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

[Garydean]

I threw 6L6GC's in last night and played it for a bit. Plates got slightly orange. Prior to plugging it in I did fire it up on the bench and took some readings with the GC's in. I tried to bias the tubes beyond -30 and the B+ got to around 711+. My line voltage last night was 122+.

Seems I can't bias the tubes without the B+ going through the roof.

But the amp sounds great......

Gary

[52 Bill]

Wow 711 volts! Without doing mods to the amp, you're really going to be testing the limits with this one.

As you increase the bias voltage, the output tubes draw less current and therefore the supply voltage drop is lessened and the voltage goes up. This is the way it always works, more current-less voltage, less current-more voltage.

Have you taken a voltage reading of the power supply with the tubes pulled? This will let you know what the maximum voltage level can be.

Edit: I just looked at the supply schematic. When you recapped this, what voltage rating caps did you use in the first filter/doubler set? The original 2 added up to a total of 700v, already below the voltages you've measured.

[Garydean]

Hi Bill, Unfortunately I did not put in higher voltage filter caps. I never imagined seeing over 700V.

When I remove the tubes where do I measure in the power supply? At the filter cap?

This ones scary....I can't let it go like this.

thanks,
Gary

[52 Bill]

Measure at the top of the filter cap totem pole, where the center tap of the output transformer attaches. Or at the tube sockets, you just need to know what the maximum available voltage is. Be careful and take a quick reading.

[Garydean]

Gottcha, will do.

[Garydean]

I popped out the output tubes to measure 809VDC on the plate socket. My line voltage is now up to 124.6VAC in this building. Threw the tubes back in and still measured 809VDC on the plates!!!!

[sportster4eva]

I popped out the output tubes to measure 809VDC on the plate socket. My line voltage is now up to 124VAC in this building. Threw the tubes back in and still measured 809VDC on the plates!!!!

That's 100 volts over the cap ratings. Be careful man.

[Garydean]

Most careful. I had it on for no longer than 5 seconds. I also measured
6.8VAC across pins 2 and 7.

[52 Bill]

That's one heck of a lot of B+.

Does anyone know if any new manufacture 6L6's are rated anywhere near 800v max plate voltage?

It seems like if you are going to get the output tube current draw down to a reasonable level, you're going to be somewhere in the area of 725 volts on the plates.

You will have to upgrade the filter cap voltages as well.

[Garydean]

I'm just guessing, or stabbing, what would it take to redesigning the rectifier, and would that be an angle so I don't mess with 800V anywhere in the amp.

[Chuck H]

I was just starting to think the same thing about this thread. I don't know if you could keep the same character to the sound...entirely. But at least it will be usable.

If you change from the voltage doubler to a four diode bridge you should get about 400 volts on the plates. A much more typical and sane voltage. You'll need to put the screens in series on the B+ rail. I think adding a choke instead of a resistor would be best to help firm up the tone after the plate voltage drop. reduce the screen grid Rs to 470 ohms. The preamp voltages will tank unless you change the dropping resistors in the B+ rail so they match the schem. Other than mounting a choke, I don't really think it would be that bad. And I don't think the tone or volume would suffer much if at all.

Chuck

[Enzo]

Just a thought, I mentioned the pink wire before, and there is none on yours, but is there any chance the amp DOES have the 100v tap on the PT primary? That would be what Japan runs it on, and if 120VAC at the primary was connected to the 100v tap, all your voltages would be MUCH higher than they should be.

Just having a hard time imagining how this amp makes 800v.

[Chuck H]

I thought about that too. But a couple of posts ago it was mentioned that there was 6.8 volts on the filaments. If the PT primary was on the 100v tap that should be much higher. If it is on the 100v tap now and it is changed to a
115v (or 120v) tap that would place the filaments below 6v.

Chuck

[Enzo]

Yeah but it is worth a shot.

[Chuck H]

The PT could have a 110v tap.

That would help. That would knock down the B+ about 60 volts!!!

Chuck