10 February 2013

Amps: re-biasing the Marshall 1923C 85th Anniversary


I had some suspects that power tubes were a tad worn, due to the use, because the sound can be weak and thin, some days. A few years ago, I've bought a quartet of matched Svetlana Winged C EL34 and, not being sure which EL34 are mounted stock in my amp, I've thought this is the time for a change.

Also, thinking on that some or other day I should BIAS those new Winged C, I bought a TAD BiasMaster with 4 octal probes. That makes the Biasing procedure easier and safer. I had some surprises, during the process.

Fighting with BIAS

Well, the 1923 is an amp based on the JCM2000 DSL50 platform and, therefore, it shares same biasing spots and techniques. To access the tubes you need, at least, to remove the upper metallic grid on the back of the combo. This allows you to swap pre-amp tubes with ease but, if you need to put those BiasMaster's octal probes, you need to remove also the wooden panel that separated the both metallic grills.

I had to tilt the combo to clearly see the guide slot in the center of the power tubes socket, to be able to insert the octal probes in the right position. To do it, I've firstly removed the old stock tubes.
Once the probes are inserted in power tube sockets, I've connected each of the two probes to the BiasMaster device and, switched it ON.

The amp was switched on, in Standby mode for a while and then, switched on the amp, leaving the tubes to get their correct operating conditions for about 5 mins, to have stabilized readings.

I've got no readings for tube 1 and, went scared!.
Switched off everything, waited for tubes to get cold, removed tubes and probes.
I've thougth about several possibilities:
  • One of the new tubes isn't working
  • One of the probes isn't working
  • One of the ports for probes in the device isn't working
And, I did the silliest thing, I've set apart the new tubes and, wanted to check the stock tubes (since I knew they were working), I've swapped the pair of probes that I was using, getting the other double-set and, I've changed the ports from 1 and 2 to 3 and 4.

After warming up the amp, I had no reading in tube 3 (previously 1), again.
I had a look to the tubes and, the left one had no light. OMG. Maybe, not properly inserted in the probe.
I've removed everything, once more and, carefully re-inserted the probes and, each tube in its probe and, re-started the test.

Wow!. Both stock tubes were very unbalanced and biased around 30 mA, what is a very cool setting.
Even that Marshall recommends 42.5 mA to Bias the EL34, everybody else says that this is really hot and, that tubes will last no long that way. It seems that the correct values are between 37 and 38 mA and that, beyond those values there is no clear tone improvement.
I've decided myself to go for 38 mA, since I've also seen the video of Eurotubes setting a DSL50 to that levels.

Even that the biasing procedure, with those probes at least, is easy, it goes a bit tricky.
Each of the two trim pots affects the bias of one of the tubes, the left one the left tube, the right one the right tube but, both biasings are highly dependent.
To raise the current level, you move the trim pot to left hand and, to lower the current you move the trim pot to right hand. But, the funny thing is that, when you rise the current in tube 1, the current in tube 2 goes down and, viceversa so, you have to check every little turn in each trim pot until you can compensate both bias values. Gets a little while but, doesn't go more complicated than that.

So, seen that my stock tubes had a wrong bias, I wanted to give a try to stock tubes, once re-biased, before replacing them with new tubes, just to check if the sound was improved in some way.

Testing re-biased stock tubes

I would lie if it say that it was a big improvement on tone. Not really. Maybe, a solid body, while as it was the sound went thin from time to time and, clearly richer harmonically content, as a bottom warm path. I had to dial down the gain to get a clearer sound.

Throwing new Winged C EL34s

Once I've succeed rebiasing the old stock tubes, was time to see if a new brand Winged C EL34s will make some difference. Maybe not, since stock tubes seem to be in a good condition but, why don't check it before closing the amp?.

This time, I wanted to go slightly lower in the bias and, biased both tubes to around 37 mA (a very usual biasing value for EL34s). I've leaved the controls as they were in the previous test.

Ok. This clearly lowered the dirtiness of the sound in clean channel.
The overall sound is nice. I've found the Delay more notable now. The signal is strong enough without distortion but, I am probably missing some 3D projection of the sound . Maybe this can probably be because  can run the amp at lower volumes with better results now.
I have the sensation that piercing highs gone away.
When I roll on the volume, the bite of the EL34 is there. Without going piercing the mid-highs explode with attitude.

Not sure which was the stock tube. Physically seems the same as the Winged C but, the tube isn't labeled the same way and has the logo Marshall, instead. Sonically, I don't hear a big gap or difference between the one and the other. The new one is quietest and has less highs or at least, they are mellower, not piercing at all. Maybe they are both the same tube but, age and use modified its natural tone.

Wondering if...

I am wondering if, now that the power tubes are correctly biased, Mullard reissues in V1 and V2 will make a difference, respect of my previous tests.
I know, I know... I should stop making tests someday.

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