1940's Hammond PA amp

[bobloblaws]

Hey all, this is the second one of these relics I've had, this one is a few years older than the first. I've changed out the filter caps and tested all the tubes as best as I could (the published test settings for my EICO 667 tester is not viable for 6SL7 and 6SN7 preamp tubes so I bought one of each to swap in/out one at a time). The symptoms are a steady background hiss and gnarly distortion when the input is above a certain level. For example, plugging in a Strat with the amp volume knob at 2 or 3 I can pluck a string moderately and the output has a reasonable fidelity, save for the hiss. But if I increase the intensity of the attack it results in a really ugly distortion sound. As for the hiss, I understand it is often due to bad resistors. I've experimented with swapping in new resistors in selected spots (e.g. screen resistors). There are also some higher wattage resistors that I didn't have suitable replacements for to try out (I have some on order). I'm uploading my hand drawn schematic including voltages I recorded. The voltages seem reasonable to me. Other than the problem(s) being down to resistors I'm wondering if the output transformer could be a culprit? Any suggestions for further troubleshooting would be most welcome.

[Justin Thomas]

As far as the gnarly distortion when the input is above a certain level, what kinds of inputs were originally intended for the PA? Your guitar could be overdriving the input right out of the gate. I'm not up on the various output levels of mics & phonos, the more common inputs of the times.

As for the hiss, it's frequently a function of too much gain. Is the hiss there with nothing plugged in and all the controls to zero? Otherwise I've had hiss from bad tubes and also from oscillations...

Check R.G.'s geofex page for extra checks on the basics. Leave the transformers as your last suspects. Look for geofex.com.

Justin

[bobloblaws]

As far as the gnarly distortion when the input is above a certain level, what kinds of inputs were originally intended for the PA? Your guitar could be overdriving the input right out of the gate. I'm not up on the various output levels of mics & phonos, the more common inputs of the times.

As for the hiss, it's frequently a function of too much gain. Is the hiss there with nothing plugged in and all the controls to zero? Otherwise I've had hiss from bad tubes and also from oscillations...

Check R.G.'s geofex page for extra checks on the basics. Leave the transformers as your last suspects. Look for geofex.com.

Justin

Yes, to be clear I'm using the input with the extra gain stage, the one on top in the schematic. I actually disconnected the second input from the tube grid to rule out those components.

Also, the hiss is omnipresent. In fact, turning up the volume knob has little or no effect on the amount of hiss.

Thanks for reminding me of the RG page, it's in my bookmarks but I'd forgotten about it.

[Justin Thomas]

Did you ever try the other input before disconnecting it?
It looks like usung that top input adds two whole extra stages of amplification. That's a LOT for the 40s. The lower one seems to be a bit more in line with a high input signal...

Justin

[Helmholtz]

Several voltages seem to be off. Too low plate voltages at couple of 6N57 plates, 3.4V at the first 6N57s grid, 5.6V at the left PI grid (where is the grid leak resistor?), low PI plate voltages, 0.7V grid voltage at the lower 6L6. I suspect several leaky coupling caps.
It makes sense to also measure cathode voltages.

[bobloblaws]

Did you ever try the other input before disconnecting it?
It looks like usung that top input adds two whole extra stages of amplification. That's a LOT for the 40s. The lower one seems to be a bit more in line with a high input signal...

Justin

I'm sure I did, briefly, but I don't remember any specifics. I could give that another try in regard to the distortion issue.

[Enzo]

Agree on the coupling caps, the 3.4v on the grid of the second stage MUST be coming through the 0.05 cap. You are likely going to have to replace every non-ceramic cap. ANy of your caps the kind dipped in wax? Those are leaky for sure. Also, that +3.4 probably explains the real low plate voltage. Although the plate resistor could have shifted way high.

The positive voltages on the 6L6 grids means the 0.5uf caps are likely leaking.

Your 6SN7 phase inverter seems to lack grid resistors. You need resistors from each grid to the cathode or to ground.

Get all that stuff sorted before remotely worrying about tone.

Transformer? LAst thing on the list, most reliable thing in an amp.

[bobloblaws]

Agree on the coupling caps, the 3.4v on the grid of the second stage MUST be coming through the 0.05 cap. You are likely going to have to replace every non-ceramic cap. ANy of your caps the kind dipped in wax? Those are leaky for sure. Also, that +3.4 probably explains the real low plate voltage. Although the plate resistor could have shifted way high.

The positive voltages on the 6L6 grids means the 0.5uf caps are likely leaking.

Your 6SN7 phase inverter seems to lack grid resistors. You need resistors from each grid to the cathode or to ground.

Get all that stuff sorted before remotely worrying about tone.

Transformer? LAst thing on the list, most reliable thing in an amp.

Yes, ,most of the remaining caps are the wax dipped type. I'll address the issues you cite and see where I am, thanks!

[TimmyP1955]

Model number should be rubber-stamped on the chassis someplace. Schematic should be available.

[bobloblaws]

Hey guys, I'm just returning to this project.

Question: I'm looking at the "V3" tube, the 6NS7 with 31V on one of the plates. The output of that plate directly feeds the grid of the other triode, no coupling cap, no grid resistor. Is that wrong? I'm wondering because it looks unusual to me but nobody mentioned that as a problem area.

[drewl]

That's normal, a couple amps do that like Marshall.

For more gain on the low input channel, you can parallel the unused half of the first preamp tube.

[g1]

Yeah, that's a fairly typical cathode follower arrangement, note the signal is taken off the cathode, and cathode resistor is quite a bit higher than usual circuits.
The marshall arrangement can be seen on the second half of the second pre tube here:
https://music-electronics-forum.com/...7&d=1547248329

[bobloblaws]

Well, I'm still fighting this beast. Changed out a whack of suspect caps, resistors, and pots. Still have significant hum and hiss. I think there has been some improvement but it's hard to know for certain because while the changes may have eliminated some of the sources of noise they presumably also increased the overall gain (evidenced by higher plate voltages on certain tubes, for example) so remaining noise is louder. That's one of my theories anyway. Before I go any further I could use some advice on something. Currently there is a fairly significant hum going on. Touching the chassis reduces the hum somewhat. Passin my hand close to certain components in and around the tone stack increases the hum. My initial suspicion was that there must be a ground problem somewhere. I also have noticed that at idle the amp is drawing quite a lot of current. I have a small meter inline on the output of my variac and it shows the amp drawing about 1A of current from the wall. I compared another 2 x 6L6 amp (Blues Deluxe Reissue) and it is drawing closer to about 0.6A. For the record, the bias seems fairly hot. I calculated each power tube dissapating about 35W, which is hot even for cathode bias I think. Could this be related to a potential ground problem?


Edit: I'm not sure what I did exactly, but now I'm not getting the loud hum, so if I had a grounding problem maybe I've inadvertantly fixed it as I tried desoldering a few things and later returning it to where it was. Anyway, I'm still somewhat concerned about the amount of current it is drawing. Or could this simply be a reality of the older, maybe less efficient technology?

[olddawg]

As Enzo is fond of saying... it isn’t rocket science. If it isn’t redplating, isn’t hot, and sounds good why worry? Remember that line voltages were significantly lower when it was built. Some old PTs are fine with it because of the old 20% design rule. Some were barely adequate at the time. Make sure you have a chassis mains ground. Play it a while... see if it smokes. That’s half the fun!

[The Dude]

............ or, raise the value of the cathode resistor and get it biased in a reasonable range. That would, more than likely, lower the noise floor also.