B+ Volts

[C Moore]

I am revisiting this after fixing some other issues.
Is there a reason you cannot drop the voltage to the OT center tap with a resistor? Just like the dropping resistors that are used further along the B+ rail? Can I use a resistor after the rectifier and before the OT CT?
I have 770 VAC from the PT and 415 VDC on the plates. Can I get that number below 400 VDC?
Thank You

[jaysg]

This is more or less the realm of the Big Ass Zener (BAZ). mouser has a pile of 50W versions from NTE that only show online...not in the catalog. Choosing a value is a ball park kind of thing. If you want to drop 20V, the bias current changes a bit and the resulting voltage isn't B+ - 20V for a 20V zener. The versions that end in 'K' can be bolted to the chassis.

[C Moore]

Sorry, but I just do not have any experience with Zener's. Are you talking about something like this?
Thank You

http://www.mouser.com/ProductDetail/NXP ... ppXUZtA%3d

[rp]

http://www.nteinc.com/specs/5200to5299/pdf/nte5240a.pdf

[Phil_S]

That thing is only 1/2W (actually 500mW, the same thing). Since the OT CT takes it's voltage from the first node and feeds the power tube plates, typical 2x EL94 amp, I think you've got, in round numbers 100mA @ 300V. That's 30W. OK, it's probably more like half that, but you have to derate things like this for safety. The other fellow who said BAZ really did mean it. This would be a honkin' big thing that needs a serious heat sink to go with it.

When you drop the B+, it has to go somewhere. It turns to heat. I have never tried this as a means to drop B+. I see this topic posted often enough, and from what I read, I don't like the side effects.

Really, the best thing is the get the right transformer. Some tube rectifiers drop more voltage than others (5Y3, 5U4).

[C Moore]

Oh wow, I missed that. Sorry.
50 watts.
Thank You

Phil -
Yeah, I think you are probably right. It would just be better to get the right PT.
Thanks Again

[Structo]

I used a big stud mount Zener on my 6V6 amp.

I put it on the power transformer CT.

I think it was a 25 watt stud. It gets damn hot too! I mounted it through the chassis for heat dissipation.

Not sure I would try it with a bigger more powerful amp.

I would be more inclined to get the proper transformer before going that route.
Anything you try is going to result in a lot of heat.

[TheGimp]

Easiest and cheapest way is to add a resistor between the output of the rectifier and the first cap which feeds the plates.

Say you want to drop to the values shown on the schematic.

Pd =12W
with 15V bias and 240 Ohms you have 62.5mA for two tubes.
Call it 315 on the plates => 12/315=38.1mA so you are below max plate dissipation.

You need to drop from 415 to 315V at 67mA (add a couple mA for the gain stage tubes) so you need 100V@67mA=1492 Ohms.

Pd =100*.067=6.7W so you need at least a 10W resistor. Best to use two 10W 3K resistors in parallel or two 750Ohm 10W resistors in series.

This will introduce a good bit of sag under load if it is class AB1, which I can't tell from the schematic, but looking at the 6V6 data sheet it looks like it is AB1.

The Schematic also does NOT show a cap at point A after the rectifier 5Y3 and it should have a 20uF. I'd add at least a 47uF after the resistor, and preferably a 100uF or a 220uF to keep the voltage up as you will have a bit of a time constant with the 1K5R plus cap value.

[C Moore]

I did add a 22/500 cap at node A. I guess it would be fairly inexpensive to try a couple of 10 Watt resistors. I just can't get over how high the voltage is from what appears to be the stock PT. I suppose it would be in the 150 dollar range to have Magnetic Component wind a PT for me?
Thanks

[tubeswell]

I used a big stud mount Zener on my 6V6 amp.

I put it on the power transformer CT.

I think it was a 25 watt stud. It gets damn hot too! I mounted it through the chassis for heat dissipation..

You need a large zener if if is a bolt-on type, because it actually picks up a lot of heat from the chassis.

Alternatively you can use one or more 9V-15V 5W axial zeners in series mounted on tag strips (banded ends pointing to ground) between the B+ centre tap and the ground. 5W is ample considering that you are dealing with a maximum of (say) 150mA on the B+ for a typical 2 x 6L6 amp, and dropping (say 15v) is: 15V x .150A = 2.25W per zener. For 6V6s it is even less (e.g.: around 70mA on the B+ in total for a 5E3, which is 15V x .07A = 1.05W per zener).

[Phil_S]

. I suppose it would be in the 150 dollar range to have Magnetic Component wind a PT for me?

IMO, this is not the appropriate dollar range for the Maggie. There is nothing special about it. You need a generic 290-0-290 +/-5% ~120-150mA with 5V @ 2A and 6.3V @ 1.7A (includes something for the pilot light).

I think I would go to Edcor for something like one of these:
http://www.edcorusa.com/category/37-xpw ... oltage=350
If you need a z-mount (lay down through the chassis) rather than x-mount (stand up), for a modest charge I believe they will get it done for you. About $50 is the right price. You may have to drill a couple of holes for the mounting bolts due to size, but I don't think that's a big deal.

As an alternative, if you have a bit of a sense of adventure and lots of patience, you can haunt eBay for a used transformer. These are generally plentiful, but you have to wait for one in the correct voltage. You can get one for $25-$40 including shipping. It may come with short leads that you have to splice on some wire, but that's no big deal.

Finally, my favorite place for used transformers:
http://www.oldradioparts.com/
He's a little cranky about email, so follow his directions carefully if you want a timely reply. I think he must get lots of spam. If you don't see what you want, just make a general inquiry because he sometimes has stuff that hasn't been listed. At his prices you can buy 3 transformers and see which one works out.

Good luck.

[cobalt]

Let's try simple.

If I remember correctly, this amp had a field coil speaker. If so, that field coil acted as a choke, yes a choke. Maybe only 3H, but a choke.
My advice is to put a 3H to 5H choke(rated to handle all the current of the entire amp) in the first node of the power supply. Maybe Hammond 159s.
It will bring down the voltage, making the rest of the dropping string more managable, and make more current available.
Please correct me if I'm wrong.

weirddave

[C Moore]

I thought a choke dropped less voltage than a resistor? Maybe right off the rectifier they are more effective?
But I really don't know, you guys have all the knowledege.....

[Phil_S]

A generic 5H choke (or about that) from Hammond is cheap. Weber has a cheap one. Used ones are cheap and easy to find on eBay. Since the inductance rating isn't critical, you might scab one into the circuit and see what it does. Just make sure it is rated for enough mA, I'd guess around 120mA.

[soma_hero]

Hired Hand,

Most power supplies we see are "capacitor input" filters. This means the rectifier goes straight to a filter capacitor and the OT CT, and then to a choke and dropping string for the preamp and screens.

What Cobalt is talking about is using what is called a "Choke Input" filter. This uses a choke immediately after the rectifier and before any filter capacitors. The output of this choke is now .707 times the input B+. With capacitor input, we get peak voltage so the output is higher, with choke input we get RMS so the value is lower.

The bummer, is you need a choke that is rated for all the current the amplifier needs. With normal capacitor input and then a choke, the choke only filters the screens and preamp so it is only 50ma or whatever. With Choke Input we need a choke that is rated for all the plate current + screen current + preamp current, likely a couple hundred mA. The plus side is lower voltage if you're trying to reduce voltage, and the voltage is much more stable and won't sag as much. A choke resists changes in voltages so it helps stabilize and reduce sag.

You can google Choke Input Filter if it's unclear.