I will ping you an email. I'm in Madrid this week and travelling for quite a while. Will work out a Saturday date if that works for you...
The gyrator seems to be working as at least you can dial the voltage. What you're not clear of is whether you get the same results with different valves. Have you tried other valves? Bear in mind these are very old DHTs and you need to be sure you;'re using a healthy 01a. Normally you would expect lower emission (in particular if filaments are starved) however your results are unusual. I have found though some CX112a which measured about 150% to 200%, way away from spec. But that is rare. I will replace the valve with a simple 39K resistor and see if you can dial the 3mA at 115V (which will reflect a functional gyrator). Then I will test with other valves.
Without pictures of your actual build is hard to say if you've done anything wrong. Posting the original design doesn't really tell much of your wiring of the Rod Coleman regulators, etc. Also measuring voltages across all parts would help.
Apologies for brief reply, I'm away from home travelling on business
cheers Ale
I did some measuring on my two 01A Gen 2 preamps, and it is indeed the tubes. That's why the anode voltages vary so much. With different tubes I'm getting anything from 102v to 180v on the anodes, and everything in between.
However, the current remains constant at 3mA in all cases, and all the other voltages in the gyrator and also the bias voltage of 4.1v seem correct. It's just the anode voltage that varies. I've no idea why, but at least you can control it by swapping in different tubes until you get what you need. We obviously need to know more here, and Ale is surely the guy to explain!
Gyrator is working perfect! I installed 39K resistor and set to 3ma, yield is about 115v like you said. It has to be something with the coleman regulator.
I connected a plate choke and got the same results, about 65v and 3ma so its not the gyrator.
Edit: Ok, I'm officially a dumb ****.... I had the ground wire on the wrong side of filament resistor. I feel like an idiot! I guess sometimes you look beyond the obvious, just wasn't familiar with filament bias. Ground is opposite of cathode bias with respects how the resistor and filament are connected. I usually never breadboard and better at building since I highlight the connections on the schematic as I build it up.
Problem solved!
I connected a plate choke and got the same results, about 65v and 3ma so its not the gyrator.
Edit: Ok, I'm officially a dumb ****.... I had the ground wire on the wrong side of filament resistor. I feel like an idiot! I guess sometimes you look beyond the obvious, just wasn't familiar with filament bias. Ground is opposite of cathode bias with respects how the resistor and filament are connected. I usually never breadboard and better at building since I highlight the connections on the schematic as I build it up.
Problem solved!
I don't quite know what you're describing here, but the Coleman regs can be connected either way round - there's some information somewhere about connecting the plus output to ground and the negative to one side of the filament, with the other side going to ground via the cathode resistor. Apparently it even sounds a trifle better that way. I haven't tried it.
I still think the variations in anode voltage are caused by the tubes themselves. Try out a bunch and see if the anode voltages vary. I just did this and yes, the anode voltage varies with the tube.
I still think the variations in anode voltage are caused by the tubes themselves. Try out a bunch and see if the anode voltages vary. I just did this and yes, the anode voltage varies with the tube.
My B+ is a rock solid 225v using a Maida regulator, R7 is 1.5K
Anode is rock solid
My B+ is 200v. R7 is 1.5K. Pretty close. I just tested a bunch of 01A and while half were around 120v, the other half were 180v to 200v. That's clearly out of range. If I use those with the gyrator it would need some different resistor values. The ST tubes were mostly fine - it was the globes that varied a lot. But there was quite a bit of variation overall, both ST and globe. Lowest anode voltage was 58, which was a freak since all the rest were over 100v. Highest was 209v. I even wondered if I needed to clean the pins on some. Haven't got round to studying how you can "revive" thoriated tubes with over voltage ....
Hmmmm.....
