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 Post subject: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 3:13 am 
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I have this thing playing, but am awaiting a better (more iron) output transformer and now have a decent 3-way speaker.

It has 347 volts on the plates of the 6L6g tubes.

Image

I had a radio chassis with a power transformer and choke, so decided to make an amplifier to combine with my Browning laboratories tuner to make a radio.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 4:36 am 
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Just curious, should the 390k plate resistor for the top 6sc7 actually be 290k? Or is there a mistake in the drawing?

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 5:23 am 
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I have 390K units in both positions. That gives 154VDC on Pin 2, the plate of the top triode section of the 6SC7 on the schematic. and 179VDC on Pin 5, the plate of the bottom triode section of the 6SC7 on the schematic (all voltages measured from plate to cathode).

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 8:22 am 
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Location: Powell River BC Canada
When you get your better output transformer suggest you feed screens
from a voltage divider. Here are the values from the old RCA manual for the
6SC7 as a phase inverter.

If your transformer can support a 5U4G, the 6L6s might work harder for you.

Attachment:
RCA  6SC7   PHASE INVERTER.jpg
RCA 6SC7 PHASE INVERTER.jpg [ 222.82 KiB | Viewed 2625 times ]
Attachment:
RCA 6SC7 PHASEINVERTER i.jpg
RCA 6SC7 PHASEINVERTER i.jpg [ 49.75 KiB | Viewed 2625 times ]

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 11:08 am 
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radiotechnician wrote:

If your transformer can support a 5U4G, the 6L6s might work harder for you.

For a new design, just use Silicon diodes. Higher rectification efficiency and reduced load on the power transformer. (And the first filter cap can be bigger---further improving the rectification efficiency)

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Wed 30, 2019 12:01 pm 
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If a 12SC7 was found, the heater could be in series with the 6L6
cathodes. That would really make a hum free unit.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Thu 31, 2019 4:38 am 
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FStephenMasek wrote:
I have 390K units in both positions. That gives 154VDC on Pin 2, the plate of the top triode section of the 6SC7 on the schematic. and 179VDC on Pin 5, the plate of the bottom triode section of the 6SC7 on the schematic (all voltages measured from plate to cathode).
I don't have much experience designing tube circuits just lots of repair and informal poking around. So help me understand, what is the benefit of running each plate at different voltages? It looks like all the other parameters are the same for both.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Thu 31, 2019 5:49 am 
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I suspect it would work better with the voltages the same, but I started with values from the radio design. I probably should get a paper copy of the RCA receiving tube manual, as it is obviously very helpful. The thing already has more gain that I really need, although that might come in handy for bass.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Thu 31, 2019 5:51 am 
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richfair wrote:
FStephenMasek wrote:
I have 390K units in both positions. That gives 154VDC on Pin 2, the plate of the top triode section of the 6SC7 on the schematic. and 179VDC on Pin 5, the plate of the bottom triode section of the 6SC7 on the schematic (all voltages measured from plate to cathode).
I don't have much experience designing tube circuits just lots of repair and informal poking around. So help me understand, what is the benefit of running each plate at different voltages? It looks like all the other parameters are the same for both.
The problem isn't the two 390k, it's that you've got a power supply filter in between the two triode plates causing B+ to be higher (or lower, depending on which is reference) on one than the other.

You're probably taking a cue from other schematics showing additional filtering for the 'sensitive' input stage but the fact is they're both just as sensitive to PS hum in the circuit you're using. The filter should be moved to the front of both, not just the one.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Thu 31, 2019 6:08 am 
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Btw, you don't need a bypass on the 270 ohm triodes common cathode bias resistor and, in fact, it might perform a little better without it. The two triodes, when 'perfectly' balanced, have a combined cathode current that is essentially DC. I.e. one goes up while the other goes down and vice versa. The cap should be doing 'nothing' because the cathode sees only DC.

In actuality, nothing is 'perfect' and paraphase inverters are notorious compromises that lends to large (by comparison) imbalances. This is where an unbypassed resistor could be of benefit because it will 'fight' the imbalance. I.e. which ever tube is conducting 'too much' will impose a cathode voltage that will retard the exuberant tube while simultaneously encouraging the laggard tube to conduct less (which is 'more' of the opposite phase). In other words, it helps to keep them in balance. Now, it can't fully correct (feedback never can) but it's better than not doing anything.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jan Thu 31, 2019 9:57 pm 
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I was hoping to learn some things from doing this, and you-all are being quite helpful.

