Been restoring guitar amps from 1960s to 1980s, and noticed how the oldest Marshalls, Laneys, Oranges, Hiwatts all ran B+ at near to 600V for EL34-equipped models.
By the mid-1970s, many of these had dropped the B+ to 460..480V. Many opinions favour the older designs for sound, so why did they drop the supply tension? Was it just to cheap out on the trafo insulation materials?
By the mid-1970s, many of these had dropped the B+ to 460..480V. Many opinions favour the older designs for sound, so why did they drop the supply tension? Was it just to cheap out on the trafo insulation materials?
In essence, it was about the tubes available.
Early EL34s were rated at up to EIGHT HUNDRED volts B+.
However, later-production tubes (Russian/Chinese, primarily- but even some of the remaining US manufacturers in the late '70s and beyond, as tubes became a lower priority compared to solid-state) had a tendency to go up in sparks, at 600v or higher.
So, many amps were dialed back, to 500v or less.
It's about not having warranty claims, from tube issues...
Regards,
Gordon.
Early EL34s were rated at up to EIGHT HUNDRED volts B+.
However, later-production tubes (Russian/Chinese, primarily- but even some of the remaining US manufacturers in the late '70s and beyond, as tubes became a lower priority compared to solid-state) had a tendency to go up in sparks, at 600v or higher.
So, many amps were dialed back, to 500v or less.
It's about not having warranty claims, from tube issues...
Regards,
Gordon.
EL34 B+
Timely thread for me. So guys asking for opinions here.
I am currently restoring a Bedrock 1200 which runs a pair of EL34 and has the plate current at 590 UNLOADED [no tubes]
Anyone have opinions on the current JJEL34 or KT77 re reliability at these voltages [ I know it will come down a bit when "loaded]
Have been trying to decide whether to replace the overheated screen supply 1k 15watt with a 25watt wire wound, move the 470ohm 5watt screen resistors up a bit to say 680ohm and leave the B+ where it is or to rework the PS to Lower the B+.
I would prefer the former if the current tubes will take it.
THOUGHTS AND ADVICE APPRECIATED
BTW Bedrocks were made in the 90s up in New Hampshire not much info on them out there and I suspect that reliability may have been their demise although they seem to be well built and thought out with the possible exce[ption of "pushing the envelope" literally. This one fell into my lap and I'd really like to get it up and running.
Mike
Timely thread for me. So guys asking for opinions here.
I am currently restoring a Bedrock 1200 which runs a pair of EL34 and has the plate current at 590 UNLOADED [no tubes]
Anyone have opinions on the current JJEL34 or KT77 re reliability at these voltages [ I know it will come down a bit when "loaded]
Have been trying to decide whether to replace the overheated screen supply 1k 15watt with a 25watt wire wound, move the 470ohm 5watt screen resistors up a bit to say 680ohm and leave the B+ where it is or to rework the PS to Lower the B+.
I would prefer the former if the current tubes will take it.
THOUGHTS AND ADVICE APPRECIATED
BTW Bedrocks were made in the 90s up in New Hampshire not much info on them out there and I suspect that reliability may have been their demise although they seem to be well built and thought out with the possible exce[ption of "pushing the envelope" literally. This one fell into my lap and I'd really like to get it up and running.
Mike
JJ KT77 will take 700V easily, so I would vote for them. And go for 1K 5W screen resistors for more safety.
Surely replace that overheated screen resistor,bumping it up to a higher wattage would be a good idea.
The cathode bias,I dunno..hard to say without seeing a schematic.
The cathode bias,I dunno..hard to say without seeing a schematic.
Plate voltage has never really been the problem with blowing the tubes... It's the SCREEN voltage being too high....
Mike...If you have a voltage doubler circuit...take the choke lead that feeds the screen supply and stick it in the middle of the two voltage doubler caps.... This will knock down the screen voltage to the 300V range..... tubes will be mucho happier...and don't worry about the high plate voltage it won't do any harm....
You could try 1K 5W resistor on each screen to keep the screen grid current running cool....
Chris
Mike...If you have a voltage doubler circuit...take the choke lead that feeds the screen supply and stick it in the middle of the two voltage doubler caps.... This will knock down the screen voltage to the 300V range..... tubes will be mucho happier...and don't worry about the high plate voltage it won't do any harm....
You could try 1K 5W resistor on each screen to keep the screen grid current running cool....
