Power resistor/sag switch..

[Jon434]

I'm new to amp electronics but have been learning a lot in the last few months and have a few ideas I'd like to try on my amps. They are modern voiced 50W push pull PCB based amps with solid state bridge rectifiers and I'd like to put in a power reduction/sag resistors with a switch (to go back and forth) in the power supply. I'd like to reduce B+ by 50V or more as well as reduce the response/general feel of the amp to be saggier and more vintage like. Similar to imitating the response of a tube recitifier. But also reducing the voltage in general and also reducing the wattage output of the tubes. Sort of like a half power switch.

Since the amps are PCB based, it'd be hard (and more risky for someone new at this like me) to go cutting traces or bridging off of the PCB board, I'd prefer to do this before the rectifier with the crimp wires rather than on the PCB. Trying to go between the rectifier and first cap would be much more complicated and harder to reverse.

So instead of going out of the standby switch into the rectifier, I want to go from the standby switch into the resistor and then into the PCB and rectifier. So..

------------

PT secondary > standby switch > resistor (with bypass switch) > rectifier > 1st two filter caps

instead of

PT secondary > standby switch > rectifier > 1st two filter caps

------------

There are obviously two taps coming out of the PT secondary and going into the standby switch and into the rectifier, so I'd need to do all of this twice. That'd mean two resistors, two switches etc. The resistors I want to use are 25W wirewound 50, 100 or 200 ohm. With the resistors bolted to the chassis to dissipate heat, with a small PC fan exhausting air through the top of the cabinet box.

With SPDT On/On switches rated 250V/10A, would these be overkill or good enough? I want to do all of the connections with crimp connectors (where possible) and these switches have those connections. I'd want to do the switching (which puts the highest stress on the switch) when in standby so maybe this wouldn't matter, but better to be safe than sorry. There are probably smaller switches that would work, but I find it hard to find them with the right crimp connections to wire them up.

I just wanted to run this idea by the more experienced in this area incase I've made any mistakes or there are any series flaws in this idea. And also ask a question about the rectification and use of these two switches.

This is a question I've never seen asked before in relation to sag resistors, so I guess it's a good one to ask. With both resistors bypassed, it should be 100% the same as stock? And with both resistors in the circuit it'd be reduced wattage/B+ voltage and current (under load) with the possiblility of running the amp at half power (depending on the correct ohms of the resistors)? So both switches on = 50% output wattage (within reason).

But what about if one switch was on and one switch was off? As far as I can tell both amps don't have a center tap on the B+ and if they were center tapped to ground, I'm not sure if that would affect this.

I was picturing the AC signal and then the rectified signal and one wave would be bigger than the other, which would be similar to when a power transformer puts out unequal voltage in it's secondary winding? The ripple after rectification would be a different shape with every second wave having more ripple. By the time this was filtered down by the filter caps and dropping resistors (or choke).. do you think this could be a workable setting for 75% power? Would the DC voltage end up being smoothed out eventually anyway? Or would this create extra noise or problems? Could it stress the filter caps more because the ripple would be bigger?

It could be nice if this worked I guess, for 50%/75%/100% power/sag option. These amps are both fixed bias and I realise that the bias would be altered a lot while doing this (and then when rebiasing that'd change the plate voltage again and it'd have to balance out).. but if this worked well, I'd also be thinking of doing external biasing probes and pots.

I'm new to all of this stuff but just want to make sure all bases are covered. If anyone has any ideas for what could be improved or any problems that this would cause it'd be much appreciated.

[pdf64]

Do these amps have bi-phase full wave, or full wave bridge, rectifiers? Schematics?
The Valve Wizard
The Valve Wizard

[Jon434]

No schematics are available (Randall and Blackstar) so all of my knowledge is based off of learning and looking at the pcbs and amps layout directly. I can't find a center tap on either PT, so I guess they must be full wave bridge?

