Last Of The Paramour (one) Mods

[Grainger49]

I'm going to make some changes to my Paramours.  They are one of the last four pair Doc put on sale just before the Paramour II came out.  So mine are very late production.

Today I got a little aggravated at myself because I hadn't adjusted the hum pot since changing tubes.  The belly of the beast is filled with stuff I added making it dangerous to adjust anything.  Today I put the hum pot shafts through the top plate.  I wish I had heard of this before assembly, there is the perfect place on the top plate in front of the power transformer and back from the input jack.

I'm happy with this, I got one amp to 1mV hum, the other to 1.8mV.  The sub woofer's plate amp has some physical vibration that hums louder than the system now.

More later, less sooner.

[Paul Joppa]

Excellent! And now you know why the hum pot is on top of the chassis in the Paramour II. (You've probably already figured out why there's an IEC socket...  :^)

Before the Paramour, and before we got into parafeed, I made a 2A3/300B amp based on the original SEX monoblocks. Must have been around 1997, documented in VALVE under the title "S.E.X. and the Single 300B" - Grainger, I'm sure you remember Helen Gurley Brown! It was the same chassis plate that was used for the first Paramour, and that's exactly the same place that I put the hum pot, for exactly the same reason.

It ran a little hot, about 18 watts, which is why we had a custom power transformer made for the Paramour. I had a bunch of cheap Shuguang biplate 2A3s; back then they had a habit of crapping out before they hit 1000 hours irrespective of the plate dissipation, so I thought "smoke 'em if you got 'em" ... that amp served me well for at least a decade.  :^)

[Grainger49]

Ha, ha!  Paula asked, "What?" when she heard me laughing over here.  I remember Sex And The Single Girl, but never read it.  Maybe I should have?  I needed a lot of pointers in those days.  I never got them either.

The lack of an IEC socket hasn't bothered me too much, just some breaking solder joints.  I feed more of the power cord into the base then flip and work on it.  I haven't found an IEC power cord long enough to reach neatly from my right amp to my P300.  If I put on an IEC it will be on the back of the base then I will mount the toroidal balun to the base and run stranded all the way to the power input terminal strip.  The solid wire broke from the balun to the terminal strip on both amps when I worked on them yesterday.

I have a short list of "upgrades" for these and will update the thread as I go.  Soon I will put a picture in of yesterday's work.

[Grainger49]

PJ, elsewhere in a thread started by xcortes, has suggested a shorting switch to short the primary (after the Parafeed cap) of the output transformer.  I think the reasoning is that the startup bleeds some DC through the Parafeed cap and magnetizes the primary of the transformer.  It takes half to an hour to clear it out and the amp to sound its best.

I mistakenly bought a pair of DPDT toggle switches with a spring return to center off from one position.  I can use these!  Thank goodness I'm a pack rat and didn't throw them out.  I will turn on the amp, hold the switch shorting the primary for 30-60 seconds then turn on the other amp.

Edit: I must have discarded the switches or lost them.  I used a SPST switch on each amp.  There was no smoke but I have to turn the switches off.  I won't forget because I'm going to be kneeling in front of the amps to turn them on anyway.

[Grainger49]

So when I changed the drivers tubes to 76s I got help from PJ with setting them up.  I got the plates to 250V and since he said +/-50V from 200V, the target, I left them there.  Then working with Paully on his next project I found I am running them pretty hot.  Maybe that is why I eat up driver tubes and not output tubes. 

A little calculating and a quick search through my resistor collection, I paralleled a resistor with each cathode resistor.  Now the combination measures 1988 ohms, each, and the voltage is 197V.  Much closer to target!

Next a bypass of the cathode cap or a replacement for it.  (probably a bypass)

[Grainger49]

If I had looked I would have seen that the cathode bypass cap is already bypassed.  One of the olides, like me, had used a 10 turn pot for the hum balance pot in his Paramours.  I got the number from him, I think ElCraigo, and put them in the other day.

I have made several tube changes, not because they were worn, just because, and I got the hum tweaked 5mV and 7mV lower than I could with the lower resolution stock pots.

