Hot Rod Deluxe Instability???

[kornowsd]

Hi Everyone;

I have noticed something on the Fender Hot Rod Deluxe and I wonder if anyone else has noticed this, as well.

I have a couple of them, right now - one modded, one not. They both do the same thing. The modded one just has it more apparent, now.

If you take the Hot Rod Deluxe and either leave the input open (nothing plugged in) or "terminate" it with a 2.7K resistor across it and do the following:

1) Set the "Drive" to 0.
2) Set all tone controls, including "Presence" to their max value.
3) Activate the "Bright" switch.
4) Set the channel to "Drive".
5) Activate the "More Drive".
6) Turn the "Drive" up toward the max value.

Eventually, the thing should start to feed back on itself generating a feedback tone in the 1kHz to 2kHz range. Even after you turn the "Drive" down, it takes a while for the amp to stabilize and recover.

Any thoughts on what one can do to stabilize this??? I remember, at one point, reading an article by Gerald Weber (Kendrick Amps) about the CBS-era power amps doing this same thing. Problem is, I can't remember where I saw that info, now and what he did to correct it.

Any thoughts on stabilizing this thing would be greatly appreciated...

Thanks.

Dar

[Strat-Mangler]

I'm no techie, so I won't be of too much help, but you could always e-mail Mr. Gerald Weber, or simply give him a call.

It's a rather specific problem you're describing. I don't wish to sound condescending, here, but how many times do you actually go through all these steps to recreate this problem? I used to have such an amp, but never noticed the problem, because I never set my tone to the maximum on every single pot, etc...

To me, it's not necessarily a problem, but obviously, if it's one to *you*, I'd suggest getting in touch with Mr. Weber. He probably will have the info you're looking for.

Good luck and welcome to the forum! :)

[Kap'n]

"It only hurts when I do this."

"Have you considered not doing that?"

Lots of four input marshalls oscillate when you dime both channels and the presence knob. So what. They sound like shit when they're set that way anyway. Has that inspired legions of folks to fix that problem?

You know I freeze my ass off when I roll down my car windows in the winter, no matter how high I turn up the heat.

[keefsdad]

I hate it when that happens!!

[bluesfreak68]

Sounds like you have either a microphonic preamp tube or some bad oscillation in the amp. Main problem on the HR Series are the 2x 470Ohm 5W Cement Resistors R78/R79 getting too hot and then burn out which gives you somewhat more then the desired +-15V on the ICs.
If you measure on Pin 4 and 8 on the TL7404 you should have only approx. 16V. IF its higher you need to replace the 2 resisitors mentioned above. Get 330 OHm instead of the 470 Ohm, this value will work also and less heat is generated by the R's.
Another possible root cause could be the Plate Load Resistors or a bad solder joint on those junky PCBs.
For replacing the PlateLoads R4, R11, R16, R22, R57, R58 you need 5x 100k 1/2W and 1x 82k 1/2W (for the PI Plate R57)....

cu
bluesfreak

[picturevalve]

Sorry, but replacing R78/R79 with 330 ohm resistors will increase the heat dissipation. There is 32V across those resistors so the power dissipated will be VxV/R :
32x32/470 = 2.2W for the 470 ohm. current = 68mA.
32x32/330 = 3.1W for the 330 ohm. current = 97mA.
Whether the extra available current will cure any instability is another matter.
I see this calculation has been posted on another thread, apologies.

[Cygnus X1]

No Thread Too Dead...to get a little more blood out of the stone.



Worry not mate.
I did the same thing when I joined up.


Welcome aboard!

[steve25]

I would bet that if you loaded all those same circuit componets on to a turret style board and used the same tubes it would not scream like that, hence its the poorly layed out circuit board issue!
One of the quotes I like form Mr weber is that relates to these amps goes like this, Good amps are built up to a standard, cheap amps are built down to a price!

[picturevalve]

Hi Cygnus, thanks for the welcome. I've just been fixing this hrd and I've read all the pages on justin's website. Can't believe he hasn't altered the reference to 470/330 ohm swap in the light of all the posts about it. It's not as if the hrd hasn't got enough problems as it is......
perhaps I should have waited until Hallowe'en to post my previous. ;)