Marshall 1987lx RI Biasing...

[rjgtr]

I just bought a 1987xl RI head. I figured I could always get MetroAmp PTP and Transformer when I was ready. Now I've biased a bunch of amps, including Marshalls, but this one is strange.

I've got a GT Bias tool, which has worked great with every other amp I've used it with. But this amp is reading only 7.5 mv instead of the more expected range of 35-40 mv. If I adjust the bias pot upwards I can't get more than 1.4. But higher than 8.5 the amp starts sounding grainy.

Is there something different about these things?

[VelvetGeorge]

Hi rjgtr, welcome to the forum.

That is strange. The 1987RI should bias the same as any EL-34 equipped amp you've used your bias tool on.

Can you open the amp up and check the bias voltage readings?

George

[rjgtr]

Thanks for the reply.

I took it back out of the box and got out the DVM. I checked the Plate voltage and it reads 446 Volts. So it should bias in the range of 35ma...

Just to rule out the Bias Tool, I hooked up for the transformer shunt method - one lead on pin 3 the other on the center tap (actually I use the HT Fuseholder - easier to get to). Got the same reading as the Bias Tool.

I checked the junction of R23 and R24 (same as R25 and R26 on the old 1987x schematic) and it reads -45.6 Volts. One thing I did notice is that R28 (same as 1987x schematic) in the bias section is 68K not the 56k indicated in the schematic. That does seem to be a big difference. My thought is that that difference could be it, but when I bias higher than 8.5 the sound gets very distorted even on 2 with one channel. The highest i can get the reading to is 14. The plates don't glow red, but it doesn't sound as good.

The amp sounds ok and is plenty loud. Strange huh?

Back to you.

[rjgtr]

Got it!

Believe it or not the OEM power tubes were bad. I had another set of Svetlanas so I put 'em in (I know I should have done it sooner!). Biased 'em to 35 and all is great. I tried 40 but like them at 35. BTW, I think they used the fixed bias resistor value for the JTM-45 (which is the 68k not the expected 56k) on this board. That means getting an EL34 past 42 is difficult. When I get out of the warranty period, I'll change it back for the expanded range.

Thanks.

[Country Boy Shane]

Amazing that we tinker for so long and it was them dang power tubes... that we all love so much.