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Low B+ voltage: Is it the xfrmr?

7/25/1999 5:58 PM
MJ Harnish	Low B+ voltage: Is it the xfrmr? Here's the story: I've been fooling with a Guild Thunderbass amp and have run into a couple of mysteries I thought I'd see if someone could clear up for me: The voltage on the B+ secondary of the PT is giving me a pretty low reading: ~190v AC. This seems really low, since I only end up with about 210 v DC after rectification. The guy who owns the amp says it was working OK before so either the PT spontaneously died (it hasn't been used since he last turned it on) or there's something funky going on. The PT has no center-tap. I've got 4 pairs of leads coming out of it (black, red, blue, and green), plus a yellow lead. Obviously the black is the primary, the red the B+ winding, the blue the bias winding (with one side grounded I get ~ -32v for the bias voltage which also seems a tad low), and the green are the 6.3v winding. I thought the yellow lead was a center-tap but it doesn't have continuity with any other lead and I can't generate any DC voltage if I hook up the B+ windings to a full wave rectifier; using a half-wave I get the aforementioned 210v DC. I believe the yellow may be a shield since it was originally connected to ground. Stock, the amp had some kind of funky rectification circuit: one red lead went to a diode and then onto the first filter cap. The other read lead went to ground and then there was another diode going from ground to where the first lead and diode met. So it basically was a half-wave rectifier with the extra diode. I had never seen anything like this before and thinking the yellow was a CT, I went ahead and rewired it for full wave rect. and of course I got no voltage. However, even if it was using a full wave rect. I'd still be only getting about 260v DC which still seems pretty low. Any idea what the purpose of the diode from ground to the other diode does and could it be responsible for raising the B+? I can't imagine that the set-up would yield close to 350v or so that I expected the B+ to be at. Any help would be appreciated. :O) MJ

7/25/1999 5:58 PM

MJ Harnish

Low B+ voltage: Is it the xfrmr?

Here's the story: I've been fooling with a Guild Thunderbass amp and have run into a couple of mysteries I thought I'd see if someone could clear up for me:

The voltage on the B+ secondary of the PT is giving me a

pretty low reading: ~190v AC. This seems really low, since I only end up with about 210 v DC after rectification. The guy who owns the amp says it was working OK before so either the PT spontaneously died (it hasn't been used since he last turned it on) or there's something funky going on.

The PT has no center-tap. I've got 4 pairs of leads coming out of it (black, red, blue, and green), plus a yellow lead. Obviously the black is the primary, the red the B+ winding, the blue the bias winding (with one side grounded I get ~ -32v for the bias voltage which also seems a tad low), and the green are the 6.3v winding. I thought the yellow lead was a center-tap but it doesn't have continuity with any other lead and I can't generate any DC voltage if I hook up the B+ windings to a full wave rectifier; using a half-wave I get the aforementioned 210v DC. I believe the yellow may be a shield since it was originally connected to ground.

Stock, the amp had some kind of funky rectification circuit: one red lead went to a diode and then onto the first filter cap. The other read lead went to ground and then there was another diode going from ground to where the first lead and diode met. So it basically was a half-wave rectifier with the extra diode. I had never seen anything like this before and thinking the yellow was a CT, I went ahead and rewired it for full wave rect. and of course I got no voltage. However, even if it was using a full wave rect. I'd still be only getting about 260v DC which still seems pretty low.

Any idea what the purpose of the diode from ground to the

other diode does and could it be responsible for raising the B+? I can't imagine that the set-up would yield close to 350v or so that I expected the B+ to be at.

Any help would be appreciated. :O)

MJ

7/25/1999 8:47 PM
R.G.	From your description, it sounds like a voltage doubler circuit. There are some voltage doubler things like that. If you 're getting a reasonable 6.3VAC across the filaments under operation, then I'd bet on a doubler circuit. The extra diode ties the first lead to ground when the line reverses, and the red lead to ground pulls the (-) of the cap (and amplifier ground) down, effectively doubling the voltage. Try it.

7/25/1999 8:47 PM

R.G.

From your description, it sounds like a voltage doubler circuit. There are some voltage doubler things like that.

If you 're getting a reasonable 6.3VAC across the filaments under operation, then I'd bet on a doubler circuit. The extra diode ties the first lead to ground when the line reverses, and the red lead to ground pulls the (-) of the cap (and amplifier ground) down, effectively doubling the voltage. Try it.

7/26/1999 11:55 AM
Farrow	I'm with R.G. on this one. Guild amps were made by Valco, (AFAIK) who also built Gretsch, Airline, etc. and they used whatever they could get their hands on. A lot of amps including a Silvertone I've got in the shop right now used 200V non-centertapped PTs. I prefer to replace this with a real PT (Bandmaster size) as it will run a bit cooler. I just redid one of those Monkey Ward amps with the 4-6L6 (cathode bias into 2 OTs!) this way and there was definitely an improvement in sound running the tubes at 450V (they were running about 385V...) Farrow http://surf.to/pharaohamps

7/26/1999 11:55 AM

Farrow

I'm with R.G. on this one. Guild amps were made by Valco, (AFAIK) who also built Gretsch, Airline, etc. and they used whatever they could get their hands on. A lot of amps including a Silvertone I've got in the shop right now used 200V non-centertapped PTs. I prefer to replace this with a real PT (Bandmaster size) as it will run a bit cooler. I just redid one of those Monkey Ward amps with the 4-6L6 (cathode bias into 2 OTs!) this way and there was definitely an improvement in sound running the tubes at 450V (they were running about 385V...)

Farrow

http://surf.to/pharaohamps

7/26/1999 1:42 PM
MJ Harnish	You've got it RG. Bruce @ Mission Amps filled me in on the details (thanks Bruce!) and provided a schematic that looks exactly like the amp did stock. I learned a lesson here: Never assume something is as simple as it appears. When I started out I assumed that it was using a full wave rectifier (I saw two diodes) and that the two 500uF/450v caps were in series to provide 500uF/900v as the first filter stage. Of course I now know that I was completely wrong and should have paid a lot better attention before moving forward... MJ

7/26/1999 1:42 PM

MJ Harnish

You've got it RG. Bruce @ Mission Amps filled me in on the details (thanks Bruce!) and provided a schematic that looks exactly like the amp did stock.

I learned a lesson here: Never assume something is as simple as it appears. When I started out I assumed that it was using a full wave rectifier (I saw two diodes) and that the two 500uF/450v caps were in series to provide 500uF/900v as the first filter stage. Of course I now know that I was completely wrong and should have paid a lot better attention before moving forward...

MJ

7/26/1999 1:49 PM
Michael Cameron	Minor slip..... two 500uf/450volt caps in series give you 250uf/900v. I figure you knew that, but just in case. Michael

7/26/1999 8:37 PM
MJ Harnish	Yep, that was a typo; thanks for pointing it out though. :O)

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