Hi @euro21
Thanks on the input.
I will make a “high level” schematic with current voltage references, perhaps it will be a tad bit easier for me for be on the same page. On the HT PSU i have got the possibility to go up the voltage up to ~500VDC, choke filter, howewer, currently have a 200mA limit.
Its 15mA for 10Y at 265V (1k Cathode resistor) and ~70mA at 380V for 300B, its biased at -76V from the bias supply and -80V that was comming out from Source follower to 300B though 100R resistor.
Thanks on the input.
I will make a “high level” schematic with current voltage references, perhaps it will be a tad bit easier for me for be on the same page. On the HT PSU i have got the possibility to go up the voltage up to ~500VDC, choke filter, howewer, currently have a 200mA limit.
Its 15mA for 10Y at 265V (1k Cathode resistor) and ~70mA at 380V for 300B, its biased at -76V from the bias supply and -80V that was comming out from Source follower to 300B though 100R resistor.
Hello,
I took the liberty to mock up a schematic using Ale's existing drawings. During the limited time for testing:
1) Substantial hum developed, I have shorted the input - to ground to eliminate that, yet need to investingate further.
2) VT-25 Rk is bypassed with a 120uF DC link, not sure if that's 100% needed.
3) Voltages for source follower are bit of from planned +22V and -214 instead of +25 and -200V respectively
4) Source follower LED is only dimly lit. It lights up brightly for a moment during HT+ start-up. Is that an indicator that I am barely reaching required 15mA?
Based on @euro21 , I need at least 395V on the Gyrator B+ (Currently 380V), to ensure 200Vpp swing (265V for the VT-25 +30V Gyrator headroom +(200 Vpp swing/2)
To increase that, I need to switch the tap on HT transformer, to go from 400V RMS to 475V RMS, yet then I need to limit the current though 300B and total current of power supply (Chokes are 200mA). Alternatively, I can try to modify the HT supply (remove one out of the two chokes/replace it with resistor ) to see where the operating HT voltage lands. in the HT supply I am using a 5u4G and a pair of C4D05120A
All in all:
Trying @euro21 modification on the Rk for VT-25 is easy, a pair of resistors and electrolytic cap, Current should be just borderline OK for the HT PSU.
Increasing the voltage on the HT PSU overall is a bit of a challenge, as have no variac to safely test. What could be the options? I am thinking of:
Cranking the 300B bias way down to be safe on the start-up,
Putting in a 5k Output transformer that I have,
Buying an array of 25W resistors (200-500R) to test,
Placing a 100mA slow-blow fuse in line with 300B 1R Rk,
Any advices to keep me (and the gear) safe are appreciated, thanks.
I took the liberty to mock up a schematic using Ale's existing drawings. During the limited time for testing:
1) Substantial hum developed, I have shorted the input - to ground to eliminate that, yet need to investingate further.
2) VT-25 Rk is bypassed with a 120uF DC link, not sure if that's 100% needed.
3) Voltages for source follower are bit of from planned +22V and -214 instead of +25 and -200V respectively
4) Source follower LED is only dimly lit. It lights up brightly for a moment during HT+ start-up. Is that an indicator that I am barely reaching required 15mA?
Based on @euro21 , I need at least 395V on the Gyrator B+ (Currently 380V), to ensure 200Vpp swing (265V for the VT-25 +30V Gyrator headroom +(200 Vpp swing/2)
To increase that, I need to switch the tap on HT transformer, to go from 400V RMS to 475V RMS, yet then I need to limit the current though 300B and total current of power supply (Chokes are 200mA). Alternatively, I can try to modify the HT supply (remove one out of the two chokes/replace it with resistor ) to see where the operating HT voltage lands. in the HT supply I am using a 5u4G and a pair of C4D05120A
All in all:
Trying @euro21 modification on the Rk for VT-25 is easy, a pair of resistors and electrolytic cap, Current should be just borderline OK for the HT PSU.
