More Amp Construction

The aluminum for those is 1/8" thick and I use Keystone 4334 brackets to assemble. This requires a custom design and quite some deviations from the stock build, so some experience is recommended if you're going to attempt this.
 
The aluminum for those is 1/8" thick and I use Keystone 4334 brackets to assemble. This requires a custom design and quite some deviations from the stock build, so some experience is recommended if you're going to attempt this.
I have used a similar keystone bracket (that’s threaded on one side) in my bottlehead builds to attach the top plate to the wood base.
I have “some” experience but I suppose that’s relative. I’m not so interested in the “tricky” things like the meters and the digital readout/volume control.. I’m more interested in the “form factor.” I’ve been wanting a 300b amp for a while, and am seriously considering 3 different kit options. I’ll probably PM you soon with some more specific questions.
 
I finished off a little pair of 300B amps last week and have had them running over the weekend.
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These feature a Magnequest TFA-2004 parallel feed output transformer, a Bottlehead Kiaju plate choke, phallic capacitors, and a healthy amount of ventilation. I used a 12AT7WA driver tube with one half as a shunt regulator. The front panel holds a volt meter to measure B+ and a current meter to keep an eye on 300B operation. DC heating for the 300B is provided by a simple LC filter. The power supply is a voltage doubler with a CLC filter having a polypropylene cap as the final C.
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Inside you can see the gigantic cathode bypass cap and large Jantzen parlalle feed coupling cap hanging out next to it. A vintage cathode bias resistor is spaced out from these caps a bit but takes full advantage of the large ventilated panel up top, and this cutout is duplicated on the bottom panel as well. There's a little V-cap copper coupling cap all the way up top as well. I ended up mounting the two power supplies on an inverted piece of aluminum to conserve on space since the future owner wanted a relatively small footprint (these are 10x14). Glass pieces glued into the side panels will let people peek inside without taking the bottom panel off.
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The buyer for these did a nice job of finding binding posts that match the powder coating (wet charcoal), and overall these are a really clean little pair of amps. Thankfully I have parts for a second pair :)
 
Always dig seeing your work @paul_b . Couple of questions. What is the circuit board for as well as the pot on the top? What is under the big aluminum plate? Thanks. Love these construction posts!
 
The circuit board is a CCS that feeds a hybrid shunt regulator that uses one half of the driver tube, then the other half is another CCS loading the other half of the driver tube to drive the 300B.

On the other side of the aluminum panel there's a bridge rectifier for the 5V filament supply, then also a voltage doubler and a pair of 220uF/350V caps for the HV rail.
 
I finished off a little pair of 300B amps last week and have had them running over the weekend.
View attachment 78759
These feature a Magnequest TFA-2004 parallel feed output transformer, a Bottlehead Kiaju plate choke, phallic capacitors, and a healthy amount of ventilation. I used a 12AT7WA driver tube with one half as a shunt regulator. The front panel holds a volt meter to measure B+ and a current meter to keep an eye on 300B operation. DC heating for the 300B is provided by a simple LC filter. The power supply is a voltage doubler with a CLC filter having a polypropylene cap as the final C.
View attachment 78757
Inside you can see the gigantic cathode bypass cap and large Jantzen parlalle feed coupling cap hanging out next to it. A vintage cathode bias resistor is spaced out from these caps a bit but takes full advantage of the large ventilated panel up top, and this cutout is duplicated on the bottom panel as well. There's a little V-cap copper coupling cap all the way up top as well. I ended up mounting the two power supplies on an inverted piece of aluminum to conserve on space since the future owner wanted a relatively small footprint (these are 10x14). Glass pieces glued into the side panels will let people peek inside without taking the bottom panel off.
View attachment 78758
The buyer for these did a nice job of finding binding posts that match the powder coating (wet charcoal), and overall these are a really clean little pair of amps. Thankfully I have parts for a second pair :)
Just, "wow!"
 
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I finally got these 845 monoblocks wrapped up. I built them a while ago, but one had an odd crackling sound that took me forever to track down. These use a 6FJ7 driver tube with the triodes directly coupled and the beefier triode loaded by a plate choke for lots of swing without worrying about current source dissipation or some giant hot plate loading resistor. The output iron is a Magnequest FS-100-PC plate choke and an FS-100-PF parallel feed output transformer. The parallel feed cap was rather hard to find, and I ended up with a 2kV rated Wima cap that advertised a 700V AC rating. The coupling cap is a CDE high voltage polypropylene, and really a lot of the parts were chosen to withstand the abuse experienced in this circuit rather than for boutique factor. An unusual design choice revolves around the glass case enclosing the iron, as I chose to bias up the plate choke's lamination stack to the full B+ potential to balance out how hard the insulation has to work.
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The 845 operating point is 940V plate to cathode and about 90mA of plate current. This puts it over the 75W plate dissipation spec for an old stock Amperex, but 15W under the 100W spec that RCA gives.

Power output is as follows:
1kHz: 29W at 5% THD and 31W at 10% THD. Maximum hard clipped output stops at 32W, so there isn't a lot of juice left to squeeze here.
35Hz: 23W at 5% THD and 25W at 10% THD. Given the nature of 10K output transformers, this is incredibly respectable.
Damping factor is just a hair over 4.

The amp is -3dB at 15Hz which is better than any 845 amp I can remember having on the bench. Up top, the amp does start to roll off a bit and is about 2dB down at 20kHz which is not shocking when dealing with iron like this. With the low damping factor of an amp like this and tweeter voice coil inductance, this does tend to result in a final product with smoother, flatter treble. Hum from the amp is a bit less than 2mV.

This is one of those projects that leaves me scratching my head a bit, as the voltage in this amp is about the same as the 833 project and it makes about 1/3 of the power. There's also the issue that I have one more iron set for this, as does the customer who ordered this amp set, and then this iron gets pretty scarce. There really aren't any substitutes that I'm seeing from other vendors that make me all that excited to build more 845 amps.
 
That's a handsome amp. Those CDE caps are very, very good for the money (and handy for the voltage ratings, too). The 942C series are my go-to caps for amps. I think they compare favorably to Teflons at a fraction of the price.
 
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