Amplifier principles for MRA5-L6

I have a few handful of Seeburg HF100Rs with MRA6-L6 amps. I’m trying to resurrect them, but even more so to understand how the circuitry works and how to troubleshoot. (I don’t have a background in electronics, but I’m studying hard.) Much of my reading has been in radio or guitar amplifier context, from all time periods, so I’m afraid I could be mixing some of the language up. There are a lot of questions; I certainly wouldn’t expect anyone to answer them all, but I would welcome any coaching.

Capacitors.
There are an awful lot of subcategories. Understanding the different materials is easy, but sometimes the understanding the different roles they play isn't. Filter, “line” filter, by-pass, blocking, coupling/ decoupling. Do any of these mean the same thing? Do they all exist somewhere in the MRA5-L6? Are there any rules of thumb about where each appears on a schematic? (E.G.: if it goes directly to ground it's a __________.) When the electrolytic caps are replaced, how important is it to locate the new ones in the same physical location? I’ve seen repair jobs with the ecaps moved, thus eliminating some wiring, but have also read that everything should be grounded in the original location.

Tubes:
I understand that both the current supplied to the cathode and the current supplied to the grid determine the current coming off the plate. What’s the difference between the effects of the grid current and cathode current? Is there any significance to the which side of the grid (on the schematic) the grid supply goes to?

AC/DC
I understand that the amplifier uses both ac and dc, that they can coexist in the same part of the circuit, and that caps and resistors can affect them separately, by letting one through but not the other. Why do some parts of the circuitry require AC or DC but not the other? Is there a rule of thumb for recognizing which current(s) should be at any part of the circuit (besides the voltages on the schematic)?

Testing, measurement and technique
Does one measure voltage to ground or across different parts of a circuit?
Are there any “rules of thumb” for recognizing which way current flows on a schematic?
Am I correct in understanding that current always flow from high voltage to lower, and that DC flows one way, but AC both?
Is there a different soldering technique when grounding directly to chassis instead of a tube or terminal strip?

What does it mean to “jump” a pin to ground?

Any expertise would be deeply appreciated.

Terry

As an experienced electronic technician, I could sit here and write a book concerning all those questions.
But.... I won't.
They've already written books on the subject(s).

To ask those questions here, you'll get a plethora of answers, or opinions,- internet answers, from people on the other side of the computer screen who could be as befuddled as you are... how would you know?
(read: don't believe everything you read online)
I've seen loads of crap tossed over the 'net as god's word, and just laugh.

Electronics Theory taught by instructors, along with classroom workshops, is the foundation for successful servicing of equipment.
It's also the tool required to understand the basic elements, and how / why they work as they do. (capacitors, resistors, etc)
Avoiding that, and relying on "others" for so-called "internet" knowledge is taking the lame path IMO.
You won't learn taking a shortcut.
If you really want the understanding to benefit from, it's not off some juke website, or snob audio site.
It's from books - written by established authors.

I've got a guy who comes into my shop with stuff all the time - a putzer - and his attitude towards "learning theory" is lame - "he doesn't have the time".
And he breaks more than he attempts to fix - but it's his stuff, luckily.
Anybody can change a burnt out light bulb or polish the dial, but lordy mama if they tinker with a screwdriver in the guts - it's gonna wind up as trash.

Terry, answers here can take pages and indeed, whole books. I'll keep it brief and in the context of the types of equipment involved here.

Capacitors.
There are an awful lot of subcategories. Understanding the different materials is easy, but sometimes the understanding the different roles they play isn't. Filter, “line” filter, by-pass, blocking, coupling/ decoupling. Do any of these mean the same thing?

