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Old 27th September 2007, 01:29 PM   #1
sgerus is offline sgerus  United States
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Default Please critique my plans for a WE91 clone.

For my next amp project, I’m planning on a circuit similar to the WE91.
OK. It starts with a pentode, and that where the similarity ends.

The input/driver is a 6sg7 with a 0c3 to regulate the g2 voltage. The power tube is a JJ2A3-40 (300B with 2.5V filament)

Here are some specific questions, see the attached schematic.

1. Is the circuit for the 0c3 correct?

2. When using a 0c3 to set the g2 voltage of the pentode, do you need a capacitor from g2 to the cathode?

3. In general, is the 0c3 worth the cost (heat) of the 15ma it burns to regulate the g2 1-2 mA

4.For the pentode, I want an octal based tube, preferably glass. I’ve looked at:
6j7gt at 3mA (cathode)
6sj7 at 6.25mA
6sg7 at 8.1mA
On paper, the 6sg7 looks like the best choice, (lowest distortion, very high voltage swing, and the most current)
Any thoughts on my choice of pentodes?

5. Is the power supply overkill…. IE the separate “g2” voltage for the oc3

Thank You -Scott
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Old 27th September 2007, 01:30 PM   #2
sgerus is offline sgerus  United States
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Here'e the psu
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Old 27th September 2007, 03:53 PM   #3
kevinkr is offline kevinkr  United States
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Default Re: Please critique my plans for a WE91 clone.

Quote:
Originally posted by sgerus
For my next amp project, I’m planning on a circuit similar to the WE91.
OK. It starts with a pentode, and that where the similarity ends.

The input/driver is a 6sg7 with a 0c3 to regulate the g2 voltage. The power tube is a JJ2A3-40 (300B with 2.5V filament)

Here are some specific questions, see the attached schematic.

1. Is the circuit for the 0c3 correct?

2. When using a 0c3 to set the g2 voltage of the pentode, do you need a capacitor from g2 to the cathode?

3. In general, is the 0c3 worth the cost (heat) of the 15ma it burns to regulate the g2 1-2 mA

4.For the pentode, I want an octal based tube, preferably glass. I’ve looked at:
6j7gt at 3mA (cathode)
6sj7 at 6.25mA
6sg7 at 8.1mA
On paper, the 6sg7 looks like the best choice, (lowest distortion, very high voltage swing, and the most current)
Any thoughts on my choice of pentodes?

5. Is the power supply overkill…. IE the separate “g2” voltage for the oc3

Thank You -Scott
I would probably choose the 6SG7 and then the 6SJ7 in that order.

Using an OC3 as a screen regulator is a great idea, shunting it with anything more than 0.01uF - 0.022uF probably isn't, at 0.1uF I would be concerned that the gas tube and cap will turn into a relaxation oscillator.

I would use a standard 300B rather than the JJ 2.5V version (2A3-40) as this will give you a lot more flexibility in terms of future tube choices. (And sadly JJ seems to be having some pretty significant QC issues with larger power tubes at the moment.) Using a 5V tube will require dc filaments (ccs recommended) for low noise - the one real advantage of the 2.5V JJ tube.

I haven't looked closely at anything else, but it looks like a reasonable starting point.

Edit:

I spoke too soon - I found some other things. (refinements mostly) The cathode bypass cap and the ultrapath cap ought to be higher in value for good bass performance. I'd recommend 80 - 100uF and for effective ripple cancelling the ultrapath cap should be sized using this relationship
C_ultrapath=C_bypass/Mu +1 in this case with 100uF a 20 - 22uF gives you a significant amount of cancellation of supply ripple in the output stage. In a similar amp I did a couple of years ago Spice predicted something like 20dB of additional ripple suppression and I got something > 15dB - a rather worthwhile improvement.

The quality of the ultrapath capacitor is quite important, I used a 20uF Solen with a 100uF ASC motor run as cathode bypass cap. A better ultrapath cap than the Solen is probably a good investment.

Gas tubes can be quite noisy, hopefully this is not an issue here, if it is a zener may well be quieter, and can be bypassed with a honking big cap if desired. Sometimes RC decoupling can be used, but R has to be small to minimize IR drop with varying screen current which means that to prevent relaxation oscillations the cap still has to be pretty small. It does establish a predictable HF pole which is a good thing.

