829B/3E29 "Triode" Push Pull Paralleled Sections

[coldcathode]

With a big tip O' the hat to Pete Millett I have jumped feet first into designing a rather simple Push Pull amp using the 829B/3E39 transmitting tube.

First, A little background as to why such a "bizarre" tube choice for a relative "beginner" to tube audio.

My first experience with Tube Audio came purely by accident after having found an old Grunow "Console" Radio in a junk pile. I liked the look of it and wanted to make it work. I found the schematic online and played with it for a while. The "sound" seemed so much better even in Mono than anything I had heard that I became intrigued with tubes.

I started picking up tubes etc at estate sales and built a couple of SE amp using proven designs off the net and "junk box" parts.

I came across an old US Army Signal Corps transmitter with (2) 829B's and a beefy power tranny in another trashpile. Having grown up watching M.A.S.H. it was so much like "Radar's" setup I had to take it home.

fast forward about 2 years and I have these 829B's along with some nice Shielded sockets and I came across Pete Millett's 829B SE and really liked that idea. Since SE Iron is relatively expensive and hard to find in "trashpiles" I held off building anything till I could really design something PP.

So I now have my hands on a set of decent PP output trannies capable of the idle current that I want to use with around 7800R P to P.

Time to start "designing".

The overall concept is plain vanilla since the choice of output tube is interesting enough. One half 6SN7 input/gain stage into the second half "split load" phase splitter. Then a 6CG7/6FQ7 will drive the 829B grids.

I am going with P to P of 7800 ohms into 8.
A B+ of 300V, Cathode Bias of about -30V for ~80mA per tube idle current.

Any comments as to whether or not I am sane?

[SY]

Depends. How many 829s do you have? If you want to run triode pp, you'll need 4 of them for a stereo amp. Pentode, you only need 2. Hint: screens are tied together and bypassed to cathode internally....

[coldcathode]

I forgot to mention that along the way I have amassed a collection of (7) 829B's all RCA from about the same years in production.

I happened to pick up a lot of boxes of "ham radio" stuff from Flea Markets and Estate Sales. Since I really have no tester to test these I can verify that the filaments are good, I figured it's a decent chance I will have (4) good ones out of 7.

Pentode would give me much more power but looking at the "triode" curves it has pretty good linearity and I will try the "triode sound" for now.

Snip out the G2 connection to the plate and supply it with 200V or so in the future is no problem if I want to go pentode.

[coldcathode]

A "first rough draft of one channel"

I am sure I made some mistakes. Let me know what you find.

Suggestions as to an interesting input tube? I have some Globe 27's "meshies" might be an interesting look?

[rman]

Hi.

Just a thought. If you bias that firs stage, the one you have no load or cathode resistor shown for, so that you have about one third B+ on the plate, you could direct couple and forget about the bias elements and grid resistor for the phase splitter.

Cheers.
Rolf.

[aardvarkash10]

Why not use the 6J5 as input triode to the 'sn7 as a LTP splitter? Should give all the gain you need and reduce the tube count by one per channel.

[coldcathode]

Originally Posted by aardvarkash10

Why not use the 6J5 as input triode to the 'sn7 as a LTP splitter? Should give all the gain you need and reduce the tube count by one per channel.

I'm not a big fan of LTP.

I should also have mentioned the "Goals" of this amp.

1: Use the 829B along with any other "funky" tubes (going for a "steampunk" theme. Brass, Black Wrinkle finish, Antique look)

2: About 15 watts with 1% distortion or so.

3: Be as weird looking as possible

[coldcathode]

Originally Posted by rman

Hi.

Just a thought. If you bias that firs stage, the one you have no load or cathode resistor shown for, so that you have about one third B+ on the plate, you could direct couple and forget about the bias elements and grid resistor for the phase splitter.

Cheers.
Rolf.

Rolf,

Something like this 2nd draft?

[coldcathode]

Now a Powersupply is needed.

This this is starting to become ridiculously over complex. Having lots of funky tubes etc. So might as well continue along that theme.

I am looking at 2 power transformers, one for the 829B's w/ Parallel 5U4GBs a choke etc.

The other for the input, splitter and driver stage.

In keeping with the "too many tubes" theme I am thinking about a VR tube regulated supply.

Heres the question: (2) 0D3's in series gets me the 300V. Can I then add an 0C3 in series with those and the 6.8mA load of the 27's to get my 405V?

If so how would you go about it?

[coldcathode]

At this point I am looking at a total of about 16 Tubes! LOL for a measly 15WPC stereo!

The power company is going to LOVE me!

The heater filaments alone are dissipating 2 1/2 times the power output!

As crazy as all this seems, I am kind of liking the idea? Certainly would make an interesting "conversation piece", not very efficient but worth a shot.