1968 vs 1969 Super Lead sound

[shakti]

Good question, and I already thought about it. The 68 doesn't have the equalising resistors across the screen caps, but during my tests I did try to sub some in, and it didn't change anything. The 69 has the balancing resistors, IIRC they are 47k 2W Iskras.

[shakti]

OK, here it is. Same settings as last time. Red light is the 68 preamp with the 69 power amp, green light is the 69 preamp with the 68 power amp. Power amp in this case means anything after the tone stack, right? So if it is the power amp acting up, it means it could be in the phase inverter, the negative feedback loop or the actual output section?

Just thought I'd mention again that the 68 has a variable feedback pot, which is a 250k pot in series with the 47k resistor, going to the speaker jack. The 69 has a 47k going to the 8 ohm tap (chose this when it was set up for Hendrix sounds). I have turned up the feedback pot on the 68 to compensate and get it approximately equivalent values (i.e. 47k/8 ohm), but the variable feedback doesn't alter the breakup/lack of clean-up.

[shakti]

My own impressions;

- red light (68 preamp with 69 power amp): This is cleaner sounding with less of the nasty distortion, but it is also a little dull/muffled sonding. Could be because of the length of the shielded wire I used. I only had two-conductor Belden wire to use, don't know the capacitance. Also, the power amp in this clip is the 6550s/12AT7. So all in all; cleaner, but a little dull and curiously, less punch than...

- green light (69 preamp with 68 power amp): This has more of the nasty high end distortion, but is not as dull sounding. Curiously, this one is more punchy feeling than the opposite. Since it was the 68 amp acting up, one would assume that the 68 power amp (or PI) is causing the nasty distortion, but also that the combination amp using this power amp would have the least punch. To my ears this is not the case.

Could it be that whatever is bad here, is causing problems both in the preamp and power amp? Could something be starving the amp of power in some way, causing both a nasty distortion/early breakup, and a lack of punch? If the problem somehow affects both the preamp and power amp, I assume it would now be present in *both* these amp setups; in the red light amp the problem is causing a lack of punch (by affecting the output of the preamp in the 68), and in the green light amp the problem is causing a nasty distortion (but the amp retains power).

Or am I over-complicating things? Tell me what you think of the clips compared to the previous ones.

[Tazin]

The problem doesn't seem as pronounced in this video compared to the others. I think the extra punch heard in the green light ('69 preamp with '68 power amp) is partly due to the 50uF-50uF preamp filter cap. The remainder maybe be due to signal cap differences (Mullard vs Wima's)?
Not sure about the nasty distortion thing.....Perhaps something with the phase inverter tube? Did you try using a 12AT7 in the '68?

[shakti]

I agree, the difference isn't as pronounced. Testing it again I'm not sure if there's really that much difference in punch and output between the amps, but the red light one definitely has that nasty thing whereas the green light one doesn't, but seems very bland and muffled. But both setups sound kind of bland now, I think. It could be that using a good length of shielded wire kills some of the response in both amps, so I'm not sure if I should read too much into it. I did try a 12AT7 in the 68 at some point, but it didn't fix the issues I started with (less punch and early breakup).

I think I should focus on the 68 from the phase inverter and out, but where to begin? I did come to think of the epic hunt for the issue with my JTM100 black flag build back in 2010. That turned out to be due to wrong colour codes on the OT leads, so I inadvertently had it severely mismatched (16 ohm cab into 4 ohm tap), messing with the NFB and causing some noises and oscillation presumably through flyback voltage going into the NFB loop(?). This one is not as obviously wrong, but I think I should check the OT winding ratio and see if the impedances match up to what they are supposed to be.

[shakti]

I decided to level the playing feel as much as possible to make sure there weren't any other factors involved. So I set up both amps with a new set of tubes that are about identical. The output tubes are short base RFT EL34s, biased the same in both amps (quite cold). I put in identical PI tubes in both amps (Tesla E83CC). Preamp tubes were also just about identical, and I have swapped those back and forth without any changes. So now the tubes themselves are completely out of the equation.

The problem remains. The red light amp is a touch muffled sounding, but that could be the shielded wire length used to connect from the preamp of the 68 into the power amp of the 69. But it cleans up quite well, and does not have the harsh distortion. The green light amp sounds shrill, has a nasty distortion that I never really get rid of even with the guitar volume turned down low. It's slightly masked once you crank the amp, but it sounds harsh and a little thin compared to the red light amp. Overall volume level is about the same, as far as punchy feeling it's a little hard to say...the green light amp (69 preamp, 68 power amp) seems to have perhaps a touch more punch but it's destroyed by the shrillness/thinness.

[shakti]

Oh, and at some point now the green light amp (with the 68 power amp) started humming quite bad...V4 and V5 were redplating, even with very cold bias. After turning down the volumes it went away again.

Any clues or opinions on where this issue might be coming from?

One very far-fetched thought; the 68 amp has the fake Cliff jacks, I bought a bunch of them at some point without knowing they were Chinese made with inferior plating to the real Cliff UK ones. I've had to replace one or two of them in other amps where I used them, but there it just stopped working altogether. I don't suppose it could cause noise like this...?

[neikeel]

Oh, and at some point now the green light amp (with the 68 power amp) started humming quite bad...V4 and V5 were redplating, even with very cold bias. After turning down the volumes it went away again.

Any clues or opinions on where this issue might be coming from?

Leaking PI output coupler on that half of the PI? Duff anti-fizz cap? You ideally need to put a scope on each output grid to compare the signals have you measured the -ve on each grid.

Are you using single 1k5 swamps or individual 5k6 on the amps?

