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Hey Mike, how about this idea?

201.161.25.147

I'm just about to jump into my Bottlehead FP3 kit, and have once again come up against the circuit I've kind of always wanted to build. I'd like to run it past you, to see what you think, and see if you might make a recommendation for the hypothetical plate choke. Here goes...

1. Basically a 76 parafeed linestage into 15K B7 OPTs.

2. Instead of attenuating the 76 grid, I'd attenuate the OPT output with either a stereo 600R unit (suggestions for brand, anyone?) or 2 mono ones.

3. A small signal grid choke on the 76 input grid

4. Plate chokes for the 76s. Is your exo-99 the highest inductance choke available? Could it be modified for more henries at 5mA? The idea here is to put the 76 in a more linear range than the C4S would afford, using an approx 150vdc B+. Or would the exo-99 be fine at 5mA, swinging a max of 30vrms?

Just some ideas, let me know what you think would work or not. Thanks! --keto

Follow Ups:

RE: Hey Mike, how about this idea? - mwiebe 11:27:47 04/30/09 (0)
Keto,
Only answering your Q 4. I built my preamp for a 26 using a 15K B7 (M6) output and I put the plate choke on jumpers so I could switch between a variety of Mike’s plate chokes. I agree with Mike’s comments that the EXO-010 sounds better then the EXO-99 when I have swapped them back and forth. The 26 at 8mA works fine with the EXO-010 in my system.

I should note that instead of the 26 I usually have some Russian direct-heated subminiature pentode on a four-pin base plugged in--usually the 1J17B (1Ж17Б, 1Zh17B), 1J18B (1Ж18Б, 1Zh18B) or 1P24B-V in that order of preference. With a pair of 9,000mAh NiMH “D” cells in parallel powering each 1J17B filament I get over 200 hours of listening before I have to switch out the batteries. With the 26 I get about 12 hours or so as I’ve been using the same 8 batteries for 5 years now. My speakers probably only go down to about 60hz as well, but I don’t miss any bass with the 26, EXO-010, 15K B7.
Matt
My take - gluca 21:09:59 04/29/09 (0)
Ciao
1. OK, I like OPTs ... that's it. you really want to get ni lams.
2. been there. go for it. search for some old langevin or daven stepped attenuator. you won't find anything better out there. in my experience controlling the gain after the OPTs sounds better when tube and load allow for it
3. I'd take an input transformer too
4. I'd stay with a CCS, cascoded FETs are darn good and easy and dirt cheap. I love chokes but ...
thanks for sharing.
gianluca

Edits: 04/29/09 04/29/09
Are you wedded to the 76? - Phil Sieg 11:14:38 04/29/09 (4)
Because if you are not, may I suggest the 112A. It's a better sounding DHT and it's easier to work with as far as load goes. You can bias the tube so that the plate resistance will fall between 3k5 and 4k making either the 10K or 15K version of the B7 an ideal load.

I used the 112A as the last (line stage) of my last preamp. Had a pair of S&B TX-102s TVCs in front and loaded it with a BCP-14. Very sweet, liquid tube. Gets you 95% of the way to a 26 without the filament hum hassles.

FWIW, the 112A replaced a 76 output stage that was C4S loaded. The OPT in both cases was a Peerless 15095.

The only potential downside is the loss of gain. Mu of the 112A is around 8.5. Still, if that works in your system, I think it's a better choice than the 76. Globe 112As (recommended) are still pretty reasonable on eBay.
- in my experience... - Jim Dowdy 14:53:57 04/29/09 (1)
  Maybe it's just me, but in my experience, the 112A is about 110% of a 26 :-)
  The lack of filament hum hassle is icing on the cake.
  - Well, ... - Phil Sieg 15:55:23 04/29/09 (0)
    I think it depends on just how much aggravation you need in your life. ;-)
- RE: Are you wedded to the 76? - keto 13:29:49 04/29/09 (1)
  Sounds really interesting. What did you do for the 5V 250mA filaments?
  - RE: Are you wedded to the 76? - Phil Sieg 14:51:12 04/29/09 (0)
    Then I was using current-regulated DC. Basically the Ronan Reg with whatever number of stages were required to deliver 5VDC at the required current. I *think* I started with a 10V 1A filament tranny. Filaments were dead quiet.
    
