Just Picked-up My 200S From the Shop

[Walt-Dogg]

I've been lazy, but I've had the amp coming up on a year, I should probably get it gone through soon, especially before tour. There's a buzz in the pre-amp that's gotten worse...

[LPB]

 So I got a finally got a chance to play through the 200S that  "I just picked-up from the Shop" and it's got a hum. It never hummed before. I think I'm going to take a look at the grounding work he did just to rule the chance that he forgot something obvious, or did something stupid that I could fix. I'm not ruling-out the possibility that something random happened to my amp in the week or so since I picked it up, either way I am still frustrated.

[george]

The PS filter caps in your Concert are under much lower DC voltage than a tube power supply. DC voltage (AC ripple) and temperature are what "wear out" aluminum electrolytic capacitors.

[LPB]

 So I yanked the chassis this morning to take a look. The polarity switch is still wired-in as it was before the grounded 3-prong power cable was installed. Is there anything additional that needs to happen in this area, or does simply installing the 3-prong cable(with the green wire securely grounded to the chassis) automatically disable this switch as the amp is always grounded.

Note: when I plugged the amp in for the first time and heard it humming I tried flipping the Polarity switch(just because) and it did nothing.

 Thanks!

Just to add-I did use the search function and there seemed to be contradicting info.

[LPB]

So I yanked the chassis this morning to take a look. The polarity switch is still wired-in as it was before the grounded 3-prong power cable was installed. Is there anything additional that needs to happen in this area, or does simply installing the 3-prong cable(with the green wire securely grounded to the chassis) automatically disable this switch as the amp is always grounded.

Note: when I plugged the amp in for the first time and heard it humming I tried flipping the Polarity switch(just because) and it did nothing.

 Thanks!

Just to add-I did use the search function and there seemed to be contradicting info.

Well, I'm pretty bummed-out.
I'm recording this weekend and was hoping to use this amp. I don't want to take it back to the same tech, there are a couple guys in the next town that have great reputations and come highly recommended from people I trust, but I don't have the time to get it there. Lesson learned.
 

[Soundmasterg]

In a proper 3 wire power plug install, the "death cap" should be disconnected and thrown away, and the "ground switch" should be disconnected completely inside the amp. Additionally, on older amps they had the fuse on the neutral side and the power switch on the hot side. These should both be moved to the hot side with the fuse first, and then the switch, and then from there it should connect to one of the power transformer primary wires, and the neutral side should go directly to the other transformer primary wire. So it sounds like that person did not install the power cord correctly.

Hum can be caused by grounding problems, but if the grounding was not changed in the amp, then the problem is probably something else. It can also be caused by bad electrolytic caps, and bad filter and bypass caps can cause other things too...low power, intemittant issues, messy electrolyte leaking inside the amp, etc. If these filter and bypass caps are original, then that would be the most likely place to start. Have these caps replaced, preferably with some new ones that will be able to handle today's wall voltages instead of the can. (search for it, there are many posts relating to this)

Greg

[LPB]

Thanks, that's the answer I was looking for. Nothing was removed, disconnected or modified aside from the power cable's green wire lying in a messy lake of hardened solder on the chassis, granted, this is the same tech that didn't suggest any cap work even though the amp was built in '67. To top it off I missed-out on an identical 200S, great cosmetic condition(no tubes and being sold as-is)for very cheap yesterday.

Well, it's been a crap week.  On the bright side, I've got some other amps to record with, and I'll also record a direct signal so I can "cab" with the 200S later-on, once it's fixed. I can discharge an amp and wield a soldering iron but it's been a while, I've become more cautious over many years of hurting myself and damaging equipment.

Thanks to everyone here! I'll post a conclusion when it happens.

