Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: guitarrock04 on July 08, 2014, 09:24:48 am
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Hello
New to the forum, but not necessarily new to tube amps. I'm working on a project and am hopeful that some advice from one more versed might save me some time and unnecessary component swaps.
The amp is a Silvertone 1482 6V6 pp. I've replaced all electrolytic caps, checked voltages, swapped tubes and speakers. I've not been able to track down a harsh fart or buzz noise when the amp is driven hard on lower notes. My first reaction was to swap the speaker, but the issue remains through 3 different known good extension cabs.
I've made a brief recording to help commicate the problem.
Silvertone 1482 Fart Demo (http://www.youtube.com/watch?v=xW6SEF5F89g#)
Any thoughts on where to look next?
Any help is greatly appreciated.
Thanks in advance
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Oscillation issue would be one concern. I would've said a bad tube, but you mentioned trying others. I am presume you tried replacing ALL of the tubes in question pre/power tubes?
A bad solder joint will sometimes cause that exact problem. I will presume you have chopsticked the amp safely looking for something like that?
I have had a couple of pf size caps go bad before and cause a harsh noise.
Since you still have this noise thru different speaker cabs, then you know it is electrical (probably) instead of mechanical.
Does it do it with the 6AU6 vibrato tube removed?
Do the voltages look good for the plates/cathodes ............etc..... ?
With respect, Tubenit
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Wow! that is a weird sound! One thing I would recomend that I didn't see mentioned, is clean the tube sockets
with a good contact cleaner. The way I do it is pull the tube, spray contact cleaner in the socket pin openings, on the tube pins and as you are reinserting tube, work the the tube up and down a couple of times before seating it. Also have you tried a different cord, guitar or plugged it in at a different electrical socket. Try the other channel and see if it does it in that channel also. If it doesn't you've isolated it to that channel in the preamp section. Seperate circuit wires to give them as much of their own space as possible--as tubenit mentioned to stop possible Oscillation. I hope you get it---I got two 1482's and love em. Platefire
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sounds like the rectifiers are sputtering.
--pete
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All,
Thanks for the responses.
-All tubes have been swapped with known good ones. This includes the rectifier tube.
-There are some small value ceramic disc caps in the tremolo oscillator circuit, but that shouldn't affect the signal, correct? For the posted sound sample, the 6au6 is indeed simply omitted.
-Filter caps were replaced with new Nichicon pieces. I used two 10uFs in lieu of the 10 and 5 originally equipped.
-There are no differences in this sound on any of the three inputs. No differences with different cords or guitars; perhaps a change in onset volume of the issue depending on the guitar.
-Tube sockets were sprayed with Deoxit, with no noticeable affect.
Static voltages are as follows: 122ac wall, 350ac PT, 366dc B+, 360 at the plates, 352 at the screens, 272dc for the preamp, 190 and 76 at the PI plate and cathode respectively.
24.1v at the 6v6s cathode resistor. In-circuit measured resistance of 285 ohms.
Voltages are high, but don't strike me as unusual. A bit hot in terms of static dissipation. Perhaps something goofy in the OT?
High level of 60 hz hum. I have indeed poked around with a skewer, with no obvious component culprit. No reduction in hum from pulling wires away from the heaters. Perhaps a 100 ohm psuedo center tap on the heater circuits would help with that?
I'm still at a loss here. I can definitely hear the potential here; this thing sounds sweet, other than the hum and this wretched noise when it's dimed.
Any additional input is again appreciated.
Thanks
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The 1482 input jacks have no wired ground to chassis but use the mechanical connection of the jack for ground. I've seen a supposably tight jack loose it's ground on these through age and corrosion. Try cleaning & re-seating those jacks or use a jumper wire from jack ground term to chassis ground to test live. Also it wouldn't hurt to use a jumper to chassis to check other grounds with the amp live to see if you hear any difference at any location. Has it had a three conductor cord installed yet? Also on an original power cord, the .05 cap to ground may have shorted. I would highly recomend a new 3 conductor cord. Platefire
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High level of 60 hz hum. I have indeed poked around with a skewer, with no obvious component culprit. No reduction in hum from pulling wires away from the heaters. Perhaps a 100 ohm psuedo center tap on the heater circuits would help with that?
I don't see a CT on the PT in the schemo.
You only need a faux CT by using 2x100ohm R's IF your PT doesn't have a CT. If your PT doesn't have a CT put in the faux CT.
Brad :icon_biggrin:
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Any success?
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Took a jumper wire to ground the input jacks. No change.
It still has the original cord for the time being. It was lower on the priority list until I track down this noise.
My inclination is that there's something going on in the power amp section. No justifiable reason. Voltages seem reasonable. I would like to clarify; There's an unused tap on the OT. I measure 2.2 & 2.4 ohmdc from across these secondaries, respectively. That's within measurement error of my cheapie DMM. Let's just assume then, that my DC resistance method can differentiate simply a higher vs lower dc resistance tap. I'm running an 8 ohm speaker on the lower resistance tap-because there was a (non-original) 8 ohm speaker in the amp when I got it. This amp should have a 4ohm speaker from the factory, correct? Am I running too high on reflected primary OT impedance, then?
