About the Pignose Brand Guitar Amplifiers:

Pignose (tm) Corp Pignose Home Page makes two fine tube amps for guitar: a 40 watt version and a 60 watt version. Their 40 watt version features a brown leatherette cover while the 60 watt version has a black face. Also, the 60 watt version adds a hi-low gain switch and a spring based reverb.

Their low cost makes them very attractive for blues harp players. Unfortunately, when played, their sound is more rock and roll than blues. This can be fixed!

First I will describe what I consider the best tube amp for harp ever made, the single 6V6 National Supro. Then I'm going to follow Joe Weber's thinking and show you how to upgrade the circuits in your 40V and 60V for blues harp.

I will dig into the Pignose circuits with instructions, drawings and photographs. However, before I do that let me insert this warning - my instructions are intended for the experienced amplifier technician not for the beginner.

The 6V6 National Supro Circuit:

My little Supro uses a 6SL7 pre-amp, a simple tone control and a cathode biased 6V6. The power supply uses a 5Y3 tube rectifier. The 6SL7 is a medium gain dual triode with curves that approximate the 12AT7. Both sections of the 6SL7 are operated with a low plate voltage, 92 and 110 volts respectively.

The volume and tone controls are located between the 6SL7 stages.

When I tested this little amp in the lab I could only get at most 3 Watts output into an 8 ohm dummy load. The voltages and bias on the 6V6 are just right for a full 5 Watts output. I measured the output transformer and found that the turns ratio represented a big impedance mis-match between the 6V6 and the speaker.

From the tube curves I found that the performance was in line with the mismatched transformer - higher distortion and lower output power. This mismatch also changes the damping factor on the speaker.

The Pignose 40 and 60 V:

Both versions start with a 12AX7 cascade pre-amp and with circuits similar to the Supro. After the pre-amp the circuits begin to resemble the Fender Bassmaster.

A 12AX7 is configured as a cathode follower. This creates a low drive impedance for the tone stack. The tone stack has bass, mid, and treble controls and is followed by the phase inverter - a 12AX7 configured as a "long-tailed-pair."

The phase inverter drives a pair of 6L6 tubes. The respective output tubes are the correct version for the specified power output. The finals are grid biased to draw about 30 mA plate current each.

The output transformer has the correct turns ratio for the selected speaker impedance, 4 or 8 ohms. At the speaker connections there is a presence control which adjusts the feedback to the input of the phase inverter.

Both amplifiers performed well on my bench. The 40V gave a solid 40 Watts output into an 8 ohm load and the 60V a solid 60 Watts.

The 60V has a high-low gain switch in the cathode of the first pre-amp tube and adds a 12AX7 for the spring reverb circuit.

Lastly, both amplifiers employ solid state rectifiers.

Some Background Information:

Joe Weber, in his book, "Tube Guitar Amplifier Essentials," offers a list of features for a Harp Amp. He suggests that:

1. The preamplifier tubes should be starved for plate voltage. We see this in the Supro circuit with only 92 volts on the first stage plate and 270K load resistors. I think part of the performance comes from the higher load resistance, but Joe says to leave this resistor (usually 100K for a 12AX7 circuit) alone and drop the voltage by increasing the supply resistance. I'm going to go with 270K - 280K load resistors.

2. There should be a paraphase style phase inverter. Joe suggests that this circuit makes the amplifier resistant to feedback squeal. It works by sending a different signal, with different gain and distortion to each 6L6. I am game to try this.

3. The final tubes should be cathode biased. We see this in the Supro too - a cathode resistor with no bypass capacitor. I can argue with this. The bypassless resistor adds negative feedback to the output stage, making it near impossible to get any overdrive sound from the final. In the Supro I believe this offsets to some extent the speaker impedance mismatch. It also serves to lower the effective plate voltage on the final tubes and thus reduce the available power output. I'm going to pass on this!

4. The final tubes should be operated in class A. Normally with a push pull circuit, one tube conducts while the other idles. With audio circuits, the idling tube is not completely cut off. This is called class AB and balances how hot the tubes get with good sound. Operating a push pull pair in class A makes everything get hotter. I see that if we reduce the plate voltage with the cathode resistor and maybe more with a tube rectifier we can increase the plate current without getting too hot. I don't see the point. I'm going to pass on this too.

