What phase inverter style has the best output to linearity ratio?

[Ray Ivers]

Iain,

The built-in 'Balance adjust' control should work as an AC balance control, but you could also make both plate resistors 180K or something, and fit a 50K pot (probably best to make this a rear-panel, plastic-shaft item) to their ends with its wiper to B+, then compensate for the DC imbalance with the Balance adjust pot (a large cap could be fitted across the 10K bias-circuit resistor to eliminate the AC-imbalance feedback to the input stages).

I've got a Sound City 120 here too - I've always thought it would benefit more from a master-volume installation than any other amp I've seen, as it's got so much gain after the volume controls.

Ray

[Zoe_N_Iain]

(Iain) Thanks, Ray.

Zoe's SC did have a Master Vol in it when she used to gig with it. Stick an overdrive pedal in the front and that amp would scream! Amusingly, the best pedal we ever found to drive it was a no-name POS that she got for £10 brand new from a little back street music shop in Edinburgh. It sounded atrocious through any other amp but absolutely amazing through the SC120.

[Arthur B.]

(Iain) And there was me commenting on 18W the other day that I hadn't seen a guitar amp with an Isodyne!

Must admit, I like the look of that PI - for some inexplicable reason. And as you say, it's direct coupled.

Zoe's Sound City L-120 is moving up the project queue slowly and we were toying with using an Isodyne in that, just to throw a curve. We want to keep the active tone controls in the preamp, of course. But a change of PI might be fun.

Don't suppose you see an obvious way to fit a variable PI balance control? We're liking the idea of being able to control the balance and, hence, the harmonics generated by the power amp to taste. What's one more knob?

The Isodyne already has a balance control. It's between the two CFs.

I experimented with the see-saw type of PI using a 12AT7. My implementation of it has an adjustable balance control.

http://scopeboy.com/poweramp.html

I never did figure out whether it sounded better or worse than other designs, though. This amp was designed with high-gain metal sounds in mind. I found that it sounded great at medium volumes for recording. But later when I took it out to jam with a drummer who was hitting pretty hard, I had to turn it right up, and I found that it kind of lost that heavy thumping bottom end you need for metal, and went mushy. (It was plugged into a Marshall 1960A IIRC.)

I think that's what the cathode followers of the Isodyne were designed to alleviate - low output and poor definition.

[Zoe_N_Iain]

Thanks for that, guys. I think I pretty much understand how to make an Isodyne that can be tuned for balance, now.

I'll draw one up when I get a minute and you can criticise it.

[Steve Conner]

Arthur, I don't know about poor output. I checked the amp with a sine wave generator and it puts out a full 50 watts before clipping. (70 with KT88s ) It also seems to clip the same way as other tube guitar amps I've seen: the tops and bottoms of the waveform flatten, and if you keep cranking up from there, the zero crossings grow flat spots from crossover distortion because the grid current pumped the bias too far negative. I concluded from that that the PI can put out more than enough drive for the power tubes.

[Arthur B.]

Arthur, I don't know about poor output. I checked the amp with a sine wave generator and it puts out a full 50 watts before clipping. (70 with KT88s ) It also seems to clip the same way as other tube guitar amps I've seen: the tops and bottoms of the waveform flatten, and if you keep cranking up from there, the zero crossings grow flat spots from crossover distortion because the grid current pumped the bias too far negative. I concluded from that that the PI can put out more than enough drive for the power tubes.

Could it maintain that output with a sgnal of wider bandwidth, like a square or triangle wave?

[Steve Conner]

When I built the amp I only had a sine wave generator. But the dirty channel turned that into a nice square wave, and the power stage seemed to reproduce that OK at levels up to what it could manage with the sine wave. I just bought a new signal generator, so I'll try it with the other waveforms.

When I compare this amp to my other one, a modded Selmer Treble'N'Bass 50, it sounds a lot darker, with less upper mids and treble. I was thinking about this and I remembered that I chose the compensation capacitor in the PI to make it stable with a 12AX7 PI tube and EL34 power tubes. IMO this is the worst case for stability since it has the highest overall loop gain.

I'm now thinking that this is what makes it sound somewhat dark and muddy. Since I use a 12AT7 PI tube now, the loop gain will be less, so the compensation cap (basically 140pF from one PI tube plate to ground) will start rolling off the treble instead of compensating the feedback loop.

