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DC coupled single ended cathode follower 6SJ7/6V6 amp project
DC coupled single ended cathode follower 6SJ7/6V6 amp project
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Old 23rd July 2019, 07:59 PM   #61
jhstewart9 is offline jhstewart9  Canada
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Default Complete SE Amp FB Pair Output

I built this little amplifier back in 1968. Since then it has been used in quite a wide variety of locations. For the past few years it has been in my workshop. Program sourc*es are an Eico HFT-90 FM receiver and a matching Eico HFT-94 AM receiver. And sometimes a CD player.

The electronics is a FB pair consisting of a 6AU6 & 6AQ5 driving into a Hammond 125D OPT. The front end is a 12AX7 connected as a Baxandall tone circuit & another 12AX7 as a preamp for a low level magnetic phono pickup up or can be switched to a mike. DC is supplied by a FW SS rectifier driven by a Hammond 270AX transformer.

I managed to build the entire amp, tone controls, and preamp into a Hammond 8 × 12 × 3 chassis. Most of the major com*ponents are on a 3 × 12 sub-chassis inside. That includes the power supply, power amp, preamp, and tone circuits. I assembled the sub-chassis on the bench. The controls, input, and output connections are mounted on the front and rear panels of the chassis. These were all wired together at the final assembly stage.

Because there is some heat to be dis*sipated, I used a set of four 1˝ diameter ventilation hole plugs mounted on the ends of the chassis. There are several ˝˝ holes in the bottom plate to allow cooling air in.

Around 1960 I built two other complete amps inside the larger Hammond 8 x 16 x 3 chassis. Both were running PP 25L6’s into Hammond 125D OPT’s. Other tubes in the lineup consisted of the regular 6SL7 type preamps & drivers. They were used by friends & never came back due to failures.

So far no electrolytic failures to report but something to consider in a poorly ventilated environment. The pages are from another era, need to be redone. For those who would prefer octals, 6SH7 & 6V6 are drop ins for this circuit.
Attached Images
File Type: jpg 3 WATT AMP RHS 9W 20C.jpg (102.0 KB, 120 views)
File Type: jpg 3 WATT AMP w Eico.jpg (92.7 KB, 105 views)
File Type: jpg 3 WATT AMP Bottom.jpg (118.5 KB, 102 views)
Attached Files
File Type: pdf 6AQ5 6AU6 Amp & PS.pdf (210.6 KB, 34 views)
File Type: pdf 6AQ5 6AU6 Amp IMD @ 2 Watts.pdf (215.5 KB, 18 views)
File Type: pdf 6AQ5 6AU6 Amp Preamp.pdf (197.6 KB, 16 views)
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Old 23rd July 2019, 08:05 PM   #62
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Dinolobe View Post
I have a Tek PT with 3 150V secondaries, 2 smallish SE 5k:8 OPTs (antique supply P-T31) and a reasonable stash of tubes. (Pretty much all common noval preamp tubes in different flavors and a few different sets of power tubes like el84, el34, 6v6, 6550...) from what I remember of your cathode follower design, wouldn’t I need OPTs with a 600 ohm primary ? I would gladly try your design if it’s possible to make it work with the transformers I already have, but if not, I would prefer to use a circuit that better fits those parts.
Perhaps something like this?
Just an idea, not tested.
The 6V6 has to have a heater winding of it's own.
Mona
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File Type: png 6V6-CommonA.png (60.6 KB, 109 views)
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Old 23rd July 2019, 08:16 PM   #63
Dinolobe is offline Dinolobe  Canada
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May I ask where is that circuit from ? Do you have access to any other information about it ?

I will let the more experienced members comment about the actual circuit. It’s a cathode follower, it has silicone involved, something that looks like a doubler in the psu, so really high voltage, but I cannot say much more. Thanks for your input !
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Old 23rd July 2019, 08:56 PM   #64
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
+630VDC raw supply.

+290VDC at the 6V6 cathode means the 6V6s absolutely must have their own heater supply lifted up to around +250V. That also means the 6SH7 (6SJ7) tubes must have a separate heater supply. Two separate 6.3v heater supplies. More complexity. Worth it?

+585VDC B+ to the 6SH7 (6SJ7). That's what enables it to swing a lot of signal volts out from its plate.
--
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Old 23rd July 2019, 09:10 PM   #65
Dinolobe is offline Dinolobe  Canada
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Does the heater supply absolutely need to be from another transformer or having multiple heater secondaries would do the job ?
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Old 23rd July 2019, 09:40 PM   #66
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
Two separate 6.3V windings on a single transformer could be used.
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Old 23rd July 2019, 09:44 PM   #67
jhstewart9 is offline jhstewart9  Canada
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The PS rectifier is simply a full bridge of SS diodes, nothing unusual except the very high & dangerous voltage. The 6V6 is still connected as a triode so can swing the cathode no farther than it would swing the plate in a normal grounded cathode circuit.

A lot of cct for very ordinary results.
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Old 23rd July 2019, 09:49 PM   #68
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
Quote:
Originally Posted by jhstewart9 View Post
A lot of cct for very ordinary results.
I have to agree.

