Still in need of cooler 6L6's....advice please

[Chuck H]

Just looked at the schem again. It shows a 100v and a 115v primary option.

But hey, if the pink wire isn't there...

Chuck

[Garydean]

Absolutly no pink wire on this PT.



2 greens
2 blues
1 white
1 red
1 black
1 orange
1 brown

Gary

[Garydean]

As I read through the threads, I'm pondering with the idea of somehow lowering the B+. I don't have experience with altering a SS rectifier or adding a choke. I do need a plan and I'm willing to do whatever it takes to get the B+ down to about 500V. I do understand it will change the sound.

[sportster4eva]

How about a bucking transformer setup before the power tranny? If you can locate a 120:12v unit with enough current capacity you could use it to drop the 12volts giving you somewhere around 112v at the input of your PT. Based on the readings you gave earlier in the thread your PT has a ratio somewhere around 6.5 to 1. That should get you back down to 700v unloaded, with the filaments being somewhere around 6.1v A 24v trans would get you down to 100v in and 650v out, but the filament voltage would be a little too low, unless you switched to the 100v tap. Do you have a variac you could test with?

[Garydean]

I do have a variac.

[Chuck H]

I can only imagine how frustrated Garydean feels. So now that it's come this far, let's burn it down:

Unless a bucking transformer of greater than 12v is used the filter caps will still be under considerable stress all the time. If a bigger bucking transformer is used then the filaments will be too low. And by the wiring given I'd say that the PT doesn't have any primary options either. Nor does it have a CTR tap on the B+ secondary so a big zener or a bucking transformer there aren't options. And obviously the amp can't be made reliable the way it is. I don't know of a rectifier arrangement that will provide 500 volts.

So, the way I see it, there are only three options.

One is to change to a bridge rectifier and series the B+ rail with a choke and different screen and dropping resistors. That should be good for about 400 volts on the plates.

Two is to change to 6550s and beef up the filters. Provided the filament winding can handle the extra current.

Three is to change the PT.

I don't think I missed anything in the details. And I wish it could be easier than that. But if I'm way off someone will be around to correct me soon enough.

Chuck

[Garydean]

Chuck, thank you!

RE: frustration.... ... lol

I like option "one" as it seems to be the least costly and gets me where I would like to be as far as high voltage.

[Enzo]

Gary. we were past the pink wire, I was looking for that yellow wire on the primary side in your drawing. The orange is the 115v and the yellow is the 100v primary tap. if it isn't there it isn't there.

[Garydean]

Thanks Enzo. I did realize that a bit later as I was re-reading everything..

[Garydean]

FINAL RESULTS

Here's what I did.... I replaced the 100/350 caps with 100/450 caps, threw in new 6L6GC tubes, and sent the amp on it's way.

The report from my customer was great. He rehearsed for 2 hours with the amp on 10 and didn't have to call the fire department. I did stop by his rehearsal room last night to have a look at the plates and no signs of orange plating...phew.
If you're in Philly on the 31st of Jan, stop by and listen to the amp.....bring a fire extinguisher..

The chilling Details
January, 31 2008 at The Fire
Philadelphia, Pennsylvania

Look at the name of the venue....haha, they have no idea.

Thanks everyone for your advice and education. Anyone wanna buy a soldering iron.....just kidding.

gary

[J Martin]

I got my 1236 with the blown output transformer fixed also. I put in a Hammond 1650R and a pair of 6550's so its got plenty of power now. The amp looked like it had been left out in somebody's back yard and had dirt caked inside. Customer emailed and said he is very happy with it.

I am only 15 mins from the Fire in S Phila.