Mesa Rectifier LDRs vs. Relays
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JCDenton6
Well-known member
The change was implemented in 2000 when Mesa discontinued the 2 ch heads and released the 3 ch heads.
APEMAN said:
I'm going to have to respectfully disagree with most of what you've posted here. Some of your comments are factually on level with the debates concerning tubes/transistors and pcb/turret builds.
To answer the specific question asked in the first post, the switching diagram with relays that you linked to was never in a mesa build rectifier, all two channel rectifiers used LDR's in the signal path. The relay diagram is an alternative option for those that wanted to build a rectifier and didn't have access to 21 LDR's.
Mesa switched to relays in the signal path and transistors in the cathodes and mute circuit with the 3 channel rectifiers and every amp since. The only J175 transistors in the direct signal path of the rectifier are for the mute circuit, and is a method still used by Mesa on all current amps, including the rectifier reborn amps and the Mark V. Relays are less expensive and have a faster switching time, as well as being a true break in the circuit. LDRs always remain in circuit. The reason you have a change in the tone of the amp when the FX loop engaged is because you are running through a bunch of parts, including two potentiometers, two tube triodes, relays or LDRS depending on the model and all the resistors and capacitors that are required to have the loop operate. Any one of those items can cause a change in tone, targeting one component is misleading.
LDR's can cause a change in tone, and the new recitifiers attempt to replicate that by adding 100 ohm resistors to the cathodes of three of the preamp stages. The reason for that is it is pretty close to the on resistance of the LDRs that were present in that position in the two channel rectifiers. So seeing as how Mesa has attempted to retain that aspect of the LDRs it must have been important for them.
Finally as mentioned earlier transistors are not used for switching in anything but the cathodes of the tubes. Every other transistor is part of the mute circuit. Any effect the transistors have on the tone will be inaudible as they are not in the signal path. If I had to guess you were using the criticisms of stomp boxes using transistor buffers at their input that are never fully switched out. That can affect the tone, but transistors are not used like that in the rectifier.
I'm not trying to argue with you, but your post was half a truth and a third of opinion.
I'm going to post a correction real quick because I was thinking about it and there are three transistors in the signal path in the loop of the three channel rectifier. My previous statements are still accurate with the exception of that correction.
Third Age Amps
Well-known member
I've designed and built amps using both LDR's and Relays and the Relays always sound better. One amp I built started with LDR's and sounded good, but different than others I had built. I disabled them and installed relays and the tone changed quite a bit.
IMO, relays are the way to go because they are simply wire to wire contacts where LDR's have a "resistor" in the signal path.
IMO, relays are the way to go because they are simply wire to wire contacts where LDR's have a "resistor" in the signal path.
elvis
Well-known member
I have no idea if relays sound better, or even different, than LDRs. I will, however, weigh in on the technical discussion, as I feel it's a bit more subjective than objective. It's easy to perpetuate mythology when throwing around engineer-y sounding terms. APEMAN, you may be completely right (or not), but you are really waving your hands on this, so your explanation is little more than a strawman.
I disagree that the resistance, or even the reactance, of the LDRs dominates the tone. I would have to simulate it in order to get a more precise understanding of the frequency characteristics, but in general:
1. The resistance is low. Most of the circuits in a tube amp are moderately to very high impedance. A change in total circuit resistance of 1% due to the LDR would be inconsequential, and the inconsistency of a few Ohms across switch events would be even less. Further, the tiny amount of inductance and capacitance would have frequency effects far outside the range of Human hearing. Also, the parasitic reactances are the same or larger for relays.
2. There is an advantage with LDRs, in that they can reduce pops during switching since they are not completely open or completely closed.
3. If the LDRs are linear time-invariant (LTI), any effect they might have can be countered effectively with gain and tone controls.
4. It's difficult to rely on testimonials. They are subjective at their face, and are also subject to placebo effect. Unless a true ABX test is used, you can't rule out the fact that the listener knew which amp had relays.
