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Old 26th July 2012, 03:12 AM   #1
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Default 808 SE Design Help!

Hey all.

Some may remember I opened a thread here about two months ago regarding building an amplifier using an 808 in SE mode. Anyway, I've started the design process and have hit a few hitches.

Right now, here is what i have. 808 run in SE with 400V on the plate and 3.5k opt. Grid bias is +30V (you'll see from the tube curves there's not really any options for traditional bias here) with +/- 30V swing on the grid provided by the Driver tube, which I decided to go with a 3C24 biased at -4V and 250V on the plate (given 350V B+ and 12.5k Ra) with +/- 4V swing on the grid provided by 1/2 6SN7GT input tube capacitively coupled.

I've designed a few small amplifiers and I've never dealt with a positive grid bias in the past such as I am with the 808. It's really frying my brain right now because I can't figure out how to achieve the +30V static voltage. So far what I have concluded:

Traditional capacitive coupling will not work, though I'm not ENTIRELY sure why.

An interstage transformer seems to be an option, with the primary essentially acting as a plate choke for the 3C24 and the secondary being fed with the +30V supply, though I don't really like this option and I have no idea what the impedance of either side should be. (Guessing it should match the 12.5k load I designed in there and maybe even just be 1:1?)

I've seen talk of this "cathode follower" business. What the hell is that? From what I've gathered it seems as if it just applies the grid as the cathode bias resistor, but how do I calculate what I'll end up with???

I would prefer an all "glass" method, but if not what am I looking at for an interstage tranny?

-Steve
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Old 26th July 2012, 04:45 AM   #2
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I think if you posted a schematic, we might have a clue about what you are building.
Otherwise, it's pretty hard to imagine.
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Old 26th July 2012, 06:55 AM   #3
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Good day Steve.

Go with the interstage believe me, otherwise you will loose money and time

Even an 1:1 ratio interstage will work fine but use a stiff driver tube biased at least 15 to 20 mA.
The 808 draw about 10mA or more grid current.

Chris
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Old 26th July 2012, 07:14 AM   #4
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Quote:
Originally Posted by satelitis View Post
Good day Steve.

Go with the interstage believe me, otherwise you will loose money and time

Even an 1:1 ratio interstage will work fine but use a stiff driver tube biased at least 15 to 20 mA.
The 808 draw about 10mA or more grid current.

Chris
While the inter-stage transformer is absolutely the best way to couple the circuit, a transformer with hi fi frequency response is super expensive.
Good coupling transformers cost a fortune.
Cheap ones have really poor low end...
And so a cathode follower is really not such a bad idea, fairly simple, lighter, and much less money. It's almost as good.
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Old 26th July 2012, 07:52 AM   #5
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Yeap money is money...

In my opinion the 2nd best option was using a dc coupled 2 stage driver using a 6EM7 tube. The first 1/2 (high cain) unit connected as anode follower choke loaded, direct coupled to the 2nd 1/2 (low gain) using a choke in cathode (cathode follower). The sound was very close to the interstage configuration.

Check also this configuration http://www6.plala.or.jp/Michi/renewa...0SE%20Amp.html

Last edited by satelitis; 26th July 2012 at 08:06 AM.
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Old 26th July 2012, 08:10 AM   #6
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Quote:
Originally Posted by satelitis View Post
Yeap money is money...

In my opinion the 2nd best option was using a dc coupled 2 stage driver using a 6EM7 tube. The first 1/2 (high cain) unit connected as anode follower choke loaded, direct coupled to the 2nd 1/2 (low gain) using a choke in cathode (cathode follower). The sound was very close to the interstage configuration.
Yeah, I mean who can afford this
Silk Input transformer 1:2+2 (Pair) - 3 pairs in stock | Diy HiFi Supply
And you know that the low frequency specs are always overstated by the manufacturer.
I mean it's really hard to find an interstage with good sounding lows.
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Old 26th July 2012, 10:10 AM   #7
multi is offline multi  Australia
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Hi:
I have made lots of amps using 805's, 809's,810, 811A. Se and PP; i have never tried the 808.
You will get just as good performance with a cathode follower as the inter-stage, in my opinion much better. i have used the 6BL7 or 6BX7. I would think with 400 volts the 6BX7 would be the best. I have made 811A PP {Russian} that have run for 17 years without ever changing the 6BL7's or the output tubes.
Phil
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Old 26th July 2012, 11:49 AM   #8
disco is offline disco  Netherlands
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- direct coupling
takes ca 100 - 150V extra high voltage for a triode driver
good LF, HF and dynamic response

- interstage
this solution can be very good (are those cheap Hammond any good?)
needs inductance (many windings) but suffers from any capacitance
Ri of 12K demands a hole lot of inductance to keep F-3dB down to 30Hz

- RC coupled gain stage
coupling caps give LF roll off and suffer from blocking when grid current arises,
Miller capacitance gives HF roll off

- gain stage & cathode follower
no blocking, can supply grid current at the expense of implementing an extra tube

Furthermore there are solid state helpers like current sources with low impedance output, gyroter loads to linearize driver tubes and one or two other tricks I can't think of right now.

Choose your pick and go ahead, you're not alone
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Old 26th July 2012, 11:53 AM   #9
7N7 is offline 7N7  United Kingdom
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Yes cathode follower is a good solution here. Given the very low input resistance of a valve biased positively, the cathode follower should be able to source plenty of current. I would probably choose 6S4 which is cheap, 8W Pa and can run at 550V. I would run plenty of milliamps through it and would use an active load, either a pentode or a mosfet to keep distortion ultra low.

7N7
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Old 26th July 2012, 12:50 PM   #10
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Hey guys, thanks for the responses.

I found a set of grid curves for the 808 and it looks like at +30V in my operating range I'll be running a constant 15mA through the grid.

I'd like to *for now* explore the interstage transformer option for the sake of simplicity.

I found a Hammond that's not "obscenely" priced (comparitively at least), I'm looking at the 126C. Hammond Mfg. - Audio - Tube Driver Transformer - (124 - 126 Series)

Let's say I go with that one, how does my 2C24 load line change? From 12.5k to 10k?

Then do I just feed the secondary from the bottom with +30VDC pulled off my PSU and hook the grid up directly to the top?
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