Increasing the voltage on the HT PSU overall is a bit of a challenge, as have no variac to safely test. What could be the options? I am thinking of:
Cranking the 300B bias way down to be safe on the start-up,
Putting in a 5k Output transformer that I have,
Buying an array of 25W resistors (200-500R) to test,
Placing a 100mA slow-blow fuse in line with 300B 1R Rk,
Any advices to keep me (and the gear) safe are appreciated, thanks.
If Ale is running B+ @ 380V with identical tubes, surely it's fine? Given that he always includes measurements from his builds, it must have been satisfactory. Are you really going to go through all the trouble of switching to higher voltage taps just for additional 15V B+ and to correct very minor voltage deviation on the source follower?!?
If the hum goes away when you short input to ground, doesn't that explain the issue? The hum either originates from the open input (this is common) or from the source. Have you tried connecting a source to see if that eliminates the hum?
The capacitor values in the PSU are barely sufficient for the 400V taps--I would not switch to 475V without finding capacitors with much higher voltage ratings (or using a couple in series).
Determining LED brightness can be a little bit tricky depending on the form factor and the angle from which you are viewing. It should be trivial to determine current draw of the follower if you have implemented this thing already? Are you not familiar with Ohm's law? Measure the voltage drop across R4 and you should be able to determine current draw.
I've followed Ale's work for a while and he obviously knows his stuff. I'm surprised by the specification of 200mA from the transformer and the 180mA total current draw of the amp--surely that means 200mA DC instead of 200mA AC power rating? So the transformer is rated to something at least 1.5x 200mA, preferably 2x, right?
If the hum goes away when you short input to ground, doesn't that explain the issue? The hum either originates from the open input (this is common) or from the source. Have you tried connecting a source to see if that eliminates the hum?
The capacitor values in the PSU are barely sufficient for the 400V taps--I would not switch to 475V without finding capacitors with much higher voltage ratings (or using a couple in series).
Determining LED brightness can be a little bit tricky depending on the form factor and the angle from which you are viewing. It should be trivial to determine current draw of the follower if you have implemented this thing already? Are you not familiar with Ohm's law? Measure the voltage drop across R4 and you should be able to determine current draw.
I've followed Ale's work for a while and he obviously knows his stuff. I'm surprised by the specification of 200mA from the transformer and the 180mA total current draw of the amp--surely that means 200mA DC instead of 200mA AC power rating? So the transformer is rated to something at least 1.5x 200mA, preferably 2x, right?
Thanks on the contribution,
Ale’s VT-25 implementation suggests 400V on the B+ Gyrator, seems right.
Good tip on the Cap rating - they are barely acceptable even now. Will rework if making a modification.
Trafo rating on the current is high, I have got 500mA i am just woried on the choke rating, which is 200mA.
Ale’s VT-25 implementation suggests 400V on the B+ Gyrator, seems right.
Good tip on the Cap rating - they are barely acceptable even now. Will rework if making a modification.
Trafo rating on the current is high, I have got 500mA i am just woried on the choke rating, which is 200mA.
Transformer is definitely up to the task then!
I wouldn't worry about damaging the chokes. My understanding is that if you exceed the current rating, you will see effective Henries drop -- i.e., poor, if any, ripple rejection. They will just pass whatever comes through without doing much to filter.
A little hard to estimate exactly what will happen with increased voltage, but I don't think you will see skyrocketing current draw. I recently converted a 2A3 amp to use SS instead of tube rectification and without ANY other changes in the circuit, the current draw on one of the 2A3s went from ~63 to ~75 mA. (Switched to JJ 2A3-40). That was with about 80 additional volts on the plate.
I bought a variac on eBay from a Chinese seller. Bright red thing. Cost about $50. Works very well and it has definitely proved handy for exactly the situations you are describing. I can't offer any help there but someone here likely could.
I do think you are probably fine with the voltages at play but the suggestion to email Ale is a very good one.
