I have just finished assembling Poindexter's 6GK5/EL34 design as a breadboard, and I am struggling to get past the white-knuckle, plug it in the wall and fire it up phase.
I'm using the PS design shown in the attachment, and so far, have powered it up through a variac.
The basic issue is lots of B+ overshoot on power-up that's not reflected in the PSUD model. Upon power-up, with a mains voltage (set on the variac) of around 80VAC or so, the B+ voltage peaks around 450V or so before settling down to the 380/320 values as expected (I crank up the variac to 120VAC after the B+ has settled). This surge lasts a good 30 seconds or so, and I imagine that if I let the PS power up on 120VAC that the B+ would exceed 500V+ for the 20-30 seconds or so. So I'm concerned that I will pop the TS-HA/HB caps with this overvoltage.
Could this be ringing due to the 2 L/C sections? It's not reflected at all in the model.
I also modeled a 5AR4 CT PS, but I appear to be on the ragged edge of the forward current rating of the 5AR4, so I started with the the SS design.
I'm using the PS design shown in the attachment, and so far, have powered it up through a variac.
The basic issue is lots of B+ overshoot on power-up that's not reflected in the PSUD model. Upon power-up, with a mains voltage (set on the variac) of around 80VAC or so, the B+ voltage peaks around 450V or so before settling down to the 380/320 values as expected (I crank up the variac to 120VAC after the B+ has settled). This surge lasts a good 30 seconds or so, and I imagine that if I let the PS power up on 120VAC that the B+ would exceed 500V+ for the 20-30 seconds or so. So I'm concerned that I will pop the TS-HA/HB caps with this overvoltage.
Could this be ringing due to the 2 L/C sections? It's not reflected at all in the model.
I also modeled a 5AR4 CT PS, but I appear to be on the ragged edge of the forward current rating of the 5AR4, so I started with the the SS design.
Thanks dsavitsk! Simple enough, that explains it. Changing both current taps to 1 ma ends up with around 540 volts.
Sounds like heating the tubes before applying B+ will solve my newbie dilemma.
So I suppose that I need a separate filament transformer and a 30 second B+ transformer delay, or delay the B+ with something on the B+ CT while still utilizing the filament taps in my primary transformer.
If I use a 5AR4 design will I still need to use some type of delay? It seems that I will since it will bring up B+ in about 10 sec or so and I need 30 seconds for warm-up.
Sounds like heating the tubes before applying B+ will solve my newbie dilemma.
So I suppose that I need a separate filament transformer and a 30 second B+ transformer delay, or delay the B+ with something on the B+ CT while still utilizing the filament taps in my primary transformer.
If I use a 5AR4 design will I still need to use some type of delay? It seems that I will since it will bring up B+ in about 10 sec or so and I need 30 seconds for warm-up.
boywonder said:
When using a solid state PS for hollow state equipment, this is a good solution to the problem. With a project using a solid state PS, I separated the heater circuit from the HV circuit, and wired in an interlock so that the HV won't come on if the heaters aren't energized. (This not only helps prevent applying HV to cold VTs which will ruin them, it also prevents the possibility of having HV present without the usual indications: glowing heaters, and making sound.) You could automate that if so desired without too much difficulty.
Your best bet is to use a separate heater xfmr. The problem with switching in the HV DC neutral is the high voltage that exists at the switch. This could present a user hazard. Given the capacitors used, you don't need a HV delay.
If you substitute hollow state diodes, you already have your HV delay. Another project I did derived the positive rail from a 5U4GB. This type, being a DH, still provides plenty of HV delay that allows all the VTs (except for the 6BQ6GTB finals -- those enormous cathodes take a good long while to heat up) to warm up before substantial HV appears. This design also included a negative rail derived from SS diodes. The negative rail does soar, but that's OK: you want the bias to appear first. It comes down to the design nominal value once the current demand goes up as the finals heat and start pulling current.
