EL34's & screen voltage

MetalHeadMike

Well-known member
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.

My current MCII is at 495 on the plates/485 screen...never measured any of the other Wizards when I had em.
 

Greazygeo

Well-known member
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.
Yep, still have my Modern Classic Road 100 Rick built me in 05. It has A OT transformer, not sure on the PT. Its 480 range plate/ screen voltage. It is ok with about anything Ive used in it. Funny the orginal set were the GT El34LS...they shorted out pretty quickly. He was surprised to hear that.

This 69 metalface has always had high voltage. Would like to get it lowered one of these days. For now the Svetlana and the variac are doing the trick.
The PT in my 68 SuperTrem started smoking, hoping to get it rewound by someone to keep it original. So far Mercury is the only recommendation Ive gotten.
In my ‘69 plexi the bias winding went out, would like to get it rewound too. Using a replacement PT in it. Luckily it still sounds the same. Lucky me. Haha.
 

fearhk213

Well-known member
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.
I was able to check the serial number & it's from '78.
 

Racerxrated

Well-known member
Yep, still have my Modern Classic Road 100 Rick built me in 05. It has A OT transformer, not sure on the PT. Its 480 range plate/ screen voltage. It is ok with about anything Ive used in it. Funny the orginal set were the GT El34LS...they shorted out pretty quickly. He was surprised to hear that.

This 69 metalface has always had high voltage. Would like to get it lowered one of these days. For now the Svetlana and the variac are doing the trick.
The PT in my 68 SuperTrem started smoking, hoping to get it rewound by someone to keep it original. So far Mercury is the only recommendation Ive gotten.
In my ‘69 plexi the bias winding went out, would like to get it rewound too. Using a replacement PT in it. Luckily it still sounds the same. Lucky me. Haha.
Interesting...yep the GT LS from that time frame are tubes I've sought out, after hearing they can take the high voltage of the Wizards and sound very good. I had the MC100 for about a year; no gigs but ran it often and loud haha...The JJ Ls came with it, tried Winged C, Siemens and the GT LS and they all did fine without issues.
I've heard Chris Merren will do a spot on rewind for you; and has great period correct new wound transformers for old/new Marshalls. I did ask him about a replacement for the T2562 for my 72 PT, I think it was around $200. But, this was pre covid.
 

fearhk213

Well-known member
Been reading up a little on variacs. Please tell me if I'm understanding this correctly:

Say my wall voltage is 122v and my plate voltage is reading 520v with EL34's biased up to 33.6 for 70% dissipation. If I reduce the wall voltage to 117v (roughly 4% less), will that also result in a 4% reduction of plate voltage? Meaning I'd then have 499v at the plates, so biasing to 35 would now be 70% dissipation.

Am I understanding this correctly with what to expect by dropping the wall voltage?
 

Fusionbear

Active member
Been reading up a little on variacs. Please tell me if I'm understanding this correctly:

Say my wall voltage is 122v and my plate voltage is reading 520v with EL34's biased up to 33.6 for 70% dissipation. If I reduce the wall voltage to 117v (roughly 4% less), will that also result in a 4% reduction of plate voltage? Meaning I'd then have 499v at the plates, so biasing to 35 would now be 70% dissipation.

Am I understanding this correctly with what to expect by dropping the wall voltage?

The bias voltage drops too, so the bias typically gets a little cooler as well...
 

MetalHeadMike

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 wonder where TAD's e34L would fall in spec? If it's able to take higher screen voltage?
 

Chubbs

Active member
On my amp at 115v my plate voltage is 440 at 105v it drops to 404. Not sure if the drop from 120-110 would follow a similar curve but any drop in input voltage would probably help.
 

Greazygeo

Well-known member
Interesting...yep the GT LS from that time frame are tubes I've sought out, after hearing they can take the high voltage of the Wizards and sound very good. I had the MC100 for about a year; no gigs but ran it often and loud haha...The JJ Ls came with it, tried Winged C, Siemens and the GT LS and they all did fine without issues.
I've heard Chris Merren will do a spot on rewind for you; and has great period correct new wound transformers for old/new Marshalls. I did ask him about a replacement for the T2562 for my 72 PT, I think it was around $200. But, this was pre covid.
Is there a way to get ahold of Chris? Would love to get those transformers back up and running. Mercury wants $500 to rewind it. I just don’t know if they would be a good choice for something like this.
 

fearhk213

Well-known member
Didn't realize they had data sheets. The EL34 B STR looks like it's up for 500v screen. They're a fatter looking EL34...interesting looking tube.
Yeah I think my best bet is grab something like a variac or brown box to knock the voltage down to that 115-117 range or so. That should get my plate and screen v around 500v or less w/out browning my sound out since that’s not what I’m trying to achieve. Then I can probably run these 500v screen tubes w/much less chance of failure it seems. Then there still the kt77 option if necessary after that.

This thread has been pretty informative for me as I had no idea reduced the incoming voltage had such a big effect of plate and screen voltage. Though now it means I also have to spend more money haha.
 

midnightlaundry

Well-known member
KT77 has the Impedance of a 6L6 where a KT88 nearly matches an EL34..
As long as your PT has the extra Heater Current, a KT88 is a great option.
 

Racerxrated

Well-known member
I
KT77 has the Impedance of a 6L6 where a KT88 nearly matches an EL34..
As long as your PT has the extra Heater Current, a KT88 is a great option.
If KT 88s are an option, I highly recommend KT 90s instead. They sound like huge EL34s; more so than KT 88s. The mids in the 90s are almost spot on to a 34; vs the 88 which are more 6L6 to my ear.
 

Racerxrated

Well-known member
I'm going to try those in my high plate voltage 2204. Thanks for the reco....
They’re not cheap; but sound really good imo. Like an 88 + 34 had a baby lol. Big lows with smooth highs; and 34-like mids. I have a pair that I used in a VHT D60. Killer. I’m going to try them in my new Naylor; hopefully it’s finished up soon.
 

Fusionbear

Active member
They’re not cheap; but sound really good imo. Like an 88 + 34 had a baby lol. Big lows with smooth highs; and 34-like mids. I have a pair that I used in a VHT D60. Killer. I’m going to try them in my new Naylor; hopefully it’s finished up soon.
Which brand are you using?
 
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