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Straange triodes

Ned Sheats

Ned Sheats

New Member
I'm still working on my PPP triode amp using 6AS7s. From what I read in the little information available on these workhorses I thought I should be able to count on a really clean 20+ W. I'm getting about 15. I wouldn't worry about 5 watts if I was building KT88s but a loss of 25% with the triodes is significant. The B+ is a solid 225V and a current of 75 ma. each section for 0 signal. I have plenty of drive from 1/2 of a 6CG7, running with a 380 DC supply, driving each 1/2 of a 6AS7. A 12AX7 input amp and a 12AU7 PI precede the 6CG7s. The 15 Watts it puts out is really smooth. I measured -1/ +.5 db 25 to 20KHz.
My problem is I believe my choice of output transformers.
Another symptom is that when I connect a 8 ohm load to the 8 ohm tap I can develop about 11 watts yet the same load across the 16 ohm tap yields 15 watts.

RCA, GE and Svetlana all specify 4000 to 6000 ohms effective load resistance depending on plate voltage and current. (I assume plate resistance means little or noting when specifying OTs). First stupid question....are these mfg specs plate to plate? After a lot of reading I assume they are.

I ordered a 2000 ohms, 50W, 250ma per side transfomer because at the time I had yet to find a source for the above information..dumb huh? I am assuming that Heybor specifications are plate to plate.

Now if specified plate load for a push pull pair is 4K to 6K, wouldnt Push Pull Parallel be 2k to 3K. That tells me I am at the low end. Would taking the load up to 3K recover my 25% power?



Still learning at the ripe old age of 72.
 
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Following closely,,, I just found a pair of 6AS7s, and built an SE mule to try em out... the OPT is the problem with these, in SE anyway,,, about 1k primary... I rewired them parallel this afternoon, still sorting out the Rk... Do you have a schematic to share ?
Too bad we are so far apart, we could collaborate in person!
 
Ned -- A few things to consider:

1. What type of bias system are you using? If fixed, the the full 225 volts of B+ is working for you. But if cathode biased, then you've already lost at least 60 volts to bias the tube, which will go against your power output goal. On the other hand if you ARE using fixed bias, and running each section with 75 ma of quiescent current and 220 plate volts (5 volt difference accounting the OPT losses), then you are operating the plates at over 125% of their Design Maximum rating. The tube will not last long operated this way.

2. Understand that any tube data sheet specifying power output is based on PLATE watts. That is, power consumed from the power supply minus power dissipated by the tube = plate power output. Therefore, if you are reading a data sheet that specifies 11 watts of power output for a push-pull configuration, that is power at the plate, or 22 watts for a PPP configuration. To get this -- or most any plate power figure -- converted into typical real world power output developed into a real world load, multiply the power power figure by 81%. That means that in this case, a PPP parallel configuration will deliver about 17 to 18 watts, with a "really clean" 20+ watts out of reach -- and this assumes you are using fixed bias, too. Cathode bias will deliver correspondingly less power.

3. When operating output tubes in push-pull parallel, the required load impedance for the tubes to operate into is in fact 1/2 that required for a single push-pull pair.

I hope this helps!

Dave
 
Yes, please post a schematic so we can better understand what's going on. But a few things come to mind - the 6AS7's requires a lot of grid drive, are you sure that the driver is able to provide the swing required? Is the B+ stiff enough to supply the high current at full power?
 
knockbill said:
Following closely,,, I just found a pair of 6AS7s, and built an SE mule to try em out... the OPT is the problem with these, in SE anyway,,, about 1k primary... I rewired them parallel this afternoon, still sorting out the Rk... Do you have a schematic to share ?
Too bad we are so far apart, we could collaborate in person!
Click to expand...
Thanks. everything is a long way from the Rio Grande Valley. LOL I have partial schematics based on parts I have used for this prototype. Nothing ready for press time.
 
My situation also... I am using 2-3 different schems, but pretty sure I have it dialed in parallel SE... Not enough gain tho... thinking of using both 6SN7 stages as drivers...
Don't want to hi jack your thread, I have one started also...
 
dcgillespie said:
Ned -- A few things to consider:

1. What type of bias system are you using? If fixed, the the full 225 volts of B+ is working for you. But if cathode biased, then you've already lost at least 60 volts to bias the tube, which will go against your power output goal. On the other hand if you ARE using fixed bias, and running each section with 75 ma of quiescent current and 220 plate volts (5 volt difference accounting the OPT losses), then you are operating the plates at over 125% of their Design Maximum rating. The tube will not last long operated this way.

2. Understand that any tube data sheet specifying power output is based on PLATE watts. That is, power consumed from the power supply minus power dissipated by the tube = plate power output. Therefore, if you are reading a data sheet that specifies 11 watts of power output for a push-pull configuration, that is power at the plate, or 22 watts for a PPP configuration. To get this -- or most any plate power figure -- converted into typical real world power output developed into a real world load, multiply the power power figure by 81%. That means that in this case, a PPP parallel configuration will deliver about 17 to 18 watts, with a "really clean" 20+ watts out of reach -- and this assumes you are using fixed bias, too. Cathode bias will deliver correspondingly less power.

3. When operating output tubes in push-pull parallel, the required load impedance for the tubes to operate into is in fact 1/2 that required for a single push-pull pair.

