Identifying Bias Resistor In 60's Ampeg Jet

[earache]

Hi - can someone help me identify the bias resistor on this Ampeg Jet schematic? I believe that it is the 22K 1W that is at the cap can, shown at middle bottom of the attached schematic. I just want to make sure.

It's the 6V6 tubed Jet and it's a nice little amp. I've replaced the old electrolytics and cap can and added a 3 prong plug. Unfortunately there is no standby so I can't add bias test points by putting 1ohm 1% resistors from pin 8 of the power tubes to ground.

Anyone have experience with these amps? Do they push the rated limits of 6V6 tubes like say a Deluxe Reverb? What should I expect for voltage at the grids (pin 5) of the 6V6's?

[captntasty]

Your amp is cathode biased...

[Bruce / Mission Amps]

Hi - can someone help me identify the bias resistor on this Ampeg Jet schematic? I believe that it is the 22K 1W that is at the cap can, shown at middle bottom of the attached schematic. I just want to make sure.

It's the 6V6 tubed Jet and it's a nice little amp. I've replaced the old electrolytics and cap can and added a 3 prong plug. Unfortunately there is no standby so I can't add bias test points by putting 1ohm 1% resistors from pin 8 of the power tubes to ground.

Anyone have experience with these amps? Do they push the rated limits of 6V6 tubes like say a Deluxe Reverb? What should I expect for voltage at the grids (pin 5) of the 6V6's?

250 ohms.
The grids are grounded so the voltage will be nearly zero.
Measure the Dc voltage from lug 8 to ground and divide that by the 250 ohm cathode biasing resistor.
That, and Ohms' Law will tell you what the two power tubes are drawing in idle current.
If the tubes are well matched, halve that number.
That new idle current number ... like .040a, times the voltage you can now measure from lug 3 to lug 8 will tell you how many watts the tubes are idling at.
I'd be worried if they were more then about 14-15 watts each.

[earache]

Bruce

"Dc voltage from lug 8 to ground" 22.2V (both tubes, they seem well matched)

22.2 divided by 250 = .088

"If the tubes are well matched, halve that number" .044

"...times the voltage you can now measure from lug 3 to lug 8"
349.5 X .044 = 15.38 watts

Do you think that 15.38 watts is a little high? The tubes are likely original, the 6V6's and the 5Y3 Rectifier are RCA and the preamp tubes are Tungsol 6SL7's. The 6V6's show a little purple glow...the amp is in pretty good shape and may not have seen much use in it's life so the tubes may still have a lot of life in them.

Thanks for the great instructions, I have read many of your postings before and have gained a lot of knowledge from them.

[Bruce / Mission Amps]

Bruce

"Dc voltage from lug 8 to ground" 22.2V (both tubes, they seem well matched)

22.2 divided by 250 = .088

"If the tubes are well matched, halve that number" .044

"...times the voltage you can now measure from lug 3 to lug 8"
349.5 X .044 = 15.38 watts

Do you think that 15.38 watts is a little high? The tubes are likely original, the 6V6's and the 5Y3 Rectifier are RCA and the preamp tubes are Tungsol 6SL7's. The 6V6's show a little purple glow...the amp is in pretty good shape and may not have seen much use in it's life so the tubes may still have a lot of life in them.

Thanks for the great instructions, I have read many of your postings before and have gained a lot of knowledge from them.

Well 15 watts with a NOS 6V6s is probably right on the edge of being OK and I wish it wasn't that high.
But that is what these amps do and why I mentioned "14-15 watts".
Assuming you are using a NOS 5Y3GT (if not, do so first), what I do with these amps is remove the 250 cathode biasing resistor and exchange it for a 270 to 300 ohm one and then go over to the power transformer and find the high voltage center tap wire which is grounded to the chassis... lift it from ground (not the heater/filament center tap, leave it connected to ground if it is)...
buy a few 5 watt zener diodes, like 7.5v, 9.1v, 12v and put one or two of those in series with the Hi-V center tap wire to lower the overall B+ voltage.
The banded end of the zener is the cathode and that side is pointed at ground when you do this... you can put as many as you want in series as long as all of their cathodes are pointed to ground in the string.

[earache]

Seems Kinda Hot...

I let the amp run for like 2 1/2 hours and the top felt pretty warm, almost too warm to touch, over the transformer.

When I got the amp I replaced the cap can with a CE Distribution 20 20 20 20 475V cap can, and changed the carbon comp resistors at the can and at the tube sockets for the same values but with 2W rated resistors. I also changed the power cord to a 3 prong, and retensioned the sockets.

Everything else seems fine except for the heat - no noises or crackling, no smoke I guess the only other odd thing is the purple glow at the 6V6's.

Is it gonna be OK, Doc??

[earache]

Posts "Crossed In The Mail"

Hi Bruce

I guess I was composing my above reply while you were sending yours. I'm gonna follow your advice about the resistor and the zener diodes, and will post the results after

...it also occurs to me that I should check the value of said 250 ohm resistor to see if it has drifted (after it cools off, of course)

Also for the diodes - are you saying that any of the values listed would work OK (7.5v, 9.1v, 12v)

TIA

[stokes]

If you want to lower the current draw by increasing the cathode resistor value,thats fine,but I wouldnt be too concerned with the 349 plate volts,many 6V6 amps are designed with that and more on the plates,and have been working fine for many years.Most 6V6 tubes can take what seems to be overvoltage.Remember the ratings in tube manuals are pretty conservative,most amps push tubes to and beyond the limits with few problems.Now before Bruce takes umbrance to my comments,I aint saying he's giving bad advice,its not,its a good solid safety measure,but that amp has been working for 50 years at those voltages,as have many others.Just doesnt seem worth the effort to add zeners in this case.

[Bruce / Mission Amps]

Hi Bruce

I guess I was composing my above reply while you were sending yours. I'm gonna follow your advice about the resistor and the zener diodes, and will post the results after

...it also occurs to me that I should check the value of said 250 ohm resistor to see if it has drifted (after it cools off, of course)

Also for the diodes - are you saying that any of the values listed would work OK (7.5v, 9.1v, 12v)

TIA

Yes check the actual value of the biasing resistor.
And any combination of zener diodes that will bring the idle current down a little is OK.
It is an easy reversible mod and your power tubes are right at the spot where they are getting hot and almost too hot...
If changing to a 270-300 ohm biasing resistor does it for you, then that's fine too.
Those amps were built during a time when it was rare to find a wall outlet that was actually 120vac, more likely less... mine is frequently 126vac now.
I don't know what yours is but that little bit of extra primary voltage on the PT can make the B+ rail in old vintage amps sneak up higher and higher.
As far as the plate voltage ratings of the 6V6s go, it doesn't make much of any serious difference what the actual B+ is as long as it is not like 450vdc, it is the combination of high plate voltage and high idle current that you need to watch out for.
But, I doubt if the amp was built thinking the 6V6 power tubes would be idling at well over 15 watts each.

Just remember the basics, voltage x current = wattage and the tubes have a safe limit of around 14-15 watts, at which time they will start to wear out much faster then if idling at say, 10 watts.
The blue glow in one or both tubes is nothing really to worry about right now.

[earache]

I have replaced the 250 ohm bias resister with a 270 ohm and the amp runs MUCH cooler now. However the owner of this amp, although he is happy about the lower temp. says that the amp is still "farting out" after it warms up for a few minutes and is played loud. He says that the amp only sounds good for a few minutes and then sounds lousy. Any ideas? Would new tubes make the difference? Or is th etransformer going south?