I have a Hammond S-6 organ I picked up; with limited knowledge I am trying to trouble shoot it - using a light bulb limiter to power up.
Here's where I'm at with it now:
When I detach secondaries from p/s, I see correct voltages on p/s. (B+ 310v, 6.3v, 25v)
When I detach just the 6.3v, I am seeing almost right voltages - like 240v/5.5/22 and the light bulb limiter glows only faintly. So I think there is a problem in there. The 6.3v reads about 0.8ohms across it so the meter reads "short" between them, but I think that's normal (?)
With secondaries attached, I see 0 on B+, 1.2v, and 8v.
With secondaries attached (limiter glows bright), When I start pulling tubes (it has 2x 6v6, a couple 12ax7, three 12BH7, and lot of 12au7s) the bulb limiter gets dimmer and the voltages go up from that low starting point. As expected, the 12BH7s pull the most current and make the most difference. With just a few tubes in there I see a little bit of glow on some of the tubes with about 4.5v on that secondary.
I'm not sure this tells me anything about the tubes per se. No one or combination of them solves anything in terms of just getting good 6.3v voltage.
I replaced one filter cap, tested the other and it seems okay. Any ideas what kind of failure this might indicate? Although there is a lot to the circuit, nothing looks blown out or short. Any insights appreciated.
Here's where I'm at with it now:
When I detach secondaries from p/s, I see correct voltages on p/s. (B+ 310v, 6.3v, 25v)
When I detach just the 6.3v, I am seeing almost right voltages - like 240v/5.5/22 and the light bulb limiter glows only faintly. So I think there is a problem in there. The 6.3v reads about 0.8ohms across it so the meter reads "short" between them, but I think that's normal (?)
With secondaries attached, I see 0 on B+, 1.2v, and 8v.
With secondaries attached (limiter glows bright), When I start pulling tubes (it has 2x 6v6, a couple 12ax7, three 12BH7, and lot of 12au7s) the bulb limiter gets dimmer and the voltages go up from that low starting point. As expected, the 12BH7s pull the most current and make the most difference. With just a few tubes in there I see a little bit of glow on some of the tubes with about 4.5v on that secondary.
I'm not sure this tells me anything about the tubes per se. No one or combination of them solves anything in terms of just getting good 6.3v voltage.
I replaced one filter cap, tested the other and it seems okay. Any ideas what kind of failure this might indicate? Although there is a lot to the circuit, nothing looks blown out or short. Any insights appreciated.
There may not be a catastrophic short at all because with tubes out voltages seem to be o.k.
But a light bulb tester is probably not appropriate to decide good or bad.
The S6 pulls at least 160 Watt from mains, and more when tubes are cold.
If you connect a 100 Watt bulb in series, next to nothing is left for the organ and the bulb is almost fully on.
So that will not work for more than an initial test against catastrophic problems.
Have a look at this web site, which elaborates in length about testing and repair of S6.
http://crasno.ca/articles/HammondS6.htm
It also suggests to install a mains fuse first, because Hammond did not care to fit a fuse at all ...
But a light bulb tester is probably not appropriate to decide good or bad.
The S6 pulls at least 160 Watt from mains, and more when tubes are cold.
If you connect a 100 Watt bulb in series, next to nothing is left for the organ and the bulb is almost fully on.
So that will not work for more than an initial test against catastrophic problems.
Have a look at this web site, which elaborates in length about testing and repair of S6.
http://crasno.ca/articles/HammondS6.htm
It also suggests to install a mains fuse first, because Hammond did not care to fit a fuse at all ...
Last edited:
The bulb limiter is only for the initial power on, and nothing else. It alters the AC line voltage to the DUT.
If there is no smoke or flames, then disconnect the bulb limiter and power normally, or with a Variac slowly.
Only check supply DC voltages after removing the bulb limiter, or else after reaching the nominal AC line with a Variac.
And do install the missing fuse.
If there is no smoke or flames, then disconnect the bulb limiter and power normally, or with a Variac slowly.
Only check supply DC voltages after removing the bulb limiter, or else after reaching the nominal AC line with a Variac.
And do install the missing fuse.
Well, I'll be damned - you're right. Thanks much for that. I had powered it up with the light bulb limiter out of an abundance of caution, and when the bulb lit up, thought it was not wise to plug it straight in, but I did just now and everything works. 😎 I have voltages above spec (to be expected from higher current wall voltage, I guess). Glad to learn that.
Now on to further troubleshooting. I'm getting only very faint sound out of it - that is probably down to tubes. Thanks so much for the link to that site -- that's where I found the schematic. Didn't expect to come across someone who is familiar with this exact model so quick!
Now on to further troubleshooting. I'm getting only very faint sound out of it - that is probably down to tubes. Thanks so much for the link to that site -- that's where I found the schematic. Didn't expect to come across someone who is familiar with this exact model so quick!