The same type & value of cap can be either a filter or a blocking element depending on the circuit surrounding it. In general, a cap where one lead goes to chassis ground is a filter. Filtering takes several forms . In a power supply we need clean DC to operate the active components. however, if connected to 'wall power" we are getting AC @ 50-60 Hertz (aka: cycles). A rectifier turns the AC (a current that changes polarity) to DC (a current that stays in one polarity) However the output from a rectifier is imperfect in that it still varies in magnitude. This would corrupt the circuit we want to power with a 'hum" or ripple. here a capacitor is used to act as a short-term battery so that when the ripple goes lower in voltage the cap fills-in the difference and maintains the voltage at a steady rate.

When the electrolytic caps are replaced, how important is it to locate the new ones in the same physical location?

In these circuits that deal with audio frequencies and AC mains power placement is not that critical. It mainly comes down to ease of manufacture, minimization of wiring and keeping the electrolytic caps away from the largest heat producing elements (power tubes and large resistors). Aesthetics sometime play a minor role as well.

Tubes:
I understand that both the current supplied to the cathode and the current supplied to the grid determine the current coming off the plate. What’s the difference between the effects of the grid current and cathode current? Is there any significance to the which side of the grid (on the schematic) the grid supply goes to?

Here I am not too clear as to your question..but in general, A tube amplifies by two parameters; Voltage gain. Current gain. Small tubes (a 12AX7) are generally only used to provide voltage gain while large tube ( such as a 6L6) provide power gain.
In these circuits it is not desirable for a the controlling grid, known as grid 1 (G1) to positive with respect to teh cathode. There are two main reasons, one is that any tube element that (goes positive) w/respect to cathode is now in effect becoming another plate, but the circuit behind that grid may not be designed to supply the power and severe distortion can result. A G1 going positive will generally attempt to draw too much current from the cathode causing "saturation' which also leads to distortion and increased tube dissipation.
In a typical audio amp a capacitor is used to 'block" the DC from the previous stage but pass through the AC which is riding on it. As capacitors age that can 'leak" DC through and disrupt the proper operation of the following stage.

AC/DC
I understand that the amplifier uses both ac and dc, that they can coexist in the same part of the circuit, and that caps and resistors can affect them separately, by letting one through but not the other. Why do some parts of the circuitry require AC or DC but not the other? Is there a rule of thumb for recognizing which current(s) should be at any part of the circuit (besides the voltages on the schematic)?

Analog audio is by nature an AC which varies by frequency add amplitude. In amps it "rides on" the DC used to operate tubes and transistors. In most cases there should be no DC on a tube's Grid 1 and a blocking capacitor is used to prevent this from happening. Schematics show proper voltages at tube and other circuits elements. Voltages that are encircled are usually AC signal voltage. UN-circled voltages are DC.

Testing, measurement and technique
Does one measure voltage to ground or across different parts of a circuit?

In most circuits you measure to ground which is generally the chassis. One clue is where the large filter caps are common-ed to.

Am I correct in understanding that current always flow from high voltage to lower, and that DC flows one way, but AC both?

Correct.

Is there a different soldering technique when grounding directly to chassis instead of a tube or terminal strip?

No, just take care not to break off the terminals when removing the old wires.

What does it mean to “jump” a pin to ground?

It simply means to connect it directly to in this case the chassis ground bypassing other means of connection or components.

ALWAYS be careful in 'jumping" circuits.

Rob-NYC

Hi All,
Great explanation, Rob--I slightly dis agree about filter placement within a chassis, especially one as large as the older Seeburg tube type amps. IMHO, what has been forgotten here is "lead dressing". This can be critical in hum prevention, and should have been/was (hopefully) calculated in the design of the amp. Moving a filter MAY upset this design perimeter, thus causing hummmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm ! Ron Rich

Ron Rich wrote:Hi All,
Great explanation, Rob--I slightly dis agree about filter placement within a chassis, especially one as large as the older Seeburg tube type amps. IMHO, what has been forgotten here is "lead dressing". This can be critical in hum prevention, and should have been/was (hopefully) calculated in the design of the amp. Moving a filter MAY upset this design perimeter, thus causing hummmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm ! Ron Rich

Datz a good one Ronnie!
Yup.

