HDRx, volume swell

[mindbender]

Whats going on? A couple of weeks ago the power tubes went out on my HDRx so I replaced them with JJ 6L6's biased at 68mV. Tonight I noticed my amps volume kept fluctuating. Got a prognosis? Thanks.

[Wally]

Mindbender, if I amunderstanding you correctly...you have biased those 6L6's at 68mv, which translates to 68ma of current draw if you are reading bias the way I think you are. This setting translates to 107.6% of plate dissipation using a plate voltage 475(just a guess).
Using 475 volts at the plate as a estimate, 70% plate dissipation would put that current draw in the area of 44ma. 70% is consider a safe and rather warm percentage of plate dissipation. Many folks consider 50-70%.
You might give some feedback as to what method of bias reading you are using. I am thinking that your tubes are biased very 'hot'. Are they redplating?

[mindbender]

Hello Wally, I'm going by "The Unofficial Fender Hot Rod Deluxe Owners Manual" site for my biasing. Tonight I rebiased at 68mV (it was at 60mV after all, Fender's recommendation) and went through three tunes with no problems. Maybe the JJ's need a higher bias than the stock GT's? This weekend I'll play a lot longer and see the result. I'm not a tech man so I appreciate any advice. Thanks Kyle

[GoldieLocks]

I just had a six month problem with volume swells in my HRDLX. It ended up being something with the circuit board loosening something (I think?). I changed tubes many times. I could also only get through about 3 songs before it would act up. The heat did something to the board I think?

After awhile mine started changing channels at random. something to do with the solid state converter or something. Its been fixed now for 4 months & working great.
(remember...I know absolutely nothing about electronics, just pieces of what my amp guy tells me - Good luck)

[Wally]

Originally Posted by mindbender

Hello Wally, I'm going by "The Unofficial Fender Hot Rod Deluxe Owners Manual" site for my biasing. Tonight I rebiased at 68mV (it was at 60mV after all, Fender's recommendation) and went through three tunes with no problems. Maybe the JJ's need a higher bias than the stock GT's? This weekend I'll play a lot longer and see the result. I'm not a tech man so I appreciate any advice. Thanks Kyle

Well, I don't know where the connections are made to yield the mv readings at the test point on those amps. I will study that the next time I have one of those in. Usually, mv readings are taken across a resistor off of the cathode to ground. The reading directly corresponds to the current draw in ma....and 68ma is too high in myworld. So, I have to think that the mv reading is a combined thing and dividing that in half will give an accurate reading of mv per tube and therefore the current draw in ma per tube. This would yield a 34 ma current draw figure. Using 420 volts as the plate voltage per the UFHDOM site yields a plate dissipation figure of 47.6 watts, which is a 'cool' setting. Some people complain of the cold sound of stock Fender amps of this type. 47.6 volts of plate dissipation is not cold, but if one wanted a bit more warmth, it is there to be had.
I use a direct method of reading the actual current draw at the tube and the actual plate voltage at that setting. I am also not averse to using my ear or the player's ear for preferred sonics and then checking current draw and plate voltage to assure that all is within the range of 'correctness'

[mindbender]

Thanks guys I I'll let you know the results, Kyle