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Old 16th August 2012, 07:25 PM   #101
eboz is offline eboz  United States
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I just checked and found that the 95vct appears to be connected correctly. Measuring the windings in series shows 185vac, as it should be. I still can't see why stacking these two supplies would not work without problems.

-MW
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Old 16th August 2012, 07:32 PM   #102
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Quote:
Originally Posted by mogliaa View Post
Been playing a bit to optimise my first version of this 4-65a SE based on Michael's design. My idea is to do 26 /46 / 4-65a. I have a 26 preamp and plan still is to maintain filament bias where possible.
The 4-65a model performs reasonably well. I did some THD measures on the 4-65a with a CCS load and if results are extrapolated is on the same lines of what should be expected.
As pe Michael's recommendations, the driver needs to provide about 200Vpp to deliver full output power given my loadline and HT constraints. As it stands I can get THD=0.88% at 14W or 0.19% at 1W.
The 4-65a is pushed close to its Pd limits: 73W (anode+screen).
I guess I can optimise the 46 to minimise distortion? I started with Rmu1=1/gm but as I increased it to 1K I reduced THD at maximum swing 0.76%. I thought that Rmu1=1/gm was the optimal value for the mu-follower. Of course I changed the Rmu without changing the bias point so this impacted the anode current, therefore I ended up with a lower Ia which set the 46 in a more linear region?

Perhaps I can tweak the 46 to find a better operating point here. It shouldn't impact that much to reduce the quiescent current as all the positive grid current is provided by the cascode?

Any suggestion of how to tweak anything else before getting into breadboard stage?

Thanks
Ale
Have you tried using a jfet instead of M2? A J310 or a 2sk170v/
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Old 16th August 2012, 08:26 PM   #103
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May need a pair as quiescent current is over Idss max on both jfets. Will it be a clear benefit to have a quieter jfet there given signal is swinging 200vpp? What is the benefit of the jfet?

Btw: I'm progressing really slowly with this amp, a lot of iron!

Thanks!
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Old 16th August 2012, 09:06 PM   #104
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Oh, I just noticed your idle current. Yes, J310 are cheap enough to get many and roughly match. You can use three of them in parallel. Still lower capacitance than DN2540. I'd try it this way (see image).
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Old 17th August 2012, 05:02 PM   #105
eboz is offline eboz  United States
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Quote:
Originally Posted by eboz View Post
I just checked and found that the 95vct appears to be connected correctly. Measuring the windings in series shows 185vac, as it should be. I still can't see why stacking these two supplies would not work without problems.

-MW
So I did more diagnostics on this and took it down to the raw supplies. It doesn't appear to have anything to do with the regulated supply. The problem is introduced when I connect the return of the 350vdc supply to earth ground. I still have no idea why this is. When not connected, the supplies stack as expected. I don't have earth connected anywhere else in the supply side during this test, except for the screen on the PT. I tested the screen just to make sure it's not shorted anywhere, and it's not. Any thoughts are greatly appreciated!

-MW
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Old 18th August 2012, 12:19 AM   #106
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Despite my best efforts, it was just a stupid wiring fault that I made.
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Old 18th August 2012, 08:27 PM   #107
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Finally I have one channel running. I had a bit of trouble with B+ coming on without blowing its fuse. I'm not sure what it was, because I double checked my wiring and then fired it again and it was okay. It's my first time using MV tubes tho, so perhaps I didn't give them enough time to warm up before the first B+. I also have NTC's on the B+ primary, which is bypassed by relay some 3s later. Just running both the input and output tubes on lower bias for now to get used to it. I'm gonna take a break on this project for a bit before finishing the other channel, so it'll be a few weeks to a month before I have stereo. I haven't noticed any wandering on the bias however, but my bias setup is a little different than Michael K's. The input tube is mixed between battery bias on the grid and cathode bias. Actually I have a DIL type chip connector that allows for different parts to connect between cathode and ground. Right now running a 1.7v AA battery and a 33ohm riken.

