step down tranny?

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randallissimo
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step down tranny?

Post by randallissimo » Thu Sep 03, 2015 6:23 pm

Hello,

My first appearance here. I was directed here from Valve Queen. I have a Marshall JTM 45/100 dual OT 40th anniversary Super 100 re-issue head in my shop. It came in with one dead White Lion Kt66, hot bias, and hum. The plate voltage was at 545v. I dialed the bias down and replaced the dead tube with one of the same rating from the Tube Store. Didn't change the hum. After going thru the amp and finding nothing wrong, I swapped out the tubes for a moment with a used set of 6L6s. Hum went away.

So now I'm looking for a new set of KT66s, and I find some brands won't fit in the bear claw tube holders, and am told others won't stand up to the plate voltage this amp is putting out. So what to do? Valve Queen says George M. suggests a step down transformer. Is there any way around this? Owner does not care to modify this amp. Thanks.

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Re: step down tranny?

Post by Haze13 » Fri Sep 04, 2015 7:32 am

What says voltage selector? There must be different taps for it... Like 110, 220, 240 Volts. On the back panel.
Don't know about Step Down transformer, but you can use a cathode bias, and the voltage drop on the cathode resistor will lower the voltage across the tube...

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Re: step down tranny?

Post by Haze13 » Fri Sep 04, 2015 7:37 am

And I think it's Gold Lion kt66;) White Lion is a good band though... :thumbsup:

P.S. None of the Gold Lion line is good for heavy loads. Their El84 will not work in the VOX AC30. Too much current, but they are not the only one when that amp is in question. Their KT88's are not ready for hard working too...

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Re: step down tranny?

Post by randallissimo » Fri Sep 04, 2015 12:12 pm

There is no voltage selector. I don't think redesigning the bias section is the answer.

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VelvetGeorge
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Re: step down tranny?

Post by VelvetGeorge » Fri Sep 04, 2015 12:55 pm

All modern KT66's tend to drop off in life expectancy over 500V B+. I've had very good luck with Valve Art branded, Shuguang made KT66's at 515-525V. That's really the max though. I don't have enough experience with Gold Lions to know their limits.

In my opinion you will have trouble with any modern KT66's at 545V. What bias current was this at? The B+ will go down as you increase the bias current, but not likely enough.

Let us know how it works out.

george
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Re: step down tranny?

Post by randallissimo » Fri Sep 04, 2015 1:33 pm

Hello George,

This amp is biased between 28 - 35 mA at 542v. (the 35mA is the new Gold Lion) It was actually quite a bit hotter than that when it came to me. So it doesn't surprise me that these tubes are having problems.

Could you talk about a step down transformer as a solution? This is something I have no experience with?

Thanks, Randall

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Re: step down tranny?

Post by VelvetGeorge » Mon Sep 07, 2015 4:53 pm

It's an auto-former wired for a fixed ratio. Just an inline transformer with male and female plugs. The 110V version, for example is a .915/1 ratio, it drops 120V AC to 110V AC. It's important to note that it does not regulate the voltage, only lowers by the fixed ratio. So if your AC runs high, say 125V from the wall, you won't end up with 110V AC out. It would be around 114V.

In the 45/100 with 545V B+ (at 32mA) you have a ratio from wall voltage to B+ of just over 4.5 Every volt from the wall translates into 4.5V on the tubes. Using the 110V step down drops the B+ approx 45 volts! Much closer to the 500V target B+ for reliability. Using a 105V step down would get the B+ in the 475V range, where any modern KT66 should live for at least a few years.

With my vintage amps I select which step down to use by measuring the filament voltage and dialing the AC supply until it is exactly 6.3V. Once dialed I can measure the AC and know what wall voltage is ideal for that particular amp. Some are at 112, some at 115, some 117V. I just use the next lowest step down value.

With my '68 Superlead 112V AC supply results in 6.3V on the heaters and 475V B+. Exactly where you want it to be.

My '66 45/100 requires a little less AC, 108 IIRC. And still has a high B+ at this setting. Though much lower than the 560V you get with 120V AC in.


There's been a lot of interest in these lately, I'll get some in production and add them to the Metroamp store.

george
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Re: step down tranny?

Post by Carbia » Mon Sep 07, 2015 6:29 pm

I was thinking in buying a variac for that purpose.

Vintage Marshalls have 220V or 240V and in Spain is 230... so the B+ is too high in 220v and too low in 240v.

With a variac I could use the 220v tap and lower the AC voltage to 220v to get the right voltage in any place.

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Re: step down tranny?

Post by randallissimo » Mon Sep 07, 2015 7:05 pm

Thank you George, that was very helpful!! I love the method of adjusting to 6.3v, that makes total sense. Approx.how much are these transformers, and where can they be had?

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