• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ampeg PF-50T tube issues are driving me insane! Please help!

mas_akala

mas_akala

Sep 1, 2018
15
17
#1
Hey all, long time mostly-lurker here. I got a great deal on a PF-50T, totally dig it, except…

It shipped with a really microphonic 12AU7 and the power tubes make this weird fizzy sputtering noise when certain notes are played (like 14th fret E on the D string).

I took it into the local authorized Ampeg guy but he's been really shady through the whole deal and seems like he's trying to make a buck off me by insanely over pricing some Groove Tubes instead of just following through with the factory warranty's replacements.

This had been going on since April and I finally got sick of it and just ordered a bunch of matched Tung-Sol tubes with the 6L6GC STR for power.

So now the amp sounds awesome and the sputtering went away, but the red biasing light doesn't ever turn off. Once you warm the amp up and turn everything on with no instrument plugged in both the green and red lights are constantly on. When I switch back to the Ruby ones it can bias as written in the manual, but sounds like garbage.

Did I get mismatched tubes? Is there some rating difference between these Tung-Sols and the factory tubes that would cause this to happen? The matched set are guaranteed so should I ask for replacements?

Hoping @JimmyM or @beans-on-toast or someone with some experience could give me some advice. Ampeg support has not been very easy to deal with as you have to use their stupid ticketing system and responding to their emails seems to go nowhere.

Thanks in advance!
 
  • Like
Reactions: DJ Bebop and EatS1stBassist
#5
mas_akala said:
Hey all, long time mostly-lurker here. I got a great deal on a PF-50T, totally dig it, except…

It shipped with a really microphonic 12AU7 and the power tubes make this weird fizzy sputtering noise when certain notes are played (like 14th fret E on the D string).

I took it into the local authorized Ampeg guy but he's been really shady through the whole deal and seems like he's trying to make a buck off me by insanely over pricing some Groove Tubes instead of just following through with the factory warranty's replacements.

This had been going on since April and I finally got sick of it and just ordered a bunch of matched Tung-Sol tubes with the 6L6GC STR for power.

So now the amp sounds awesome and the sputtering went away, but the red biasing light doesn't ever turn off. Once you warm the amp up and turn everything on with no instrument plugged in both the green and red lights are constantly on. When I switch back to the Ruby ones it can bias as written in the manual, but sounds like garbage.

Did I get mismatched tubes? Is there some rating difference between these Tung-Sols and the factory tubes that would cause this to happen? The matched set are guaranteed so should I ask for replacements?

Hoping @JimmyM or @beans-on-toast or someone with some experience could give me some advice. Ampeg support has not been very easy to deal with as you have to use their stupid ticketing system and responding to their emails seems to go nowhere.

Thanks in advance!
Click to expand...


Here's what the manual says about both Red and Green Bias LEDs on:

"Both on all the time Possible bad Bias Control or bad tubes Have unit checked by a service technician."​

Also

"If the green and red LEDs are lit, the amp is under-biased (clockwise) and too much current is flowing to the power tubes. "​

Since the amp biases fine with the original tubes, there is a good chance the bias control is fine. However the circuit may not have enough range to dial to new tubes down into the proper current range. This is something that may be fixed by swapping out a bias ranging resistor, so the bias can be set to a higher negative voltage. But a better solution IMHO is to get tubes that are graded to work in the circuit.

Not exactly sure how the bias monitoring circuit is set up, but I believe the green LED will not come on until both tubes reach their minimum target current. This is an "AND" logic function. If A and B satisfy the minimum current target, you get green.


Then I believe the red LED comes on if either tube exceeds the max current limit. This is an "OR" logic function. If A or B satisfy the max current target you get red. The Green lights stays on, because both tubes are above the minimum current target.

Is the amp humming? The reason I ask is if the tubes are not matched it will produce sort of a dull roar. With this problem it may be possible to get red without green. I believe red without green means one tube is over the max current target and one tube is under the minimum current target.
 
  • Like
Reactions: EatS1stBassist, jw23mind and nbsipics
#6
Give Ampeg's customer service department a call and explain what's going on. Maybe they can help you by recommending a better choice for getting this fixed. You shouldn't have to pay for a service tech's inability to fix any product that's under warranty.
 
  • Like
Reactions: Haroldo, InnerCityBass, kesslari and 9 others
#7
Something I would check with original tubes. Does the sputtering occur if the amp is not placed on the cab. It's possible the pins in one or more of the tube sockets is loose, so vibration creates an intermittent connection.

The size of the pins in the tubes varies somewhat from one company to the next. In particular, I have seen lot's of posts that suggest the pins in some or all JJ tubes are smaller than other brands.
 
  • Like
Reactions: EatS1stBassist
#8
Wasnex said:
Here's what the manual says about both Red and Green Bias LEDs on:

"Both on all the time Possible bad Bias Control or bad tubes Have unit checked by a service technician."​

Also

"If the green and red LEDs are lit, the amp is under-biased (clockwise) and too much current is flowing to the power tubes. "​

Since the amp biases fine with the original tubes, there is a good chance the bias control is fine. However the circuit may not have enough range to dial to new tubes down into the proper current range. This is something that may be fixed by swapping out a bias ranging resistor, so the bias can be set to a higher negative voltage. But a better solution IMHO is to get tubes that are graded to work in the circuit.

Not exactly sure how the bias monitoring circuit is set up, but I believe the green LED will not come on until both tubes reach their minimum target current. This is an "AND" logic function. If A and B satisfy the minimum current target, you get green.


Then I believe the red LED comes on if either tube exceeds the max current limit. This is an "OR" logic function. If A or B satisfy the max current target you get red. The Green lights stays on, because both tubes are above the minimum current target.

Is the amp humming? The reason I ask is if the tubes are not matched it will produce sort of a dull roar. With this problem it may be possible to get red without green. I believe red without green means one tube is over the max current target and one tube is under the minimum current target.
Click to expand...


The only time a noticeable sort of hum comes on is if the bias dial is turned toward fully clockwise. If it's in the middle or lower it sounds pretty good. Both lights are constantly solid though. I guess I stupidly assumed all 6L6GCs would be interchangeable, but maybe that's not the case?
 
  • Like
Reactions: EatS1stBassist and Wasnex
#9
agedhorse said:
Give Ampeg's customer service department a call and explain what's going on. Maybe they can help you by recommending a better choice for getting this fixed. You shouldn't have to pay for a service tech's inability to fix any product that's under warranty.
Click to expand...

Calling just led to some Line 6 voicemail, but I can try the ticketing system again.
 
#10
Wasnex said:
Something I would check with original tubes. Does the sputtering occur if the amp is not placed on the cab. It's possible the pins in one or more of the tube sockets is loose, so vibration creates an intermittent connection.

The size of the pins in the tubes varies somewhat from one company to the next. In particular, I have seen lot's of posts that suggest the pins in some or all JJ tubes are smaller than other brands.
Click to expand...

I thought I tested this, but I'll try again. That may have been before the microphonic 12au7 was sorted out.
 
  • Like
Reactions: ficelles and Wasnex
#17
I have the Tung Sol 6L6GC STR tubes in my PF-50T. Sounds fantastic. And, I had no trouble correctly adjusting the bias. Also, I had no sputtering problem with the Ruby tubes. Betting there is something else wrong with the OP's Ampeg.
 
#18
I have the original power tubes in my PF-50t at the moment... retubed the preamp with reissue Tung Sols but even with the originals it works fine. Do you have a recording of the sputtering noise?

I had a great response from Ampeg the one time I emailed them for support, that was a while back though.
 

Latest posts

Top Bottom
Back