Bias issues

[myker]

hello, i have an ampeg v2 on my bench right now http://www.schematicheaven.com/ampegamps/v2powamp.pdf" onclick="window.open(this.href);return false;, i cant get the bias voltage to stabilize. i have replaced the coupling caps, the diode, and many of the components in the bias circuit. I start the amp up at about 35ma then as it warms up i start to hear humming and the current climbs to -135. I have a 500k pot where the 75k ohm resistor was and if i keep turning down the bias as it rises it will stabilize only for a second then continue to rise. I dont have any 7027a's so i have tried two different sets of 6CA7's and 5881's with the same results. Any help would be greatly appreciated. Thanks in advance!
-mike

[chubs]

Last time I had something like that happen to me... the HT fuse was blown... You could not see it by looking at it... but it was gone...

[myker]

if the HT fuse was blown i wouldn't have a solid 500 vdc on the plates, but i do so i dont think its that. but i will monitor the positive and negative dc to be sure that they are not both creeping on me. oh crap, this amp doesn't have a HT fuse, only mains. Nevertheless, i will check both +&- are not fluctuating as the amp warms up. thanks for the idea, chubs!
mike

[chubs]

Why don't you slap a HT fuse in there while she's openned up... can only do good at this point... if it blows... you know something is up.

Is the bias layout the same as a marshall? Have any pics?

[stoo]

hmmm.. How long do you wait till you set the bias? You have to wait till the tube starts to do it's tube thing before adjusting the bias. The filament has to heat up the cathode so that electrons start boiling off. Now the tube is "working". I'd say around 2 or 3 minutes min.
Stew

[myker]

hmmm.. How long do you wait till you set the bias? You have to wait till the tube starts to do it's tube thing before adjusting the bias. The filament has to heat up the cathode so that electrons start boiling off. Now the tube is "working". I'd say around 2 or 3 minutes min.

at two or three minutes i would have no tubes left, i fear. the plates turn bright cherry red. i can actually hear a click before the supply runs wild on me. I have biased tubes many a times, never before experiencing this. I have tried to let the tubes warm and have managed to get a few minutes on them by riding my bias pot, but it gets to a point where the tubes are either red plating or will not pass signal. I can keep it at 35ma but the sound is farty and motor boat sounding. then when i try to increase to get a better sound it goes red.

thanks, keep the ideas coming guys!
-mike

[SDM]

Some things to maybe think about here:

A 500K pot will let the bias circuit put out much more than -100V (max bias caps are rated for or what any tube would need). So if bias pot was set to 500K, the -voltage would be quite high, and the time the bias circuit will need to stabilize/charge will be increased as well. So may have taken about 5 seconds before (with stock 75K) to stabilize, now could take around 20 seconds or better to start stabilizing. If you have the pot wired up as a variable resistor (two lugs connected together -an outer lug and a middle/wiper lug - to one end of where the 75K was, the other outer pot lug to where the other end of the 75K was), this slow stabilizing problem will go away once you get the pot set back towards the stock 75K value. However, If you instead wired the bias pot up as a voltage divider (one outer lug to where one end of the 75K was, wiper out to rest of bias circuit alone, other outer bias pot lug to other end of 75K former location), you'll still see the long delay for bias voltage to stabilize, even for (eventually) correct - bias voltage needs.

So I'd use a 100K pot if you have it, more than you need, should keep your -V safely under -100V and thus bias filter caps safe. No 100K pot around, just wire a 100K resistor across the 500k bias pot to knock it down towards a more reasonable total value (83K would be total here, should be plenty but if need a bit more - V, use a 120K resistor). Be sure the pot is wired as a variable resistor as described above (middle/wiper lug connected to an outer lug on one end). Since the filter caps may be old and hard pressed here (very possibly exceeding their max -V rating with the 500K pot alone), check them out well/replace if in any doubt.

Also be sure the sockets are wired safely/correctly for 5881s (be hesitant with Sov's at that voltage or other cheap subs), El-34s or whatever you use. 7027s are basically 6L6-GC, BUT they have internally connected pins 1-4, pins 5-6, so occasionally you can see the sockets themselves wired or rewired to take advantage of that. Probably don't need to worry about that in an Ampeg, but definitely worth a good look. If okay for 6L6/5881 types, you may still need to add a jumper between pins 1-8, if not already there, and definitely increase the screen resistor values for EL-34/6CA7 use. Be sure to remember to not stick a true 7027 back in with this jumper added.

May want to be sure the PT is handling the extra EL-34/6CA7 heater current draw okay too. No overheating, heater voltage not sagging too much, though an Ampeg PT is probably fine with them.

Also may want to bias a little colder at those voltages unless using stout tubes/tube types.

Some ideas at least, hope anything helps.

[myker]

thanks, steve!
i do indeed have the pot wired as a variable resistor an connected as you described. when i get home ill use the smaller pot and hope it fixes.
thanks again to everyone!
-mike

[myker]

ok guys, heres the update...i bought a bias probe and cheap multimeter from the guy over at eurotubes so now i have the probe on and i am starting the amp, waiting for a few minutes for it to warm up and starting up the amp and through the sweep of the pot the tube draws no current. sometimes it will register and fluctuate between 9ma and 100+ma really fast then go away. does this make sense to anyone?
thanks,
mike

[yladrd61]

The 7027 has a different pinout than the EL34, 6L6 Type Tubes. You either need to rewire your tube sockets or find some 7027s I think JJ and Sovtek are making them.

[yladrd61]

Looks like SDM already has you covered on that