1975 Twin Reverb in for Repairs

[Platefire]

This twin MV Silverface shown in the attached pix's. SN# B 02511=1975. The customer said the vibrato didn't work and they had already tried a new tube. Well in spite of what he said I tried a new 12AX7 in it and the vibrato started working fine.

It's just that while I was testing the vibrato that I noticed one of the tubes was red plateing. The tubes are International C Servicemaster USSR?? Not familar with those. I went through my used tubes looking for 6L6GC found a used chinese pulled out of a Pignose G40V. Replaced the red plater with that one and seemed to fix it and then I noticed another one on the other side of the PP set red plating. So then I got my bias master out and did a check on all four tubes. The mA ran something like this: 15, 20, 12, 30. I did put the other pig chinese tube in to replace the other red plater and it seems to be running presently without red plating.

First impression is that it must be faulty tubes red plating with that cold of bias. I might would pull the chassis and adjust the bias but with that far of a spread of readings I would think the solution was to install a new matched quad set of JJ's and then re-bias. This is my first Twin ever to work on, so maybe I can get some old twin hands to chime up!   The amp sounds pretty good just playing at low volume, nice and quiet running. Nice clean tone. Haven't really cranked it up any. I know faulty coupeling caps could cause that too. Platefire

[HotBluePlates]

How tight are the octal sockets? Are the tubes nice and snug in them, or do they rock around easily?

Open the amp up, and give it a look-over. See if anything appears obviously wrong. If not, measure the bias voltage at pin 5 of each 6L6, and consider whether the octal sockets need retensioning.

The only way I can think of output tubes passing only 15mA, but redplating on occasion is if the tube sockets are very loose, and the output tube bias is intermittent.

A second possible (but in this case, unlikely) cause is if the coupling caps from the phase inverter were leaking. I'd be very surprised to see intermittently leaking caps in that amp, because there's no good way for there to be movement/flexing of those caps in this amp.

[eleventeen]

I'm very suspicious of the combination of conditions you describe; red-plating with such cold bias and the intermittent tremolo. Maybe there was something wrong with the trem circuit that futzed with the bias supply. IEC Servicemaster Tubes...I think these would have to be at least 20 years old, probably more. (Good Gawd, a 1972 amp is FORTY years old!! Can you believe that?)

You seem reluctant to pull the chassis....I think I would try to see if I could re-create the red-plating by turning the tremolo on and off a bunch of times and varying the speed & intensity.

[HotBluePlates]

Unless the trem circuit is modified, it is not connected to the bias circuit in that amp. The Twin uses an optoisolator trem, and the trem only connects the raw bias voltage to the oscillator to turn the oscillator off hard if the footswitch allows it.

Meaning: this particular trem circuit should not turn on at all unless the footswitch is plugged in and engaged.

If you aren't using a footswitch, suspect a modification (probably a poor one) that has altered the trem oscillator to function all the time. Depending on how the circuit may be monkeyed, it might be possible for the raw bias voltage to be shorted, killing output tube bias, and redplating the output tubes. However, for this to be the case, all the output tubes would probably redplate, rather than just one or two.

So it's probably worth a look inside.

[Platefire]

I've had this amp sitting around for several days and just got it out tonight to do initial testing. The Vibrato problem was not intermitant but wasn't working and now it is with a new tube. This Vibrato unit I don't think is bias related but uses the bug! It will not work without the footswtich being plugged in--so it's still original. Yelp I am a little hesitant on pulling that big old chassis and I will but not tonight.

I'm going to contact the customer tomorrow about the additional problem before I dive in and let him give me the go ahead because time is money and who knows how much extra effort it will require. Platefire

[Platefire]

Ok got this twin chassis out now for inspection. It is a AA769 with a bias balance pot. Two out of the four tubes were red plating, so I'm trying to determine what is causing that. I did some initial testing that proves out to be way un-balanced. Reading was as follows:

Bias voltage= -50.2 inside pair 6L6's/-54 outside pair 6L6's

Bias mA=49.5 inside pair/24 outside pair(inside average 24.25 mA tube/outside average 12 mA tube) This is what I would consider way too cold bias setting to be red plating??
 
