Power Tranny Voltage Question Help Needed!!!

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Zenabi
Junior tube assistant
Posts: 1
(4/17/04 6:33 pm)
  Power Tranny Voltage Question Help Needed!!!
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 I have a 400-0-400 power transformer. It's the hammond 278CX. Here are the specs.
VA Total: 454VA
Heater: 6V3-37.8VA 6A
Windings: 800VCT--416.2VA (465mADC-RATED)
400V--1,040mA or 800V--520mA
565VDC-735mADC 1k13VDC-376mADC

I'm trying to make a 100W Marshall type poweramp.
I need 497V at Plates and 495V at Screens.

After doing some research, I found out that my power trans might be outputting too much voltage.

Is there anything I can do?

Thanks.

Edited by: Zenabi at: 4/17/04 11:16 pm
 
Leevi
Junior tube assistant
Posts: 135
(4/18/04 1:30 am)
  Re: Power Tranny Voltage Question Help Needed!!!
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 If you are going to use semiconductor rectifier (two diodes+CT) the theoretical voltage after the rectifier and filter capacitor is 1.4x400=>560V. In practice the voltage is lower because of the load that your amp will cause.

If you use EL34 in your power amp the anode voltage wont be a problem because the maximum value on EL34 is 800V. What comes to the screens you´ll probably use ~1K screen grid resistors which will drop the voltage. If you want to reach exactly the 497V at plates then you have to use power resistors or zeners to drop the voltage. E.g. two 50 Ohm/18W resistors in serial connection will drop the voltage at 400mA current 40V.
/Leevi
 
6G6
Junior tube assistant
Posts: 158
(4/18/04 8:08 am)
  Re: Power Tranny Voltage Question Help Needed!!!
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 No need to get too worked up about it.
That 560V will be more like 530V with a load.
The EL 34s will still be happy and your output may increase some.
Despite the fact that they provide exact voltage readings on the schematic, in real life it will hardly ever read exactly what they say, anyway.
 
Zenabi
Posts: 2
(4/18/04 9:38 am)
  re: rectifier?
--------------------------------------------------------------------------------
 Thanks guys,

Are you suggesting that I should not use a Full Wave Bridge Rectifier?

Thanks.
 
Leevi
Junior tube assistant
Posts: 136
(4/18/04 11:10 am)
  Re: re: rectifier?
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 As you stated before there is a center tap in your PT (400-0-400). I would use two diodes instead of bridge. If you want to use bridge the input voltage to the bridge must be 400V. If you connect the bothe 400V wires to the bridge you´ll have 1.4x800V=1120V after the bridge and cap which is too much.
/Leevi
 
jbrew73
Junior tube assistant
Posts: 117
(4/18/04 11:23 am)
  Re: re: rectifier?
--------------------------------------------------------------------------------
 you should not use FWB if you do you will have over 1000volts D.C. i'm assuming you may be new to this hobby from the number of post you have made(2). you may be better off to buy a marshall 100 watt tranny from www.hoffmanamps.com/index.html this would make it a lot easier to follow standard schematics without having to modify the circuit to fit the transformer you have now.
if you are an experienced amp builder please ignore my comments as you are probably aware of the changes that need to be made
 
Zenabi
Posts: 3
(4/18/04 12:02 pm)
  RE:
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 You guys are totally right.
I'm new to this hobby.
I've been wanting to complete this amp a year ago.
During this year, I've been doing a lot of research as to how things should be done.
I kinda got the idea that using a FWB would be doubling the voltage.

What I'm not aware of, is whether the Type of Solidstate recitifer affects tone or not. I do not intend to follow the schemetic I have fully. I've already have some changes made to it already.

This forum is a very good forum. I just found this forum yesterday, and already have learnt many things. Great forum, and great Users like you guys.

So what rectifier are you guys telling me to use? I don't seem to be able to find it in the Hoffman Library.
I'm just afraid that it would affect tone.
The reason why I'm using the Power Transformer, is because London Power recommended it. Since I'm putting in the Power Scale, I just want to use parts that they are suggesting.

Thanks, once again.

Edited by: Zenabi at: 4/18/04 12:03 pm
 
Tiny Daddy
I will work on all amps
Posts: 619
(4/18/04 12:29 pm)
  Re: RE:
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 You could install a tube rectifier AND diodes with a SPST switch that selects the diodes. The tube may be connected all the time and would not need to be switched.
 
Zenabi
Posts: 4
(4/18/04 2:57 pm)
  re: rectifier?
--------------------------------------------------------------------------------
 I dont' think I need a tube rectifier. But thanks for the info.
I was reading on the Hoffman library and found that the Full Wave Bridge Rectifier outputs approx. Half of voltage of the Full Wave Rectifier.
I do notice that the Full Wave Bridge Rectifier on that page does not require a center tap. Would it be possible if I just connected the center tap somewhere else (for example, in between the first 2 capacitors connected in series) so that the B+ would have roughly approx. 400*1.4 V ?

Please Excuse any of my stupid questions.

Edited by: Zenabi at: 4/18/04 2:58 pm
 
Tiny Daddy
I will work on all amps
Posts: 620
(4/19/04 7:09 am)
  Re: re: rectifier?
--------------------------------------------------------------------------------
 "connected the center tap somewhere else (for example, in between the first 2 capacitors connected in series) so that the B+ would have roughly approx. 400*1.4 V ?"
NO you will end up with 800*1.4V.

If you want to use the bridge, do not connect the bridge negative to anything, it is not used. Then ground the center tap and you are there. This connection uses only 2 of the diodes in the bridge.

 
Zenabi
Posts: 5
(4/19/04 12:52 pm)
  re:
--------------------------------------------------------------------------------
 Would that be considered as a half wave rectifier than?
Or would it still be considered as a full wave rectifier.

Is this what you're suggesting?
red
D-D
| |------B+
D-D

What do I do with the other Red? ground it? isolate it?

Thanks.

Edited by: Zenabi at: 4/19/04 12:55 pm
 
Zenabi
Posts: 6
(4/19/04 1:12 pm)
  re:
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 I suppose I could use the Full Wave Rectifier instead of the Full Wave Bridge Rectifier, right?

Edited by: Zenabi at: 4/19/04 1:16 pm
 
Leevi
Junior tube assistant
Posts: 137
(4/19/04 1:49 pm)
  Re: re:
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 Zenabi,
Check the following circuit below, I would do in that way.
/Leevi

www.kolumbus.fi/risto.kiv...lyH30S.PDF
 
Zenabi
Posts: 7
(4/19/04 2:31 pm)
  re:
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 IC IC.
Thanks.

Now, I'm a bit confused as to why London Powers suggested the usage of a 400-0-400 power tranny for a 100W (4x el34)amp.

Perhaps it has something to do with the amount of current needed?

Edited by: Zenabi at: 4/19/04 2:48 pm
 
 
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