I hope this helps!
dcgillespie said:
Ned -- A few things to consider:

I am using both cathode (tube balance) and fixed bias as recommended by the mfg.Rk is only 150 ohms with 250ufd as Ck for each triode. Fixed bias runs about -90V to give me 65 MA per tube (50% of max). The performance (sound quality) is excellent but I think I maade a mistake ordering a 2K transformer. Do you think changing it to 3K would provide an additional watt or three ;). Powersupply is not an issue the 0 to full power voltage fluctuation is 2 V. Its a big toroid. Do you think I am correct in assuming Hybore specifies P to P resistance. Thank you for your answers. I want to get started on the stereo version but noit until I solve the transformer issue.
Click to expand...
Click to expand...
 
OK. I'm somewhat lost here. Your original post indicates that you are running 75 ma "per section", but you last post indicates 65 ma total "per tube". If you will please clarify this issue.

If you run about 60 ma in each section (120 ma for the tube), that should put you well into Class A operation territory, if not entirely. At 65 ma per tube however, you're into class AB which typically requires a lower P-P impedance. I would at least try bumping your quiescent current up to 60 ma per section and see how the performance is at that point. The driver would likely endorse this move as well!

Dave
 
Follow on to Dave's question, with the output power that you are getting, I think you are running 65mA per section, or 130mA per tube with both sections wired in parallel, is that correct? Also, by chance, would any of you gentlemen have a copy of C.G. McProud's "High Quality Amplifier With the 6AS7G" that was published in the 1948 March issue of Audio Engineering? The copies that I see online all are missing three key pages... urgh.
 
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All you guys. Thanks! I had to take a day off. Here is where I am right now.
Plate volts 210 #1
Plate current 85Ma per triode
Cathode volts 13 (150 ohm bypassed w 300 ufd (each triode separately) #2
Fixed Bias (g1) -85
Drive (g1) 200V pp
18 Watts b4 clipping
No feedback
Xfmr is 2000 ohm P to P, 50 W, 250ma per side
#1 supply drops 1 volt at full output
# 2 recommended by GE for fixed bias operation

Attached (hopefully) is a 1947 RCA spec sheet from for the 6AS7 note that the plate voltage I have to use it calls for about 11 watts per pair but with the higher bias I am way into the AB range. Sorry.. cant figure out how to insert a Jpg here. Unfortunately my files dont have URLS and I havent a clue how to do that. It's a shame we cant just upload a jpg from our computers

So, noting that the specs for 11 WPC per pair or 22 for PPP and that the impedence is 4000 for a pair and therefore 2000 for PPP (my xfmr specs) should I quit beating my head against the wall?
The one phenomenon I cant figure out is why I can generate 18 watts across an 8 ohm Load on the 16 ohm transformer tap and only 12 watts with the same load across the 8 ohm tap.

Thanks again, If Ive missed anything obvious..Let me know. I do accept constructive criticism.:whip::whip::whip::whip:
 
you can upload things from the computer. Drag and drop it into the reply box. At the bottom, under the text you'll see an option to insert either the full image or as a thumbnail. Pick yer poison.
 
I think the tubes are biased very hot since 0.085 x (210-13) = 16.75W, which is well over the 13W Pdiss limit. The idle current should be set to 60mA per unit as shown in the RCA app note shown below.

The reason that the output power is higher with the 8Ω load on the 16Ω tap is because the load seen by the tubes is halved, so the peak current is much higher, but the distortion is higher as well.

The 12W with 8Ω load on the 8Ω tap is however lower than expected, it should really be over 20W as you said - that is something worth looking into. Wild thought... what if the OPT is mis-labeled or spec'd, so the 16Ω tap is actually 8Ω, and the 8Ω tap is actually 4Ω, etc.?

6AS7G%20AN.png
 
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-125 volts to idle at 100ma with 250 on the plate? Wow, that sure does need a hell of a bias supply. Needs quite a fair bit of grid voltage swing too.

And does it really need a 2.5k cathode resistor in addition to the grid supply? Seems like with 100ma across 2.5k, you'd have the cathode at 250 volts unless I've completely lost my math understanding here. I can't quite see how having cathode voltage equal to plate voltage is gonna work, unless this is just way funkier of a tube than I can get my head around.

*edit* derp, voltages would be measured with respect to cathode, so that means the actual plate supply is 500v to let the cathode run at 250v ? I guess that -125 would still be referenced to cathode, so you'd actually be able to run +125v with respect to ground and have it work. Unless of course its either the massive cathode resistor or fixed bias like most other things.
 
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I'm not understanding the pri. Z situation, is the pri. side of the OPT center tapped? I'm confused as usual. I'm in a break period (due to a damn bathroom remodel) of building a pair of triple 2A3s in parallel, so I'm watching this close too. I'm hoping for a clean 8 or 9 watts with distortion dependent on tube matching.
 
Guys, the figures shown in the app note are for both units under class A push-pull operation, i.e., not for a single unit unless marked.
 
I get that its for both sides in push-pull, but my question still stands regarding both the high value cathode resistor and the significantly negative grid supply. I've just not seen both used together, and certainly not at those values.
 
Ah, I see, I think it should be read as using one or the other, not both at the same time.
 
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