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Im having trouble with a rebel 20, the little sucker has a low level hum that is constant gets worse when el84 are pulled, the 84 will only make to about 15mv The 6v6's will make it up to 33mv. I have checked everything bias voltage is around -55v at the r216 next to the bias diode.. ohm'd out all the resistors on the tube board they are all correct -34v at 6v6's, -17v at el84's...and good voltage on the plates.. Ive read another post with similar issue and a filter cap job cured it so he said but the bias caps were done first and made no difference..Anyone have any expertise on these little pita's......... Ive seen the schematic posted on here already it is better than the one they sent me more legible so I will post it. Could this be just tubes that are not in the correct range? hum goes down when I turn the bias down on the 84's its quiet at like zero mv Or a bad ground somewhere?, though Ive checked the whole board with a chopstick The amps Bias had not been touched as it still had the green goop on it and the 84's were at 5mv and the 6v6's were around 19mv though I cant believe they set the bias this way at factory would they ? any info would be helpful thanks people
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Last edited by Smitty02; 08-29-2013, 09:02 AM. -
Unmatched tubes will hum if they are to far apart and one could also redplate if the lowest one is biased up to a higher wattage. You need to find out "exactly" what each tube is pulling current wise. The bias range seems fine from what you have posted. 6v6's typically bias up around -35 to -40 and EL-84's at arounf -14 to -17. The wattage of the tubes is what can change and each tube has it's own amount of current it can pull which will determine it's wattage in dissipation. If they are way out of wack they will hum.KB -
Quote:"has a low level hum that is constant gets worse when el84 are pulled."
What's up with that?
If it hums without output tubes in, you may have a power supply problem. (excess B+ ripple. something else is pulling it down)
Either that or the output transformer is toast.Comment
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I saw that also and questioned it but wasn't sure of the operation of the amp if tubes were supposed to always be in respective sockets are not but there are a few other things in the post that didn't make sense either. The bias was ok at the resistors as -34 and -19 but later says they are at 5mv and 19mv. I'm not sure of the realationship between mv and ma but to me that says tubes are inconsistent with those readings however there may be other problems as you say so maybe it would be a good idea to post all of the tube voltages on each tube from chassis ground.Originally posted by Jazz P Bass View PostKBComment
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This amp has two sets of output tubes EL84s and 6V6s connected to the same output transformer. There is a control on the front that allows you to send the signal to either or both sets of output tubes.
Like everything else, tubes are always the first thing to check. What happens to the hum when you leave the EL84s and pull the 6V6s?Comment
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I will try to explain better an i can post voltages....Amp runs and is making sound the voltage at the test points is WAY lower than spec the el84's tp is at 5mv factory setting the tp still had the glue on it ...the 6v6 bias is at 19mv factory set Friend gave me a set of mesa el84's I tried to set bias for both sets and neither will come up to spec the highest the el84's will go is 15mv... the highest the 6V6 will go is 35mv....So neither set will come up to the factory spec printed on the board if I turn the el84's up all the way to 15mv amp hums pretty bad if I pull a tube it gets worse...the el84 pins are 1-9 334v,,,,2.6mv 343v 2.9mv, -3.4mv -3.4mv 5.3mv -17v 2.2 both el84 tubes are very close in measurements except pin 9 one is pin 9 2.2v the other pin 9 is 3.6v is this makes a difference ? other than that both are consistent with each other.....the 6V6 tubes are both the same very consistent with each other and are pins 1-8 11.6vdc 3.2vac 341v 336vdc -34.5 1.5 3.4 11.5mv I hope this helps and I didnt confuse again....the tubes are all good tubes as to matched I know the mesa el84's are matched the 6v6 are JJ's and I dont know if they are matched but they test good the pre tubes all test good but v2 is very microphonic but I guess these amps have mucho gain... I dont have a good set of 6v6 tubes to try in it
.. Amp is operational and my friend gave it to me to check out cause of the hum but the power tube board all looks good save for the Bias issue... the -voltage is -55v at r221
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I will assume that the mV settings that you are referring to are read at the test points that connect to the cathode resistors of the power tubes. Have you tested the values of the cathode resistors?Comment
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No I have not I had to put this one on the back burner but work on it when I get the time and have not pulled the boards out of it yet I was hoping to get a clue as to where or what may be going on..This amp dont look like no fun I am not real familiar with them, schematics without values ?? The Blend knob which i thought may be a issue I did check a lot of transformer voltages and they seem good. I am finishing do a Bassman, converting to 864 bias and NFB ala machine guns article.. Seems to be a good conversion but it really lowered the volume output to me. I also did the NFB to ground switch mod and he wants a mid pot also... Not really sure if i like the Machine gun mods the voltage divider to the Pi is 100k and 82k...its putting 420v on pin6 and 282v on pin 1 which dont seem right to me ?Comment
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Just to update and close out this thread, a cap job cured the issues. for one the main filter cap was shot. and the rest were all suspect after that for me so I did them all with Panasonic's it fixed the amp however the boards and layout, space on these amps are a pain, with double sided traces and sleeved connection points just plain no fun to work on, delicate yes ! Plus I dont want to go into how cheap the caps are in these amps probably pennies each for Egnater if that....Ive seen this brand before and failed they were, conclusion great little amp people love them if he spent more money on good parts Im sure they would be a bigger hit than they are...Ive worked on several of Egnaters amps and it always come to bad parts or QC if they would spend more on better caps I think these would be excellent amps..Comment
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I have an Egnater Renegade sitting here waiting for a friggin' power transformer.
They are on backorder.
It is a weird tranny with a 12-0-12 for the opamps.
Customer says he is willing to wait.
And wait we will.
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