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50W Valve Head build


JapanAxe

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Completed the 6.3V AC wiring for the heaters and the pilot lamp, and continuity is good. I've also installed the bulb and fuses and fitted the knobs - as the circuit is based on a JTM45 I went for a 'stealth Marshall' look.

 

I've made myself a check-list ready to commission the amp tomorrow morning!

 

QyZWHqj.jpg

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It lives! I shouldn't be so surprised, this isn't my first rodeo...

 

I ran through all my checks this morning. It turns out B+ barely gets above 500V in standby so maybe I could have got away with the single 100uF/500V cap. The two 100uF/450V are safer though and there is no lack of smoothing in the power supply. When I first brought the power up with valves in, I got a sudden squealing when the mains (via my Variac) hit about 120V, so I swapped the OT primaries over and that went away.

 

Bias was a bit hotter than I wanted even at minimum setting, so I swapped out a resistor and that brought it into range. Plate dissipation on each KT66 is now about 56% of maximum which is perfect for a grid-biased amp. I initially had loud oscillation when the Master volume was up at maximum (Gain at zero) but that turned out to be caused by the meter probe clipped to the earth bus!

 

I plugged in a bass, and even through a One 10 this is loud, much louder than the B15 clone that I built. There is also a fair amount of overdrive available from the Gain control. There are a couple of outstanding issues:

 

(1) The input socket isn't shunting to earth properly when there is no plug in it. It's a recycled Switchcraft from my parts stash - hopefully some judicios bending will fettle it.

(2) The response of the tone controls is weird. I know the FMV tone stack can be pretty interactive, but this is all over the shop. I'm going to go back to the schematic, the original layout, and probably Chapter 11 of Merlin Blencowe to bottom this one out!

 

I'm going to fit the bottom plate and cover now and try it through my BF Super Twin...

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36 minutes ago, JapanAxe said:

It lives! I shouldn't be so surprised, this isn't my first rodeo...

 

I ran through all my checks this morning. It turns out B+ barely gets above 500V in standby so maybe I could have got away with the single 100uF/500V cap. The two 100uF/450V are safer though and there is no lack of smoothing in the power supply. When I first brought the power up with valves in, I got a sudden squealing when the mains (via my Variac) hit about 120V, so I swapped the OT primaries over and that went away.

 

Bias was a bit hotter than I wanted even at minimum setting, so I swapped out a resistor and that brought it into range. Plate dissipation on each KT66 is now about 56% of maximum which is perfect for a grid-biased amp. I initially had loud oscillation when the Master volume was up at maximum (Gain at zero) but that turned out to be caused by the meter probe clipped to the earth bus!

 

I plugged in a bass, and even through a One 10 this is loud, much louder than the B15 clone that I built. There is also a fair amount of overdrive available from the Gain control. There are a couple of outstanding issues:

 

(1) The input socket isn't shunting to earth properly when there is no plug in it. It's a recycled Switchcraft from my parts stash - hopefully some judicios bending will fettle it.

(2) The response of the tone controls is weird. I know the FMV tone stack can be pretty interactive, but this is all over the shop. I'm going to go back to the schematic, the original layout, and probably Chapter 11 of Merlin Blencowe to bottom this one out!

 

I'm going to fit the bottom plate and cover now and try it through my BF Super Twin...

Very little of that made sense to me - it's all word salad, but still interesting to follow the thread. Does it go up to 11, though?

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33 minutes ago, Crusoe said:

Very little of that made sense to me - it's all word salad, but still interesting to follow the thread. Does it go up to 11, though?

 

No numbers on it at present so I'd have to say no!

 

I've just had a blast with it in my living room. I think I've found the settings where it sounds best (from L to R in pic: Master, Bass, Mid-shift, Treble, Gain) but I may well re-work that tone stack. Unfortunately the oscillation at full volume is back, or it could be the pot wiper coming off the track when full clockwise.

 

Sounds best with my Fender Precisions, doesn't seem to like my Dingwall so much!

 

eDUSGmf.jpg

TrsQ0qy.jpg

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I’ve been giving the tone stack some thought. I really like the James tone circuit on the B15 and I’m now toying with the idea of fitting one here, with the addition of a shift control. It would be quite fiddly as there are a lot of components to mount to the pots and I don’t have space for a dedicated tone stack board. Watch this space!

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9 hours ago, SubsonicSimpleton said:

A friend of mine built a homebrew amp based on one of the champ circuits (he plays bass and DB no skinny string), and tried a few things for the tone stack eventually settling on a baxandall circuit which works great - I spent some time twiddling and couldn't get a bad sound out of it, might be worth considering.

Yep, the Baxandall is very close to the James.

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  • 2 weeks later...

Got to grips with this today:

 

(1) Fettled the input jack so that it shunts to earth when no jack is inserted.

(2) Replaced the master volume pot. The sudden loud buzz at full volume is gone - let's hope it stays that way!

(3) Replaced the tone stack with an enhanced version of a James tone stack (see pic below). The enhancement takes the form of a pot replacing the fixed resistor between the treble and bass pot wipers. At its centre setting it is equal to the Ampeg value, and adjusting the pot either way shifts the frequency centres of the controls. I may never use this, but the alternative would be a blanking plug!

 

The amp seems to be working great now. I will have a living room session with it through my Super Twin tomorrow. For now, time to print some panel labels...

 

dkn0FxO.jpg

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  • 3 months later...

This project has been parked for a while. I had last week off and finally got round to measuring the output power - bang on 25W! Hmm... not quite the 50W amp I was hoping for (and yes I know about watts and decibels). I woke up in the middle of the night and thought of some things to check.

 

Thinking about it, the Modulus OT is rated at 30W so I shouldn't be surprised. I'm still puzzled though - how does the Ampeg PF-50T get 50W out of its much smaller transformers?

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