Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on July 09, 2012, 02:57:39 pm
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I got my SE El84 Little Bear amp out this week and have been jamming on it a bit. The thing works good but I'm noticing a ratteling along with the note especially using the bridge pickup. The sound is similar to a ratteling speaker would make but I know it's not the speaker. I use this same cab with other amps and don't have this problem. It has three basic settings:
Normal=no NFB, one bypass cap on first preamp stage
Boosted=no NFB, two bypass caps on first and second preamp stage
Adjustable NFB= Normal plus NFB, 50K pot in series with NFB resistor(I'm not sure if that 150K NFB resistor is correct--I may have changed the value.
A clue may be is it don't make the noise with the NFB engaged??? The schematic is attached. Any help appreciated---it sounds really good except for this--would liked to get it fixed! Platefire
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Did you swap out the tubes?
The NFB-rattle relationship is a mystery, though. I have no clue about that. I have read other people complaining about similar noises only to find it is an inadequate filter cap in the power supply, but this is a pretty new amp.
Sorry I can't be of any help. :dontknow:
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I guess I sould at least try another preamp tube even though it's operating quiet as a mouse--other than the unwanted noise. Platefire
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Nice litle amp . It is not your problem but I suggest to install a artificial center tap to heater with two 100 ohms resistor .
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EL84s are prone to rattling too.
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I would try a voltage divider between the second amplification stage and power tube
/Leevi
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Hi Plate, a mate gave me his PEAVEY to do some minor mods to and it had a rattle as well, had the same problem with the tubes and a closer look could see little white flakes floating around inside. The amp came with JJ's in it and were about 7 years old. Replaced with new JJ's and quiet again :icon_biggrin:
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Well as Sluckey said apparently that's something that must be pretty common with EL84's. My Boogie has 4 but so far no problems with that. I'll definately remember that issue now! Plate
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Platefire , you are not sure of the schematic regarding grounding !!!!!!! And the rest of the circuit ? Is right ?
Don't you think a right schematic is a must to help you ?
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-it operating very quietly--so I guess I leave well enough alone.
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We'll I'm kinda embarrised---was the EL84. I overlooked my own rule, to check basic things first.
I tried a different preamp tube, no change. I had a vintage RCA EL84 in there that I was real proud of but I put a JJ in there and it cleared up.
I had the same problem recently with my Goodsell Super Seventeen combo. It uses an old Hammond AO-35 chassis with the tubes hanging upside down. I have a really sweet sounding set of old RCA 6BQ5 tubes that I love. In the Goodsell they rattle a bit (or one of them does). If I set the amp on its side the rattle stops. I popped in a set of Electro Harmonix EL84 tubes and total silence when upright. I have set the 6BQ5 tubes aside for special occasions and recording.
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I have found that amps sometimes need NFB to get rid of "rattling" or "fizziness" in the speaker. I think there is nothing wrong with your amp, it just needs some type of NFB. PP amps have the option of a suppression cap after the PI. I guess the equivalent for an SE amp is a conjunctive filter after the power tube. (I have not tried conjunctive filtering.)
My Vibrochamp is similar topography to your amp. It has, and needs, 2 types of NFB: 1) a 330pF cap across pins 5 (signal) & 8 (cathode); and 2) global NFB. This follows the AB764 schematic. I tried a re-build with no NFB and had speaker issues. Replacing these 2 forms of NFB cured it.
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I don't know what it is with a set of JJ EL84's I got becuse I usually love JJ power tubes but this is the second amp I've tried them in lately and in both made them brittally bright to the point is was too much. They are pretty old so they might of been when they were in a earlier expermental mode. I didn't throw my old RCA away---it may do fine in another amp.
On the schematic, I did that several years ago on my at work CAD program before I retired. If I'm going to keep this amp, I need to build a ExpressSCH type file and double check the info with the as-built circuit. That's one of those things on the back burner.
This amp is good little recording amp. Done some recording with it in the past with very good results. Platefire
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You guys are just going to force me to tear down my amp and re-draw the schematic correctly---arn't you???? :BangHead: Hay it's most likely wrong on heater grounds and PT ground. I was learning when I drew it years ago. I'm still learning! I repent!!! Please forgive me! It works----for now that's good enough for me. I got other irons in the fire. Please accept my humble apology for posting an incorrect shematic. I'm nabbed, I'm caught, I turn myself in to the amp police! :notworthy:
INFRACTION--RUNNING AN AMP ON A NON-WORKING SCHEMATIC
BTW-think I should try decaf?
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think I should try decaf?
It's not time for decaf till we say it is!!!
:laugh:
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Hi Plate-
A quick scan of the schem and this is only to help so take it for what it is...a reminder to help for later???
1. Need balance resistors on resevior caps - also will serve as "drain resistor" when off
2. Need a power resistor or choke btwn resevoir and 1st plate node
3. What no 100r or 1k screen resistor!? ;)
*don't mess with Murphy's law and leave her be.
Take care,
Keo
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Your killing me! :violent1:
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If this amp is built in-correctly----then that's its advantage because it sounds awesome. Don't mess with success! :l3: :l4: :laughing4: :happy1:
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This amp was the first Diode full wave doubler power supply I ever built. I thought I was at Rocket Scientist level then :l2:
Also this is the amp I once plugged the EL84 in the 12AY7 socket and the 12AY7 in the EL84 Socket---smoke did rise!!! The first power rail resistor was in distress and changing colors. I caught it in time before any damage done. :BangHead:
Honestly it's the quietest amp I have regarding operational noise. When I recently changed that EL84 out it helped the performance a bunch. The old RCA EL84 might of been a victim of the tube mix up. :dontknow: The thing gets loud for a single EL84---ask my wife! :cussing: I would say it gets as loud as my 1482. Platefire