I now have my chassis and output transformer, so can mount the transformers and chokes, and decide if I want diodes or a rectifier tube. My concern with diodes is that B+ could go too high for the 6L6 tubes, but that is where a copy of the receiving tube manual would again be helpful. I plan to drive one Renegade 8" 3-way speaker which is rated for 300 watts: https://www.amazon.com/Renegade-RX830-8 ... B0045V5WLC

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 4:36 am 
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That is a nice speaker. A pair of 6L6 tubes driving that speaker wont do it
justice. That speaker wants a screaming solid state amplifier with a high damping
factor like a modern version of a Dynaco 120.

AB2 with a separate power transformer for screens, and driver pentode will
give you 45 watts, your circuit about 20.

Your amplifier is a sweet tone unit wanting to drive efficient speakers
like the kind they made in 1950.

Looking at the sqwawk and tweet in that triax, I would say a
Phase Linear driving might shatter a wine glass. However, a sprinkler
over it might be wise.

Love the project. Love the speaker. :D

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 5:03 am 
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Try the changes in this schematic:
Attachment:
6L6amp.png
6L6amp.png [ 38.84 KiB | Viewed 2474 times ]


The 390K plate load resistors are too high in value and the 270 ohm cathode resistor is too low. This causes the 6SC7 plates to run around 65V, and this causes the stage to "run out of gas" and start clipping before the output stage can be driven fully. The plate loads are now 150K and the cathode resistor is now 470 ohm. I changed the 12K resistor in the voltage divider for the inverter to 10K for better balance. It will probably have to be a potentiometer so best balance between the halves of the 6SC7 can be obtained and get equal amplitude signals to the 6L6 tubes. The 6L6 cathode resistor is 250 ohm in line with the recommendation in the RCA tube manual. The 1K screen dropping resistor is now 12K to get closer to the recommended 270V on the 6L6 screens. I powered the inverter stage off the full 350V, but it could be decoupled through a resistor and capacitor if desired. I assumed a 6600 ohm plate-to-plate impedance for simulation purposes. In simulation the power output tops out around 15W. The input grid of the 6SC7 needs a resistor to ground. The coupling capacitor to the inverter section of the 6SC7 is unnecessary because the 6L6 grid and the voltage divider are at nearly zero volts since you're using cathode bias on the 6L6s.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 6:00 am 
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FStephenMasek wrote:
I have 390K units in both positions. That gives 154VDC on Pin 2, the plate of the top triode section of the 6SC7 on the schematic. and 179VDC on Pin 5, the plate of the bottom triode section of the 6SC7 on the schematic (all voltages measured from plate to cathode).

Of course you do. The bottom triode plate is fed B+ through the 390K resistor, but the top triode is fed B+ though its 390K resistor plus the 100K resistor and 20uF filter. The bias current of both halves of the 6SC7 being pretty close to the same by the common cathode resistor as Flipperhome pointed out, the zero-signal plate currents would be close to the same. The extra 100K in the top triode supply would drop the additional voltage. What's the plate current? (179-154)/100K = 0.25mA
John

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 6:20 am 
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OldWireBender wrote:
FStephenMasek wrote:
I have 390K units in both positions. That gives 154VDC on Pin 2, the plate of the top triode section of the 6SC7 on the schematic. and 179VDC on Pin 5, the plate of the bottom triode section of the 6SC7 on the schematic (all voltages measured from plate to cathode).

Of course you do. The bottom triode plate is fed B+ through the 390K resistor, but the top triode is fed B+ though its 390K resistor plus the 100K resistor and 20uF filter. The bias current of both halves of the 6SC7 being pretty close to the same by the common cathode resistor as Flipperhome pointed out, the zero-signal plate currents would be close to the same. The extra 100K in the top triode supply would drop the additional voltage. What's the plate current? (179-154)/100K = 0.25mA
John
Hold your horses there Tonto. :wink: I didn't say anything about the (static) bias currents being equal with unequal plate voltages. In fact they won't be equal and the common cathode resistor (vs separate Rk) will only exaggerate the difference. That will, in all likelihood, also affect the gain and, hence, balance (signal). Then there's inherent gain differences between the two triodes and, of course, the resistor divider driving the second tube is also critical for balance. It's not a trivial task getting 'good', let alone 'perfect', balance with a paraphase inverter.

Btw, my suggestion was to move the PS filter to before both triodes so plate voltage would be equal.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 12:55 pm 
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Erich Loepke wrote:
Try the changes in this schematic......

