Current Question Full wave bridge vs. Full wave

[cbass]

I have a PT The amp it came out of Gives out about 500VDC after The FWB rectifier. The PT has a centertap Not being use obviously.

Judging by the physical size and the tube compliment it ran.I figure it rated for atleast 225ma

If I ground the CT and Use a full wave rectifier I know I will Have aprox. 250VDC
So Does the current supply capabilities Double When using CT and full wave?
Or stay the same?

[Phil_S]

Look at the PDF attached to this thread. All answers are there!
https://tubeamparchive.com/viewtopic.ph ... +rectifier

[cbass]

You were supposed to give a simple explanation that a dumbass could understand.Not more readin

So would A PT rated a 100mA with a FW rectifier be derated to 62mA when using a FWB rectifier?

[Phil_S]

You were supposed to give a simple explanation that a dumbass could understand.Not more readin

So would A PT rated a 100mA with a FW rectifier be derated to 62mA when using a FWB rectifier?

Sorry, but I think the answer depends on the particular PT, cap input, resistive load, choke....didn't want to get into all of that. Assuming 100-0-100 @ 100mA is the AC secondary output, you get with cap input:

FWB: 1.4 x 200 = 280VDC @ 62mA
FW: .7 x 200 = 140VDC @ 100mA

There's no free lunch. Current goes up, voltage goes down, or vice-versa! Also, I find this is less than perfect. Typically, I find voltage output is about 94-96% of what I expect from the calc and I think this is due to ordinary losses due to heat, resistance, etc.

For the current rating, unfortunately the only way I know to prove it for sure is the let the smoke out. So, in testing an unknown PT, I've learned to live with testing for intended use. I have a breadboard rigged to run a PT (currently taken apart for a rebuild) into a FW silicon rectifier with a couple of caps and a screw-type terminal strip that lets me change the resistive load. As the load gets smaller in Ohms, the current is increasing. I often start with 10K Ohms and work down, watching and smelling for signs of heat/smoke. The test load gets very hot, way too hot to touch. I have several 25W rated sandblock wirewound resistors I use for this, sometimes using them in series or parallel. By measuring the voltage across the test load, and applying Ohm's law, you can determine the amount of current flowing. I plug the PT into a surge protector, and I hope it will trip off in the event of an overload, but it really needs a proper line fuse also, which is one reason I will get around to rebuilding it one day. Remember, you will smell something wrong way before you see smoke. Keep on hand on the off switch and remember that any hesitation will probably let the smoke out.

Good luck!

[tubeswell]

I have a PT The amp it came out of Gives out about 500VDC after The FWB rectifier. The PT has a centertap Not being use obviously.

Judging by the physical size and the tube compliment it ran.I figure it rated for atleast 225ma

If I ground the CT and Use a full wave rectifier I know I will Have aprox. 250VDC
So Does the current supply capabilities Double When using CT and full wave?
Or stay the same?

I'm figuring that your HT secondary measures about 0-350VAC when loaded, in which case it'll be about 175-0-175VAC (loaded) with a 2-phase FW rectifier (and will deliver about 250VDC with SS rectifier). But you can get a better idea about that if you disconnect the rectifier and measure the VAC on each end of the HT winding with the CT grounded (It'll be a bit higher unloaded). And yes the maximum current draw from 2-phase FW rectification will be 2 x what it would be if you lift the CT and use FW bridge.

[cbass]

Thanks guys.I shoul;d have been more clear but I was assumming capacitor input load.

The amp ran 2 6l6gc a 4 12ax7s The PT ran nice and cool even after extended periods of loud playing.

I am trying to figure if I might be able to run 6-6v6gt's Class A with around 250V B+

[katopan]

I meant to get back to this for ya Cbass. Short answer is if you reckon it ran at least 225mA with a bridge recto then you'll get at least 360mA out of it full wave CT.

[Phil_S]

I meant to get back to this for ya Cbass. Short answer is if you reckon it ran at least 225mA with a bridge recto then you'll get at least 360mA out of it full wave CT.

I was actually thinking it might be the other way around, 225mA with the FW, in which case, I'm not seeing 6x 6V6. 360mA ought to be enough, so I hope I'm wrong. For this PT to be 360mA it must be a sizable hunk.

I'd hate to see the smoke out of it in load testing. Be careful!

[73GTV]

I meant to get back to this for ya Cbass. Short answer is if you reckon it ran at least 225mA with a bridge recto then you'll get at least 360mA out of it full wave CT.

I was actually thinking it might be the other way around, 225mA with the FW, in which case, I'm not seeing 6x 6V6. 360mA ought to be enough, so I hope I'm wrong. For this PT to be 360mA it must be a sizable hunk.

I'd hate to see the smoke out of it in load testing. Be careful!

I read a book, "The How and Why of Guitar Tube Amps as "Gar" sees it" By Gar Gillies. Garnet amplifiers. The first thing in that book is every power supply usage from a multiple of different transformers for the first 65 pages...Hint Hint!!

[Reeltarded]

Ceeb.. what amp was it?

I think it's like Phil said on the current cap, but maybe even lower.

[cbass]

Its from an old Peavey amp.Its running a pair of El34's FWB retified right now with no problems.Maybe I'll just back it off to 4 6v6 .

This isn't the amp the pt came out of but it's a similar setup and it shows Hi low voltage switch.
http://www.webphix.com/schematic%20heav ... ge_vtx.pdf

Its not a huge transformer but its decent sized

[Reeltarded]

Worried. The similar Marshall only has capacity for 150mA.

I would cross ref other amps they made with the same PT and try to extrapolate what they thought it was capable of doing.

[ToneMerc]

Worried. The similar Marshall only has capacity for 150mA.

.

Robbed from the Tweed Bassman PT

TM

[Reeltarded]

Tweed Bassman? Never heard of it..

[cbass]

Seems pretty beefy for a 150 mA PT.I have a 125mA PT and this is definatly bigger.LIke I said it runs pretty cool and there isn't a ton of voltage sag when you hit a chord I'd be doubtful that it could supply 460ma Though.It'll be a while before I get around to doing anything with it anyway.

I don't have an express OT But I'd say this one is similar in size the laminations aren't as thick

I think it;ll be safe for 4 6v6 anyway.