Thanks again
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in the clip you're playing a low freq. note, after a few of hits of that low freq. note, then you hear what sounds like rectifier sputter.
i think you are a victim of piss-poor design driven by economics.
the 6X4 has a 75mA MAX DC output rating (23W @ 325V). if this schematic (http://tubedata.milbert.com/sheets/127/6/6X4W.pdf) is close to your output stage's draw then you're output stage is already drawing more then the rating of the 6X4 at idle. roll in the screen current at full volume and you're probably around 90mA.
so at 38mA / 6V6 * 2 = 76mA @ 350V = 26.6W so, at idle you're already exceeding the 6X4 specs, plus you haven't allowed for the small bottles.
http://tubedata.milbert.com/sheets/127/6/6X4W.pdf (http://tubedata.milbert.com/sheets/127/6/6X4W.pdf)
IMO you have a couple of options:
tune the coupling caps of stages to roll-off at higher frequency and add some (10uF? 20uF?)more filter.
replace 6X4 with a 6CA4/EZ80 and use up to a 50uF first filter stage cap. to do that you'll need to replace the 7 pin socket with a 9 pin. easily done with a unibit and 1/8" bit and i doubt the additional 400mA in filament draw will be an issue.
http://tubedata.milbert.com/sheets/127/6/6CA4.pdf (http://tubedata.milbert.com/sheets/127/6/6CA4.pdf)
lastly, i can't help but think this amp should have been designed with a 5Y3 or 6CA4 in the first place.
--pete
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Well I don't know? but both of my 1482's run fine on the 6X4. I think it would be a good idea to try another 6X4 if you have one. You can't get OT Ohms by measuring across the OT secondaries. To determine Ohms, you have to apply voltage to the OT primaries and read the output of secondaries and apply the results to a formula to determine Ohms of an unknown OT. On an original 1482 OT secondary set up, the secondaries are wired to a terminal strip by the OT. The terminal closest to the power tube is 4 Ohms, center term is ground and the normally unused terminal closest to the speaker baffle is the 8 Ohm terminal. To change it to 8 Ohm just simply move the speaker wire from outside terminal(by the power tube) to the unused terminal by the baffle. Leave the center terminal speaker wire(ground) in place. Then it's ready to go for 8 Ohm load.
Regarding filter caps, in my all original #1 1482, it has the original can capacitors and #2 1482 with modifications has been replaced with Sprague Atom's filter caps, new power resistors and coupling caps and both operate quietly with little operation noise. So something has gone astray to cause the hum in your amp. I dare say it's probably something very simple, but problem is finding it. In trouble shooting sometimes you have to be very patient and long suffering to finally find the problem. Wish I could be there to help you because I would like to know what the problem is too? Platefire
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Use troubleshooting flow charts, e.g.: http://geofex.com/ (http://geofex.com/) >
- Unintentional and ugly distortion (http://geofex.com/ampdbug/distort.htm) > Sound cuts out or squawks on loud notes:
- Failing coupling capacitor (http://geofex.com/ampdbug/coupling.htm)
- Failing (http://geofex.com/ampdbug/distort.htm)plate resistor (http://geofex.com/ampdbug/resistor.htm), cathode bypass cap, cathode resistor or grid resistor
- Intermittent ultrasonic oscillation (http://geofex.com/ampdbug/osclate.htm)
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This is some good trouble shooting info on tracing hum.
http://www.geofex.com/ampdbug/hum.htm (http://www.geofex.com/ampdbug/hum.htm)
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All,
Reviving an old post. Have some time to tinker ov r holidays. I've made little progress on this issue. I put the amp back together and have been using it (infrequently) as is. In keeping the volume down, the issue isn't present.
What had occurred to me was the possibility of solid state diode(s) as a band aid for the issue. Without an understanding of the physics as to how sputter occurs, is there a means by which a 1n4007 can be used to supplement the capacity of the existing 6x4? I'd prefer a reversible mod to the existing tube lineup.
As always, any help greatly appreciated.
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This is a temporary test to see if the 6X4 is your problem.
Just pull the 6X4 and lay it aside. Now solder the cathodes of two 1N4007 diodes together. Leave one lead full length. You will have three full length leads. Bend the leads so they will fit into the 6X4 socket. Insert one anode into pin 1, the other anode into pin 6, and the cathode lead into pin 7.
The diodes will replace the function of the tube. B+ voltage will be higher but you can get an idea if the 6X4 is the problem. When done, just pull the diodes. If this fixes the problem you can simply solder the diodes on the bottom side of the tube socket.
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Having just listened to the thumping, I'm thinking that there may be an issue in the tremolo circuit. After rebuilding my 1482, the tremolo thumps a bit, even when off, but not too badly.
Maybe try disconnecting the tremolo circuit, and see if the thumping goes away.