Hello, I have a concern over the 6L6 voltage regulator used to stabilize the output tube screen grids shown in the attached schematic. Seems like the Vhk is very excessive and, I'm not sure of the extent I can go with heater elevation. The first thing I am considering is a separate 6.3 VAC transformer for the 6L6 (I don't have a 5VAC tap on the power transformer and it is my understanding that the 6L6 CF should be run off of a separate PT winding). Should I also do the heater elevation? Any other specific recommendations? Can't say how long or how hard it was run like this. I have not run the amp much as it needs to be refurbished, but it sounds pretty good considering. I have seen one other very similar amp that Mr. Evans built that did have a separate winding for the 6L6, but this one has all the tube heaters running off the same winding. Any advice is appreciated. Cheers
Yes, you're right: As the 6L6-GC cathode voltage of 375 V well exceeds the heater to cathode voltage rating of just 200 V, this tube indees requires a dedicated heater widing. I wonder what the amplifier's designer might have been thinking...
Anyway, this is not a voltage regulator, as there's no reference involved. It might be ocnsidered a tube powered voltage divider.
Best regards!
Anyway, this is not a voltage regulator, as there's no reference involved. It might be ocnsidered a tube powered voltage divider.
Best regards!
Schematic says it's 375V "regulated" but there's neither a Zener nor a capacitor at the grid. Strange.
Advice? Remove the 6L6 regulator and use a more modern one. Either the original Maida regulator or an updated one like the 21st century Maida regulator. This will be a much higher performance regulator, which will translate into better sound quality. I have experience with 6L6 and 6DR7 based regulators, and the LM317 based Maida regulator is much better. I have used them to regulate the screens and front end in Dynaco Stereo 70, Mk 3 and several home brew amps. The tube regulators are more rugged, but are otherwise inferior as regulators.
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Thank you hifiamps. This sounds like a practical option worth looking into. I don't know if there is a practical way to deal with the existing 6L6 and ensure reliability. Of course, I have seen where many like these amps and still use them over half a century after they were built. I've had offers for mine but, for now, I want to keep it. My current project. I also like to give the builder the benefit of the doubt for what he did but, unfortunately, he is no longer amongst us so I can't ask him. Thanks again for the response. That may be the way to go.
The amp maker did not warrant the tubes. So "not my problem!" The tube may not fail for many months, even years: heater-cathode failure is slow and statistical. If you got lucky (no imperfections in the insulation) they may out-live you; the Vh-k rating covers the tube-maker's fanny for the inevitable flaws. As you say, these amps did not all die an early death.
I would leave it alone until the pile of burnt 6L6 got too big. Even in this tube-stressed world, one 6L6 is not a big running cost. The 7027s are probably more of a hassle today. (Observe different pinout and premium 6L6GC will probably serve.)
I would leave it alone until the pile of burnt 6L6 got too big. Even in this tube-stressed world, one 6L6 is not a big running cost. The 7027s are probably more of a hassle today. (Observe different pinout and premium 6L6GC will probably serve.)
Hi I would be tempted to fit a separate heater source.
Another option would be to leave the circuit as is and make up a plug in FET source follower, it could with the addition of one extra capacitor also function as a capacitor multiplier greatly reducing the screen supply voltage ripple; the basic component count would be 1 x power FET, 1 x 15V zener (gate to source), 1 x gate resistor (100 ohms) , 1 x capacitor (cap for multiplier), 1 x octal plug, 1 x heatsink, wire, solder, hardware etc.
Original operation could be restored by unplugging the solid state device and fitting a 6V6.
BTW the FET dissipation could be reduced by fitting a suitable wire wound resistor in its drain as only working headroom would be required for the power FET.
Regards Ken Kranz
Thanks Tubekwk,
The FET circuit sounds like it may do the trick; reliable and at little cost. I guess it's time to break out the old electronics books and read up on FETs. Regardless of which option I go with, this is the type of advice I am looking for. Glad I picked this forum to present the problem I am trying to solve.
Cheers
The FET circuit sounds like it may do the trick; reliable and at little cost. I guess it's time to break out the old electronics books and read up on FETs. Regardless of which option I go with, this is the type of advice I am looking for. Glad I picked this forum to present the problem I am trying to solve.
Cheers
The quality of modern IC regulation will significantly improve the sound quality compared to the other options. I have built Maida regulators on breadboards that are 2-3". The largest components are the 1uF mylar film caps (and they are necessary for optimal sound quality, because they effect the transient response). The main pass transistor (or MOSFET) that drops the voltage dissipates the heat, and I mount these directly to the chassis. It serves as the mount for the board. The other modification is that the wide voltage adjustment provided by the 20k 5W pot isn't needed. Calculate the resistors for the voltage, and then add a trimmer in parallel to one for fine adjustments. The large resistor should be a wire wound for dissipation and voltage stability (if the resistance changes with heating, the voltage will drift a bit). Also, don't run the screens at the voltage rating, drop it by 10-25V. Test your regulator IC at low voltage and be sure they regulate properly. My first LM317s were from Radio Shack, and the regulator voltage would drift. Then I realized that the LM317s were defective.
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This ShoCo is a guitar amp, not a hifi amp. It has come down to us through the ages because it "has a sound". I don't see why it needs to be changed (except health and safety issues (no, I'm not over-concerned for health of a 6L6)).
Isn't there the danger of collateral damage if the screen supply fails?
If there is room for a small 9VA 6.3V transformer under the chassis, then that is compensating for the change in quality of current tubes, and doesn't change the design. If it can be attached without having to drill holes, it is reversible too.
If the 6L6 stays, I'll install a small heater transformer for it even though I have not seen any excessive Vhk levels on the other tubes. I also plan to put in the standard safety upgrades and install some power supply and output transformer protection since it has none at the moment. Of course, most of the amps I have worked on in the past have never had internal fuses and such and I have never had a problem. I guess there is always a first time. There is no concern about modifying this amp as no one has told me that it is a collectable and it looks like someone beat me to drilling into the chassis anyway. I just want a nice clean sounding, reliable amp, and this is supposed to fit the bill (one reason for the 6L6). This amp has some unusual circuity in it compared to Fenders and Marshalls and it's easy to get info on them. More knowledgable people on this forum, and the books by Merlin Blencowe, give me the options and make this a safe and doable undertaking.
Thanks again and cheers.
Thanks again and cheers.
