Fired Up!

[Billy Batz]

Hey thats how Weber does it. Thats where I got the idea from. Some times you can go too far from a sonic aspect. I notice that more with EL34s. They get a real tin-like quality to them when you go too far. And the drive startes to stutter a bit like a fuzz in a way. Hard to describe. Like that choked sound with fuzzes with the batteries running out? Its like just a bit of that mixed in.

[shakti]

Well, my own JTM45/100 is finally up and running, ending a many years-long quest for *that* tone. Basically, the moment I heard "Spoonful" from Fresh Cream some 17 years ago, I knew I had to have one eventually...

Wow. I am gobsmacked with tone. Even though I know (and hear) that the amp needs some serious break-in (I only have about an hour on it yet), the sustain, punch and clarity were almost revelatory! I could hardly stop the amp from sustaining, even with the guitar volume backed off. That was one of the coolest things - reducing the volume resulted in a cleaner sound, but almost all of the sustain was there still!

I used a *erren OT and Marstran PT which has a dual voltage option - 490 and 560V. I currently have it set for 490V, but with the wall voltage down a few % I was getting 475V on the plates of a quad of GECs. I admit it - I'm a tube snob, and just knew I would never be satisfied unless I tried GECs. But the dual voltage option allows me to safely use Shuguangs as well. Can't wait to try those GECs at 560V and with hotter bias - I biased them rather cold for burn-in.

A few minor questions, though:

- I'm getting some rather serious ghosting. I'd say it's comparable to my JMI Vox AC30. I have F&T axial 30uFs and a BC 16uF in the preamp - I was going for Clapton/Fresh Cream specs, so I decided to go with low filtering. Now I kind of wish I'd gone with the BC 33uF ones...or would they have made any difference? Anyone else have this problem? Does it get better with burn-in? I didn't form them, but assumed that new, off-the-shelf F&Ts would be OK. Also, I don't have the snubber cap on there yet. Does this affect the way the filtering works?

- I'm also getting a very faint hum with the amp on, all controls on 0 and no instrument connected. It does not increase significantly when turning up the controls, so it's not necessarily something I need to worry about, but neither my JTM45 or JMP50 have any hum whatsoever, so I do want to track it down if possible. I guess I should upload a photo when I can get one to let you all see if you have any suggestions on improvements of the grounding scheme, but in short, I have two major grounding points; the one by the input which gets the input grounds and the first one or two from the board, and on V7, which gets the mains ground, center tap, filter board and pot ground bus (including the rest of the ground wires from the preamp board).
Oh, and I guess I should mention that I have the PT rotated 180 degrees compared to what most people have. I decided on that since I thought it allowed for the cleanest wire routing. The HT side is on the power and standby switch side. Then the 120/220/240 wires run from the rear side straight over to the voltage selector. The heater wires also run from the rear side a very short distance to V7. I was curious about this - is it better to keep heater wires as short as possible to avoid interference with other wires, or is it better to keep them long and twist them a lot to get the cancellation effect? I twisted them all tightly, but I also tried to keep them as short as possible.

Lastly, MAJOR thanks have to go out to George for making this possible, to Billy Batz and gutpile for the instructions and layout diagrams, and to everyone else here on the Metroamp forum for their advice and enthusiasm.

Cheers!!

[Flames1950]

- I'm getting some rather serious ghosting. I'd say it's comparable to my JMI Vox AC30. I have F&T axial 30uFs and a BC 16uF in the preamp - I was going for Clapton/Fresh Cream specs, so I decided to go with low filtering. Now I kind of wish I'd gone with the BC 33uF ones...or would they have made any difference? Anyone else have this problem? Does it get better with burn-in? I didn't form them, but assumed that new, off-the-shelf F&Ts would be OK. Also, I don't have the snubber cap on there yet. Does this affect the way the filtering works?

I'm doubtful that the 33uF BC's would matter much. Both the 45/100's I built with BC's ghosted pretty good. And honestly, it's the nature of the beast -- it's a 100watt amp with only 30-32uF filtering on the mains and less on the screens. Even Marshalls with 50uF in each of those spots ghost (Basses worse than Leads perhaps.)

