Seeburg Amp MRA5-L6 in a V200

[jeffinmn]

In 2002 I got a Seeburg HF R100 which had been refurbished. The mech had been cleaned and lubricated, the amp and selection receiver had been recapped and the pin bank disassembled, cleaned and put back together. It worked and sounded great. After a few months I found a V200 on Ebay and purchased it. I rebuilt the amp and selection receiver, cleaned and lubricated the mech and had much of it rechromed. After that it also worked and sounded great.  It's my favorite by far between the two jukeboxes. I've been collecting Zenith radios from the late 1930s since 1980 so I've had a lot of experience recapping equipment and reading schematics. 

I noticed that the MRA5-L6 sounded a little better than the HFMA1-L6. The main difference is that when a loud record is compressed or volume limited the highs are reduced more than the other frequencies so the sound is somewhat muffled. If I leave the compressor turned off then it sounds great.  I noticed the similarity between the two amps so I put the amp from the R100 into the V200 and the V200 sounded great with the compressor on. I purchased another MRA5-L6 on Ebay, rebuilt it and put it into the R100 so it also sounded great. 

The HFMA1-L6 has been sitting on a shelf because it's the original amp for the V200 and I've hung onto it ever since. I'd appreciate input here from the experts as to the difference between these two amps and if what I did makes sense. Is one of these amps better than the other? Why would the compression circuit on the HFMA1 sound not as good as on the MRA5? If I got the compression problem fixed I'd be tempted to use it in the V200 since that's the box it's intended to be in. 

Thanks,
Jeff

[Ron Rich]

Jeff,
My GUESS -- would be--- it is onacountabecause --the HMFA -1 has a problem--bad tube/resistor value changed, or wrong version of the HMFA-1, or because,
Seeburg engineers considered the cabinet part of the sound system and did not consider swapping amps---Ron Rich

[robnyc]

Jeff, ALL of the single Tube AGC's in seeburgs do affect the highs more than bass. It is a primitive voltage dependent shunt circuit and the reactance tube (SK7 or BJ6) tube capacitance has an effect. The SK7 circuits also introduce some non-linearity when they are attenuating around 11db -but that goes away again at higher levels.

None of this should be too obvious though.

One thing to check for is to be sure that G1 voltage never exceeds the cathode on the SK7. If this happens the circuit loses control and a weird phasy & bassy distortion results. This voltage can be measure at the time constant (the 1 mfd cap)but even a 10megohm meter will load the circuits and bring down the actual voltage a bit -so a quick meter that will read before the time constant drains down too much is necessary.

Check or sub C18. This is the link that couples the reactance tube to the circuit.

Make sure all resistors in that area are within tolerance.

It is essentially the same circuits as the later MRA series -so you can compare voltages and other values.

Finally, I assume you are using the same Sk7 and other early-stage tubes for these comparisons.

RobNYC

[jeffinmn]

Hi Rob,
It's been years since I had done this comparison. When I get back in town I'll install the HFMA1 into the V200, install the tubes that have been in the MRA5 and see what happens to the high frequencies. If possible I'd appreciate it if you'd list off the resistor numbers I should check.


Are there other tubes besides the 6SK7 that can increase the compression function? When I first brought home the R100 I tested all the tubes and found a  different tube where the 6SK7 was supposed to be. I removed it and installed a 6SK7 which reduced the compression, and if I remember right, the speed of the compression. I set the tube that was originally in the location that the 6SK7 is supposed to be aside but with my other jukebox tubes but I'm not sure which one it was. I'd like to try that tube again so see how it changes the compression function. 

[robnyc]

Jeff, the 6SK7 came in both glass and metal envelope.

I have listed as subs:
6AB7
6AC7
6SG7
6SJ7
6SH7
-However, the SK7 is a remote cutoff pentode. This means a wider range of action from the control grid and that is important here. I experimented with some subs back in the early 1990s and found that they often had too-sharp curves that resulted in rapid changes in gain when used in this circuit.

"Are there other tubes besides the 6SK7 that can increase the compression function?"

Analog gain control circuits are all based on attenuating an excess of gain.So, essentially, anything ahead of the 6SL7 AGc amp and rectifier can add to compression by producing added gain. I did experiment with replacing the rect section of the SL7 with a silicon diode, but didn't keep any notes on the results. In theory this would increase the agc due to less loss across the sil diode vs. the diode-connected section of the tube.

"If possible I'd appreciate it if you'd list off the resistor numbers I should check"

Just the resistors in the immediate area of the SK7...but it is good practice to check all resistors when it can be done with disconnecting them. I generally just test voltages with a know-good set of tubes I keep for that purpose.

I'm interested to hear how this all comes out.

RobNYC

[jeffinmn]

I appreciate you sharing your knowledge and will definitely post the results of what I do when I get back home.

Two other related questions:

I have a brown remote volume control MRVC-1 connected to the MRA5-L6 amp in the V200. I don't have the remove volume control MRVC-2 for the HFMA1. I saw in the V200 manual that it says not to use the MRVC-1 on the V200. I'm wondering why? Are the specs different on the volume control pot? The amps seem so similar.

I'm also using a Seeburg Remote Amp HFA1-L6 to power some ceiling speakers. I have it connected to the MRA5-L6 via the 8 ohm speaker connection. I believe the HFMA1-L6 only has connections for C.V. speakers at 70 volts. Is there a way to connect the Seeburg remote amp to the HFMA1-L6?