Well, Mattie,
You should know by now, that I am not always rite--er, write--er, correct, BUT, Im NEVER rong !!
Ron Rich

Ron Rich wrote:Well, Mattie,
You should know by now, that I am not always rite--er, write--er, correct, BUT, Im NEVER rong !!
Ron Rich

Au Contraire, you are always right....far right.

Rob

Ok, let’s see if I got this.

The amp receives AC current, which reverses polarity and “flows” in both directions simultaneously. Except for the filaments/ “heaters”, which run on a separate AC circuit, the tubes need an unwavering one-way current to operate properly and without introducing distortion.

The rectifier tube, choke, and filter capacitors serve to convert the incoming AC into the “flat” DC that the tubes need.

Meanwhile, the audio signal enters the amplifier as a very small AC current, which “rides” the DC current from the pre-amplifier, in my case the 5879. At various steps along the circuit, other capacitors serve to block the DC current, leaving only the audio signal, which becomes progressively stronger and “purer” en route to the power amplification stage— on the MRA5-L6, the 12AX7 and 6L6Gs.

Various resistors ensure that the grid voltage isn’t more than the cathode voltage (“positive with respect to cathode”) because if it was, it would pull electrons that should go to the plate. Insofar as the audio signal is concerned, the grid is the input and the plate is the output. Blocking capacitors ensure that the grid receives only the AC audio signal. (These capacitors are also called “coupling” capacitors?)

Plate current is both AC and DC. The DC current is one way to the plate from the rectifier. Capacitors ensure that that no AC current is fed to the plate except that which comes from the tube. Because AC is a two-way current, it can proceed from the plate to the next stage while the plate simultaneously draws the DC.

I have some follow-ups, which I’ll share if I have it right so far.

Terry, this is all essentially correct --in the context of this typical type of amp called an "R-C" coupled amp (R= resistor, C=capacitor).. Where signal and DC currents are routinely kept separate.

In many types of amp circuits the stages are DC coupled where the plate or collector currents sometimes called a 'product" (AC & DC) from the previous stage is present in the grid or base of the following stage. This is much more common in transistor circuits, but has advantages in both tube and transistor designs.

Various resistors ensure that the grid voltage isn’t more than the cathode voltage (“positive with respect to cathode”) because if it was, it would pull electrons that should go to the plate.

Basically correct in the context of a simple tube such as a triode (12AX7). Many tubes such as the 6L6 and 5879 have additional grids. These can be at various voltage potentials and are used to accelerate the electrons, suppress certain undesirable traits or just provide additional control input.

Since you have the basics i suggest you do a search for each of the tube types in your amp and read the descriptions. That is the next logical step in getting a deeper understanding of amplifier operation.

You are going about this work in the right way by acquiring knowledge so that you can perform real service and not just 'tinkering".

In that same vein, a friendly warning; these amplifiers contain lethal voltages and currents. While working on a hot chassis is often the most efficient way to troubleshoot it is also the most dangerous to both you and the device. Make sure you understand safety measures and proceed with caution.

Bring on the questions....

Rob

Except for one thing--I think you have it correct--I don't know how audio, can be "purer"-- what goes in (should/hopefully) come out ! Ron Rich

Terrific. That's a breakthrough. You've helped me tie together a number of things I more or less understood separately.

I've invested a great deal of time in mapping out the amplifier as it appears physically and aligning that with the schematic. I don't think I could get comfortable with the schematics any other way, and I realize that I have to "see" the circuitry both ways in order to service them safely and skillfully. It's a slow process.

I haven't done any testing under power yet in part because I want to make sure I know what I should expect to find at any point, but I'm pretty close to being ready. My guess is that it's a lot easier with alligator clips than the probes that my multimeter has.

I'll bring on the circuitry questions shortly, but I have a few about technique that others could probably weigh in on also.