[Tooboob]

A 50V drop will model some tube rectifiers. You need to ask yourself if this is responsible for some sonic magic. It certainly reduces headroom, but the decision to go with an SS rectifier might have included a reduced transformer output voltage as well. Folks have good results simply using a power resistor. It doesn't do much until you crank the amp, if it's too low in value it has little effect, and if it's too high, you can get motorboaty problems. You're certainly looking in the right resistance range.

To get a 50V drop with just a resistor, you'll probably find that the resistor is too high valued (won't sound good). It's better to use a zener solution for part of the voltage drop, but I question the utility, unless you're trying to retrofit an ss rectifier into a currently tube-based solution, with minimal tonal impact and without stressing the tubes more. Weberspeakers.com and others sell solid-state tube rectifier replacements.

Understand that if you drop the voltage at the bridge/rectifier location, all the supply voltages in the amp drop. The preamp output will show the effects too.

Also note that if you drop the supply voltage 12%, your bias voltage to the power tubes will also show that error (it probably won't change to track). With the resistor(s) engaged, the power tubes will run cooler - with a bit lower bias current.

Because of all this, trying to achieve really significant power reduction through this technique isn't simple. You can catch the supply to the output transformer and drop voltage there, but voltage and current are high, you need an active circuit, and the bias voltage will need to track.

The first tube amps had comparatively low value caps, chokes, and rectifier tubes. The sonic effect of your resistors will depend somewhat on the values of the high-voltage storage caps.

But yeah, a couple of power resistors and a DPDT switch will give you a bit of sag. It's a good idea to use high-voltage wire and cover exposed stuff with heat-shrink tubing. It may take two layers of heat-shrink to achieve a 500V rating.

For related mods, you can look into switching the power tubes to triode mode, adding a choke, or switching off or pulling two power tubes in a four-tube amp (change the impedance switch too, at least if you're anal).

If you put the resistor just before the cap(s), and use the switch to short it, you get a clean SPST solution with a single resistor.

Sounds like you're leery. I've never met anybody who claimed to have been killed by a tube amp, but it can happen. Maybe you'll only suffer nerve damage. Google "tube amp safety", etc., and read everything until it's repeating so much that it gets boring. This one's fun and useful, but incomplete: Safety Tips For Working On Tube Amplifiers

One hand in your pocket, discharge, and don't get nervous or you'll die. Respect, not fear. Happiness is a clean bench.

[Jon434]

Thanks for those hints, it helps a lot.

Definitely not taking it lightly, both in terms of the safety of the amp and also myself. I'll need to read it again and again until it sinks in but I think the experience of it does help, once you've gained it (without hurting yourself). I saw a good video a few weeks ago about a transformer guru who only works alone because he doesn't like to be distracted. It's easy to focus on your own but when others are involved you start multi tasking and becoming careless, things like that are on my mind a bit as well.

Understand that if you drop the voltage at the bridge/rectifier location, all the supply voltages in the amp drop. The preamp output will show the effects too.

I guess you could say I'm trying to mimic the effect of a variac and turning down the volts from 120 to 90.. similar to the way EVH did back int he day with the Plexi. Even though it's 240V 50hz for me here. Trying to immitate that (to some extent) but also knowing that using a resistor won't be the same thing but it could be something similar.

Soldano released a Hot Rod 25 recently, which was basically a 50W amp with 6L6's and using similar transformer sizes (large).. but dropping the plate voltage down using lower taps on the secondary of the PT. Dropping the plate voltage for the 6L6's from 450-500 down to below 400. With what you said above, I guess he would have factored in and tweaked screen/preamp voltages or other aspects to make sure all is good. But his way is also not easily changeable and his amps aren't cheap. And I do like the high voltage high headroom sort of sound as well.

So I guess I'd like to attempt something that wouldn't be as good, but would also be changeable back and forth. Sort of like a voicing switch (for more sag) or similar to coil tapping a pickup on the guitar. Just another voice that you can use for the right situation. If it didn't work out well, I could always just leave it off (or remove everything and revert back to stock).