If I were to try to find a real cause for tonal difference with LDR vs. relay, I would look for reasons why the LDR might not be LTI. For sure there are a wide range of signal levels in the preamp. So for the front stages, the small signal levels probably see a quite linear resistance in the LDR. However (at the risk of starting a myth, since I have never measured this), the large signal levels in the loop and output circuits might see a nonlinear, signal-dependent resistance IF the LDR has a big voltage coefficient on its resistance. It's certainly plausible, but again, hypothetical.
And there is really no question that the LDRs are simply not that reliable. That's reason enough to avoid them.
I disagree that the resistance, or even the reactance, of the LDRs dominates the tone. I would have to simulate it in order to get a more precise understanding of the frequency characteristics, but in general:
1. The resistance is low. Most of the circuits in a tube amp are moderately to very high impedance. A change in total circuit resistance of 1% due to the LDR would be inconsequential, and the inconsistency of a few Ohms across switch events would be even less. Further, the tiny amount of inductance and capacitance would have frequency effects far outside the range of Human hearing. Also, the parasitic reactances are the same or larger for relays.
2. There is an advantage with LDRs, in that they can reduce pops during switching since they are not completely open or completely closed.
3. If the LDRs are linear time-invariant (LTI), any effect they might have can be countered effectively with gain and tone controls.
4. It's difficult to rely on testimonials. They are subjective at their face, and are also subject to placebo effect. Unless a true ABX test is used, you can't rule out the fact that the listener knew which amp had relays.
If I were to try to find a real cause for tonal difference with LDR vs. relay, I would look for reasons why the LDR might not be LTI. For sure there are a wide range of signal levels in the preamp. So for the front stages, the small signal levels probably see a quite linear resistance in the LDR. However (at the risk of starting a myth, since I have never measured this), the large signal levels in the loop and output circuits might see a nonlinear, signal-dependent resistance IF the LDR has a big voltage coefficient on its resistance. It's certainly plausible, but again, hypothetical.
And there is really no question that the LDRs are simply not that reliable. That's reason enough to avoid them.
elvis
Well-known member
No way to do that. Everything you record, and everything the audience hears from the house system, went through semiconductors. Nothing inherently evil about them, you just have to recognize their limitations in any design. Tubes have WAY worse limitations. Which is why we love them :shock:
Any chance you'll use your engineering powers to throw an LDR on a curve tracer or a scope and let us know what's actually wrong with them? :idea:
Any chance you'll use your engineering powers to throw an LDR on a curve tracer or a scope and let us know what's actually wrong with them? :idea:
Third Age Amps
Well-known member
Actually, elvis has a point. I thought the head sounded better with relays, but I also think Randy Rhodes tone sounds like he playing a screaming weasel through a transistor radio.
I appreciate the subjective look at the devices. And as for "semiconductor-y" parts, if you use a pedal to boost your tone ... All sand.
I appreciate the subjective look at the devices. And as for "semiconductor-y" parts, if you use a pedal to boost your tone ... All sand.
This sounds condescending and sarcastic to me. I'm not trying to flame, I'm just calling it as I see it.APEMAN said:
Yup.Third Age Amps said:
Dom
elvis said:
This is pretty much what I was getting at. Any effect would be pretty minimal and more subjective based on a persons expectations.
I use relays over LDRs as much as possible. I built a slo100 clone with the NFB switching of a rectifier and used relays because they were less expensive, easy to wire up and had minimal affect on the amp. It is essentially a two channel slo with 100 ohm resistors on the cathodes between the capacitors and ground and a 20pf capacitor before the grid of V1B as in the three channel rectifiers. I built it for someone who wanted an amp similar to the dual rectifier. I also utilized elevated heaters for the power amp and tubes V3/V4/V5, and regulated heaters for the first pair of tubes. Switching is done using relays. The overdrive channel has a feedback circuit is like a Mesa Tremoverb and has a modern and vintage setting.
With that being said, I have designed an amp that is the two channel rectifier using all relays except for three LDRs. I feel those LDRs are important to the two channel rectifiers character.
