B+ to high, how to bring it down question

[dju]

hi all,
I am trying to make a Marshall 1987 style circuit with el84 power tubes. I am using a mojotone 18w power transformer that is supposed to be 120VAC-->290V-0-290V. looking at the Marshall 18w schematics they are getting around 325V after the rectifier which is a tube rectifier. I am using diodes for my rectifier like on the 1987 circuit. my first problem is instead of getting the 290-0-290 on the secondies of the PT I am getting closer to 300-0-300. there are only 120V and 240V on the primary side so I couldn't have messed that up. Plus I double and triple checked the color code and wiring. after my diode recifier circuit I am getting ~423VDC which is way to high.
what is the easiest way to bring the voltage down? put a power resister right after the recifier to drop ~95V?
what is the best way if the easiest is not the best?
I hope the easeist is the best way as I just ordered several values of 5W and a couple of 10W power resisters that will hopefully resolve problem number 1.

I am an amatuer, not a designer. however I am in the process of learning so any help ya'll can give to help me learn as I go is appreciated.
I probably bit a bit to much for my first project but hey, it should be a good learning project by the time it is done.
I'll probably have more problems to get resolved but I figured this is the first one that needed to be fixed before I worry about the bias circuit or any other part.

thanks for your help,
dj

[Reeltarded]

Measurements all taken without tubes before bias though, right?

[Gaz]

Another thing to keep in mind that the 18W design is cathode biased, and the plate voltage is measured relative to the cathode. With the cathode voltage up around 13 volts, it effectively lowers the the voltage the plate sees by that amount. Still, the 18W run El84s really hard to begin with.

The obvious thing to do is add a zener, but if it were me, I'd just use cathode bias and a tube rectifier to get the B+ at a more reasonable level.

Better yet, just get the right PT, and save the one you have for another project.

[LeftyStrat]

Your voltage will drop when the PT is loaded to somewhere around 360v. I've seen designs run el84's as high as 400v. I have an amp with jj el84's running around 380v. The tubes have lasted over a year with regular use.

Others might have different experiences, but if it sounds good, who cares, current production el84's are not that expensive.

[Bob S]

Lefty's right - if that makes sense
Couple of years ago I bought some expensive 7189's to cope with a high B+
Now I just use 6P14P's - cheap, sound great & so far indestructible.
Your loaded B+ should be fine.

[Gaz]

7189s are a good idea for sure. EL84 plates can handle the upped voltage, but watch those poor little screen grids glow! Maybe some 2.2K screen grid resistors are in order. If you do decide to just punish those tubes, however, make sure either a) the amp has good fusing for when those screen melt (literally) or b) play really gently like LeftStrat

[sliberty]

You mentioned that your secondary seems a bit high - 300v instead of the expected 290v. This could be caused by your line voltage. It might be 123-ish instead of 120. Whenever you take secondary readings, you should always check your primary readings as well to know what you are dealing with. And this will vary by day and by location. If you take your amp to a club, you might be getting 117v in, and maybe 280 on the secondary.

[Prairie Dawg]

I wouldn't worry about the voltages too much until you get r built. Then you can see what they're going to settle at given tube choices. In addition I wouldn't worry about the odd ten volts on your HV winding. Fender always gave its working voltage measurements as +/- 20 per cent. Millions of amps later they seem to have been on to something.

I too would use a tube rectifier just because they're cool. Using a massive dropping resistor to get rid of voltage is like killing ants with a sledge hammer. It's overkill and not an elegant solution to a problem you do not know exists just yet.

[Phil_S]

+1 on Prairie Dawg's thoughts. 5Y3GT will drop the most voltage. The Sovtek that is called a 5Y3 is not that, so should be avoided if what you seek is the voltage drop. There is plenty of old stock USA made 5Y3GT around. They sell cheap on eBait -- you can buy a few in case you get a dud, but I've done OK so far.

[dju]

hi guys, the amp is already built but not working at the moment. being a beginner I don't know for sure were to start to detemine what is going on so I decided to start with the power supply. my line voltage was measured at 121.9VAC. So this drives up the output on the secondaries.
I measured the voltages on the plates (need to get a few more voltages also) V1 pin1=183v,v2 pin1=188v, v3 pin1=366v v4 pin7=429v, v5 pin7=429v.
now for more background on what I mean when it is not working at the moment. there is no sound/hum/noise when turned on. I did accidentally get some sound out of it when I had just strummed a chord and deicided to turn it off and for ~2 seconds there was sound that quickly faded away. I found that I would get sound for ~2 seconds and would fade away whenever I would turn the amp on or off. I have not set the bias yet either. I haven't tried to set it yet as it is not yet at least somewhat working.
below is a link to my schematic I am using. it is a modified (unfortunatley by me) version of the Marshall 1987 circuit. I took clues from varies 18watt circuits that I had hoped would work here. any suggestions to change that might make things work better are appreciated.
http://i1224.photobucket.com/albums/ee3 ... dified.gif
if anybody needs any information that might get this thing up and running let me know and I'll get it.
thanks for all of the suggestions and comments and hopefully I'll get this thing running some day.
dj

[Tillydog]

Try setting your bias - you may be pleasantly surprised. (Have you included 1 ohm resistors in V4 & V5 cathode connections (or one 1R resistor in the combined connection)? they're not on the schematic.)

If not, post some clear pictures of all areas of the circuit & people will have a look.

What voltage do you have on the junction of the 2 220k grid leak resistors on V4/V5 ?

Andy

[LeftyStrat]

... play really gently like LeftStrat

The Guitarist Tunes Up by Francis Cornford

With what attentive courtesy he bent
Over his instrument;
Not as a lordly conqueror who could
Command both wire and wood,
But as a man with a loved woman might,
Inquiring with delight
What slight essential things she had to say
Before they started, he and she, to play.

[dju]

Try setting your bias - you may be pleasantly surprised. (Have you included 1 ohm resistors in V4 & V5 cathode connections (or one 1R resistor in the combined connection)? they're not on the schematic.)

If not, post some clear pictures of all areas of the circuit & people will have a look.

What voltage do you have on the junction of the 2 220k grid leak resistors on V4/V5 ?

Andy

hi Andy,
I do not have the 1ohm resistor at the moment but I have some on order that I plan to add. when I had my compu-bias probe attached it showed that there was no cathode current. this probe shows plate voltage, cathode current and calculated wattage http://www.compu-bias.com/ . I knew that with no cathode current there was a problem but I still need to figure out what is up.
I will have to check the voltage of the 220K resistors tonight and let you know.

thanks again for the help,
dj

[Phil_S]

Can you give us a full voltage chart for all tubes? Remember for the preamp tubes and PI, there are two triodes in each bottle. You gave plate readings for only one triode in each. While you are at it, give us the B+ readings at the + side of each filter cap, too.

You can omit the 6.3VAC on the heaters -- that's just clutter when posting, but do make sure heater voltage is present at all tubes.

[Firestorm]

If there's plate voltage but no cathode current, then either your cathodes aren't grounded or your bias voltage is so negative the tubes are always cut off.