EL34's & screen voltage

fearhk213

Well-known member
I reached out to TAD about what EL34's they'd recommend for an amp that sits around 520V at the plates w/the screens around the same. I don't know the exact screen reading, but it's on an older modded Marshall and I've always heard Marshall screen voltages tend to only be a few volts below the plates. Anyway, they recommended the TAD EL34-STR Redbase. However, the datasheet says 500v at the screens. To be safe I have always bought tubes that say they will handle the same or higher voltage than what my amp puts out, but since they recommended these it got me scratching my head. I guess I don't really know how absolutely critical it is for the screen voltage to be at or lower than those datasheet numbers. Would grabbing these tubes just be asking for early failure or is this in more in the range of "it's close enough"?
 

fearhk213

Well-known member
other than winged C`s or nos mullards ?
Overall, I've had good luck with winged C's in this amp and others, but over the years I've had them fail in other amps with less screen voltage than the one I'm talking about. I just popped in a set in recently, biased them up, and one of them crapped out after about 30-45 mins. I wasn't even pushing the volume that hard. Super annoying. I haven't gone the NOS Mullard route before, though.

Anyway, I just figured I'd just reach out to TAD and what they recommended, which lead me to post here asking about how vital it is to stick to those data sheets numbers. I actually just looked up the EL34 Winged C datasheet numbers. I could have sworn that was 600v at the screens, but those say 500v.
 

Fusionbear

Active member
Either use a variac to get the plate voltage to 490ish or go the KT77 or K88 route. I have a diy 2204 build that sits at 520+. I use KT88's with tonal success. As always ymmv
 

MetalHeadMike

Well-known member
I was told by several tube vendors that if your not winding the amp up and cranking volume that a PT that rates a bit below screen/plate voltage will likely be fine. Don't know how accurate that is.
 

Fusionbear

Active member
I was told by several tube vendors that if your not winding the amp up and cranking volume that a PT that rates a bit below screen/plate voltage will likely be fine. Don't know how accurate that is.

I tried that, never had good results, tubes blowing.... I can't afford to replace tubes that frequently....
 

fearhk213

Well-known member
Either use a variac to get the plate voltage to 490ish or go the KT77 or K88 route. I have a diy 2204 build that sits at 520+. I use KT88's with tonal success. As always ymmv
I was leaning towards KT77’s. I’ve never messed with a variac. How far would you expect to have to drop the wall voltage down to get the plate v down that far? I’m assuming you rebais with the new voltage?
 

Nigel

Well-known member
I was leaning towards KT77’s. I’ve never messed with a variac. How far would you expect to have to drop the wall voltage down to get the plate v down that far? I’m assuming you rebais with the new voltage?
The JJ opens up over 500v! Go for it!!
 

Racerxrated

Well-known member
JJ EL34L are the only new production EL34 that can handle that voltage which is why Rick puts them in Wizard amps. I’ve had good luck with those, older Siemens and Winged Cs as well in the MC100 I owned. Another option but hard to find are the GT EL34LS which aren’t made anymore..they had larger heat wings than the current version which are basically the JJ EL34L
 

Chubbs

Active member
Not exactly a direct comparison but many el84 20w amp run 15%+ above recommended plate voltage and near or at the screen voltage max. I'd think you'd be fine considering your barley over.
 
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SpiderWars

Well-known member
What is the amp? And what is the AC voltage you are plugging into?

I have a homebrew '69 Super Lead that is at about 520vdc when plugged into the 123-124vac wall voltage. It has arced an EH EL34 (and blown the HT fuse) when cranked at that voltage. But the tube survived and is still going. However I built that amp for variac use so I run it at 90vac or even less and it hasn't ever had any issue there. But if anything, a 115-120vac regulated supply wouldn't hurt.
 

fearhk213

Well-known member
What is the amp? And what is the AC voltage you are plugging into?

I have a homebrew '69 Super Lead that is at about 520vdc when plugged into the 123-124vac wall voltage. It has arced an EH EL34 (and blown the HT fuse) when cranked at that voltage. But the tube survived and is still going. However I built that amp for variac use so I run it at 90vac or even less and it hasn't ever had any issue there. But if anything, a 115-120vac regulated supply wouldn't hurt.
It’s a Cali mod on a 70’s super lead (don’t have it front of me right now to check the serial number for the date). It’s sort of unique one though in that it was built with some older era parts that Marshall still had around like it’s lay down transformer.

I have it plugged into a voltage regular. Depending on what the wall a/c is feeding the furman, which affects what tap the Furman is on, it’ll swing between 118-122, but 120-121v is the typical range.
 

