Johnk_10's SVT VR relay mod.

[reddi]

Do the majority of the CLs have this problem?

[JimmyM]

Originally Posted by reddi

Do the majority of the CLs have this problem?

it's not the cl's that have the problem. it's the vr's. btw, did we ever find out if loud fixed that problem?

[NickMonti]

Originally Posted by Jerrold Tiers

They ought to do that mod at the factory........ i.e. get rid of the relay.

One day I'll ask George M why that circuit is even in there.

BTW, if it was still SLM and not Loud, there would be a Service Bulletin on that, very possibly to just bypass it. Way I see it... forty years of SVTs can't be wrong.

Please do just that!
That would be awesome and important information.
Could deserve a sticky in Amps, as does this thread.

[StuartV]

I had my SVT 2 Pro cut out on me several times in one gig, back in October. Switched to my backup amp for the rest of the night. My SVT has worked fine ever since (but haven't been back to that venue).

I have been assuming my 2 Pro was cutting out because of low voltage, and was planning to have my local shop bypass that thing. Some of you have said the VR is the only SVT that has this relay.

So, the SVT 2 Pro does not have the low voltage relay? I wonder why mine was cutting out at that one gig (which I KNOW had serious issues with their electricity).

[Rickett Customs]

Originally Posted by StuartV

I had my SVT 2 Pro cut out on me several times in one gig, back in October. Switched to my backup amp for the rest of the night. My SVT has worked fine ever since (but haven't been back to that venue).

I have been assuming my 2 Pro was cutting out because of low voltage, and was planning to have my local shop bypass that thing. Some of you have said the VR is the only SVT that has this relay.

So, the SVT 2 Pro does not have the low voltage relay? I wonder why mine was cutting out at that one gig (which I KNOW had serious issues with their electricity).

Jarrold or JohnK could verify that for sure.

[blastjv]

I don't know if this will tell youfor sure, but I did the mod on my VR and the only noticable difference that you could check for without opening up the amp is that before there was a delay when you first powered up during which you could not switch out of standby, but after the mod you can switch out of standby immediately after turning it on. You should still give your amp time to warm up before doing this and actually playing, but as a quick check to see if it has such a delay I would hope that would be OK.

Don't know how helpful that is.....

-JV

[Rickett Customs]

Originally Posted by blastjv

I don't know if this will tell youfor sure, but I did the mod on my VR and the only noticable difference that you could check for without opening up the amp is that before there was a delay when you first powered up during which you could not switch out of standby, but after the mod you can switch out of standby immediately after turning it on. You should still give your amp time to warm up before doing this and actually playing, but as a quick check to see if it has such a delay I would hope that would be OK.

Don't know how helpful that is.....

-JV

This is very true, about there being no delay (after the relay bypass) with the standby,as soon as you fire it up. After the mod, it becomes a manual standby, with no "safety" switch.

One of the main characteristics of the relay, is the fact that if the voltage is sagged below 115v, It won't come out of standby (LED will stay red). 230v folks needn't worry about this mod though.....

[johnk_10]

for mine, it was 112 volts and below where it wouldn't come out of standby. at 115 volts, the relay worked fine.

but i prefer a manual standby anyway (like the vintage SVT's). if you were playing and the source voltage dipped below 112V, it would automatically return to the standby mode, and wouldn't come back on until it reached 112+ volts, which was unacceptable to me.

[Rickett Customs]

Originally Posted by johnk_10

for mine, it was 112 volts and below where it wouldn't come out of standby. at 115 volts, the relay worked fine.

but i prefer a manual standby anyway (like the vintage SVT's). if you were playing and the source voltage dipped below 112V, it would automatically return to the standby mode, and wouldn't come back on until it reached 112+ volts, which was unacceptable to me.

Yeah, I had that happen at 2 gigs, and that was it...........

[blastjv]

Happened once in my rehearsal space...that was enough...

-JV

[okcrum]

Originally Posted by johnk_10

CL's definitely don't have the bias relay circuit, or at least, every one that i've seen doesn't. AFAIK its exclusive to the VR.

True, the only relay in the CL is the time delay on the standby switch. It's powered by the DC preamp heater voltage, so I suppose it could get flaky if that got too low.

[quadra]

Just to be sure on the last step of the mod.

After the mod, J26 slot on the PCB will be left unplugged, correct?

[Rickett Customs]

Originally Posted by quadra

Just to be sure on the last step of the mod.

After the mod, J26 slot on the PCB will be left unplugged, correct?

Yes, "J-26" is left disconnected on the board.

And also thank Johnk_10 on your way out, it was his mod

[smur89]

I just did this mod today on my 230V VR. I had all the same problems you 120V lads were having, and this mod fixed them for me too.

Though all i needed to do was swap the J26 connector to J28. Easy job done in 20mins and have a functioning amp again!

More pictures here: https://www.facebook.com/media/set/?...2815226&type=1

Thanks for the pictures and Jonhk for your tutorial.

[StuartV]

Originally Posted by johnk_10

CL's definitely don't have the bias relay circuit, or at least, every one that i've seen doesn't. AFAIK its exclusive to the VR.

My SVT 2 Pro has behaved in the past exactly as if it has this circuit.

Can anybody confirm whether it does or not? I need to have it serviced anyway and I want to know whether to ask my tech to look for it and disable it.

[johnk_10]

I don't believe that it does.

[StuartV]

Hmm. Okay. Thanks, John!

[Jerrold Tiers]

IIRC the same exact board is used for AC connection on most of the units.

Usually it is both a sort of interlock, that forces the HV off if the filament voltage is not present, and also forces it off if there is a fault in the output tubes. Therefore, if the voltage is low enough to look like loss of filament, OR if the voltage drop makes the amp do something that looks like a fault, then it might open for a voltage drop. I have not looked at it in detail, but suspect that it would need a pretty good drop.

This is different from what it does in the VR.

But yes, the relay IS present, and it is possible for it to open and cut the HV.

[StuartV]

Thanks, Jerrold! (I'm assuming you're responding to my question)

I was using my SVT 2 Pro on a gig a while back where I was told in advance (by a guitarist friend who'd played the place before) that the house power was really crappy. The bassist in the band that opened for us was using my rig. During the first few songs, my amp completely stopped outputing any sound for a second or two at a time, a few times. Then, it stopped completely for a couple of minutes. I had my Class D SS head there for a backup, so I just moved the instrument cable and cab cables over to it and used it the rest of the night with no problems. And my SVT worked fine after that, once I got it home.

Ever since, I have suspected that the 2 Pro has some kind of circuit that puts it in standby when the AC voltage drops too low. From what you're saying, it sounds like there could be. But, I'm not clear on whether it would be a good idea to disable it. It sounds like it works differently than the way the VR works, so maybe it would be a bad idea to defeat that part of the circuit?

[Jerrold Tiers]

Defeating the relay would also defeat the protection stuff. Given that older units don't HAVE that, and do fine, there isn't any real "guaranteed amp-killing gotcha" to bypassing it, but it DOES mean the tube protection won't work, and that you could turn on the HV with no filament power.

Both of those are true of all vintage SVT, so........