Need a project idea for higher voltage vintage iron

[chopstuck]

I have a old mystery PA that ran a pair of 6l6's and 6sn7 and 7b7 preamp tubes with a 5u4 rectifier. The secondaries on the PT measure 408-0-408. It has a whopping great choke (86 ohm) the size of it's hefty OT. It also has a smaller choke thats marked 500 ohms but measures 33k, (I think it's fried.) I believe the whole amp took it's B+ voltage after the big choke. It was hard to tell, a lot had burned up in there. The whole PS had only three 10 mfd can caps that had screw on rings to secure them to the chassis for filtering. I imagine that chokes were cheaper than good high voltage capacitors in the 50's - the era I'm guessing this amp came from.

I was thinking of a country-ish percussive amp with edge in the 40+ watt range using this iron.

I have read about the standard PI filter and this amp looks like it had the big choke before the first node and the small one between the first and second. I'm interested in what anyone would think the V+ drop may be.
I'm shooting for equivalent to a 370-0-370 PT.


When loaded and possibly used this way what kind of power tube plate voltage could I expect ? I was hoping for a RT 66 style amp with an additional preamp channel and wanted to try this chassis. It had a lot of really good parts that I would like to recycle.

[rfgordon]

This iron might make for a cool build. OK, as a general rule of thumb, taking an "average" 5AR4 as the reference, you can expect around 1.25 times the HT voltage to come off the recto as B+. So, for your PT, that's 1.25 x 408 = 510. Now, with a 5U4, you might get down to 490 VDC.

That may seem pretty big B+ for 6L6, since the data sheets will say 500 VDC as max recommended plate voltage for audio applications. I would venture a guess that your PA is cathode bias--yes? In cathode bias, the voltage the tube "sees" is plate voltage minus cathode voltage. So you could, if you wanted to, run a pair of 6L6s cathode bias, even with this high a B+, as long as the cathode resistor is big enough, say 330 ohm shared.

If you opt for 6L6s, I would lower the screen supply voltage by putting a 20 mF filter after a 10 k to 20 k resistor off of post-choke B+. Then definitely put 1k5 or 2k2 5 watt resistors on the screen pins. Lower screen grid voltage and current limiting will keep the tubes much happier in a high B+ environment.

On the other hand, this voltage range works really nice for a pair of KT88s or 6550s, either fixed or cathode bias, though they may draw too much filament and/or plate current. However, a single KT88 or 6550 is extremely cool-sounding, and would be safe to use given your voltage and the unknown current handling capacity. This would safely free up some HT current in case you wanted to put in a BF-style reverb circuit using a 12AT7.

Happy soldering!

[Jana]

You might want to take a look at a schematic for a Sunn Model T. If memory serves me right, it had a couple of chokes in parallel (to increase the current rating) before the feed for the plates. I don't know what the calcs are, but you get a lower voltage with a choke input filter (no cap before the choke). If you do filter before the choke, make sure it is small, 10 to 20 uf, that will give you some filtering before the choke but won't raise the voltage all that much.

Something else I have done is this: rectifier diodes> 10uf> 50ohm/50watt resistor> 100uf, plates> choke> 50uf, screen feed > etc.

for a pair of 6l6's you can expect to see about 15 to 20 volts less at the plates than if that 50R resistor wasn't there. It is also kind of a "sag" thing since it drops more voltage as the power draw goes up with volume.

I did this when I had a transformer that was giving me about 510 volts and I wanted to get things down below 500 for the plates. I didn't want to have a butt load of caps in series with bleeder resistors to meet voltage limits of 500 volt caps. Only the first section was two 20 uf caps in series to give me the 10 uf.

You could also check out the Weber Copper Caps, I have never used one, but on his website he gives some choices of various voltage drops, some up to 50 volts.

[M Fowler]

Chopstuck

Is that an old Stromberg-Carlson PA amp you have?

Mark

[chopstuck]

RF Gordon and Jana, thanks for the input. I believe the plan then is to have a choke input filter. A single 20mfd after the choke at the first node for the plates then a 10k-20k resistor and 20mfd for the screens and the 1.5k to 2.2 k on the screen pins. Then another 20-40mfd for the preamp tubes. Does that sound right ? Does limiting the the second filter to 20mfd decrease the screen voltage vs a 40-100 mfd filter at that node ? Do I run another smaller choke before the screen suppies or will the 10k-20k resistor do that job well enough? I plan to run cathode bias. (Seems squishier with that great tweed bottom as the PS recovers.)

I'm thinking back to the Fender TV Pro that used the field coil speaker as the big choke. I just looked at the old schematic again and it shows the field coil to ground and not in line to the nodes. Does this use provide any PS filtering at all ?
BTW, The amp had no markings it was just a chassis. I will try to post a pic later.

[chopstuck]

delete double- sorry

[Jana]

The cap values you mention seem a bit low, I think you might have some issues with motorboating and hum. The cap after the choke impacts the voltage much less than the one before it. You can try it like this but maybe leave yourself open to the option of increasing the filtering. If it was me, I would have 100 uf after the choke feeding the plates, 50uf for the screens, and 50uf for the preamp node. But this would take out some of the squishy factor. I like solid and no hum, hence the larger values. Some like squishy and can live with some hum, that's okay too.

A 10 to 20K resistor for the screens instead of a choke should do fine.