Maggie 192-5

[C Moore]

I have a couple questions about this amp. There is a buzz, louder and faster than a hum, that pretty much disappears when I touch the chassis. Is that indicative of a bad ground on a tube or cap? This is with no guitar plugged in.
I have 790 VAC on the PT secondary. Does that seem like a lot for this little guy? It has a 5Y3 rectifier. The 4.7k and 10k 5 Watt PS dropping resistors are getting damn hot. If I have done the math corectly......
The 4.7k is dropping 120V (490-370). That makes for right around 3 Watts.
The 10k is dropping 255V (370-115). That is about 6.5 Watts. Perhaps I should switch to 10 Watt resistors? Obviously, the 10k at 5 Watts is too small.
Do these voltage drops seem "Normal" to you guys? These old PTP amps are pretty time consuming compared to a turret or eyelet board.
A schematic is in the attachments.
Thank You

[marcoloco961]

I have 790 VAC on the PT secondary. Does that seem like a lot for this little guy?

Since the highest voltage I see in the schematic is 318V, I am going to guess that the voltages are high. Are you sure the PT is original, or did someone switch it also?

[C Moore]

I have 790 VAC on the PT secondary. Does that seem like a lot for this little guy?

Since the highest voltage I see in the schematic is 318V, I am going to guess that the voltages are high. Are you sure the PT is original, or did someone switch it also?

I am not sure if it is original, the wires look like they have been on there forever. But they might have been somewhere else first. But yes, the voltage numbers are way overboard. Let me see if I can get a part number, or take another picture.....
Thanks

[C Moore]

I do not know what the original PT should look like, but here it is.
The numbers on the Lams are.....391628
And the numbers on the cover.....R-5164A

[marcoloco961]

I do not know what the original PT should look like, but here it is.
The numbers on the Lams are.....391628
And the numbers on the cover.....R-5164A

I have no real knowledge of transformer #'s, so those could or could not be the right tranny #'s.

My observation was the fact that the schematic you posted shows the B+1 at 318 Volts, the first dropping resistor only drops 5 volts down to 313 according to schematic, the second resistor only drops 33 more volts. From what I have been reading the unloaded voltage after rectification is approx. 1.4 times the single voltage (i.e. a 300-0-300 would be 300*1.4 or 420V unloaded) As a 300-0-300 is approx 585V across the secondaries. At 790 V on your secondaries, I would assume it to be closer to a 400-0-400 Tranny. For the voltages stated in the schematic I would expect to see them use closer to a 250-0-250. It should produce about 350V after rectification, unloaded. Slightly less after the load is applied.

I cannot think of any thing that would cause a Transformer to produce a higher voltage than it was designed to produce. (Unless a bad, or wrong value filter cap could cause this). This is what leads me to believe it is not the original spec tranny. It would also explain why the dropping resistors were overheating. They are obviously dropping a much higher voltage then the schematic intends.

[C Moore]

This amp was built in 1949, so anything under the sun could have happened to it by now.
Maybe I will check the classifieds for a PT, or maybe check with RJ.
That just can't be the right tranny.....can it???

[Phil_S]

I'm pretty much in agreement with marcoloco's math. I have some questions:
Are all tubes in sockets? Readings will sink when there is a load. Maybe that PT is original and is just "soft".

What rectifier? 5y3? 5Y3GT? Sovtek 5Y3? These are different tubes with different voltage drops. In particular, the Sovtek doesn't drop 50-60v like the real 5Y3 or 5Y3GT.

On the other hand, those voltage drops across the dropping resistors worry me. They are dropping large amounts of voltage. Either something is pulling a lot of current (bad tube, unintended ground) or the measurements are wrong. To be sure, clip your meter probes, one on each side of the resistor to get an accurate read on the drop without having to calc the difference. Though it should be the same, let's make sure because what you report is weird. It suggests the circuit is pulling about 23mA after the power tubes and this is way too much for a 6SL7 and a 6N7. I'd guess 5-10mA is as much as those two tubes are capable of demanding.

With tubes in sockets, can you give us a voltage chart of plates and cathodes? On the power tubes include the screens, too.

The schematic says 110V. 120VAC line voltage is 9% higher. Adjust your thinking accordingly. This does not account for what you report.

I also get nervous when you say the hum disappears when you touch the chassis. I think this means you are the ground. Stop doing that. Make sure your VAC feed is in there correctly done. The hot leg (black) should be fused before the switch, and then to one side of the primary. The neutral goes to the other primary wire. The green redundant ground goes into a ring lug that is placed on a dedicated bolt through the chassis. Leave enough slack on the green wire, in the event of a pull out, it will be the last wire to pull away from the chassis.

It looks to me like maybe the HV CT and the green ground are sharing a lug. Why bother grounding if you are going to provide a direct path from the secondary to the line feed? Fix this, please!

[Dr-Joned]

What are the wires/wire that has blue heatshrink on it?

