A few miscellaneous questions.

[robnyc]

Those amps were designed for ample gain if using a redhead original mono spikes. The entire gain structure is predicated on the approx 30mv output from that cart on a 'loud" record.

So, what styli are you using and are you asking for more volume or more -gain- as in the ability of the AGC to level loud and low cut records effectively?

Seeburg amps were designed to drive the output finals to a point slightly below audible distortion assuming normal input and fully effective AGC action. If you want more -volume- to drive external speakers louder, it is best to take a line out at the mute or volume control to an external amp of any desired power.

FWIW: Since I only used the later carts on my location machines, I needed more gain to make the AGC fully effective. At first I built an op-amp (LM84 based) pre-preamp and kept the 5879. This worked well, but with heavy use I noticed that the 5879 tubes were deteriorating and at that time (mid-late 80s) there seemed to be no new production. I adapted a typical 12AX7 preamp design from the RCA tube manual I liked that model as it used no feedback and since we now had two stages i could modify the equalization to boost the treble response and set the gain to the ideal output of the preamp.. I continue to use these in the V-VL and punched the chassis some of the 201 (HFMA2) to replace the crappy single transistor pre there -on others I just bypass the existing pre with an external.

Modding the 5879 based pre's for 12AX7 is not much work since the socket and most voltages are readily available.

Point is, you have a number of options here.

RobNYC

[nyjb]

I am using the redhead with stereo "dog-leg" stylii. I have a few other cartridges that are considerably louder, but I prefer the sound of the redheads.
I'd like both more volume and more gain; as it is, there's very little AGC action.
(Is there a difference between AGC & AVC?)

Could increasing the grid load resistor offset the decreased output of the stereo stylus? If I did that, would I need to alter the bias resistor also?

I'm willing to try the modification of the preamp.

Separately, I have an amp on which the bass is really muddy. It's most noticable when treble control lowest, naturally. Where should I look to address it?

Lastly, does the AVC switch have any direct effect on the tone?

Thanks!

[robnyc]

The "dog leg" styli have substantially less output. This results in little/no AGC action.

There really is no simple fix (that I know of)for the lower gain of this amp when using anything other than original mono styli.

"Separately, I have an amp on which the bass is really muddy. It's most noticable when treble control lowest, naturally. Where should I look to address it?"

Is this in relation to another amp tried on the same speakers? IOW: Does another amp of the same vintage sound better? There are a number of fixes but first it is helpful to ascertain if anything is actually wrong, or you are just noticing the shortcomings of these older designs.

"Lastly, does the AVC switch have any direct effect on the tone?" Indirectly. The switch just introduces some attenuation to prevent overload and distortion if the AGC is switched off. Since the AGC works by reducing gain on loud records (attack) and just allowing more gain on low records (release) if it is switched off sometimes this will lead to a volume control setting that places it outside of where most of the bass boost would occur. Similarly, styli/carts with lower output can cause this as well due to a lack of gain in the preamp. A weakening set of tubes are another cause of weaker bass. Due to the nature of this AGC design where the 6Sk7 tube acts as a shunt to reduce gain, the tube's capacitance comes into play and reduces treble a bit when it is conducting heavily. It is not a sophisticated circuit and there is a it is a tradeoff. I find the leveling action essential in commercial service and helpful in home use so you live with the slight dulling (only a few db).

"(Is there a difference between AGC & AVC?)" Technically, yes. AGC can describe any kind of circuit that controls via servo action (feedback or shunt) the level of some sort of signal or current. RF-IF AGC are used in analog TV and radio receivers to produce better reception but generally have little effect on "loudness at the speaker". AVC is specific to audio level.

For jukeboxes the term AVC is more accurate. I'm just more used to seeing these as AGC circuits.

Rob

[Ron Rich]

Guy's,
The "dog legged" type needles sold for the red head vary greatly in output--I don't understand why, but they do. If you want "good ones", purchase only from "legitimate sources"( listed above in FAQ's), as some of the eBay sellers sell "junk", The good ones are NOT cheap ! Also the good ones require that the needle pressure be set at 4.5 grams.
Ron Rich

[nyjb]

Good info.  I'm pretty sure the dog-legs I have are the high-quality ones, but I didn't know they needed a different tracking force.

I haven't been able to find the steps for modding the 12ax7 to replace 5879.  I'm guessing Rob's put it out there somewhere.

I'm puzzled by the change in value of grid-load resistor.  for mra5-l6 it was originally .1 meg, then changed to 47k.  that's a huge difference, not a tweak.  the grid load resistor also changed dramatically.  I'll leave this alone if you think I should, but is there any risk in increasing that resistor a bit?

Lastly, for Rob, you recommended adding a horn, and later suggested that you don't replace the 5" speaker.  Where do you add the horn?

[nyjb]

Hate to be kinda dummy, but I got a few common mode chokes for the 12" crossover. It's rated at 1.2 amps. It has 4 pins, which I didn't expect. How do I know which to connect to what?

[Ron Rich]

Hey--Watch out !!--I'z da Head dummy in charge here" And the only answer I can give you is "RTFS" --- lol
Ron Rich

[nyjb]

Not after your title, Ron; for a "dummy", you do a pretty doggone good job. That's why I always RTFJA. LOL.