Replacement capacitors and resistors range tremendously in price. Are there manufacturers to seek or avoid, or all they are more or less the same?
What do you recommend doing to prepare a post or pin for soldering? Do you remove existing passive components at the posts, or leave the posts and connect new components to the old wires?
I invested in a soldering station with adjustable temperature. Where is the "sweet spot?" Do you use different temperatures for different parts of the job?
Is there any formulation of rosin-core solder that you'd recommend?
Do you have any suggestions for affixing replacement e caps to the chassis? One well-regarded rebuilder suggests installing a terminal strip, which makes a lot of sense to me.
Lastly, how do I test the chokes?

IMHO, electronic parts, today are not brand dependent. All seem to have problems, once in awhile, but much less so then in the past. Price is not a determining factor, as far as I can tell. At one time,I waz learnt,, that electronic parts had a 90 day period of failure expectancy--- If viewed in a graph ( which is what I recall seeing) the 50-60 day time frame was when the most failures happened--trailed of drastically, after 90 days--.
On Chokes--this is measured in "micro Henerys" (ies ??). If you do not have a meter to measure them, a simple very low resistance reading should be expected. Ron Rich

I’m a little confused perhaps because I’m seeing amps-- again, the MRA5-L6-- that have been modified in slightly different ways. Some changes are pretty easy to understand, like using different points for grounding. Complicating it further, Seeburg revised the schematic and parts list a few times. I would assume the last update is the best one to use; I don’t see any changes except changing the values of a few passive components, and adding some in already existing circuits. If anyone has an informed opinion about this, or other generally useful “tweaks,” I’d love to know.

I intend to make an extension of the jones plug and speaker plug so I can test the amp on the bench. Would the extra few feet of wiring have any significant effect on the circuits?

Does anyone know a simple way to check a tube socket?

When taking dc voltage readings, in some cases I get a steady reading, in other cases it fluctuates. Is this more likely to be the result of a poor connection or a poorly functioning capacitor, or something else? Are there conditions under which one would want the reading to change? That seems counterintuitive to me. I’m using a digital multimeter.

The notes direct one to measure DC with J3 grounded and AC with it ungrounded. Does this mean that anytime one wants to take AC measurements, they must physically disconnect the grounding wires? Note 8 says to take signal voltages with 5 megohm input A.O.V.T.V.M. with 30 M.V. at I & C input at J4… SPKR SW 1M 16 W POS. PIN 1-3 J4 SHORTED. Could I have a translator please?

Terry, re: Seeburg mods on these amps, -most- seem to be centered around the AGC ckt. These were intended to reduce the "runaway" that could happen if the 6Sk7s G1 went positive due to a very loud record. The result of that was a weird, phasey distortion and blasting. This would normally only occur using the mono spike styli. What i found was that under heavy drive conditions the drop across the SK7 G-2 resistor (R43 here) would be excessive causing teh tube to lose control because it was unable to conduct enough at that point. I simply tested for a different value of that resistor and used one that held up better. I'll have to check on one of the amps in my V-VL's on location. In any case, if you are using modern styli, I would bother with it.

I intend to make an extension of the jones plug and speaker plug so I can test the amp on the bench. Would the extra few feet of wiring have any significant effect on the circuits?

A few feet will have no effect but, why bother? Just jump pin 2 on the chassis female to ground and connect a decent 8 ohm spk to Pin 1 and chassis ground (or Pins 3&4).

Does anyone know a simple way to check a tube socket?

With the amp powered and signal going through, slightly rock the tube in-question and look for any effect. You should also look for this on the RVC and speaker plugs.

When taking dc voltage readings, in some cases I get a steady reading, in other cases it fluctuates. Is this more likely to be the result of a poor connection or a poorly functioning capacitor,

There are a number of things that can cause this to happen:

1) If the amp is being driven you should see some DC fluctuation due to the loading of the power amp section. Test DC voltages with no drive.

2) Fluctuating AC mains voltage. These are unregulated PSU's so what goes in -is what you get out of the rectifier. If some voltages are steady that is probably not a factor here.