Plate voltage=440 inside pair/440 outside pair

I checked the coupeling caps to the power tubes for any DC leakage but didn't detect any. From what I see so far I tend to blame the tubes for the problem but sure am open to any other possibilities I may not see. I've attached a pix of this twin power section. Platefire

[HotBluePlates]

That looks like a bone-stock 70's Twin Reverb.

How were you measuring the idle current?

[Platefire]

I was using the OT shunt method.

Edit: As a futher check I will use this I=U/R  alternate shunt method described as follows:

A further alternative to the shunt method

However, there is a second method, set up similar to the shunt method, but which measures voltage instead of current. After first shutting off the amplifier AND draining the filter capacitors, the DC resistance of one side of the output transformer is measured from the center tap to the start of the coil.

Next, the amplifier is turned on, and the voltage drop over the same half of the
transformer is measured. The setup is exactly like that of the shunt method above,
except that a volt meter is used rather than an ammeter.

Since the DC resistance of the transformer (which is constant) leads to a voltage drop,
Ohm's Law can be used to calculate the current flowing through the transformer:

Total Idle Current (I) = Voltage drop over the transformer (U) / Resistance of transformer (R)

 As with the shunt method, the calculated current is the total current flow through all the
tubes on the side of the output stage feeding the measured half of the output
transformer, and it must be divided by the number of tubes per side, in order to obtain the
current flow for a single tube. An alternative is to multiply the desired value for a single
tube by the number of tubes and compare to the value measured, making adjustments
as necessary.

Formula  I=U/R

R=Resistance accross OT center tap and secondarys=37.5 Ohms inside pair tubes/35.1 Ohms outside pair Tubes

U=Voltage Drop accross OT Center Tap and secondarys=450VDC on Center tap, Inside pair tubes plates 447 with voltage drop of 3 volts, Outside pair tube plates 448 with voltage drop 2 volts

I= Inside pair, 3(voltage drop)x37.5(resistance)=112.5 mA(I)=56.25mA per tube
    
  Outsuide pair, 2(voltage drop)x35.1(R)=70.2 mA(I)=35.1 mA per tube

[sluckey]

I= Inside pair, 3(voltage drop)x37.5(resistance)=112.5 mA(I)=56.25mA per tube
   
  Outsuide pair, 2(voltage drop)x35.1(R)=70.2 mA(I)=35.1 mA per tube

No, that last step is wrong. I=E/R (current = voltage divided by resistance)

So, I_{inside pair} = 3v (voltage drop) / 37.5Ω (resistance) = 80ma   (40ma/tube if tubes are perfectly matched)

Or, I_{outside pair} = 2v (voltage drop) / 35.1Ω (resistance) = 57ma  (28.5ma/tube if tubes are perfectly matched)

[Platefire]

Thanks, thats' why I posted the proceedure, so if I got it wrong, someone would correct me. Only thing, I hate to be a dummy--but I can't see how 3 divided by 37.5=80 unless you scoot some decimals around--maybe you could explain that to me.  

I adjusted the bias balance pot where both sides now have -51.7 negative voltage

I also re-installed the original two tubes that were giving me a red plating problem before except I changed their locations. They still red plated in the new locations. So I pulled those two tubes and I put the replacements back in that was used before that wasn't red plating and they are still not red plating--holding up good after an hour burn.

So based on what I've done so far, to me the issue seems to be the tubes. Do you agree??? Platefire

[John]

3/37.5=.0800

Using a calculator is the onliest thing I'm good at. :)

[sluckey]

but I can't see how 3 divided by 37.5=80

I didn't say that. I said 3v/37.5Ω=80ma.

unless you scoot some decimals around

That's exactly what I did. 3v/37.5Ω=0.08A. Thats AMPS. But one amp is 1000mA (milliamps) Therefore, 0.08A x 1000 = 80mA. You must scoot (or shift) the decimal 3 places to the right when multiplying by one thousand.

