Input Supply Voltage and Bias?

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revolver1
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Input Supply Voltage and Bias?

Post by revolver1 » Fri Apr 09, 2021 1:56 pm

Hi all,

I built a 69 Super Lead probably back around 2010, I lost that amp so a few years later I got back into building and have got my self a new collection going.

I think back to that amp and the new ones don't sag the way the first one I built did. I'm using the same trannies but there are some differences that should be mentioned to how I'm setting it up at the moment.

One of the amps I built is essentially the same as the first one I built , only differences being this time I used 1/2watt and 1watt resistors instead of 1w and 2w. Also as I will explain im on the 230v input tap and before I was on the 240v tap.

I also used to use 10 gauge strings back then and now I'm using 9's so I guess that must have some impact on getting it to sag but....

I was reading some threads about Variacs and some of the guys were saying they bias the amp up on wall voltage at 70% then turn the Variac down with out re-biasing to around 90vac (USA) to get the sag.

I live in London and the voltages can get a little out of hand here with a wide variance, still with in regulations but I've seen it as low as 234Vac and as high as 254Vac. With that in mind what I used to do was get up really early before the Underground and Over Ground trains would start up to bias at a steady worse case voltage because I was paranoid of blowing an out put valve and taking out the P.T. I seem to remember it being around 249Vac at that time of the morning but then most of the time I'd be using the amp in the day it would be back around 240 - 244Vac.

I'm wondering what you all think? Would this be a similar effect to what is happening when the guys are using a Variac? What I liked about it was the sag wasn't really all that noticeable until I kicked in some kind of boost then it would fatten out and sag and was really harmonically rich which it just doesn't seem to do now.

It's a bit extravagant I know but I had a great job at the time and I knew I'd probably never financially have the opportunity again so because of the power issues I picked up a Hewlett Packard laboratory test equipment power supply. It's like the Kikusui unit you've probably seen on those rig rundowns of AC/DC, Billy Gibbons and Joe Bonnamassa. So you can completely set and controle the input voltage supply characteristics of the amp rig. I got the Hewlett Packard version because it was a refurbished unit with a guarantee of a line that was going to be discontinued so it was going considerably cheaper but essentially does exactly the same as the Kikusui and can be refurbed in the future.

I picked up a Furman Surge protector of ebay and boxed it all up in a shock proof flight case, made up all the cabling and it really does the job kicking out totally clean stable power.

So I've sort of settled on running it at 234Vac 50Hz mostly because of voltage issues with 3 other amps I built.

I did a Hiwatt DR504 and ended up using a Merc P.T but the voltages out of it are ridiculous so I set it to the 240V input tap and running it at 234Vac the heaters are good and the plates are around 500v. Running EH 6CA7 sounds great.

I also built a TrainWreck Rocket using an Edcor P.T but its a 230vac unit. I had to put in some big aluminium heat sink resistors bolted to the chassis in the heater lines to bring the voltage down a touch for the heaters and pilot lamp and adjust the cathode resistor on the out put tubes to adjust the bias for 11.4w dissipation on the el84's but it is working well on 234Vac. It was on the brink before but this brings it back in line and is the compromise with the Marshall's.

So to the Marshall's, if I run the Marshalls on 234Vac on the 240v tap the heater voltages are a bit too low and the pilot lamp doesn't work but if i put them on the 230V tap they are spot on. I get 6.4Vac at the start of the chain and 6.2Vac at the end so its slap bang in the middle and the pilot running off the 120v tap works fine.

It would work at 236Vac on the 240v tap but that would be too much for the Hiwatt and Rocket so it's a compromise that's not ideal. With 234Vac input on the 230V tap it's an over voltage of 4v which probably won't help much trying to achieve the sag but straingely all the voltages seem bang on throughout the amps and the only other option would be to change the P.T's on the other two amps which one day I may well do, work permitting.

Back to the original point would it make any sense to set a voltage of say 239Vac and bias the Marshall's then operate the amps on 234Vac? Would that get the sag effect I was getting with my first plexi?

What do you think?

danman
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Re: Input Supply Voltage and Bias?

Post by danman » Fri Apr 09, 2021 7:06 pm

Sag would be a result of the power supply running out of juice when pushing the amp hard. PT's with a higher ma rating won't sag as much as PT's with a lower rating because they are able to supply more juice when the power supply requires it. What you are describing is more about the amp seeing a lower voltage throughout the entire circuit. Everything from the preamp to the power amp will see lower voltages if you lower the wall voltage, and this in turn can make the amp a bit spongier and browner sounding. You can also mimic this effect without lowering the wall voltage by simply raising the value of the preamp dropping resistors in the b+ rail. Many amps came with a single 10k, two 10k's or a 10k and 8.2k resistor for the preamp dropper. Raising this value up slightly will lower the voltages throughout the preamp while the rest of the amp is still seeing normal voltages. Lowering the filter cap value on the screen node also gives you that saggy feel.

The differences that you are experiencing between the two amps is likely due to running one on the 240 tap and another on the 230 tap. The 240 tap will cause lower filament voltages and lower b+ voltages throughout the amp. This alone will cause the saggier feel that you experienced. Filter cap values can also vary quite a bit from their stated spec so they may also play a role.

revolver1
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Re: Input Supply Voltage and Bias?

Post by revolver1 » Sat Apr 10, 2021 3:47 am

Hi Danman, thanks for that. It gives me alot to think about, it's actually great just to have a chat with someone about it.

I actually have my 1987 on the bench at the moment and I'm going to swap out one of the 8.2k's for a 10k to see if that tips it in the right direction. This is going to sound pretty stupid but I managed to get a full set of Iskras for that amp that I was extremely proud of. The only 10k I have is a Piher. I was over come with a bizarre pang of guilt over swapping it out so its sat on the bench for two days. I need to get over myself and give it a shot.

