I made the mistake of ordering 20 Chinese made 812 all but two arrived with broken filaments none of the 811A's or 572b's in the same parcel had the problem, costs a lot to send back to China.
I would not chance on Chinese 812A's. I find 812a's have excellent sound PP, but expensive for USA NOS.
Phil
I would not chance on Chinese 812A's. I find 812a's have excellent sound PP, but expensive for USA NOS.
Phil
From the schematic it looks like the operating point is 780V at 60mA with a grid voltage of about -9V. Depending on the output transformer impedance, you are going to need to drive the grid to +40V to get full power. Looking at the RCA data sheet it clearly shows the grid will pull about 32mA. None of your driver circuits can deliver that kind of grid current.
Well I hate to hear that. All these 812A's I have are RCA NOS. I have about eight of those and they are all pretty evenly matched as far as grid current & plate current when I put each in the test build.
I already have are some 5k and will try those.
On the bench, I'm just using a 12VAC power transformer which I measured to be about 1600 ohms @ 1kHz. Mids and highs pass just fine on that but the lows really suffer.
Can someone point me to calculating this more precisely and save me some time reading?
Yes, I realize this, but the speakers I'm using are very efficient full-range Fostex and to be truthful, this is plenty loud for me and my wife's listening.
It really has a fine amount of volume in the room and also the kids can't drive it into square waves with "lil Wayne" when we're not around.
And I also know I could do the same thing with a 6L6, but I like seeing the big bulbs, and I appreciate you reading the thread and offering the comments.
After listening a while and knowing you are correct concerning the driver circuits (after all I want this thing to last a few days), I modified it to make sure the grid current is satisfied and I also like the extra volume. Thanks for all the inputs.
New schemo :
An externally hosted image should be here but it was not working when we last tested it.
Been there; done that. (Attached)
I had a 812 SET in the works that never got completed, as getting the OPT proved almost impossible. I couldn't find a winder who couldn't guarantee that the sonic performance would actually be good enough for better than a guitar amp. That wasn't what I needed. Getting OPTs for SE is a good deal harder to accomplish, given that you have a core magnetization problem that doesn't arise with push-pull operation.
This loadline shows some 16W of output, but in practice, you're not gonna see that at the secondary. That isn't just copper and iron losses either. Real speaker loadlines open into ellipses, and will hit plate current cutoff before you see 16W, so you'll hear clipping distortion sooner than the simplistic loadline would predict. Still, the low H2 estimate indicates good linearity, and likely excellent sonics, if you had an OPT that could perform.
Since this must be a Class A2 design, given the u-Factor and plate characteristics, you're also gonna require hefty grid drive. For that: MOSFET source follower is about the best option: much lower Zo, and more than enough current sourcing capability.
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Well I finished it up and ended up using the config of the 6SL7/812A combination. With my sensitive speakers, it was the most sonically pleasing of any other combination. There's not a ton of watts, but the plate current is happy and so are my ears. I had a pair of 5k OPTs and am using those as the loads.
I do have a hum that does not change at any volume and after doing a bit of reading, I believe it's just the nature of the beast when you use a directly-heated cathode with AC filaments. I may at some point switch those to DC, but even with DC, there is still a bit of filament noise that I can hear. My wife tells me I have "dog ears" anyway.
Oh, I found that there are a ton of little 60 to 100 VA toroidal transformers at various "for sale" sites that work great for winding your own separate filament lines. I paid $10 for the one I'm using and it's a great little item.
Thanks for all the tips and help during this. It was a fun learning experience. I'll post a photo later on, but I used the same chassis as I had before, its just now has a 6SL7/812 instead of the 6C4/6146.
I do have a hum that does not change at any volume and after doing a bit of reading, I believe it's just the nature of the beast when you use a directly-heated cathode with AC filaments. I may at some point switch those to DC, but even with DC, there is still a bit of filament noise that I can hear. My wife tells me I have "dog ears" anyway.
Oh, I found that there are a ton of little 60 to 100 VA toroidal transformers at various "for sale" sites that work great for winding your own separate filament lines. I paid $10 for the one I'm using and it's a great little item.
