Thanks Ron, I didn't mean to suggest that the hum was not coming from the speaker. Rob had suggested earlier that I try connecting another speaker to see if the hum was coming from the amplifier or perhaps the speaker itself. As the hum was also present in the second speaker, I'm assuming it is bein...

Sounds like 120hz to me. I guess I could post an audio sample if it would help. I attached an external speaker as you suggested, and the hum is present there as well. The output tubes are new and were supposedly a matched set, so they should be OK. The hum is also present even with no output tubes i...

OK, I finally had a chance to try some things. I moved the center tap from the speaker plug and connected it directly to ground, which didn't help. I also tried to increase the first two filter caps from 15uf to 22uf, which also didn't help. All tests were done with only the rectifier tube installed...

Thanks for all the suggestions guys. So if I'm getting hum with only the rectifier tube in, I should start by looking at the B+? And the possible causes are: Bad ground on the center tap of the B+ Bad filter caps. Lower resistance on the field coil. Anything else I should look at for this scenario? ...

Thanks Ron, 2. Depending on how your external source is connected, and "cable type/routing", it can cause hum. For certain, you will hear "noise" in between selections, if you have "dead time", as you have no mute (squelch, if AVC equipped) function. The hum is still pr...

I guess warm is a relative term. Right now I have the amp power bypassing the play control relay so I can connect my ipod to it and listen while I'm working. The hum is definitely coming from the speaker and the amp.

Now that the weather is a little warmer and I no longer have to run the heater in the garage, I've noticed that there is some hum present in the amp of my Seeburg 100B, and I can't seem to get rid of it. It isn't super loud, but it is noticeable when no music is playing. The hum is still present if ...

I just wanted to thank those of you who replied and give a quick update on this. I finally finished replacing the caps and most of the resistors in this amp, and got the new tubes necessary for the AGC. The amp sounds really good, and the AGC is working fine. When I was rebuilding the amp, I did not...

You are looking at the feedback resistor (430K) the added one is R9 (HFMA-1) @100K. Yep, you are right. There is a 100K resistor in that location. I also noticed that C4 has been increased to .05 mfd, as it is on the newer model amps with AVC. Thanks again for all your help. I put in an order today...

Thanks Rob. 1) there -has to be- a resistor between C4 (amp) and the connection from the reactance tube(SK7). The tube functions as a voltage controlled resistor and is part of a divider at that point. I just double checked this, and there is no resistor there. The .06 mfd. capacitor is connected di...

Thanks Ron and Rich, I found a schematic for the MRA3 (SN above 14,000) and spent some time today comparing the AVC section to what I have in the MRA1. I attached a revised schematic, redrawn to show the 6SL7 instead of an empty socket, and laid out like the Seeburg schematic is. The only real diffe...

Thanks Rob. The chances that I missed a connection or two in the schematic I attached are pretty good, things were packed in pretty well and I didn't want to disconnect anything at this point.

So you think the 6SJ7 currently in place should be a 6SK7, and the missing tube is a 6SL7?

Thanks Ron. This isn't a separate box, someone actually added two tube sockets to the amp chassis (where the ID plate usually is) and soldered the components in underneath.

Does anyone have the schematics for the add on box? Maybe it is similar?

Hi Guys, I'm working on a 100B with an MRA1-L6 amp. At some point, the amp has been modified with two additional tubes, which I am assuming are some type of AVC circuit. One of the tubes is a 6SJ7, but the second is missing, so I'm not sure what it should be. I've attached a schematic of the added c...