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Post subject: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 10:27 am
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I have a 1968 TR with Altec 417-8H Series II speakers and new JJ matched power tubes. I have had a recurring problem of the bass cutting out on the vibrato channel while playing. The sound goes from great/usual, to super thin and tinny. I've had it in the shop (certified Fender repair shop) three times in the past, most recently 2008. Each time they soldered "faulty" connections and the final time soldering any "possible faulty connections". Problem has surfaced again. Question is whether I am just chasing a typical problem with older amps or could there be another possibility? For example, I plug my guitar in Ch2 Vibrato and it sounds great for a couple seconds, then bass cuts out. Unplug, plug back in, sounds great for a few seconds, then sh*t. It is intermittent at this point but don't want it to crap out at worst moment. Could it be something as simple as the input jack? When cable is plugged in there is no slop or wiggle. Any help would be greatly appreciated!


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 10:40 am
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Kinda sounds like a hinky solder point in the tone stack of the Vibrato channel. Did you try changing out V2 (12AX7)?


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 11:00 am
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Thanks for quick reply BMW2002Ti! I haven't switched out V2 and don't have a spare lying around. Can I swap out another of the preamp tubes to use as test? Also, its intermittent. In fact right before posting I checked the amp and of course no bass drop out! Like bringing car to garage with strange noise only doesn't do it with mechanic sitting next to you. urgh!


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 11:04 am
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Go ahead and try another 12AX7 from another position. Maybe V1, since your problem seems to be on the Vibrato and not Normal channel.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 11:24 am
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Are you using the vibrato channel's bright switch when the problem occurs?

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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 12:07 pm
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i will try swapping V1 when it goes fritz next time. So I am sure, V1 is the preamp tube all the way to the left when viewing from back of cab? If it turns out that swapping tubes resolves the issue (because you are correct that I don't have the problem on the normal channel), its a simple matter of a bad preeamp tube? Seems to me i had them all changed ~ 10 years ago....maybe time for new ones anyway?

Have you heard of this type of problem with old amps. Makes sense to some degree that older solder connections can crack or come loose over the years but seems I am dog chasing its tail.

Arjay, Yes i normally have the bright switch on but have tried turning it off when bass drops. No effect.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 18, 2015 5:43 pm
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Photos of those solder "fixes" maybe nice to see. It's rare to see a BF or early SF era Fender amp with bad solder points. At least any should have been fixed a long time ago. Once done correctly, the solder should last a LONG time. It doesn't rust or oxidize, like other metals.

You may have a bad coupling cap or resistor in the tone stack. Once the amp warms up, maybe the continuity those that device changes. Or bad pot in that part of the circuit. May need to do voltage & continuity tests.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 7:34 am
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I'm not certain what exactly has been re-soldered as invoice only says "faulty connection on capacitor soldered" and second invoice 'soldered several faulty connections'. I used to use the TR as a head to power a 4x12 half stack and actually set the TR on top of the cab. I suspect all the vibration over the years led to a number of degraded connections. Have not stacked it for several years. I haven't let it run to full hot to see if the problem occurs again. if does I'll pull out guts and and take some pics to send. Thanks again for you ideas!!


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 8:08 am
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Pics may help. Like these of the general circuit board and PSU caps board.

Image


Image


Image


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 8:17 am
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Thats great! Thanks! While i am very "handy" I have never had to check solder connections. My presumption is the connections look clean, no crud or debris, component leads show no corrosion, and each component is firmly in place. Is inspection for "firmly in place" simple a gentle wiggle to see if the solder connection or component lead does not appear loose?

What is still confusing to me, is that when the drop off occurs, if I unplug my guitar and then replug, the bass is back for a moment or two and then drops out again. Is it possible the problem lies with input jack. I can't think of reason why bad jack would act this way but have you heard of something like that happening?


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 9:55 am
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Ther symptom you describe about getting sound for a few seconds after replugging your guitar could be from DC getting into the grid of V!. When you unplug, it would ground the grid and bleed off the DC. When you plug back in, DC slowly rises and the sound fades. I haven't heard of it only affecting the bass though.
The problem could be coming from active electronics upstream from the amp. If there is a leaky coupling cap in a pedal or active guitar preamp, that could be the source of the DC. It might cause a big pop when you plug in.
There's no real source for the DC in the eyelet board of the amp unless someone has resoldered it with conductive paste flux. That can be cleaned off
The 1.5 K grid resistor could be intermittent. That might cause a problem such as you describe, If the cathode has no ground.
You can probably short the #2 input jack momentarily to ground the grid. If the sound comes back, it's DC on the grid. Or use a voltmeter. The voltage will probably fall as the meter itself discharges the grid.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 11:56 am
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Thanks. The bass drop off is immediate, not at all gradual. When it first occurred several years ago, I could actually tap the top of the amp and bass would return for a time. After having the the soldering done the problem disappeared for a couple years, then returned, had additional solder work done, and now ~ 5 years later problem is back. Urgh.

Regarding DC getting into V grid, we're getting into territory beyond my knowledge but when I recently checked to see if problem was still there, i had simply plugged my guitar directly in. Normally I play through a VOX ToneLabSE (which has a small preamp valve in it) so will try that and see if the source is somehow result of my pedal board.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sat Feb 21, 2015 7:37 pm
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kevink wrote:
But when I recently checked to see if problem was still there, i had simply plugged my guitar directly in. Normally I play through a VOX ToneLabSE (which has a small preamp valve in it) so will try that and see if the source is somehow result of my pedal board.


If this problem is happening when you plug the guitar (any guitar, right?) --- straight into the amp, it's not the pedal board. The bass cutoff is only on the VIBRATO channel. Never on the NORMAL channel. Right?

Tapping = sometime fix usually means bad connection (solder) somewhere. It could be the socket holding one of the tubes in that channel. Ever have them replaced?


Thanks! :D


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 10:03 am
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OK, so if you need to work from the outside to try to diagnose it
There is a 1meg resistor on the input jack that is the dc ground path for the grid. It gets its ground from the chassis ground on the input jack. I know you probably already have, but try tightening the nuts on the input jacks. If they turn more than 1/4 turn, that might be it. Pounding on the amp with results suggests this might be the cause.
Otherwise....
To test for DC on the grid ov v2 vibrato channel, find a 1/4' plug with no cord or a very short cord. Plug this into #2 input.
Plug in the guitar to the #1 input. Keep the volume low. When the symptom occurs, short the tip to ground. There will be a loud pop as the grid is discharges, but the sound should now return for a moment after you remove the short.
If this is what happens, and you have no active electronics in the guitar, then the DC is internally generated. If unplugging the guitar fixes it, then it must be in the first stage. Either the grid is going positive or the cathode is losing ground.
If you take it to a tech, have the 1meg and 1.5k resistors replaced as well as the cathode capacitor. While he's in there, replace the bass limiting resistor in the tone stack as well. It's all of $2 parts. Then thououghly clean that end of the board and the V2 socket with solvent to remove flux.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 10:16 am
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TimsAudio wrote:
If you take it to a tech, have the 1meg and 1.5k resistors replaced as well as the cathode capacitor. While he's in there, replace the bass limiting resistor in the tone stack as well. It's all of $2 parts.


......And seventy-five bucks worth of labor.

:mrgreen:

Arjay

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