Click the image to open in full size.

-MW
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Old 19th August 2012, 07:31 PM   #108
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Hi Michael,
Superb look, that is a nice amp. Difficult to comment without schematic, however I'd probably apply HT with a separate switch when 4-65a Is hot enough. Are u using dc filament regulators?

I'm coming slowly on this project, at the moment have designed all LT and filament regulator supplies. Got all iron so far, so will build 46 driver first and do some tests cheers
Ale
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Old 19th August 2012, 08:20 PM   #109
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Quote:
Originally Posted by mogliaa View Post
Hi Michael,
Superb look, that is a nice amp. Difficult to comment without schematic, however I'd probably apply HT with a separate switch when 4-65a Is hot enough. Are u using dc filament regulators?

I'm coming slowly on this project, at the moment have designed all LT and filament regulator supplies. Got all iron so far, so will build 46 driver first and do some tests cheers
Ale
Hi Ale,

I am glad to see your interest in this amplifier, as I always enjoyed reading about your efforts in the 26 preamp thread and your blog. I have a schematic that I used to get a rough idea of bias points and behavior of the amp. The actual schematic I am still working on, as it's quite involved. There is a standby circuit that is turned on by a key switch, so I can be sure others won't be able to operate it if I'm not around. The B+ will only turn on after the heat and bias comes up, and it also enforces a 2min delay. B+ can then turn on through two 10ohm NTC's in series, which are bypassed 3s later. The amp can stay on heat+bias mode as long as you want before hitting the B+ switch. In theory after the MV tubes are settled, I should be able to leave the B+ switch "on" and then just hit the heat+bias switch and the amp will take care of itself, but I've not tried that yet. I am using Rod Coleman's DHT heater and also the SSHV1 for the 4-65a cathode supply. I used shielded coax for signal throughout the amp, and much work to have the d3a or other high mu pentode strapped triode as behaved as possible. It's a CRCLC into a LM317 in CCS mode, which is floated around 55v from the SSHV supply, and ceramic caps right at the heater pins shunting any hf noise to ground. You can see that the topology is a little different in that the cascode mosfets are not folded. At least in simulation, this appeared to look better to me in terms of headroom and distribution of harmonics (lower in 3rd and higher in 2nd). I actually used two 1n60 mosfets, and dn2540 was used only for the CCS. I am not that knowledgable in the SS department, so it's quite possible my choices are less than ideal in the mu-follower. So far I can say that the music is sounding excellent, and the amp is extremely quiet. I did test and scope about every section of the amp, except for B+, before assembling it into this huge two chassis affair. I have multi taps for B+ at 650, 750 and 850v, currently running at 650v. I can certainly say that anyone using Eimac tubes would be wise to get the plate cherry red, because it does sound dull and awful when the plate is dull and grey! I'll have the schematic soon, and get the other channel finished a little down the road.

Cheers,
-MW
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Old 20th August 2012, 01:57 AM   #110
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Quote:
Originally Posted by eboz View Post
Finally I have one channel running. I had a bit of trouble with B+ coming on without blowing its fuse. I'm not sure what it was, because I double checked my wiring and then fired it again and it was okay. It's my first time using MV tubes tho, so perhaps I didn't give them enough time to warm up before the first B+. I also have NTC's on the B+ primary, which is bypassed by relay some 3s later. Just running both the input and output tubes on lower bias for now to get used to it. I'm gonna take a break on this project for a bit before finishing the other channel, so it'll be a few weeks to a month before I have stereo. I haven't noticed any wandering on the bias however, but my bias setup is a little different than Michael K's. The input tube is mixed between battery bias on the grid and cathode bias. Actually I have a DIL type chip connector that allows for different parts to connect between cathode and ground. Right now running a 1.7v AA battery and a 33ohm riken.

Click the image to open in full size.

-MW
Woot!

I can't wait to see the final anode wiring.

I pinned the photo of your amp on my Vacuum Tube pinboard (Pinterest)

Cheers!
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