Tab v74 110v to 220v step up transformer issues. Please help.

[outraged]

Hi,
first off thank you to anyone who reply's!

I have two racked Tab v74's in a dual rack that was just racking two v72's. I am in the Usa and am trying to use a step  200w step up transformer from 120v to 220v and every time it turns on the V74's for 2 seconds before the 2.5v fuse's breaks. Even if only one v74 is plugged in the fuse goes out.

The step-up transformer Does power my Calrec compressors fine. I'm guessing I don't have a large enough step up transformer to handle the initial power surge from Tube gear?

I'm just confused because a seemingly reliable studio said he was powering two v72's with this same rack and transformer everyday.

Am I missing something here? Doubt its its the v74's because the do power up before popping the fuse.
Should I just get a larger step up transformer or just try larger voltage fuses first?

Thank you so much for your time and insight

 

[Bo Deadly]

What's a "2.5V fuse". You mean 2.5A?

Is this the circuit?:

Are you talking about the fuse labelled 0.1A?

Have you replaced the large electrolytics or rectifier diodes?

 

[outraged]

What's a "2.5V fuse". You mean 2.5A?

Is this the circuit?:

Are you talking about the fuse labelled 0.1A?

Have you replaced the large electrolytics or rectifier diodes?

Thanks for the reply,
yeah sorry its a 250v, 2.5amp fuse and I powered the transformer through a medical grade power conditioner. Everything inside the V74's was recently gone over, tested and looks good as new so I doubt its something to do with them ( I know you can never be sure with these type things though). Also tried each v74 in the rack individually and I get the same thing.

I'm guessing its the step-up transformer because it runs really loud like its being overdrawn compared to when I run it through the Calrecs Vca comps which obviously take way less current (especially considering tubes gears massive current draw to turn on).

Tomorrow I will try 10amp, 250v fuses and/or pick up a larger step-up transformer to see if that changes anything. If not, I'll take it to some tube technician in the Denver/colorado area and hope they have experience with 65 year old German tube stuff lol.

No way I have the technical experience to troubleshoot these when I can't even read the german schematic's.
Also need someone to connect the attenuator's on the rack to the v74's for volume adjustments so taking it in may be my best bet. I guess..

Anyone know a good German tube tech that can do this without making me go bankrupt? Its one wire out of the attenuator's so I can't be that hard, right?

Thanks again,

 

[Bo Deadly]

I'm not sure I would try a larger fuse. The fuse is there for a reason. If the unit is even remotely close to that schematic, there's no way it should be drawing more than a few hundred mA. And note that the PT is stepping down so the current through fuse is going to be less. Meaning 220 on the primary and 6.3 on the secondary is 220:6.3 so 300mA x2 tubes is 600mA / (220 /6.3) = 17mA through the fuse for one unit's heaters. Even if the fuse is in the 120 path (where is the fuse exactly?), that's still only 32mA. I'm not a tube guy so maybe my back-of-napkin calculation is crap. And yeah, when cold heaters draw more. But 2.5A? No. As Fonze would say, something is wr-wr-wrong. It's just wired incorrectly. Something is shorting out.

 

[outraged]

I'm not sure I would try a larger fuse. The fuse is there for a reason. If the unit is even remotely close to that schematic, there's no way it should be drawing more than a few hundred mA. And note that the PT is stepping down so the current through fuse is going to be less. Meaning 220 on the primary and 6.3 on the secondary is 220:6.3 so 300mA x2 tubes is 600mA / (220 /6.3) = 17mA through the fuse for one unit's heaters. Even if the fuse is in the 120 path (where is the fuse exactly?), that's still only 32mA. I'm not a tube guy so maybe my back-of-napkin calculation is crap. And yeah, when cold heaters draw more. But 2.5A? No. As Fonze would say, something is wr-wr-wrong. It's just wired incorrectly. Something is shorting out.

You were right squarewave! Thanks!
I ended up pulling apart the enclosure and found another step-up transformer inside set to receive 110v. Sounds like the person I bought it from had no idea what was going on here power-wise since they told me it needed an external step up transformer:

Anyway's I plugged into this transformer and both modules work and sound incredible.  Ran a bunch of sounds through both v74's with no issue! Sounds absolutely brilliant. Never heard such classy high and low end come out of tube gear and super quiet as well.

Thanks again for all the reply's and I guess the thread is done unless someone knows something about properly connecting the attenuation switch to these(??)

 

[Michael Tibes]

I guess the thread is done unless someone knows something about properly connecting the attenuation switch to these(??)

I am not aware of any standart attenuation switch, are you referring to something concrete or do you mean attenuation in general? There are obvious ways to change the gain: the wiring of the input and output transformers and the gain trim pot. But all this is related to internel wiring or the wiring of the connector.

One more thought: If I get you right you did run the units with 2 step up transformers? That means that they saw 440V at the mains input? I wonder if this might have done some damage?

Michael

 

[inkster]

Most V74 modules have a split primary power transformer that can be rewired for 110(120) volts. And if you omit some of the feedback components and trimmers you will have a very nice preamp and they make a wonderful DI.