When is a PT too hot?

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aiyiadam
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When is a PT too hot?

Post by aiyiadam » Thu Jan 29, 2015 4:46 pm

Hi,

I have just finished my build. It is 50W plexi inspired amp with VVR for power tubes and PI. I had to tweak a lot, but now I am actually happy with the tones I am getting from that amp.

My PT was built by a local transformer producer in Istanbul and I am starting to think that it isn't big enough for my amp.

The temperature on standby reaches somewhere around 113F( 45C) and stays there. And when I am playing with the B+ at 410V, I am seeing temperatures around 120F in an hour. The tranny at 120F is still comfortable to be held for 10 seconds.

I have already a installed a fan. When it is running at 4.5V it reduces the temperature just a little. Maybe 1-1.5F.

So.. Is temperature of ~125F (52C - I would probably reach that in summer easily ) normal for a PT?

Thank you

Evren

Haze13
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Re: When is a PT too hot?

Post by Haze13 » Fri Jan 30, 2015 2:59 am

How did you ordered that transformer? He told you that it will hold on or it was your specs? You can calculate the power of the transformer that is needed. Current * voltage = Power for Filaments and B+. Different power tubes needs different current. 6l6 - 0.9A and KT88 - 1.6A... So for a safe operation of PT 6.3*4 (2A per Power Tube) + 6.3*1 (1A for 3 12ax7s) = 31.5W just for filaments. For B+ current swings 0<I<2I in theory... ~40mA at idle for one el34 in a push-pull => 80mA at idle for a couple => 160mA MAX in theory => 410*0.160=65.6 Watts. More or less your PT must to supply 30 watts for filaments and 60-65 watts for a B+. 90-95-100 watt for a El34 or kt88, a little bit less for a 6l6 :) Is that what have you bought?

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Fri Jan 30, 2015 4:24 am

We had done some similar calculations. Theoretically my PT should have a maximum output of ~190W and the insulation should withstand more than 150C(302F- That is what the builder suggests). But I wouldn't really like this PT to be too hot, when it is giving me the 190W. And I am also not that sure about the insulation. :)

The PT has the outer dimensions of 80mmx96mmx65mm (Outer dimensions without considering the coil) , which is smaller than f.e. the classic tone tranny 40-18033.

At this point, I am unfortunately also questioning the abilities of the builder. I was disappointed with the choke and the OT he had built. I wouldn't be surprised, if there is something wrong with the PT.

I have tried to upload a few pics here. Unfortunately it didn't work out. :(

And since I have made some more measurements, let me be more specific about the temperatures:

My PT seems to reach an equilibrium at 45C in about 60-70 minutes, which doesn't change much in the standby mode. If I push the amp the temperature goes a little higher.

I have also tried to measure the power consumption of the amp with a simple device, which should be reliable enough.

At the moment of powering the amp: ~120VA
Standby: ~45VA
Playing at full voltage: ~72VA

Interestingly the power consumption seemed to me a little low. :?

Joshabr1
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Re: When is a PT too hot?

Post by Joshabr1 » Sun Feb 01, 2015 1:14 pm

I would say if it ain't smoking it ain't to hot. Owned several old marshall some of them get really hot. They are fine.

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Sun Feb 01, 2015 7:01 pm

Thank you for the replies.

It is not smoking yet. :)

There is a temperature classification on the mercury magnetics website, which considers my transformer as hot (and "angry"). They also claim that "hot" is a bad sign. Anyway, some of this stuff is obviously just marketing.

After having used the amp a couple of more days, I also don't expect some major PT failure. If something does happen I'll just get myself a better PT and probably build a new chassis.

Let me also put here the link of a few pics:
http://s1070.photobucket.com/user/aiyia ... t=6&page=1

Haze13
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Re: When is a PT too hot?

Post by Haze13 » Tue Feb 03, 2015 2:19 am

There are 2 secondary windings, one for B+ and one for filaments. If any one of them is working at the limit, the power trannie will get hot. Hot PT makes more noise than a cold one -> more filtration is needed. It's also can kill it faster. Classic Tone is what I used, and don't see any problems with it.

What that transistor is doing?

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Tue Feb 03, 2015 4:28 am

When I turn the amp on, the PT makes a mechanical noise and I also hear a slight humming on standby. The humming might be due to the bias voltage on the power tubes or the PT effecting the OT.

Other than those things, the amp is really quiet. But I have a lot of filtering. The filtering of the preamp and the Power tubes + PI are after the first filter caps separated. For the preamp I have an additional 50uF+50uF.

I also wanted to have a classic tone PT, but didn't want to give that 60-70$ for shipment.

The two Mosfets are for regulating the voltage (VVR). I also use an additional transformer, which feeds from the filament supply and gives 50V for a low impedance bias circuit.

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Re: When is a PT too hot?

Post by Haze13 » Wed Feb 04, 2015 6:54 am

What voltage do you regulate with a mosfets? B+ or heaters?

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Re: When is a PT too hot?

Post by Haze13 » Wed Feb 04, 2015 6:58 am

So big Transformer is the Power Transformer, for B+ and Filaments, right? Small one for filaments also? If so,is it for power tubes or preamp? Mosfets on B+ or filaments? After which transformer? Small one or the big one?

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Wed Feb 04, 2015 9:31 am

It is a typical VVR circuit. The dual pot regulates the B+ (from 30V to 410V) and the bias voltage (from 2,5V to something like 32V).

The PT is for the B+ and filaments. (P:220-230-240V, S:3.15V, 0, 3,15 - 330V-0-330). In the typical 1987X circuit the bias circuit is fed from the high voltage tap. The bias circuit has a high impedance and with power scaling this leads to a problem: if you change the B+ and the bias voltage with the dual pot, the bias voltage doesn't follow the B+ as fast as it should.

Since my PT doesn't have a low voltage tap for the bias voltage, a possible solution is (as suggested by KOC) taking a 220V -24V 5W transformer and connecting it to the filament taps of the PT. With the small transformer giving 50V, I could reduce the resistor values in the bias circuit accordingly.

Haze13
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Re: When is a PT too hot?

Post by Haze13 » Thu Feb 05, 2015 1:39 am

Well... It's possible that your PT working on it's maximum. Like an 9VDC adapter that has max current of 100mA, and you pulling all of it or may be more, it starts to heat up and transformer inside is making humming noise.

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Thu Feb 05, 2015 11:45 am

Yes, you are right. My PT is obviously smaller than the Classic Tone Plexi PTs. I guess the heater current is too much for this transformer.

I'll just keep on playing until it breaks.. :rock:

Haze13
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Re: When is a PT too hot?

Post by Haze13 » Sat Feb 07, 2015 1:31 pm

5881 and 6L6GC style tubes draw 0.9A when el34 ~1.4A. Your tranny could last longer if you'll move to those tubes :) 6.3 watts less than El34...

aiyiadam
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Re: When is a PT too hot?

Post by aiyiadam » Sun Feb 08, 2015 5:26 am

Thank you.

Using power tubes with a lower heater current is also a good idea. I will order a couple of them. It would also be good to hear the tonal changes with the different tube types.

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