That said, improvements could be made by looking at the grounding some more. Sort through Rockstah's Putfile stuff for the grounding diagram that Steve "SDM" helped him with -- it may help, or certainly can't hurt!!

The "snubber" cap is actually for rectifier noise, which often isn't an issue with today's rectifier diodes anyway.

[MacGaden]

Well, my own JTM45/100 is finally up and running, ending a many years-long quest for *that* tone. Basically, the moment I heard "Spoonful" from Fresh Cream some 17 years ago, I knew I had to have one eventually...

Wow. I am gobsmacked with tone. Even though I know (and hear) that the amp needs some serious break-in (I only have about an hour on it yet), the sustain, punch and clarity were almost revelatory! I could hardly stop the amp from sustaining, even with the guitar volume backed off. That was one of the coolest things - reducing the volume resulted in a cleaner sound, but almost all of the sustain was there still!

I had an original

[gutpile]

Congrats Shakti & MacGaden... I am STILL blown away at the tone I get from this amp !!! This quickly became #1 in my arsenal!!! I only wish I could get clips to do the tone justice!! BB's tone on his clips are amazing...

Mike

[Billy Batz]

Finally got some unattenuated playing in for a while at a friends house. It really is an amazing sounding amp.

[shakti]

I finally got around to looking at the grounding again. The problem was easily solved by moving the wire from the pot ground buss to the grounding point by the inputs. That grounding point now gets the grounds from the two inputs, the first ground wire from the board, and the ground wire which I have coming off of the ground buss. I attached that to the ground buss by the volume 2 pot.

The amp is now very quiet indeed!! With all controls at zero there is no ground hum. There does seem to be a tiny bit of hum when I crank the bass above 3, but I hardly ever do that anyway... But I came to think of one thing; this is the first build I've ever done with PEC pots. Since they have quite long shafts, I had to add washers on the inside so the shafts wouldn't protrude so far out on the outside. However, that means that the little tap on the pot casing doesn't touch the chassis, like it would normally do and which it does on all my other builds. Is this a problem? Normally, the ground buss would touch the casing of all the pots, and the pot casing would touch the chassis via that little tap I mentioned. Now the buss doesn't touch the casing on all the pots, and the tap doesn't touch the chassis. Just trying to determine if this might be a problem?

The amp actually seemed to ghost a tiny bit less after this change, but it might be my imagination. I have got a set of Tube Amp Doctor 33uF ones that I want to try to see if that improves things.

[Billy Batz]

Normally youd bend or break that tab so its out of the way. I mean its purpose is to be mounted into a pot hole with a second smaller hole to the side of the main shaft hole, the purpose being that since the tab goes in that smaller hole the pot wont rotate when you tighten it or if it loosens a bit. It will always stay in position.

[shakti]

I see. So it shouldn't make a difference whether it touches the chassis or not, then?

What about the other difference between using the ground buss you did in your instructions, vs. the "regular" one where it gets soldered to the back of all the pots? That would mean that the pot casing is grounded on all of them, which I guess is not the case with your method?

I did notice that the mid control on my amp seems to do very little. I would even be hard pressed to tell whether it does anything at all... I doubt that I could have overheated it, I got that ground buss and the leads soldered onto the pots very quickly and neatly. If I did overheat them, then the PECs flat out suck if that's all it takes to ruin them... Are you other guys getting a big difference between 0 and 10 on the mid pot? On all my other builds the mid pot does a *lot* more than on this one...

[shakti]

Update.

I went ahead and installed the 0.05 snubber cap. After I had sorted out the grounding issues the ghosting actually seemed to diminish. Or maybe I just got used to it? I'm pretty sure it lessened.

Anyway, curiosity got the better of me, so I swapped the 30uF F&Ts for T.A.D. 33uF ones. I can't say with 100% certainty that the ghosting was further reduced, but it's definitely acceptable now. I also switched to the 560V taps on my Marstran PT. I still don't have it properly broken in, but it's a beautiful sounding amp! Strat with G12Hs was just fantastic! I suspect I will prefer the sound with G12Ms with Les Paul, however.

