Amp problem

bluesoul · Post by **bluesoul** » Thu Dec 11, 2014 2:00 pm

VHT Standard 36 (4 el84). I am chasing my tail trying to figure this out. At medium to loud volumes, when I dig in on the strings...I hear what sounds like a short or cold solder joint noise. I have had the chasis out and probed & tapped everything with a wood dowel...nothing bad happens.
So I figure maybe I need to retention power tube sockets ( I have cleaned them). BUT...this is the odd part: I pull the 2 outer tubes and crank it...dig in and play hard trying to make it act up and it is fine. I put those tubes back in and do the same with the inner 2 tubes...again no problem.
This only happens when all 4 tubes are in the amp.....even at less volume than when I only have 2 tubes in.
Also..if I set the head off the cab it still does it.
Appears not to be tube sockets or vibration from the cab.

Any idea's?

bluesoul · Post by **bluesoul** » Tue Feb 03, 2015 10:49 pm

Still having this problem...anyone?

herbvis · Post by **herbvis** » Tue Feb 03, 2015 11:55 pm

have you checked voltages with a multimeter yet?

bluesoul · Post by **bluesoul** » Wed Feb 04, 2015 8:32 am

I have not dome any voltage readings. With a little guidance I could. What do you suggest?

Here is another bit of info:
I have 4 sets of matched el84 tubes. 2 sets of JJ (1 new and one I am using in a Delta Blues). 1 set of Chinese and an older set of Sovteks.
If I use the Sovteks...everything works fine. I noticed the pins on the sovteks are slightly beefier. Swapping back to any of the other sets and I have the issue.

I have re-tensioned the sockets. What puzzles me the most is why it acts up only with 4 tubes and not 2. Which leads me to believe the sockets are fine.

If you think checking some voltage readings might help...let me know!!!

Thanks

Strat78 · Post by **Strat78** » Wed Feb 04, 2015 10:20 am

Double check the bias, it could be considerably low.

bluesoul · Post by **bluesoul** » Wed Feb 04, 2015 12:23 pm

Thanks....I will check that out.

bluesoul · Post by **bluesoul** » Sat Feb 07, 2015 10:45 am

I have not taken in readings on bias...but looking at the tubes glow...they do look to be running cold. Also...the bias cathode cap and resistor pairing for each tube set up tubes is...Cathode cap 470ohms (should be fine)...resistors 250ohms. 250 seems pretty high. Maybe a swap for 180ohms?

neikeel · Post by **neikeel** » Sat Feb 07, 2015 12:01 pm

bluesoul wrote:I have not taken in readings on bias...but looking at the tubes glow...they do look to be running cold. Also...the bias cathode cap and resistor pairing for each tube set up tubes is...Cathode cap 470ohms (should be fine)...resistors 250ohms. 250 seems pretty high. Maybe a swap for 180ohms?

Whoa there!

You are talking about Swapping out your cathode bias resistor without measuring your tubes working plate voltages and the voltage across the cathode cap ad resistor thus working out what your plate dissipation is

bluesoul · Post by **bluesoul** » Sat Feb 07, 2015 12:44 pm

Well no, not without measuring and doing calculations first...just suspecting the bias is running cold. Just wandering if the 250ohm resistor looks a bit on the high side. I plan to get the meter out today a see what the plate voltage looks like.

danman · Post by **danman** » Sat Feb 07, 2015 2:33 pm

You cannot tell how hot or cold the power tubes are running simply by looking at the heater glow. Some tube's heater filaments glow brighter than others and this has nothing to do with plate dissipation. The only way to know for sure is to take the readings that Neikeel suggested and do the math for the proper dissipation. If you are concerned about your filament glow then you need to check heater voltages to make sure that everything is within spec.

bluesoul · Post by **bluesoul** » Sat Feb 07, 2015 3:54 pm

Plate to Cathode Voltage = 347
Voltage across Cathode resistor = 14.01
resistor value=250

Dissipation = 76.7%

Opinions please!

neikeel · Post by **neikeel** » Sat Feb 07, 2015 5:03 pm

bluesoul wrote:Plate to Cathode Voltage = 347
Voltage across Cathode resistor = 14.01
resistor value=250

Dissipation = 76.7%

Opinions please!

14.1v across a 250ohm gives total cathode current of 56mA, but that is for 4 tubes, divide that by 4 which is 14mA

so less than 50% dissipation at about 350v (of course we included screen draw too so maybe a little colder, unless I missed something (do all 4 EL84s share same resistor?)
Presume your cap is 470uF

So yes you could go down a bit on the resistor value (180ohm) and re measure based on your numbers

bluesoul · Post by **bluesoul** » Sat Feb 07, 2015 5:24 pm

4 tubes do not share one resistor. Yes cap is 470uF...2 of them, each with a 250 ohm resistor. So, it looks like 75% right?

it would not appear the problem I am having would be a bias issue would it?

neikeel · Post by **neikeel** » Sun Feb 08, 2015 9:41 am

No it would not (I could not find a schematic for a VHT standard 36.

Odd why it should be ok with all 4 tubes in.

Presume you tried different tubes and a different PI (driver) tube too?

bluesoul · Post by **bluesoul** » Sun Feb 08, 2015 7:37 pm

It is when I use 4 tubes is when I hear the problem. If I use the 2 outer by themselves or the 2 inner by themselves...no problem.

Yes...swapped tubes. pre and power. With the amp powered I have wiggled power tubes as well. I keep coming back to sockets. hard to re-tension el84 sockets! I may replace the sockets and see if that works...a little time and not much cost. but....I would expect a socket issue to reveal it's self when pulling the outer or inner. I don't know....it has me perplexed.

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