Power resistor/sag switch..

[Jon434]

Is this wiring better? A lot of this stuff is starting to make more sense lately, but seems best to be 100% sure of all the minor details to avoid any potential problems. It still confuses me a bit, in terms of visualing the schematic vs the real wiring, where it sometimes seems like 2 different languages. Especially once double sided PCB's get involved.

The resistor could be chassis mounted (or on a separate little breadboard) with the wires only on the PCB, if having the resistor hanging in this way isn't ideal but it was more for the purposes of drawing it easier.



To summarise about the B+, you said that it would be ok to do it my original way since there is no center tap, so it could be ok but you'd still advise to do with just one resistor off of the negative half of the rectifier to first filter caps ground? The reason I chose against the idea of something like this originally, is that it's working on the PCB directly, where as the other way is with spade connectors and less chance of a mistake on the PCB or identifying the right cap etc. The caps are in pairs, I just showed one in the schematic to simplify. But even separate from that.. if there IS no problem with the original way due to no center tap.. wouldn't that be 2 levels of resistance to choose from instead of 1? 150R vs 300R vs just 150R or 300R? I could imagine liking different levels depending on the master volume level.

[Jazz P Bass]

Not too sure what you are after here, (sag?) but if you keep lowering the B+, you will be affecting the preamp voltages also.

[Jon434]

I don't mind if it lowers the preamp voltages, it'd be a learning experience to see what values influence things. The resistors are 25W chassis mounted, and should be easy to swap one value out for another.

What I'm after is lower B+ voltages (similar to variac effect in some way but built into the amp), the option to control the sag (because both the amps I want to do this with are very tighly focussed and modern voiced with high value caps and have little sag). And just to try different levels of voltage to find out what I like. I want to add a choke because I bought them when upgrading to a Classic Tone output transformer and adding a choke should increase voltages a little and reduce sag even more, so it makes this idea even more appealing. I thought that while having the chassis open to put in the choke, this would be a good opportunity to try something else at the same time. And if they are both switchable it makes it easier to revert back. It's not so much about lowering volume but just the response and feel of the power amp to be more vintage sounding and adding a different voice. Lower B+, screens, preamp plates and extending the bias range to work in all of these situations. The heaters wouldn't be affected, but I wouldn't mind if they were. Suhr runs SL68 at 5V heaters in variac'd mode.. some say the cathode stripping can be an issue but he doesn't think it does (can of worms for debate etc). I've read of some people using a variac for 20-30 years and never having a problem and others saying flat out you just shouldn't do it. Quoting tube manuals and graphs, but anyways.. this doesn't apply to me as AC6.3 supply would be untouched. I'm also turning one of the amps from a combo to a head, and needing to make a new front panel.. so there's also other things to focus on. Maybe I can visit lower heater voltages in the future one day.

Lower voltages, less headroom, saggier squishier response etc. More like the Soldano Hot Rod 25 has (which is always running in a built in variac mode). I'd like to experience the effect of having the plates starved for voltage a bit and see if I like it. But switchable, so that if it doesn't suit the moment, can be reverted back to stock. I don't want any permanent changes, just experiments to build some knowledge. I can't say how effective this will be in practice but I like the idea of trying it. Something similar to the switches on the back of the Mesa Mark V or Triple Rectifier. Where you can go from 460 plate to 420 to 390 plate voltage depending on the combination of switches you change. Pentode/triode is another option that Mark V uses (and tooboob also mentioned this) or solidstate/tube rectifier, or 90v or 120v (bold/spongey). This isn't the same thing I want to do, but a similar idea in some ways. I don't know of any amp top line amp makers that use a choke switch but it's something I'd like to try and some people have done it. Some people hate extra options, but I like the option of using them similar to coil tapping or coil splitting pickups. Also.. line voltage can vary sometimes.. some places it can be really high and some places really low, so I like the idea of having control over it with a switch instead of carrying a variac or power conditioner around all the time.

I also just want to learn more about my amps by experimenting to see how these things feel in practice, compared to other people using them. This is how I found what necks I like, what pickups I like etc. But also want to be safe and take it bit by bit. There is a lead time of 4 weeks before I can use parts that I order, so trying to plan everything ahead of time and understand the ideas behind everything properly. It's been very fun to learn how all of these amps work and the different ideas behind them. I read 17 pages chapter on power transformers by ValveWizard last night (the same site pdf64 linked), and not once was center tap mentioned. Some info is very hard to find, so thanks to those who have helped explain some of these things. And a lot of amp information is inspired by vintage designs and current amps are harder to understand with less information about them.

