Had another look at it last night. With no load on the boost converter I'm able to set the output to 300v. As soon as I power the circuit, the voltage readings are unstable and jump around and the adjustment pot becomes unresponsive.
Measuring the voltage at the valve pins gives changeable readings. I think I might just give up on the boost converter all together. It's a pity though, I like the idea of it as they're small (and cheap!). I'm wondering if I could test the boost converter by putting another load of some sort on it and seeing how it behaves to rule out something else in the amp circuit causing the erratic output under load?
Measuring the voltage at the valve pins gives changeable readings. I think I might just give up on the boost converter all together. It's a pity though, I like the idea of it as they're small (and cheap!). I'm wondering if I could test the boost converter by putting another load of some sort on it and seeing how it behaves to rule out something else in the amp circuit causing the erratic output under load?
Only had some 2k2 10w resistors... wired them in series which measured just over 9k on my multimeter. disconnected the amp circuit and connected the resistors between +/- output on the boost converter and measured the voltage. Still all over the place. was reading 400+v, the trim pot does nothing to change the voltage output. Think its defiantly the boost converter now.
I have a mains transformer which would be ok for the HT but its heater winding is only wound to give 6.3v. I have a load of these TV tubes which need 13v for the heaters. I'm wondering if I could use this transformer with a diode rectifier after the heater winding to give me DC and then use am LM317 to step that up to the required heater voltage? I'm guessing even if that was an option, it would be restricted to 1 valve rather than a stereo setup? Transformers configured like that seem plentiful on ebay, probably salvaged from old mono amps. Its hard to find something that gives the required HT and the 13v needed for the TV valves I have. I have quite a few PCL86 and 82's so ideally I'd like to put them to good use rather than buying triode/pentode valves with 6,3v heaters instead.
https://www.amazon.co.uk/gp/product/B08R357JWD/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Ordered one of these to try, only a tenner so not the end of the world if it doesnt work . Specs are:
Looking at the specs for a PCL86,
Pentode 41.0 mAs (guessing that means Anode mA?) , Triode 1.2 mAs. so, total load of 42.2mA??
Worth a shot even if I need one per valve. Takes up a lot less space on the chassis than a MT and more flexible perhaps?
Ordered one of these to try, only a tenner so not the end of the world if it doesnt work . Specs are:
- This module is used on security products, stability is fairly good.
- Output current: 1~20mA.( If you want to add current, pls put insurance below the transformer replaced the 0-ohm resistor, maximum 50mA, but does not recommend increased.)
- Output Power: 5 W
- Input Voltage: DC 5V-12V
- Size: 6.4x3.1cm/2.52x1.22""
Looking at the specs for a PCL86,
Pentode 41.0 mAs (guessing that means Anode mA?) , Triode 1.2 mAs. so, total load of 42.2mA??
Not entirely sure what that means....
Worth a shot even if I need one per valve. Takes up a lot less space on the chassis than a MT and more flexible perhaps?
Each PCL86 heater requires 15v at 300mA, but the cold load is higher i believe, so I think your boost converter will not manage that.
Edit: I thought it was for the heater.
Why don't you look for some secondhand toroids? A doubler or quadrupler with 60VDC, for example, and another for the heaters. They are often cheap on my local auction site.
Edit: I thought it was for the heater.
Why don't you look for some secondhand toroids? A doubler or quadrupler with 60VDC, for example, and another for the heaters. They are often cheap on my local auction site.
I thought you had a separate setup to power the heaters? If you use your HV tfmr to knock up a quick HT supply, say a bridge rectifier & those two big caps you have plus a wallwart to power the heaters that should get the circuit going and give you a sanity check.
It is possible if the trmr has a big enough power rating to wind on a few 10's of turns to get your heater voltage either by pulling off the 6v winding or just winding a few turns on top of it if there is the space. I can send you some wire if you need it.
If not I might have a 15/16v wallwart in my shed, I'll have a look tomorrow.
It is possible if the trmr has a big enough power rating to wind on a few 10's of turns to get your heater voltage either by pulling off the 6v winding or just winding a few turns on top of it if there is the space. I can send you some wire if you need it.
If not I might have a 15/16v wallwart in my shed, I'll have a look tomorrow.
Yeah, I have been powering the heaters from the 20v laptop power brick, stepped down with the LM317. I was just thinking it would be nice to run the whole thing from the MT I have if possible.
I was thinking the same thing as you, for the time being, to continue run the heaters from the 20v power brick/LM317 and try and use the MT for the HT. 1 valve in mono just to play around with it and get some sound out of it. Which, I'd pretty chuffed with as a start 😁.
I was thinking the same thing as you, for the time being, to continue run the heaters from the 20v power brick/LM317 and try and use the MT for the HT. 1 valve in mono just to play around with it and get some sound out of it. Which, I'd pretty chuffed with as a start 😁.
If I wanted to work out the resistive load on HT for a PCl86 (for example in comparing specs on a boost converter to the valves specs to work out if it was suitable).....looking at this:
Pentode:
mAa = milliamps Anode? (41.0)
mAs = milliamps screen? (10.5)
Ah = Heater amps?
Triode:
Ah = Heater amps?
mAa = milliamps Anode? (1.2)
So... If that's right, (ignoring the heater loads as they're powered separately), I'd add all them values together to get 52.7ma? So, I'd need a boost converter capable of at least 52.7ma?
