Quick question, Sunn Sorado bass amp B+ voltage

[Diverted]

Hi all,

Just working my way through a Sunn Sorado bass amp that came to me blowing fuses. One output tube was shorted, GZ34 shorted as well (too bad, a really nice Mullard). The amp had never been touched. Replaced electrolytics and output stage coupling caps. It's playing fine now.
My question: B+ at 122VAC house voltage is around 550, a good bit north of where it should be per schematic (500). It's running new Gold Lion KT88s. Tubes are biased around 60 percent.
 I'd like to bring the B+ down a bit and was thinking about using a few zener diodes from the HV center tap to ground. I have never worked on an amp of this size/power before. I have some 10v 5w zeners on hand, was thinking about stringing 2-3 of them in. Will these be up to the task or should I go with higher wattage zeners?
Or, would a dropping resistor after the rectifier be a better option?
Thanks!

[shooter]

If it's sounding good and you're not exceeding the 88's Pdiss, play it, maybe add a 100ohm G2??.  IFF you wanna experiment; With a cap/choke input, I would be inclined to "shift" the taps left, adding a "new" C, but you'll wanna make the 2W current "B" tap something closer to 5 -10W.

[Diverted]

Thanks! Don’t think I want to add another filter cap on the power reel. Just one clarification? What do you mean by G2? I get the 100 on my part. Thank you!

[sluckey]

G2 is screen grid.

I would not mess with zeners or resistors. Bias the tubes hotter and that will suck the B+ down a bit. I would be more concerned about the voltage rating on those filter caps!

[Diverted]

OK, thanks. I wasn't thinking the original responder was referring to screen grid resistors when he said G2; I was thinking the response had to do with my question about dropping overall B+ voltage. I'll add some in; one question, I see 470 ohms a lot there; why 100 in this case?

Anyway, so my original question remains: Would the 5w zeners be up to the task of reducing B+ voltage? Or should I use a dropping resistor after the rectifier but before the first filter cap? Thanks!

Ted

[shooter]

so my original question

I would not mess with zeners or resistors. Bias the tubes hotter

If it's sounding good and you're not exceeding the 88's Pdiss

[Willabe]

Anyway, so my original question remains: Would the 5w zeners be up to the task of reducing B+ voltage? Or should I use a dropping resistor after the rectifier but before the first filter cap? Thanks!

I would not mess with zeners or resistors. Bias the tubes hotter and that will suck the B+ down a bit. I would be more concerned about the voltage rating on those filter caps!

I would try a bucking transformer. Here's the link to Sluckey's web page scrap book. It shows how to build one.

I'd try that and if still needed bias the power tubes a little hotter like Sluckey suggested.   

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[Diverted]

Great idea. I have a few 120 to 6v 5a transformers. Thanks.

[sluckey]

That is a well designed amp, especially the power amp. And the power tubes can easily handle that B+. You may want to read up on ultralinear operation a bit.

[Willabe]

The bucking transformer would give that amps B+ filter caps a little more breathing room, like you said, they could use it.   

[Diverted]

The bucking transformer would give that amps B+ filter caps a little more breathing room, like you said, they could use it.   

I was pretty worried about that too. So I made the first three filter caps 68+68, 39+39 and 39+39, each in series, each cap at 350V, with 220K smoothing resistors. So the first three caps are effectively 700V caps :)
The final cap is 19@50V and is only seeing around 345.

You're right. I know very, very little about UL output transformers.

Thanks!

[Willabe]

I wasn't thinking the original responder was referring to screen grid resistors when he said G2; I was thinking the response had to do with my question about dropping overall B+ voltage. I'll add some in; one question, I see 470 ohms a lot there; why 100 in this case?

Because that amps OT is ultraliner, it has screen taps that feed the power tube screens. The taps help limit current draw by the screens, it helps protect them. The taps are used instead of a screen grid R, sometimes companies use both, the tap and a G2 R.

I think shooter was saying to add 100R for a little more screen grid protection because the B+ is a little high.

Can't hurt. I'd probably add them. New production tubes are not always able to take the stress as NOS tubes can. 

[Diverted]

Final question: All I've got on hand for 100 ohm caps are 2w. Will this cut it? I do have a bunch of 5W sand 250 ohm resistors as well.

[Willabe]

Final question: All I've got on hand for 100 ohm caps are 2w. Will this cut it? I do have a bunch of 5W sand 250 ohm resistors as well.

I'd use the 5w sand with those big KT88's. The 250 ohms, I don't think you'd hear any difference.

Some guys like to use larger R's for G2, 1K and sometimes even a little more. A larger G2 R can/will add some compression.   

[Diverted]

Will do, thank you. Final question:
As for the polarity switch, should I just take it out of circuit? I have a three prong cord on there. Thank you!

Ted

[sluckey]

I would leave it.

[SoundmasterG]

Since you used series connected caps of the first couple nodes, there is no reason to drop the B+. As Sluckey said these are very well designed amps, based on the Dynaco Mk. 3 power amp and Dynaco PAM 1 preamp. They are also UL, which makes them different than just about any other amp you will usually see. I wouldn't use larger than stock screen grid resistors as you will begin to really limit the power of the amp. UL isn't like the usual amps you would see like a Fender. Why reduce the B+? The amp will be bulletproof now that you replaced the caps. It is higher than the schematic because when the amp was made the wall voltage was centered around 110-115 instead of 120-130 and the power transformer was designed with the lower voltage primary in mind. All vintage amps have higher voltage than the schematic these days. Its not a problem. Adjust the bias up hotter as the others mentioned and the B+ will go down. Get the bias up around 65%-70% of the max dissipation of those KT88's and it will purr along nicely.

Greg

[Diverted]

Thanks Soundmaster et al.

I'll just let it roll as is then.
I originally thought about dropping the B+ because I didn't originally think a bucking transformer would be the best option. At regular house voltage the rectifier filament and heaters are right where they should be, around 5.3 and 6.3vac, respectively. So dropping the overall voltage into the amp would bring those down; wanted to leave them as close to spec as possible. I was just concerned overall because having never used KT88s before, I wasn't sure that the new stock Russian-made Gold Lions I got for it could handle the 550 or so plate voltage biased at 60 to 70 percent. I'm used to seeing 6V6s and 6L6s with 100-200v less on the plates. I was in uncharted territory! So, thanks for the suggestions. I've never dealt with an ultralinear amp before and it's definitely a different animal. The thing's a beast.

Ted

[PRR]

KT88 is good for at least 600V plate and screen.

[Diverted]

Excellent. Thanks for the info on the screen/plate voltages. Good to know.

[SoundmasterG]

When in doubt, look up the datasheet. This is one from New Sensor, the maker of your KT88's, however it looks like they just used a vintage schematic and put their company info at the bottom of the sheet to me.


https://www.newsensor.com/pdf/genalex/kt88-genalex.pdf


Here is one from the original M-O Valve KT88.


https://frank.pocnet.net/sheets/086/k/KT88.pdf


Most of the new tubes can handle voltages close to the vintage examples. There are some that can't, like most EL34's today can't handle the 800V plate like the vintage Mullard datasheets say, but many other types can handle what the vintage datasheets specify.

Greg