[Soundmasterg]

Thanks, that's the answer I was looking for. Nothing was removed, disconnected or modified aside from the power cable's green wire lying in a messy lake of hardened solder on the chassis, granted, this is the same tech that didn't suggest any cap work even though the amp was built in '67. To top it off I missed-out on an identical 200S, great cosmetic condition(no tubes and being sold as-is)for very cheap yesterday.

Well, it's been a crap week.  On the bright side, I've got some other amps to record with, and I'll also record a direct signal so I can "cab" with the 200S later-on, once it's fixed. I can discharge an amp and wield a soldering iron but it's been a while, I've become more cautious over many years of hurting myself and damaging equipment.

Thanks to everyone here! I'll post a conclusion when it happens.

You're welcome!

It is a good idea to be cautious and safe around high voltage electrical equipment. Lord knows I shocked myself enough when I was getting started to try to be extremely careful every since.

The basic gist of what I was suggesting with the cap changes is to replace all electrolytic caps in the amp. This can be done with an original type can cap, but if you do this, these caps are over their voltage recommendations because the wall voltage is higher than it was when the amp was made. This causes a shortened lifespan for the can cap. A better approach is to leave the can cap in place for cosmetic reasons, but disconnect it electrically, and use discrete caps inside the amp, wired in series for the first two stages (with a resistor in parallel with each one). This will allow a higher max voltage rating, and will also allow you to change the cap value for the first two stages easily if you want to get better filtering out of the first two stages. This is a great idea for a bass amp. The originals were filtered pretty low by today's standards because back them, electrolytic caps were expensive. To do this requires the use of terminal strips and some holes drilled, wires added, etc., or you can also use an SDS cap board available from Triode Electronics. These bolt right in the amp and give the higher filtering as I suggested for a bass amp. The one caveat to going with higher filtering is that a solid state rectifier (or Weber Copper Cap) must now be used instead of the tube rectifier. Again, this is a great idea for bass. Do some searches here and you will find it all spelled out as far as these cap changes.

If you are interested, I would like to sell one of my Sunn amps. It is a 1970 Sonic 1/200S head, and is in very nice cosmetic shape, and electrically it is completely rebuilt. There are some tasteful mods to it such as what I suggested for the caps, and a couple others that I did too that have major benefit. I'm not in a hurry to sell it, but I would like to see it go to someone who would use and appreciate it. I could always keep it myself but I have a 2000S that can pretty much duplicate the sounds so I don't need it anymore. If you are interested in knowing more, then PM me and I will be happy to share more.

Greg

[Johnny Guitar]

Greg, why can't you use GZ34 with SDS cap board?  And what is the difference between a GZ34 and the 5AR4, are they interchangable?
Thanks,
Johnny G.

[Soundmasterg]

Greg, why can't you use GZ34 with SDS cap board?  And what is the difference between a GZ34 and the 5AR4, are they interchangable?
Thanks,
Johnny G.

If you go by the specs in the tube amp manual, the GZ34/5AR4 has a max value of 60uf for a cap that it feeds. The vintage GZ34's can handle that but many new ones can not, or they can handle it for awhile, and then fail. Today's GZ34's aren't very good quality if compared to NOS ones. There are really Sovtek/EH/TungSol reissue and JJ, and Chinese ones and thats it. The JJ's and Russian ones are a cut above the chinese ones for reliability, but they're aren't that great either.

Here is the SDS link: http://store.triodestore.com/sdslabmk3cap1.html

The SDS cap board uses for 50uf sections, so it will provide some hum reduction over stock, but it will also change the sound. The preamp and phase inverter sections are RC coupled to their filters so increasing the filter cap value changes the actual tone of any stage that is supplied by those caps.....these sections will have more bass response that stock. You may like it, you may not. They do use series connected caps so there is a high enough voltage rating for the filter sections with this board and that is a good thing. The other good thing about it, at least in Sunn's bass amps is that it fits into the amp easily and is less work than installing individual parts. And you could always get the board and then use different value caps if you wanted to.

The GZ34 and 5AR4 are the same tube btw.