Millivolt signal and tube sockets

[indianajo]

The Hammond H-100 organ has a hauntingly beautiful reverb feature that, combined with the "celeste" vibrato, simulates a stone cathedral. When it works. It exports the signal 8v PP to a spring, then brings it back as a 10 mv signal to an EF86 for extensive amplification. Schematic is here. It only works when the tube has been recently rocked in the socket. Socket has been cleaned, tube has been cleaned, I would like to be done with tube rocking and cleaning and push the organ against the wall. My experience indicates that 10 mv signals do not cross tin plated contacts reliably. Tube sockets are tin plated, as are tube pins. So I would like to solder something in this area to boost the signal before tube amplification. Also though the EF86 has it's fans (currently being talked up in "differential amplification" thread) it is $42 each. I would like to get rid of it and replace it with a soldered device, perhaps a FET transistor, or a fet plus a 12AX7 that is easy to buy, or somesuch. Complete Hammond H100 schematic is in an H100 service manual on archive.org, in case I didn't manage the attachment properly. It is schematic sheet 41, in the upper right. The Hammond schematics do not explicitly show how they handle the filament, and although it is obvious by inspection that it is ac, whether the EF86 is 12.6v or 6.3v is a mystery to me. I am afraid to tip the chassis over with the power on. There is 120 VAC running through the chassis, so I could replace the tube socket with a little 12VAC or 24 VAC transformer to generate transistor type voltages. I have 2n7000 Fets left over from another project, also 2n3904's, and tube sockets and 12AX7's. I'm afraid of just soldering to a glass tube, and I would like to keep the 250 VDC below the chassis. As the gain increases and decreases with popping noises, I believe the grids are connecting and disconnecting as I rock the tube. Any comments, advice?

[leadbelly]

Frankly, it still sounds like a bad socket to me. Cleaning socket contacts won't fix loose ones.

[gain-wire]

Well, congratulations first on your instrument... Funny Boston is plating on the Radio... The B3 plays on silently in the background of "Longtime".

Anyway, I would not recommend soldering on the tube pins itself, you will damage the inside. I do not think it is the socket. IF you do think it is, replace the socket. After all the circuit was meant to work this way.
Here are my suggestions, in order of ease:

-replace EF86 tube
-check phono jack on reverb tank, check all soldering joints on tube socket, I expect something else is loose.
-replace electrolytic capacitors (after nearly forty-five years, they are due)
-replace tube socket.

I wouldn't recommend adding any solid-state amplification, or any amplification, for that matter, it will ruin the originality of your organ and alter the sound. Maybe not drastically, but mixing solid-state and tubes in a vintage organ? nah.

Check Antique Electronic Supply for a new (vintage) tube.

[indianajo]

The electrolytic capacitor was changed with a 2006 220 uf 10 wv unit three weeks ago. Every other electrolytic cap in the organ was changed, also. Inspection revealed that the "console-tone cabinet" switch on this unit had never been soldered on the console end, even though a factory tech replaced 3 electrolytics in this chassis in 1977. I saw no other bad joints. Removing the recently sprayed and erased RCA plug and reinstalling it does not change the symptom. Wiggling the trimpot doesn't change the volume much. Rocking the tube does, and changes the volume resulting in 2 discrete steps. H100 organs have a resale value somewhere between $100 and "boat anchor" because, in part, they have enough bass volume to knock the preamp tubes out of the sockets, as well as this tube and the various volume trimpots wipers off the trace. Found a trimpot to be replaced with a soldered resistor on the vibrato chassis today. Reviews on the web of the H100 condemn "bad tube sockets" pretty blatently, although I don't think these sockets are any worse than the ones in my ST70 amp or PAS2 preamp. What is different is the speaker is attached to the amplifier chassis, and one input signal is 10 mv. Antique Electronics Supply, tubesandmore.com, is charging $41 an EF86 and don't even list 7199's or 7591 power tubes. When I can afford $100 in new 450 V caps for my tripplett tube tester, I will test the EF86, but I don't think the tube is at fault. The reason the entire power amp hasn't been pitched and replaced with a couple of surplus car amps is money, plus the amp chassis is "interesting" with the EF86, the 6BQ6's, 7199's, 7591's, expensive coupling transformers, etc. I went to the garage and dug out some RCA 7895 and 7586 "nuvistor's" hoping they were solder in tubes, but they were miniature metal tubes that also require tiny sockets. While I could change out this one socket, I do have a couple of workman's in shrink wrap, I am dubious that a 10 mv dry signal can reliably jump the gap no matter how new the lead tinplate is. Then there is the problem of the 12AX7's in the preamp the get knocked loose. At least, there I have a schematic of the transistor amp that Hammond replaced it with on the H200. Thanks for looking.