I'm going to try cool the bias just a bit and see what happens.

The other thing is to see how much I can bump the input voltage on the Rocket to see if I can get enough voltage to switch the plexis up to the 240v tap. I'm mostly concerned with the tube recto on that one but let's see.

Lots to think about and try, suppose I've just gotta get in there and get my hands dirty.

Thanks DM.

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Re: Input Supply Voltage and Bias?

Post by danman » Sat Apr 10, 2021 8:04 pm

From your post, it sounds like you may have two 8.2k's resistors in series which gives you around 16.4k of resistance leading to the preamp section. Switching to a 10k may not change the voltage enough to notice a difference. try to get the reistance up to 20-22k with either a single resistor (2-3 watt is fine) or two separate resistors that equal 20-22k. This should give enough of a voltage drop to change the sound and feel a bit.

Another spot you can use to add some squish and sag is the screen grid resistor on the power tube sockets. Try something around 2k on each socket and see if you like the change.

revolver1
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Re: Input Supply Voltage and Bias?

Post by revolver1 » Sun Apr 11, 2021 5:10 am

Gotta say a big thanks Danman. Yesterday I had the 1987 on the bench and thanks to your suggestion I took a quick look at the filter cans and sure enough I'd completely forgotten I built it with a 50/50 can on the P.I/Screens.

So I've swapped that out for a 32/32uf. So now its 100uf mains and 32/32 preamp 32/32 P.I/ Screens. I think that will help a bit.

Your right, I had 2x 8.2k droppers in there, I've pulled one and replaced it with a 10k. Like you said it may not be enough of a difference but I'll test it first and maybe pull the other 8.2k out for another 10k.

Today I want to bump my supply voltage up to 236Vac, switch the amp up to the 240Vac tap and see where my heater voltages are at. If it's in spec I'll re bias and see what it's like.

I've got a good feeling I'm headed down the right track now.

I did however change the .0022 cap out for a .0047 because I didn't think it had enough bottom end, not too sure if that will work now with the 32/32uf but let's see.

After that I need to take a look at the Rocket and see how that sits on the new supply voltage. I think that P.T is the fly in the ointment and I probably need to swap it out for something more suitable.

Total PITA but at least now I'm looking at it in a different way, trying to make a compromise is a total waste of time I'd rather have my Marshall's running at their best and do what ever I need to do to fix the other amps at a later date even if it means sourcing new P.T's.

I'll let you know how I get on, just gotta wait for a day that the neighbour goes out to give it a blast.

revolver1
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Re: Input Supply Voltage and Bias?

Post by revolver1 » Sun Apr 11, 2021 12:32 pm

So that was an interesting day. Got the 1987 hooked up on 236vac input voltage on the 240v tap.

The heaters came in at 6.16vac at V5 and 6.10vac at V1. So V1 is just above the -5% cut off point so I guess that's O.K.

Previously the plates were at 237vdc and now after settling for an hour they are at 419vdc biased at 65%. So that's quite a difference.

Really need to play it to reach a verdict but I'd be suprisee if it doesn't soften up alot more like this.

Need to see where the Rocket sits at tomorrow, it needs to be played but I'm finding this all really quite interesting.

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Re: Input Supply Voltage and Bias?

Post by danman » Sun Apr 11, 2021 6:26 pm

Are you saying the voltage at the v1 plates went from 237vdc to 419vdc after changing the resistor value. Something isn't right because the voltage reading should have come down throughout the entire preamp if you installed a larger resistor value.

revolver1
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Re: Input Supply Voltage and Bias?

Post by revolver1 » Mon Apr 12, 2021 3:41 am

Sorry no, I meant the output valve plates.

The result is with 236vac input voltage on the 240v tap the voltages through out the amp are as low as they can be without the heater voltages dropping below the -5% threshold.

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Re: Input Supply Voltage and Bias?

Post by danman » Mon Apr 12, 2021 6:36 pm

Gotcha.

revolver1
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Re: Input Supply Voltage and Bias?

Post by revolver1 » Fri May 21, 2021 12:39 pm

Finally got a chance to give it a blast today. I tried the Rocket on 237vac but it was just too hot so I'm back running the Marshall's on the 230v taps and running them at 234vac. The more I think about it when I used to run them on the wall voltage on the 240v tap it wouldn't be uncommon to see 244vac out of the wall so I think running them at 234vac on the 230v taps has got to be much the same. It's not ideally how I'd really like to do it but at least I can run all my amps like this.

So the good news is it's not the input voltage but the filtering that wasn't doing it for me.
The two amps that were bothering me was my 1987 and my Superbass. I'll address what I did to the Superbass in the other thread I have going in this section. So this is about the 1987.

I am an idiot because I had forgotten my original plan was to try some different filtering configurations but I kinda forgot all about it. Then thinking it over I hadn't considered how stiff the 50w filtering is with 100uf on the mains.

I put in dual 32uf cans all round so 64uf mains, 32uf screens, 32uf P.I and 32/32uf on the preamp. The only other difference is a .0047 cap in place of the .0022 to let a touch more bottom end through.

Gotta say I'm really liking it. The .0047 sounds pretty good, it hasn't totally lost the brightness so it still cuts but it's just slightly more woody and sounds a little more classic. All my pedals sound a lot more naturall, they seem to blend better and sound more organic rather than as before they seemed to thin things out and sound harsh.

The amp was totally silent with 100uf on the mains and now there is the tiniest bit of noise with 64uf but you really have to listen close up to the speakers to notice it so I'm very happy with that. So all in all I'm really pleased with the results. It blooms in a more musical way having lost the one dimensional hard edge, its altogether more pleasing to play.

Great result. :thumbsup:

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