PT question

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MikeR670
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PT question

Post by MikeR670 » Fri Apr 21, 2017 6:52 am

Please solve my mystery.......I have a pt that read 400-0-400. Put through an ss full wave rectifier I then tested dc output on the b+ line coming out. Measured voltage between that line and ground. It was 360 volts. Seemed right at about 45% of the a.c. 800. No choke, no caps etc. Perfect I thought, for my plans........hooked it through a choke and two 50uf 450v caps - voltage after that now measures 540!! How can that be? How can I run through a filter and decoupling section but still keep my 360 volts?

I also noticed that when I put it further through three nodes of 20uf caps with 10k 5 watt resistors, the voltage remained at 540 everywhere, instead of reducing in stages, like on the trainwreck power circuit schematic.

What am I missing, it's driving me crazy.....?! :roll:

frankdrebin
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Re: PT question

Post by frankdrebin » Fri Apr 21, 2017 9:07 am

You're lacking electronic basics. That voltage is normal. First, you have to explain your project, then it's possible to give some advice about it.

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Stevem
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Re: PT question

Post by Stevem » Fri Apr 21, 2017 10:50 am

What you have found with your 540 volts is pretty much right, I come up with 560 by the math , but more than close enough for a tube amp.
If you use a tube recto like a 5ar4 you will have about 500 volts, and if you use a 5U4 you will have about 480 volts once the power supply has a load on it.

Also with no load on the power supply , as having no tubes in the amp you will see that 540 volts all the way thru to the first gain stages plate load resistor .

No load = no voltage drop from the power supply, and as the power supply see's more load like when your Crank a amp out , so will the power supply voltage sag down from that 540 volts or whatever voltage you have once the amp is idling on all of its tubes!
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MikeR670
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Re: PT question

Post by MikeR670 » Fri Apr 21, 2017 4:13 pm

Got it, thanks Steven! Carrying on........:)

Stevem
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Re: PT question

Post by Stevem » Fri Apr 21, 2017 4:58 pm

A decent PT will have no more then a 10 to 12% voltage regulation range.

What I mean by this is let's say you have a PT rated for 150 ma on its V+ winding, and with no load on that winding you have 400 volts, and the amp is a push pull fixed bias deal.

Your amp build has things all loaded up and idling at 110 ma.

Then you Crank the amp out and your right at that 150 ma draw on that winding, the effect of this should be that your V+ should have dropped down no less then 352 volts, this is 12% regulation.
A good thing to do when you get the amp up and running is to write up the details of voltage and current for all sections of the amp for future reference.

Note that in regards to tubes be they preamp or output that the stronger they are, the more current they will pull out of the power supply and this will make for lower voltages being read through out the amp with the exception of Cathode voltages.
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MikeR670
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Re: PT question

Post by MikeR670 » Fri Apr 21, 2017 5:44 pm

Thanks Steve, I'll reference those voltages and currents as I get things going. Still a few problems to solve before power up, but I'm getting close.

You mentioned changing the order of choke and caps - I only have all 450v caps: one 50uf, one 37uf, one 30uf and three 20uf. Should I put the 50 and 37 in series after the choke for b+1, then a 6 or 10k resistor onto the 30uf for b+2 - and then three further cap and resistor nodes with the 20 s, for b+3, 4 and 5?

Then add the bleed resistor like 66k 2 watt across the first two series - I don't have two large caps the same values, I notice that circuits tend to show any series caps being the same values. Can you still add those load sharing bleed resistors on two caps in series but different values?

Are those resistors hooked up across each cap pos and neg? Maybe I could size those resistors individually to suit each series cap - using 50/C?

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Re: PT question

Post by Stevem » Fri Apr 21, 2017 6:29 pm

I never posted anything up to you in regards to a choke.
What is it you are building, do you have a amp in mind, or a schematic you are working from that is close to what you are using for a outcome?
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MikeR670
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Re: PT question

Post by MikeR670 » Fri Apr 21, 2017 7:06 pm

Sorry, my mistake - getting conversations mixed up ha!

I'm building the trainwreck express based on the Kelly 90 schematic from here. And I found a turret board layout drawing designed to work with it. I'd like to stick close to the b+ voltages it suggests, also so I can use a spare pair of 6v6 tubes I have.

