Throwing metal film resistors in Fender amps

[Wil]

Update :
The hiss comes from the preamp. No hiss with V6 removed. (no sound at all obviously)

But now I have a strange problem with the normal channel and still that low volume.
The normal channel makes a lot of loud parasitic noises. The volume, treble and bass pots act very strangely. Treble and bass get the higher output at about 3-4 and then, past than, they cancel each other. Same thing for the volume pot, about 3 is the max output which is weak... Fully counter clockwise, on 10, there's a weak output.

Weak output on the vibrato channel too. Not a rectifier problem (tried a GZ34 and a 5u4gb), power tubes are known working ones, and preamp tubes were swapped with known working ones, no improvment.
I may have fried something with my experiments. What parts should I check ? (already checked the 82k and 100k in the power section, they measure OK).

[Wil]

Piece of frustrating amp history
Well I'm back with more problems and frustration than ever...
Some time ago, I build a Deluxe reverb based on the Hoffman schematics with all the problem you know I had.
I wanted to start again from 0, what I did.
I finish the amp, it sounds absolutely great, like the first time I build it. But there's a little buzz, some I go and rewire cleanly the power transformers wires. I put the amp back in the cabinet, and, no vibrato channel. Well that surprised me. The normal channel works fine.
I change the .1uF cap next to the last 100k plate resistor. No improvment, worse, I hear deterioration (bit more hiss) in the normal channel.
The same scenario as with the first build. Once finished, it sounds great but there's a little problem, but if I touch anything after than in the amp, it goes noisier and out of control.
Concretly
Now the first thing to do is making the vibrato channel work.
Volume on 10, there's very little sound. With NFB off, it's not even at conversation level.
Tried to measured voltages on a couple of things and plate resistors showed some let's say resistance... War resistance. When trying the first one, it made a "pop" and a hole thought the aluminium sheet I put over the blue and brown OT wires (to pin3)... Yes a hole when there's no contact ! Second one actually set fire to the pin 3 of V7... A shiny blue flame, had to blow to make it stop. You understand I stop at the second one.
I believe the power tubes are know fried as is my motivation. It works fine at first, just a little problem but if I reopen it, it screwed. Happened twice in a row...
Surprisingly, everything is still working after the fire and flames...
More symptoms from the vibrato channel.
There's a distortion accompanying the low volume. The more reverb, the more distortion (but the reverb seems to work fine). Since the reverb works fine, it seems to me that the signal going into the reverb is fine too, the reverb has enough of it to fully go though its spectrum.
Could the problem be located in the wet/dry signal mixer (the 3M3 reads 3M3 but is a 1/2W) ? According to me, it looks like it's after the reverb, maybe in the recovery stage.
Are those usual symptoms ?
EDIT : I did the reverb on both channel mods and the problem is now also on the normal channel. So it's not "V2-related".
NExt step: trying to disconnect the vibrato.

[tedmich]

you should really look at your lead dress and routing internally, moving wires with a dielectric probe (chop stick) while on can really help run down a little buzz. Schematics say nothing about routing/lead dress and its a real mystery to most first time builders. Reproducing a tried and true schematic in a free form rats nest can be a real bad experience. Rerouting early stage audio signals and/or replacing them with shielded cables can help a ton. These designs were never low noise, and are often NEVER no-noise, so lose the CD quality expectation. Shouldn't buzz though....good luck!

[Wil]

Thank you for the reply (from a lefty player too!)
The amp was working super well but had a little buzz, so I rewired the power section and move some preamp wires to get them closer to the chassis (which I read is a good practice) and since then low low volume ! Nothing was changed in the preamp section, no solder was altered... that's so strange !

[Justin Thomas]

I know from experience -misrouted wiring and a ground wire soldered in the wrong spot (as in, lumped with the wrong bunch of other grounds) can produece the effects you describe. I had to rebuild a Bassman 50 from scratch and moving
preamp wires around killed some hum, and chqnging the location of ONE ground wire fixed the volume and cancellation and screeching issues. Good luck, keep at it, and don't work while tired or frustrated. It'll be worth it.

If you can, get lots of pics of known properly working unmodified originals. It may not be right "by the book," but it's a proven layout that works. It's the only way I fixed that Bassman, even though I do 't know why it did.

Justin

[Wil]

Thank you Justin ! I believe I'm underestimating the importance of lead dress. I'll try and try and try then !
I use a lot of shielded wires, do you think the place the shield of the cables is wired is important ?
Thanks

[tedmich]

Thank you Justin ! I believe I'm underestimating the importance of lead dress. I'll try and try and try then !
I use a lot of shielded wires, do you think the place the shield of the cables is wired is important ?
Thanks

YES!

(another) great RG article on the subject:
Lead Dress in Tube Amps

[eschertron]

Thank you Justin ! I believe I'm underestimating the importance of lead dress. I'll try and try and try then !
I use a lot of shielded wires, do you think the place the shield of the cables is wired is important ?
Thanks

Hi Wil,

I've been following your thread for a while, I wish you the best with your amp!