This is a great move.  I waited 20 minutes after starting to listen to do the final trim.  Should have rechecked at the end of the listening session but didn't.

I had also wired one of the "primary shorting switches" improperly.  (reply #3 above)  My Paramours are a mess inside and it took 4 or 5 comparisons to see my mistake.  I am now shorting the transformers at startup every time, shut down too just in case. 

I still go through ear warmup but I am certain that the transformers are not molested on startup.

[najo49]

Grainger, Could you walk me thru setting the hum pot on Paramore. I just do it by ear, Is there another way?

[Grainger49]

I use the meter method, others listen.  But you can reach a lower hum if you use the meter.

Short the inputs, I use a pair of alligator clip jumpers, one for each channel.  Attach your meter to the speaker leads.  I put the meter leads through the holes in the 5 way binding posts and tighten the plastic part down on them.  This is a hands free method to read the magnitude of the hum voltage.

Set your meter to AC volts, the lowest range if it has ranges.  Turn on the amplifier and let it warm up for 15 minutes.  If you are just measuring hum, this is what you have.  Posting be sure to include the milli-volts (mV) reading.

Start moving the hum balance pot one direction.  If the voltage goes up change directions.  When you get it to lower find where it begins to climb again while moving in the same direction.  You have just passed the null point.  Now very carefully make minor adjustments to the pot to get the lowest number, milli-volts, you can on the meter.  It is best if you use a 4 digit digital meter.  This gives you the greatest accuracy.

The stock pot is wire wound and the limit of resolution is "one knuckle."  That means one wrap to the next wrap of wire.  They are good pots but don't have the resolution that a 10 turn pot has.  The half way point between knuckles shorts two windings together.   This is why I went to the 10 turn pots for my Paramours in the hum balance circuit. 

[Paul Joppa]

For what it's worth, we now "sandwich" a 10-ohm pot between two 22 ohm resistors, giving 5 times as much resolution.

[najo49]

Thanks for the info  both of you. One question is it all right to play the amp w/o speaker hooked up? I have not done it thinking I would damage things.

[Grainger49]

Yes, I do it whenever I swap the speaker leads for adjusting absolute phase.  These are no feedback amps.  They are fine without the load on the secondary of the output transformer.  My Stereo 70 is NOT!

[Grainger49]

I'm wondering....  Now that I can afford it....

What if I put a film cap on my cathodes.  The stock cap is 220uF, haven't looked at the voltage. 

I'm looking for large value film caps and I have found a 100uF@400V Obbligato film power supply cap.  I am hoping not to need two of these. 

What is the effect of lowering the value of the cathode bypass but increasing the quality?

[adamct]

Grainger,

Consider using a couple of the Icar caps I used in the power supply of my Crack. Check the links at the bottom of the (very long) first post in this thread. At only $10 each, it is a much more economical solution, and I have been extremely happy with how well they worked in my Crack. They are available in both 200uf and 250uf (not to mention 300uf) versions.

Regards,
Adam

[Alonzo]

Hi Grainger,
I've put 100's (film)on the 45 amp and on a 2A3 amp I have.  I went as low as some 51Uf Axom caps.  I gave it a 2 week listening period and changed back to the 220.  Seemed to lose some of the bass definition, some of the classical music I listen to wasn't as clear or as deep (these were the only things changed in the systems).  Now I'm on 350 JJ's electrolytics, these are just in the break in period but it doesn't sound worse.  I went away from film due to size considerations and I think a film would be wasted here (maybe a bypass, I've got some 3.3 Solen's I can put in).  Is it critical for a film here?  I was thinking of going up to around 470uf to see how it sounds.  Please post the results of you Obbligato experiment.
Alonzo

[Grainger49]

Adam,

I'm going to have to consider the ICAR caps.  They will have to stick out the back of my bases like a... I can't come up with an analogy.   

Did you link a source for them?

Alonzo,

You have convinced me that the value is critical.  The above linked ICAR might be just the thing for both of us!