Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 13, 2016, 01:29:22 pm
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Guys - I have a '68 reissue twin reverb on the bench. All the tubes check good. When I turn the unit on, and then plug a cable that is connected to a guitar to either trem or custom inputs, irregardless of volume setting, the amp emits a loud POP. When plugged in a playing through the amp it will emit loud crackle/pop sort of like when power tube is about to fail. But the power tubes check out. I haven't got under the hood yet but am heading that way. Anyone ever have an amp exhibit these symptoms?
Thanks
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Ok out on the bench. I hook it up and it sounds better. I tightened up the ground points to make sure, and pulled and re-seated the cables coming from the controls to the main board. The popping on cable insertion stopped. So I tap on the 6L6s individually and one of them is definitely more fragile than the other three. When I tap it lightly I get that sound that it was making. So looks like I'm going to have to replace that pair and rebias. Or try just one and see.
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This amp had a newer pair of GT-6L6-S tubes in it and an old set of GT-6L6-S tubes that looked like they had seem some real good heat. labels fell off in pieces, printing on tube changed color. An unbalanced set of pairs. Once a fairly matched quad is in there and power dissipation and bias figured correctly, all is well.
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So it's fixed?
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Did you just take down 2 of your posts?
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Thanks Willabe - NOPE not fixed at all.. I set the bias to 32 mills and then plugged in a strat and played and after a few minutes the nasty loud noise came back. Sounds like a tube arcing. Something to do with the high voltage. I have never seen this before, so kind of perplexed .
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I set the bias at around 65% of 25 watts or close. Yes I took down those posts because it's acting up again.
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Ok, mods have a few extra buttons to click on and I thought I goofed up. :laugh:
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OK now when I take it off standy, there is a LOUD thud. heres a schematic
http://support.fender.com/schematics/guitar_amplifiers/65_Twin_Reverb_schematic.pdf (http://support.fender.com/schematics/guitar_amplifiers/65_Twin_Reverb_schematic.pdf)
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The schematic says 85 watts minimum, and it shows 6L6GC so I suppose this amp is probably 120W at full power 30 watts per tube. But to be safe I am going to stay at 65% of 25watts PD. The schematic is for an earlier reissue, this is a silverface 68 custom, but very much similar inside I think. So I think the schematic is OK to go by. There are no test points in it, or none marked TP this or that. But it's pretty easy to find your way around in it. It's pretty clean actually. I was chop-sticking and one filament wire just broke cleanly off. To get to it to solder it back on I had to remove the fuse holder. When I went to take the wire off the bottom spade terminal on the fuse holder it just broke off. Lovely! Some previous person had bent it down. A no-no with these cheap chinese parts. So I had a fuse holder that I made work.
When it warms up fully now and you take it off standby it makes a big ole thud. And I am about to call it a day on this amp and pick up tomorrow. I will post some voltages on the 6L6s
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OK i took readings on the 6L6 sockets there is still a nasty loud POP when switching off standby and back to standby is a little pop
V7
pin 3 - 458.5v
4 - 457
5 - -50.22
8 - 0v
V8 -- the same as V7
V9
pin 3 - 456.8v
4 - 457
5 - -50.23
8 - 0v
V10 - same as V9
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btw I see all the test points now have to scroll down on the schematic page
voltages TP 27 - 404dc .013ac
TP28 - 456dc 1.045ac
TP29 - 455.6dc .035ac
TP30 - 446dc .020ac
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I'm a little concerned that the screen voltage isn't lower than the plate v. Pop could be standby switch going bad.
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That amp is a PCB? Could be a (partially) broken trace that when it heats up it acts up?
Pop could be standby switch going bad.
If you have a spare standby switch change it.
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Yes PCB did you look at the schematic I posted? Well I took out two tubes and ran the newest ones only ( I don't have a new matched quad) and I just played it for about 15 minutes in half-power mode and it sounds awesome . So I guess I still have a tube issue. It looks like all 4 tubes that were in this amp when I got it not good. I had the two tubes on the inside sockets. Is it OK to try them on the outside sockets?
Thanks Willabe, I really don't like standby switched unless they are implemented properly and that would be "after" the first PS cap. I need to talk to the owner and ask if this switch has been popping for him or did it just start doing that.
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Yes PCB did you look at the schematic I posted?
Yes.
I had the two tubes on the inside sockets. Is it OK to try them on the outside sockets?
Yes, you can do that. :icon_biggrin:
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I put the two good tubes in the two outside sockets and played it for a bit and it sounds excellent. So I need a new quad and I think that's all that's wrong with this amp. I've never fooled around or owned an amp
that has a quad output section. Believe it or not, after all these years this is the first one I have been inside.
Concerning yesterday - have you every had a day where it seemed nothing seems to go right? Well that way yesterday for me. That's why I just stepped back and left it alone for another day.
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Concerning yesterday - have you every had a day where it seemed nothing seems to go right? Well that way yesterday for me. That's why I just stepped back and left it alone for another day.
That's wise. :icon_biggrin:
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you every had a day where it seemed nothing seems to go right?
maybe the residues from the gravity waves, since we're a couple weeks out from a full moon :icon_biggrin:
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https://www.youtube.com/watch?v=heK8QjhWGag (https://www.youtube.com/watch?v=heK8QjhWGag)