Make sense now?

[Platefire]

OK, 1 Amp=1000 mA, .08 Amp=80 mA. Yeah thanks, I understand now. I'm kind of hard headed especially with math. But I'm going to figure out this bias thing becuase I've been stumbeling all around it for years, but now I've got to get it right. Thanks for your Patience!

So back to the amp! So am I crazy or is that little bit of difference/variation in the OT resistance on each side causing the off balance in bias and I need to correct it by varying the negative voltage balance pot more to one side to balance mA Idle so they will be the same on both halves---or if the tube are that far out of balance would that be impossible? Who is the culprit here that needs to be fixed? Platefire

[sluckey]

You must have some good spare tubes to work on this stuff. Tubes should always be at the top of the list of culprits. Plugging a good matched quad of tubes in that TR will tell you a lot. I know it's expensive to keep good tubes around but if you're gonna have customers, you're gonna have overhead too.

You're tracking down unbalanced tube issues but the method you're using only gets the combined current for a pair of tubes. Then you're assuming the current divides equally between the two tubes. You need to KNOW what the current is thru each tube.

[Platefire]

Well when your right your right! Four new balanced 6L6's would tell me a whole lot on this amp. Kinda shooting in the dark without them.

Also this amp has the Ground Switch/Death cap in place. Don't know if I should make an issue out of that or not? Right now it's pretty much all original. Platefire

[Danskman]

One of the best ways to get idle current per tube is to solder a 1ohm / 2 watts / 1% precision between pin8 and ground on EACH power tube socket. This way, you ever know the idle current per tube. On these Twin, there's a little bit of braided wire to get the cathode grounded; I unsolder them on the socket side, solder the resistance between pin8 and ground and use it to get the idle current (30mV DC measured on pin8, for example, means 30mA idle current). Once the amp is repaired, i get these resistance out or let them in place for a future use.
OT shunt can be good in single tube PP poet amp, but in your case, doesn't give you the right thing.
You ALWAYS need a matched quartet of poet tubes if you want to get this kind of amp repaired. JJ's are a very good choice, coming from a reputable seller (Tubedepot or CE distribution are OK and their prices are OK, too). If it was me, I would modify the bias control circuit, in order to vary the bias voltage itself, as a bias "balance" doesn't balance anything if the power tubes are defective or badly matched; you can find instructions on how to do this on Doug's web pages.
Another thing to do is measuring the bias voltage on each power tube socket, without the tubes. This way, you know if the grid one resistance is OK (could have been destroyed by a flash in the tube, who knows?).
Hope you'll soon find the solution.
best regards,
Danskman

[sluckey]

Also this amp has the Ground Switch/Death cap in place. Don't know if I should make an issue out of that or not? Right now it's pretty much all original. Platefire

I'd explain the safety issues and benefits of installing a three prong grounded power cord versus the original 2 conductor power cord and ground switch. Let the customer decide. Many shops probably have a set fee for replacing the 2 conductor cord and push this as a safety issue.

[Platefire]

Oh, it does have a three conductor power cord hooked up correctly but also with the ground switch/cap in place. One thing I would say about the ground swith is it is a three position: A/Off/B selection. Looking at the AA769 schematic it doesn't show the switch as a center off where the cap connects to the center term, but with with the center position being off, it would have to be a center off switch.   

Every amp repair that I do, I actually provide a repair report that includes schematics, what I found regarding overall condition, mods found, repaires I performed and recomendations regarding the amp. I will note in my report about the dangers of the ground switch/death cap and recomend in the future that the switch be left in the off position or either be disabled/abandoned in place so it can't be activated.

