Skipping the PT?

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sepulchre
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Skipping the PT?

Post by sepulchre »

I've got a couple of 50B5 Pentodes pulled from an old mono Hi-Fi. I looked at the data sheet for them and saw they only require 120V or so on the plate. So I'm wondering, could you get away without using a PT? I mean, couldn't the AC from the wall be rectified, smoothed a bit and used as is? Of course everything would need proper fusing and general safety precautions should be in the mix.

I'm sure this is some how nuts, but to eliminate the cost of a PT is enough to make me ask.
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martin manning
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Re: Skipping the PT?

Post by martin manning »

Millions of AA5 radios and quite a number of small guitar amps were made that way, and for exactly that reason. If the chassis is isolated they can be made safe, even better with a three-conductor power cord. Filaments were powered directly from the line voltage, with all tubes having the same current draw. 2x 50B5 + 2x 12AX7 would be a good match (adding up to 124V), and with the 12AX7's running 12V they have the same 150 mA current draw as the 50B5's.
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TUBEDUDE
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Re: Skipping the PT?

Post by TUBEDUDE »

I prefer the transformer providing the isolation. Just sayin'.
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sepulchre
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Re: Skipping the PT?

Post by sepulchre »

Martin: I thought I remembered older equipment being wired so. I know that it easily posed a threat if a short occurred. But as you said, 3 prong cords have brought safety a long way. But still . . .

T-Dude: After contemplating this awhile I believe I share your view. I think it's safer to be isolated by a PT. Plus it makes standard heater feed simpler.

I was just thinking how inexpensive it would be to produce a nice little amp. A customer is considering having a 5 watter built so I was looking at various options.

Thanks guys!
R.G.
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Re: Skipping the PT?

Post by R.G. »

(1) Yes, it's entirely possible. In fact, there was a set of tubes, probably the AA5 set, where the heater voltages added up to about 110Vac (the AC line voltage most common back then) and went into a standard radio set. I believe the radios were the real design intent, and the guitar amps were a side benefit.
(2) Those guitar amps are generically referred to as "widow makers".
(3) "General safety precautions" in the mix translates as "use an isolating transformer. The radios were set up with no externally accessible metal at all as I remember. Some hot-chassis (the other term for "widowmaker") radios and TVs used audio transformers on the speaker output so a headphone jack could be added. Guitar amps had another problem. TVs and radios were output-only devices, as their inputs were radio waves. Guitar amps have a "grounded" shield braid cord connected through hard copper links to the guitarist's fingers, and with single ended inputs, this could not be effectively isolated. Hence "widowmaker". Get the plug in the AC wall socket backwards, hold your guitar and touch anything that was earth ground and you're toast - literally.
(4)
martin manning wrote: Sun Dec 10, 2017 12:18 pm If the chassis is isolated they can be made safe, even better with a three-conductor power cord.
UGH... Yes, it can be made theoretically safe, as long as everything works exactly right. I'm a weenie about things like this, having designed power supplies for computers for some years and being fed horror stories by our safety inspection people. My personal level of bravery puts hot chassis equipment about on a par with kissing a pet rattlesnake.
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jjman
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Re: Skipping the PT?

Post by jjman »

One of the sites I frequent forbids posting schematics using that approach, even with a disclaimer. They probably also forbid discussing a new build using it. Of course each site can make its own rules.
If it says "Vintage" on it, -it isn't.
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martin manning
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Re: Skipping the PT?

Post by martin manning »

Certainly not something I would recommend building for sale, and that was not mentioned in the OP.
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xtian
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Re: Skipping the PT?

Post by xtian »

I say we get into Tesla coil guitar amps. Twenty thousand volts can’t be that dangerous, right?
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M Fowler
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Re: Skipping the PT?

Post by M Fowler »

And we could transmit our signals around the world.
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Re: Skipping the PT?

Post by sluckey »

I got mine! Anybody else?
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martin manning
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Re: Skipping the PT?

Post by martin manning »

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johnnyreece
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Re: Skipping the PT?

Post by johnnyreece »

Here's one I did a while back (somewhat) using this thought of design. Note: I did use an isolation transformer, but it's much cheaper than your standard PT.

http://ampgarage.com/forum/viewtopic.ph ... l6#p343980
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martin manning
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Re: Skipping the PT?

Post by martin manning »

That’s a good point about a small isolation transformer or simple PT with no filament winding being cheap as compared to a dedicated tube amp PT, and that opens the door to using a couple of cheap tubes for the output (50L6, 50C5) and two 12AX7’s with series heaters.
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pompeiisneaks
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Re: Skipping the PT?

Post by pompeiisneaks »

Yeah everything I've read says never build them without an isolation transformer. Even with a good earth connection to the chassis, you can't guarantee someone isn't going to have a house with no earth or a poorly setup earth, so it's a ton safer to have the isolation transformer.

~Phil
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JMFahey
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Re: Skipping the PT?

Post by JMFahey »

martin manning wrote: Sun Dec 10, 2017 12:18 pm Millions of AA5 radios and quite a number of small guitar amps were made that way, and for exactly that reason. If the chassis is isolated they can be made safe, even better with a three-conductor power cord. Filaments were powered directly from the line voltage, with all tubes having the same current draw. 2x 50B5 + 2x 12AX7 would be a good match (adding up to 124V), and with the 12AX7's running 12V they have the same 150 mA current draw as the 50B5's.
Sorry Martin :( WRONG advice.

1) AA radios and some TVs were made that way because live chassis lived inside a fully insulated box either wood or bakelite, and there was NO WAY a user could touch anything metallic inside, only pot shafts through bakelite knobs or switches, same thing.

But a Guitar player is by definition connected to chassis :shock: through the strings and guitar metallic parts.

2) so you can not "make the chassis isolated"

3) a 3 conductor power cord will blow fuses or trip GFI, whichever reacts faster, so it does not help here.

4) but ... but ... but ... "it was done decades ago and nobody complained"

Well, decades ago you could smoke inside a Hospital, up to the 30´s or early 40´s (same era when these "things" were popular you could buy a full auto machine gun, just 17 years ago you could buy dynamite and detonators at a farm supply shop without background checks, and many other fun things I won´t mention but today all that is illegal, including making hot chassis guitar amps.
Even publicly suggesting they are reasonably safe or usable as is.

In a nutshell: add the proper isolation transformer and grounding if dealing with vintage stuff, and start with a proper PT if building new stuff.
Design/Make/Service Musical stuff in Buenos Aires, Argentina, since 1969
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