Amp getting hollow and harsh - component drift?

[Zhangliqun]

Have had this Hotrod Deville (2x12) for 10 years. Just in the past 4 months or so it's been starting to sound harsh, with hollow midrange, no warmth, cardboardy. It's weird -- by no warmth I mean, the bass is there, the top end is not really all that bad. It's the upper mids, say around 2-3k, that sound awful. No more bell tone on the neck or shimmer and violin top end on the treble.

Power tubes dying? No. The punch is still there, and when the power tubes start to go on these amps (and probably most tube amps) it actually gets a little sweeter, but raspy and the punch is just gone. Plus I put new tubes and rebiased and it still sounds the same. The preamp tubes look fine too.

My best guess is component drift, the cap or caps in the tone control section. This happens to amps as they get older and it can be good or bad depending on your tastes, but in this case flat-out bad. It's cold and brittle, like playing through a PA power amp.


Any suggestions short of buying a new amp? (This is my main pickup tester and I need it to be giving me a righteous read.)

 

[GoldenVulture]

Re: Amp getting hollow and harsh - component drift?

Have had this Hotrod Deville (2x12) for 10 years. Just in the past 4 months or so it's been starting to sound harsh, with hollow midrange, no warmth, cardboardy. It's weird -- by no warmth I mean, the bass is there, the top end is not really all that bad. It's the upper mids, say around 2-3k, that sound awful. No more bell tone on the neck or shimmer and violin top end on the treble.

Power tubes dying? No. The punch is still there, and when the power tubes start to go on these amps (and probably most tube amps) it actually gets a little sweeter, but raspy and the punch is just gone. Plus I put new tubes and rebiased and it still sounds the same. The preamp tubes look fine too.

My best guess is component drift, the cap or caps in the tone control section. This happens to amps as they get older and it can be good or bad depending on your tastes, but in this case flat-out bad.

Any suggestions short of buying a new amp? (This is my main pickup tester and I need it to be giving me a righteous read.)

Electrolytics are probably the first to decay/die. Ten years old or more, it's a likely possibility.
I've just replaced most of the electrolytics in my amp. It gave the amp just that bit more presence and killed stray low level noise. I did the filter caps a while back.

- http://www.schematicx.com/schematic/fender-hot-rod-deville-guitar-amplifier-schematic/ -

 

[Zhangliqun]

Re: Amp getting hollow and harsh - component drift?

The big gray cylinders are the filter caps, I know. Do I just replace all the gray cylinders and those are all the electrolytic caps? If so, I'm sure Mouser has them but I'd need to know what to look for. (I'm no good at reading schematics.)

 

[GoldenVulture]

Re: Amp getting hollow and harsh - component drift?

The big gray cylinders are the filter caps, I know. Do I just replace all the gray cylinders and those the electrolytics?

Yes.

Filter Caps are usually a can looking thing [ or two ] mounted to the chassis with the teminals facing into the chassis. I can't find any images showing that. I'm not familiar with the amp so it may not have any mounted can types.
The gray caps in there are power supply caps, ranging from 100V. to 500V. That's the type of ones I've just replaced. Be sure to replace them with the correct voltage [especially the 500V. rated ones.] I looked at the circuit. there's a few 500V types in there.
Also be sure to get the polarity correct +.........- otherwise they will explode and splattered caps are a pain in the bum to clean out. Not to mention the other damage it could do to the amp and you.
6 caps cost me around $40 Aus. but it's money well spent.

Make sure your power is unplugged before touching the inside and If you know how to, drain your caps.

I'm sure Mouser has them but I'd need to know what to look for. (I'm no good at reading schematics.)

The Cap values and Voltages should be on the caps themselves. EG 100uf [MFD]. 500V

 

[Chickenwings]

Re: Amp getting hollow and harsh - component drift?

.....yes drain the filter caps first! (noone wants to die for their amp!)
i reckon if it is the caps then usually its only the big filter camps that need replacing but its very unusual for them to die after 10 years. Maybe take it to a tech and get him to check all of them and the transformers as well...it might save you a lot of frustration and time.
p.s. you probably know this site already:
http://www.justinholton.com/hotrod/

 

[zzmoore]

Re: Amp getting hollow and harsh - component drift?

"The preamp tubes look fine too."
What do you mean when you say they "look" fine.?

 

[Gearjoneser]

Re: Amp getting hollow and harsh - component drift?

Yeah, eliminate the possibility of it being a weak pre tube. Borrow them from another amp if you need to.

 

[donnievaz]

Re: Amp getting hollow and harsh - component drift?

"The preamp tubes look fine too."
What do you mean when you say they "look" fine.?

I think the odds are that it's gonna be a preamp tube. The caps should be fine in a 10 year old amp, especially if it gets regular use. I'd sure as hell swap a few preamp tubes before doing a cap job.

 

[Fresh_Start]

Re: Amp getting hollow and harsh - component drift?

My first bet would have been power tubes and/or bias. What is the voltage reading at TP30 and what's your B+? Did you bias the new power tubes for somewhere between 21 and 25 watts each? How did you determine the bias point?

The stock bias setting looks pretty darn "cold" to me for a 6L6-GC. About 14.5 watts per tube which is only 50% of max for a modern 6L6. (485 volts B+ *times* 60ma => 29 watts for both tubes. 60ma from 60mv shown at TP30) Too cold a bias setting could produce the tone you're describing.

Schematic and layout: http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_DeVille_schematic.pdf

Please don't touch the insides if you don't know how to drain the high voltage out of the power rail filter caps.

Honestly, replacing components on a cheaply built PCB without knowing which ones are bad is just asking for trouble. I'm not trying to be rude - just realistic based on trying to keep my Blues Jr. functioning. Plus power rail filter caps wouldn't affect a specific frequency band. On top of that, there are a LOT of transistors, FETS, etc. inside that amp which could have decided to start acting funny and you'd need an oscilloscope and a ton of experience to figure out where the problem is if it's one of those solid state gremlins.

Preamp tubes should last forever but they don't always. Taking a known-good tube from another amp and replacing each preamp tube in the HR Deluxe 1 at a time should identify a bad tube.

Hope that helps,

Chip

 

[Zhangliqun]

Re: Amp getting hollow and harsh - component drift?

I'm familiar with draining the filter caps. Learned how to do it for replacing some resistors that had cracked and replacing the input jacks.

Preamp tubes tend to get all dusty and burnt looking on the inside when they go bad in my experience. Mine still look brand new.

G Vulture -- in your photo at the top center of the PC board are two vertically (relative to PC board) mounted cans.

Keep in mind everyone that there is no loss in punch or output or any indication that signal is weakened at all. The quality of the tone is different, that's all.

 

[GoldenVulture]

Re: Amp getting hollow and harsh - component drift?

I'm familiar with draining the filter caps. Learned how to do it for replacing some resistors that had cracked and replacing the input jacks.

Preamp tubes tend to get all dusty and burnt looking on the inside when they go bad in my experience. Mine still look brand new.

G Vulture -- in your photo at the top center of the PC board are two vertically (relative to PC board) mounted cans.

Keep in mind everyone that there is no loss in punch or output or any indication that signal is weakened at all. The quality of the tone is different, that's all.

Yes, PCB mount caps [Radial ]. Just a different package to the others [ Axial ] .

This happens to amps as they get older and it can be good or bad depending on your tastes, but in this case flat-out bad. It's cold and brittle, like playing through a PA power amp.

This suggests the Bias has drifted.