Sunn Sorado question/problem. Help?

[paulbystereo]

So I just picked up a 1968 Sunn Sorado head from craigslist and I couldn't be more thrilled. Couple of questions for those of you more well versed in Sunn than I.

1) The rectifier tube socket was sealed off and looks like it could have been done from the factory not just aftermarket based on what the bottom of the tube socket looks like. Has anyone seen anything like this before?

2) After a few days of playing, it started making some wretched noises. Could this be the electrolytic caps? Or something worse? It has the original RCA 6550's in it, but I've never heard bad tubes sound like this.

Here are a few pictures and a youtube link of the noise.

https://www.youtube.com/watch?v=iAMIQBCiOMM

[loudthud]

A 68 Sorado would have come with A GZ34 and KT88 power tubes. It looks like the power transformer has been replaced. The screw heads should be on the outside of the amp with the nuts on the inside. The transformer should have two yellow wires going to the rectifier tube socket. The fuse holder and knobs have been replaced and that hole on the front panel near the standby switch is not original. Any amp this old should be recaped.

Is there a small paper label glued down inside the head box?

[Ryan Phelps]

The gooped-up rectifier socket was not done at the factory. All of the electrolytic caps are original and should be replaced. This may or may not solve the weird noise, but needs to be done regardless. The 550 volt-rated can caps available are still marginal in my opinion, given the higher wall voltage we have today. The solid state diode rectifiers (used in place of the tube rectifer that the amp was designed for) will also increase the voltages delivered by the power supply, further exacerbating the problem. I would recommend using discrete capacitors on terminal strips inside the chassis. Use a pair of caps in a totem-pole configuration with 2 watt bleeder resistors for the first position like Fender used.  Also replace the bias and cathode caps. Have this done by a tech as lethal voltages are involved.

Good Luck!

[Ryan Phelps]

Oh yeah......replace that 2-prong AC cord with a 3-prong cord and get rid of the death cap while the amp is being serviced.

[paulbystereo]

Thanks so much for the replies guys. Wouldn't the grease pencil on the inside of the chassis indicate it's '68? I attached a picture of the tube chart and it seems like someone swapped the KT88's out for 6550's and ditched the rectifier tube at some point. I'll make sure to have the bias and cathode caps replaced along with the filter caps.

[BassHog]

Yeah, that's not the original power transformer. Looks like the PT went out, and someone replaced it with a PT that didn't have 5V taps for the rectifier heater so they had no other choice but to convert it to solid state rectification. If you are going to keep it that way then I would also recommend recapping the head and using caps higher than the stock 525V cap can. I know folks here disagree with me but I have had great luck with the 550V(600V surge) authenticap/dynakitparts cap cans. Or you can just use discrete caps inside the chassis.

[Ryan Phelps]

I agree with loudthud and BassHog re: the power transformer. Now I'm going to beat the dead horse regarding the filter caps.
Your amp was designed to use a 5AR4/GZ34 rectifier tube which naturally drops the voltage delivered by the power transformer. Solid state rectifiers do not create a voltage drop like a tube rectifier, so your amp is going to see a higher voltage. Your amp was also designed around 117VAC wall voltage. Now, 120VAC is the standard, but some areas of the country can have as high as 125VAC. Add these two voltage bumps together and the DC output of the rectifier can easily exceed the 550 volt rating of the can capacitors available today.

For example, I just restored a 1967 100S that uses a 5AR4/GZ34 rectifier (not solid state diodes) and has the stock Sunn power transformmer. Per the schematic it is designed to deliver 500VDC. My wall voltage is typically between 122VAC - 124VAC. The rectifier output, under load, is 560VDC....exceeding the working voltage rating of a can cap. Also understand that the surge rating is just that....to take the instantaneous voltage spike when the amp is first turned on....not the continuous working rating.

I don't doubt that BassHog and others have had good luck with the can caps, given the right circumstances. In my situation a can cap would not last too long. And I don't like to stress parts to their limit.

Assuming you are going to keep the existing power transformer (and the solid state rectifiers) ask your amp tech to check the both the DC surge voltage when the amp is switched on, and the stable voltage once the existing caps are charged. This will tell you the stress your caps will be under and you can select your re-capping strategy accordingly. Also remember that we don't know the voltage output of the replacement power transformer....it could be higher or lower than the stock Sunn transformer.

Good luck with your amp repair!