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Giulietti = Ampeg?

By Sid of Stone Marmot

Jan. 25, 2009

Warning: To keep this article a reasonable size, I purposely skipped listing all the safety steps I had taken while troubleshooting this amp. There are some very high and potentially lethal voltages and currents in these tube amps. If you are not intimately familiar with the safety precautions necessary to troubleshoot tube amps, don't even think of trying this yourself. Take the amp to a qualified electronics technician. The amp isn't worth your life!!

Someone recently gave me a Giulietti vacuum tube amplifier. It is 18.5 x 14 x 10 inches and weighs less than 22 pounds. It has a 12 inch alnico Jensen P12S speaker in it, date coded spring of 1960. It has five tubes: two 6SL7s, two 6V6s, and one 5Y3. It has three controls: Volume, Tone, and Tremolo, all scaled from 0 to 10. Figures 1 and 2 show the front and back of this amplifier.

Figure 1 - Giulietti tube amplifier front

Figure 1 - Giulietti tube amplifier front

Figure 2 - Giulietti tube amplifier back

Figure 2 - Giulietti tube amplifier back

I never heard of Giulietti. But the 6SL7s, which are octal tubes like the 6V6s, sounded familiar, like something Ampeg might use in one of their older amps. So I looked through my various schematics and found a number of Ampeg amps that did use this tube.

I then popped the back off this amp. A schematic is glued to the back cover. The schematic looks like an exact print of the Ampeg J-12B Jet schematic, except, as shown in Figure 3, the lower corners were neatly cut out. (A bigger schematic is here.) The lower left hand corner would ordinarily be where the Ampeg model number would be, whereas the lower right is where the Ampeg logo would reside.
Figure 3 - Giulietti tube amplifier schematic

Figure 3 - Giulietti tube amplifier schematic

So, I did a little research. It turns out that from about 1958 to 1961 Ampeg built amps for two accordion manufacturers, Giulietti and Noble, which they then rebranded and sold as their own amplifiers. Consequently, this Giulietti amp I have is really a 1960 Ampeg Jet.

Since the amp is almost 50 years old and I was told it hadn't been turned on in at least 15 years, I just the assumed the power supply filter caps would be bad, the number one problem with amps this old, and replaced them before turning them on. Now many people would just try it as is without replacing these caps. If the caps are then bad, they could literally blow up and/or take out some other parts with them at power up. But even it the amp does appear to work, the caps are probably rather leaky and are pulling down your supply voltages.

I recently had this experience with a 1971 Ampeg V4 I acquired. I powered it up as is, after replacing a few visibly bad parts. It worked. But all the power supply voltages where lower than expected, with the highest being about 520 Vdc instead of the about 545 Vdc expected. The amp was measured at 110 Wrms max, more than the 100 Wrms advertised rating but less than the about 130 Wrms max these amps usually put out. I replaced the filter caps and the supply voltages all rose to what was expected, with the max power now measuring the 130 Wrms expected. These leaky power filtering caps could also have an impact on the amp's tone, though that is much more subjective.

Figure 4 shows the back of the amp opened up with the power supply filter caps replaced , though I left the original can in, unconnected. After replacing these caps, I powered up the amp and it sort of worked. The tremolo wasn't working and it distorted with any volume at all and was very raunchy when turned up to two. This didn't seem right so I further investigated the amp.

Figure 4 - Giulietti tube amplifier back open with filter caps replaced

Figure 4 - Giulietti tube amplifier back open with filter caps replaced

I suspected leaky coupling caps. This is a very common problem in old tube amps and is easy to test for. The input grid of every non-rectifier tube should be tied via a resistor to a DC voltage (usually ground, or a minus voltage for fixed bias power tubes). There should be no DC voltage drop across these grid resistors (a few tenths of a voltage is usually acceptable, though not ideal). This power amp is cathode self biased, with the cathodes of the 6V6s tied together and then going to ground through a 250 Ohms, 10 W resistors. This means the input grids of these tubes should be tied to ground through some resistors, which they are. But I initially was measuring around 55 Vdc at the grids of these tubes instead of the 0 Vdc expected. All the other input grids had the expected DC voltages on them.

