Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on April 16, 2012, 02:48:37 pm
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Every amp or so I get an amp that doesn't like EL84s. Not a particular circuit, but an amp that I have built with a circuit that has digested EL84s just fine in the past. Usually it will be frequency specific, kind of a hash or background distortion. I have built ten or twelve TW Rocket type amps and 2 have shown this and now I have an AC30TB that I am finishing up and it does not like an open "B" string. Does anyone else have this problem? It's getting to the point that I am not sure I want to mess with EL84s any more........which is a damn shame because the Rocket is a great amp.
Suggestions?
BL
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I'm sure you already tried tube swapping between "EL84-friendly" and "hash-nasty" amps, right?
I wonder if there is any microphonic effect in the output tubes. Additionally, you might try whopping big grid stoppers on the output tubes, to see if they help. By "whopping big" I mean 56k-100k.
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HBP...... Yeh that's what I usually end up doing, it's just a PITA because I know that who ever changes tubes later will have the same problem. And I don't mind keeping extra tubes, but 4 or 5 sets of new power tubes just in case I get a finicky circuit seems.......... not reasonable. I need to figure out a fix.
drgonzonm...... Edcor and Hyboer not sure if any were MM.
BL
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I notice this as well when i build the same amp over and over again like the November amp. But i think i found a preamp tube that is really warm and sounds way better than most 12AX7's on the market today. I tried the JJ Gold pin spiral filiment tube s about 5 months ago and have noticed a big difference in the entire amps tone vs using regular JJ or other 12ax7 preamp tubes. Of course a pair or quad of 1958 Bugle Boy EL84's would be nice too. JME
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I've had the same issue on a recent 36 watt build.The only variable is the OT in my case.
I tried the zener diode Paul Ruby fix but it didn't work at all.Tried large grid stoppers and it worked a bit but not all the way.
So I tried a filter on the plates of the EL84's.It is detailed in an old Guitar player tech article.
It involves two 5 watt 1.5k resistors directly on the plates and then filtered with a pair of 8.2meg balance resistors and a pair of .01uf caps.It filtered out the offending high-frequency that was making the OT ring and make nasty distortion sounds on some notes.
It worked perfectly.
Google 'smooth your sonic ride' An old guitar player magazine tech article.
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Does this happen with all brands of EL-84s or are you using a particular brand?
If you haven't tried Ei EL-84s, it might be worth picking up a pair. They were just the ticket in my Blues Jr. (I know that their 12AX7s have a bad and deserved reputation for going microphonic but I haven't had the same problem with the EL-84s.)
Chip
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My favorite "new" tubes are the mil Saratovs. Best tone, warm not bright or spikey like JJs can be, that being said I have tried a few different brands and had trouble with all. I have swapped Saratovs and been good.
Phsyonoodler: thanks for the heads up, I'll give it a try.
BL
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Saratov? Is that a nickname?
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Saratov is the name of the Russian factory that made the tubes.
In the past at least, Saratov was one of maybe 5-6 factories that might have made the tube you bought with the Sovtek label. I haven't bought any Russkie tubes in a while, so I'm not familiar with how you go about buying tubes just from Saratov. Of course, Svetlana set up its own distribution to the US a while ago.