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I've tried two tanks, a Mox short-tank 3-spring long decay, and an OC Electronics Folded Line. Both sound fine, but I still have to turn up the level control further than I would on a Fender amp. The amount of reverb available certainly doesn't reach surf levels! I wonder whether the driver could be made to push the tank harder, for more 'dwell' I suppose.
Here's what the reverb driver circuit looks like now:
Try changing the orientation of the reverb tank, it's most likely just hum pickup from the return transducer. Make sure the Output end is at the opposite end of the amp from the power transformer first...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Great, I'll try some lower values of Carvin R24.
See my update post - it was my Variac! Roger that about tank orientation though.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
At the moment I am finding that whatever I do to the Carvin cathode resistor R24, I always end up with about 0.5W dissipation from the two 12AT7 triodes - reducing R24 increases the quiescent current, but also causes a bigger voltage drop across R25, and these effects cancel out.
I realise the circuit has its limitations but I just want to get the most out of it without cooking anything. There is enough reverb available for regular playing - if I want to go all Dick Dale, i will break out the Princeton reverb!
I may try a higher value of C26 may allow more lo frequencies through so I may give that a go.
Interesting, I would have expected more power with the lower R24 values.
You may need an oscilloscope to find the optimum value, in that case - you're looking for the greatest signal swing between cutoff and clipping across R25.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I have had R24 down to 295R (combination of existing and piggy-backed resistors) and that gave me:
B+2 331V
12AT7 plates commoned at 125.8V
12AT7 cathodes commoned at 1.25V
By my calculation this gives a combined quiescent current of 4.14mA and dissipation of 0.515W.
Unfortunately I don't have an oscilloscope, except for a DIY thing off eBay that was a complete waste of £17!
In the scribbles below, (1) is the original Carvin design using half of a 12AX7. Plate voltage is spec'd as 210V, which would make the plate current about 2.7mA and dissipation just over 0.5W.
(2) shows a second triode added in parallel, with resistors.
(3) is electronically equivalent to (2), so if I had just used both halves of a 12AX7, it seems to me that R24 and R25 should have halved in value to keep both triodes properly biased.
Obviously a 12AT7 has different characteristics, but I feel like I'm heading in the right direction here. It's a bit late to get the soldering iron out now, so in the meantime I have hot-glued any flapping leads, and have dug out my eyelet punch with a view to making a leatherette bag for the reverb tank.
I’ve been busy with other stuff the rest of the day but this evening I hope to make and install the reverb tank bag. Then I’ll call it finished.
Pics tomorrow!
Here's the finished article. At low volumes there is enough hum that it bugs me, but once you crank it, all such considerations evaporate. I'm l;ooking forward to trying this at an open mic next week, as long as that doesn't turn out to be illegal.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Thanks again
EDIT: see post below!
So I'll be trying the circuit with a paralleled 12AX7 as the driver, or maybe one of the 5963's that I've got lurking in the bottom of my valves box...
I was moving the MOD reverb tank around to see whether I could find a less hum-prone position for it, but I couldn't. I already had it oriented with the output transducer further from the PT.
I tried an old Folded Line tank instead, and bingo, no hum!
How are you finding the Rocket style amp ?
I have been revisiting my Express as I wanted to build a new cab for it and I have tamed the oscillation I was getting after reading a hell of a lot of the old posts across at Ampgarage.
I am thinking of building a Rocket (or even a VVR Express) this winter and wondered how you like it, and what kind of character it has?
The Express is a bloody loud amp - even with a pair of 6V6s in it ( at 400v plates ) It can go from sparkly clean to roaring overdrive from the guitar volume, but its not something I would dare take to one of my usual club/pub gigs tbh !!!! I'm looking for somewhere here in North Yorkshire where I can crank my amps to properly road test them.
cheers
PeteC
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I might see if anyone on the board has a rehearsal space in N Yorks that would do.