I understand it's hard to follow this thread because I ramble on about it but some of info about tube amps is hard to find, and any info or ideas anyone has is always appreciated.

[R.G.]

Go here: power supply diagrams.
A half-wave rectifier charges a filter cap once per AC cycle. The diode blocks the reverse half wave.
A full wave bridge rectifier with four diodes is a clever way to let the diodes connect BOTH half-waves to the filter cap when they happen to be "pointing" in the right polarity.

A full wave, center-tapped transformer winding has the same result as the full wave bridge, but does it by using a center tapped winding. The center tap is usually attached to the negative side of the first filter cap. Two diodes are used, and the diodes each let through current from their half of the winding only when it's the right polarity to charge the cap, blocking it otherwise. Most all-tube equipment used the FWCT circuit because then only one rectifier tube with two sections was needed, and this was cheaper than putting in the three tubes it would otherwise need.

In any case, if you look at it from the point of view of the first filter cap, there are two wires attached to it from the rectifiers: the one that feeds pulses into the (+) side and the one that sucks current pulses out the (-) side. The transformer and rectifiers don't know or care which side of the filter caps is "ground" or eve what "ground" is.

We care, because the tube amplifier circuits need ground to be the cap's (-) lead. So we connect the amplifier signal ground there.

On the other hand, you're wanting to sag the DC voltage in the first cap by inserting some resistance, inductance, whatever, in series with the wires feeding current into and out of the first filter cap. You can do this several ways.

- insert ONE resistor in series with the wire feeding current into the (+) capacitor lead. This works, but has the disadvantage that both sides of this resistor and any switch used are floating up at the few hundred volts of the first filter cap (+) lead.
- insert one resistor in series with each of two diodes feeding the (+) side of the capacitor lead. Each resistor now carries half of the current pulses into the cap, so they dissipate much less power each, but switching them in and out is complicated.
- insert ONE resistor in the lead carrying current away from the (-) lead of the first filter cap. The (-) lead is still at ground, as it's connected to the chassis and signal ground, so the side of this resistor away from the filter cap (-) is now negative with respect to chassis ground, but it's only the amount negative that the current times the resistance makes it, probably in the range of 20V to 50V. It has the same power as the resistor in the first option.

[Jon434]

On the other hand, you're wanting to sag the DC voltage in the first cap by inserting some resistance

I'm more interested in just dropping voltage all together (the option of having it for the right situation), if I had the choice of two sets of primary wirings like a 240V and 200V I'd do that, but the PT's I have doesn't have them. As far as I know, using a resistor to do this (as opposed to other options) would still create some sag under high load regardless of whether it's before or after the rectifier, but since the amp is so modernly voiced and with high value caps, I figured this could be a good thing. I was originally going to use a voltage regulator circuit that are small pcb based (the ones which have pot, mosfet and diode etc) but this seemed a simpler easier way to implement/experiment with. The idea of simulating a tube rectifier isn't bad either, I guess I'm just trying to say that I'm not trying to simulate an exact rectifier tube type or anything like that.

Just trying to explore the options, because doing anything directly to the PCB would be harder to reverse and revert back to stock if needed. Both amps have no publically available schematics which makes finding the right point harder. I could draw it out in full detail if needed.. there's 220K resistors of each cap, there's sets of two caps in series, and also 2 1uf snubbing caps in the rectifier (one for positive side and one for negative side I think). Whatever happens, I may end up drawing up proper full detail schematics for them both anyway at some point. To help with this, or just as a reference point for the future, but it's a lot to take on at first. I'm not against doing it post rectifier, I'm open to it but it's also a lot more complicated to do it this way. There is also the idea eschertron suggested of using the fuse's place on the PCB and moving the fuse elsewhere which is also interesting and easier to implement.

