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Old 5th April 2008, 10:12 PM   #21
Hi_Q is offline Hi_Q  England
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Could you not extend the bias adjust circuit as per the attached pdf, by having independent adjust you would not have to have critically matched tubes. Just set each pot E1 to E4 for correct bias.
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Old 10th April 2008, 02:11 AM   #22
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Well, I got my Oscilloscope today from UPS. I set up a my amp and a speaker cab and performed the same bias set-up procedure as before(cathode current test) but this time I monitored the output sine wave from the line out jack with the scope and the same 8 Ohm cab connected. At idle the tubes only showed a difference of 3mA of Cathode current comparing the worst tube to the best tube. The sine wave was from my laptop signal generator. I got the crossover distortion dialed out as close as possible with my "calibrated eyeball" while still remaining under the max dissipation ratings on the clean channel.(5000mV across the 99.2Ohm Cathode resistors= 50.4mA Cathode current *440V Plate voltage =22.1W output power) The amp sound better than ever!!! I then switched the output tranny primary winding from the output tubes pin3. I did the prescribed "howl" test after triple checking the schematic, tranny hook-up diagram and color coding. When reversed blue to red and red to blue It creates a crazy buzzing/fast pulsing type of overtone, therefore I am 99.999% sure that the schematic, hook-up diagram and color coding are correct and that I followed them. I then connected a 440Hz(Key of A) sine wave from my Laptop signal generator and observed both the inverted and non-inverted outputs of the 12AT7 phase splitter. I did notice an odd Notch in the neg portion of the sine wave that was directly connected to the eq. I turned all of the knobs to zero and the notch disappeared. I also noticed that the sign wave was much wider in the peak than in the trough?? I then tried connecting the Global Negative feedback wire and got a sign wave that was more like a shark tooth and jumped in amplitude. I could not sync on it with any scope setting. it also appeared to be more like a 2 or 3 phase signal than a standard sign wave?

I really feel that I am looking
at a set of mismatched tubes and that maybe the farting sound that I am hearing is simply one side of the push pull going in to conduction before the other and conversely the other side coming out of conduction befeore the other as well as preamp oscillation as seen in the notches within the sign wave before the power section that were cleared up by EQ trimming. The High eq had the most effect on the notch. Thoughts??

One question still remains....I am leaving in the 100Ohm Cathode resistors on the power tubes(6l6gc) While leaving the circuit in accordance to the schematic I attached earlier. This is identical to a Mesa dual rectifier power stage with the exception of the 12ax7 that they use for a phase splitter. Is this a poor circuit design to begin with and/or will the cathode resistor cause long term problems in the poweramp operation??

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Old 10th April 2008, 04:50 PM   #23
cerrem is offline cerrem  United States
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I will give this some thought...and post back...
If there are any additional wierd things the amp did or those as well...

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Old 11th April 2008, 01:14 AM   #24
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I do not have too much to add to the excellent advice given so far, but perhaps some additional observations:

Those safety diodes across the OPT primary; can you just make sure that they are OK? They probably are, but it would be frustrating to hunt all over the show just to find later that those were not in order!

Then I have not tested the 12AT7 part of the amp, but off the cuff I frown on the large difference in anode load resistors for equal output - 68K and 100K? From my experience that seems to be somewhat large - you might try a 20K pot temporarily on the 68K side, just to make sure the outputs are equal. Some of your observations seem to indicate 2nd harmonic distortion somewhere.

But I would put my money on feedback instability (after the above has been checked) as was suggested earlier. Not just on the low-f side (motorboating) but also h.f. Here the scope will be invaluable. As you might know NFB needs to be optimised per OPT. It does not necessarily mean a poor OPT if there is trouble here, but h.f. phase shifts can cause havoc. A square wave generator would be a great help - but you might know all this!

Regarding the drifting of power tubes with high Rgs I have some experience. I also use two 6L6 pairs p. p. in a 100W fixed bias amplifier, in a circuit where I was forced to use Rgs of 470K. I servo-stabilize G1 drift, and in the process measured unchecked G1 drift of up to +4V (because of the 470Ks) at an anode current of 50mA and 25W dissipation per 6L6. (My bias voltage around -46V.) That would rather certainly (if one could use such an expression!) have caused runaway in the long run. I would guess that you might have a problem with your values.

A suggestion here: For cathode bias, if one bypasses with a high enough value capacitor, then for most music conditions operation close to fixed bias will result because of their storage behaviour. I know folks frown on electrolytics here, but with physical sizes these days it is quite practical to use 10mF (that is 10 000 uF) or more, preferably made up from a few caps in parallel. Just a thought.
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Old 29th January 2009, 12:14 AM   #25
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So, over the last 10 months I have taken this amp on the road and played almost a hundred gigs spanning 8 states. I used it as a clean power amp for amplifying my guitar processor. Having over 150 hours of run time, It was time to re-tube. I re-tubed, re-checked bias and got interested again...........and that is where my problems began again.

I started to dive in to the oscillation problem on the overdrive channel. It sends the tubes into massive current draw like 75-90 mA! After trying most of the tips in the previous posts last time and coming to a decent result I thought the problem was with the OT. I then removed the 100 ohm cathode resistors and added 1 Ohm precision resistors to retain cathode current measurement ease. It seems that the output transformer is just not compatible with this type of amp. For a refresher it is a Hammond 1650T with ultra linear taps, and the amp is a 6l6gc quad push-pull w/ fixed bias and GNF removed. I was wondering one thing.....what are you supposed to do with ultra linear taps if you do not use them??? How exactly should I dress the rest of the wires from the output trasnformer??? where should I ground it exactly?? Currently it is grounded directly to the chassis under the underside of the OT. I noticed that I if I move the secondary wires around even a centimeter i get all kinds of Low frequency oscillations and such. Additionally the distortion sound is harsh due to the oscillations and excess tube wattage dissipation I presume?...? If i use the clean channel, all is well and the cathode currents are normal, around 27 mA. @ 450 V on the plates.

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Old 1st February 2009, 02:42 PM   #26
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i have not gone all the way back in past posts, but the effect of moving the secondary wires around strongly suggest that you have unstable NFB - unless those wires run in close proximity to inputs, but I do not think the construction would have been done that inconsiderately.

You very likely have some h.f. oscillation which can easily manifest as motor-boating in relaxation oscillator fashion. As RichW has indicated, one is really in the wilderness without a signal generator and scope. Things are attempted the wrong way round by switching (rolling) OPTs, apart from being expensive. It is quite unlikely that you will be unable to get proper stable operation with all but the lousiest OPTs. But each has its own set of h.f. characteristics which must be included in NFB design, and this is easily done with the aid of a square wave signal and oscilloscope.

If not using the screen taps, just leave the leads out of the way. If they are long, roll them up and tape, or cut so that you can use them again. General advice on dressing is to keep signal wires (especially low level signals) away from power leads or output circuits. There is no hard and fast rule, but neatness gives benefit over a rat's nest kind of appearance.
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