On that note, I'd also like to have a switch going between resistor and choke which in this case is a 5W 270 ohm ceramic resistor between the B+ and screen supplies. I'm not sure how much each would interact with each other, (lowering voltage and how it would affect the choke) but if they were both switchable it would be easier to experiment. The ironic thing is all of this might not be noticeable as much at lower volumes after all of this trouble but nice to have those options ready for the right moment I guess. And an experience to learn some things.

The first tube amps had comparatively low value caps, chokes, and rectifier tubes. The sonic effect of your resistors will depend somewhat on the values of the high-voltage storage caps.

Both of these amps are very modern voiced, sterile if you are used to playing a Fender Tweed or Plexi I'd guess. I guess I like the idea of bringing some vintage flavour to them, but yeah.

The caps are 350V 330uf in sets of two, so I guess that'd mean they are in series for 700V 165uf? The B+ supply is 330V or more for each tap. That's for the Randall (6L6). For the Blackstar, the values and caps are similar but slightly different and an overall higher B+ voltage and higher plate voltage with the EL34's. Plate voltage is 500V for the Blackstar I think, with the EL34's. I haven't checked the voltages of my own amps personally though just seen others reference them. Wish these companies would give out schematics.

But yeah, a couple of power resistors and a DPDT switch will give you a bit of sag. It's a good idea to use high-voltage wire and cover exposed stuff with heat-shrink tubing. It may take two layers of heat-shrink to achieve a 500V rating.

By a weird coincidence I bought 10m of 18awg, the same guage that's used on the transformer taps and also some heat shrink tubing a few days earlier for something else, so I guess it's good to have them on hand.

Thanks for your advice. Hopefully no nerve damage.

[Jon434]

Here's a schematic of the part I'm not sure about. It's not my exact amp but simplified to be very similar. It confuses me to think what would happen to the rectified signal if one resistor was on, and the other was off. And if this would increase noise or ripple further down the supply. Or if it'd get smoothed out by the caps/choke/resistors. Having both resistors on or off seems like it would work ok since it would be equal on both sides, but not sure about having only one on.. and if this would be a good thing or not. I know with PT primaries it's bad if they are unequal but this inbalance is on the secondary side. Would it be averaged out to 315 if one was off? Would be it 300 on one side and 330 on the other, or would the rectifier see it as two sets of 315? Would this stress the caps or PT?

Here is the schematic.



The bottom part is the part I'm not sure of. The reason I'm trying to find to plan ahead for this is because I'd want to choose different resistor values for if I'd want to run both resistors at the same time, or having the option to just use one.

[eschertron]

The second drawing is a B.A.D. idea. You won't get a significant voltage reduction from stock with this, as each other half cycle will charge the reservoir cap up to the full voltage. what will happen is that you will stress the PT (providing essentially all the power from half the cycle), and stressing the reservoir cap too. Don't do it.

Look at where you have the fuse in the schematic. perhaps you have an opportunity there to jumper off the PCB there to a piggyback board with a resistor [and a fuse!] and then back to the PCB. you could experiment with different value resistors without making any permanent changes to the amp as it is.

[R.G.]

In general, if you are mucking with reducing B+ or sagging it or something similar, it's safer (in the sense of you not dying) to do this on the negative side of the wiring headed back to the rectifiers. This removes the majority of B+ from the stuff you're working on. You might think about sticking your sag resistors between the rectifier negative (which is the minus side of a bridge, or the center tap of a CT winding) and the first filter cap negative. It has the same effect as sticking it on the top side of the first filter cap, but is a few hundred volts nearer ground.

[Jon434]

Thanks guys, something to think about. It makes sense that it'd put more stress on the PT/caps to have it unbalanced on one side like that. So I guess it needs to be both resistors at once or just once after rectification.

I like the fuse idea, I think it'd work well on one of the amps (which is more similar to the schematic), but harder to implement on the other where the fuse/PCB layout is a bit different.

What are the dangers to doing this, after the caps are drained and all connections unplugged? Would it be only if it was wired wrong? Or if there was a short? Could the switch become dangerous to touch? If it was wired correctly and there was heat shrink tubing on all of the solder/crimp connections, are there any other dangers? Above, do you mean safer when working on it (putting this stuff in) or do you mean after it's already been wired in and is working?