I'm still not sure of the harm of using two resistors on ac side, since this would just be mimicking a lower voltage winding, that comes out of the PT secondary? There was talk above, that having one resistor on, and one resistor off would create an inbalance, and then another reply saying that it'd be ok, since there is no center tap. I'm still not 100% sure on that, I could have misunderstood it, but using both on a dpdt switch and having both on or both off.. I'm not sure where the harm is other than higher voltage going through the resistor and switch (which for example, the standby switch already has).

I'm not saying it's the ideal solution overall, just that it's easier to implement and/or remove if I don't like it. Since no one criticised the choke wiring, hopefully that was wired up ok, as I don't want to do anything until it's 100% sure and planned out. The pic below, if I am following right, the yellow circle is first option, the blue crosses are the second, and the red cross is the third option? It's not mine, but taken from a pdf guide about sag resistors. So there'd be a lot less voltage across it with the third way, so that's what makes it the most ideal option? The resistor/choke switch would still have 400V B+ across it though?

[Dave H]

...I'm still not sure of the harm of using two resistors on ac side, since this would just be mimicking a lower voltage winding, that comes out of the PT secondary?

There's no harm in it. It's just less elegant as EEs like to say. Why use two resistors and two switches when one resistor and one switch will do just as well?

On your schematic I think the blue and red cross locations are incorrect, The red cross certainly is. There are four places you could put a single resistor. Where the 150R is now, where the standby switch is, in series with the CP11 wire and between the 'blob' where CR8,9 anodes connect and the 'blob' where C31 -ve should be connected to ground (missing on your scm). The last option is the preferred one because it has the resistor at a lower voltage.

[Jon434]

I can understand that there is a lack of elegance. It's just that since I'm in an earlier stage, I'm less worried about that (for now). The two switches and resistors, I thought that having one and one off could give added flexibility (two different settings) like none, medium and full (resistance) if this could be safe to do so (since there is no center tap), and also that the two resistors could share the load in terms of temperatures. If there was only one setting I liked/used, the other would be pointless and it'd be easy to simplify it and go with one and save some space. I thought that two settings could be better because at higher volumes, it's going to sag more. So at lower volume more resistance could be better, and at high volumes less resistance would be better.

In the pdf I got that schematic from, he talks of linking the bias resistors in with the switch (DPDT) so that it could have the bias 'track' with the switch. So that's another upside for only one resistor, the simplicity of wiring that in with one side of the switch being the power resistor and the other side being the bias resistors. So that when the switch is flicked, the bias current remains the same on both settings. Without that, the bias would drop some ma's and would need rebiasing or sound much 'colder'? And if you rebias at the sag setting, then flicking the switch back can put the tubes in danger of over current?

Thanks for the clarification in terms of the schematic Dave, helps a lot. Yeah the ground, is cropped out to the right of image. Since no one has mentioned the choke image, hopefully there is no error in that one. I guess you guys aren't keen on the choke switch idea for the same reason of the other resistor, having a switch with so much voltage running through it. Ideally, it'd be choke with no switch and just one switch on the lower voltage part between the two blobs. Maybe after experimenting, if I never used the choke switch, I could remove it and revert to only the choke for safer use in the long term.

[R.G.]

I'm more interested in just dropping voltage all together (the option of having it for the right situation), if I had the choice of two sets of primary wirings like a 240V and 200V I'd do that, but the PT's I have doesn't have them. As far as I know, using a resistor to do this (as opposed to other options) would still create some sag under high load regardless of whether it's before or after the rectifier, but since the amp is so modernly voiced and with high value caps, I figured this could be a good thing. I was originally going to use a voltage regulator circuit that are small pcb based (the ones which have pot, mosfet and diode etc) but this seemed a simpler easier way to implement/experiment with. The idea of simulating a tube rectifier isn't bad either, I guess I'm just trying to say that I'm not trying to simulate an exact rectifier tube type or anything like that.

OK.
Some comments on resistors as voltage dropping elements is in order then. Resistors have a voltage across them that varies directly with the current, according to Ohm's Law: V = I * R , or the voltage is equal to the current times the voltage. In the B+ of a tube amp, the voltage drop will be variable, being higher when the amp is putting out more power (* for class AB amplifiers, anyway) so this is how you get "sag", a voltage drop when the power goes up.