Last edited:
You may also want to consider adding a 1N4007 in series with each plate of the 5U4 rectifier valve - this is often refered to as the 'yellow sheet mod'. It adds some insurance to the power transformer if the 5U4 fails.
The other simple mod is to add a 10 ohm current sense resistor in each 6V6 cathode in the output stage, as a simple way to measure and confirm that the 6V6's are operating with similar 'matched' cathode idle current - that improves the sound quality, but can importantly indicate if one 6V6 is hogging all the current and could then cause collateral damage to the output transformer.
The other simple mod is to add a 10 ohm current sense resistor in each 6V6 cathode in the output stage, as a simple way to measure and confirm that the 6V6's are operating with similar 'matched' cathode idle current - that improves the sound quality, but can importantly indicate if one 6V6 is hogging all the current and could then cause collateral damage to the output transformer.
Ah so thanks. So a 10ohm between cathode and the OT? And I'm looking for what? Current at idle?
I tried the phono input of the amp and get the same low level output, so that narrows it down some. It may be one or all of those three tubes, unless some capacitor could cause weak output - I replaced two of the five filter caps. Not sure how to test the OT itself. Mainly want to use this as an amp.
I am also interested in whether this can be modified to change the octave of the oscillators. I know nothing about tube synths.
I tried the phono input of the amp and get the same low level output, so that narrows it down some. It may be one or all of those three tubes, unless some capacitor could cause weak output - I replaced two of the five filter caps. Not sure how to test the OT itself. Mainly want to use this as an amp.
I am also interested in whether this can be modified to change the octave of the oscillators. I know nothing about tube synths.
Last edited:
The 10 ohm resistor is meant to measure the cathode current of the 6V6, so it is inserted between cathode terminal 8 and 0V, I suggest you read up on bias adjustment and setting idle bias for a 6V6.
Your S-6 is a bit more complicated than a typical valve amp as the negative bias voltage is generated using a 'back bias' technique, which I'd suggest you read up on, as it can be initially confusing, and you will need to appreciate in order to confirm the powering circuitry is working ok.
Your S-6 is a bit more complicated than a typical valve amp as the negative bias voltage is generated using a 'back bias' technique, which I'd suggest you read up on, as it can be initially confusing, and you will need to appreciate in order to confirm the powering circuitry is working ok.
The 10r resistors go from each 6V6's cathode to ground as Tim says. You want to use some decent resistors for this 3W rated made by a decent manufacturer like Ohmic or Welwyn. Welwyn W21 wirewounds are perfect.
Andy.
Andy.
It would be much simpler to rewire the front end and bias circuit to something more conventional.
You need to have a decent chassis, P2P wiring, you have a great playground there.
You need to have a decent chassis, P2P wiring, you have a great playground there.
What's the symptom your organ presents?
Anyway, the S-6 is a rather complex circuitry, with it's 22 freely running Hartley oscillators. It's packed on two rather large chassis, plus the PSU. What's your measurement equipment? At least you need to check all DC voltages and compare them with the service manual/circuit diagram.
I struggled to fix mine, and as I learned via Youtube from it's weird sounds, I decided to part it out eventually.
Best regards!
Anyway, the S-6 is a rather complex circuitry, with it's 22 freely running Hartley oscillators. It's packed on two rather large chassis, plus the PSU. What's your measurement equipment? At least you need to check all DC voltages and compare them with the service manual/circuit diagram.
I struggled to fix mine, and as I learned via Youtube from it's weird sounds, I decided to part it out eventually.
Best regards!
I'd say 3W is a bit of an overkill for those 10 ohms resistors. R*(Ia^2) will be pretty low.
Yeah, you don't necessarily want to oversize them too much. Cathode resistors are very cheap very effective fuses. They dont have to open stupid amounts of voltage.
Working well now, but power output seems low - may be weak power tubes. Using a test tone into the phono in, I measured AC voltage off one speaker (6.2ohm) at 2v, so that's 0.5watt output I guess - should be more like 12 watts I think. I haven't replaced the seconds can of main filter capacitors, I'll see if that impacts anything.
Was planning to just take out the power supply and make a Princeton type amp, but with it working and sounding nice it's a shame to part it out. Would be nice to be able to input guitar/bass at an earlier, higher gain stage and use it as a little amp. It's not going to sound like a Fender but could be useful. With music program into the phono in, sounds pretty good as a small mono amplifier through a decent speaker.
Would be really interesting to pitch shift everything down an octave or two because the two-note bass pedal sounds great but the keyboard doesn't have that range.
Yeah interesting you notice that. Mine is actually slightly different (S-6 vs S-4 model). Was trying to figure out how to get to that VR (it's in a box), because the service manual actually refers to it as "adjustable feedback condenser C237 serves as a factory set volume adjustment" level. Maybe mine is drifted out or set too low?