A 5AR4 will give even more HV delay since it's an indirectly heated type.
Thanks Miles. The two issues that I presently have with the 5AR4 design are:
1 Getting to the target voltages without exceeding the forward current of the 5AR4. I cannot seem to get below .75A on the PSUD sims, and I'm assuming that that's the limit. Dual mono's look do-able, but with double the current draw for stereo I'm struggling. If I am interpreting the PSUD error message correctly, I am on the ragged edge for forward current for the 5AR4. Please enlighten me if this is not the case.
2. As mentioned above, don't I still risk overvolting the caps if the rectifier comes up over 10 seconds or so and the output tube filaments take 30 secs? It seems at 10-15 secs I'll have full B+ on semi-cold output tubes overvolting my PS caps.
Attached is the 5AR4 PS. The error message says that the recitfier forward current is .78A
1 Getting to the target voltages without exceeding the forward current of the 5AR4. I cannot seem to get below .75A on the PSUD sims, and I'm assuming that that's the limit. Dual mono's look do-able, but with double the current draw for stereo I'm struggling. If I am interpreting the PSUD error message correctly, I am on the ragged edge for forward current for the 5AR4. Please enlighten me if this is not the case.
2. As mentioned above, don't I still risk overvolting the caps if the rectifier comes up over 10 seconds or so and the output tube filaments take 30 secs? It seems at 10-15 secs I'll have full B+ on semi-cold output tubes overvolting my PS caps.
Attached is the 5AR4 PS. The error message says that the recitfier forward current is .78A
boywonder said:
That is always a risk, and a very good reason to be generous with filter capacitor voltage ratings. When I used a 650Vct power xfmr with a 5U4GB, I made the first resevoir capacitor from two 68uF / 300Vdc units in series, to give the design 34uF total capacitance, and for a generous voltage margin. Since the HV could go to a maximum of 458Vdc, you can see why even though the design nominal voltage was 350Vdc. 450Vdc capacitors just might
Same treatment for the negative rail: 47uF / 300Vdc units in series. Electrolytics are cheap in comparison to what they may take out if they fail. When wiring in series for higher voltage, be sure to parallel each capacitor in the series string with 100K resistors to ensure voltage balance.
Well, for giggles I tried the 5AR4 PS design since I do not have a means presently for providing a separate filament transformer, and doing the human powered delay on the B+ CT is giving me a slight case of the willies.
The design is pretty much the PSUD 5AR4 layout above, with the addition of a pair of 55 ohm Rs in each leg of the transformer secondary. This got my peak forward current just below the .75A limit of 5AR4 and the DC current just under 250ma, at least on paper.
On the actual breadboard, I had an interesting light show inside the recitifier shortly after power up and a B+ voltage of essentially zero, so I guess that's what arc-over looks like. I used a beater ruby 5AR4 for the test.
So now I'm back to the SS design, and I understand why Poindexter shows SS rectification in the stereo PS schem and 5AR4 rectification in the mono PS schem. I am also planning using series connected caps in the PS just for some safety margin.
How does the ST-70 (and other old stereo amps with high B+ and current) successfully utilize a 5AR4 for B+ rectification?
The design is pretty much the PSUD 5AR4 layout above, with the addition of a pair of 55 ohm Rs in each leg of the transformer secondary. This got my peak forward current just below the .75A limit of 5AR4 and the DC current just under 250ma, at least on paper.
On the actual breadboard, I had an interesting light show inside the recitifier shortly after power up and a B+ voltage of essentially zero, so I guess that's what arc-over looks like. I used a beater ruby 5AR4 for the test.
So now I'm back to the SS design, and I understand why Poindexter shows SS rectification in the stereo PS schem and 5AR4 rectification in the mono PS schem. I am also planning using series connected caps in the PS just for some safety margin.
How does the ST-70 (and other old stereo amps with high B+ and current) successfully utilize a 5AR4 for B+ rectification?
