Picking a PT for 2EL34 build

[Alan0354]

Hi guys, I am starting to look for a PT for my Marshall build. It is going to have two EL34. I bought the Classic Tone Marshall OT 3.4K already. Now I need a PT. This is the one I am looking at:

https://taweber.powweb.com/store/022798intsch.jpg

It is only $50, 660VAC with 200mA. 6.3V at 6A and 5V at 3A. This will give me 450VDC which is what I have right now. My main question is whether 200mA is enough. I am NOT worrying about the power output, 40W is plenty for me already. Attached is the Aiken load line for 100W Marshall. This is for one tube and 850 ohm load line.....which is 3.4K primary. As you can see, the peak current is 500mA at the point of load line intersect with the 0 grid curve. This mean the RMS current is 0.707X0.5A=353mA. This is over the limit of the transformer. But at the same time, guitar signal is not continuous at peak amplitude.

Also, when I look at Classic Tone PT for 50W Marshall, it's only 150mA!!! So the question what is the safe limit when choosing a PT?

Thanks

[frus]

This mean the RMS current is 0.707X0.5A=353mA.

why did you multiply by 0.707?
people tend to make square waves, especially from Marshalls, and that's your worst case scenario
anyway, the PT is not rock solid like that, it sags A LOT under load, so try the curves for some lower Vg2 (this one is for 450V right?)

[Alan0354]

why did you multiply by 0.707?
people tend to make square waves, especially from Marshalls, and that's your worst case scenario
anyway, the PT is not rock solid like that, it sags A LOT under load, so try the curves for some lower Vg2 (this one is for 450V right?)

I use sine wave. For square wave, that's even worst, it's full 500mA.

Even though it sag to say 400V, you basically just shift the load line down vertically. So you intersect at say 400mA, that's still way too high.

Classic tone site is down, I can't get the spec of the 50W Marshall PT spec. But this is Weber PT for Twin. It's only 400mA. So this means it's only barely enough for two EL34 according to the load line. Something is missing here.

https://taweber.powweb.com/store/022756sch.jpg

More important is the PT I chose is for 40W amp like Super Reverb/Bandmaster reverb etc.

Thinking back, I should have got the Classic Tone Marshall OT with 4K primary instead of the 3.4K to lower the wattage and the current requirement. In fact, that will be the same as Fender OT and it got to be safe.

[jazbo8]

The 100W marshall uses 4 tubes, so you can divide the 353mA by 2 which comes to 176.5mA, so call it 180mA and if you want some sag, round it down lower to say 150mA. Or just buy a replacement transformer for the Marshall 50W, done.

[Alan0354]

The 100W marshall uses 4 tubes, so you can divide the 353mA by 2 which comes to 176.5mA, so call it 180mA and if you want some sag, round it down lower to say 150mA. Or just buy a replacement transformer for the Marshall 50W, done.

In the Aiken's graph, the load line is for 850 ohm and intersect the 0 grid at 500mA. That graph is for a single tube. How do you get 352mA and still divided by two?

The reason I pick this PT is because it's half the price of any Marshall type PT. And more important, the Classic Tone Marshall 50W PT is rated only 150mA, is even lower than the one I picked. The Classic Tone site is up and here's the spec for the Plexi 50W:

http://www.classictone.net/40-18095.pdf

[jazbo8]

You are right, I forgot he was just using one tube from one side: "The yellow line is the loadline for one tube on a side (3.4K p-p, or 850 ohms per tube per side). The green line is the EL34 25W max dissipation curve, and the red line is a 50W dissipation curve."

About the Classic Tone PT, I guess you got to be a bit careful when you crank up the amp with its 150mA rating, it might be ok for short bursts, but if run continuously at high level, it would probably heat up and self-destruct. Alternatively, you may want to dial down the bias to keep it in check.

[Alan0354]

You are right, I forgot he was just using one tube from one side: "The yellow line is the loadline for one tube on a side (3.4K p-p, or 850 ohms per tube per side). The green line is the EL34 25W max dissipation curve, and the red line is a 50W dissipation curve."

About the Classic Tone PT, I guess you got to be a bit careful when you crank up the amp with its 150mA rating, it might be ok for short bursts, but if run continuously at high level, it would probably heat up and self-destruct. Alternatively, you may want to dial down the bias to keep it in check.