The question isn't about the "FS", it's about Rob's crossover. The chokes in the unit and on the "FS" have two points of connection; the ones I got for the crossover have four.

[Ron Rich]

I no No-thing !! I am inno-sent ! Ron Rich

[robnyc]

nyjb -I don't know why you would be attempting to use a CM choke for a speaker crossover. Those are primarily intended for noise/ripple cancellation in power supplies. I guess you could use specific values to cancel noise picked up in long speaker cables.

There is no harm in experimenting with it here if you only use it in-series with the load. Some things to consider are:

1) What is the inductance in each coil.

2) What is the effective series resistance.

You'll get more predictable results by sticking to the values listed in the links and using components designed for the purpose.

The phono pre schematic is here: www.flickr.com/photos/90641375@N06/37682800724/in/dateposted-public/

I chose this because of the common tube type (12AX7) and the easy to mod equalization -which is replaced here with a pot between the two stages. IIRC: I eliminated the two cathode caps as there was more than enough gain w/out them. In the Seeburg amps of this period the 5879 is a buffer producing straight gain and the RIAA eq is in the stage afterwards. I wanted more gain and be able to adjust it so as to not over-drive the the AGC. This design also offered multiple points where eq could be modified to increase treble -though i ended up doing that at later stages.

Rob

[nyjb]

I wasn't sure which type of choke to order. The mark of a rookie in a hurry. Not sure which links you are referring to, or which type of choke is best for the crossover. Perhaps I need to slow down and revisit my texts. Besides the jukebox manuals, I rely on a few books for radio servicemen for the fundamentals of amplifier circuitry, but they don't really address modifications. Brilliantly written, though. Guitar amp resources are plentiful, but they tend to be too sophisticated for me.

I'm enjoying the photostream, too.

As always, I am grateful for your counsel.

[robnyc]

Ok, I went back over the discussion and it looks like the link never came through:

www.parts-express.com/resources-crossover-component-selection-guide

Since we are dealing with a total load of 16 ohms (two 8's in-series) i suggest approx 4mh choke.

Rob

[nyjb]

I've looked at the preamp schematic, and think I've got a good plan for wiring it.  I do have a question or two about it, though.

1.) The schematic doesn't have a value for the grid load resistor; it just says "see note 3."  What do you suggest?
2.) The cathodes of both sections are grounded with 2700 ohm resistors and 25uF/25V capacitors in parallel.  The schematic shows the cap in the first section to be polarized, but not the second.  Is this correct, or an oversight?
3.) Should the value of the grid load resistor for the 6SN7 (R37 on the MRA5-L6) change after replacing the 5879?
3.) Are there any changes in the circuitry of the 6SN7 that follows?
    a.) On the MRA5-L6 schematic, the plate of the 5879 appears to be connected to the plate of the amplifier stage of the 6SN7 through R10, R13, R14, R15 and C7, C8, C9.  That is, the plates are grounded through the some of the same resistors and capacitors.  Does this represent some kind of "link" between the two plates, or should I think of it as two separate routes to ground that happen to share some components?  I think the latter.  If so, I'd ground the plate of the 12AX7 as on the pre-amp schematic, remove R10, R13, and C7 (which grounded the 5879 plate), and leave the rest alone.  Is that correct?
    b.) The schematic has no DC voltage measurement for the plate of the 1st stage of the N.  Any idea what to look for?

[robnyc]

"1.) The schematic doesn't have a value for the grid load resistor; it just says "see note 3." What do you suggest?" _ I just kept the 220K that was there and moved it to the G-1 pin thta I decided to use on the 12Ax7.

"2.) The cathodes of both sections are grounded with 2700 ohm resistors and 25uF/25V capacitors in parallel. The schematic shows the cap in the first section to be polarized, but not the second. Is this correct, or an oversight?" -Given that this is DC you could use polarized or non. I eliminated these two caps as there was more than enough gain once the EQ is removed.

3-A-B-C. Just go with the circuit as described in the schematic, but remove C-1 R-1 and the 0.01 @ 600v we are using this new pre as simply a gain stage, the EQ comes in the following stage. Replace the 0.68M-ohm at the input of the second stage with a small pot to adjust gain. IIRC I used a 1 meg trim pot -the arm goes to the G1 of the second stage.

RobNYC

"3.) Should the value of the grid load resistor for the 6SN7 (R37 on the MRA5-L6) change after replacing the 5879?" No, nothing needs to be changed downstream. We are simply replacing the input buffer/preamp.

[nyjb]

Hooray. It's up and going, and is substantially stronger, with more avc action! It also has pretty heavy and muddy bass, and now I hear all sorts of vibrations around the place. I've been reading some other similar threads, so this isn't a big surprise. I haven't put the chokes on the 12" speakers yet.

Any suggestions for bass management besides looking at first grid load resistor and the volume pot? Do you recommend any changes to the specs of the volume pot? I expect I'll be replacing them.

This is a big breakthrough! Thanks Rob for shepherding it along.