• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

300B tube amp mA meter question

inperson

inperson

Active Member
I just bought this amp about a month ago. Long story short, the mA meters line up dead center (50) for all the tubes I used so far (Shuguang and Valve Art) but when I use mesh plate Full Music 300B n/c tubes the meters don't match and the sound is also unbalanced. I don't know why. The amp is self-biasing the only adjustments I can make are some hum pots behind the 300B tubes. Yes, they are hum pots, not biasing pots. I asked the seller about it and they just said Full Music tubes aren't good. I also exchanged the first pair of Full Music 300B tubes for another pair but got the same bad results. The amp sounds great and I feel those mesh plates sound best but they don't seem to work with this amp. The amp uses two 300B tubes, 2 OB3 tubes, one 5U4 and two 6SJ7's.

Any ideas? I would love for someone to give their thoughts!

The second picture shows the best setting the Full Music tubes reached in the meters, but usually they are not this close. I have left the amp on to warm up for over 30 minutes but no change in the meters.
 

Attachments

  • PICtT0054.jpg
    PICtT0054.jpg
    72.8 KB · Views: 86
  • PICT0069.jpg
    PICT0069.jpg
    55.6 KB · Views: 95
  • PICiT0048.jpg
    PICiT0048.jpg
    87.5 KB · Views: 93
  • dlz013.jpg
    dlz013.jpg
    121.8 KB · Views: 99
Last edited:
Register to hide this ad
inperson said:
I just bought this amp about a month ago. Long story short, the mA meters line up dead center (50) for all the tubes I used so far (Shuguang and Valve Art) but when I use mesh plate Full Music 300B n/c tubes the meters don't match and the sound is also unbalanced. I don't know why. The amp is self-biasing the only adjustments I can make are some hum pots behind the 300B tubes. Yes, they are hum pots, not biasing pots. I asked the seller about it and they just said Full Music tubes aren't good. I also exchanged the first pair of Full Music 300B tubes for another pair but got the same bad results. The amp sounds great and I feel those mesh plates sound best but they don't seem to work with this amp. The amp uses two 300B tubes, 2 OB3 tubes, one 5U4 and two 6SJ7's.

Any ideas? I would love for someone to give their thoughts!

The second picture shows the best setting the Full Music tubes reached in the meters, but usually they are not this close. I have left the amp on to warm up for over 30 minutes but no change in the meters.
Click to expand...

You need to get a matching pair. Tubes are often sold in pairs to make sure that they work well together.
 
for_p1 said:
You need to get a matching pair. Tubes are often sold in pairs to make sure that they work well together.
Click to expand...

Both pairs I got were matching pairs. That the problem.

Do Full Music 300B's have a different operating parameter than other 300B tubes? Anyone know what the n/c stands for on 300B n/c?
 
Mesh plates sometimes are not to spec of the amplifier. Do you have a pair of carbon plates? I would try those and see if the problem continues.
 
Fast996 said:
Mesh plates sometimes are not to spec of the amplifier. Do you have a pair of carbon plates? I would try those and see if the problem continues.
Click to expand...

I thought of this but the Carbon plates are way too expensive. Double the mesh plates and don't have that cool glow :smoke:
 
schematic shows the 300Bs are cathode biased and no
way to operate or change so the meters are mostly for
show (fixed bias allows static change where the meters
are useful). the 4R/5W resistor share the current up into
the heater divider network so my guess is that the meter's
internal resistance is much larger than the 4R (usually is)
and even if there's large variation in the current - there's
less affect on the meter's display.

I wouldn't worry too much about this since there's no user bias
adjustments as it would be for fixed bias.

I like the fact that the screens are regulated.

Enjoy the amp. I have a 829 amp that I have no idea
what's inside. It plays well.

Others can jump in and suggest changes if you want a
working bias circuit and therefore working meters.

Bob
 
Bob said:
schematic shows the 300Bs are cathode biased and no
way to operate or change so the meters are mostly for
show (fixed bias allows static change where the meters
are useful). the 4R/5W resistor share the current up into
the heater divider network so my guess is that the meter's
internal resistance is much larger than the 4R (usually is)
and even if there's large variation in the current - there's
less affect on the meter's display.

I wouldn't worry too much about this since there's no user bias
adjustments as it would be for fixed bias.

I like the fact that the screens are regulated.

Enjoy the amp. I have a 829 amp that I have no idea
what's inside. It plays well.

Others can jump in and suggest changes if you want a
working bias circuit and therefore working meters.

Bob
Click to expand...

Yeh, I agree with Bob. If it really bothers you though, I'd start at the input with power off and caps drained, and measure resistance at several points comparing right and left channels. Could be a way out of spec resistor on a driver tube even.
 
I would guess that the meshplate tubes are drawing a bit more grid current than the others. The spec for maximum grid resistance in self-bias (i.e. cathode bias) is 250K per the WE data sheet. They use 330K in the circuit, which might be fine for most tubes, but it might not cut it for these. To verify this, you can short out the 330K grid leak resistors with clip-leads and turn the amp back on (no hard can come from this, but no sound can come out). If the tubes bias up fine, then it is larger-than-typical grid current. Not necessarily a problem with the tubes, since the circuit uses a larger-than-spec grid leak resistor (if they do bias up fine then).
 
Thanks for the input guys! Some is more technical than I can handle, but I did go to my friends company and he told his tech to check all operating points of the 300B mesh plates to find the truly perfect matching pair. He said most people just check one point to match a pair. I am not sure I am talking about this correctly, I hope you know what I mean. Anyway, the tech checked and now I have a very balanced sounding amp. The meters still are not matching up but the sound is and that is what I was wanting!

One more question. I just got some Mullard EF37A's to use in the preamp stage and I also ordered, and got today, some Sylvania OB3 tubes to use instead of the Chinese WY2P tubes. But I noticed that the OB3's are 6 pins and the Chinese WY2P's are 4 pin tubes. I thought I read that they were the same tube. What's the story with this? I am guessing now that they can't be rolled. Is that right?
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom