Another 832 Amp Begins its Life

[WA4SWJ]

Hi Gang,

After months of not working on it, I just finished my 832A Amp today and fired it up. Worked the first time. Picture is here:


It's really ugly underneath but it sounds just great. I'm using some Hammond 1608 OPT's, a Hammond 370FX power transformer and JJ ECC81 driver tubes. The 832's have just a slight red glow on the plates with about 335 volts. I used the same circuit that Tom used with some slight changes to electrolytic capacitor values.

Very nice sound and it's fun to watch the little "R2D2 looking" tubes do their thing, but ya gotta be careful with those exposed plate leads. I did not choose to illuminate them with blue LED's underneath - might do that sometime but most likely not. I've still got to tune the feedback with some square waves, but that will come later. It sounds great as it is. I'll post an underside view and some more pictures maybe tomorrow. I still have to finish the wood base with something - maybe tung oil. I have some of that.

Great fun!

[TomMcNally]

Very cool looking Ed ! Do your 832A's run hot ? Mine sure do.
It's definitely a good sounding amp.

... tom

[SteveH]

Very very cool !

[Shannon Parks]

Ed, where did you get that chassis? If that is unfinished aluminum, tung oil might not be a good idea. I think I'm going to use some one-step stain & urethane on mine.

[WA4SWJ]

Thanks guys. The 832's are hot but I don't think they're hotter than normal. The plates are just slightly red, which is how I like to run most of the power tubes I have. I have not checked the current flow yet. Maybe in the next few days. All appears well and as I mentioned, it sounds great!

[WA4SWJ]

Shannon,

It's a Wellbourne Labs chassis. I was going to finish the wood base and not the aluminum. It's walnut and it's unfinished at the moment.

[EWBrown]

The RCA 832A data sheet indicates that "the plates show no color when tube is operated at maximum CCS or ICAS ratings".

I've been "running the numbers" on the original design, and decided that the VA and PI stages are being run far too lightly, and that the 832 cound go upto around 300VDC at the plates, with around 35 mA plate current each section, and cathode biased, somewhere TBD between 10 and 15VDC.

The Tube CAD "numbers" for the 12AT7 VA / PI stage look better, with around 325VDC @ 4 mA for the PI, and 232 VDC B+, at 1.25 mA, and a plate voltage of 107V for the VA section (one more dropping resistor, no biggie).

The PI section will hold 4 mA, and a grid voltage of 107VDC for B+ voltages anywhere between 315 and 370VDC, using 27K 2W K and A resistors, hence the "juggling" to get 107 VDC on the VA plate, and sticking to common resistor values. I'd post the actual VA stage resistors, but I have all that at home, and my memory isn't exactly "photographic"... Yellow_Light_Colorz_PDT_09

Until I prove otherwise, I believe it was 1K for the cathode resistor, 100K plate resistor (to 232VDC) and then 68 to 75K to the 325VDC dropping resistor and the requisite 22 uF / 450VDC bypassing caps.

I'd keep the 832As under 10 watts max plate dissipation (for each section) :o Just to play it safe...

/ed B in NH

[WA4SWJ]

Hey Ed,

Does this mean you're getting ready to build an 832 amp? Assuming so, would love to see it when you're done! The plates on mine are just ever so slightly red. You have to look at them in a dark room to notice. The amp sounds great and I still haven't tweaked the feedback network yet.

I might mess around with the VA and PI some as you've suggested. Maybe this weekend. Work gets in the way of fun!

[EWBrown]

I've been considering putting together one of the 832 amps, and have been running all the "paper scenarios". I now have all the parts that I will need, except for the free time, which will increase for me when the long NH cold and lousy winter strikes...

I thought of kicking it up a notch, with 815s or 829Bs, but then the 832 design has more appeal to me...

I've also been toying around with various 6S4A and 12B4A designs, both PSE and PPP, though that is a lot longer out for now.

/ed B in NH

[alexg_ht]

I did the 832 PP amp and I like the sound of it. B+ around 260VDC, biased at around 33ma. Driver tube is a 6j5.

I really like the sound of it, until I did a parafeed 12b4 using a 6n1p for a driver. This flea powered amp has very good midrange/vocals, though does not have enough grunt to drive my Fostex 127e.

Last week, I just did a 6080 parafeed using 12at7 as driver. Of all the three amps, I really like the 6080 parafeed for the vocals and better dynamics and clarity.

But believe, they all sound good!

[hbi]

Greet, wherefore you don't try and push-pull?
Yet I compliment on you realization. Holds all such!
All good the current si in filaments!
Hbi