Marshall Short Part II (old subject new thread)

[sweatyk]

Once again thanks for pointing that out to me..............sorry for being slow right now. But here it is.



COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = Started at +9.1mv and climbed
across R96 = Started at +9.1mv and climbed

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.32V
across R118 = Started at +20mv and leveled at +91mv
across R96 = Started at +20mv and leveled at +92mv

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = +2.2mv
R96 = +2.2mv
HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = +2.4mv
R96 = +2.4mv


I certainly hope I have it all on there now.

[DrGonz78]

Certainly looks to me like that 10k resistor did the job and the idle current should be well in the acceptable range now. I am pretty sure that you can now jump to step 3 on Juan's guide line. With light bulb limiter still connected, connect speaker and check those same measurements. Also, double check for DC on the output just to see where it is sitting before connecting the speaker.

[sweatyk]

OK one last thing before turning in for the night. Reattached the 10K resistor across R74 and read across speaker out. I got 16mv when I first turned it on then it dropped slightly.

[J M Fahey]

So far very good.
I *specially* like that bias current (measuring voltage across ballast resistors) now is *stable* , with minute variation from cold to hot.
The old one seemed to go into thermal avalanche.
Now you can recheck with direct (non limited) connection to wall power and if fine, play loud and check at, say, 2 , 5 and 15 minutes (previously setting all controls to 0)

But I guess it will hold now

[sweatyk]

Juan I only had a few minutes today to do this. I removed the limiter and plugged directly in the wall. These are the cold readings:

TR8 & TR10 base to base = +2.17v
R118 = +2.2mv
R96 = +2.2mv

That's all I have time for today. Next will be after 5 to 10 hot correct? Then it's play time if all goes well?

[sweatyk]

JM could you clarify something before I move forward? You said......."Now you can recheck with direct (non limited) connection to wall power and if fine, play loud and check at, say, 2 , 5 and 15 minutes (previously setting all controls to 0)"
In all of my testing the controls are set to zero which you refer to in parentheses. So I can assume if you say play loud then adjust them accordingly as normal. I know it sounds like a dumb question.......I just like to be absolutely sure before moving forward with this. And are you looking for the same reading points.....TR8 & TR10 base to base, R118 & R96? I have the preamp tube in and speaker hooked up and ready to go.

[Jazz P Bass]

Jumping in:
What you want to see is a steady reading.
One that does not change by much at extended times.

[g1]

Yes, play with your normal settings, set to zero when checking voltage. R118 & R96 are voltages to check.
Also listen to the clean setting sound at low volume, this is where you would notice any objectionable crossover distortion from too cold bias.

[sweatyk]

OK first round of testing. Tube in, speaker hooked up, no limiter measuring across R118 after playing for 2, 5, and 10 minutes on clean channel with gain at about 4 and master volume around 3. All control set to zero for voltage reading.
2 min = +2.5mv
5 min = +2.6mv
10 min = +2.6 to 2.7mv

After it cools down some I will do the same with R96 but I'm confident and hopeful that they will reading the same numbers.

Good way to spend a rainy day in northeast Florida

[sweatyk]

Second testing on R96:
2 min = +1.9mv
5 min = +1.9mv
15 min = +1.9mv

Same as last round.....all on the clean channel

[DrGonz78]

Sure looks stable to me. How does it sound?

[sweatyk]

Not sure what the problem is here on the forum. If I do not log in I can see DrGonz's last post to this thread. If I log in it's not there. Why is that? Anyway to answer his question it sounded pretty good considering it's hooked to my bench test speaker which is an 8 inch peavey blue marvel. Once I'm confident that it is ready I will install it back in the combo with it's nice Celestion speaker.

Guys is there anything else I need to be doing other than playing this thing? Can we sum up that after I replaced the outputs with the TIP's that it created the secondary issue with the bias voltage? If so that is key information that should be passed on for others that change out the BDV's for these others.

[DrGonz78]

Yeah I think this thread gets it pretty well documented and especially if you report it all in good working condition in say the next few months. I also am confused by the TIP142/147 being the issue in relation to the BDV64/65. I only state this in relation to the other thread that I had linked to about similar problem. In that thread it appears that he had a very similar issue with biasing the amp and had to change one of the bias resistors. In that case, most importantly, he had the original BDV64/65 transistors in place and not the TIP142/147.
http://music-electronics-forum.com/t5913/

Not sure why when you log in you can't see my posts, but I will confirm that this did happen to you one other time. Strange.

[g1]

Sometimes the new posts don't seem to update in real time. I occasionally only see half of a new post, then if I look a few minutes later, there is more of the same post.

[J M Fahey]

Ok, it seems to be working very well.
I donīt think thereīs a "TIP vs BDV" problem at all.
Simply that different batches of transistors (or tubes or anything else) do have some spread in their parameters, so all amps *should* have a bias trimmer.
Not always the case.