Rescued a Spectrum II

[gui_tarzan]

It's alive!!! Boy does it make noise without a guitar plugged in. With a guitar plugged in it's quiet and sounds really good.

All I had to change initially were the cap can (used the JJ can and mounted it on its side inside the chassis) and the two 50/150 bias caps. So I will drain the caps and start going through all the jacks and see if I have anymore noise. There was a lot of corrosion in the output jacks from sitting in the guy's garage for two decades. The pots will also be replaced and then anything else I find out of sorts will be changed.

A couple of things I'm not familiar with are the molded black caps, are they similar to the Fender blue molded caps? Should I change those as well? Also the Mallory caps with transparent cases, what is the replacement suggestion for those? I attached a picture of those circled with red and blue respectively as well as the caps I replaced circled in brown.

Also I used a 5AR4C and a matched pair of 6550s. It came with a KT88 and a 6550. It seems like the voltage is a touch high (I'll post those readings tomorrow night), would a GZ34 help that (even though they're the same)? My understanding is the 5Y3GT/5U4 isn't a suitable replacement.

Once I get everything worked out it will be for sale, it's really loud and I don't need another really loud amp.

[Ryan Phelps]

The JJ multi-section can cap is probably rated for no more than 525 volts (?). Don't expect it to last too long as I suspect the rectifier is delivering 540 volts or more (check it and let us know). Only use a 5AR4 rectifier tube (it's the same as a GZ34 .....just American vs European designation). No need to change all of the coupling and tone caps unless they are leaking DC voltage. You could change the 250 uf cathode cap on the 12AX7 as it is an electrolytic and they do dry out. Check the shorting input jacks. If the amp has a lot of hum without a guitar plugged in, the jack may not be shorting to ground due to corrosion. Try cleaning or replacing the input jacks.

[gui_tarzan]

That is correct, the JJ cap can is 525. I was hoping to drop the voltage just a touch, it seems like it was around 440/440v coming out of the secondary at the rectifier jack but that was without any load. I'll have to check when I get home tonight.

[Soundmasterg]

That is correct, the JJ cap can is 525. I was hoping to drop the voltage just a touch, it seems like it was around 440/440v coming out of the secondary at the rectifier jack but that was without any load. I'll have to check when I get home tonight.

No easy ways to have the voltage drop in the amp and still have it stock. The higher wall AC voltage these days is what causes the higher than schematic voltages in amps that were made for a lower AC voltage. Go to 125V AC instead of 110V AC and the DC voltages in the amp change too, but it isn't a linear thing....meaning that just because your AC voltage changed by 15V doesn't mean the DC will also. Often it is much more, which then exceeds the rated voltage for the caps. In a Sunn, the first three stages are all over 500V, which can be problematic with some caps. That is why I recommended to use series connected discrete caps on this forum in the past as that is the only way to ensure the caps are at a safe level in relation to the voltages in the amp.

Greg

[Isaac]

Right. It's not linear, it's more or less proportional. So, if input voltage goes from 110v to 125v, the DC will not go from 500V to 515V. Instead, it will be more like 125/110*500=568V.