wjdunham wrote:
One simple thing to check, depending on the finish on your chassis - did you make sure to grind the all of the ground connections down to the metal and ensure good contact, sure sounds like you've got some weak grounds both in terms of the noise issues and voltages. I've used both those transformers with the multi-point Dumble style grounding scheme and the amps are dead quiet and voltages are right on the specs.
Thanks Bill. I'm using a bare aluminum chassis, but I've just removed and checked all chassis grounds and they're all secure.
Loaded w/ ~100mA at idle, I'm still getting 357-0-357 out of this 325-0-325 PT, giving me ~475 plates.
I did notice that with tubes out, no load, the secondaries only go up to 363-0-363. Doesn't that seem like a small jump from loaded>unloaded?
The plates(and HT after rectification)on the other hand, jump from 475>505, loaded>unloaded.
This seems very suspect that the differences before and after rectification are not proportional. The only thing inbetween are the SS diodes....could they be causing this?
If the PT is overrated, for instance, a transformer for a 100 watt build, but is now being used for a 50 watt build, it won't pull down as much as it would in a 100 watt build. I realize I'm stating the obvious here, and I certainly don't mean to sound like I'm talking down to you, Smoke. It's just that sometimes we glop over these sorts of details - I know I do
No worries at all, Lou. Both PTs I've put in here are 200mA. I figure I'm drawing ~90mA at idle, but I figure that would load the PT down a good bit, at least enough to where I wasn't getting so much extra voltage from the secondaries.
This 325-0-325 PT should, under no load, be putting out 1.4*325=455VDC , but I'm getting more than that(~470VDC) with tubes. What are the chances I've received 2 bad trannies in a row?
I think there are some transformer manufacturers who spec secondary voltages at load. So I wonder if the AC would indeed measure 650V if you were to load your B+ to 200mA?
JazzGuitarGimp wrote:I think there are some transformer manufacturers who spec secondary voltages at load. So I wonder if the AC would indeed measure 650V if you were to load your B+ to 200mA?
Indeed, both the Hammond and CT are both rated loaded. The Hammond was rated 710 unloaded. Throw in 122 wall and 5% tolerance and there's my 720, half loaded, I guess. Ive just never had to estimate higher than an unloaded rating to target B+ for a tranny advertised for a particular tube set. Frustrating. I need to just get over it and move on with zeners, or higher rated caps
Ok I haven't given up quite yet. Concerning SS diodes, if they were to be damaged by overheating when soldering, could they exhibit higher rectified voltage? I'm unfamiliar with what happens with diode failure. When I spliced a bridge together my voltages dropped to near target , but when I actually put them on the board, soldered quite closely together, they jumped back up.
Smokebreak wrote:Concerning SS diodes, if they were to be damaged by overheating when soldering, could they exhibit higher rectified voltage?
No, you only get about 0.7V drop across each diode, and there are only two active diodes on each half-cycle - so 1.4V max drop. If the diodes started to somehow rectify without a voltage drop, you'd only gain 1.4V on your B+.
It looks like your options are to either use a zener to get the B+ down, or, if you have room on the chassis and a 5V rectifier tap on the PT, you could switch to tube rectification.
John_P_WI wrote:Not to be a smart ass here, you are testing the voltages with the stand by in the play position???
Yep. I've tested both ways. The plate voltage of course drops considerably when in play mode, but the AC at rect only has a difference of 5 volts or so between standby/on(or tubes in/out)which is confusing me.
That's is, DC drops when tubes pull current, but AC barely drops
1) The ac voltage is measured at rms, or 0.707 of the peak to peak voltage, your caps when NOT in the play mode are hanging on to or near the "peak" voltage rectified (BTW, there is very little amount of "power" just under the "peaks"). When flipping to play mode, your caps are being pulled down by the load to around the average or 0.707 (rms) x ac of the leads - the diode drop IF the load and transformer are matched. The large drop when flipping the stand by to play is caused by the caps not being able to "hang on to the rectified peaks" due to the load.
2) Mentioned above, a voltage of 470 was measured when 455 was expected. This is 15 v or 3.2% above expected and is a fairly good regulation. Actually a theoretical, 650 vac x .707 = 459.6 minus 1.4 (2 diode drops) = 458.2 v, which is even less error than the expected 455.
3) The wall voltage was measured at 122 vac, when the transformer was designed for 120 vac? If so, this is 1.7% greater "than design" ac input into the transformer. Subtracting the 3.2%-1.7% gives us an expected 1.5% overage on the output IF exactly 120 vac is applied at the input, assuming that is what the transformer is designed for. 1.5% is spot on in the unregulated world. Try this with a variac (lower the ac input to exactly 120 vac) and see what happens.
Verify that everything is biased as it should be, I'm guessing this is the correct voltage that you will have and everything is working as it should.
John_P_WI wrote:
2) Mentioned above, a voltage of 470 was measured when 455 was expected. This is 15 v or 3.2% above expected and is a fairly good regulation. Actually a theoretical, 650 vac x .707 = 459.6 minus 1.4 (2 diode drops) = 458.2 v, which is even less error than the expected 455.
Thanks John, I'm still wading through this information a bit, but concerning the 455/470 numbers, while I expected 455VDC unloaded, and agree 470 would be fine, I'm getting 470 plates loaded .I believe unloaded I was up in the 505 range. This is what seems so out of whack.
I noticed in lord preset's thread that we've got the same high plate voltages on the same amp, but I've got the 325-0-325 Classictone and he's got the 355-0-355 classictone(both 200mA I assume) This is what I'm having a hard time wrapping my head around, as an example.
Structo I'm measuring Plates in the amp on DC setting plate to chassis. For AC I'm measuring red and black probes across the secondaries, which is exactly double of what I get from either secondary to ground reading, meter set on AC.
I hope I'm not being too difficult here.. Just trying to understand
SB, the MCI transformer that I used for my 5B6 build was supposedly 350@117V, but actually was 403VAC unloaded at 124V mains, which was about 30 volts too high for 124. However, once it was loaded the voltages dropped right in range to the target circuit.
I rarely use an off the shelf power transformer anymore.
ToneMerc wrote:SB, the MCI transformer that I used for my 5B6 build was supposedly 350@117V, but actually was 403VAC unloaded at 124V mains, which was about 30 volts too high for 124. However, once it was loaded the voltages dropped right in range to the target circuit.
I rarely use an off the shelf power transformer anymore.
TM
Yes! This illustrates my problem, as loading does not bring it down for me.
What sort of transformer would you recommend to get my plates down to ~430? 720CT 200mA apparently is not working for me, though it's "spec".
