PT, rectify AC to DC

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joCCe
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Location: Oslo-Norway

PT, rectify AC to DC

Post by joCCe »

Hi all

I'm new here and this is my first post. What a great place you have here!

I'm building an AC30 - like amp for clean sounds with upped filtering. I've built a few kits before, but this is my first build from scratch. I'm basing the build on the Weber 6V30 scheme and I'm using the normal and brilliant channels only.

I'm in the process of sourcing parts and I'm trying to find the right PT for the task. My target is around 320V on the plates and I've spent some time searching for posts and gathering info on this subject. The obvious would be to use the Weber WPT30. The Weber WPT30 is specified to 280-0-280.

The 280-0-280 rating confuse me. I'm using a GZ34 rectifier and from what I've read the factor to use is 1.31 to get from AC to DC. Using that factor, a PT with 245-0-245 should get me in the ballpark, 245 X 1.31 = 320.95V. However, since the WPT30 is specified with higher secondaries, there is something I'm missing in my calculations…

I guess the B+ drops when all the tubes are in and up and running, but how to I calculate the final B+ going to the EL84 plates?

Thanks
Joakim
surfsup
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Re: PT, rectify AC to DC

Post by surfsup »

I'm using a GZ34 rectifier and from what I've read the factor to use is 1.31 to get from AC to DC. Using that factor, a PT with 245-0-245 should get me in the ballpark, 245 X 1.31 = 320.95V.

The rectified voltage is pretty much what will be on the plates since the OT won't drop much voltage. So I'm not sure how much the GZ34 affects the voltage drop (doesn't seem like much with a 1.31 factor, if that is correct). Or you can just use diodes and skip the recto tube but it seems like a 245/245 will get you in the ballpark for 320 on the plates.

EDIT: looks like the GZ34 has an internal resistance of around 85 ohms at 350 volts:

http://drtube.com/datasheets/gz34-philips1958.pdf
Last edited by surfsup on Mon Oct 03, 2011 1:03 pm, edited 1 time in total.
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Masco
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Re: PT, rectify AC to DC

Post by Masco »

Typically using the AC-30 3 channel circuit, the 280-0-280 will give you between 340-350v B+, which is fine. If you don't want to order some custom iron to suit your desired plate voltage, you could power scale the output or use a VVR and dial-in any voltage you want. They're cheap and easy circuits that I highly recommend.
joCCe
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Re: PT, rectify AC to DC

Post by joCCe »

Thanks guys!

I'll check out the VVR.

Thanks
Joakim
big_teee
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Re: PT, rectify AC to DC

Post by big_teee »

Classic Tone has a 260-260 TrainWreck Transformer.
It's a standUp PT, though.
Look it over.
http://www.classictone.net/40-18065.html
http://www.classictone.net/40-18065.pdf
Their prices are good.
Later,
Terry
Roe
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Re: PT, rectify AC to DC

Post by Roe »

originals had 280-90vac on the secondaries adn 320+v on the plates. I'd go no higher than 320v with the 50r cathode resistor
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Masco
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Re: PT, rectify AC to DC

Post by Masco »

The early Haddon PTs had a 285-0-285 secondary, the later Wodens were 280-0-280. If you use 120v input here in the US, you'll have 350+ volts B+. I've been in dozens of Jennings AC-30s, never had one give me 320v per the schematic. If you use a 120 to 220v step-up xfmr and set the AC-30's voltage selector to 245, you'll get closer to 325-330v B+.
Always some taming needed here in the U.S.
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rp
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Re: PT, rectify AC to DC

Post by rp »

joCCe wrote:I'm building an AC30 - like amp for clean sounds with upped filtering.
I assume you've already looked over the Trainwreck Rocket pages? If you haven't you've come to right place.
joCCe
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Re: PT, rectify AC to DC

Post by joCCe »

big_teee wrote:Classic Tone has a 260-260 TrainWreck Transformer.
It's a standUp PT, though.
Look it over.
http://www.classictone.net/40-18065.html
http://www.classictone.net/40-18065.pdf
Their prices are good.
Later,
Terry
Yes, I've concidered one of those, but they are HUGE and weigh in at 100lbs :D
Roe wrote:originals had 280-90vac on the secondaries adn 320+v on the plates. I'd go no higher than 320v with the 50r cathode resistor
I'm curious if you managed to land the 6V30 you built on 320V with the Weber WPT30 without a zener or vvr?
Masco wrote:The early Haddon PTs had a 285-0-285 secondary, the later Wodens were 280-0-280. If you use 120v input here in the US, you'll have 350+ volts B+. I've been in dozens of Jennings AC-30s, never had one give me 320v per the schematic. If you use a 120 to 220v step-up xfmr and set the AC-30's voltage selector to 245, you'll get closer to 325-330v B+.
Always some taming needed here in the U.S.
Isn't that because the original Vox PT's were made for 110VAC primaries and not 120VAC? I'm wondering if the current production tubes are up for the task or if I should try and land around the 320V mark.
rp wrote:
joCCe wrote:I'm building an AC30 - like amp for clean sounds with upped filtering.
I assume you've already looked over the Trainwreck Rocket pages? If you haven't you've come to right place.
The amp I'm going for is the Bruno Underground 30. It's the normal and brilliant channel of the 6V30 with 47n PI caps and 40uf filtering. Lower henry on the choke too, I guess. It may turn out to stiff, so I'll have to experiment. I compared the rocket / UG30 and the brilliant channel looks identical to the rocket. The PSU is different though.

Thanks
Joakim
Roe
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Re: PT, rectify AC to DC

Post by Roe »

yes, the 6v30 gives me 315-20v. I tried higher voltages but prefer 320v with the 50r k. It should be keept in mind that the original PTs were rated for little current, meaning that they ran hot and sagged more than many new PTs
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