Thanks for all the tips and help during this. It was a fun learning experience. I'll post a photo later on, but I used the same chassis as I had before, its just now has a 6SL7/812 instead of the 6C4/6146.
Well there she is - basically the same chassis sa before, just different bulbs.
An externally hosted image should be here but it was not working when we last tested it.
I've been pondering the 120Hz hum I can hear with my head up in the speaker and I'm going to try this solution when I get some free time.
Effects of AC Heating Power Applied to Directly Heated Triodes
Effects of AC Heating Power Applied to Directly Heated Triodes
I drew up a PP loadline for the 812 (attached). Since RL= 2000R per phase, you would need an OPT that could match 8000R to your secondary load. The h3= 3.7% estimate is looking pretty good. As for how this actually sounds, that would depend on the actual harmonic profile of the distortion in practice. You probably don't need more than enough gNFB to take the edge off.
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Hi, I know this is a little off topic, but not by much... Linked is a schematic for the Gotham PFB-150 cutter amp:
http://www.vintagewindings.com/gen pop/8299543VW8335/ProAudio1/Gotham PFB-150 Cutting Amplifier.pdf
I knew about the PFB-150WA amplifier, made to link up with the Grampian cutter head, but Gotham also made the PFB-150WD version of this amp which was made for high fidelity music reproduction. Both used a General Radio torroidal output transformer which can be configured for several primary and secondary impedances.
Link to the torroidal output transformer spec sheet:
http://www.ietlabs.com/pdf/Manuals/GR/942-A Output Transformer.pdf
I used to think this amp used the output transformer configured for 6600 ohms primary linked to a push-pull pair of 811A output tubes but in actuality, the output transformer in these amps is configured for 1650 ohms primary impedance for the pair of 811A tubes. This seems very low for 811A tubes, even though they are operating in class A2. Can anyone shed any light on this? Thanks...
http://www.vintagewindings.com/gen pop/8299543VW8335/ProAudio1/Gotham PFB-150 Cutting Amplifier.pdf
I knew about the PFB-150WA amplifier, made to link up with the Grampian cutter head, but Gotham also made the PFB-150WD version of this amp which was made for high fidelity music reproduction. Both used a General Radio torroidal output transformer which can be configured for several primary and secondary impedances.
Link to the torroidal output transformer spec sheet:
http://www.ietlabs.com/pdf/Manuals/GR/942-A Output Transformer.pdf
I used to think this amp used the output transformer configured for 6600 ohms primary linked to a push-pull pair of 811A output tubes but in actuality, the output transformer in these amps is configured for 1650 ohms primary impedance for the pair of 811A tubes. This seems very low for 811A tubes, even though they are operating in class A2. Can anyone shed any light on this? Thanks...
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I have one of the C core output transformers made for the Grampian cutter head amp I checked it and also sent the readings to Sowter in England the plate to plate worked out as 2.200ohms P To P.
I have built the 811A amp using 5k somewhere I have the data for 811A at 750 volts that recommends 5 k p to P. for 150 watts. I have only used 600 volts has enormous power and Dynamics.
There is an article in Glass audio 2/95 using very high plate to plate loads for 812’s and 811’s at 572 volts and getting very little power sounded terrible. Unfortunately lent the magazines and never got them back.
I have built the 811amps less the RIAA and using large toroid or c core power transformers transformers with very good regulation. easily give 100 watts better than 6550 amps and the tubes are so stable
Trying toroids Out puts made by TOROIDY.PL TRANSFORMATORY with the 811a
I have built the 811A amp using 5k somewhere I have the data for 811A at 750 volts that recommends 5 k p to P. for 150 watts. I have only used 600 volts has enormous power and Dynamics.
There is an article in Glass audio 2/95 using very high plate to plate loads for 812’s and 811’s at 572 volts and getting very little power sounded terrible. Unfortunately lent the magazines and never got them back.
I have built the 811amps less the RIAA and using large toroid or c core power transformers transformers with very good regulation. easily give 100 watts better than 6550 amps and the tubes are so stable
Trying toroids Out puts made by TOROIDY.PL TRANSFORMATORY with the 811a
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