You have to keep in mind these are very old valves, and when you are starving the filaments the following takes place:
1. Emission is reduced, see the curves at the top of the diagram belo
2. Anode resistance is increased
3. Transconductance is reduced
4. Mu is kept the same
you are fixing both anode to cathode voltage with the gyrator and you're pretty much fixing the grid to cathode voltage due to the filament bias as the anode current feedback is minor in the filament bias arrangement. Hence you should expect different biasing points with different valves if their emission is degraded (even before applying starvation to the filaments). Only way is to readjust the gyrator voltage to dial the desired current
Only caveat is though, that you need to do the following 2 tests to make sure the old lady is performing as expected:
1. Output swing test. If the valve is close to the anode saturation due to poor emission, this will be visible as high distortion and compressed output.
2. Frequency response test: this is key. I found valves with some internal leakage that seems to be ok at DC but then they have an unexpected dip at 5 or 10kHz when the treble should go up to 50-100kHz with the gyrator depending on the load.
3. Listening test, never fails!
Hope this helps?
Ale
1. Emission is reduced, see the curves at the top of the diagram belo
2. Anode resistance is increased
3. Transconductance is reduced
4. Mu is kept the same
you are fixing both anode to cathode voltage with the gyrator and you're pretty much fixing the grid to cathode voltage due to the filament bias as the anode current feedback is minor in the filament bias arrangement. Hence you should expect different biasing points with different valves if their emission is degraded (even before applying starvation to the filaments). Only way is to readjust the gyrator voltage to dial the desired current
Only caveat is though, that you need to do the following 2 tests to make sure the old lady is performing as expected:
1. Output swing test. If the valve is close to the anode saturation due to poor emission, this will be visible as high distortion and compressed output.
2. Frequency response test: this is key. I found valves with some internal leakage that seems to be ok at DC but then they have an unexpected dip at 5 or 10kHz when the treble should go up to 50-100kHz with the gyrator depending on the load.
3. Listening test, never fails!
Hope this helps?
Ale
#5 - max emission is reduced significantly, and exceeding this rating damages the tube.
Hi Ale
I guess this could happen to other builders than myself, since I have quite a few old 01As that are measuring 200v on the anode. As I said, the other voltages are all fine - just the anode.
So what do you suggest for resistors R4 and R7 (R mu)? Can these values be changed to accommodate these 200v tubes? Or indeed other resistor values?
And do you recommend using the standard 250mA at 5v, not starving them?
I hope some combination of all this could result in some modifications so I can use these tubes with the gyrator. There's always interstages, but I'd like to use the gyrator.
Andy
I guess this could happen to other builders than myself, since I have quite a few old 01As that are measuring 200v on the anode. As I said, the other voltages are all fine - just the anode.
So what do you suggest for resistors R4 and R7 (R mu)? Can these values be changed to accommodate these 200v tubes? Or indeed other resistor values?
And do you recommend using the standard 250mA at 5v, not starving them?
I hope some combination of all this could result in some modifications so I can use these tubes with the gyrator. There's always interstages, but I'd like to use the gyrator.
Andy
Its hard to tell until I get this thing built in a chassis and do some real listening. I like it so far, lots of gain, probably a little too much for my amp, I wish I could turn down the MU a couple notches. The breadboard tube was very microphonic, more so than my 26 (RC coupled) in my 300b amp, but that could of been the tube. I bread boarded on the carpet floor and could hear the tube ringing just walking across the carpet. I purchased about a half dozen tubes that were somewhat matched so I'll try them once i get this thing built up. I'm doing some cadd drawings now so I can send to CNC guys, I hate drilling chassis plates... I'll be posting different phases of the build.Glad to hear you fixed it. Now, do you like the sound of the stage?Cheers Ale
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Hi Guys Got 2 questions : 1. Re gyrator board, signal caps are recommeded as 0.1 or 0.22 uf. being of such low value, would it not heard the bass response of the preamp ?
2. Re 01a, read that variations in the tube is great & that would affect the sound of the tube especially the high frequency. If this is true that would mean that we would have to purchase lots of tubes to test, is the correct ? My interest are in the ST versions only.
Thanks.
2. Re 01a, read that variations in the tube is great & that would affect the sound of the tube especially the high frequency. If this is true that would mean that we would have to purchase lots of tubes to test, is the correct ? My interest are in the ST versions only.
Thanks.