Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on October 24, 2010, 04:40:26 pm
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I have read Doug's info on Biasing. In all my amp experiance I never had much problem with red plateing before. I know it can be a tube or either and bad coupeling cap leaking DC into AC area.
I've got TAD 6L6 GC tubes both closely balanced equal.
I recently played the amp and it wasn't sounding good. I checked the bias and it was way cold--I think around 25 Mv and I re-biased it to 35 Mv and sounded great. After jamming a good while on it amazed that it was sounding so good, I noticed is was losing power and getting staticy so I immediatly looked at the tubes and they were red plateing so I shut it down. When it cooled off, I fired it up and biased it down to about 32 Mv. It was sounding good there and did good for a while but today started loosing power and getting staricy and shure enough it was red plating again.
I have tried some of my other used tubes for a short while with no problem but they are so unbalanced as far as 35 to 25 apart and don't sound as good as the TAD's. I'm thinking the next step is to pull the chassis and check the coupling caps for leakage. Any suggestions is appreciated! I need to get that sound I was getting at 35 Mv without the redplate. Platefire
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I'd put new 6L6s in it.
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Ok Agreed! I haven't got a brand new set but I pulled a set of hardly every used set of JJ 6L6 GC's out of my old Dukane conversion and intalled those. I've currently got them biased at 36Mv/32.5Mv and watching them. I know if it red plates with the JJ's it will have to most likley a problem in the amp--then. Right now the JJ's are sounding great with the present bias. Thanks, Platefire
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Weird Stuff Here! I've been keeping this amp out testing it from time to time to make sure it wasn't going to have another occurance of red plating. No more of that but yesterday I fired it up and it blew its 1.5 amp fuse. I didn't have another 1.5, so I stuck a 2 in there and blew that. No smoke ore evidence of anything burning. I went to town and got some more 1.5's and stuck one of those in there and it has been doing fine ever since. The amp is working great right now.
Also another thing the bias setting seem to be drifting. I had both tubes set for the JJ 6L6's close to 35 Mv and when it was blowing fuses I checked it at about 30 Mv and once it seemed stabelized again, I reset it back close to 35.
Could that be a fluxuation of housewhole current--I live in a rural area? I'm beginning to wonder if I don't have a short somewhere in the amp circuit--something touching that shouldn't be. Platefire
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I think the tubes are just settling in. Turn it on and let it idle for an hour. Adjust the bias. Leave it running and recheck the bias about once an hour for all day. Does it finally settle in?
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Yep! I been fiddeling with it. I'll check in one time and it close to 35Mv and another time lower closer to 31 or 32. Has to be a fluxtuation in house voltage. It sounds and reponds great, so I think I will leave it where its presently set. No more burnt fuses! Thanks, Platefire
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One last note on this. With the new JJ 6L6's being re-biased properly this amp sounds better than it ever has. I truly believe the original TAD tube performance was holding the performance back for what ever reason? I played it last night in a large room with the volume only at 2.9, no raw control on and master volume full up where it was actually operating like an original 65 Deluxe reverb and the response and tone was phenomenal. Now I know why that amp was so popular!
Plate