My basement play room consists of the V200, a desk top computer and 6 pinball machines. I also have 3 of the 3WA wall boxes connected to the V200 including one that takes half dollars. The wallboxes are in other room of the basement. I enjoy this room very much.

[robnyc]

" I saw in the V200 manual that it says not to use the MRVC-1 on the V200" The older amps used a two-gang pot that varied bass separately from the full range. The HFMA use a dual tapped 25K-ohm pot and accomplished all the bass boost & control within the two taps on that pot. IIRC the RVC plugs are different -did you change it? No harm will be done but you won't get the normal bass control action or volume curves.

"I'm also using a Seeburg Remote Amp HFA1-L6 to power some ceiling speakers. I have it connected to the MRA5-L6 via the 8 ohm speaker connection. I believe the HFMA1-L6 only has connections for C.V. speakers at 70 volts. Is there a way to connect the Seeburg remote amp to the HFMA1-L6?"

I'm not familiar with how those amps are intended to be connected. Normally when I connect an external power amp (in nearly all my locations) they connect via the taps in this pic: www.flickr.com/photos/90641375@N06/39761405971/in/dateposted-public/

Green line is Pin 9 -chassis ground -used in all cases.

Red line is Pin 1. This gives and output that is not controlled by the jukebox volume or bass control (local or RVC).

Blue line is Pin 7. This one is controlled by the juke volume control and bass control as well.

These connections are desirable in that they remove the output stage of the main amp from the feed to the booster amp.




Note that if "some speakers" means more than one 8 or 16 ohm speaker you may be overloading that amp. For an 8 ohm load you can connect two 8's in series or two 16's in parallel.

RobNYC

[jeffinmn]

I have not installed the HFMA1 in my V200 yet. With the information you have provided I will not use the MRVC-1 with it since it's not compatible due to the different value of the volume pots.

[jeffinmn]

Yesterday I removed the MRA5-L6 from my V200 and installed the HFMA1-L6 in my V200. I also removed the tubes from the MRA5-L6 and installed them in the HFMA1-L6 as Ron suggested. I found that I like the sound of the HFMA1-L6 better than the MRA5-L6 because it has, to my ears, more treble and better over all tone. I did have to turn the bass control to 3 otherwise the bass was overpowering, and I like bass. I ran the bass control at 4 in the MRA5-L6.

Now the problem I have is that I don't have the remote volume control MRVC-2, but I see in the service manual the diagram for it. I think I can build one. Jukeboxparts.com sells a 25k ohm pot with two taps that is listed as the proper replacement. I'm wondering if anyone here has used this pot and if it sounds the same as the factory pot. The manual shows that the cable is a 6 conductor cable plus shield connected to the pot at point A. Four other wire are connected to the other lugs on the pot and then the two remaining wires connect to a cancel switch.

Does this sound reasonable? Any suggestions for me?

[robnyc]

Jeff, I used several of the dual-tapped pots from Vern Tisdale back in the mid-late 1990's. They are good quality and have more accurate curves than most of the originals that I ran out of. Taps are @ 7k-ohm and 17k-ohm.

Tisdale's are slot type (as original)and appear to still be $25: www.verntisdale.com/Seeburgpage.htm

RobNYC

[jeffinmn]

I called Vern Tisdale's phone number which was answered by his son. Vern died in 2013 and the son has been selling off the remaining parts. He sold the last volume control for a V200 last week and he doesn't know what Vern's sources were for his merchandise. If you need anything I'd call him to see what he has left. Do you have any other suggestions for me?

Thanks for providing the tap values. I'm going to email Jukeboxparts.com and see what value the taps are at on the pot he has. I'm also having trouble finding a 7 pin plug to connect the wires from the cable to.

[robnyc]

I was aware that Vern Tisdale died but figured that his son might still have some pots.

As for the male 7 pin, here are some suggestions:
www.leedsradio.com/parts-sockets.html
www.etsy.com/listing/514923719/7-pin-miniature-plug-fits-7-pin

It is a 7 pin miniature tube male plug.

If you are unable to find one online, you can always remove the existing jumper dummy and solder leads from the socket to a terminal strip. If all else fails I'll see what I have here. I don't expect to be setting up any more vinyl locations.

RobNYC

[Ron Rich]

"Guyz, Anyone who knew Vern, knew he was a GREAT "BS'er" ! If you wanted the truth, his wife was the one to ask !
Anyway when he got the volume controls, he told me that he got them from the OEM factory. At that time, they were in the process of closing their doors, as the owner wanted to quit. Vern told me he bought all he could get--at that time-- Ron Rich

[jukeboxmarty]

Durfee has pots, but the shafts are not stock.

needles4jukeboxes.com/volume_control_pots.htm

[robnyc]

Feb 20, 2018 0:07:50 GMT robnyc said:

I was aware that Vern Tisdale died but figured that his son might still have some pots.

As for the male 7 pin, here are some suggestions:
www.leedsradio.com/parts-sockets.html
www.etsy.com/listing/514923719/7-pin-miniature-plug-fits-7-pin

It is a 7 pin miniature tube male plug.

If you are unable to find one online, you can always remove the existing jumper dummy and solder leads from the socket to a terminal strip. If all else fails I'll see what I have here. I don't expect to be setting up any more vinyl locations.

RobNYC

On the Leeds link -this is what you are looking for:
7 pin minature plug -fits sockets for 1L6, 6AU6 etc
good for making tube adapters and test jigs with
screw on back shell - NOS $8.00

RobNYC