Sunn Sceptre Head 1969

[baddog]

I picked up one of these heads and changed all of the electrolytics except the cap can which is on order. This is the one with , 6550 tubes , reverb and trem but no mid boost.
I ran the feedback loop to a switch on the back of the amp and installed a standard fender input. I also disconected the polarity switch and installed a grounded cord. I have read a little here on mods to this amp and learned a bit about it. the reverb pot is scratchy and the reverb seems to be a little noise . By noise i meen that i get an amplified sound when I turn the reverb up + the scratchy pot thing. I realize by looking at the schematic that their is roughly 16 v on the pot but im not sure if this would cause the scratchy sound. I might just change the pot for the heck of it. What can I do to quiet the amp down. Im used to fender amps that quiet untill i hit a note on the guitar. I have 450v on the plates of the 6550 tubes with the amp biased at 48ma...
I'm going to check the couping caps for dc leakage and i might have to try star grounding or something. Are these amps normally noisy ?

Thanks
baddog

gut shots

[riz]

These photos were taken BEFORE you worked on it, yeah? I see a bunch of old electrolytics in there... Your heater wiring could be rewired as a twisted pair for less hum. You didn't post a schematic, but it looks like there is at least one transistor in there, and sometimes those old solid-state circuits have a lot of background noise. Someone else would probably be better to elaborate on that...

[baddog]

Yup the pics where taken before the parts swap. I'll have to look at the filament wires and see what I can do to clean them up a bit. I still haven't received my new cap can yet . I'm hoping that helps a bit withthe hum. Your probibly wright about the a ss effects causing noise in the amp.

[soundmasterg]

BD, you may get more response by posting here: The sunn Discussion Page - Index . That seems to be the Sunn hangout rather than this section....

Greg

[loudthud]

These amps aren't very low noise considering the low gain. I'd wait for the replacement cap before doing much work on the hum. Make sure the choke isn't shorted, it should be about 50 ohms. The output tubes need to be matched for lowest hum.

Check this thread: http://music-electronics-forum.com/t3137/ It covers most of the problems with these amps.

[baddog]

These amps aren't very low noise considering the low gain. I'd wait for the replacement cap before doing much work on the hum. Make sure the choke isn't shorted, it should be about 50 ohms. The output tubes need to be matched for lowest hum.

Check this thread: http://music-electronics-forum.com/t3137/ It covers most of the problems with these amps.

thanks for the replies .... Ill check back after I replace the CapCan and Ill check out the other thread for ideas..

Thanks Again'
BD

[baddog]

Well just for craps and giggles I took a 600v 40uf weber electrolytic cap and installed it on the rectifier before the standby switch and ran it to ground on the back of the chassis. I disconnected the choke from the 30uf spot on the cap can and ran it to the standby switch. I have 590v on the rectifier / capacitor junction on standby

with stand by off B+ Readings

rectifier 544v
pin 3 6550 536v
bias 40ma

The amp is allot quieter now .... Cool

Could I use 2 x 20uf x 350v caps in parallel instead of the 40uf x 600v?????? With the 2 x 20uf that should = 700v ????

BD

[tboy]

Could I use 2 x 20uf x 350v caps in parallel instead of the 40uf x 600v?????? With the 2 x 20uf that should = 700v ????

No, the voltage rating would remain the same with the caps in parallel. If you put them in series it would double the voltage rating, but the capacitance would be halved to 10uf. You'd need two 80uf @ 350v caps in series to get 40uf @ 700v.

[baddog]

That's what I thought but I wasn't sure....

Thanks

[baddog]

Tried the amp out this morning and it doesn't sound half bad Good reverb and trem with very little noise I dug through some stuff and found 2 x 100uf E-caps and did the totum pole thing for the first filter stage ( will change them out when the new ones arrive) and left the other 3 in the cap can connected. I do get some static when I take my fingers of the strings but I'm guessing it has something to do with the poor grounding scheme in the amp. I did get a kind of sizzling sound through the speakers after I ran the amp for a little bit I wonder if it need new power tubes I'm going to change out the preamp tubes later and see if that helps... I have to work tonight from 6pm to 6am so i don't think ill mess with it until Saturday... I haven't had a chance to crank it up yet and see what happens or how it sounds:-thnk

BD

[baddog]

Well I plugged the amp in again and now something weird is happening. I didn't hear the sizzling noise but when I turn the amp up the volume tapers of after 4 or 5 and I and hear low pulse before it happens. I tried the feedback loop switch and I did get a warmer tone but no increase in volume. I tested the amp with an 8ohm speaker load in the 8 ohm jack and I'm niow playing it through my Fender twin speakers at a 4ohm load. Maybe that jack is dirty or something . I hope its not a OP transformer issue...hmmmm I did notice that the 5ar4, power tubes and PI are glowing pretty hot but the preamp and reverb driver tube are glowing normal.... The amp doesnt seem to be redplating but If i we rto gess I would say its close by the look of it... IM going to change the other caps in the cap can section to axial caps and see if that helps. Changing the 30uf out made a big difference maybe the other caps are failing....

[baddog]

All is well in the amp I changed out the last E-caps and the volume has returned to where it should be. Just goes to show that you cant take old E-cap for granted ... Change them and don't look back I didn't get to check out the amp cranked but I could tell the volume was there just by turning the dial....The amp is noisier with the FB loop disconnected Short of buying new isolated input jacks , isolated speaker jacks and redoing the grounding in the amp I think its as good as its going to get. I think I'll look at the grounding a little and maybe move the first filter stage so its after the standby switch . Maybe that will help with the popping I get when turning of the amp.... I might have to change or clean out a couple pots that are scratchy .

How do I wire the power switch to get the light to work wright? Do I just jump a wire from the on side of the switch to the light terminal on the back of the switch so it sends power to the bulb when the switch is turned on? Not famillure with this switch ...

The trem pulses when its turned down. Is their a fix for this???

Thanks
BD

[soundmasterg]

All is well in the amp I changed out the last E-caps and the volume has returned to where it should be. Just goes to show that you cant take old E-cap for granted ... Change them and don't look back I didn't get to check out the amp cranked but I could tell the volume was there just by turning the dial....The amp is noisier with the FB loop disconnected Short of buying new isolated input jacks , isolated speaker jacks and redoing the grounding in the amp I think its as good as its going to get. I think I'll look at the grounding a little and maybe move the first filter stage so its after the standby switch . Maybe that will help with the popping I get when turning of the amp.... I might have to change or clean out a couple pots that are scratchy .

How do I wire the power switch to get the light to work wright? Do I just jump a wire from the on side of the switch to the light terminal on the back of the switch so it sends power to the bulb when the switch is turned on? Not famillure with this switch ...

The trem pulses when its turned down. Is their a fix for this???

Thanks
BD

The proper wiring when going to a 3 wire power plug is to have the hot go to the fuse, then the power switch, then to one transformer wire, and the neutral goes to the other transformer wire. The green goes to chassis ground on its own lug. With the Sunns and the light, I couldn't figure out how to make the light work when it was wired that way, and ended up wiring the fuse to hot and the switch to neutral, and wiring up the light like it was before in the stock setup. It isn't ideal but it works.

Greg

[Gtr_tech]

Replace *all* the lytics on the rev/trem board. When the large filters go south, you'll get strange things happening that are related to the trem.

[baddog]

They have all been changed but I might change out those 1000uf again. Ratshack has some so I might try it and see what happens .