Hi, I've been building a stereo 6p15p amp based on the "zen triode" topology, but with a power supply based around a "mystery transformer" (which was in a previous life powering a mono push-pull amp based around 6CM6, rectified by a 6AX5) being my main constraint.
W/150 ohm cathode resistor, It measures 290v plate-to-cathode @ 30ma which is roughly 8.7w...
I want to get a couple more watts of dissipation.
using one of those "online calculators" and I conservatively estimated that 100 ohms will get me there.
However, when I changed it to 100 ohm, the volts have dropped to 266v plate-to-cathode @ current increase of 36ma which is roughly only 9.6w...
Before I get more aggressive with the resistance-dropping, could my power supply be my limiting factor here? what am I not considering? do I just go ahead and try a lower resistance? Or should I ditch the 6AX5 tube rectifier for solid state? Should I try more capacitor in the power supply ? (within the limits of the rectifier of course)
W/150 ohm cathode resistor, It measures 290v plate-to-cathode @ 30ma which is roughly 8.7w...
I want to get a couple more watts of dissipation.
using one of those "online calculators" and I conservatively estimated that 100 ohms will get me there.
However, when I changed it to 100 ohm, the volts have dropped to 266v plate-to-cathode @ current increase of 36ma which is roughly only 9.6w...
Before I get more aggressive with the resistance-dropping, could my power supply be my limiting factor here? what am I not considering? do I just go ahead and try a lower resistance? Or should I ditch the 6AX5 tube rectifier for solid state? Should I try more capacitor in the power supply ? (within the limits of the rectifier of course)
I think the main problem is the power supply. It had to supply about a 100ma for the mono pp 6cm6 . Now it has to supply 2 pp amps with the EL 84 equivalents which means you need a 200ma supply. This should explain the voltage drop.
If you would make Rk still smaller the voltage drop will increase and possibly you'll never reach the 12w anode diss. you're looking for. Plus if you would that will be at a much lower Vb with 2 consequences:
Suppose the pt is able to supply 200ma the voltage drop may be caused by a too small first supply electrolytic and/or, if anode voltages are taken from a resistor or a choke, the dc resistance of either . In that case (improbable I think) you can switch to diodes fe. Get the impression your pt is too light.
As to Rk values. I would stay with the textbook values - 130 ohm common Rk at Vak= 300v. Ia at idle= 36ma is 10,8 w anode dissipation but at full power it becomes 48ma
In case you want 100% class A of your pp amp the common Rk should be 68 ohm but Vak should not exceed 250 v ; each tube now draws slightly more than 60ma (Ia + Ig2) making a total of 240ma for a stereo version and Po=12w x 2
If you would make Rk still smaller the voltage drop will increase and possibly you'll never reach the 12w anode diss. you're looking for. Plus if you would that will be at a much lower Vb with 2 consequences:
- Power output is very dependent on Vb. At Vak 300v it's 17w, at Vak 250v 11w etc. so very little output power remains when Vb goes down further;
- The power transformer will almost surely overheat.
Suppose the pt is able to supply 200ma the voltage drop may be caused by a too small first supply electrolytic and/or, if anode voltages are taken from a resistor or a choke, the dc resistance of either . In that case (improbable I think) you can switch to diodes fe. Get the impression your pt is too light.
As to Rk values. I would stay with the textbook values - 130 ohm common Rk at Vak= 300v. Ia at idle= 36ma is 10,8 w anode dissipation but at full power it becomes 48ma
In case you want 100% class A of your pp amp the common Rk should be 68 ohm but Vak should not exceed 250 v ; each tube now draws slightly more than 60ma (Ia + Ig2) making a total of 240ma for a stereo version and Po=12w x 2
I should have been more specific that a by “zen triode” topology, I meant it’s a SE design, so i’m still a 2x power tube design despite adding a second channel, so I figured that the power requirements would remain similar-ish. I even dropped a tube since I no longer need a phase inverter.
I did just briefly try subbing in the 6AX5 for a couple of SS diodes… The 6p15p was seeing around 430 Vak so I shut it off immediately…. and i think that’s way too high to get to the 300 range with a resistor so I think I need to get a different PT with a lower voltage. Current one is approx 365-0-365 and the design calls for 300-0-300 with SS diodes.
Do you know what B+ voltage and current the mistery transformer supplied while powering the PP 6CM6 pair? Is this a choke input PS? It sounds as if the power transformer runs out of steam when you asked for more current and dropped the output voltage from 290V @60 ma (for the two channels) to only 266 V at 72 ma. The fact that you measure the current transformer as 365-0-365 Vac unloaded, and then only get 290 V after regulation when loaded with 60 ma sounds like it cannot supply enough current.
