Too much bass on Seeburg J

Thanks Rob, I really do appreciate the help. This has me stumped. I will order the new volume control today from Vern Tisdale. I need to order some motor mounts for my Wurlizer 2104 anyway and I see he has them too.

One question I have is about chassis ground, versus equipment ground in this unit. Most of the connections are made to equipment ground, which are insulated from the chassis. However, if you notice on the schematic, there is a terminal strip beside the 5879 tube that shows the equipment ground and chassis ground connected together at that point. In fact, that terminal strip is not insulated from the chassis, which makes the entire strip actually chassis ground. In the actual amp, that terminal strip is located very close to the 5879 tube, which is close to the bass control. I noticed that C-30 on the bass control is connected to that terminal strip at the 5879 tube, which is actually chassis ground. Would it make any difference in the bass if this cap goes to chassis ground, rather than equipment ground? It was done that way originally, because the caps on the bass control are .15uf caps, so they are fairly large (particularly the old paper caps replaced). To spread them out, the tech obviously connected to equipment ground at different locations. As I mentioned in my earlier message, it makes a big difference in the readings I get on the volume control taps, dependent on where I access the equipment ground. It would seem the readings should be the same, as the equipment ground should be common throughout the amp. Any words of wisdom you can give me on that and do you think the chassis ground would affect the bass sound?

Any other thoughts you have are appreciated. I will order the VC and continue tracing the circuit in hopes I find something.

Julian

Julian, it is considered good practice to place all low level signals ground returns onto a common buss. The reason is that in the presence of higher level/current signals a minute resistance in the ground can lead to feedback, modulation or oscillation when the higher current signals cause a slight resistance that floats the ground ever-so-slightly.

There should be NO difference in reading between that ground buss and chassis ground. Also, because the bass section comes at a stage of much higher signal level -nothing- from that stage should be connected to the low-level ground buss. For that matter, there should be no difference between any ground readings.

In the past I have found -many- occasions where riveted grounds become defective due to slight corrosion that developed over the many decades.

I have several locations with multiple wallboxes that required several auxiliary transformers to power all of them. Originally I use the existing daisy chain method as designed. However I found that I was getting wrong selections occasionally and after some tests I found that the ground connections that looped through all those transformers had developed resistance at the crimps and rivets inside the cases, you could actually see voltage appear at the ground when the wallboxes were drawing current. What should have been a solid ground was now floating slightly and interfering with the signal.

This happened on two locations.

After running an external ground strap to all of the transformers the problem was solved.

So, eliminate and difference of ground potential at that point and get any later stage components off that buss.

Rob

Julian,
All grounds are common at the input plug--also, just found a note regarding "grounding"on that model--It notes that the common for the standee resistor should be grounded at the ground lug near J-2 ( I assume that there was some problem discovered ?) Ron Rich

Thanks guys,

I am going to change those late stage leads off of the buss and see if I can determine why there appears to be a difference in potential. I will also check my readings again. If I am not mistaken, there was a difference in readings from the various points of the equipment ground throughtout the amp. If that is the case, would assume it would indicate a faulty connection somewhere. First I will verify again and see what I get.

I did verify the remainder of the connections in the amp and found no wiring errors according to the schematic. All wiring matched the drawing. One thing I did not do is change all of the ceramic caps. In my radio restoration, the ceramic caps are usually good and I thought they were probably good here as well. Perhaps that was a false assumption. I also changed several tubes that went when the amp failed. Perhaps one of the replacement tubes is faulty, although they all test good. I am using metal 6L6 tubes. They might change the sound some, although I would not think they would change the bass to this degree.

I will let you know what I find on the ground readings.

Julian

Julian,
Unless the customer insists, and is willing to pay for the extra time, I don't change the ceramics either--except in the case of "Solar" branded ones.
Ron Rich

I have finally finished with mySeeburg J and now have great sound. I wanted to take a minute to give an update on my findings, in case it might help someone in the future with a similar problem.

As it turned out, there was no one silver bullet that fixed everything. The issues were apparently caused by the increased output of the amp, after I rebuilt it, issues with the tubes that I had replaced and small problems that had accumilated over time. Although all the tubes tested good on my tube tester, as is always the case, the final test was how they performed in that particular amp.

1. - I first traced all circuits in the amp, according to the schematic and verified each of the components to be the correct value and to be terminated at the proper place. There were no wiring errors, or incorrect values found.

