|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
62.127.57.34
Have a Musicman HD130 guitaramp with OP-amps in preampsection and a 12AX7 as amp+Consertinasplitter and four EL 34 (new EH) in the poweramp. With an oscilloscope sinussignal is fine all the way to and including the grids of the powertubes. But at the anodes and speakeroutput thereīs kind of a step/shelf on both slopes of the sinushalves . When I plug in my Fender, I can hear distortion . Have changed all filtercaps to a bit higher capacitance and coupling caps. Anodevoltages are 700 V, bias about -40V , grid 2:s 350 V . Anodecurrent each tube 25mA. Please tell me what You think is wrong . Thanks for Your opinion.
Follow Ups:
The crossover distortion can be eliminated by runing closer to class A - higher bias current - but at 18W (700*.025) there isn't much room for increasing it. I don't know about EH, but some modern EL34s won't last long at the rated maximum of 25W. I wouldn't go over 30 mA... and I'd check the power transformer temperature.
That amp was designed around 6CA7, the fat-bottle, American equivalent of EL-34.It's confusing because some tubes have both designations.
The tall skinny (true EL-34's) will self destruct in that amp due to the high plate voltage.
You need true 6CA7's for reliability.
The waveform you describe is crossover distortion. If the amp is correctly biased, it should not be present. Factory specs call for 0.5V across the 10 ohm cathode resistor. That translates to 25 mA/tube.
Hope this helps! Al
Thanks for Your answers. I have changed positions with the powertubes, rebiased but no improvement. The cross-over remains on the scope. The crossoverdist. starts too early..... on quite normal sound from the speaker. (one JBL D130 , 15") . The same result from another cabinet with two 8 ohm, Weber Califonia 12" in parallell. Can there be something wrong with the outputtransformer ? A cuople of windings in contact, inside ? The amp is 33 years old. Would it help to change the OT for a Hammond 1650N or 1650T? Perhaps in ultralin. connection? I want the sound to be as clean as possible. Anyway a Musicman HD 130 with new Electro Harmonix EL34 shall be able to give a loud, clean sound , donīt You think? Thanks for an answer. Stewen in Sweden
My Musicman HD130 guitaramp has 270 AC for the powertubes. And voltagedoubling with 2 diodes and two 100uF/450V caps in series. That gives 350 VDC to the screengrids and 700 VDC to the anodes. Isnīt that called halfwawerectification? As I wrote earlier I have 25mA current through each EL 34. (EH). Problem: crossoverdistortion. Wonder if I should change to full rectif. with 4 diodes in a bridge ? 270 AC will give at least 385VDC ? Can I use this voltage instead to the anodes ? It will also mean that no caps have to be in series so total uF for filtering will become two times the present value. With perhaps 50mA current through each tube? In that case, can I expect less crossoverdist? Please give me an advice what to do. This amp must be able to play louder without dist. than it does now . Regards Stewen
This post is made possible by the generous support of people like you and our sponsors: