Install a phase invertor trimmer?

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M Fowler
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Re: Install a phase invertor trimmer?

Post by M Fowler »

Tweaked D'lite schematic:
Tweeked_DLite_Amp_1v1.jpg
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norburybrook
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Re: Install a phase invertor trimmer?

Post by norburybrook »

pompeiisneaks wrote: Fri Aug 11, 2017 8:29 pm My general understanding of a trimmer adjustment would be that you get a known input sine wave, and get a scope on either side of the output of the PI after the coupling caps, before going into the power stage, and measure the peak to peak voltage of the sine wave. One should be larger than the other, dial the pot until it gets to be roughly the same. I would think this may vary depending on the PI tube, as well, so if you replace it, it would be slightly out of balance again. I don't know how critical it would be to get it exact. OTOH, If you don't have a scope, you could likely use a volt meter and set it to AC and measure at the same point and try to dial them to the same voltage and that should also work no? I've never read or seen how to do it, this is just how I imagine you'd be able to do it.

~Phil
Phil,

I always thought the idea was to get about 6 or 7volts difference between them with the highest voltage the plate nearest the output end of the board.

That's what I've always done and seems to work :D


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Roe
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Re: Install a phase invertor trimmer?

Post by Roe »

xtian wrote: Fri Aug 11, 2017 5:45 pm
Roe wrote: Fri Aug 11, 2017 10:08 amBalancing the phase inverter is very important for getting the right tone and response, particularly at full distortion (when NFB does not help) and if you have problems with ghosting.
I would like to learn more about how to adjust the PI trimmer. What do you look/listen for?

My #183 is working very nicely--I hear nothing to complain about. But I never touched the PI trimmer, ever. Just soldered it in, set it mid-way, and left it.
look for redlating, listen for ghosting. I try to maximize power, punch and a fast response. Test at full power with little or no NFB
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stelligan
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Re: Install a phase invertor trimmer?

Post by stelligan »

norburybrook wrote: Sat Aug 12, 2017 10:36 am
Phil,

I always thought the idea was to get about 6 or 7volts difference between them with the highest voltage the plate nearest the output end of the board.

That's what I've always done and seems to work :D


Marcus
This always works for me ^^^^^^
I shoot for about 6 volts, then tweak by ear. I will play the clean channel up at fair volume and pluck long tones up and down around D on the B string. Listening for sustain and "bloom". Not very scientific but, hey....
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pompeiisneaks
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Re: Install a phase invertor trimmer?

Post by pompeiisneaks »

Oh interesting, so a specific amount of difference is what helps balance but also accentuates some of the say 2nd order harmonics? Something like that?

~Phil
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norburybrook
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Re: Install a phase invertor trimmer?

Post by norburybrook »

pompeiisneaks wrote: Sat Aug 12, 2017 8:58 pm Oh interesting, so a specific amount of difference is what helps balance but also accentuates some of the say 2nd order harmonics? Something like that?

~Phil
yep, that's what I've always thought :D it's worked for me...........................so far :D


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Charlie Wilson
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Re: Install a phase invertor trimmer?

Post by Charlie Wilson »

Hello, I have always been a little baffled at the 6V(DC) reference for setting the trimmer as the trimmer balances the gain of the PI. Tubes can be all over the place with being matched or balanced. Even a matched triode can have unbalanced gain on either side. Because of this, it seems to me that a DC reading of the voltage on the plates is irrelevant. From information provided by Max it seems that the intent of the trimmer is to create a balanced output section. It also is likely that Dumble uses a harmonic distortion analyzer at the output to accomplish this. I have also read(and I think it was Tony that stated this) that a balanced output section encourages 3rd order harmonics not 2nd. Many of the guys like the sound of unbalancing the PI which will encourage 2nd order harmonics and maybe with a matched/balanced PI tube they are the most encouraged at a 6V difference. :D
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lovetone
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Re: Install a phase invertor trimmer?

Post by lovetone »

i'm with Charlie.
I all ways use a distortion meter and adjust for minimum distortion of the output stage.

I inject an 800Hz signal directly in to the power amp with the output stage running at 50% output power so 25w for a 50w amp. This yields around 0.3% THD this gives a nice sustain on clean and OD and at the right volume level gives a pleasing bloom.

Before this I would run the output stage until it clips and then adjust the PI pot for equal clipping, using 1Khz and a scope. Just for interest I used the distortion meter on an amplifier set up this way and found that it was set to minimum distortion so either of these methods gave the same results.
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Re: Install a phase invertor trimmer?

Post by martin manning »

Charlie Wilson wrote: Sun Aug 13, 2017 2:21 am Hello, I have always been a little baffled at the 6V(DC) reference for setting the trimmer as the trimmer balances the gain of the PI. Tubes can be all over the place with being matched or balanced. Even a matched triode can have unbalanced gain on either side. Because of this, it seems to me that a DC reading of the voltage on the plates is irrelevant. From information provided by Max it seems that the intent of the trimmer is to create a balanced output section. It also is likely that Dumble uses a harmonic distortion analyzer at the output to accomplish this. I have also read(and I think it was Tony that stated this) that a balanced output section encourages 3rd order harmonics not 2nd. Many of the guys like the sound of unbalancing the PI which will encourage 2nd order harmonics and maybe with a matched/balanced PI tube they are the most encouraged at a 6V difference. :D
Measuring AC signal at the PI outputs misses any unbalance in the power tubes and the OT, which could be significant, especially if the tubes are not matched in Gm. In the amp lovetone checked those elements were evidently a small piece of the THD.

There is a theory that says a slight miss-match produces an increase in second harmonic distortion, which is seen by some as desirable. If you have a distortion analyzer the procedure would be to find minimum distortion and then move from there to a nearby point where the second harmonic increases. The mismatch that maximizes this response is reported to be on the order of 2 dB.

I have seen the recommendation of 6-7 VDC difference in plate voltage repeated often here, but this value would definitely depend on how well the triodes in the PI tube are matched. Note that when using that method, it is the PI's input triode that should have the higher plate voltage.

In a simulation, a perfectly matched tube in LTP configuration with miss-matched plate resistors (100k and 110k, 10k trimmer centered) shows a difference of a bit under 4 VDC, and the outputs are within 0.1 dB (second triode output is higher gain). Miss-matching the plate resistors further with the trimmer (at 90% rotation, 111k and 119k total) produces a 6.5 VDC difference in DC plate voltage and a 0.6 dB miss-match in the output. This was done with no NFB. The DC values given will not be affected but this, but the difference in the AC response would presumably be reduced.
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Re: Install a phase invertor trimmer?

Post by Charlie Wilson »

Hello, I would like clarify for my sake and reference Martin's first paragraph. It seems to me that Lovetone's method for balancing the PI without a harmonic distortion analyzer does take into account mismatched tubes and output transformer. If you are looking at a sine wave on a scope you are downstream of the tubes and transformer. With my Mickey Mouse scope the only way I can see if both sides of the wave are equal when injecting a signal is to push the output to clipping. Dummy load gets pretty hot. :D Lovetone I am curious why do you choose that specific frequency as a test signal? Also if you don't have a HDA you could use Lovetone's method for the trimmer on the Tweedle Dee.
CW
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martin manning
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Re: Install a phase invertor trimmer?

Post by martin manning »

Looking for equal clipping at the speaker output would assess the whole power amp's symmetry, as described in lovetone's last paragraph. Setting the trimmer by looking at the PI outputs does not. That is a technique that has been described here, so that's why I made a note of it. You can also read DCV across the power tube cathodes if you have 1R current sense resistors (short the two on each side together for 4-tube outputs), and that will bring the power tubes into it. I think any of these techniques could be applied to a cathodyne PI, except a common cathode bias resistor on the outputs would prevent using the cross-cathode voltage method.
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martin manning
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Re: Install a phase invertor trimmer?

Post by martin manning »

A caveat- when looking for equal clipping at the output, I suppose it's possible that the power tubes could hit grid current limiting at unequal drive voltages, and so that would not be a balanced condition.

I would AC couple the scope to the output, and position the trace at zero on the graticule with zero signal. Then, with signal applied, adjust the trimmer for equal positive and negative peaks.
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Re: Install a phase invertor trimmer?

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There are many ways to go about balancing out!.. quickest and easiest method I have found is this one. :D Originally posted by Gil Ayan a way's back..You can download some kind of free signal generating software on your laptop/Tablet or they may have an app for it if you don't have a generator.(I made my own cable Min to 1/4 straight to front of the amp).A meter screwdriver and a couple alligator clips..You will have to have the 1 ohm resistors in the cathodes and matched output tubes since we are balancing current draw on the tubes on each side of the OPT primary (in relation to Gnd).BTW IMO..As long as you have a fairly well made (newer) output transformer I wouldn't worry too much about the winds being too out of balance there.After I get everything set up it only takes about a min or so to set it using this method..I have a 100W power resistor for the load and when I am done the resistor a little warm..The more you do it the quicker it becomes so I don't really worry about cooking my test load
Good Luck and Thank You Gil!!


1. Tie together the cathodes of the power tubes that are in phase with each other (i.e., the tubes that have their plates connected to the same B+ wire coming off the OT's primary).

2. Connect a digital multimeter set to DC Volts (yes, DC!) to each pair of cathodes, and set the scale to millivolt's. You will be measuring the voltage difference between the two pairs of cathodes.

3. Inject a 1 Vrms 100 Hz signal into the amp. I keep the amp in clean mode with the preamp volume turned down low and the tone controls where I normally set them (it will not matter much where you set them anyway, all I try to do is to not overdrive anything).

4. With a load connected to the speaker jack, set the output level (master volume) to the point where the amp dissipates about as much power as you think it dissipates when you normally use the amp. My rule of thumb is that at gigs I may be running at 50W tops, so into an 8-ohm load I will have a 20 Vrms output (P = V x V/R, so V = SQRT (PxR) = SQRT (50 x 8.) = 20). If you play your amp at lower volumes you may want to shoot for say 20W, which translates into 12.7V, etc. The output level will change the results/readings.

5. The multimeter's reading should be on the order of a few milivots. Turn the PI trimmer until the reading is 0 mV; 0 may go by quickly, so look for the reading to shift from + to - or vice versa. When 0 is reached, each pair of cathodes is at the same voltage with respect to ground and the two sides are therefore balanced. If the reading doesn't cross over, try another tube in the PI slot; it could also mean that your power tubes are not matched.

This method does not take into account any further imbalances introduced by the OT but has, per my experience, consistently yielded excellent results. I hope it works for you too.

Tony
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martin manning
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Re: Install a phase invertor trimmer?

Post by martin manning »

talbany wrote: Wed Aug 16, 2017 4:26 am...(I made my own cable Min to 1/4 straight to front of the amp)...
Tony, it's not clear from Gil's description but I believe the test signal should be injected at the loop return to provide a symmetrical sine wave at the PI input.
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Re: Install a phase invertor trimmer?

Post by talbany »

martin manning wrote: Wed Aug 16, 2017 10:26 am
talbany wrote: Wed Aug 16, 2017 4:26 am...(I made my own cable Min to 1/4 straight to front of the amp)...
Tony, it's not clear from Gil's description but I believe the test signal should be injected at the loop return to provide a symmetrical sine wave at the PI input.
Hey Martin

In sec 3 Gil mentions
I keep the amp in clean mode with the preamp volume turned down low and the tone controls where I normally set them (it will not matter much where you set them anyway, all I try to do is to not overdrive anything).

I think his reasoning here is try to set the amp to where he normally plays it live (around 50W range)..I imagine you could also inject the signal in the loop return and scope the output to the same dissipation range and set it there with the same results..I am not exactly sure why he prefer's to hit the front end?? :D

Tony
" The psychics on my bench is the same as Dumble'"
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