I'm working on a build that I started as a winter project a few months ago. Yes, it's noisier than I'd like, and I'm tracking that down slowly...

The advice I'd like to offer is that when an amp is not working the way it should (buzz, low or no signal, etc) I track it down to the area of the circuit the problem seems to be coming from and inspect very carefully to make sure the circuit follows the schematic. If it's correct, often re-flowing a solder joint or two in that area will take care of the problem or at least improve it. Note that "tracking the problem down" for me involves troubleshooting with ears as well as meter.

I can also attest that 'chop-sticking' will work wonders on a circuit. Placement of any sensitive wire (shielded or not) is critical. Shielding helps, but it is not the cure-all. I think, also important is WHERE the shield is landed. I land my shield where the LOCAL circuit references it's signal ground.

[Wil]

I read and re-read RG keen's geofex and try to apply it as good as I can.

Hi Wil,

I've been following your thread for a while, I wish you the best with your amp!

I'm working on a build that I started as a winter project a few months ago. Yes, it's noisier than I'd like, and I'm tracking that down slowly...

The advice I'd like to offer is that when an amp is not working the way it should (buzz, low or no signal, etc) I track it down to the area of the circuit the problem seems to be coming from and inspect very carefully to make sure the circuit follows the schematic. If it's correct, often re-flowing a solder joint or two in that area will take care of the problem or at least improve it. Note that "tracking the problem down" for me involves troubleshooting with ears as well as meter.

I can also attest that 'chop-sticking' will work wonders on a circuit. Placement of any sensitive wire (shielded or not) is critical. Shielding helps, but it is not the cure-all. I think, also important is WHERE the shield is landed. I land my shield where the LOCAL circuit references it's signal ground.

Thank you for your input. Can you show me a picture of your build ?
I'm little afraid by chop-sticking since I was "electrocuted" once. (Did it with a pencil with no lead).
The thing is, it seems like, everytime I try something new on the amp (like simply moving wires or resoldering a couple of them) the amp loses volume, dynamics and get noisier.
I simply don't get it... Even if sometime I don't use the soldering iron, I get more problems... That maybe in my head, though...

[eschertron]

I'm little afraid by chop-sticking since I was "electrocuted" once. (Did it with a pencil with no lead).

I bought a package of bamboo skewers (for cooking) and there are so many that I grab one fresh from the package when I want to poke around. I'm pretty sure the forum members here will tell you that pencils are a no-no.

[Wil]

Here s a picture of my build. Today I manage to reduce some hiss and buzz. But the amp is too thin, lacks bass, and misses 4-5 decibels of volume. Tried another rectifier and another set of powertubes, still the same problem.

[eschertron]

'Thin' plus 'lacks bass' plus 'low volume' sounds like a recipe for a bad tone capacitor. The problem I have with the dipped style caps (like the brown ones you have) is that I can look up the cap codes, but I don't know them by heart. I am quite likely to put the wrong value into a spot if I don't measure the cap on a meter before installation and double check my build. I'm not saying that's what happened, but I'd suspect - if it was my build - that a cap of the wrong value got into the build and is now causing the amp to lose all the low end.

Using another amp (hifi or guitar, doesnt matter) and a pot to attenuate high voltage signals, tap into the circuit just at the grid resistor at stage 1, stage 2, etc. Can you follow the signal to the point that it becomes thin? This will help narrow the trouble spot. Be safe while bringing potentially high voltages out of the chassis, and turn down the attenuator pot before moving or landing the wires.

[Wil]

'Thin' plus 'lacks bass' plus 'low volume' sounds like a recipe for a bad tone capacitor. The problem I have with the dipped style caps (like the brown ones you have) is that I can look up the cap codes, but I don't know them by heart. I am quite likely to put the wrong value into a spot if I don't measure the cap on a meter before installation and double check my build. I'm not saying that's what happened, but I'd suspect - if it was my build - that a cap of the wrong value got into the build and is now causing the amp to lose all the low end.

Using another amp (hifi or guitar, doesnt matter) and a pot to attenuate high voltage signals, tap into the circuit just at the grid resistor at stage 1, stage 2, etc. Can you follow the signal to the point that it becomes thin? This will help narrow the trouble spot. Be safe while bringing potentially high voltages out of the chassis, and turn down the attenuator pot before moving or landing the wires.

Thanks for the answer. All caps are the right value at the right place since it sounded super nice the first time i fired the amp up with the right volume. I don t really understand ...

[eschertron]

Sorry, I misunderstood. I thought a lot had been changed since it started out.

[gui_tarzan]

Good luck, keep at it, and don't work while tired or frustrated. It'll be worth it.

Truth! I learned that the hard way.

I'm pretty sure the forum members here will tell you that pencils are a no-no.

That made me laugh out loud! Hahahaha! Yes, graphite is a pretty good conductor!

This is the first I've read the thread and there is a TON of good information here. Thanks guys!