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In Reply to: RE: You may be overloading the transformer. posted by Al Sekela on January 15, 2008 at 15:08:53
The PT does not seem to run abnormally hot and it takes a while for it to get hot. The OTs get a little warm themselves, but a good bit of the heat comes from the KT-88s which are close to the PT and OTs. After a couple hours of use the PT is only hot enough that you 'almost' can't keep your hand on it.
My Fluke meter says I'm at 119.1 volts AC at the wall receptacle.
The amp is self biasing, but does have a 1/4" jack to check it through. I should wire up a plug and see where its actually at. Doesn't seem to be too high since there's no faint redness on the plates
Follow Ups:
The difference is slight, 1.6 amps versus 1.5 amps, but if the power transformer supplies the tube filaments, this might be the source of your issue.
The self-bias circuit might also be causing the KT88s to draw more plate current. This would be worth checking. Perhaps the circuit is out of adjustment.
Most transformers have a small amount of mechanical hum under no- or light-load operation. The hum can get loud if the transformer gets close to maximum design load, or if there is enough harmonic distortion in the AC (the so-called 'DC on the AC' problem). Your comment that the transformer gets almost too hot to touch after a while suggests the transformer is running close to its maximum load. Adding more filter capacitance is not a good thing to do, as it increases the resistive losses in the transformer windings.
Another bit of information to add is that when turned on, the PT has a very faint hum, but when the KT-88s are on for around 30 seconds and are heated enough to produce music, then the PT gets louder. When I install 6V6s, there is no increase in PT loudness once they're warmed up. Also, there is a small transformer under the chassis.
Sounds as if I should not get a quad of EI KT-90s as I was planning to do(if I can find them). Might be too much of a strain on the PT. Alternative plan is to try a set of the Gold Lion KT-88 reissues. The current Winged C Kt-88s are beginning to near the end of their life (testing 75-80 where 65 is the minimum).
I will buy a 1/4" plug and check the bias this weekend.
Another question......According to the owner's manual, I can use 6V6s in this amp (plus 6L6, EL34, KT-88, & 6550), but they (6V6s) seem to get very hot quickly. Could the amp be too beefy for them and cause them to run hot or is it that the glass envelope is so small and very close to the heat source.
The 6V6s sound great and the filaments seem to be glowing normally, but I have not used this tube before I got this amp and do not know what to expect. Thanks for your help.
Your observation,
"Another bit of information to add is that when turned on, the PT has a very faint hum, but when the KT-88s are on for around 30 seconds and are heated enough to produce music, then the PT gets louder. When I install 6V6s, there is no increase in PT loudness once they're warmed up."
confirms that it is the B+ current that is overloading the power transformer. It takes about 30 seconds for the tubes to warm up enough to draw their full plate current.
It is not safe to use such tubes with your amp. Either the transformer has something wrong with it, the bias circuit is not adjusted properly, or the power transformer is simply too small. I would look into the bias settings and consult with the factory.
Sorry that I don't have experience with the 6V6 to say whether it runs hot. It may be the case that the auto bias is too high for that tube type. Generally, running tubes at high bias improves the sound at the expense of tube life. The tube is only rated for 14 watts of plate dissipation, so it may well be overloaded in your amp. Yes, the smaller envelope is part of the power limitation in the tube design.
Al,
I'd forgotten to mention that the amp is running in triode. I don't know if that would make any difference, but thought I should mention it.
I'm going to buy the 1/4" plug today and check the bias. I'll let you know how that checks out. By the way, the amp is a AES AE-25 Superamp and puts out 15 WPC in triode (no switch to go to ultralinear). I love the sound it puts out, except for the mechanical one.
I checked the bias this afternoon with the KT-88s in. The manual calls for 285ma and I got 280ma on my Fluke meter. The 6V6s ran at 185ma and the EL34s were at 232ma. Looks like the bias is okay. What do you think?
Also checked for DC on the incoming AC line and got -0.009vdc. Not sure if I checked that correctly, just set my meter for VDC. Got 119.3 on VAC.
I'm going to build one of those DC blockers and see if there is any difference. They're inexpensive enough to put together, so if it doesn't make any difference I'm not out much. I also bought two boxes to build a set of High Resolution Speaker Cable Filters....looks like a good tweak.
Thanks again for all the help.
Regards, Larry
Your observations regarding hum from the power transformer, how it tracks with tube type, and how it comes on when the EL-34 tubes warm up, all suggest the EL-34s are overloading the power transformer.
It still could be the case that you have a slight problem with DC on the AC, that the EL-34s are revealing. Do your test with the DC-blocker, and see if the EL-34s no longer cause the increase in mechanical hum. If they still do, then avoid using them or check into a replacement transformer.
Good luck!
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