[leadbelly]

Originally Posted by indianajo

While I could change out this one socket, I do have a couple of workman's in shrink wrap, I am dubious that a 10 mv dry signal can reliably jump the gap no matter how new the lead tinplate is.

This is just nonsense and not science. You do realize that EVERYTHING electronic was made with tubes for the first half of the last century, including phono & mic preamps, test bench equipment, medical devices, and so on. 10mV is no sweat.

[kevinkr]

The tubes falling out can be fixed with simple spring clamp assemblies that you can get from a variety of sources online. (eBay, AES, New Sensor, etc.)

You can generally find EF86 for a lot less than $40 if you look around. You might find some on eBay, locally at radio shows, and hamfests - both are often a good source for these as well.

Junk tube mobile radios (Motorola taxi and police radios) often found at hamfests may provide tube mounting hardware that will help keep things in place in you errant organ. (And they are usually going super cheap.)

The socket is most likely bad and needs to replaced.

You may have to drill out rivets or resort to other subterfuge to mount the retaining clips, but they will solve the problem of tubes falling out due to vibration.

Leadbelly is correct, probably your issue has to do with a cheap, worn out socket. When looking for a replacement a good gold plated socket may help.

Cleaning the tube pins with a hard eraser or a fine emory board may also help. The tube itself may have an intemittent internal connection that is restored when you futz with it.

[Michael Koster]

Is the electrical contact potential, which arises when two metals having different work functions are in contact, significant wrt. millivolt level audio signals? What is the difference in work function between the tin-lead plate on a socket barrel and the copper-invar of a miniature tube pin? Isn't it just a tiny DC offset anyway? Isn't it really a battery, not a "diode" as some imply?

Hmmm

I'd replace the socket ;-)

When metallic connections loosen, there is an ohmic range formed by various salts, semiconducting oxides, and tiny metallic cross-sections e.g. tin whiskers. If a semiconductor is involved, these can rectify signals (like the detector in a crystal radio)

A healthy socket connection will make a gas-tight metallic bond that will pass microvolt level signals without distortion.

cheers,

Michael

Ps I don't have that album anymore but I remember some great work on that B3 immediately preceding "Long Time"

[cbdb]

Please dont add solid state. Thats like putting an electric motor in vintage car. Every year we loose a few more of these wonderful instruments. Try to restore it as close to the original as possible. Not ownly for posterity but it will be worth a lot morer $$$ in the long run.

[gain-wire]

Originally Posted by cbdb

Please dont add solid state. Thats like putting an electric motor in vintage car. Every year we loose a few more of these wonderful instruments. Try to restore it as close to the original as possible. Not ownly for posterity but it will be worth a lot morer $$$ in the long run.

morer?
Well, I didn't want to say it, but he's absolutely right. Once it's modified, it's difficult to put back to its original state.

Anyway, it is your thing. I would definitely replace the socket. I have an old Tektronix 543A that can amplify microvolt signals with ease, so it isn't that the circuit isn't capable or just barely.

[EC8010]

If it's the socket, it's loose pins. On most sockets it's possible to tighten the pins by careful poking with the tip of a small screwdriver or needle file. That's easier than replacing the socket - and nobody can accuse you of it not being original.