Cut Crazy Amp in Two or Put Big PT on It and Keep it Whole?

[The New Steve H]

I'm sorry I never have a normal question.

A while back, I went nuts and made a 4x6BM8 amp, loosely based on the Little Wing plus the 5F6A Bassman. My genius idea was to rig up TWO output transformers, with the idea that I could run them separately. To get 15w, turn one volume control up. To get 30w, turn both up.

It did work, but it was a really silly idea.

The amp has a great singing tone, but it lacks punch. I figured this was probably caused by heavy taxation of a fairly small PT. I have a bigger PT I can put in it.

Now I'm wondering: should I use the big PT, or should I just yank one OT and make a 15w amp?

Hard to decide.

I guess I can yank two tubes and play it as a 15w amp and get some idea how it would sound after being cut down.

[cbass]

You can hook up two identical OTs in paralell if you wanted for more headroom. Are you wanting astereo amp?. are you sure you're overtaxing the PT ? Does it get hot or sag alot?

[sluckey]

A pair of 6BM8s is only gonna give you about 6 or 7 watts. Not much punch. More like a little poke.

[The New Steve H]

It has been so long since I built this amp, I forgot how many watts it put out. I thought it was 30, but it was actually 15. Thanks for the correction.

I played it with two tubes removed, and I decided the best thing was to yank the other tubes and the extra OT, which was situated poorly anyway. The amp sounds a lot better now, and I lost a lot of hum caused by the OT position.

The idea was not to have a stereo amp. It was to have variable power.

The amp sounds wonderful, but the natural distortion gets muddy when I crank the volume and turn down the PPIMV. Something new to figure out.

[The New Steve H]

Apparently, the PPIMV reduces the feedback signal, so you get muddy bass as the volume comes down. I have checked into ways to get around this, but the amp is sounding really good. I stuck a 5AR4 in it to replace the 5Y3, and I put a Tube Amp Doctor 12AX7 at V1 to replace a 12AY7. The results are great.

I have always loved 12AY7's, but I think that's because most 12AX7's sounded crappy to me. TAD's seem much smoother.

I wish I knew where to find them in the original Chinese packaging so I could avoid the TAD surcharge.

[teemuk]

By attenuating signal from the phase inverter you are reducing "open loop gain" of the power amp. Since ratio of negative feedback is that of open -and closed loop gains (gain with global feedback path enabled), the reduction of open loop gain simultanously reduces the magnitude of negative feedback applied.

Negative feedback has prominent effect on overall distortion characteristics (threshold, softness of clipping, etc.) and also effects amp's output impedance and therefore its damping factor and frequency response to reactive loads.

So adjusting a control that tampers with open loop gains not only controls the magnitude of signal hitting power tube grids (and consequently magnitude of their overdrive) but it also changes the character of the amp entirely at its extreme settings. The effect is so distinct its the most common critique against PPIMV scheme, or any other concept that allows to adjust open loop gain.

And since point of negative feedback loop is to make the operation less dependent on open loop circuit characteristics, the negative feedback will try to "fight against" the PPIMV settings as much as it possibly can. e.g. If there's distortion in the output signal the point of NFB is to generate "error signal" and its that error signal that PPIMV controls and that clips the power tubes. More distortion in output clips them more (beacuse of the error signal within the closed loop), less distortion clips them less. So, NFB also makes the PPIMV control very "insensitive": Throughout a wide range of its dial you get very mild audible effects from the control, and when NFB can no longer fight the PPIMV adjustment, the sought after effect(s) appear abruptly.

IMO, open loop gain -based controls work ideally only when the circuit is genuinely "open loop".

[Leo_Gnardo]

I have always loved 12AY7's, but I think that's because most 12AX7's sounded crappy to me. TAD's seem much smoother.

I wish I knew where to find them in the original Chinese packaging so I could avoid the TAD surcharge.

Might try JJ 5751 for a lower gain choice, not terribly expensive. Tubesandmore/Antique has 'em. FWIW they also offer a low-gain 12AX7 but I haven't tried it yet. Don't know whether this last is engineered differently or if they just sell deficient-gain culls this way. Anything's possible in this crazy tube world.

TAD has to ship from China to Germany, then test/label/package, then ship to USA. Besides profit, you gotta pay for extra shipping & glitzy packaging.

EH has 12AY7, cost a lot, and I've found them to have a high percentage of noisy/ringy tubes. The good ones are good but I try to avoid 'em due to cost/headache factors.

[The New Steve H]

Thanks for the info on feedback. I fiddled around with bypassing the feedback resistor, but it made things worse.

My documentation on this amp is what you might call "sparse," so I'm not totally sure what's in it. I measured the resistance on the feedback resistor, and I got about 4K. I was expecting 56K, so I suppose it's time to root out my notes and drawings.

The amp sounds great at normal practice volume or with the PPIMV on full blast, so it's not an urgent problem. It's only an issue when the amp is turned down more than usual.

I was wrong about the 12AX7's. They are actually Svetlanas. About eleven bucks each on Ebay. Very good for the sound I like.

In case anyone cares, the TAD's are actually Shuguangs.