Choke question

[Bob D]

Since it was all I had laying around, I used a Hammond 156G choke (9h, 40ma) in a Marshal style homebrew and I have very low volume (about 1/2 of what it should be). I think that it may be due to the ma rating of the choke. In this amp, the choke see's the power tube screens, the pi and the preamp tubes. It has 1 ef86 and 4 12ax7 tubes. Assuming 3ma for the ef86 and 5ma for each half of the 12ax7s and 8ma for power tube screens I get ~51ma which greatly exceeds the 40ma choke rating. Could this cause my low volume problem? Is my choke actually "choking" my amp?

[trobbins]

Extremely unlikely imho.

You should have measured circuit voltages, given it is a home brew - were they all ok at idle, and how much did they drop when you applied signal?

[mort]

As a noob, I'm going to guess that overfeeding the choke past its rating could severely effect its ability to conduct voltage, causing a drop, and probably heating it up, which would exacerbate the voltage drop and current rise until eventually the choke fails. And if there IS a significant enough voltage drop to the tubes, then output would be reduced across the board. Somebody tell me if I got this right...

[Bob D]

Choke question

All voltages check out at idle. I don't have any measurements with signal applied.

[trobbins]

Perhaps need to check signal levels achieved for each stage, and fault-find if not normal. This would include using a resistive load, and checking waveforms as they pass through each stage and for when sine waveforms start to distort with increasing signal level, and if possible also checking the actual voltage gain levels of each stage.

[loudthud]

Is this a 50W or 100W amp? Figure about 9mA for the whole preamp. Measure the voltage across the first B+ dropping resistor and divide by the resistance. That will give you the draw of all the downstream tubes. Measure across the screen dropper resisitors and that will give you the screen current. I doubt you are exceeding 40mA at idle. When you are pounding away and clipping the amp, that's when you will exceed the current rating. Under that condition the inductance of the choke drops very low and only the resistance of the coil (300 ohms) does any filtering until the wire overheats. Then poof, no picture, no sound. There must be some other problem to cause such a big voltage drop like a bad or unconnected capacitor.

[Bob D]

Thanks for the replies.

Unfortunately, I don't have a scope so I can't look at waveforms. I think I will order a properly sized choke for this amp. I'll report back with the results when I replace it.

I'm still wondering what the symptoms would be for an under sized choke.

Anyone?

[loudthud]

The problem could also be one or more open rectifier diodes. Post some DC voltages.

[Bob D]

To narrow down the problem I ran a patch cord from the FX send of this 50 watt amp (amp A) into the FX return of another 50 watt amp (amp B). There was plenty of volume. Then I ran a cord from the FX send of amp B into the FX return of amp A and there was low power. This tells me that the problem on amp A is from the FX return thru the power amp.

Then I hooked up a switch to bypass the FX loop on amp A. The low volume is there when the FX loop is engaged or bypassed. This rules out the FX return and tells me that the problem is from the PI thru the power amp.

My voltages at idle are as follows (tube rectified btw):
12Ax7:
pin1 213
pin2 19
pin3 35
pin6 210
pin7 20
pin8 35

6L6 (both tubes are virtually identical)
pin1 -41
pin3 411
pin4 408
pin5 -41
pin6 410
pin8 48mv

[Jazz P Bass]

What's up with pin 8 on the 6L6s?
Should that not be at ground?

[loudthud]

I think he has the output tube sockets wired blackface style where the grid stopper goes from pin 5 to pin 1 and the screen resisitor goes from pin 4 to pin 6.

What are the B+ voltages at the filter caps?

Try the amp with the power amp feedback disconnected.

[Bob D]

you are correct, I wired the output tubes blackface style.

I disconnected the feedback resistor and the amp got louder. This told me that I was applying negative feedback.

I'll measure the filter cap voltages tomorrow.

Thanks for taking the time to comment!

[Bob D]

Loudhud - that was a pretty good estimate on the current draw! I measured 90 volts across the 1st dropping resistor (10k) for 9ma draw.

Voltages at filter caps are as follows:
Vp = 414
Vs = 410
pi = 316
fx loop = 281
12ax7 pre1 = 263
12ax7 pre2 = 258
ef86 pre = 280

looks pretty good to me.

Comments?

[loudthud]

The voltages look ok. Unless they drop significantly (>10%) when the amp is played, the problem must be elsewhere. The experiment you did interconnecting two amps at the effects loop tells us there is definately a problem somewhere. Disconnecting the feedback should have stopped any oscillation. When you play through the new amp's power amp, is the signal clean when is should be, or distorted all the time?

Things to look at:

Voltages with big drop when playing. Clip your voltmeter to B+ voltages in the power amp and PI so you can watch as you play. Do any of them drop more than 10%?

Monitor the voltage an pin 8 on the power tubes. When you play a note the voltage should jump up from like 45mV to perhaps 200mV. Both tubes should jump about the same amount.

Output transformer. Check the ohms of the primary. The resistance from B+ to each tube plate should be close to the same, maybe 50 ohms on one side and 60 ohms on the other. And the resistance from one plate to the other plate should be the sum, 110 ohms in the example. Make sure there is no short on the secondary side between taps or any tap to ground. Next to impossible to measure with an ohm meter, you may have to disconnect some of the wires to make sure there is no short. Any wires you didn't know what to do with so you just grounded them?

Parasitic oscillation (ultrasonic). Check AC voltage across the speaker with no signal. It should be only a few millivolts. Not likely because the bias would be very hard to set if the power amp was oscillating.

[Bob D]

Well I finally found some time to work on this amp.

I swapped the choke with a new 150ma choke and it did not fix the low volume problem.

Running an old amp's clean channel into the new amp's fx return - the new amps power section had low power but stayed clean.

I monitored the voltage across the 1 ohm resistors and it was 48ma at idle and jumped to about 75ma when I hit a chord with the amp turned half way up. Both tubes jumped the same amount but nowhere near the 200ma you predicted. I would think that if I bias at 70% dissipation at idle (48ma), I should jump to about 70ma when I hit a chord. Do I have this wrong?

I still need to check for oscillations and B+ voltage drop tonight.

Any thoughts on my bias current at idle and under load?