I wouldn't worry about damaging the chokes. My understanding is that if you exceed the current rating, you will see effective Henries drop -- i.e., poor, if any, ripple rejection. They will just pass whatever comes through without doing much to filter.
A little hard to estimate exactly what will happen with increased voltage, but I don't think you will see skyrocketing current draw. I recently converted a 2A3 amp to use SS instead of tube rectification and without ANY other changes in the circuit, the current draw on one of the 2A3s went from ~63 to ~75 mA. (Switched to JJ 2A3-40). That was with about 80 additional volts on the plate.
I bought a variac on eBay from a Chinese seller. Bright red thing. Cost about $50. Works very well and it has definitely proved handy for exactly the situations you are describing. I can't offer any help there but someone here likely could.
I do think you are probably fine with the voltages at play but the suggestion to email Ale is a very good one.
I use my (CCS loaded D3a, SF, 300B, 5k:8) amp from 392..394V B+ for a decade. No runaway, no failure.
The operating point of 300B is 385V, 70mA.
If you use solid metal plate 300B (my is EH300 gold grid), it's safe operating point.
I suggest to use few hundred kOhm resistor from 300B grid (practically SF out) to -214V. At startup (or SF failure) it's closing the power tube.
100mA fuse in cathode is good idea.
If you want to raise B+, try to use larger first cap (in accordance with the rectifier datasheet). I use 5U4GB with 4.7uF first capacitor (then 33R-47uF-10H-47uf//15uF).
p.s
1.75A is too much for 10Y. Is it a typo?
p.s. 2
If you use 15H 200mA choke with -significantly- larger current, the inductance will be decreasing (and the dissipation increasing). If the current slightly exceeds 200mA, practically nothing happens.
sample: 5H 100mA choke at different current.
p.s. 3
I would never use 450V capacitors at -near- 400V loaded voltage! At startup, when power tubes are cold, the voltage even exceed 480-490V!
The operating point of 300B is 385V, 70mA.
If you use solid metal plate 300B (my is EH300 gold grid), it's safe operating point.
I suggest to use few hundred kOhm resistor from 300B grid (practically SF out) to -214V. At startup (or SF failure) it's closing the power tube.
100mA fuse in cathode is good idea.
If you want to raise B+, try to use larger first cap (in accordance with the rectifier datasheet). I use 5U4GB with 4.7uF first capacitor (then 33R-47uF-10H-47uf//15uF).
p.s
1.75A is too much for 10Y. Is it a typo?
p.s. 2
If you use 15H 200mA choke with -significantly- larger current, the inductance will be decreasing (and the dissipation increasing). If the current slightly exceeds 200mA, practically nothing happens.
sample: 5H 100mA choke at different current.
p.s. 3
I would never use 450V capacitors at -near- 400V loaded voltage! At startup, when power tubes are cold, the voltage even exceed 480-490V!
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Noted.
Noted. Now 2uF, I believe I have some 5uF DC-links, questions is just of the PCB lead spacing.
Yes, 1.2A as far I can remember.
Fixed the Typo on the schematic. Will be able to do more testing only next weekend, thanks all for the contribution.
Still confused with the SF voltage. I have read Ale's instruction document but kinda lost. Can you explain how you come up with -214v and +22v.
If you use 370V, 70mA operating point, the bias voltage is -76 .. -80V (depends of tube manufacturer), suppose that -80V.
The fix biased A1 limit is at 0V DC grid voltage, in this case the grid swing is 160Vpp.
Then -this type- source follower need circa 20V headroom at both swing end, so +20V and -180V requiring.
If you want use 300B in A2, +22V is insufficient.
For A2 capable 300B amp I usually use FET/SiC FET source followers CCS or resistor loaded to -220V, and much larger positive supply: +220 ...400V.
The fix biased A1 limit is at 0V DC grid voltage, in this case the grid swing is 160Vpp.
Then -this type- source follower need circa 20V headroom at both swing end, so +20V and -180V requiring.