[C Moore]

This is all my fault. I can't have 115 VDC on the screens.
There are two different schematics (maybe 3) for this amp. I went from using a choke to using just two dropping resistors and I have wired the OT wrong in the process.
I will report back after I change a wire and attach the PT HV ground.
Sorry about that, my fault, a waste of your guys time.
Thanks
The blue heat shrink is the 6 Volt CT. I went with 100 ohm resistors instead.

[C Moore]

Basically the mistake I made was not checking the B+ nodes after going from the "field coil" schematic to the schematic I have posted on this thread. Essentially I had both ends of the B and C nodes connected. So the B and C filter caps were both terminated to the screens and the preamp plates.
The readings are a lot better now, and there is not all that voltage dropping across the resistors. But the numbers are still quite a bit higher than the print.
PT sec..... 765 VAC
Node A.....400 VDC
Node B.....371 VDC
Node C.....338 VDC
Power tube cathodes are 23 VDC to ground.
Another potential problem I am having in switching from the other schematic is that it has a filter cap at Node A and the schematic I posted (it is the schem I am trying to follow) does not show the extra 22/500 filter cap. I thought it would be a good idea to keep that cap at Node A, but maybe I am wrong? Is that cap potentially adding to this "extra" voltage that I seem to have? I guess I thought it would aid in making the amp run "quieter".
Thank You

[marcoloco961]

Basically the mistake I made was not checking the B+ nodes after going from the "field coil" schematic to the schematic I have posted on this thread. Essentially I had both ends of the B and C nodes connected. So the B and C filter caps were both terminated to the screens and the preamp plates.
The readings are a lot better now, and there is not all that voltage dropping across the resistors. But the numbers are still quite a bit higher than the print.
PT sec..... 765 VAC
Node A.....400 VDC
Node B.....371 VDC
Node C.....338 VDC
Power tube cathodes are 23 VDC to ground.
Another potential problem I am having in switching from the other schematic is that it has a filter cap at Node A and the schematic I posted (it is the schem I am trying to follow) does not show the extra 22/500 filter cap. I thought it would be a good idea to keep that cap at Node A, but maybe I am wrong? Is that cap potentially adding to this "extra" voltage that I seem to have? I guess I thought it would aid in making the amp run "quieter".
Thank You

If there is several different versions of the schematic, it is possible that they made changes to the design, one of those being a higher B+. Your readings are much more reasonable now. I would leave the 22uF Filter cap at the B+1 (A). I have always seen one there. Your rectifier tube suggests no more than 32 uf on the A node. I haven't done the actual math, but a 22 uf should be close enough I would think. Sounds like you could give it a try and see how it sounds. You should be able to tell pretty quick if the elevated B+ is going to be an issue.

[Phil_S]

People make mistakes. No biggie. It is good that we helped you fix them, even if we didn't do very much.

The voltage drops appear to be appropriate for the tubes. The math says there is about 6mA at node B (3mA for the screens) and about 3mA a node 3 (for the two dual triodes). These are more like what one would expect, and the 33v drop between nodes B and C is right on spot with the schematic, even though the absolute numbers are higher.

See if you can find a old stock 5Y3, not the 5Y3GT. I'm assuming you've got a 5Y3GT. The change in tubes should drop another 10v.

The power tubes are biased rather hot. I think I would raise the cathode resistor from 240 to ~300 ohms. As soon as you change Rk, all the voltages will change, so it is not so easy to choose precisely. You might also try 270 ohms.

Did reworking the line feed ground take care of that thing where you were touching the chassis?

[C Moore]

Hey Phil -
Yeah we all make mistakes, but if there is a problem AFTER you change something.......I should have looked harder.
Anyway....the rectifier is a Phillips 5Y3WGTA.
The nearest ground for that CT has the 2 x 100 ohm resistors, so I thought that was a bad idea (was it?). I connected that CT to the ground of the first filter cap. I figured they share the chassis for electron flow, so if I connect them at that ground, it is the same as the chassis. Am I right about that? And yes, you are right, the line noise is all but gone now.
I have a 250 ohm at the cat resistor now. I will see what I have to swap in there.
Thank You

[Phil_S]

Don't know about that 5Y3WGTA, but I'd leave it.

That artificial CT for the filaments with a pair of 100z resistors should be a separate ground. You made the right call on not using it.

There is a bit of science on ground schemes. I suspect on an amp as simple as this, you can get away with breaking some of the rules. If it hums let us know and we can work on that.

Thanks for the update on the line ground. I am very glad to have cleared up that unsafe connection.

Keep at it. This should be a good sounding amp.

[C Moore]

Don't know about that 5Y3WGTA, but I'd leave it.

That artificial CT for the filaments with a pair of 100z resistors should be a separate ground. You made the right call on not using it.

There is a bit of science on ground schemes. I suspect on an amp as simple as this, you can get away with breaking some of the rules. If it hums let us know and we can work on that.

Thanks for the update on the line ground. I am very glad to have cleared up that unsafe connection.

Keep at it. This should be a good sounding amp.

10-4
Thank You