2. - I then replaced the 6SN7 tube, as this is one of the tubes I had replaced after the amp went down. Replacing that tube did reduce the amount of distortion I was experincing, although it did not completely eliminate the problem.

3. -I ordered a new volume control from Vern Tisdale, as Rob suggested and installed it. That corrected the poor response of the volume control.

4. - I reconnected several ground points on both equipment ground terminations and chassis ground. Again, some slight improvements.

5. - I replaced both 6L6 metal tubes with 6L6G glass tubes. I was able to get a closely matched pair of Slyvania 6L6G tubes. These tubes have a great sound in this amp.

6. - Using the information Ron provided, I rewired the amp to be a J version. The only difference was 8 component values (3 caps and 5 resistors). I felt that since this is a J unit and the J amp was designed for this unit, the changes were warranted to make the box back to it's original design.

At this point, all distortion is gone in the amp and the sound is great. Each of the changes seemed to bring some improvement, with a really nice sound change in the 6L6 tubes. The only issue now was the amp was so much stronger than before, the bass was causing vibrations within the cabinet. The vibrations were even with the volume at around 1/2 full open. To resolve this probem, I just started looking for any loose cabinet part I could find. I found vibrations with the program holder missing 2 clips, diffuser glass missing a couple of rubber gaskets on the end and the upper interior trim was loose at one point. Also, I pulled the two 12 inch speakers and found that sometime in the past the gaskets were torn up and removed, so the speakers were mounted directly to the plywood cabinet. I replaced those gaskets which softened the bass sound.

So final report is after all of the small fixes above, I now have a great sounding Seeburg J. I never realized how much sound I was losing, until I rebuilt the amp and fixed all of the small problems that had accumilated over time. I hope this update helps someone else in their efforts to improve the sound of their unit.

Julian

Julian,
Whew--glad you "got-it" ! Thanks for posting this--
You probably still have one thing left to do--however---
Take a "Sharpie" pen and mark a "J" on that ID plate, so that in about 100 years, when it needs re-doing, your great grandson, knows it was "converted". BTW, one quick question--the ID tag plate--is it metal or paper on that amp ? Ron Rich

Hi Ron,

It is metal and, as you suggested, I have written a J on the plate and filed a copy of the schematic you sent and the parts list (both marked with a J) in the bottom of the cabinet to the side of the vent screen. It should remain perfectly safe and handy for the next time it is needed. Thank you very much for your help in solving this problem and thanks to Rob for his help. This forum is a huge help when you have an issue to solve with these old boxes. I think many would be scrapped and destroyed if there was no place to seek advice. Please keep up the good work.

Julian

Julian,
Thanks for the info on the tag--someone was attempting to tell me the other day, that most of those were paper--I still do not believe that to be true--but I have seen paper ( or maybe vinyl ?) used--just thought that was someone's repro---- Ron Rich

Julian, delighted to read of success. The metal 6L6's were probably really weak, I don't believe metal 'L6s have been made in over 60 years.

You mentioned that the speaker 'gaskets" had been removed, that is odd and I wonder if the speakers are originals or replacements. The originals should have phased disconnect lugs. One a pip type and the other a flat spade on each speaker. I ask because on several machines I have bought the originals were replaced and 8 ohm speakers were use instead of the proper 16 ohm. This will overload the amp if the power switch is set to maximum (20 watt) and make weak tubes sound a lot worse...unless you are a guitar player .

Rob

Hi Rob,

These speakers do have the phased lugs you describe. I believe they are original. It appears someone had removed the speakers at some time and when they took them out had torn up the gaskets. Portions of the gaskets were on the speakers and portions were stuck on the cabinet, but the pieces did not make a whole gasket on either speaker. It was obvious they had been removed at some time. I thought they may have been removed when someone was working on the amp, so they could use the speakers for testing. Of course, that is no more than a guess. The cones appeared to be in really good shape, but one thing I did not mention in my recap is that I went ahead and recentered the cones in both speakers, while they were out, just to be on the safe side and to be sure I was not getting any distortion from a misaligned cone. I recentered them the same way I do a radio speaker by throughly wetting the cone of the speaker and then placing them with the cone facing up for about 24 hours to dry. That will usually recenter any misalignment in the speaker. I use a soft brush to apply clean water and take care to wet the cone only.

Thanks again for all of your help in solving the sound issues with this amp.

Julian