The easy advice it to get a new power transformer, but there is a chance that with SS rectifiers you will be OK with the mystery transformer. The momentary high voltage of 430v you measured with the SS diodes is probably before the 6P15P tubes warmed up and started conducting. My guess is that it will drop after the tubes start conducting and settle to some 10-20 V higher than the 290 V you got with the 6AX5. If you are brave enough try it again and watch for any red coloring on the plates/screen before turning it off if the supply voltage does not come down. Or find another 6.3 V heater transformer to heat the tubes before applying the high voltage. Typically such turn-on delays are not necessary, but the 6P15P screen can take only 1.5 watt dissipation.
Good luck.
The easy advice it to get a new power transformer, but there is a chance that with SS rectifiers you will be OK with the mystery transformer. The momentary high voltage of 430v you measured with the SS diodes is probably before the 6P15P tubes warmed up and started conducting. My guess is that it will drop after the tubes start conducting and settle to some 10-20 V higher than the 290 V you got with the 6AX5. If you are brave enough try it again and watch for any red coloring on the plates/screen before turning it off if the supply voltage does not come down. Or find another 6.3 V heater transformer to heat the tubes before applying the high voltage. Typically such turn-on delays are not necessary, but the 6P15P screen can take only 1.5 watt dissipation.
Good luck.
I did let the tubes warm up with a 100ohm resistor after the diodes as an attempt to drop it a bit, but even then, I still saw voltages across the tube that were close to 400 even after warm up. So I put it back to the 6AX5 tube rectifier setup. And yes, it’s got a 10H choke. I think it just doesn’t have transformer.. just measured the temperature and it’s pretty toasty at about 50 degrees celsius. so it’s working pretty hard.
This 290-0-290 I pulled out of an old TV might just do the trick for the SS route… Looks like it even have the same bolt pattern! just a bit taller. I’ll save the current one for a project that requires higher voltage.
This 290-0-290 I pulled out of an old TV might just do the trick for the SS route… Looks like it even have the same bolt pattern! just a bit taller. I’ll save the current one for a project that requires higher voltage.
Original condition, 290V/30mA is equivalent 9667 Ohms.
New condition, 266V/36mA is equivalent 7389 Ohms.
6mA rise of current causes 24V of drop. Power supply is like a 4,000 Ohm (!!) source.
At zero current we would expect a rise of 30mA*4K or 120V, making 410V. (Not clear if "430V" is a start transient or if there is more to account for.)
Theorem: If source and load resistances are near-equal, quite large change of either will cause very little change of Power.
The MOST you can do in this system is 10.51 Watts at 4K bias condition (205V plate at 51mA). Note that this is also 10.5 Watts of heat in the supply!! (We usually like the waste heat in the supply to be much lower than the useful heat.) For 10% change of plate dissipation (10.5W to 9.5W) we can swing the load from 2K to 7.8k. 10% change of plate dissipation for 4:1 change of load, the Theorem looks good.
Arbitrarily raising plate dissipation (if possible) does not ensure more audio power or better sound.
However this near-matched condition does have one advantage: you can stick almost any dang thing under that cathode and the tube won't melt. It may sound bad, or over-volt, but 10.51W is all it can get with this supply.
I think you found the right transformer! Good luck. Let us know how you progress and how you like your Zen clone.
Thanks! 🙂
Any idea how many watts the output tubes in a Zen is supposed to dissipate?
I'm tempted to just leave it alone... since dropping the Rk from 150 to 100 ohms seems to have made the thing sound better.
The design objective was to make an "acceptable" amp that was more of a "boho home decor" piece rather than a serious amp. Something I can safely run the lowest common denominator of output tubes I find lying around, like the pair of 6CW5 I found in an estate sale junk box - which is also where I found this chassis originally, comes with a neat tube cage too!
Any idea how many watts the output tubes in a Zen is supposed to dissipate?
I'm tempted to just leave it alone... since dropping the Rk from 150 to 100 ohms seems to have made the thing sound better.
The design objective was to make an "acceptable" amp that was more of a "boho home decor" piece rather than a serious amp. Something I can safely run the lowest common denominator of output tubes I find lying around, like the pair of 6CW5 I found in an estate sale junk box - which is also where I found this chassis originally, comes with a neat tube cage too!
6P15P tubes are plentiful and inexpensive (before Putin’s invasion of Ukraine), so, if it sounds good use it, and replace it when necessary. They should last a long time if you keep the combined plate and screen dissipation under 12 watts.
Remember also that your 6CW5's are completely other tubes. Philips designed them for OTL purposes, or, to say it more precisely, they derived them from the older UL84 pentode and called it EL86.
Best regards!
Best regards!
Well, I just received a couple pairs of 6p15p’s in the mail so i probably don’t be sticking anything weird in for a while. that being said, the transformer does seem pretty warm…. close to 60degrees C… Otherise, i’ve put quite a few hours on it now and loving the way it’s sounding with my work-in-progress speakers. Definitely going to leave it alone for now.
I just want to thank everyone here who has provided me great information!
I just want to thank everyone here who has provided me great information!