As I was preparing to do some tube swapping/testing I disconnected my D-lator from the loop.
Other times I've done this, the sound would fade in and out for a while, then stay on.
This time, instead of fading in/out, the tone went from thin/trebly to normal, then settled on normal.
When I plug a small patch cord through the loop it sounds normal.

So I have a weird problem with those jacks, its seems. The change from in to out or trebly to normal is not instant, it fades over a second or two.

I'll experiment tomorrow with the tubes I have and note the measurements.
It might be that I have 2 bad tubes in there. The one from each brand that was drawing a lot of current.
When I put the pair that's in there now, I had to turn the bias down quite a bit.

It seems no matter what, V4 gets noticeably hotter than V5.
I just tapped the tubes and got some noise, so I'm suspecting a cold solder joint somewhere.
Or it might have been because the tube is bad.
Got to get some zzzz's now.
Thanks for your help.

Jon

I don't think it sounds any better.  Both a matched pair of JJ's and a mached pair of Mesa's were way out of wack. I'm pretty sure the Mesa's have few hours on them.

FWIW, here's a couple of samples of how it sounded in the first set and the second set.

http://www.afgm.ca/JesusJustLeftChicago_Clean.mp3

http://www.afgm.ca/LittleWing_Distorted.mp3

Is the voltage on pin 5 dependent on the tube that's in the socket ?
Right now the bias is reading about 41 in V4 and 35 in V5.
I would get different readings if I swapped those tube positions.

So if the bias is not equal, should pin 5 still be equal ? Or within what tolerance ?

Thanks for the (quick) response.   Yes, I think I can manage that.  According to the chart I have, pin 5 should measure -42 V and pin 4 should measure 441 V, is that correct ?

I'll open her up and see what gives.  Seems strange that a supposed matched pair are so grossly unbalanced.  I just found 2 tubes that got a closer balanced reading - but I'm still getting some distortion on the clean channel - though its subtle - not really noticeable at bedroom level, but noticeable at anything higher.
THe OD channel is either unaffected, or the added distortion is too subtle to detect on the OD channel.

In the middle of a gig, I suddenly noticed my clean wasn't clean anymore.
It wasn't really dirty,  just not as clean as it was before. 
Swapped power tubes and V1, to no avail.
When I got home, I put back the old (JJ) power tubes and measured the bias.
One tube looked normal at about 36mV, but the other tube measured around 70 !
I know the bias was fairly well balanced when I installed the tubes.
I tried the other pair (Mesa) of power tubes again, and found the same imbalance.
I ended up using 1 Mesa and 1 JJ and the bias for 1 is 35 and the other reads 41.
Both sets (JJ & Mesa) were bought as matched pairs, and as I said, when first installed biased correctly.

I built the amp, but I'm not a tech - just a monkey that can follow instructions.
Any ideas or advice ?

Great advice. Thanks Hyweig.

I've played a few gigs with just this one mod and the amp sounds better all around.
No flabby bass.  The OD is still a bit hard or stiff as it always was at gig volumes, but its not bad and certainly no worse than before the mod.

Played the last gig with Deep Off the whole time - never felt the need to switch it on.

I'd like to try some of the other mods - like the slope resistor.  Hywelg gave me some info on what parts to get, but when I went to Mouser to find it - there was a bewildering array of similar parts, so i had no clue what to order.

Does anyone here have any extra parts they'd like to sell?  That way I could be sure I'm getting the right items.

PickMaster, THANK YOU.

Just got home from my first gig with the V1-NFB mod.
I'm very, very pleased with what its done to the  amp.
In short - my Strat grew a new set of bigger balls.
Another bonus was that the level differences - Deep on/off, PAB on/off were less drastic.

I thought I'd leave the deep on, but found myself switching back and forth for different songs.
I've done that on some other gigs, but now it just seems to work a lot better.
So now I have twice as many tones available as I would if I left the Deep switch in one position all the time.

What do you other OTS guys do with your Deep switch at gigs?

Hywelg,
Thanks for the info.
I'll try to source those at my local electronics shop.
Otherwise, I'll have to order from Mouser or similar, in which case I'll have to figure out what else to order.
I don't really want to place an order and pay shipping for 1 resistor!!!!

PickMaster,
Thanks for the speaker tip. I'm pretty happy with my pair of G12H30's, but still willing to experiment.

I performed the V1 NFB mod, and so far, I'm liking it a lot.
Ever since I got the amp, I couldn't decide if 'my' tone was with the Deep switch on or off.
I like the ballsyness when its on, but I always wished the boost had a lower center frequency.
Now, with the NFB mod, it makes the Deep setting sound less honkey and more useable.
Its just like you said - fatter, a bit more compressed, a bit more gain.  Its really making my Strat sing better.
Can't wait to try it at a gig.
Thank you.

I'm probably going to try the slope resistor mod next.  I've never seen those plain brown resistors before.
What are they called? and what wattage?  Can I substitute something I have laying around - like a regular 1/4 watt color coded type.

Tone is in the ear of the beholder.
D amps may be based on Fender designs, but they are NOT Fenders.
If you want blackface tones - stick with your blackface amps.

But why do you feel the need to insult owners of D style amps who are very happy with them?

Muchos Gracias, Dr.
I'll report my results/opinion in the next while.
My next gig isn't until next week, and I can only REALLY tell what's what after a gig or 2.
Like the EVM's - sounded good at bedroom levels, but got somewhat harsh when pushed a bit.
I had 2 of them - one was quite a bit darker sounding than the other.  At home, I thought the darker one had more mojo, but on the gig, I hated it.  The brighter one sounded more 'right', but in the end I didn't like the tone of either of them enough to put up with the weight.  (I'm getting old!).

Love your playing and tone, BTW.

Thank you Dr.  Yes, I'll put a switch.  Just wasn't sure if any signal should still flow through that path.
Your mod calls for a 3-way.  So I assume the tone with the other (470pf) cap is sort of half-way between the .05uf cap and no connection.  Is that right?

hywelg, thanks for the advice.
I'm going to tackle the mods one at a time so I can gauge the effect, and I want to start with the easiest ones.

So my question at the moment is: Do I replace the NFB cap with wire, or just remove it from the circuit?
Thanks