[Justin Thomas]

So, might we be able to use a "Sag Pot" in the negative side, and twiddle it halfway through a song? I'm just running on P=IV, imagining the current stays the same but a few less 100V = less power, so the pot won't die a gruesome death? I just know most pots are not high-wattage items. Also opens the door to some creative knob names and ranges...

Justin

[Dave H]

Thanks guys, something to think about. It makes sense that it'd put more stress on the PT/caps to have it unbalanced on one side like that. So I guess it needs to be both resistors at once or just once after rectification.

The two 150R resistors are in series with the transformer winding. It only needs one 300R resistor and one switch. I'd put the 300R resistor between the bridge -ve and first filter cap -ve (ground) as R.G. said. The standby switch also could be single pole.

You are wasting the extra smoothing of the choke by having the 270R shorted across it. The choke could be 10 times 270R at 120Hz. I don't think it's a good idea to open circuit the choke when it DC flowing through it.

[Dave H]

The second drawing is a B.A.D. idea. You won't get a significant voltage reduction from stock with this, as each other half cycle will charge the reservoir cap up to the full voltage. what will happen is that you will stress the PT (providing essentially all the power from half the cycle), and stressing the reservoir cap too. Don't do it.

That would be true for a centre tap two diode full wave rectifier but this is a full wave bridge with no centre tap. It will be fine.

[eschertron]

That would be true for a centre tap two diode full wave rectifier but this is a full wave bridge with no centre tap. It will be fine.

I'll need to prove that to myself, but what you said makes sense, since the voltage and current are referenced only through the two wires of the transformer. Thanks for the catch!

[Jon434]

I don't know enough about this experience wise to say either way.. which is why I want to learn/experiment with switches and make topics like this and find out. Over time, you figure out what you like but it's a long road to get to that point. It was like that a bit for scales/chords and how to setup the guitar the way I like it but years later it's all easy now. I hope that one day, this stuff could be like that.

I don't know the difference (in practice) of a center tapped PT vs one that doesn't have one, only that the latter is cheaper to make. I've seen people say for example when dropping the heater voltage you can just put a resistor on one side of the 6.3V secondary, but I didn't know if this applies to the higher voltage. I don't understand enough about how the center tap influences the PT, so that it would be bad in one situation, but ok in another.

You are wasting the extra smoothing of the choke by having the 270R shorted across it. The choke could be 10 times 270R at 120Hz. I don't think it's a good idea to open circuit the choke when it DC flowing through it.

I've seen suggested by some to use a choke and resistor via a switch, but have no experience with it personally. Are you saying that it's pointless to have both, or just that it's not a good idea, with the way that I've wired it in that schematic? Would there be a better way to wire it? There is also the third option of choke + resistor and tbh I really wouldn't know which one I would like unless I tried them (with a switch) to find out. If I did that, it'd only be to bring the screen voltages back to the pre choke value. Because a choke can let more voltage through, and going over 450V for 6L6 isn't recommended?

What I'm thinking is the sag resistor + the choke resistor = lower voltages and saggier for more vintage tone but also a bit noiser. Sag resistor bypassed and choke = high headroom, high voltages and better bass response.. more modern tone but also less noise. Some combination between the two.. not sure. I have weeks to figure this stuff out and won't be touching the amp until 100% sure of everything, just needing to order the parts in advance, as they aren't available locally. The 10m of awg 600V I ordered might take another 3 weeks to get here. But separate from that, I find it interesting to learn this stuff. So thanks to those for sharing the info. Much appreciated.

[Dave H]

...I've seen suggested by some to use a choke and resistor via a switch, but have no experience with it personally. Are you saying that it's pointless to have both, or just that it's not a good idea, with the way that I've wired it in that schematic? Would there be a better way to wire it?

You could wire it with a changeover switch to select either choke or resistor, that way the resistor isn't across the choke in 'choke' mode.

Unlike the B+ winding centre tap the heater winding centre tap doesn't have current flowing in it. It is only used to reference the heaters to ground so you can get away with using a single dropping resistor.