8417 Power Tube Conversion

[garytoosweet]

KT88's installed. Pin 1 on the KT88 had no connection.

Amp biased for 70% dissipation, ended up needing -33V bias voltage. So in the end the mod was unnecessary, however it will ensure future KT88's can be biased properly for future owner.

@toddgw, good advice. Coupling caps test OK. They are Orange Drops, will leave them in there.

I will admit I didn't think of ordering retainers. The tubes hang upside down in this amp and the bear claws that are installed don't do a thing for these KT88's. Just get squished over their massive base. So I'll have to pull this chassis out one last time when those arrive.

Played for a couple hours and it's sounding so much better than with the 5881's.

Appreciate everyone's input.

[martin manning]

Nice. I was trying to get the bias trimmer centered at 36V so there would be plenty of range on either side. What does the phase shifter sound like? I've read that it's basically a Maestro PS1-A.

[garytoosweet]

Yes I believe it is exactly a PS1, and sounds as such. Very nice. Cool amps, will probably be more collectible in the future.

[thehero]

I'd never heard of these amps before. Anybody know what that PI circuit is called?

In any case, the bias supply can be modified as shown to use KT88's.

edit: Rev 1 has a change to the negative terminal connection on the second added 10u cap. It now sees a lower voltage, from raw negative bias to -35V. Otherwise, all the same.

Working on the bass version of this amp, it has the same power circuit with 8417s. If I wanted to replace them with EL34s, would these changes be close to what I'd need to do? What would you recommend?

[martin manning]

Working on the bass version of this amp, it has the same power circuit with 8417s. If I wanted to replace them with EL34s, would these changes be close to what I'd need to do? What would you recommend?

You could do that, and the bias circuit mod would be the same, except leave R80 at 22k. As I noted above, you will have to miss-match the speaker load, i.e. 8Ω speaker on 4Ω tap. I think I'd go with KT88's and avoid any messing around with that.

[thehero]

Working on the bass version of this amp, it has the same power circuit with 8417s. If I wanted to replace them with EL34s, would these changes be close to what I'd need to do? What would you recommend?

You could do that, and the bias circuit mod would be the same, except leave R80 at 22k. As I noted above, you will have to miss-match the speaker load, i.e. 8Ω speaker on 4Ω tap. I think I'd go with KT88's and avoid any messing around with that.

What causes the need to miss match the load? I'm not doubting you, just curious to understand it.

[martin manning]

You need load impedance that is high enough to prevent the EL34's from dissipating too much power. See here: http://www.valvewizard.co.uk/pp.html

[thehero]

Thank you!

[thehero]

I’ve done the conversion for KT88, I’m using 6550s though. Any idea what could be blowing C51? Also, since I got it without power tubes I don’t know if it was in working condition before I started messing with it. Since The changes go directly into c51, the simplest answer is that I likely did something wrong. Any help would be appreciated.

[martin manning]

Are the diodes all oriented correctly?

[thehero]

I’m pretty sure they are. I will double check though

[thehero]

Diodes are oriented correctly

[thehero]

If you can understand the poor layout here, this is how I arranged everything, using existing unused traces. Maybe you see something dumb I did that I’m not noticing? Also, D3 and D4 should be 1n4007s? I put those in. The ones that were there were unlabeled, but looked really bad so I replaced them.

[martin manning]

It's hard to see what you have done there in altering the PCB. Is there any possibility you reversed the electrolytic? Other than that, I think I would disconnect the additional doubler circuit and get the low voltage supply working, and verify that all the supply voltages generated from that winding are good. That goes to your statement saying you're not sure that the power supply was working properly when you started. It would have been good to power it up without tubes and checked the supply voltages before making any modifications.

[thehero]

I disconnected all of my alterations and it looked good. Nothing blew, -33v at c51-. Let it run for about 5 minutes, everything good. Reconnected the modification and c51 blows again. I’ve triple checked electrolytics and diodes. I’m stumped. The cap blew before I got a measurement on it to see if it was still what it should be.