Chris
Controlling the Screen voltage
I'd heard that ScreenGrids were weak on the recent Pentode & Beam Tetrode production.
The 1960s guitar amps put only a choke & a resistor between the 600V B+ and the screengrid, so this terminal gets about 570V. Even the mighty Mullard EL34 is rated at only Vg2=500V max, so sparks must certainly be expected from the weaker designed valves.
I offer my customers a reduced screen voltage, implemented with a voltage divider and (900V) power-FET source-follower. 400V Vg2 seems a lot kinder to the EL34s, and they sound really good like this too. Only downside is that the gain is reduced because lower Vg2 means lower gm. But you can usually fix this by adding a capacitor to any unbypassed preamp cathodes - there's usually at least one like this.
If you add 10uF/450V at the FET gate, you can do without the power supply choke, too - a FET gyrator gives excellent hum rejection at the screengrids.
Marshalls usually use 1K 5W at G2, combined with 5,6K at G1. Makes sense to follow that - even with 400V on the screengrids, I believe.
But if modern EL34s can be run OK at 600V/(400V G2), what about modern transformers? Modern Marshalls seem to have a less than perfekt record of trafo reliability. Anyone got experience running replacement manufacturers' trafos at 600V+?
I'd heard that ScreenGrids were weak on the recent Pentode & Beam Tetrode production.
The 1960s guitar amps put only a choke & a resistor between the 600V B+ and the screengrid, so this terminal gets about 570V. Even the mighty Mullard EL34 is rated at only Vg2=500V max, so sparks must certainly be expected from the weaker designed valves.
I offer my customers a reduced screen voltage, implemented with a voltage divider and (900V) power-FET source-follower. 400V Vg2 seems a lot kinder to the EL34s, and they sound really good like this too. Only downside is that the gain is reduced because lower Vg2 means lower gm. But you can usually fix this by adding a capacitor to any unbypassed preamp cathodes - there's usually at least one like this.
If you add 10uF/450V at the FET gate, you can do without the power supply choke, too - a FET gyrator gives excellent hum rejection at the screengrids.
Marshalls usually use 1K 5W at G2, combined with 5,6K at G1. Makes sense to follow that - even with 400V on the screengrids, I believe.
But if modern EL34s can be run OK at 600V/(400V G2), what about modern transformers? Modern Marshalls seem to have a less than perfekt record of trafo reliability. Anyone got experience running replacement manufacturers' trafos at 600V+?
Thanks Guys, exactly the kind of info and iseas I was looking for.
Will go with the 1k 5w on the screen.
I have a 1k 25w to replace the supply resistor.
There is a bias tap and supply in this unit [sorry no schematic available, this is one of the earlier PTP on turret board ones. There is a schematic out there for the later PCB ones but there are some subtle differences]
I will be checking the bias when I get things rolling. There is a trimmer there [not individual tube adjust, but adjustable for both tubes] and I may install individual bias adjust. I memory serves me it was measured at about -57vdc [unloaded] when I did my original tests.
I am going to try to lower the screen supply voltage and appreciate your suggestions... Think adding the FET is slightly beyond my poor analog psyche though so I will explore the choke modifications suggested.
I do have a set of the JJ EL-34L on hand and at the price may go ahead and give them a try after repairs and mods..upgrade to the 77s if necessary.
Thanks again...keep it comming
Mike
Will go with the 1k 5w on the screen.
I have a 1k 25w to replace the supply resistor.
There is a bias tap and supply in this unit [sorry no schematic available, this is one of the earlier PTP on turret board ones. There is a schematic out there for the later PCB ones but there are some subtle differences]
I will be checking the bias when I get things rolling. There is a trimmer there [not individual tube adjust, but adjustable for both tubes] and I may install individual bias adjust. I memory serves me it was measured at about -57vdc [unloaded] when I did my original tests.
I am going to try to lower the screen supply voltage and appreciate your suggestions... Think adding the FET is slightly beyond my poor analog psyche though so I will explore the choke modifications suggested.
I do have a set of the JJ EL-34L on hand and at the price may go ahead and give them a try after repairs and mods..upgrade to the 77s if necessary.
Thanks again...keep it comming
Mike
JandG said:
😀
I think that Marshall was the original cause for the oft-repeated phrase: "It sounded really GREAT, right before it blew up!"...
And the folks are correct- it's not plate voltage, but screen voltage, that's the probelm. Should have been more descriptive in my original post.
Regards,
Gordon.
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