What you have described is wanting the voltage to go down, period, regardless of the power level, simply subtracting some voltage from B+. There are circuit elements that do that, the most common ones being zener diodes, or composite circuits using "amplified zener diodes" to withstand more voltage or power. These do *exactly* what you describe. For one circuit that does this, see
http://geofex.com/Article_Folders/mo...osfetfolly.htm , down at the very bottom of the article. You can use this to simply subtract some amount of voltage from the B+. You can switch it out, or use more than one circuit or more than one zener so you can switch different voltages. [Note: this is a power circuit, and the Zener (if that is what you use alone) or MOSFET (in the MOSFET version) must be able to get the heat out or it will burn up. ]

It's worth noting that it is *possible*, with careful design, to make a combination of two zener circuits, with diodes and such to make a "bidirectional zener" that will subtract off a fixed voltage from what goes into the rectifiers in BOTH polarities of the AC signal. But it requires competent electronic design to do this and have a reliable circuit. It's not as simple as wiring in a resistor, but it acts fundamentally different.

I'm still not sure of the harm of using two resistors on ac side, since this would just be mimicking a lower voltage winding, that comes out of the PT secondary?

It mimics a higher resistance winding, not a lower voltage one. They're different.

There was talk above, that having one resistor on, and one resistor off would create an inbalance, and then another reply saying that it'd be ok, since there is no center tap. I'm still not 100% sure on that, I could have misunderstood it, but using both on a dpdt switch and having both on or both off.. I'm not sure where the harm is other than higher voltage going through the resistor and switch (which for example, the standby switch already has).

There is a non-intuitive principle in electronics; for a series string of things, if you don't look at or use any of the voltages or currents in the middle of the string, the order of the parts in series does not matter. The voltages at the ends of the string and currents into and out of the string are the same, no matter what the order. For a single, non-CT winding, one resistor on either side is the same as two resistors of half the resistance, one on either end. But again, this introduces sag, not a lower voltage.

I'm not saying it's the ideal solution overall, just that it's easier to implement and/or remove if I don't like it.

It would be even easier to implement and remove changing out the volume knob for a different kind of knob. It, too, might not do what you want to do, but it would sure be easier to switch out.

Since no one criticised the choke wiring, hopefully that was wired up ok, as I don't want to do anything until it's 100% sure and planned out.

There is a big difference between no one saying it won't work and even one person saying it will work; and that is fundamentally at odds with your desire to have everything 100% sure. You're not going to get 100% sure from surfing the internet. Much of the stuff on the net is just plain wrong, and some of it is malicious. It is up to you to be able to filter out what's plausible from what's pure cr@p.

Reading your posts, I'm worried about your ability to do the modifications you're proposing without getting shocked, possibly severely. Is there a qualified tech you can take this to? I'd be willing to provide over-the-net advice to the tech, too.

[Jon434]

I could have chosen the easy way but that would have been sending the amp interstate. I chose to do it this way because I have the time and willingness and while it is a whole lot of learning and extra work, it can pay off in the long run.

I'm not worried about being shocked, I'm worried about damaging the amp rendering it all a waste of time or something negative instead of something positive. I do have concerns about some of this stuff but it's a concern of wanting to be safe rather than a fear of never attempting it. I am a guitar player and my hands are valuable. To have any damage for either the amp or myself makes the whole thing counter intuitive. Causing a new problem with the amp.. for example problem a - leads to problem with b - leads to problem with c - and getting out of hand. That's more what I am afraid of. I'm used to this sort of thing in other areas of life, I am familiar with the processes it's only that in this case, amplifiers come with the higher voltage risk and the knowledge is highly technical.

Step 1 is biasing the tubes in a safe way. Step 2 is putting in choke and new output transformer (no switches). Step 3 is adding a switch for the choke. Step 4 is adding connectors to any leads/wires so that the PCB's could be take out without having to desolder all the time (there's only a couple led wires that need this, at this stage). Step 5 is external biasing points for the amp, as well as a bias range switch if the potentiometer is out of range with EL34 vs 6L6 (for example). Step 6 is this power/sag resistor switch idea. Step 7 is looking at the voltages of the whole power supply as a whole and balancing them out how I like them. Step 8 is getting into the preamp side but not sure I'll ever want to do this. Step 9 is building a DIY amp kit from scratch, and again I'm not sure if I'd ever want to do this. My motivation for a lot of this comes from the power amp side, rather than the preamp side but maybe one day that would change. Each step has a different level of complication and safety and I wouldn't want to go to the next unless I felt comfortable. There are safe ways to do things and sloppy ways, and I've been trying to learn and plan out the safe ways. I see for example people making mods for some of this stuff and they use under guaged wire, no heat shrink and I'd be using all of those things. As well as current limiting bulb and with a multi meter on hand before touching anything. To take every precaution. There is no rush for any of this either, I wouldn't go onto the next step unless feeling comfortable and if this risk is to high I wouldn't do it at all. First part of any of these steps is to have the knowledge and plan it out. I appreciate those that help me to do this. The more knowledge, the more that it helps me to assess if it would be worth it or not or how to go about it. But I do want to attempt the early steps (especially) as I consider the experience and knowledge that would be gained from it to be valuable. That is the main motivation as opposed to the mods themselves.