5. There should be a tube rectifier. Tube rectifiers have a current to voltage relationship that is more complex than a simple resistor. When an amplifier is played hard there is a momentary slump in output power which makes the sound more percussive at first and then tend to level the loudness of the tone immediately following. This sounds great for harp. It also lowers the available output power by about 1/3.

6. Adding one of my own: the speaker should be mismatched to the final tubes. For the Pignose this is simple enough, plug the 8 ohm speaker into the 4 ohm jack.

The Modifications:

1. Starving the Pre-Amp: I began by fixing a 3 tab terminal strip under the transformer mounting screw nearest the V1 circuits. Step 2 I lifted the B+ end of the two 100K plate resistors for V1 from the PC board and soldered 180K ohm resistors from the lifted end to an ungrounded tab on the strip. I also soldered a 10 mF 400 WV filter capacitor between this tab and ground. For Step 3 I added a wire from the B+ terminal on the filter board over to the other ungrounded terminal on the strip. Step 4 I used a decade resistance box to determine the proper dropping resistance value to set the V1A plate to 85 volts and then permanently mounted the closest 1/2 watt value resistor to the strip.

2. Paraphase Inverter: I copied the paraphase inverter from a Fender(tm) Deluxe 5D3. I built the circuit on a small piece of perforated prototype board and hot glued it into the chassis. In order to make it fit I removed the unused capacitors from the Pignose circuit board. Because the Fender circuits have a DC feedback loop and Pignose circuits use grid bias I added 0.22 mF decoupling capacitors in both the 6V6 grids.

3. Tube Rectifier: I only added a tube rectifier to the 60V and not the 40V. There is no clean place to mount it on the 40V chassis and I did not want to reduce the power output any further. Step 1: For the 60 V I began by unsoldering and removing the filter capacitor board and cutting and drilling the tube socket hole under where it was. Step 2: I mounted my 5 volt filament transformer on the underside of the chassis and using screws from the existing power transformer. I had to modify the filament transformer mounting holes to get this fit. Step 3: I wired the rectifier socket following Joe Weber's instructions from his book, including using series 1N4007 rectifier diodes. Add an extra wire from one of the 5U4 plates, you will need it later for the minus bias supply.

4 & 5. These amps sound so good so far I think I'm going to pass on these, at least for now!

6. Plug the speaker into the 4 jack: An easy change that I think works best on the 60V.

But First a Few Words About Feedback:

Feedback is what happens when the gain of your amplifier exceeds the loss in sound from your speaker back to your microphone. Gain is greater than loss.

Every stage I've played on, there was no sound baffling anywhere; all the walls were hard. This is the ideal situation for feedback.

Seems that, to continue my experience, amplifiers start to feedback somewhere around 20 Watts output, but with careful control of the frequency response and a degree of "crunch", this can be raised to around 40 Watts.

Thus, the reason that the boutique amplifiers seem to be feedback resistant is because their circuits have good control of frequency response, they have a good crunch sound, and their output power is limited to about 40 Watts.

By frequency response I means that the peak or hump where the greatest gain occurs is set to between 200 and 300 Hz. By "crunch" I mean that when the signal gain is set with the volume controls, the circuit begins to soft clip at a fairly low gain setting.

Indeed is possible to test an amplifier and microphone combination by setting them up and seeing, by measuring with a sound level meter, how loud one can play before feedback begins.

I will do this when I get to my results.

The Results:

Results: For the 40V I starved the pre-amp and converted to a paraphase inverter. For the 60V I starved the pre-amp and converted to a tube rectifier.

For the 40V, starving the pre-amp added a lot of crunch to the sound and the paraphase inverter rounded the sound - this amp sounds great. The un-equal gain from the paraphase reduced the available output power, before clipping begins, from 40 Watts to 32 Watts. It is likely that a change in bias on the 12AX7 would give it more headroom. The inverter change was no silver bullet for feedback though.

For the 60V, starving the pre-amp added crunch to the sound. Because this amp has a much higher B+ voltage, Rx, the starvation dropping resistor had a higher value than with the 40V. For some reason this change seemed to help a lot with feedback resistance. Adding the tube rectifier gave the amp a lot of punch and also seems to round the sound. This amp also sounds great.

What's next is to take these amps out on the road and play them. Over the next three months that's exactly what I am going to do.

Drug Intervention
Drug Intervention