Attached is a rough schematic of my old console amp that I use for my computer listening. It actually runs quite well after replacing the Cs and Rs and removing the tone control section (which was a convoluted arrangement in a feedback loop). Note there is no direct feedback loop now . I'm not going to push the envelope too much with this, as the tubes are almost unobtainable now.
The main reason for this message is to see if there are any glaring mistakes in the design (or my alterations to it; I don't have an original schematic).
Note that the triode section gets its anode supply from the pentode screen. Would there be any reason not to take them from the B+ point instead? It would make measuring screen and anode currents easier.
The output stage only runs 32mA anode current by my calculations of drop across the o/p trans. Sounds a bit low to me, but these are old tubes not NOS. I think it only gets to about 2W out into 8ohms before it starts to clip.🙁
Thanks, Gary
The main reason for this message is to see if there are any glaring mistakes in the design (or my alterations to it; I don't have an original schematic).
Note that the triode section gets its anode supply from the pentode screen. Would there be any reason not to take them from the B+ point instead? It would make measuring screen and anode currents easier.
The output stage only runs 32mA anode current by my calculations of drop across the o/p trans. Sounds a bit low to me, but these are old tubes not NOS. I think it only gets to about 2W out into 8ohms before it starts to clip.🙁
Thanks, Gary
Attachments
The only thing I see is you left in the 330 pf cap that rolls of the high end to maintain feedback stability. You do not need it. Just adds to high frequency distortion would be my guess.
Doug
Doug
Yes, I noticed that today when I was drawing this up. I guess I should have removed it as the response is down 3dB at 10kHz (and 100Hz which I blame more on the small o/p trannys)
Gary
Gary
The front end and screen grids both benefit from being isolated from the B+.
You could add a separate RC for the front end if you want.
The 1 Meg pot may be contributing to the HF roll off as well. There is no real reason to use anything bigger than 100K. Having said that, the Dinaco ST 70 had a 250k grid leak because the pentode front end shielded the input from the miller capacitance. If I were a betting man the input triode is high mu, and has a high voltage gain. Sorry, too late at night to look it up. That virtual cap probably interacts with the pot to roll off the highs.
Another thought is to replace the RC on the input with an LED string that gives you about the same voltage drop. It will eliminate an RC time constant cheaply may be better than most caps.
You may be right about the LF roll off being due to the transformer. It's that or the cathode bypass caps.
HTH
Doug
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That's a leftover from a divider network I thought I might need to match the computer output. It really doesn't do anything now.😛
Well, you can send some my way if you like😉
Seriously, the best price I have seen for these tubes NOS is about $18 USD each for an unnamed brand. None at any price available here in New Zealand.
If my tubes give out (I still have a couple of spare used ones), I'll rebuild the amp with ECC82/EL84 as there is room on the chassis.
Gary
The pots were a leftover from the original console, as I didn't have a dual gang one to replace it. Might have to hunt down a lower value one.
I'm still trying to get my head around the LED cathode bias idea, it's something new to me. As the cathode DC point is only about 0.8v on the triode section, can I just stick an LED in there on its own and lose the R & C?
Actually, I measured both channels, and one cathode is 0.8v and one is 1v DC at idle, and the anode voltages are 140v and 113v so I guess one tube is down a bit. Would the LED help correct that?
I trained on radio valve gear for a year, but that was 35 years ago, and haven't done anything since with them until now, so I'm re-learning things from scratch 🙄
Short answer, yes.
Longer answer is that you need to pick a diode with the right voltage drop for your circuit. IIRC, the infer-red LED drop about a volt. you could also use small signal diodes for lower voltage drops. The point is a diode has a low dynamic resistance and a fixed voltage drop and looks much like a RC from AC and DC perspective but much faster overload recovery / less memory and does not have the Dielectric absorption issues of a capacitor. Its a drop in, parts are inexpensive, and its worth a try at some point.
Doug
OK, I removed the 300pF from the anode of the triode and the HF has audibly improved.
I stuck a red LED on the cathode of each triode, but that gives me 1.5v DC. I have no idea what the acceptable range is
However at least the two tubes have the same voltage now and it may have picked up a little at the LF end but it's hard to tell without measuring. I will try and find the 3db points again during the weekend.So, 4 less components and the sound is better. Does this mean that if I take everything out it will be perfect? Just kidding 😀
Gary
Thanks for pointing that out Dave. I actually meant to say ECC81, as I have a number of used ones of those that I can experiment with.
At 1V on the cathode, the current through the tube is about .6 mA. If you up it to 1.5V, current is going to be more like .4 mA , which may be more than a bit lower than optimum.
Different colors LED have different drops. Infrared LED is the lowest at about 1V, blue is the highest at about 3V.
Silicone diodes are .7V, and Schottky diodes are about .3V.
Its just another tool.
HTH
Doug
Is there any particular reason to use LEDs for this? Or would stacking signal diodes accomplish the same thing e.g I can get 1v by stacking a silicon and a germanium that I have in my junkbox.
Anyway, it sounds really nice with the red LED in there 😀
Can I pull the LED trick on the output stage? At the moment 5.8v across the 150 ohm cathode resistor, so just under 40mA. Seems too high for LEDs on a continuous basis.
It's amazing how much of the theory I have forgotten. After 25 years in the electronics servicing industry, and the last 9 years away from it 😕
Gary
Anyway, it sounds really nice with the red LED in there 😀
Can I pull the LED trick on the output stage? At the moment 5.8v across the 150 ohm cathode resistor, so just under 40mA. Seems too high for LEDs on a continuous basis.
It's amazing how much of the theory I have forgotten. After 25 years in the electronics servicing industry, and the last 9 years away from it 😕
Gary
You can indeed use whatever diode you have.
LED are nice because they tend to be quiet.
Glad to hear you are enjoying the red LED. 🙂
The RLD is a PP amp and mine is SE, but I guess the same applies.
Paralleling LEDs is hairy and means some characteristic matching.
Might look at a simple CCS instead, I have a few LM317s here.
Gary
I'll spring for an IR LED I think.
FWIW, I bumped the pentode cathode bypass up from 100uF to 1000uF (overkill I know, but I didn't have anything in between). Certainly brought up the LF end, but I don't know if I like the sound, somewhat "loose". Probably trying to saturate the output trannys now 🙁
However, the no-name 10" woofers I have in these big sealed cabs probably aren't helping in that regard. Tonight I'll try the amp on my Pro9TL's just for kicks 😛
Gary
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