I now have my breadboard up and wobbling. I am back to the SS PS. I ordered some filament transformers today, so for now the amp is still on life support (variac). I fire it up at around 100VAC and then crank the variac up to 120VAC after 30 secs or so.
I also rebuilt the PS using two of all the 450V TS-HB's in series with 220K R's across them. Now I don't squint and stick my fingers in my ears when B+ goes above 500V at start-up.
My latest issue is that I cannot get the bias current to go below about 150ma (75ma/tube) for either pair; I run out of bias pot adjustment. I can only get about -23V out of the bias adjustment pot, so this is most likely why my bias current is too high. Will increasing the value of the 22K R after the bias pots help this? Any other ideas?
I also rebuilt the PS using two of all the 450V TS-HB's in series with 220K R's across them. Now I don't squint and stick my fingers in my ears when B+ goes above 500V at start-up.
My latest issue is that I cannot get the bias current to go below about 150ma (75ma/tube) for either pair; I run out of bias pot adjustment. I can only get about -23V out of the bias adjustment pot, so this is most likely why my bias current is too high. Will increasing the value of the 22K R after the bias pots help this? Any other ideas?
Attachments
LOL, it makes a big different when one zero didn't pan out 
I meant 100K // 27K which should be equivalent to 21K.
When you decrease the resistance in this position, you will get a little more -ve. From Poinz diagram, 27K drops 36V => approx 1.3ma
1.3ma x 21K = approx 28V, 36 - 28 = 8, so you should see another -8V , i..e up to -31V. You then can adjust the pot to get to -30. Once you find the right combo to give you the right range, you can replace the combo with one resistor. You can also increase the 22K, but I am lazy, adjust one is easier than two, and it is also easier to undo one change
I meant 100K // 27K which should be equivalent to 21K.
When you decrease the resistance in this position, you will get a little more -ve. From Poinz diagram, 27K drops 36V => approx 1.3ma
1.3ma x 21K = approx 28V, 36 - 28 = 8, so you should see another -8V , i..e up to -31V. You then can adjust the pot to get to -30. Once you find the right combo to give you the right range, you can replace the combo with one resistor. You can also increase the 22K, but I am lazy, adjust one is easier than two, and it is also easier to undo one change
Well, that reduced the current by about 10ma/pair of tubes. I was at 150-160ma/pair, now it's 140-150ma when I run out of adjustment.
I suppose that I can increase the 22K to about 30K to bring everything back up (down?) a little.......That will bring the total R that the -66V tap sees back to where it was.
I suppose that I can increase the 22K to about 30K to bring everything back up (down?) a little.......That will bring the total R that the -66V tap sees back to where it was.
Hey, looks like it is moving in the right direction.
Yes, you can increase the 22K to 30K to get more -ve.
Based on Poinz's voltage measurement, 1V across the 10 ohm resistor suggest 100ma per pair. Since your B+ is only 380VDC, you can consider to stop at 110ma. I am running 60ma per EL34 on my Eico HF-87 that was converted to fixed bias for a couple years. So far so good, no firework.
Yes, you can increase the 22K to 30K to get more -ve.
Based on Poinz's voltage measurement, 1V across the 10 ohm resistor suggest 100ma per pair. Since your B+ is only 380VDC, you can consider to stop at 110ma. I am running 60ma per EL34 on my Eico HF-87 that was converted to fixed bias for a couple years. So far so good, no firework.
Well, since this thread started, I've successfully breadboarded and built mono block versions of this amp, and have been using them as my main amps for over a year.
To get the range of the bias pots in their happy place, I had to swap the 22K and 27K resistors on either side of the pots, making the bias voltage a little more -ve. I'm presently running reissue Mullard EL34s at 400V/60ma.
I've also accumulated the iron to built another EL34 MM (James 6225 5K/20W OT) and the 6V6 version (Hashimoto 8K/20W OT), just need to find some time.............
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