[eschertron]

Even accomplished surgeons lose a patient once in a while!

If your primary concern is for personal safety, then by all means, continue to seek assistance here on this forum, and at your local library, among places. If your next pressing concern is to "do no harm" to the amps that you have, then my advice - based on my not knowing anything about your skill levels with meters, soldering, and electronic theory - is to hone your skills on other devices before attempting a modification on your prized amps. Even something as simple as practice soldering and unsoldering on a broken clock radio or VCR will provide you with experience to improve your technique and raise your confidence.

For myself, as a youngster with no familiarity with the science of heat transfer and no intuition about matching thermal power to the task, I burned several circuit boards and damaged many components before becoming equal to the challenge. From a cranky old guy to someone who seems eager to learn: if it's the experience you want, then start on something simple and relatively easy - in my mind a kit like a 5F1 - that you can build, then modify, to test the effects of your proposed changes. I know it sounds like drudge, and there are things that you can do to your existing amps that are non-scarring, but as R.G. pointed out, the things that are easy may not be the things that you want or need.

And that brings up my point. Drive your mods by credible and tested theory, not mere hypothesis. We have the benefit of standing on the shoulders of giants. In order to get the most of that benefit, we need to see where the giants have been (understand the theory of vacuum tube design that has existed for lifetimes) and see where they are headed (understand the theory as well as the ideas behind what has been done to amp design in the last few years/decades).

Hope that helps!

[Justin Thomas]

Agreed. Your concern for thehealth of the amps you currently have is screaming "go buy a gutted & trashed old junker head to learn on!" Anything with two trannies, a choke, two power tube sockets & three preamp tube sockets will do. Any added parts are a bonus. You burn it up, so what? The fact that schematics are unobtainium also yells: are these amps still under warranty? What are they, exactly? Maybe someone HAS the schematics & might share.

Pick up an old trashed Silverface Fender - plenty of them have already been hacked into mosquito-fuzz "Marshall" clones anyway. IOW, messed up. Put it back to stock, then tweak it till it totally falls apart. You can build any circuit into it you want - BEFORE you try it on the amps you really like. Bonus: probably a whole lot more room to work with. Added bonus: no SS relays/channel switching/reverb/whatever to worry about screwing/burning up. Simple rules for learning. Me? If I wanted to lower voltages, I'd kludge in a crappier PT, as long as the heater spec is good.

Don't potentially sacrifice an amp you like in order to learn on. My Concert & Bassman are off limits - I love them too much to risk screwing them up.

Justin

[Jon434]

Yep thanks, all good points. My skills in practice with those tools are at an beginner/intermediate level but I'm a quick learner and do have the time to do it right. The amps are Blackstar HT-60 (which I'm turning into a head) and Randall RD45H. A simpler way to put it would be a British/Marshall style modern amp and a Fender/Peavey/Mesa style modern amp. From what I've seen, the schematics available for these types of amps are the kind that techs do when attempting to repair (with no help of the company) but I haven't found any that are similar to mine. The closest I could find is a Blackstar Artisan which is a more vintage/Plexi/PTP style amp. I'm not complaining about Blackstar though as they were very helpful and encouraging when I emailed back and forward with them a few weeks ago. Asking the designer specific questions for me, which I didn't expect.