So at least one of the capacitors feeding the input to these 6V6 power tubes was leaky. Since the tremolo modulation is injected here, there is an extra 1 Megohm resistor here between the connection of the 270 kohm resistors coming from each 6V6 input grid and ground, with the tremolo modulation being applied across the 1 Megohm. That means that three caps are feeding this circuit, a 20 nF from each side of the phase inverter circuit and a 100 nF from the tremolo circuit. Any one or more of these caps could be leaky.

An easy way to test which one is to lift one side of each cap one at a time and see if the problem goes away. This does render the audio portion of the circuit inoperable, but the DC biases should still work properly. It turned out that all three of these caps were leaky.

But replacing all three caps didn't totally fix the problem. I was still measuring about 5 Vdc at the grids. The tubes could be leaky, but I swapped them out with known good tubes and that did not solve the problem. So I measured the resistor values. The two 270 kohm resistors were measuring around 300 kohms, which is acceptable for this circuit. But the 1 Megohm was measuring about 7.5 Megohms, way too high! So I replaced this resistor. Now the power tube input grids were measuring around 0.3 Vdc, which is acceptable.

But the tremolo still wasn't working. Frequently, the values of the three caps feeding the grid for this tube have drifted too far from nominal values for this phase shift oscillator to have the proper phase shift around the loop to work. But I noticed that the plate voltage was way off, about 40 Vdc. I measured the plate voltage of the other tubes and the one side of the phase inverter (pin 5) was also way off, about 55 Vdc. Table 1 shows the expected voltages listed on the schematic on the back of this amp. All the other plate voltages seemed reasonable.

Table 1 - Voltages listed on Giulietti schematic for various tube pins

Tube Function l Pin Vdc l Pin Vdc l Pin Vdc
6SL7 preamp l 5 140 l 6 1.8 l
6SL7 tremolo l 2 110 l 3 1.2 l
6SL7 phase inverter A l 2 140 l 3 1.6 l
6SL7 phase inverter B l 5 135 l 6 1.4 l
6V6 power amp l 3 310 l 4 300 l 8 18

So I measured the resistance of all the plate resistors. These plate resistors often drift with time and use to voltages far off their original, usually on the high side. The tremolo plate resistor was measuring about 1.6 Megohms instead of 270 kohms and the plate resistor of the phase inverter side in question was measuring about 700 kohms instead of 120 kohms. Both these reading were way too high. The plate resistor for the phase inverter side that appeared to be working was measuring about 320 kohms instead of 120 kohms, which is also high and should also ideally match the other side of the phase inverter. So I replaced the all three of these resistors.

Now the amp is 100% functional. It puts out about 12 Wrms. It still breaks up early at around 3 on the volume control. But it is mostly clean up to there and has a much smoother sounding distortion when it does break up.

The amp is rather bright. This is probably due to the Jensen P12S, which is a 13 W speaker with the smallest alnico magnet of any 12 inch Jensen made for musical instrument amps. I did try the amp with other speakers and you can get a lot more bottom out of the amp with the right speaker.

The plate resistor of the preamp stage was measuring about 164 kohms, a little high for this 120 kohm resistor. But this is probably acceptable. A lot of the “mojo,” the special sound of these old vintage amps, probably comes from parts that have drifted from their design specs. This can be good or bad and is one reason why a lot of supposably identical vintage amps don't sound the same. A lot of the best sounding classics may be ones where the component values have by pure chance drifted into a sweet spot, whereas the ones that didn't sound so sweet have already been scrapped or rebuilt. So the classic amps we are hearing are the good ones that survived with the bad ones already being weeded out. So I'll try the amp for a while with this value, and a few others, off from ideal. If after some time I don't like it, I can always change them then.

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