Is this Mordor glow on my KT66s normal?

[ezietsman]

What is your B+ and pri of the OT?

B+ is 485, Primary is 7500Ω

[Roe]

What is your B+ and pri of the OT?

B+ is 485, Primary is 7500Ω

this can be hard on screens, esp. if the screen voltage exceeds plate voltage

[ezietsman]

What is your B+ and pri of the OT?

B+ is 485, Primary is 7500Ω

this can be hard on screens, esp. if the screen voltage exceeds plate voltage

So should I increase the screen resistors some more? Keep in mind I'm running KT66.

[neikeel]

B+ is 485, Primary is 7500Ω

Keep in mind I'm running KT66.[/quote]

Where is the OT from (is it a JTM45RI OT)?

Normal B+ for those is around 400v and an original 450v.

[ezietsman]

Yes, its a JTM45 RI OT.

[Haze13]

According to Gnalex KT66 datasheet, screens should be around 390v for 510v b+ and 41,5mA on a cathode... How high are yours?

[ezietsman]

According to Gnalex KT66 datasheet, screens should be around 390v for 510v b+ and 41,5mA on a cathode... How high are yours?

Mine is around 10V lower than the plate, so 475 for a 485 B+, my voltages matches up very well with the 50 watt documentation, except that they're all a bit higher because of the Classictone 18033 PT I have.

[ezietsman]

OK, judging by their glow in a dark room they are MUCH dimmer than they were with the 470Rs (and of course I didn't have the 1k flying resistor because I didn't see it), so the 1ks did have an effect. I guess I could install a 1k flying resistor and put the 470s back or just leave them be as is.

[Haze13]

http://i1276.photobucket.com/albums/y47 ... dbxajg.jpg

Less than 400v for the 50watts - fixed bias

[ezietsman]

http://i1276.photobucket.com/albums/y47 ... dbxajg.jpg

Less than 400v for the 50watts - fixed bias

Well, you won't have these numbers in a Marshall. Besides, the screen voltage won't do anything to the screens, its the current that heats them up and kills them. I think I'll try a 1k flying resistor, and if that doesn't affect the sound then I'll leave it be, else I'll leave it in its current state, they should be ok with the 1k screen stoppers.

[Haze13]

510v is a bit higher than a 485... 80mA per two tubes is not reachable? it's only 40 mA per tube, cab be done in a regular 50 watt marshall. Try it with a 400v on the screens. RC for lowering the voltage and 470 to the screens... I'd try if I had that problem. Some thing is heating up more than it should be, that's for sure. When you play, current through the screens is rising too.
Have you tried another set of tubes?

[ezietsman]

510v is a bit higher than a 485... 80mA per two tubes is not reachable? it's only 40 mA per tube, cab be done in a regular 50 watt marshall. Try it with a 400v on the screens. RC for lowering the voltage and 470 to the screens... I'd try if I had that problem. Some thing is heating up more than it should be, that's for sure. When you play, current through the screens is rising too.
Have you tried another set of tubes?

What I meant was, in that datasheet, you have 500V on the anodes and only 400 on the screens. In a Marshall the screens are not at 80% the voltage of the anodes, they are much closer. So to make it match that, I will need to severely deviate from JTM45 or 50 watt circuitry, which I don't think is the answer.

At the moment, I have 1k of resistance between the screens and the screen supply, this is the 50 watt spec, however, in the JTM45 there is 470R per screen and another 1k that is shared between them, I *think* this amounts to an effective 2k5 per screen. Now, because I'm never going to run this amp clean, but driven pretty hard as a rule I think I should just add the extra 1k flying resistor too. This will increase the effective resistance a lot more so the screen current under heavy loads will come down sharply. Just doubling them already had the effect that instead of bright yellow they only glow orange. I really need to measure the current but don't have the equipment.

So this is my plan, add another 1k of resistance that is shared between the two screens, and if the amp still sounds fine then I should have plenty of protection for the screens. I'll have more than a stock JTM45 has, at any rate, and maybe that'll be ok because I have more voltage and more gain too. Does this seem like a reasonable argument?

[neikeel]

What I meant was, in that datasheet, you have 500V on the anodes and only 400 on the screens. In a Marshall the screens are not at 80% the voltage of the anodes, they are much closer. So to make it match that, I will need to severely deviate from JTM45 or 50 watt circuitry, which I don't think is the answer.?

You are well off stock Marshall JTM45 and JMP50 voltages already. A stock JTM45 has 450v on output plates (a RI around 400v), a a JTM50 with GZ34 around 420v, a solid state rectivied plexi JMP50 about 420-440v and later ones (early metal panels) are anywhere from 360 to 400v.
The Classictone PTs seem to come up consistently high in this respect.

[ezietsman]

Totally. I don't think the higher voltages are hurting though, my only issue is the screens drawing too much current under load. They aren't dropping their voltage under load as much as the plates so I probably just need to add more resistance to them to make that happen. Maybe this is a consequence of the higher voltages (or maybe even the lack of a choke).

[Haze13]

First. Classictone has a PT for kt66 tubes, last time I've checked... 3-4 months ago.
Second. New kt66 tubes are not the same as the old ones, MAY BE some brands but there are a lot of them that do not even look like a kt66 from the inside - so you can not treat them as a regular KT66. So, what exactly are you using?
Third. You have not tried to use what you have as written in a datasheet, so may be you will like the sound of it more than you have right now. There were really smart people who wrote these datasheets, so don't throw the idea just because it's not as the marshall scheme says. Just try and put good filtration for them, as they should be more or less stable...