SpiderWars

Well-known member
It’s a Cali mod on a 70’s super lead (don’t have it front of me right now to check the serial number for the date). It’s sort of unique one though in that it was built with some older era parts that Marshall still had around like it’s lay down transformer.

I have it plugged into a voltage regular. Depending on what the wall a/c is feeding the furman, which affects what tap the Furman is on, it’ll swing between 118-122, but 120-121v is the typical range.
You might not like the tone with a variac on that amp. You'd just have to try it, variacs are cheap.

Another thought...blasphemy to some...would be to replace the PT with something new and sell that prized Marshall iron to someone willing to endure whatever needed to use it (like always using a variac). Some dude building a for-variac-use-only-brown-sound amp would probably pay dearly for it (not referring to me...necessarily).
 

fearhk213

Well-known member
You might not like the tone with a variac on that amp. You'd just have to try it, variacs are cheap.

Another thought...blasphemy to some...would be to replace the PT with something new and sell that prized Marshall iron to someone willing to endure whatever needed to use it (like always using a variac). Some dude building a for-variac-use-only-brown-sound amp would probably pay dearly for it (not referring to me...necessarily).
Yeah I don’t think I could bring myself to do that haha. Maybe I’ll just nab a variac to see how that does and some kt77’s since those seem to hold up and see what option is best.

When you drop yours to 90v at the variac what does you plate voltage end up at?

Any particular variac(s) I should look at?
 

Greazygeo

Well-known member
JJ EL34L are the only new production EL34 that can handle that voltage which is why Rick puts them in Wizard amps. I’ve had good luck with those, older Siemens and Winged Cs as well in the MC100 I owned. Another option but hard to find are the GT EL34LS which aren’t made anymore..they had larger heat wings than the current version which are basically the JJ EL34L
Nope the screen voltage is only rated for 450v. Plate voltage is 800 but so are most other tubes too. I just went thru this trying to find tubes that would last in my 69 Metalface at 520+ on the screens....The Svetlana was what I came up with that would work. So far so good. Also lowered the wall voltage to 110 with a variac to be safe.
 

Racerxrated

Well-known member
Nope the screen voltage is only rated for 450v. Plate voltage is 800 but so are most other tubes too. I just went thru this trying to find tubes that would last in my 69 Metalface at 520+ on the screens....The Svetlana was what I came up with that would work. So far so good. Also lowered the wall voltage to 110 with a variac to be safe.
Interesting. I know you have/had some Wizards? I would think they’d be able to handle the higher screen voltages on Wizards? Or maybe they aren’t as high as I think? The MC100 I had was from 2010, with the C transformers; the PT measured at 560v (no tubes installed). I actually have a period correct PT arriving today, a T3556 for my 72 SuperTrem. Right now it has a Dagnall T4145 from a late 70s SL. I hope my Siemens will last in it…they did fine in the Wizard.
 

scottosan

Well-known member
I reached out to TAD about what EL34's they'd recommend for an amp that sits around 520V at the plates w/the screens around the same. I don't know the exact screen reading, but it's on an older modded Marshall and I've always heard Marshall screen voltages tend to only be a few volts below the plates. Anyway, they recommended the TAD EL34-STR Redbase. However, the datasheet says 500v at the screens. To be safe I have always bought tubes that say they will handle the same or higher voltage than what my amp puts out, but since they recommended these it got me scratching my head. I guess I don't really know how absolutely critical it is for the screen voltage to be at or lower than those datasheet numbers. Would grabbing these tubes just be asking for early failure or is this in more in the range of "it's close enough"?
I’ve had great luck with Ruby El34BHTs, which were designed for high screen voltages. Unfortunately, I haven’t been able to find any since COVID.
 

Racerxrated

Well-known member
It’s a Cali mod on a 70’s super lead (don’t have it front of me right now to check the serial number for the date). It’s sort of unique one though in that it was built with some older era parts that Marshall still had around like it’s lay down transformer.

I have it plugged into a voltage regular. Depending on what the wall a/c is feeding the furman, which affects what tap the Furman is on, it’ll swing between 118-122, but 120-121v is the typical range.
The lay down transformers in the early 70s are the same as the standup versions. Just installed differently. They were mostly the T2562; but could have also been the T3556 or T3517. All 3 had the same ballpark voltages-550 or even more. The reason they can go in up or lay down is the wiring; all wires on one side unlike the T4145 which have wires on both sides. So your transformer may be original to the year. I’ve been searching for an original for a while for my 72 and just scored a T3556. It’s going in very soon.
I got most of this from the Marshall forum.
 
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