One issue remains: the middle pot seems to do much, much less than on my other Marshall builds. I used a PEC 25k pot, I went over the wiring and that seems to be right. When I measured the pot between the two lugs (not ground), at 10 I got 0 as expected, but I could only get it down to around 12k with the pot at 0. Is that because I didn't measure it isolated from the circuit? Are there any other possible reasons for this? Could a bad 0.022uF cap cause this? I reflowed the connections and changed V2, but no change.

BTW, I tried to form the caps "Larry"-style; I put two 47k/1W resistors in series to simulate 100k/2w, and inserted them between the rectifier cathode and the first filter cap. Even after 1 hour there seemed to be no change in voltage; I got 390V on the rectifier side of the resistors, and 133V on the other side. If I got this right, the voltage drop was supposed to slowly approach zero, right? Sorry, but I didn't have the patience...these caps were brand new, so should be formed already. But just for reference, did I do it right? I did actually leave preamp tubes in so I wouldn't exceed the voltage rating of the preamp cap.

[shakti]

Did some more troubleshooting and measurements today, comparing it to my JTM45. The mid 25k PEC pot is definitely faulty.. Wide open it measures 0, of course, but turn it all the way down and it reads 9k one time, 11k the next time, 7k after that... So there's definitely something wrong with it. I didn't measure it before putting it in, so for all I know it may have been troublesome from the start, but as I said, if that's all it takes to ruin it, I'm swearing off of PECs for all eternity!

The important question is: how paramount is it to change the pot? I run mids at 10 virtually all the time, so does it make a difference then?

[thousandshirts]

if that's all it takes to ruin it, I'm swearing off of PECs for all eternity!

Did you solder to the back (casing) of the pot? If so -- as Larry says -- the fault is sitting in front of the amp!

[shakti]

No, I was well aware of the problem with soldering to the back of the pot, so I did the buss the way BillyBatz shows it in the instructions here. Of course, as I had to to solder other leads to the ground buss as I went on, the buss would have been heated, but I find it hard to accept that just that little bit would ruin the pot. I went in and out pretty quick. All the other pots are fine. So I guess it *could* have been faulty from the start, but we'll never know.

The important question is; do I absolutely have to change it? I run it wide open 99% of the time anyway. Changing just one pot is a major hassle because of the way the grounding is done.

[thousandshirts]

No, I was well aware of the problem with soldering to the back of the pot, so I did the buss the way BillyBatz shows it in the instructions here. Of course, as I had to to solder other leads to the ground buss as I went on, the buss would have been heated, but I find it hard to accept that just that little bit would ruin the pot. I went in and out pretty quick. All the other pots are fine. So I guess it *could* have been faulty from the start, but we'll never know.

I see. I measure all my pots before installation -- I must say the PEC's are the most accurate I've seen. Did you measure yours beforehand? Not that you should really have to. But I suppose you could have returned it. And if you've just soldered to the lugs as described, with everything else properly connected in the amp, really, you should still attempt to return this pot, I think, for a replacement. It sounds more and more like the pot was the problem from the get go ( ).

The important question is; do I absolutely have to change it? I run it wide open 99% of the time anyway. Changing just one pot is a major hassle because of the way the grounding is done.

No, you don't have to change it if you run it wide open all the time. But you could snip the buss bar right at that mid pot's lug, and simply add a small section of buss bar ( a splice if you will ) when you install the properly working one. I too run my mids pretty high up, usually at 10, and otherwise just using it as a kind of 'cut' control. I know what you mean about the replacement job, what a pain in the ass. But it would bother me more to know that I had a poorly working mid pot in the amp than it would bother me to see a little splice on my buss bar whenever I opened up the amp. If it was me I'd have to replace it.

[HEAVIER THAN HELL]

That clip BB posted was great, I'm excited to know the 45/100 is gonna give me the tone I've had in my head all these years.

I think the Crossroads tone would be much closer using the bass channel though with the treble and presence up around 8+