Dear Forum,
does anybody have a sixty-five amp that has been converted to the newer style power transformer but with the original PI tube intact? The conversion, documented in an MM service bulletin, includes swapping out the power hi-off-lo selector for a DP style switch and adding a 2-resistor voltage divider to switch the bias voltage for the power tubes.
I could use some reference measurements and just observations as I have an amp on the bench with weird symptoms.
Thank you in advance.
Sincerely,
Lars Verholt
Sorry brutha'. The only 5's
Sorry brutha'. The only 65's and 130's I'VE worked on had the original transformers and switching. I wondered if the new scheme was designed to be compatible with the euro market. I think all the exports use that system, don't they? Haven't looked. Mike.
Not Export
Hi Mike,
just to put the info out there: No, this has nothing to do with export type amps. You are correct, some export type MM had DP power switch in the back where the regular US version's ground switch is and the power selector switch on the front had no 'off' in the centre. However, that is separate from the modification that MM specified in the service bulletin when replacing an old style PT with hi/lo switch on the primary to the later style PT where hi/lo happens on the secondary.
Cheers,
Lars
No DPDT switch for me
I stopped at step 9 when I changed my transformer. I don't know exactly what the DB-2 modification is so I stopped there.
Following the instructions up to that point got my amp running.
A shorted cap killed my transformer so just getting it working again made me happy. I got my transformer from Mojo, it is the 100-65 version.
I still have distortion in the Low power position but it seems to be OK in Hi power. I am just getting new tubes and am waiting for the 1 ohm resistors so I can see what the current flow as I adjust the Bias. The set to 1/2 volt does not really tell you much. I got the JJ 12ax7 and my matched Ruby tubes are on the way. If the current is much different I guess I will have to change some resistors, that I have not checked, and see if they are ~equal in value.
If you know what the DB2 mod. is let me know. Mike
DB2 mod is changing to solid state PS
Hi Mike,
you would more than likely get distortion (cross-over) in the lo setting without the mod I mentioned in my OP. If you simply switch the plate voltage to the EL-34s but leave the bias the same the amp will be at a different point on its transfer curve and will likely run way too cool. The mod described by MM says to make a voltage divider that cuts the negative bias voltage in half when running the amp at the lo setting. In my particular amp (the only one with this configuration I have ever touched or heard) it sounds great in the lo setting but it is biased quite a bit hotter so the signal is actually louder than in the hi setting at the same volume control setting - but it does distort easily, maybe a bit too much.
Cheers,
Lars Verholt
If solid state PS means the
If solid state PS means the solid state driver board, an I think it does. I was right to stop at step #9. I have the 12ax7 but do not have the SS driver board. The drawing in the service bulletin shows the DB-1 Driver Board, confuses me. This step is why I almost did not change the PT myself, but I kept reading until I convinced myself to do it. If you think i is just to cool a bias try adding a pot to the voltage divider so you can adjust the low power setting all by itself. Might be worth a try.
The PT was bad and the MM now does work in Hi power mode but not Low power.
I am having the distortion problem you describe and very low audio in the low power position. I think all of the voltages look correct in Hi vs Low power position. I just hope it is a flaky 6ca7 causing this, I will know when mine gets here. I do intend to take a real good look at all of the components in the high power section to see if I see anything burned/cracked/whatever.
Hi power Low Power
766VDC 536VDC
380VDC 268VDC
352VDC 248VDC
+44 -44VDC +44.4 -45VDC
+17.84 -17.42VDC +17.89 -17.48VDC
Pin 5 of 6ca7 L -33.43 R -33.43VDC
Thanks,
Mike
With all due respect...
Mike, you are reading the service bulletin wrong. It states 'Only complete the following steps if the amplifier contains a 12AX7A driver tube and has not had a DB-2 modification'. In other words, to correctly replace the power transformer in your amp steps 10 through 12 are also necessary. Your issue in the lo setting is almost certainly due to the bias being way too cool.
Sincerely,
Lars Verholt
PS
I agree that a trimmer to set the lo power bias would be a valuable addition - if nothing else as a temporary measure to find what the actual sweet spot might be.
Cheers,
Lars Verholt
all done
Lars,
I just put in new finals (Matched Ruby EL34BSTR). I also added the 1ohm resisters so I can really see what is going on.
I think the distortion is less at the higher voltage tap. I was running ~-16.4VDC now its ~21.4VDC.