BTW, nice workshop system! The amp is really nice, and I love the EICO tuners. That stuff used to be everywhere, available for peanuts. Now it's collectors' gold. Times change...
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Old 23rd July 2019, 11:32 PM   #69
Tubelab_com is offline Tubelab_com  United States
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Quote:
from what I remember of your cathode follower design, wouldn’t I need OPTs with a 600 ohm primary ?
The optimum load impedance for a given tube depends on it's peak current capability, it's ability to pull it's plate as close to the cathode voltage wise as possible, and the B+ voltage. The 6336 and a triode wired 6LW6 sweep tube both work best in the 200 to 250 volt range for B+ and have current capabilities in the 800 mA to 1.4 Amp range. This criteria is what drives the use of a 600 ohm OPT. 10 to 15 watts of triode SE cathode follower are possible. The limiting factor is the same as in most SE amps, the maximum tube dissipation.

Put a 6V6 in the same amp and that peak current capability drops to 105 mA, while the rated B+ voltage rises to 315 volts, and many users including Fender totally ignored this spec and went over 400 volts. This criteria predicates a 5K or higher OPT, but the maximum power output in SE triode is still about 2 watts from a 6V6.

The reason many people including myself experiment with cathode follower amps is that the voltage amplifier, and the current amplifier functions are now performed by two different tubes......What DEVICE would you use for the "best choice" for a current amplifier? The most important spec for the device in a follower application is Gm, so the logical choice would be a big fat MOSFET......uh, but I might get called Transistorlab again if I bring that up, but I tried it and it does work quite well. Just remember that the voltage across that part will swing to twice B+ in normal operation, and nearly 4X B+ in an overdriven guitar amp where the speaker is operated at or near it's resonant frequency. Use a 900 volt fet in a 170 volt amp, but use a 100 ohm OPT and get 25 watts. That required a BIG heat sink on that old Fuji mosfet (obsolete but still stocked at Allied).
Quote:
The 250k ohm plate resistor means it would drop 250V with only 1mA across it. With B+ of 350V this would leave only 100V at the 6SJ7 plate.......I was thinking the plate voltage should be more like 150V. With 3mA plate current, that would mean something like a 33k plate resistor to drop 100V, leaving 150V at the 6SJ7 plate. The screen grid load resistor might be something like 220k(?).
This circuit comes from the 1940's. Leo Fender was trying to maximize the gain from the 6SJ7. The gain in a pentode is determined by the load impedance and the Gm of the tube. Gm varies with current, more current, more Gm.....more current requires more B+ voltage, or a lower load resistor which reduces gain. For any given tube there is only one combination of load resistor and current for maximum gain at any given B+ voltage......but there is a fairly wide range of "near maximum" gains with different combinations of load resistors and tube current.

That old Fender circuit runs the 6SJ7 in near starvation to get maximum gain. This kills high frequency response since the output impedance is very high, and requires a high AC load impedance. This is why there is a 1 MEG volume pot despite the 500K maximum spec for grid circuit resistance in the 6V6 tube.

I prefer a more sane load resistor with a few mA of tube current. Need more gain, use a tube with higher Gm. The previously mentioned 6EJ7 is a good choice, but it's a 9 pin miniature tube. Other tricks can be used like bootstrapping the plate load so the tube sees a very high AC load. I have achieved stage gains of over 1000 with a 6AU6, but microphonics become the limiting factor in this case.

Quote:
6AC7 is a good one, and keeps the metal tube vintage thing going.
I have collected a box full of metal tubes for a little MetallicAmp guitar amp project I plan to build.....All tube, no glass. I found 8 X new metal 6V6's in a box full of metal tubes that were about to be trashed at a hamfest several years ago. There are also 2X metal 6L6's, but they look well worn. Maybe 20 or so small signal tubes including some 6AC7's.
Attached Files
File Type: pdf 2SK3678.pdf (115.6 KB, 9 views)
File Type: pdf 2SK3675.pdf (120.0 KB, 1 views)
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Old 24th July 2019, 12:35 AM   #70
rongon is offline rongon  United States
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
Okay, I'm not having an easy time with this. But here's what I have.

It doesn't try to DC couple. It's based on a 300V B+.

It looks like it's not going be a low distortion type of thing. LTspice says 0.57% THD at 1W out into 8 ohms, 1kHz. Not exactly hi-fi, and this is with both local plate-grid NFB and another 6dB of global NFB. Open loop THD is downright scary.

LTspice says max power is 5W into 8 ohms at 2.3% THD at 1kHz.

I don't know if this is good or not. It's the best I can seem to do at this point. I've attached the .asc file if anybody wants to play with it in LTspice.

I hope it's of some use.
Attached Images
File Type: png SE6V6-6SJ7-LocalNFB+GlobalNFB_00.png (41.0 KB, 87 views)
Attached Files
File Type: asc SE_6V6GT_6SJ7_Local+GlobalNFB_01.asc (4.3 KB, 6 views)
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