    Today I would scrap the SS regulators in favor of filament chokes with a cap input only if I needed it to get the required voltage. Make sure the choke is the last element before the filaments. I want to stress that I'd make that change not because I think current regulated filaments are "bad", but because my design philosophy has shifted toward simplicity and as few active components as necessary. But that's just me.
RE: Hey Mike, how about this idea? - danlaudionut 23:10:22 04/28/09 (3)
Keto

Since no one else is answering I'll give it a go -

1) A 15K OPT on a tube with ~10K output impedance
is not a recipe for good sonics IMO.
Mike specs them at > 200H and then parallel them
with the plate choke so ~150H - barely enough.
But I never like having that steep of a slope
of a loadline (ie. almost vertical).
I like the sound when it is almost horizontal.
It will give you a gain of ~5.5
from the preamp - is that enough?
Is it worth it?

2) A 600R pot on the output would seriously load
the output WITHOUT anything connected to it.

3) Small signal choke on the input,
well I am in favor of grid chokes
on ANY tube ANYWHERE.

4) The EXO-99s are Mikey's highest inductance plate chokes.
I have a pair on my 12B4s - talk about overkill 8^D

IMO #1 and #2 will seriously hamper the 76 tube.
That's my take on this anyway.

DanL
- RE: Hey Mike, how about this idea? - keto 09:56:40 04/29/09 (0)
  Hey DanL,
  
  thanks a lot for the ideas.
  
  I've gone back and looked at the 76 plate curves and done a lot of rethinking, keeping in mind the "toughest load" situations of either 15K or 20K, even though from Mike's post, the Cobalt 15K B7's will be "lighter", especially if in parallel with just a C4S (my thinking -- and maybe the 15K or 20K models would apply for the Cobalt 15K B7 + the appropriate plate choke?). I'll do this on an existing platform, rather than change the amazing looking FP3 kit I've been staring at. Given a mu of about 9, there wont be a plate voltage swing of more than 20vrms, probably. I'm seeing that given the C4S plate loads + 300V regulated B+ that are on the existing platform, deepest class A would look something like this:
  
  @5mA, 15K load, 170Vp-k, -8Vg 50vrms swing
  @5mA, 20K load, 185Vp-k, -9Vg 70vrms swing
  
  On the 76 data sheet, plate loads of 50K and 250K are mentioned, so it is a logical stretch to apply (even) a (nominal) 15K load to the plate, but the numbers look okay to me now to give it a try.
  
  I've got to order some parts before I try this out. I'll keep the exo-010, 1:1 input transformer (thanks Mike) and not loading the secondary output in mind as I proceed.
  
  Thanks again, --keto
- RE: Hey Mike, how about this idea? - mqracing 09:00:21 04/29/09 (1)
  Hi Dan:
  
  You framed the questions very well...
  
  I'll make just a few comments.
  
  Under your first point--- the reference L of 200H is for M6. In Keto's intended application I would opt for a nickel based core which will increase the L severalfold.
  
  Re: loadlines and "nominal" impedances and etc--- please see the following post;
  
  http://www.audioasylum.com/forums/magnequest/messages/8798.html
  
  I agree with you Dan--- I would not load the secondary and, instead, would do the volume control on the "front side" of the voltage amplifier.
  Will also make finding a suitably good quality attentuator\potentiometer much easier.
  
  Another suitable plate choke--- physically smaller than the EXO-99's and less expensive--- is the EXO-010. The 010 is rated at 275 henries--- which at 20 hertz would give you a load impedance of a bit over 34K. I keep thinkin' that if it were my project I would first consider using the 010. Smaller core, smaller winding lengths equals less capacitance that must be driven.
  
  grid choke on input side of voltage amp--- yep--- though I might also look at using a 1:1 with 10K volume control (think high quality Penny & Giles 10K stereo unit).
  
  Thanks Dan for responding. I've been sidelined a bit--- seems I can work a few days and then must give my ribs a few days "off"--- just saw my family doc yesterday and will soon be seeing a muscle, bone, and joint specialist. This shit has to get better sometime soon.
  
  MSL
  
  Builder of MagneQuest™ & Peerless™ transformers since 1989
  - RE: Hey Mike, how about this idea? - danlaudionut 09:11:54 04/29/09 (0)
    Mikey
    
    Glad to help.
    
    Yes the tube's load is the primary L
    in parallel with the secondaries load.
    that makes for an almost vertical LL.
    
    I forgot about the EXO-010.
    I have a pair loading my 6P5GT drivers
    on my SE 6L6G amp.
    
    Hope you do get better.
    Use some wisdom on activities.
    Most men have problems with that.
    Me included !
    
    DanL

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