The specs from the one I've used are:
Input Current: 5A; Quiescent Current: 15mA; Output Voltage: ±45-390V; Output Current: Maximum 0.2A; Output Power: 40W
0.2 Amp = 200 Milliamp.....
So..... it should have been well up to the task?
Pentode:
mAa = milliamps Anode? (41.0)
mAs = milliamps screen? (10.5)
Ah = Heater amps?
Triode:
Ah = Heater amps?
mAa = milliamps Anode? (1.2)
So... If that's right, (ignoring the heater loads as they're powered separately), I'd add all them values together to get 52.7ma? So, I'd need a boost converter capable of at least 52.7ma?
The specs from the one I've used are:
Input Current: 5A; Quiescent Current: 15mA; Output Voltage: ±45-390V; Output Current: Maximum 0.2A; Output Power: 40W
0.2 Amp = 200 Milliamp.....
So..... it should have been well up to the task?
You have just enough current available from the boost module to be able to bias up the triode and pentode halves of the tube. When you start to drive the input of the amp to power a speaker load, It'll be a whole different story, and likely you will overload the boost module. A power supply using a decent mains transformer would likely be a better solution for your needs.
Yes, just as a sanity check to confirm the amp circuit is working and to give yourself a bit of of a boost. When designing & building valve amps you hit problems and it's easy to get dispirited, a nice sinewave or some music out of the speaker does wonders to cheer you up.
Should have, but didn't by the sound of things. If it can't handle a 9k resistive load it's either faulty or a pile of doggy chocolates.
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Right. So usibg the MSI s transformer, after bridge rectification I'm getting about 330v DC (offload). I need a to get this down obviously.
Looking at the table for the PCL82, I need 230 if PCl82 (or 230 for the 86).
Tube resistance is 18k,
I'm Reckoning I need something around an 18k resistor to bring the voltage down to 170?. Can anyone give me any pointers on the best way to do this? And also how to work out the smoothing capacitor (s)?
Wood the two to capacitors I already had after the boost converter, wired in parallel with the 390 resistor work ok or does this need re designing?
Looking at the table for the PCL82, I need 230 if PCl82 (or 230 for the 86).
Tube resistance is 18k,
I'm Reckoning I need something around an 18k resistor to bring the voltage down to 170?. Can anyone give me any pointers on the best way to do this? And also how to work out the smoothing capacitor (s)?
Wood the two to capacitors I already had after the boost converter, wired in parallel with the 390 resistor work ok or does this need re designing?
You will have to iterate. First you have to determine the loaded voltage supply. Youknow the current consumed by the tubes, you mentioned it above. You know the voltage to be dissipated - 330V. I (from your calculation) = V (your power supply)/R (the load). Try different values till you are consuming that current in the load, then you can calculate the resistance you wil need to act as a dropper by calculating the resistance dividor for the circuit. E.g. if you used 100K to set the load, and the voltage was still 300V, then splitting the resistor into 33k and 67K would drop 100V and leave 200V. (Ifd the circuit required 200V then it would replace the 67K resistor).
The snag is that the current draw is not consistent unless you have a class A amplifier, so you will get lots of voltage 'sag' using this approach on dynamic music. But it gets you going.
I think the problem with your booster philosophy is that you are buying too cheap units. They are used a lot I have seen here, but they are beefier devices.
The snag is that the current draw is not consistent unless you have a class A amplifier, so you will get lots of voltage 'sag' using this approach on dynamic music. But it gets you going.
I think the problem with your booster philosophy is that you are buying too cheap units. They are used a lot I have seen here, but they are beefier devices.
I haven't had much time until now to look at this again. I'm not quite able to follow what you mean here but it seems like really good information.
Can you explain that again. Particularly what you mean by try different values. What's the equation used here?
"Try different values till you are consuming that current in the load". That's what I can't get my head around
Thanks
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If you have a good idea about the total operating current of your two tube stages, you can directly get the needed dropper resistor value by just using Ohm's law. R dropper = delta V/I bias. You will need a good storage cap on the downstream side of your dropper resistor for decent regulation for the pulsating audio load on the output stage.
Current draw is about 43mA per channel, HT is 320v approx, Vmax is 250v so you'd need a 1k6 for one channel or 820r for the two. This is a bit much for one RC filter, you may be better using an RCRC filter and splitting the dropping resistor in two, say two 330r's and put up with a bit higher HT. Then & again as the PCL82 is a tough little valve designed for use in TV's with high transient voltages you may just decide to run them as is, especially if you have a few spare.
Well it's built and working. I'm very presently surprised how good it sounds. Way surpassed my expectations. Got a a bit of hum to sort out. But the sound is great. Driving some Castle Clifton bookshelf speakers at the moment and sounds lovely.
One odd thing is after say 10, 15 minutes, the output volume decreases to nothing. After leaving it off for a few minutes it comes back on.
In still using the laptop power brick for the heater 16 volts (stepped down with buck converter).
I'm using a mains transformer diode rectification for HT. The mains transformer doesn't seem to be under much stress. It's getting warm but I can hold my hand against it.
Very happy to have got this far!
One odd thing is after say 10, 15 minutes, the output volume decreases to nothing. After leaving it off for a few minutes it comes back on.
In still using the laptop power brick for the heater 16 volts (stepped down with buck converter).
I'm using a mains transformer diode rectification for HT. The mains transformer doesn't seem to be under much stress. It's getting warm but I can hold my hand against it.
Very happy to have got this far!