4x6550 stereo power amp

[Bob-I]

..... in my stereo 4xKT88 amp.

Bruce, if you're willing to share the schematic I'd appreciate it, if not I understand, but could you take a look at my first go at this schematic?



A friend of mine used to use a tube power amp for his PA and asked if it was possible to build one. I have iron that I happened on cheep a few years ago, 2 Hammond 1650R UL output xformers and a Hammond 287CX, a monster 400-0-400 capable of 465ma

I'm thinking that I can use PP 6550's or KT88's, a single gain stage and a cathode follower into the LTP PI. I have 4 Sovtek 6550's I got in the same deal years ago. I actually started building this then but got a bit nervous about it so benched it for awhile.

I have concerns of course.

1) Is the B+ going to be too much? If I use a full wave like I drew I can expect 600V. Can 6550's handle that kind of voltage? I know most are rated at about 500V and I have 4 Sovtek 6550's that I can use for this. Under a load that may be more like 575V but I'm calculating closer to 600.

2) Where can I insert a gain control? After the CF?

3) I'm questioning the filtering. The largest 220uF cap I can find is 385V so 2 would be 770V.

4) I drew up 2 rails, once for each pair of 12AX7's. Will the single B+ be ok, or will there be crosstalk between the 2 OT's.

5) I have 2 standby switches, then I started worrying about the load on the B+ changing. I'm thinking that if this amp is ever needed in mono mode I'd have to connect a dummy load, the SB would prevent this need.

I only drew up one channel, of course I'd duplicate this.

http://www.patmedia.net/bob-ingram/SPAschem.jpg


A few comments on the schem.

The CF is there just because I have 1/2 12AX7 to use. Comments welcome. Maybe it should be after the PI? Maybe I don't need it at all?

The tail and cathode resistors are reversed on the PI, I'll fix that later.

The NFB is not drawn... oops


Thoughts?

[Satamax]

Hi Bob, not Bruce here. Just me.

Nice looking schemo. But why thge CF? I'd use it as a gain stage instead witha master volume between the input stage and the gain stage. Caps wise, http://www.turnstyle.com/nsc/catalog.asp?raster=58CE.0x4D2870C09E3359D7.6E04.0.A667&category=cancaps

You've got dual 200 at 500V in LCR types.

And 2X100µ in a can at 500V in F&T. to make up a 200µ, both of theses are good imho. If you want i still have 4 new LCR 200µ@500V 40mm cans. PM if you want them.

Bye.

Max.

[DirtyGeorge]

This looks like a good candidate for a Marshall Major circuit. Replace the CF with a concertina PI and turn the LTP into a diff amp and your there.

Ray Ivers seems to like that circuit and that's good enough for me.:D

DG

[Bob-I]

Caps wise, http://www.turnstyle.com/nsc/catalog.asp?raster=58CE.0x4D2870C09E3359D7.6E04.0.A667&category=cancaps

You've got dual 200 at 500V in LCR types.

And 2X100µ in a can at 500V in F&T. to make up a 200µ, both of theses are good imho. If you want i still have 4 new LCR 200µ@500V 40mm cans. PM if you want them.

Bye.

Max.

500V isn't going to be enough. The DC unloaded will most likely be upwards of 700V so I'll need to caps in series. These only have a common ground so I can't use them. Good thought though.

[Bob-I]

This looks like a good candidate for a Marshall Major circuit. Replace the CF with a concertina PI and turn the LTP into a diff amp and your there.

Ray Ivers seems to like that circuit and that's good enough for me.:D

DG

Ok.... I don't know anything about the concertina PI. Is that the one used on the 200watt Marshall's where a single triode is used? Like this schem?

http://schematicheaven.com/marshallamps/major_1967u_lead_200w.pdf

What design criteria is required? I can troubleshoot an LTP easily so I'm concerned about using something I no nothing about.

[Satamax]

I think the one with the concertina is this one http://www.schematicheaven.com/marshallamps/major_200w.pdf tho, may be the one Ray likes is the one you linked to. The one you linked to hasn't got a full concertina, in that it doesn't use the cathode output of the triode as a phase spliter output.

[Bob-I]

I think the one with the concertina is this one http://www.schematicheaven.com/marshallamps/major_200w.pdf tho, may be the one Ray likes is the one you linked to. The one you linked to hasn't got a full concertina, in that it doesn't use the cathode output of the triode as a phase spliter output.

Thx, I'll read up on this one. Looks interesting.

[Amp Kat]

IMO the long tailed pair or differential splitter would be better as it's more balanced than the concertina. For guitar applications the concertina would be ok. The whole reason is for Hi-Fi you want the least amount of thd% possible and a close match on your output tubes. That's why amps like the marantz 8B Fisher SA300 have such high regards to tube matching. Hum is a big issue with tubes and closely matching them and keeping the PI and preamp as linear as possible a step in that direction. Just my .02

[Bob-I]

IMO the long tailed pair or differential splitter would be better as it's more balanced than the concertina. For guitar applications the concertina would be ok. The whole reason is for Hi-Fi you want the least amount of thd% possible and a close match on your output tubes. That's why amps like the marantz 8B Fisher SA300 have such high regards to tube matching. Hum is a big issue with tubes and closely matching them and keeping the PI and preamp as linear as possible a step in that direction. Just my .02

I think I'm going to stick with the LTP, I know it better and know how to balance it.

So that gets back to the question of the unused triode. It's obvious that the CF is not required so I've redrawn the schem using it as a gain stage with a gain control. I also corrected mistakes pointed out earlier.

http://www.patmedia.net/bob-ingram/SPAlayout_noCF.jpg

[Amp Kat]

That looks good Bob and the only issue may be the 6550 matching. If you measure equal ma's side to side and can get it pretty close with a really well matched pair of tubes your good. If you can't get it real close you may op for individual bias pots on each tube because you have to figure over time they will start to drift apart and the hum will get more noticable when they do. It just depends on how it bothers you and if not then your cool.

[Bob-I]

That looks good Bob and the only issue may be the 6550 matching. If you measure equal ma's side to side and can get it pretty close with a really well matched pair of tubes your good. If you can't get it real close you may op for individual bias pots on each tube because you have to figure over time they will start to drift apart and the hum will get more noticable when they do. It just depends on how it bothers you and if not then your cool.

Good points, thx.

IME not only do the tubes drift, the OT may not be balanced in DC resistance but is perfect in AC impedance.