YAOB (yet another one-zero-two build)

[aflynt]

I brought the bias down to 38ma. Any lower and the amp starts sounding too dry for my tastes. This brought the overall B+ voltage up to the 430 range so I think I'll stick with this PT for now. I also got the FET ironed out. The 2k worked perfectly bringing the voltages to around 22/11 on either side of the 10k.

I'm very happy with the sound of the amp. I'm finding that I kind of like the mid boost off which surprises me a bit since I usually like lots of mids. The natural sound of the guitar seems to come through better with it off.

About the only thing the amp doesn't do which I wish it did is feedback more easily. I still have to locate a matched triode 12AX7, though. With the one that's in there I'm only getting about a 5 volt difference on the plates of the PI with the trimmer maxed.

Eventually I'd like to try to implement some sort of brightness reduction circuitry on the switching jack of the effects loop so that with the loop cables unplugged it gets darker. I might put an LNFB switch on the back too since there is an empty hole there.

-Aaron

[B]

Eventually I'd like to try to implement some sort of brightness reduction circuitry on the switching jack of the effects loop so that with the loop cables unplugged it gets darker.

-Aaron

This sounds like a really good idea. I'd be curious to see how you implement it. Mostly so I can steal it for my own build.

[ampdork]

Interesting.. I just peeped the layout Tony did and I must have mis communicated something to him because on the Bludo I serviced there was clearly high end reduction circuitry around the loop jacks such that with no loop I guess it would simulating it... definitely knocking the highs way down....
IIRC correctly it was a 220K between the jacks instead of bare wire... a 500pF (may have been 250pF.. I'll have to check) cap to ground on each jacks switching tab... so with nothing plugged in at all you would have a pF cap to ground, 220K series resistance and then another pF cap to ground on the second jack... LOTS of high end getting tweaked with that.

When jacked up to a loop or patch cabled none of that would be "seen".

I think this was brandons addition to the circuitry and not what is in Ford's amp but seems that's pretty much what you want to do...

cheers

[aflynt]

Interesting.. I just peeped the layout Tony did and I must have mis communicated something to him because on the Bludo I serviced there was clearly high end reduction circuitry around the loop jacks such that with no loop I guess it would simulating it... definitely knocking the highs way down....
IIRC correctly it was a 220K between the jacks instead of bare wire... a 500pF (may have been 250pF.. I'll have to check) cap to ground on each jacks switching tab... so with nothing plugged in at all you would have a pF cap to ground, 220K series resistance and then another pF cap to ground on the second jack... LOTS of high end getting tweaked with that.

When jacked up to a loop or patch cabled none of that would be "seen".

I think this was brandons addition to the circuitry and not what is in Ford's amp but seems that's pretty much what you want to do...

cheers

So something like this, then?

-Aaron

[ampdork]

exactly.:O)

[aflynt]

exactly.:O)

Thank you very much! I'll give this a shot once I acquire the parts.

-Aaron

[dimitris]

Aaron the way you have wired it the 220k resistor plus the left cap will be always in circuit even when the loop is used.I think that the right side of the resistor should be connected to the middle lug of the right jack.

[Structo]

So it has two switching jacks?

Interesting about the extra cap.

I would love to one day see some gut shots of Fords Dumble.

[aflynt]

Aaron the way you have wired it the 220k resistor plus the left cap will be always in circuit even when the loop is used.I think that the right side of the resistor should be connected to the middle lug of the right jack.

Thanks! I think you may be right. I fixed the diagram and uploaded a new version.

-Aaron

[ampdork]

my bad...I haven't used a switchcraft in forever. ;O)

[aflynt]

I worked on the visual appearance a bit. I printed up a little logo sticker and cut a tolexed panel off an old amp to cover the transformers up. I think it looks decent enough for now until I get a proper head-shell. I think I probably need a bit more ventilation on the front panel, though. The thing gets hotter than heck with it on. It's attached via velcro, and I wound up removing it at the last job to get some airflow. I also think I might have to actually replace the PT for real this time.

[img768]http://www.aaronflynt.com/wp-content/up ... 24x768.jpg[/img]

-Aaron

[aflynt]

I put the Showman PT in to replace the low output one from the Quad Reverb. The good news is that the B+ is now 470VDC unloaded and 450VDC loaded. The bad news is that it's acquired a bit of a nasty hum when cranked up.

I'm going to try mounting it flush with the chassis on the bottom (it's got 4 nuts spacing it away from the bottom right now). If that doesn't work I'll try spinning it around 180 degrees and twisting the heater wires.

I wound up having to grind a bit of the bell down to fit it on inside the chassis (it was hitting one of the PS board standoffs). Underneath the bell the transformer was covered in this nasty sticky black tar-like substance with cardboard embedded in it. I had to remove a buch of the tar (and with it the cardboard) to get it to fit in the chassis. Hopefully I didn't mess it up.

-Aaron

[Structo]

I know you probably did but did you check the bias after installing the new PT?

If one side is way off from the other it can create hum.

Or the new PT is not shielded as well.

Check your AC lead dress (heaters, primary lines)

Also check your standby switch wire routing.

[aflynt]

I know you probably did but did you check the bias after installing the new PT?

If one side is way off from the other it can create hum.

Or the new PT is not shielded as well.

Check your AC lead dress (heaters, primary lines)

Also check your standby switch wire routing.

Thanks for the ideas!

I did re-bias it. Definitely needed it. I also finally increased the bias range by putting a 47k on the pot. It definitely needed that too. Other than the hum, it actually sounds pretty decent biased colder than I had it before.

One tube is reading about 1 or 2 ma lower than the other 3. Not sure if that's causing the issue, though as it was the same with the old, non-humming, PT.

I think it will extend the primary lines. The leads were a bit shorter on this PT and they are routed a bit less neatly than before. I also forgot to twist the PT filament wires before I soldered them to the first Power Tube.

-Aaron

[Structo]

Also, does this PT have a center tap on the filament winding?

If so and you have it grounded, I would lift that and try a couple 100R 1W resistors.
One from each side of the heater circuit to ground for an artificial center tap.

Sometimes the wired center tap is not real close to the middle and causes an off balance circuit which hums.

Or install a humdinger pot to have an adjustable hum balance.

I wouldn't worry about a 2-3ma difference in bias, it's when it gets more than about 10ma that you start hearing things.

You can also try swapping the power tubes around in different sockets to see if that balances them any better.
Usually taking a tube from one side to the other from push to pull is the way to go there.

Try a bit lower on the bias as well, these amp generally like a colder bias.