If the two triodes have a shared (unbypassed) cathode resistor, then it's starting to look like a differential amplifier. As such, it seems that it would want the other grid grounded and not fed from a divider.....or, as a minimum, the divider ratio would be much less?

More generally, how about some feedback?
--small, unbypassed, shared cathode resistor for the output tubes
--feedback loop to the input triode (no shared cathode resistor for the triodes in this case.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 2:06 pm 
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pixellany wrote:
Erich Loepke wrote:
Try the changes in this schematic......

If the two triodes have a shared (unbypassed) cathode resistor, then it's starting to look like a differential amplifier. As such, it seems that it would want the other grid grounded and not fed from a divider.....or, as a minimum, the divider ratio would be much less?

More generally, how about some feedback?
--small, unbypassed, shared cathode resistor for the output tubes
--feedback loop to the input triode (no shared cathode resistor for the triodes in this case.


Since the tubes draw essentially no grid current, the impedances at the grids really don't need to be equal. That's more of a problem when using BJTs, such as at the input of a transistor power amplifier or op-amp. For best performance a differential amplifier needs a high impedance current source in the tail; either a high resistance to a high negative voltage or a BJT current source circuit to a lesser negative voltage like the bias supply for the output tubes, if one was in fact used in the circuit described here.


I tried in simulation to add a feedback loop to the input grid through the grid leak resistor, but it didn't do much, probably because in simulation the voltage sources are ideal (zero impedance) and the feedback through the 100K grid leak is simply shorted to ground through the voltage source. In real life the feedback would depend on the source impedance of whatever device was connected to the input, not a good thing. I don't know of any good way to get a feedback loop around this circuit due to the shared cathode resistor of the 6SC7. If using a tube with separate cathodes (12AT7 would work with few or no changes except for using separate cathode resistors) then feedback would be able to return to the cathode resistor of the input tube. Schade feedback around the 6L6s wouldn't do much either since that method works best with pentode drivers and their high plate resistance. See the "50 watt amplifier" in the back of the RCA tube manuals from the 1960s; the one using 6CB6 drivers for an example of Schade feedback.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 01, 2019 6:51 pm 
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Thanks everyone for participating. I have a variety of tubes, so can consider other options. especially since I've not yet made any holes in the chassis, a BUD 406 which is 7" x 9". The power transformer is a key limiting factor, as I do not want to buy a larger one.

radiotechnician wrote:
That is a nice speaker. A pair of 6L6 tubes driving that speaker wont do it
justice. Love the speaker.
I found those speakers last year and used one in my Crosley 58 Buddy Boy, where it works nicely drive by just one '45 tube. I also used one in a Gloritone cathedral.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Sat 02, 2019 2:37 am 
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The output transformer I have has the primary taps for ultra-linear operation.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Sat 02, 2019 2:44 am 
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pixellany wrote:
Erich Loepke wrote:
Try the changes in this schematic......

If the two triodes have a shared (unbypassed) cathode resistor, then it's starting to look like a differential amplifier. As such, it seems that it would want the other grid grounded and not fed from a divider.....or, as a minimum, the divider ratio would be much less?

More generally, how about some feedback?
--small, unbypassed, shared cathode resistor for the output tubes
--feedback loop to the input triode (no shared cathode resistor for the triodes in this case.
A "differential amplifier" would have a long tail or (even better) current source under the cathode, not simply an unbypassed Rk, although the 'see-saw' action is precisely what I was suggesting "might help" overall balance. However, the effect is way to small for a 'real' differential amplifier, which is why you can't ground the grid (as you could with a diff amp) and have it work worth spit. The second (inverted) phase would be miniscule. For reference, this is what a "long tail pair" looks like.
Attachment:
ACltp1.jpg
ACltp1.jpg [ 31.94 KiB | Viewed 2395 times ]

Rt is very large and (when large enough) acts as current source (note, this requires either a very high B+ or a negative supply at the bottom of the long tail). Of course, a 'real' current source (using either a tube or transistor[s]) can be used and, as would be expected, improves performance.

The same applies to the output stage (which is why push pull outputs need a preceding inverter). Now, it could use an unbypassed Rk ***IF*** the tubes were operated Class A but the vast majority of output stages, especially pentodes, are either Class AB or Class B. In that case, when one or the other tubes goes into cutoff (actually, well before) there's no longer an inverse phase to sum with the signal, so cathode voltage is no longer DC. Which is why it needs to be bypassed.


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