1940's Hammond PA amp

[Justin Thomas]

In my limited experience, tubes hum a bit when biased too hot, usually right before they start redplating. I actually take excessive hum as a good possibility my bias is out of whack or gone. I'm with The Dude - put the bias in a more reasonable range. It may help the hum.

Justin

[g1]

Agree with above about the cathode resistor.
What variant of 6L6 are in there? Old ones (like that would have come with new) are even lower max. dissipation. As you say, 35W is very hot even for a modern 6L6 running single ended, let alone an old type in push-pull.

[bobloblaws]

Agree with above about the cathode resistor.
What variant of 6L6 are in there? Old ones (like that would have come with new) are even lower max. dissipation. As you say, 35W is very hot even for a modern 6L6 running single ended, let alone an old type in push-pull.

Yes, I'm def gonna change the cathode resistor and tame it some, per the consensus opinion. For the record it has relatively new 6L6GC's.

[The Dude]

A few things you may or may not already know:

1) Don't forget to subtract the cathode voltage from the plate voltage when calculating power dissipation.
2) When cathode voltage increases because of a resistor change, make sure the cathode bypass cap is appropriately rated.
3) Keep the cathode bypass cap away from the cathode resistor so that heat doesn't cause the cap to fail prematurely.

Just noticed, no cathode cap in this schematic, so ignore the last 2.

[bobloblaws]

1) Don't forget to subtract the cathode voltage from the plate voltage when calculating power dissipation.

Yes, I factored that in. Here's my calculation, roughly the same on each tube:

Vp = 339V
Vk = 20V
Rk = 183.4 ohms

I = 20/183.4 = 0.109
P = (339 - 20) x 0.109 = 34.77W

[g1]

Your drawing shows that as a shared cathode resistor, so that 35W would be divided by 2 tubes, I think.
(I guess the 183 ohms is what the 150ohm marked resistor measured?)

[bobloblaws]

Your drawing shows that as a shared cathode resistor, so that 35W would be divided by 2 tubes, I think.
(I guess the 183 ohms is what the 150ohm marked resistor measured?)

Shoot, you're right, I forgot about that. Oops.

False alarm then, bias is actually pretty conservative.

At this point hiss is more of a problem than hum. On Keen's troubleshooting page it talks about resistors on the B+ line being potentially problematic in terms of hiss, particularly around 10K ohm. The only one I haven't changed is the 270K. I changed all the plate resistors in the preamp stages based on info from the same page. The resistors I haven't changed are the 6.9K on the 6L6 screens/OT center tap, the 6L6 cathode resistor, the 100K grid resistors on the power tubes, cathode resistors on V3A (470R) and V4A (470K) and V3B (22K) and V2 (2 x 3.3K), the 1Meg resistors between V2 plates and the tone stack, and the 2.2M grid resistor on V3A.

That's actually a lot of remaining resistors. I stopped short of replacing those simply because they weren't mentioned specifically on Keen's page and initially my goal was to keep it somewhat vintage. However, one of the first things I changed was the resistors on V1 and I seem to recall noticing that changing the cathode resistor made a difference in amount of hiss.


http://www.geofex.com/ampdbug/hiss.htm

[The Dude]

Agreed. I'm coming up with about 17W per tube, which is fine. No need to change the cathode resistor. I initially understood you had 35W per tube.

Edit: <simulpost>

[bobloblaws]

Agreed. I'm coming up with about 17W per tube, which is fine. No need to change the cathode resistor. I initially understood you had 35W per tube.

Edit: <simulpost>

Yeah, sorry about that, my bad.

[The Dude]

It would be helpful to know what stage the hiss is coming from. Can you remove tubes one at a time starting with V1 and tell us if/when the hiss is gone?

[bobloblaws]

It would be helpful to know what stage the hiss is coming from. Can you remove tubes one at a time starting with V1 and tell us if/when the hiss is gone?

Yes I can. I'm working on that right now as a matter of fact. Removing V2 eliminates the hiss. For starters I'm going to temporarily swap out the cathode resistors.

[The Dude]

Do you still have V2 second grid disconnected?

[bobloblaws]

Do you still have V2 second grid disconnected?

No, I hooked it back up, last week I think.

Anyway, changing out the 3.3K cathode resistors on V2 did the trick! The hiss is gone.

While I have your attention, there is something curious going on with the "high gain" channel which I first noticed yesterday. First of all I should mention that the problem with hum that I mentioned yesterday and then retracted, (where it hums even louder when I move my hand close to the tone stack etc.), appears to be intermittent. A couple of hours ago I was convinced it had somehow been corrected. This evening when I fired it up it was doing it again. So I'm not sure if these two things are related but I'm also seeing an aproximately 13K oscillation on the output, basically a clean sine wave, when the volume control is turned all the way down, and with no input source. As soon as I move it to say "1" on the dial it disappears. Any idea what might be going on there? I also noticed a discrepancy whereby yesterday the digital scope told me the frequency was just under 13K, today it's just over 13K. Sure enough, I verified with my variac that changing the the AC input voltage changes the frequency. Lower line voltage = lower frequency oscillation, for whatever that's worth.

[The Dude]

It was meant to accept a mic level input, so there's lots of gain there. I'm wondering if it's just picking up something in the room while it's opened up. Is there shielding in the head/cabinet that's not there while you're working on it?

On a side note, I noticed there are no cathode bypass caps at all in this amp- likely again because it's designed for a microphone. My guess is that there's plenty of/maybe too much low end. You might try adding a cathode bypass cap to, at least, the first gain stage(s) to tame the lows a bit if that is a problem.

[Enzo]

I think the mic input back then was expecting a crystal mic.

Note your input jacks do not have grounding shunts. They sit there acting like antennas.