If you want use 300B in A2, +22V is insufficient.
For A2 capable 300B amp I usually use FET/SiC FET source followers CCS or resistor loaded to -220V, and much larger positive supply: +220 ...400V.
My setup at the moment is 389V and -82V fixed bias, If I want to add SF, what is suitable voltages?
I saw Ale 300B versions have -200V and +25V so I thought it would work on mine as well.
I saw Ale 300B versions have -200V and +25V so I thought it would work on mine as well.
Try to increase positive supply.
In my practice the higher - at least +100V - voltage gives lower distortion near/over A1, mostly in HF region.
In my practice the higher - at least +100V - voltage gives lower distortion near/over A1, mostly in HF region.
Rod, while reading post 32 in this thread I realized that you are now in V5 of your fixed bias reg. I have V2 w/ 300b and 45, could you explain what have been the changes up to the newest version and if you consider it worth to upgrade (like with your v9 heater regs)?
The V5 bias regulator kits are upgraded in all the measurable parameters:
Example: Bias V5.1 for 300B at -80V; 39kΩ grid bias resistor, typical performance:
Noise 20Hz .. 20kHz <10µV rms typical
DC stability: 0.1V typical, 25 .. 50 °C
measured harmonics: -147dB [ie: ca. 50nV] for 78V pp drive signal;
It's a true negative regulator - only the ground is shared between input and output, and this means 50/60Hz breakthrough is more controlled, compared to positive regulators preseed into negative bias service - no spurs visible at 50/60 or 100/120Hz or any harmonics.
Bias V5.1 .. select Docs to see the manual.
I haven't had any reports of direct comparisons with the V2 regulator kits, so it wouldn't be right for me to make claims about the sound, but constructors like it enough to make repeat-orders.
Example: Bias V5.1 for 300B at -80V; 39kΩ grid bias resistor, typical performance:
Noise 20Hz .. 20kHz <10µV rms typical
DC stability: 0.1V typical, 25 .. 50 °C
measured harmonics: -147dB [ie: ca. 50nV] for 78V pp drive signal;
It's a true negative regulator - only the ground is shared between input and output, and this means 50/60Hz breakthrough is more controlled, compared to positive regulators preseed into negative bias service - no spurs visible at 50/60 or 100/120Hz or any harmonics.
Bias V5.1 .. select Docs to see the manual.
I haven't had any reports of direct comparisons with the V2 regulator kits, so it wouldn't be right for me to make claims about the sound, but constructors like it enough to make repeat-orders.
It took way longer than I expected to go back to this project.
Update:
Increased the B+ to 405V,
VT-25 is now running 265 on the Anode and 510R/330uF Autobias, ~25mA per tube.
Switched the output transformers to Monolith Magnetics SX-11 (5k/8)
Not my 300B's (KR AUDIO ELECTRONICS) are running +390V on the Anode and -81V Fixed bias, which should be. ok according to the load line, as far as I understand (attached). I have tried to measure the voltage drop across the 1R Cathode resistor, yet I am getting confused readings, from 25mV (right) to 95mV (left). Can it be just my multimeter being inaccurate? Both readings are way of the scale..
https://www.tubeampdoctor.com/media/pdf/c3/05/b4/kr300b.pdf
Update:
Increased the B+ to 405V,
VT-25 is now running 265 on the Anode and 510R/330uF Autobias, ~25mA per tube.
Switched the output transformers to Monolith Magnetics SX-11 (5k/8)
Not my 300B's (KR AUDIO ELECTRONICS) are running +390V on the Anode and -81V Fixed bias, which should be. ok according to the load line, as far as I understand (attached). I have tried to measure the voltage drop across the 1R Cathode resistor, yet I am getting confused readings, from 25mV (right) to 95mV (left). Can it be just my multimeter being inaccurate? Both readings are way of the scale..
https://www.tubeampdoctor.com/media/pdf/c3/05/b4/kr300b.pdf