how to drop plate voltage &nb

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Leevi
Junior tube assistant
Posts: 119
(3/23/04 3:57 pm)
 how to drop plate voltage
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 The plate voltage is about 390V i.e. too high in my VOX based power amp. The reason for that is the PT that gives 325V-0-325V on the secondary. There is a 20 Ohm resistor placed after the SS rectifier. I know that I can drop the voltage by increasing this resistor value but I don´t like hot resistors in my amp so are there any other ways to do that?
/Leevi
 
ganzonimx
Hey get your own solder
Posts: 438
(3/23/04 5:56 pm)
 Re: how to drop plate voltage
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 Your coul drop the voltage with a zener diode between the center tap and ground, with the cathode to ground, but dropping voltage always will produce heat with either a resistor ore a zener diode.

I suppose the resistor would give you some sag, the zener will not do that.

Cris
 
GroundhogKen
Forum Moderator
Posts: 2334
(3/23/04 6:30 pm)
 Re: how to drop plate voltage
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 Four choices:

Hot Resistor,
Hot Zener,
Hot Tube Rectifier,
Hot Weber Copper Cap Rectifier.


Ken
 
boogielicious
Junior tube assistant
Posts: 58
(3/23/04 7:08 pm)
 Re: how to drop plate voltage
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 To add to Ken, the Weber Z Cap will drop the voltage and give the characteristics of the rectifier you want. I've got a Z cap for my 5F6-A that drops the B+ about 40 vdc with GZ34 sag and it works great. And it's only $25.

Scott
 
lowtech
Senior tube assistant
Posts: 184
(3/24/04 12:08 am)
 Re: how to drop plate voltage
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 Here's something different to think about...
www.geofex.com/Article_Fo...ntvolt.htm
 
Leevi
Junior tube assistant
Posts: 120
(3/24/04 3:35 am)
 Re: how to drop plate voltage
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 Thank you for the fast responses. What is the most suitable
zener for that. I should drop the voltage about 90V with 200mA. The zener should be a special one for this purpose.
/Leevi
 
Leevi
Junior tube assistant
Posts: 121
(3/24/04 7:40 am)
 Re: how to drop plate voltage
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 A correction to my previous message: the 200mA current will certainly not go through the zener diode. The voltage should be dropped from 390V to 300V. I don't have any idea how much current will go through the zener diode when dropping this amount of voltage.
/Leevi
 
bluesbear
Hey get your own solder
Posts: 351
(3/24/04 10:17 am)
 Re: how to drop plate voltage
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 If a GZ34 Weber Copper Cap gives a 40 volt drop, I'd guess a 5Y3 Copper Cap is going to be pretty close to the 90V drop you're looking for.
Dave
 
lowtech
Senior tube assistant
Posts: 185
(3/24/04 11:39 am)
 Re: how to drop plate voltage
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 That is quite a lot of voltage ( 90vdc, approx 25% of your existing B+) to try to drop. If you're gonna use zeners, your gonna end up with a mess of 'em in series in order to handle all the dissipated power ( heat) involved. Whatever you end up with you're going to waste abunch of power in the form of heat.You'd be better off with a transformer that produced less secondary voltage in the first place. As far as copper cap type configurations go, drop Ted Weber an email, and tell him what you're trying to do. He should be able to let you know if its feasible, and what its gonna cost. He's a pretty good guy to work with.
 
Leevi
Junior tube assistant
Posts: 122
(3/24/04 2:45 pm)
 Re: how to drop plate voltage
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 You are right, 90VDC is too much to drop. I just wonder one thing: Max Va of EL84 is according to data sheet 300VDC. In the schematic (below) of a top boost AC30 from 1992 there is the plate voltage set to 355VDC! I have used in my tests 390VDC and the tubes (from JJ) work still fine. What is the limit?
/Leevi

www.flintholm.dk/vox/VOXA...0VTB92.HTM
 
6G6
Junior tube assistant
Posts: 143
(3/24/04 4:00 pm)
 Re: how to drop plate voltage
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 That's good to know.
How long did you run 'em at 390V?
I have heard that the EL84M is supposed to take more abuse than others.
Perhaps the newer EL84s are able to stand more than the old ones, sort of like the current 6V6s?
 
jc
I only work on Fender's
Posts: 685
(3/24/04 5:13 pm)
 EL84M
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 I run the sovtek el84M at 390vdc all of the time. I have also run the standard sovtek el84 at 390vdc. They survive but I have not tried to play for hours with them in.
It sounds like it would be easier to change the tubes.

I have bought el84Ms from this guy (he is in europe) and he is reliable.

cgi.ebay.com/ws/eBayISAPI...gory=14947


 
Leevi
Junior tube assistant
Posts: 123
(3/25/04 12:42 am)
 Re: EL84M
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 I have been running the amp with 390V in total maybe couple of hours. After that I have added a power resistor after the rectifier. I´m now when the amp have warmed up on level 365V. Nice to hear your comment jc. I begin to believe that the overvoltage is not a problem with today´s EL84. Thanks also for the tube link. I´ll save money (tax, customs) if I order from him since he is located in Europe.
/Leevi
 
jc
I only work on Fender's
Posts: 689
(3/25/04 7:28 pm)
 Re: EL84M
--------------------------------------------------------------------------------
 I get a lot of mileage from telling people that I buy surplus russian military supplies from a guy in Lithuania off of ebay...

always good for a funny look from someone.

 
 
Leevi
Junior tube assistant
Posts: 124
(3/26/04 12:07 am)
 Re: EL84M
--------------------------------------------------------------------------------
 Lithuania will be a member in European Union from 1st o May which means that the trading becomes easier. I understood that the selling is based on auction.
/Leevi
 
jimf
Junior tube assistant
Posts: 8
(3/28/04 4:44 pm)
 7189, aka EL-84M
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 I've read that the Sovtek EL-84M can be used as a replacement for the 7189. The 7189 has a max voltage of 400V. So if the EL-84M can really be a sub for the 7189, then you should be fine.

I bought a pair from these folks:
www.thetubestore.com/russianel84m.html

I'm trying to rebuild an odd old amp... a Gregory Bass Sixty... It running around 400V. If I ever get it running I'll tell you if the tubes hold up.

Jim
 
Crispycircuit
Junior tube assistant
Posts: 14
(3/28/04 10:50 pm)
 Voltage drop?? Choke input.....
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 Why not use a choke input after the rectifier????
 
bnwitt
Senior tube assistant
Posts: 304
(3/28/04 11:00 pm)
 Re: EL84M
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 Jc,
I guess you mean KWtubes huh?
 
Leevi
Junior tube assistant
Posts: 126
(3/29/04 12:27 am)
 Re: EL84M
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 JJ seems to work fine on 365V. I have used them now several hours without problems.
/Leevi
 
 
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