Input Supply Voltage and Bias?
Posted: 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?
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?