which is a cheap price for just the transformers and speakers!
It will be interesting to have another and compare them.
My first SuperTwin's background noise has more burble, crackle, pop, bit of hiss than hum. I can probably get rid of much of the remaining hum, as there are much better techniques now. I replaced one tube who's cathode corners always glowed red. I suspect it's the old-fashioned carbon-composition resistors that are the cause of the random noises, and IMHO a major area of improvement in new amps. It's not rocket science, but there are a lot of resistors! I would consider making new high-quality turret boards and swapping them in, but the original caps look pretty good, kind of like dark blue copies of the much-beloved epoxy-encapsulated Sprague orange drops.
My first SuperTwin's background noise has more burble, crackle, pop, bit of hiss than hum. I can probably get rid of much of the remaining hum, as there are much better techniques now. I replaced one tube who's cathode corners always glowed red. I suspect it's the old-fashioned carbon-composition resistors that are the cause of the random noises, and IMHO a major area of improvement in new amps. It's not rocket science, but there are a lot of resistors! I would consider making new high-quality turret boards and swapping them in, but the original caps look pretty good, kind of like dark blue copies of the much-beloved epoxy-encapsulated Sprague orange drops.
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The background noise is in the power amp section, as it's still there with the master volume off. I could pull pairs of tubes, but I think it's the resistors. My dad's Harman Kardon Citation II always had the same background noises, as did everything of the era. Yet I hear modern boutique amps with more gain and they're completely silent; of course they don't put out 280 watts, so by comparison even this signal to noise ratio would be quieter at that low a volume. It would be much more versatile if the noise was less. I could burn off some output with an Aiken Reactive Load to practice at home, but I already have practice amps.
Cathode follower clipping. With the different bias the stage "starves" in current and goes to cutoff on negative halfwaves. Result is HARD clipping of the negative halfwave and nothing else. Plenty of asymmetric clipping distortion and those "musical even order harmonics". ;-)
...That is, if you believe any of the remarks concerning musicality of different types of harmonics. In that regard, this amp is a good eye opener.
...That is, if you believe any of the remarks concerning musicality of different types of harmonics. In that regard, this amp is a good eye opener.
There are just SO MANY tubes and resistors! I definitely got my money's worth just in transformers. Or dollar per pound, best deal in a hand-wired tube amp. I'll see how the new amp sounds.
Good thinking though, if I pull pairs of tubes and move 'em around in different sockets that should tell me a lot about the tubes and the resistors local to them, and be a lot easier than trying to tell what's going on in a complete circuit via an oscilloscope. I might even try the tubes in my champ if I just want to find the ones that are noisy at idle. Yessir, now I've got a logical approach to start with.
Good thinking though, if I pull pairs of tubes and move 'em around in different sockets that should tell me a lot about the tubes and the resistors local to them, and be a lot easier than trying to tell what's going on in a complete circuit via an oscilloscope. I might even try the tubes in my champ if I just want to find the ones that are noisy at idle. Yessir, now I've got a logical approach to start with.
I am going to purely guess here, but I would suspect the biasing circuit for the power tubes. Any noise on that DC will show up at the grids of power tubes. Check electrolytic and suggest replacement, and resistors on that circuit replace with lower noise metal films, you can up the wattage on them. Use a high quality pot if it's going to be an adjustable bias circuit. Have a look at the screen resistors for possible replacement too.
What "Noise"? 60Hz hum? 120Hz hum? Hiss? Crackle/rumble? Every one of those is from a different thing, and so would have not only a different source but also a different cure. You mentioned several in passing, but which are you trying to solve at this moment?
Isolate the problem. The noise is power amp, so you mean after the master volume? OK, that means it is in the V4 stage, the phase inverter, or in the V5 stage, the drivers, or in the power tube stages. Could be the tubes themselves or something else in one of the stages.
So pull V4. Noise stop? If yes, then that stage is central. If no change, then pull V5. If that stops it, then something about the V5 stage is key, and if neither stops the noise, then the power tubes are suspect, or their circuits. Sounds simplistic, but it narrows down your search.
First thing for any suspect stage is a different tube.
V4. The two 220k plate resistors are most likely for noise like hiss and crackle.
One powerful technique is to ground the signal path. Just a clip lead to chassis will do that. If it stops the noise, you have at least found part of its path, the noise is before what you grounded. But what if the point you want to ground is at 200v or 50v or anything not ground level? Use a cap. I have a lot of 0.047uf/630v film caps around, so I grab one, but the value is not critical. Ground one end, and have a free clip wire on the other. Now we can "ground" a point with this cap. A cap to ground will shunt off a signal, even a noise signal.
And you can shunt a resistor with a cap in similar fashion.
V5. again possible noisy cathode resistors.
Power tubes. You can power up the amp with just one of them in at a time to check them individually. And you can take a tube you know is good and move it socket to socket to check them. Surely ripple on the bias supply will make hum on the power tubes.
But start with tube pulling. Pull V4 and then V5, and you ought to have a pretty clear idea where the noise is coming from. Then by "grounding" point to point along the signal path, you can narrow it down further.
As to all those resistors, oh please... The phase inverter has only seven, the driver stages each have three. Each power tube has two. There may be 100 resistors in the preamp, but they are not making our power amp noise, so...?
Isolate the problem. The noise is power amp, so you mean after the master volume? OK, that means it is in the V4 stage, the phase inverter, or in the V5 stage, the drivers, or in the power tube stages. Could be the tubes themselves or something else in one of the stages.
So pull V4. Noise stop? If yes, then that stage is central. If no change, then pull V5. If that stops it, then something about the V5 stage is key, and if neither stops the noise, then the power tubes are suspect, or their circuits. Sounds simplistic, but it narrows down your search.
First thing for any suspect stage is a different tube.
V4. The two 220k plate resistors are most likely for noise like hiss and crackle.
One powerful technique is to ground the signal path. Just a clip lead to chassis will do that. If it stops the noise, you have at least found part of its path, the noise is before what you grounded. But what if the point you want to ground is at 200v or 50v or anything not ground level? Use a cap. I have a lot of 0.047uf/630v film caps around, so I grab one, but the value is not critical. Ground one end, and have a free clip wire on the other. Now we can "ground" a point with this cap. A cap to ground will shunt off a signal, even a noise signal.
And you can shunt a resistor with a cap in similar fashion.
V5. again possible noisy cathode resistors.
Power tubes. You can power up the amp with just one of them in at a time to check them individually. And you can take a tube you know is good and move it socket to socket to check them. Surely ripple on the bias supply will make hum on the power tubes.
But start with tube pulling. Pull V4 and then V5, and you ought to have a pretty clear idea where the noise is coming from. Then by "grounding" point to point along the signal path, you can narrow it down further.
As to all those resistors, oh please... The phase inverter has only seven, the driver stages each have three. Each power tube has two. There may be 100 resistors in the preamp, but they are not making our power amp noise, so...?
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