drz400 wrote:
What if you DC bias the Heater center tap to lets say 70VDC
Good for the hum and heater to cathode voltage no???
Sure, if you do it for the one stage that has it cathode "way up in the air." But if you do it across the board, now you're stressing all the other cathodes (which probably have 2-3v on them) as well.
--mark h
Why would the cathodes be stressed? That's just the standard DC heater lift idea - many people use it to get rid of noise.
Yes, quite right. That one uses a 12AU7. This one uses a 12AX7. I'm having trouble finding good examples using our common triode tubes. If anyone has good examples, I'd love to see them, as well as guitar amp examples using some of the other tubes like, what is it, the 6N1P?
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I have used a cascode input stage in my superlead. Sounded quite good and it was easy to put into the amp. It used a 100k plate bypassed with a 250pf cap and a .022 coupling cap into a 250K potmeter. The other values are the same as the schematic you've posted. 12ax7, so beware of the voltage... It is the firefly rev.3 schematic. I hope this helps,
Jelle
Randy and I were working out the values for a cascode stage to replace an EF86 in one of my designs back in 1999. I am glad to see that he went with it! I, on the other hand, had a DC offset that I never bothered to resolve and quickly went back to using the EF86. I believe that my problem had more to do with using a 12AX7 than the actual circuit.
IMHO, the 12AX7 is not a good tube for use in cascode stages. The tubes traditionally used in this circuit topology are the 6DJ8, 6922, and even the 6N1P. I believe that Randy is using the 12AT7.
Here are the heater to cathode ratings for the common preamp tubes:
12AT7 -- 90v
12AX7 -- 100v
12AU7 -- 180v
It looks like the latter is the safest bet. Even though it has a low mu, the cascode circuit has so much gain, I don't think it will be a problem.
That said, the discontinued Aiken Tomcat uses a single 12AT7 as a preamp tube, so it seems extremely likely that that's the cascode one!
mhuss wrote:
That said, the discontinued Aiken Tomcat uses a single 12AT7 as a preamp tube, so it seems extremely likely that that's the cascode one!
No, I don't use a cascode in any of my production amps.
The Tomcat has a 12AT7 in a normal two-stage cascaded setup. They sound fabulous in the first preamp stage if you design them properly, but only about 1 out of 25 is usable in terms of microphonics, so I don't recommend them.
OK, howabout this. Is this a workable example of using a 6DJ8 as a self-bias cascode input stage, which might perform similarly to a pentode?
I am considering trying this type of mod on my 18W on the "Normal" channel in place of the paralleled triode, to get a bit more gain and pentode-like tone.
Do I need a resistor between the plate and cathode of the two halves rather than directly connecting them?
What about that bypass cap from the grid of the top triode? Is 1uF the correct value or should I go with .1uF or something?
I presume that 820 ohm cathode res. will work here?
Firefly uses a similar circuit for the cascode boost stage but with a larger cathode resistor.
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Well never content to just sit around and let amps sit idle, I went ahead and whipped this up. It sounds extremely good but has too much gain for my 18W. I think I need to do something to tone it down. Sounds great up to about "4" and then just TOO MUCH and it gets raspy. Time to do some tuning
Anyway, the quesiton is ... is this an example of the self-bias cascode like RA was talking about?
I am running the cascode stage of the firefly (ECC83) in front of a low gain ECC83 10-15 W SE amp.
The original schematic uses a 220p cap bypassing the anode R for a roll off of the high freqs.
Since I had no capacitor of that value when I built the cascode stage, I inserted another pot&cap to ground on the output to tame the highs.
Gain is just right for me
Try increasing the cathode resistor or inserting some Rs in the signal path.
maybe YOU don't need a bypass cap for the cathode resistor...
Well ... I really just rewired the tube socket in the 18W to achieve this without adding any components to the turret board. The original circuit had 820/25uF cathode and 100K plate and both halves of a 12AX7 paralleled. This sounded very good up to about 8 or 9 on the volume knob, after which it became a bit buzzy, but really didn't offer a whole lot that the other channel didn't have (other channel being the trem channel with just a 100K/1.5K 12AX7).
So what I did is just rewire the socket, leaving the 820/25uF and the 100K in place on the turret board and added my 1M resistor across pins, added the 1uF cap from grid to ground, and changed the heater wire, moved another wire and bingo. Plug in the 6922 and play.
As a result, the amp is a lot more chimey and open sounding at low volume settings (1-2) than it was before (it was kind of dry and a bit muddy sounding at lower volumes with the parallel triode arrangement), and with the vol on "4" it has more crunch than it had before on "10".
So now that I have broken spades so to speak, I will go ahead and swap a couple of board components. Probably try 47K plate and 2.2K cathode resistors and see if that turns the gain down so that it has more than one notch of "clean" range on the pot, and you can dime it without really honestly thinking it's going to catch on fire.
This has been a fun raed. I like where it's apparently going. Dehughs,sir, what's the Latin in your signature translate as? Everytime I've read one of your posts I feel it says something about me.[guilt?] I never really studied up on some Latin befores. cheers
All excellent things are as difficult as they are rare__B Spinoza
cheers
