Sure.
Here is a version...I like the possibilities current mirrors give, and to me the pros outweigh the cons. (No capacitor, wideband response, gain and bias (of the output tubes) in one, load resistors placed at any voltage level, etc)
For simplicity only a single 6AS7 is shown, and for simulations the load is 32ohms. 4 tubes in parallel is fine for 8ohms.
Many think the cathode resistance in the output is too high, but I've tested and believe it both sounds better, and provides better harmony between the parallel tubes.
The mirrors can be modded in all kinds of ways, cascoded, wilson style, base drive with another pnp's emitter to the mirror bases, etc etc.
The load/gain resistors can be varied without any change in distortion, just gain. U can experiment with various values for max symmetry, but I cannot hear distortion if it is slightly off, since it's all benighn second order.
The bias is adjusted with a potentiometer in series with the LTP tail resistor.
Btw, do u know why a CCS in the tail of the PI is actually worse in this circuit?
Cathode followers driving the output helps increase power, obviously, but I prefer the sound of the lesser powered CF-less version.
I don't use NFB, R13 is not there, but if u like, R13 can be up to 2kohms...actually, u decide, it can be any value...
SemperFi,
V3 & V4 are the same tube right? top is 123 bottom 456? Correct? Thx
Thx Stu,
I am wiring an octal socket tonight with bare solid wire that i can plug into the 9 pin socket of the 12ax7 up front. I am going to do this as mentioned above to and leave all original components in place just to see what the 6SL7 might generally sound like. I will chime in later tonight.
Seems to me that Kevin had one of our kits back in the old days, very similar to what you posted. We use a differential cascode with CCS for the voltage gain.
The advantage of this approach is even-order distortion cancellation, and almost nothing above the 3rd harmonic.
6C33s will hold together OK if you precondition them for abour 72 hours. If you don't do that, you will loose twice as many out of the box!
Your amps certainly have set the standard and by all reports are superb. I thought I might approach the input stage a bit differently. I thought the 6SN7 operated as you have it, was running a little low in current, but a nicer sounding tube probably can't be found. The 6SN7 used as a CCS in the tail, I considered doing with a cascoded pair of DN2540s.
While my current HF amp (custom build in a Citation II chassis) uses a Hedge circuit with cathode followers, I've grown less fond of the cascode because of its non-existent PSRR, lack of AC balance adjustment and very high output impedance. The cascaded input, DC coupled, of Covi's design appeals to me on those grounds. I would certainly use the aforementioned CCS in lieu of the LM317, and perhaps select a different tube lineup.
The driver stage of the Covi amp is a bit of a mess as well. The 6DJ8 is really overtaxed voltage wise. The PowerDrive circuit seems ideal here, and much simpler.
I have a small pile of 6C33Cs laying about, so maybe it's worth a try. At worst, i could refit those 6AS7Gs.
Stuart
Hey desperate!
May I assume that the reason you didn't post your results last evening is that you were up all night listening to the wonderful sound of your modification?
I hope it's not because you're in the hospital with a shock...
No got it done but kids u know so will play tonight and report back over the weekend.
If you have a differential cascode, you eliminate the PSRR issue. The output impedance does not have to be high- some things about the cascode are counterintuitive, and evaded even its designer! For example, depending on what tube is on the bottom, often you can increase Gain, linearity and bandwidth all at the same time by **increasing** the current (lower Rp value)! I've certainly seen this, so don't assume that a cascode has to have a high output impedance- you could be very wrong.
If you have a good CCS, the AC balance issue is of no consequence. A bad CCS and all your performance goes out the window...
To precondition any tube, the plate, grid and cathode are tied to the filaments and the tube lit up (no B+ obviously) for whatever time it seems to need. 72 hours seems to do it for the 6C33.
It just isn't true that you have to run the 6SN7 at a high plate current in order for it to sound good. Take a look at this loadline for the 6J5 (a singleton that's basically half a 6SN7). For this design, the 6J5 is running with an IPq= 1.0mA. The THD estimate is quite low, and this definitely sounds quite good.
I've used cascoded BJTs to make the active tail loads. Works great, and the AC balance per phase is excellent.
When used as an LTP splitter/amp, these are not problems. With an active tail load, the cascode LTP doesn't require any external AC balance adjust. As for the high output impedance, that's no worse than what you get with small signal pentodes, and becomes a non-issue when used in conjunction with cathode/source follower grid drivers.
That was the major problem: very few examples of cascoded LTP hollow state amps, though this is used quite frequently with solid state designs. So I went ahead and gave it a try, using 6BQ7s to make a single stage splitter/front end amp. It worked just great, with excellent sonics.
The way I've seen it described is to run the plate at ~100Vdc, adjust the plate current to: Ip= 400mA. Operate until you don't see any white specs on the cathodes.
Attachments
It did not work actually it took the amp down checking now on what happened. All was connected correctly checked 4 times but it killed it. Will figure out why and report back later.
Thanks for the preconditioning information.
I do use good CCSs.
Is the AC balance of the output stage unimportant? I thought the AC balance control in the input stage should be used to balance the complete amp. Is this wrong?
Thanks for your input,
Stuart
Never used the 6DJ8, however, it's the same design principle behind the 6BQ7: a type designed for cascoding to make VHF small signal amps for TV/FM xcvr front ends. 6BQ7s are available cheap, don't have that "audiophool pedigree" that drives up cost, and still work great as cascode LTPs, and as straight LTPs as well. (George has used them thusly).
Besides, the relatively enormous cathodes (for a small signal triode) add glowey bottle kewelness points. Just watch out for the 6BQ7s with series connected heaters: these tend to be microphonic and ring like bells. The parallel heater Motorolas and Sylvanias don't seem to be microphonic.
Hey desperateaudio (DA?),
I know you're familiar with the pinout of the 12xx7 series, but the 6SL7 end can be really confusing with it's odd pinout sequence.
Hope you get it straightened out soon. Best of luck.
Stuart
Miles,
I actually have a bunch pulled from old mobile radios. Naturally, most are Motorolas. I'll give 'em a look. Thanks for the tip.
Stuart
Hi there, I may have caught this a little late. This will be my first tube build and it looks like a easy to follow project. My question is if anyone would be willing to share with me the part numbers and source for the all the transformers for this build. I always find this part difficult for me!
Cheers
Cheers
post # 21 in this thread cnpope posted this info
http://www.diyaudio.com/forums/tubes-valves/175247-otl-designed-tim-mellow-4-6c33c-3.html
yan6-
While you certainly can use the Hammond #'s from post #21, you'll save a little money picking similarly spec'd transformers from Antek. If you prefer EI core transformers, check out Edcor's line. Fairly priced and high quality.
Stuart