I do want to start off slow and slowly work up to the more advanced things, it's just that buying some of these things have long lead times, and wanted to have the parts on hand for when the right moment comes (if it ever comes), rather than decide to do something and then have to wait a while. That means trying to spec out the right parts (like the right types of switches, the right voltage ratings, the right wattage of resistors etc) and also the right guage of wire etc. I may not ever use some of them but wanted to get them all in advance anyway and can also sell them off if they go unused. That's the main reason why some of this stuff is premature, in terms of being ready to do it vs wanting the information/theory of what I'd need (in that future situation). Some of the changes are quite simple, like replacing an output transformer which has spade/crimp connectors and is relatively simple but others are more complicated. There are a lot of aspects to this stuff, like understanding the theory behind them, planning it to function correctly, safety and also having to do it in practice separate from a page (working with your hands etc) and they all have their own elements to it. In how they are separate disciplines.

I did take a part an old PC power supply recently and the caps still had voltage in them years later, so I drained those and plan to use that PCB as a practice tool, for soldering on and off of PCB's without damaging the traces to develop a confidence of working on PCB's directly. I hadn't considered the old radio or older tube amp idea, but that's a good idea. It's hard to replace the PT's on both of these amps because they have weirdly spec'ed secondaries. The B+ and heater supplies are standard but the other one is strange using pin connectors like you'd see on a PC motherboard.

Some of the changes I originally wanted to make are simple, one I forgot to mention is leds/12V fan with mini switches running off a pedal supply. Putting a choke in is relatively simple if the caps are drained and you do good soldering and can drill a hole in the chassis etc, but the more you learn, the more the ideas (possibilities) start to grow. I never expected to learn about amps in my lifetime, but it's been interesting to do that recently especially going back to the early roots of Fender and Marshall. It's interesting to learn even separate from ever touching your amp or modifying anything.

I'm not in a rush to get to those later stages, just open to the possibility of them.. if the earlier ones go well. One amp is under warranty, the other isn't but I'm not concerned about warranties, this might be a weird viewpoint I'm not sure but I've never been a fan of warranties. For many years I think I've only had to use a warranty once, and have never understood why people always want to buy an extra 3 years warranty for something (my uncle is like this). It's a hassle to have to send it out, if it's repaired (as opposed to replaced) there is a chance it will fail again and if you believe or have a fear that a product is going to fail, why buy it? Maybe I'm just lucky but to me most products are to a high standard these days it's even better if you make good choices. I've only had to use a warranty once in 10+ years. If it fails, I consider that it was a bad choice and next time, I should pick a better one. Once I know it isn't DOA, I consider it my responsibility, maybe that's why the idea of modifying it doesn't bother me. I've scratch built/modified PC's for many years, so maybe that's where that belief comes from. All of this is just my opinion, for other aspects of life they could be valuable and I understand the need for them.

[eschertron]

It's hard to replace the PT's on both of these amps because they have weirdly spec'ed secondaries. The B+ and heater supplies are standard but the other one is strange using pin connectors like you'd see on a PC motherboard.

Many of the newer amps will have secondaries for control circuitry, etc., and as you point out it is not standardized. That's part of the allure of the vintage equipment; standardized heaters (6.3 and 5) and a HV winding of your choice.

Some of the changes I originally wanted to make are simple. Putting a choke in is relatively simple if the caps are drained and you do good soldering and can drill a hole in the chassis etc, but the more you learn, the more the ideas (possibilities) start to grow. I never expected to learn about amps in my lifetime, but it's been interesting to do that recently especially going back to the early roots of Fender and Marshall. It's interesting to learn even separate from ever touching your amp or modifying anything.

I'm glad you've become acquainted with the legacy. Part of the "standing on shoulders" that I mentioned. Study the evolution of these amps to see when the chokes (for example) were present; when they weren't. This forum and others can help decipher the why about the designs and help you decide if there are certain design modifications that you will like in your amps. My problem (and one that I think is common) is that I hear about something I don't have, and want it 'just to see' if I'd like it. This leads to a lot of unnecessary work (if referring to mods), or to having a lot of stuff that I don't want or need. Better to have a clear understanding of what the thing will accomplish - by knowing what amps are out there that are like yours but have the 'thing', being able to hear the difference between an amp with and without the 'thing' - by being able to minimize the effect of any other engineering variables.

Having a good idea that you'll like what you've worked so hard for makes the success that much sweeter, too!

[Jon434]

Yep, makes a lot of sense.