3) Yes, a leaky cap can cause this to happen. One hint is that it will get worse as the amp warms up. In any case, ALL the paper couplers should have been replaced before power-up. These amps are 60 years old.

4) If you are using an auto-ranging DVM sometimes a slight fluctuation will cause it to 'hop" or hunt when the voltage reaches the top or bottom end of one scale and bleeds into the next upper or lower one.

The notes direct one to measure DC with J3 grounded and AC with it ungrounded. Does this mean that anytime one wants to take AC measurements, they must physically disconnect the grounding wires?

On that amp J3 is a speaker plug. It will have no effect on DC readings but the jump must be there for there to be B+ . There must be a load of 8 ohms or more across the speaker terminals any time the amp is being driven. the other plug that will affect DC readings is the Mute-Squelch (j4). If Pin3 is not grounded a squelch voltage will be applied to the SK7 that will cause it to conduct heavily (as if it was coming out of stylus setdown). this will in-turn affect the DC voltages around that tube. It will also attenuate the signal voltage readings downstream of that tube (as it should). One useful test here is to remove the grounding connection on Pin 3 while feeding signal and note how the level on the output changes. Heavy signal will cause little/no change. low signal will see the level get lower as the squelch voltage builds up w/Pin 3 ungrounded.

Note 8 says to take signal voltages with 5 megohm input A.O.V.T.V.M. with 30 M.V. at I & C input at J4… SPKR SW 1M 16 W POS. PIN 1-3 J4 SHORTED. Could I have a translator please?

-5 megohm input A.O.V.T.V.M.- That was based on the old, analog meters. In some cases a less sensitive meter would load the circuit and distort the readings so they spec what is a minimum acceptable load impedance. Any VTVM or modern DVm will exceed that so consider it a "legacy spec".

with 30 M.V. at I & C input at J4

That should read: With a 30 millivolt input at 1KHZ (1000 cycles) at J6 which is the cartridge input. Replace the & w/ a KC and it will make sense.

SPKR SW 1M 16 W POS. PIN 1-3 J4 SHORTED

They are specifying a load across the speaker terminals with the wattage set to "16" and the AGC squelch shorted as if in play mode as i explained above.

These are just the standard test setup parameters that should be on every service schematic

Rob-NYC

Please correct me if I misunderstand:

G1 refers to the "control" grid, which must always be negative with respect to cathode. In this case, it's on pin 3 of the 6Sk7. G2 is the "screen" grid on pin 6. G3 would be the "suppressor" grid.

Pin 2 on the chassis female, which I'll jump to ground, is pin 2 of the speaker plug's socket. Pins 1, 3 & 4 here are also on the socket.

J3 is the two prong speaker plug inside the cabinet.

"jump" as a verb means to actually, physically make a connection using wiring. "jump" as a noun can refer to the connections made when a plug enters a socket (like P2 & J2)

on the mute-squelch, by removing the grounding connection, you mean physically disconnecting it. in terms of the J3, one can unground it just by unplugging it.

by "drive," you mean to power the amp with an input signal and speakers connected.

and a few questions:

Perhaps my terminology was incorrect, because I've seen "Jones plug" in reference to a broad range of connections. I was thinking of a cable that would allow me to connect the amp to the receiver in order to power it on the bench, basically a four-prong extension cord. As it is, I'm sliding the whole thing in and out of WSR in the door each time.

I don't have any kind of signal generator to use in lieu of connecting the rca plug from the mech. is there a simple, lo-tech / lo cost signal generator available for this purpose? again, I'm looking to untether the bench and cabinet.

On the schematic, around the first section of the 12AX7 there are a number of circled AC readings-- 0.8, 17, 110, plus 0.24 in a box. The 0.8 seems to be for pin 2 and the 17 for pin 1. The 110 I'm not so sure about and I don't know what to make of the .24 in its rectangle. The box could just be there because there's no room for a circle. But where would one look for that reading?

Thanks again, Rob-NYC.