I'm doing the breadboard thing, and have a single-ended 6BQ5 amplifier circuit lashed up. I'm swapping OPT's on the fly, and am currently using the 5K primary tap with the 6BQ5.
Probably not optimum, but oh well...

I have the output tube pentode connnected, with a 4700 ohm R and 100 uF cap for the filtering of the screen supply.
Feedback from the secondary using a 6800 ohm R and a 470 picofarad comp cap to the 12AX7 cathode.

The James 6123HS has a screen grid tap. At 5K primary impedance, the chart shows B-SG at 35%.

Now a couple dumb questions.

Is (B-SG) 35% of the B+ tap voltage?

And if use the screen grid tap, do I just hook the tap up to the 6BQ5's screen grid and that's it?
Or should there be some de-coupling involved?

I'll be swapping to 6V6, which is one of the specced tubes for the 6123HS, but the dumb questions still apply.

Thanks,
Mike.

hey-Hey!!!,
First of all, there are no dumb questions. Now onto yours:

Is (B-SG) 35% of the B+ tap voltage?

If your tap is 35% of the way towards the plate from the B+ connection, it will be 35% of the plate voltage AC. It will actually be a few volts higher than the plate because there is less IR or copper DC loss( as in 35% of the length of copper ).

Is your screen supply voltage close to your plate supply now?
cheers,
Douglas

hey-Hey!!!,
First of all, there are no dumb questions. Now onto yours:

Is (B-SG) 35% of the B+ tap voltage?

If your tap is 35% of the way towards the plate from the B+ connection, it will be 35% of the plate voltage AC. It will actually be a few volts higher than the plate because there is less IR or copper DC loss( as in 35% of the length of copper ).

Is your screen supply voltage close to your plate supply now?
cheers,
Douglas

Ok, then the percentage is determined in AC?

I have roughly a 30 volt difference between the screen and plate, depending on where I set the voltage with the regulated DC supply. A 33 volt drop at 300 vdc, 26 volt drop for 250 vdc, for example.
The current meter on the supply is running at around 48-52 milliamps, depending on where the voltage is set.

Ok, then the percentage is determined in AC?
Yes. In theory at DC the B+, plate and screen grid are shorted together as the inductance of the winding is ineffective at DC. In practice there is the winding resistance to consider.
should there be some de-coupling involved?
You will probably want a 100 ohm resistor at the screen grid terminal to suppress oscillations but other than that, no.

You will likely also need to re-bias your stage.

I'm only vaguely following what you guys are talking about, but I'll re-read it a few dozen times to see if more sinks in...

Mike, where did you find the specs for your James transformers? I have a pair of the 6112HS, and precious little information on them.

Also would be curious to see a schematic for what you're putting together if one exists - I'd like to clone my little Olson amp someday, and it runs 12AX7, (2) 6BQ5's, EZ81 rectified. Looking for similar circuits out of curiosity.

""Ok, then the percentage is determined in AC?""

"Yes. In theory at DC the B+, plate and screen grid are shorted together as the inductance of the winding is ineffective at DC. In practice there is the winding resistance to consider."


I kind of know what you and Douglas are trying to explain, but I'll just end up asking more questions. When I should be google searching on the topic, or digging around here for the few books that might have related information.
(Dunno if RDH3 has info on screen windings, gotta be something in there.)



"You will probably want a 100 ohm resistor at the screen grid terminal to suppress oscillations but other than that, no.

You will likely also need to re-bias your stage."



Ok. Will using the screen tap change the bias, and this is why you are mentioning to re-bias?

I'm thinking the 5K primary impedance tap isn't really the most optimum primary impedance for 6BQ5, and should just swap to 6V6 to see how that sounds.

While the James OPT has good bass, the mid and top-end doesn't have much life to it. Well, not as much as when I swapped to a Magnavox SE output transformer.
There isn't as much bass as the James, but there isn't too much difference.
The mid and top-end is nice and airy, and more relaxed sounding if that makes sense.



Redboy,

""Mike, where did you find the specs for your James transformers? I have a pair of the 6112HS, and precious little information on them.

Also would be curious to see a schematic for what you're putting together if one exists - I'd like to clone my little Olson amp someday, and it runs 12AX7, (2) 6BQ5's, EZ81 rectified. Looking for similar circuits out of curiosity.""



http://euphoniaaudio.netfirms.com/ea/nfoscomm/catalog/pdf/6112HS.pdf



I don't have a schematic. It's kind of a the 197 series Magnavox SE 6BQ5 amp circuit I suppose....but then again, not really.
I'm just noodling around basically, using a tube regulated DC supply.

The James are a little overkill for 6BQ5 at 20 watts, or 6V6 for that matter.
I'd like to find a output tube that would use at least 15 watts of that 20 in some sort of single-ended amp circuit.

Thanks for the replies.

James 6123HS and Magnavox SE 6BQ5 OPT:

""Ok, then the percentage is determined in AC?""

"Yes. In theory at DC the B+, plate and screen grid are shorted together as the inductance of the winding is ineffective at DC. In practice there is the winding resistance to consider."


I kind of know what you and Douglas are trying to explain, but I'll just end up asking more questions. When I should be google searching on the topic, or digging around here for the few books that might have related information.
(Dunno if RDH3 has info on screen windings, gotta be something in there.)



"You will probably want a 100 ohm resistor at the screen grid terminal to suppress oscillations but other than that, no.

You will likely also need to re-bias your stage."



Ok. Will using the screen tap change the bias, and this is why you are mentioning to re-bias?