You can't use a cap of meaningful size on the screen to the cathode or ground with a gas tube because of the oscillation issue, and in any case the source impedance of the gas tube is extremely low. You do need to make sure the cathode is well bypassed to ground for best linearity.
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Old 27th September 2007, 04:18 PM   #4
kevinkr is offline kevinkr  United States
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In terms of OC3 current you could probably get by with a little less than 15mA, but this or even higher current will lower the internal impedance of the gas tube. You have plenty of voltage margin so the tube should "strike" reliably.

I don't see any particular problems with the psu design, you might want to check it in psud if you haven't done so already..

Arcol makes great little heatsinkable aluminum cased power resistors for chassis mounting that cost a lot less than the Dales they compete with. They're available from Mouser.

Good ASC motor run caps for cathode bypass and power supply stuff is available from midori meadows on eBay. (Highly recommended.)
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Old 27th September 2007, 06:44 PM   #5
sgerus is offline sgerus  United States
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Thanks Kevin,

I committed to the 2.5V filament for the power tube (custom made T1 on order).
So that leaves me with the Sovtek 2a3 runing at about 21 watts, take my chances with the JJ (that was original plan), or spend about $375 a pair for EML 2a3-s or KR 2a3.

I will change the 0C3 to run at 10mA with a .01uf cap.

What brand of capacitor would you recommend for the “ultra-path”… in place of the Solen?

Also, what do you think about just using the 50-100uf ultra-path capacitor and omitting the cap on the cathode bypass resistor?
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Old 27th September 2007, 09:25 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by sgerus
Thanks Kevin,

I committed to the 2.5V filament for the power tube (custom made T1 on order).
So that leaves me with the Sovtek 2a3 runing at about 21 watts, take my chances with the JJ (that was original plan), or spend about $375 a pair for EML 2a3-s or KR 2a3.

I will change the 0C3 to run at 10mA with a .01uf cap.

What brand of capacitor would you recommend for the “ultra-path”… in place of the Solen?

Also, what do you think about just using the 50-100uf ultra-path capacitor and omitting the cap on the cathode bypass resistor?
I tried just the ultrapath configuration and found I preferred the two cap configuration, and with a single cap you loose the ripple canceling inherently possible with this configuration.

In terms of something better than Solen I was thinking the Clarity Cap SA would be one possibility, the Erse Pulse X another - both are available from a variety of sources like www.diycable.com, and Clarity from www.e-speakers.com The nice thing about these caps is they sound good and aren't very expensive unlike many other boutique brands out there. ASC motor run caps on eBay are cost effective and pretty good sounding in this configuration - a bit soft on the highs if used by themselves.

Looks like you have some good options for later with both EML and KR making suitable replacements for the 2A3-40..

Sounds like a cool project, be sure to post details as it all unfolds, pix of course would be great.

Kevin
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Old 28th September 2007, 05:12 PM   #7
coresta is online now coresta  France
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Hi I wouldn't go with 6SG7 as these are "semi remote" pentodes
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Old 28th September 2007, 05:41 PM   #8
sgerus is offline sgerus  United States
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Last night I tested the 6gs7 with the 0C3 voltage regulator. It worked fine, but didn’t sound any better than the 6j7 (the current amp).

So I have decided not to use the 0C3…. Not worth the effort or the extra heat you get from dropping 10-15mA over 300V.

This weekend I will do some “blind” test of different pentodes. A few weeks ago I tested a 6sj7 / 6sl7 “mu-stage”. I could actually hear a difference, with the mu-stage being the winner. I just my switch my plans for the input/driver based on this weekends experiments.
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Old 28th September 2007, 06:34 PM   #9
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How about an EF37A. High quality, lo-noise drop in octal replacement for a 6J7 with grid cap also. My Mullards have nice red paint on glass envelope.

Quite different is the 6AG7 metal octal or the 6CL6, 9 pin glass miniature. Those swing large and are used to drive video transmission lines at 75 ohms. I was planning to try the 6AG7 as a driver in a PSET multiple 6080 design.
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Old 15th October 2007, 11:54 PM   #10
sgerus is offline sgerus  United States
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Kevin / Bob,

I purchased the 100uf and 20uf caps for the ultrapath per Kevin’s recommendation and the Mullard EF37A per Bob’s. The EF37A’s look good. The red paint will look good next to the red letters on the JJ-2A3-40’s….. and I got them for about $15 each from the Tube Depot!

I’m thinking on running the EF37A’s at:
B++ 370V
Plate 210V at 4mA
G2 at 100V at 1mA
Cathode at 1.5V at 5mA

According to the EF37A data sheet, that works out to Av=76 with .6% 2nd Harmonic Distortion, with the plate at .8Watts.

1 Watt is the max for this tube, so I’m running on the hot side?

Any comments on the operating point?
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