EL34 screen supply - where can I take it from?

[jakehop]

Hi all,

I've recently aquired a Simms-Watts PA200 amplifier. The KT88's were swapped for EL34's. Now, there's about 620VDC on the screen grids. The datasheets for the KT88's specify a max of 600VDC, and the datasheets for the EL34's specify a max of 425-500VDC.

I've previously installed MOSFET regulators in old Marshall's, to get the voltage down at about 400VCD for EL34's, to increase their lifespan, but here I think the voltage is too high for that kind of a job.

Can I take the screen grid supply from the middle of two of the series filter capacitors? Or should I just get some KT88 and not think about it again?

Kind regards, Jake

[loudthud]

You can't use the middle of the two series filter caps unless that point is fed by the center tap of the HV winding. If you can find 1000V Mosfets that would work or you could use two 500V parts in series. Kevin O'Connor uses that in many of his designs. IMHO stick with KT88.

[jakehop]

Thanks for your reply. You're probably right. However, there's an unused spot for a switch, so I'm gonna make the screen grid voltage switchable.

By the way, in my experience you do not need a device that can withstand more than the voltage it is dropping plus a reasonable amount for safety, if you're using a series regulator. I use a FET + some resistors and a zener. If I'm dropping 48V I'll use a 100V device, with no problems the last 5 years.

[Enzo]

Getting away with something is not the same as it being a good idea. In typical operation there may only be 220v across the part, but every time you fire up the amp from cold, the filter caps downstream of it are empty, and for a moment they are a short circuit to ground. SO for that moment, the whole B+ voltage is across the device, until they charge. That has the potential to stress the device. Doesn't mean it will blow the part up, it just means the potential is there for the stress. And that is why we design for the whole voltage and not just the part we are immediately looking at.

[jakehop]

Yes, of course, don't get me wrong - I'd never use a cap that's too low. Here, I'm talking about a FET. The zener and resistors keep a constant potential across the FET, so I don't think it's compareable. Please, do prove me wrong. I'm in here to learn!

[Wilder Amplification]

Yes, of course, don't get me wrong - I'd never use a cap that's too low. Here, I'm talking about a FET. The zener and resistors keep a constant potential across the FET, so I don't think it's compareable. Please, do prove me wrong. I'm in here to learn!

Enzo wasn't talking about the caps!

What Enzo is saying is that for the time duration that the filter caps are initially charging up on power up, the FET itself will drop the full supply voltage across it and the drop will gradually lessen as the caps charge up. As such it needs to be rated for AT LEAST the supply voltage (I say full supply + 25-50% at least) so that the FET isn't stressed when the caps are initially charging.

[jakehop]

Really?! That's not what I've been measuring! As I wrote, the regulator is made so it drops a specific voltage, of the B+, in series with the screen grids.

[loudthud]

In the case of the "amplified zener" approach to dropping the screen voltage you only need a breakdown voltage slightly larger than the zener voltage and enough current capacity to charge any down stream caps. You used the term "regulator" so I jumped to the conclusion you meant a series pass regulator.

[jakehop]

Ahh, yes, my fault! But great that my theory pretty much holds up. I should be more clear in my terms.

Kind regards, Jake

[Gtr_tech]

That amp is what it is.....and its designed to take advantage of the capability of the KT88 output tube. I'd leave it as is. It'd make a hell of a bass amp and they're not that common anymore so putting in a bunch of oddball mods will kill any resale value it has. And make reliability a problem. Stuff in some good quality 88s, bias it into the SOA, and get some earplugs. Fresh filter caps is a good idea too to get it to 100%.

The only mod that *may* be beneficial is if the inputs are setup for mic level signal, you may need to tame that first stage gain a little to be guitar friendly.

[jakehop]

Yeah, I'm keeping it as close to stock as possible. However, I don't HAVE KT88's right now, and a quartet is pretty pricey here in Denmark, but you're probably right. I'm gonna fix it up, and see what my bank account holds for my "special purchases". It does sound very good with guitar!