MusicMan 2275-130 Bias question

[Prattacaster]

That is what those two caps are for: isolating two points at different DC voltages, while passing signal between them

Of course. The positive side of those e-caps are on the bias supply side.

I expect a half a volt of so there for that reason.

Yes, and it is .588vdc.

Both the 65 and the 130 are now finished and both work great. The 65 has the 12ax7 PI and you can tell. The overdrive sounds early marshallish without dynamics. Pretty good overall tho. The cleans on both amps are great.

[boots]

This is an old thread, but I have one of these amps on my bench and have a similar problem. It seems like the tubes are biased too cold, and the trimpot won't get me in the ballpark. The amp sounds pretty good when cranked, but sounds gravelly and fizzy as the notes die out at low volumes.

I read this thread and became familiar with the cathode biasing arrangement. The voltage across R56 & R61 varies from 111 to 132 mV when running the trimpot from one end to the other. Both resistors read 4.5 ohms. So the max current I can get through each resistor is 29 mA. So each tube is idling at about half that, so roughly 15 mA per tube. For an EL34, 70% of max should be closer to 25 mA (assuming B+ is 700V--that's a little beyond the range of my DVM).

Also, I tried measuring current using the OT shunt method, and came up with about 10 mA per tube. In either case, it doesn't seem right. Am I missing something here? It seems like I should be able to get those tubes to idle at a lot more than 15 mA.

The tubes are actually E34L's (not EL34), so I'm not sure if that has a bearing on anything. I don't have four EL34's to swap in.

Thanks to anyone who can shed some light on this. Hopefully I'm just missing something simple.

[g1]

Forget the 70% thing, these amps run the tubes very cold. If you are not seeing crossover distortion on a scope, don't assume that is your problem. It could be something else, even a bad speaker.
From the sounds of it, you have the tubes running very hot, or I'm not understanding your measurements.
You should measure around 25 millivolts DC across each of those resistors. If there is no effect when turning the bias trimmer then there must be a circuit problem.
Attached is the factory bias info.

[Enzo]

Hi, it is an old thread indeed, which is a good rason to start a new thread for your amp. Also link or post the schematic you are using. I only have this one on paper in my storage unit.

or an EL34, 70% of max should be closer to 25 mA

This is not a conventional amp, note the cathode drive and your control grid is at a positive voltage? Throw that 70% stuff out the window. Look at your schematic, there are extensive notes in the lower right, they tell what measurements to expect. So yes, you are missing something. it is normal for very low idle current. This amp runs a lot closer to class B than you are used to.

EL34, E34L doesn;t matter.

Your amp sounds OK and works? great. Low level gravelly fade out? First thing I think of is voice coil rubbing on a speaker. Connect different speakers to it and see if the distortion remains.

[boots]

Thanks fellas,

Sorry to dredge up such an old thread. I do think I understand the cathode drive arrangement, but not sure I understand why the amp was designed to idle at such a low current. Thanks for the Service Bulletin, I hadn't seen that.

So it does indeed appear that the amp idles hotter than intended, and I can't get it to adjust down to the specified 25 mV across the 3.9 ohm resistors. I'm inclined to try replacing IC8 as discussed earlier in the thread. The chip in the amp appears to be the dreaded 4558.

Thanks for your comments. If I continue to have trouble, I'll start a fresh thread.