I'm thinking the 5K primary impedance tap isn't really the most optimum primary impedance for 6BQ5, and should just swap to 6V6 to see how that sounds.

While the James OPT has good bass, the mid and top-end doesn't have much life to it. Well, not as much as when I swapped to a Magnavox SE output transformer.
There isn't as much bass as the James, but there isn't too much difference.
The mid and top-end is nice and airy, and more relaxed sounding if that makes sense.


IJames 6123HS and Magnavox SE 6BQ5 OPT:

hey-Hey!!!,
I am not sure if you'll need to re-bias. It depends on what voltage your screen winding is at to begin with. Since it looks like it was lower before ( dropping R, and de-couplng cap ), you should check the idle/zero-signal condition.

5k SE is fine, just as 10k a-a PP in Class A is fine. Both present the same load to the tube. My PP 6V6 amp runs 10k load, and it likes it plenty...:) Your comparison at the end leaves me a bit confused as to just what you're comparing to what.

Also, keep asking questions, not asking is the stupid action...:)
cheers,
Douglas

Thanks for the link, Mike. That picture's pretty darn funny, too!

Another question now that I'm thinking of it. Are those terminals on the James transformers meant to soldered, or should I be using some sort of connector?The James are a little overkill for 6BQ5 at 20 watts, or 6V6 for that matter.
I'd like to find a output tube that would use at least 15 watts of that 20 in some sort of single-ended amp circuit.What about 6C33C? I was over at member ejfud's place the other dCCay, and had the chance to listen to his Almarro (http://www.audiokarma.org/forums/showthread.php?t=95342). 18 watts per, I hear.

hey-Hey!!!,
That 5k load is good for a moderate voltage 845. How much idle current is it gapped for? What ever the gap, run the 845 at 700-800V at that current and see how your power output goes. Or an 813 at 500-600V since you've got a U-L tap...:)
cheers,
Douglas

The James 6123HS is a universal OPT with 2.5K, 3.5K and 5K primary taps.
Primary inductance for each primary tap is, 16H for 2.5K, 22H for 3.5K, and 31H atr 5K. At 60Hz.
(I'm guessing the inductance is determined at 60Hz frequency.)

Screen grid percentage is 50% for 2.5K, 42% for 3.5K, and 35% for 5K.

Primary DC current is 120 milliamps. (I assume this is the gapped idle current...)

You can solder to the taps, there is solder on mine.

Is the screen grid winding a ultralinear type grid winding, or just a screen winding?
I've had some RCA single-ended output transformers that had a screen winding, but it wasn't for UL.
They were just a winding to help aid in filtering if I recall. A way to get a bit of better linearity out of a cheap output tranformer. I could be wrong.....

When I had the James OPT in circuit, I wasn't using the screen tap.
I had a dropping R and de-coupling cap coming off the main B+, and then right to the screen of the 6BQ5.
I hadn't checked the voltage on the screen tap while the circuit was powered up. Should have I suppose....

I was comparing the James OPT against the Magnavox OPT in the same 6BQ5 circuit with no changes to the circuit. Just cocking around, basically.

Thanks for throwing out some options on output tubes.
I've pondered 811 and maybe 2E22.

My DC supply is only good for 350 volts at 200 mils, so I would have to build up a power supply for anything with 500-800 vdc.
No problem there, I've got a few hefty PS supply transformers that can swing that. Chokes, damper diodes, regulator tubes and whatever else to use.

6C33C.... Don't those have a rather lowish plate resistance?

Thanks,
Mike.

Argh.....nice long reply and I lost it......
I'll try later tonight. Man, I hate swing shift........

Primary DC current for the James 6123HS is 120 milliamps.

hey-Hey!!!,
A 6C33 is a low mu and low plate Z tube. For reasonable circuits, figure something like 1k to 1k5 at 150-200 mA. You can increase voltage and decrease idle to take better advantage of your OPT, but that has its own price.

You've definately got a nice OPT for one of the transmitting tubes if you want to wring out some more power. Nice options to look into later when you feel like expanding the project.
cheers,
Douglas

hey-Hey!!!,
A 6C33 is a low mu and low plate Z tube. For reasonable circuits, figure something like 1k to 1k5 at 150-200 mA. You can increase voltage and decrease idle to take better advantage of your OPT, but that has its own price.

You've definately got a nice OPT for one of the transmitting tubes if you want to wring out some more power. Nice options to look into later when you feel like expanding the project.
cheers,
Douglas

I had thought I had replied to this, but the AK site did the big switchover....

Yep, one of these days I'll have to pony up for a pair of higher wattage output tubes for these OPT's. Triode or whatever....a good 15 watts per side would float my boat fine.

I got this old radio chassis, must be close to 70 years old. A seven dollar yard sale find, with tubes and parts included. Someone pulled the chassis from the console and made it a component I suppose.
It still had the fake plastic wood faceplate from the original console. Minerva was the name.

I'll have to make another top-plate, but this chassis is pretty big and would make quite the boat anchor stereo SE amplifier. I hope it's big enough...it is a good 4.5 inches deep. With a bottom plate, too.....

I do have seperate PS transformers that would swing enough voltage for a pair of monoblocks, but I would have to buy two chassis enclosures. And maybe some chokes, maybe not....

Too many projects, too much pondering......I should be getting my Eico ST-70 going. At least I finally figured out which plate lead on the OPT's is inverting and non-inverting........

Mike

I can maybe come across a pair of Shuguang 845's real cheap locally.

It seems the James 6123HS would work with a 845 at the lower spec voltages.