Heater voltages too high?

[Bernardduur]

Hey all!

The heater voltages in my Super City DEA200 is 6,99V. Too high. What can I do to lower em? The unit has no voltage selector switch (guess it set at 220V) and no center tap on the heater taps.

[Bernardduur]

I solved it by a tip from this website. Added 2 6A diodes side to side on one side of the line. Making it a perfect 6.3V AC.

The tip was described as having 2 pairs of diodes on each line; I only used one pair. Could this hurt? The diodes are warming up but no real heat!

[6267]

As long as the diodes are within specs, it'll be just fine.

Even if the diodes die, there's only two ways they can do that, open or short.
Open will leave the heaters off, which will be fairly evident and shouldn't kill the amp or anything (not great for the cathodes, but they'll make it.)
Short will bring the heaters back up to the original 6.99v.

Are you measuring with tubes in circuit? An unloaded 6.3v winding can get as high as 7 something volts until it's properly loaded...if you were measuring unloaded, you may not even need the diodes!

[Bernardduur]

Thanks!

I measured loaded, also unloaded btw. The unloaded is even higher, the loaded is 6.99V. With the 2 diodes I get to a perfect 6.3V AC. Great!

[Bernardduur]

As another problem, the plate voltage really is raised. I now measure a voltage of 900V on the plates............

Can I lower this by adding a resistor + cap in line?

[6267]

That is dangerously high! It sounds to me like something is wrong with the power supply. What output tubes are you using? Until you figure out why, try not to power it up since the filter caps could pop at those voltages...

Are both output tubes properly grounded (or connected to cathode resistor)?
Is the amp a FW Bridge rectifier (I "found" out the hard way that these double when I tried one last week)
I'm assuming that's a loaded voltage. What's the transformer's AC voltage supposed to be?

The resistor thing may work, but that's a lot of voltage & power to eat to get back to the sane level of say 450v. I'm thinking something ain't right.

[Bernardduur]

I think I should've be more informative



Can I load the plates line down by adding a resistor + cap to the main line?

The output of both the lines are 450V.......

[Alf]

Bernard , I think you may have added up 2 x 450 volts = 900 volts

Could it be that the negative side of C1 should be connected to ground instead of to C5 ?

By the way why are there two xformers ? Is this the way they were built ?

Alf

[Bernardduur]

Bernard , I think you may have added up 2 x 450 volts = 900 volts

Could it be that the negative side of C1 should be connected to ground instead of to C5 ?

Alf

No, this the design that was in the amp and the owner really wants it to be the same......... it was designed to bring 790V to the plates and approx 1/2 of that to the screens. I guess that the raise of the primairy voltage over the years gave this as a result.

I use it with 4x EL34's btw.

[Alf]

Voltages have gone up over the years.


I can agree with you on that. My Guyatone with a 220 volt transformer had also 6,7 volt on the heaters and the platevoltage was about 510 so I am using a variac to get the voltage down again to 220.
This was for me the cheapest solution. Brought the heaters down to 6.3 again.
And the platevoltage to about 480.

Alf

[Bernardduur]

Yes, I wanted to do that too but the new owner really can't use that.......

I got the heater voltages down nicely. Now only the plate voltages.......... I have no real experience with these type of PSU's so I don't know what would happen. Guess I'll try it in a minute

[6267]

I hope that the caps are rated or properly seriesed with balancing resistors, but I guess it's been working, so they are either tough as hell or already correct. For just dropping say 10-100v, you could try:

-zener in the center tap of PT(hv)
-resistor in center tap of PT(hv)
-back biasing (this might actually be a great option, since you have B+ to spare, just use a GOOD resistor)...the description is on Aiken Amps http://www.aikenamps.com/BackBiasing.html . It's a good way to lower B+ and get some richer tones...first time I saw it was a hammond organ amp.
-you could also "lift" the output stage cathode & grid by 100v or so, but that will require a seperate filament winding for just the power tubes. You'll need to bias this with DC to keep from destroying the Heater-Cathode insulation. This will be tricky if anything is DC coupled into the power stage.
-use a smaller first filter cap (I think, PSUD will confirm)
-use a choke on the HV supply, that's probably the ideal choice here.

That's the list of ways to make the HV lower that I'm aware of...there's probably even more tricks up someone's sleeve though...I've just started working with bridge rectified supplies and my conclusion was that I was avoiding them until I needed around 900v .

[Albert Kreuzer]

I agree that the problem comes from mains voltage being too high. Try R.G.'s Vintage Voltage Adaptor:
http://www.geofex.com/Article_Folder...t/vintvolt.htm

This also should take care of your heater voltage problem (without the diodes).

Way cheaper than a variac.

Cheers,
Albert

[Steve Conner]

+1 on the Vintage Voltage thing. I built one into my Crown stereo amp, and it really needed it, since the highest voltage I could set the transformers up for was 220, and the line voltage in my house is nearly 240. I used a 240:15V transformer to knock 15V off.

See if you can spot it :-) http://scopeboy.com/crown-sxa/IMGP1422.JPG

[Bernardduur]

Thanks all!

I'll have to find a suitable transformer and then try this method! great!