That's the reason I choose the Weber because it's 200mAhttps://taweber.powweb.com/store/022798intsch.jpg

[frus]

Even though it sag to say 400V, you basically just shift the load line down vertically.

but the curves also shift down when the screen voltage is lower, so the intersection should be even lower
I can't find something like this for EL34, it would be useful:

http://www.triodeel.com/6l6gc_p7.gif

[Alan0354]

but the curves also shift down when the screen voltage is lower, so the intersection should be even lower
I can't find something like this for EL34, it would be useful:

http://www.triodeel.com/6l6gc_p7.gif

Right, That's why I did say instead of using 500mA, let's use 400mA instead in post #3. Still that is way too high.

I think I am missing something major because it is a FACT that all the PT for Fender 40W amps are 200mA PT. If you proportionally increase to 50W, that will imply 20% increase. So 240mA should be plenty safe.

That's why I can kick myself not buying the 4K primary OT. That will automatically reduce the power from 50W to about 40W and it will be like any Fender. Who needs 50W now a days? 30W is plenty already. Maybe I should just use the Vibtrolux OT with 4K primary because I did compare the sound, it didn't make any noticeable difference between the two OT.

[danman]

Hello everyone...new member here and I just wanted to share what I learned recently when shopping for PT's for a couple of 50 watt Marshall style builds I did. I looked into a couple different PT's from Marstran and Classictone when shopping for the builds and went with the Classictone on both. Both companies modeled their 50 watt transformers after the original Dagnalls that Marshall used from the late 60's and onward. The current rating on the secondary did seem a bit low to me also but both companies assured me that this was what the originals were rated and that it was a primary part of the Marshall sound. The sag from the PT under heavy load was necessary in recreating the classic crunch and compression the amps are known for. I was a bit skeptical myself but after using both amps for a couple of years now I can say that I haven't had any issues with the Classictone iron. I'm using the 40-18054 in my 1987 circuit and the 40-18095 in my 2204 and both amps sound as good as any of the real Marshalls I have owned. I hope this can help you in your decision.

[Alan0354]

Hi Danman, welcome to the forum. This is very helpful. That's kind of what my guessing as I cannot explain by the rating.

At this point, I kind of think the Weber I choose is good enough, it's for a 40W amp and is rated 200mA. I just hope experts here can confirm this. This one is almost half price of the classic Tone and have better rating.

[big_teee]

I used the classictone marshall iron,on my first build.
http://www.classictone.net/40-18023.html
The only issue I had was very high voltage with the 345-0-345 Iron.
Nearly 500 volts in play mode. The Caps are 500v rated
The second build I went to the Fender Iron and used this one on the second one, on the modified 18W chassis.
Fender Power Transformer, Vibrolux, Vibrolux Reverb, Tremolux, 022723 & 125P26A, 120V
It turned out around 460v in play mode.
It is a 325-0-325 PT.
I couldn't tell too much difference in sound.
As far as Sag, with diode rectifiers, I don't hear any.
Both transformers do a adequate job.
The Dreamtone Plate calculator says you need 142ma, with 2 EL34 and 3 12ax7s.
PT Current Draw

[eschertron]

90VA (25W per tube plus 40W output) / 450VDC = 200ma so that part looks good to me?

However, using Interactive Valve Data Sheets with the 3.4k OT, I couldn't get a decent solution. 5k was better for output power at the higher voltages. Was the 3.4k OT for 4 valves?

[Alan0354]

90VA (25W per tube plus 40W output) / 450VDC = 200ma so that part looks good to me?

However, using Interactive Valve Data Sheets with the 3.4k OT, I couldn't get a decent solution. 5k was better for output power at the higher voltages. Was the 3.4k OT for 4 valves?

Thanks for the link, I have been playing around with it a little. The 3.4K is for 2 tubes only as the plate load for each tube is 850 ohm.

Yes, I saw in the lower right corner that the output power is only about 10W. That's way too low. I have my amp with 4K primary, it's is so much louder than my 20W amp that was measured 23W.

[Alan0354]

Anyone can tell me how do you get from the Aiken 500mA max to using a 200mA PT? What kind of assumption do you use to justify the PT?

I understand 200mA will work, I just want to learn what is the assumption.