One very far-fetched thought; the 68 amp has the fake Cliff jacks, I bought a bunch of them at some point without knowing they were Chinese made with inferior plating to the real Cliff UK ones. I've had to replace one or two of them in other amps where I used them, but there it just stopped working altogether. I don't suppose it could cause noise like this...?

I doubt it.

[shakti]

There's no DC leakage on the output side of the output couplers. I have desoldered them at the output end and measured.

By measuring the negative on each output grid, do you mean the negative bias voltage? I have individual 5.6k grid stoppers (or maybe 6.8k, can't remember).

Unfortunately I don't have an oscilloscope. This is a case where it would certainly be handy.

[shakti]

Quick update, haven't had much time yet. I've tried to replace the PI filter cap, no change. So after replacing, one by one, all the filter caps in the amp (though not all at the same time) I think I can pretty safely rule those out altogether as the cause of my problems here. Same thing with tubes.

To sum things up, there is a problem with my 68 build which has probably been there from the start, but may have gotten worse lately. It is happening somewhere after the tone stack, and it is causing a slightly harsh distortion with early breakup, a slight lack of punch and a slight lack of dynamics. The amp retains almost full power, it is only slightly less output than the 69, but it appears to have more gain and a lot more compression.

Since the tubes appear to be cooking nicely and the output is almost the same as my 69, I am thinking the power amp is less likely to be the problem, so I think I will focus on the phase inverter next. Any suggestions on measurements that can be easily done (remember I have no oscilloscope)? Or I could just start replacing caps one by one and see...

[shakti]

Pulling my hair out over this one, slowly...

I tried disconnecting the feedback loop on the 68. No change in the fundamental problem (early breakup, slight harshness, lack of punch). Then I replaced, one by one, all the caps in the PI with some Mallory caps I had. So both the 0.022 input cap, the 0.1, the two 0.022 output caps, the 47pF fizz cap and finally the 0.1 presence cap. No change. I even replaced the 100k PI plate resistor as the one I was using was an old Iskra pull, but no change there either.

I suppose the next step is to look even closer at the output transformer. The primaries measured OK as mentioned.

I did take some voltage readings to compare the 69 and 68, values below:

68:
Plate voltage 515V
Screen voltage (before 1k grid resistors) 503V
Phase inverter filter cap 358V

69:
Plate voltage 513V
Screen voltage: 508V
Phase inverter filter cap: 358V

68 V3:
pin 1: 235,4V
Pin 2: 23,3V
pin 3: 40,7V
pin 6: 242,1V
pin 7: 21,1V
pin 8: 40,7V

69 V3:
pin 1: 219V
pin 2: 24,2V
pin 3: 42,4V
pin 6: 233,9V
pin 7: 22,5V
pin 8: 42,4V

Amps are biased similarly, very much on the cold side with current draw via 1 ohm resistors around 25-27mA, so they don't sound great at that bias, but the difference is the same as it always has been between the two amps in overall sound/attack.

Two things I notice; screen voltage is lower in the 68 comparatively, whereas the other voltages are very similar. Also, in the 69, the voltage drop across the PI plate resistors is bigger. I forgot the swap the PI tubes to see if that explains it, but I was thinking it could explain the difference in output (i.e. less voltage drop in the 68 = less output)?
See anything else that stands out?

One last thing; when I took readings on the cathode and grids of the PI tube, I checked to see what happened when I turned up the amp volume. On the 68, that sent it into a loud oscillation at high volume settings, but the same phenomenon did not occur in the 69.

[Carbia]

well, 68's does sound a little less loud than the '69's and compress more, 69's are tighter. This is normal.
Less filtering in '68's causes more voltage drop when the amp is loud.

The difference in screen voltage can be due to different chokes or even different current draw in different quads of el34's

[shakti]

But the specs are identical in mine except for the preamp filter cap. 50uF mains, 50uF screens, 100uF PI. And in my videos you can hear the distortion in the 68 that does not happen with the 69. Something is going on with the 68. It's somewhat subtle, but it's there.

If everything else fails I will try to rebuild the amp, at least from the PI and out. But even tah would be a bit of a crapshoot.

Some kind of parasitic oscillation perhaps? The OT secondaries are routed between V6 and V7 on the 68.

[shakti]

Double post

[shakti]

OK, I looked into the OT. I have the same OT in both my 68 and 69, so I could compare.

On the primary side, I measured between CT and each primary (pin 3) wire. On the 68 I get 17.1 and 15.7 ohms respectively. On the 69 I get 16.7 and 15.5 respectively. Seems OK to me?

Measuring voltage on the secondary, I calculated primary impedance for each ohm setting (4, 8, 16). On both OTs, I get almost exactly 1.7k for each setting, so that seems OK too.

Can I conclude from this that the OT is good, or could it still cause problems even with those readings?

The only things I haven't yet tried (from the PI and out) is to replace the 220k bias splitters, the 82k plate resistor and the bias caps, or the grid stoppers on output tubes (pin 5). But bias voltage is nice and stable and equal on all four tubes.

I am running out of suggestions now...got any more? I am tempted to rebuild the amp, in case I have a parasitic oscillation. In the below thread is a photo of how it started out, since then I have had the cap board in and out a few times and replaaced the 33uF caps on the screens with 100uF ones, and the PI cap was changed to a 50+50. I have the OT secondaries routed between V6 and V7, so they are running parallell with the heater wires for a bit. I did that to separate the primaries and secondaries. Tried lifting them away from the heater wires, but couldn't hear a change.

http://forum.metroamp.com/viewtopic.php?f=7&t=36954