But then again, maybe my Toaster amp actually has a flat response, and the tone stacks in the T'n'B have a treble boost even with all the knobs set halfway. Again, something to check out when I get my new generator, I guess

steve

[Merlinb]

Or just drop in a low ra valve to reduce output impedance to improve driving performance when grid current sets in. 12AY7 is a popular choice to replace the ECC83.

And if you can't get the big negative voltage as discussed above, just use a pentode for a constant currrent sink instead of a resistor tail.

[Guitarist]

I found this circuit which looks like it sounds very clean, simple and no transistors (which are OK but a misunderstood phenomenon). As long as these can drive the tubes to full, clean power it should work as well.

I saw a few builders use this split 12au7 which is very similar to the concertina (is that right) ala Orange but it must be cleaner and better balanced or all these cats wouldn't've used it.

[Arthur B.]

I found this circuit which looks like it sounds very clean, simple and no transistors (which are OK but a misunderstood phenomenon). As long as these can drive the tubes to full, clean power it should work as well.

I saw a few builders use this split 12au7 which is very similar to the concertina (is that right) ala Orange but it must be cleaner and better balanced or all these cats wouldn't've used it.

It's yet another variation on the Williamson amplifier. This time, the feedback is applied to the concertina as well.

[kg]

I think the most awe inspiring metal sound I ever heard was on a recording of Ken Gilbert's Big-Ass Guitar Amp system, which had (iirc) a 400W tube power amp firing through two 4x12"s fitted with EV speakers and stuffed with that acoustic blanket material they put in hi-fi speakers. I almost felt sorry for the poor drummer and bassist trapped in the room with it.

awww, you're making me blush.

fwiw, the driver circuit of the baga was pretty straight-forward: 1x 6n1p diff amp, sourced from symmetric split rail power supplies, plain old resistive cathode and plate loads, cap coupled to 4x el84 cathode followers, directly coupled to 4 banks of 3 kt90s each. the el84 cf's have constant current sinks in each of their cathodes.

in this way you could feed the input stage either a single ended or differential signal.

ken

ps don't feel sorry for the drummer--he's the reason i needed a big ass guitar amp in the first place.

edit: baga preamp schemo: http://ken-gilbert.com/images/pdf/BA...iverschema.pdf

[Steve Conner]

OMG, it's the man! I'm not kidding about the BAGA, it reminds me of someone gunning a V8 engine with no mufflers.

For a while I was all inspired to build a bass version of it, but it turns out the band I play with just now is quiet enough that I can use my old Toaster head for bass. I got a big 2x15" high efficiency cab to help it out. I can get the meter showing about 150mA of plate current before the singer starts complaining the bass is too loud.

This is kind of relevant to the thread I guess because the Toaster head is the one that uses the see-saw PI I posted earlier. I'm not sure the output is exactly "Clean" but it seems to have plenty. I think KT88s are fairly low gain and need a lot of grid drive, so my PI must be doing all right.

I went back to EL34s though. I tried the KT88s driving an 8 ohm speaker with the 16 ohm tap, and I never really got much more power, because the B+ just sagged drastically. I was expecting 100w but I only got about 70. Using a 16-ohm load they didn't really do anything that 6L6s or EL34s couldn't.

[kg]

Using a 16-ohm load they didn't really do anything that 6L6s or EL34s couldn't.

well, kt88s do tend to drain the wallet a bit quicker, don't they?

[Steve Conner]

Well, I thought so but it turns out the Svetlana 6550Cs and Shuguang KT88s aren't too pricey. A pair of these tubes costs about $10 more than a quad of 5881WXTs and does about the same power.

I found out that 100 watt hi-fi grade output transformers sure do drain the wallet though

[Smitty]

Bruce,

Is that the chi town copy of the Musicmaster Bass driver transformer? I wonder if that's got enough oomph with a 12AU7 running in parallel to drive the grids of EL34s just enough positive to round off the clipping I'm getting with a convnetional LTP. I'm wondering how far up fc goes during conductance.

I'm also looking at the monster from the 300 PS. Although it would work nicely, it seems like overkill. I really dig the split secondary. I like dual bias.

Perhaps I'm looking for Baby Bear's Driver

Goldilocks