Stevem
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Re: PT question

Post by Stevem » Sat Apr 22, 2017 10:25 am

Is the Kelly fixed bias and can you post up a schematic?
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M Fowler
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Re: PT question

Post by M Fowler » Sat Apr 22, 2017 12:57 pm


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RJ Guitars
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Re: PT question

Post by RJ Guitars » Sat Apr 22, 2017 2:37 pm

MikeR670 wrote:
Fri Apr 21, 2017 7:06 pm
Sorry, my mistake - getting conversations mixed up ha!

I'm building the trainwreck express based on the Kelly 90 schematic from here. And I found a turret board layout drawing designed to work with it. I'd like to stick close to the b+ voltages it suggests, also so I can use a spare pair of 6v6 tubes I have.
540 volts on the B+ is really hot for an Express and a near impossibility for a set of 6V6GT's. The best sounding Express amps are generally more like 400V on the plates. It might be a cool sounding amp with a set of EL34's...

You can use a zener diode to reduce the initial voltage out of your PT. However, if you have access to a PT with a greater current rating and a lower initial voltage on the secondaries I think you would find it' would all turn out a little more friendly.

Lot's of good guys and knowledge is available on these amps over in the TrainWreck section. YMMV

MikeR670
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Re: PT question

Post by MikeR670 » Sat Apr 22, 2017 9:12 pm

Thanks guys, really appreciate the help!

I guess I'll have to accept that my tranny will not be any good for a 6v6 amp - I think I'll carry on and try using my 6550 tubes. I've installed a variable bias setup, so if I change the tranny out later it could all be made to work with other tubes.

I might put in the switch to kill power to one 6550, to idle it and reduce output.

I'll swap out the 220k/ 220k for a pair of 150is, hopefully that's all I'll need to do to adapt that schematic for 6550s. Power to the preamps should all be reasonably close - as the old pa ran similar after I jumpered several channels while messing around with it. Before I stripped it to pieces ha.

Who knows what this beast will sound like but I'll give it a shot. I figure if I can make it work and sound reasonable at this stage, then at least I've built my first amp and learned a lot for next time :)

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Phil_S
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Re: PT question

Post by Phil_S » Sat Apr 22, 2017 9:29 pm

MikeR670 wrote:
Sat Apr 22, 2017 9:12 pm
I might put in the switch to kill power to one 6550, to idle it and reduce output.
This isn't a good idea. It won't work as you think and it is likely to cause other problems. In a push pull amp, you really need two tubes operating. You'd be better off with a quad of 6V6 and pulling one from each side. Remember doing this doubles the output impedance requirement, so you need to change the speaker. However, even if you do this, the perceived drop in volume will only be about 10%. I wouldn't bother. You might think about trying a pentode/triode switch.

Stevem
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Re: PT question

Post by Stevem » Sun Apr 23, 2017 11:01 am

Even though 6V6s and 6550s are both what's termed Beam tetrodes they sound very different when cranked up unless your running China made 6550s!
A EL34 is a Pentode and due to there higher gain need less drive signal to reach the same output power level as most any Beam tetrode.
Since your doing all of this for the love of sound then do it right, sell that PT that you have and get the needed one,do not do things half Assed unless you can settle for the half Assed results that you will surly have in the end!!!
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Phil_S
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Re: PT question

Post by Phil_S » Sun Apr 23, 2017 12:23 pm

Stevem wrote:
Sun Apr 23, 2017 11:01 am
...Since your doing all of this for the love of sound then do it right, sell that PT that you have and get the needed one,do not do things half Assed unless you can settle for the half Assed results that you will surly have in the end!!!
Well said Steve! What we have here is a classic, "When the only tool you have is a hammer, every problem looks like a nail." Getting the right tool for the job is always the best way to get something done. Either build something appropriate to the PT or get a different PT. Also, all along, there's been no discussion of the output transformer, for which the spec keeps changing each time we talk about a different configuration.

The latest post about turning off one tube suggests there is a desire for a lower powered amp. If this is the case, look at lower powered tubes, not 6550. Or maybe you get the correct transformer set for a single ended build using a 6550. There are lots of possibilities. None of them has to cost an arm and a leg. It's just that whatever you do should meet your desire for a certain tonal pallet. Tell us, if you were just starting from scratch, what is it that you really want?

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