Bias Voltage readings
left right Y Bias tap V
Hi 15mv 17mv .38v 21.44vdc
Lo 21mv 22mv .5v 21.57vdc
Notice the Hi setting are lower than the Lo settings so I set the low to the recommended settings that I have seen on the forum. I am just letting the new tubes burn in for a while and will check the setting again after a while. I intend to close it up as it is unless something changes. I also noticed at the lower bias voltage the difference between the finals was higher. That could have been before I set the highest Y level to .5v because it was high when I started measuring. Let me know what you think, you are much more experienced that I am with these amps and I know yu can read.
Mike
Hmm
Hi Mike,
The known reference point is the 500mV at point Y in the hi setting and with full bias voltage. The tap used in the lo position should yield a slightly higher voltage at point Y because the tubes are on a different part of the transfer curve. The 500mV at point Y on hi, as you know, corresponds to a combined current draw of 50mA for two tubes. If the two tubes are fairly similar (i.e. matched) we should get around 25mA per tube. I think my own amp yields about 750mV at point Y in the lo position (so 37.5mA per tube roughly) with the bias at 50%. Your idea of simply adding a couple of resistors to allow for a selection of bias voltages relative to the 'master' hi voltage is great and I am going to experiment when I have time. I think that the MM logic back then was that if you ran the amp in lo mode, you really wanted the distortion - so they gave it to you. Let the amp cook on the bench with no signal for at least 30mins before setting the reference bias.
Cheers,
Lars Verholt
Readjusted “Y” to .5v on Hi
Readjusted “Y” to .5v on Hi with new RUBY TUBES EL34 BSTR matched set 9/14/2017
Left Right “Y” Bias voltage
Hi 20mv 21mv .500vdc -20.25
Lo 26mv 26mv .610vdc -20.36
Love the looks of these readings. Lo power distortion is minimal as I tried it. Put it all back together. Now to play it and see what I really think. I am not very good at guitar and have a tin ear so I am sure i will be happy with it. I'd love to have another one to play with now, got the bug. Can't read, can't play, tin ear but had fun working on it. Mike
Lars, Thank you for reading
Lars, Thank you for reading the bulletin for me. (I'm not trying to be smart). Too late for that. I thin I just read what I wanted to see. I got a switch today had the resistors, but no pots in my stock. I am sure just putting this in will be better than what I am getting in the Low power mode. I am getting ~34vdc on the grey wire, half of that is ~17 volts is that about what you are getting? I assume you need a little more? 10K & 12K = ~18.8v 10k & 15k = ~20.4v. Think that would be enough? Let me know if you try some experiments. I just reread (dangerous for me) your posting and I am going the wrong direction. I will try to get mine done tomorrow so we can compare notes.
Thanks again for getting me straight. Mike
I'm torn :)
You are right on the path I have been down in my mind: Why not use, say 8.2 and 12k instead of 10/10 - but the amp sounds so darn good with 10/10 in the lo position. My band's FB page has a silly little video with mostly talking and yours truly doing a few notes just testing that the amp sounds ok. That's with the amp in lo, ch2 volume at 5 and master at 8. The video was posted Sep 3 or so (show was on Sep 2). Anyway - that is a pretty good indicator of what my amp sounds like with just a good quality OD pedal and a ditto Stratocaster. I will for sure post figures here when I get around to experimenting some more.
Cheers,
Lars
Did it
Lars,
I got everything put together today and first impression is the amp is working in both Hi and Low power. As I said earlier, I didn't have a pot so I split the difference. I used a 10K ~2K ~2K 10K voltage divider. I am at the mid point and it sounds OK. The middle step is -14.3vdc the next step I have is -16.4vdc. I could not reach the first resister to see what the first step is but I will guess ~ -12vdc. It makes it a little bit messy using 4 resistors but as long as it is together no body will ever know.
I will check it out more later and see if I can get it into distortion as easy as you seem to be able to do.
Thanks for getting me straight. Mike
Low power does distort
Lars,
The Low power mode does seem to distort more than when in Hi power. The bias in Low power on the 6ca7 is almost double what it is in high power .5 vs .9. I moved my DPST jumper to the next resister and the bias is now ~.4 Hi power and ~.5 Low power. This seems to be better but family came home and I could not really test the distortion. I think it is still there but I hope it is less than before. I will let you know. Is this sort of what you have been seeing?
Mike