I have already buttoned the amp up ready to go. I will recomend he buy a new quad matched set of good quality power tubes. Meantime I might need to use some of my proceeds from these repairs to buy some new 6L6's, EL34's and 12AX7's to have on hand for next time. I especially appreciate your help! Thanks, Platefire 

[Platefire]

Danskman

  I did notice the brades to ground on the cathode pins. That would be a pretty easy fix to get a bias on all four tubes. Well I've already put the amp back together. Will be taking it to customer tomorrow. I doubt the customer would be open to modify it anyway. Most owners think in terms of not modifying them not to degrade the future sell price.
  I'll tell you what--that twin sounds pretty dog gone good even with the un-balanced power section. The MV works good enough for my purposes. If it wasn't so so heavy, I could get use to that sound & response! Not bad at all. Platefire 

[birt]

if you unsolder the braided wire from the sockets and solder the 1ohm resistor between that wire and the socket it's a mod that is perfectly reversible if the owner wants to sell it as a stock twin.
the same goes for the death cap. if you unsolder one side and insulate it it's still there as an original part.

[sluckey]

Oh, it does have a three conductor power cord hooked up correctly but also with the ground switch/cap in place.

FYI, once the amp chassis is properly grounded to earth with a 3 conductor power cord, the 'death cap' is a non issue.

[Platefire]

Well since we are discussing the death cap, if the death cap capacitor blocks DC and allows AC to pass---with a two conductor power cord---at what point or what change has to happen to make the chassis hot to be a danger? It seems with the cap in place it would be allowing one leg of the AC to be tied to chassis continuously---I know the is Elementry Watson, but again I'm hard headed and if I'm not sure, I just have to ask! Also how would the presence of the ground nilch the death cap. Platefire

[sluckey]

Well since we are discussing the death cap, if the death cap capacitor blocks DC and allows AC to pass---with a two conductor power cord---at what point or what change has to happen to make the chassis hot to be a danger? It seems with the cap in place it would be allowing one leg of the AC to be tied to chassis continuously---I know the is Elementry Watson, but again I'm hard headed and if I'm not sure, I just have to ask! Also how would the presence of the ground nilch the death cap. Platefire

A few basics...

1. A .047µF cap looks like a 56KΩ resistor to the 60Hz line frequency.

2. The purpose of the 'death cap' is to act as a cheap line filter. When connected between the line side of the incoming AC power and ground, it will pass high freq noise, glitches, etc. to ground. The net results is that buzzes and other noises that may be present on the AC line will be eliminated, or reduced. Line filters are good and will be found in most all high end, commercial, or military equipment.

3. The 'death cap' does not deserve it's reputation. It's not the culprit and I'll refer to it as a line filter from now on. The real culprit is the fact that the amp chassis is not connected to earth ground when you use a two conductor power cord.

Now here's the problem when using a two conductor power cord and a line filter... Suppose the cap shorts as they sometimes do. (That's why the voltage rating is usually 600v or higher.) There's a 50/50 chance that the cap is connected to the line (hot) side of the AC power. Since one side of the cap is connected to the amp chassis, 120VAC is applied to the chassis thru the shorted cap. This is not a problem to the amp. The real problem is that your guitar strings are also connected to the chassis and now they are HOT. That's a bad thing!!! That 120VAC will be looking for a path to ground thru your fingers and bare feet if you're playing a beach party.

That whole problem is solved by using a power cord with a dedicated wire (GREEN) that connects your chassis back to earth thru the house wiring. For the shorted cap scenario, the AC line has a zero ohm path back to earth and all the current will flow thru your green wire back to earth. You are protected. And hopefully a fuse or house breaker will operate and prevent the house wiring from burning up.

So, I say properly install a three conductor power cord and leave the line filter alone. It's a good thing, even if a little misunderstood. My radar system has hundreds of AC line filters. All cabinets have line filters at the AC input. Then each sub chassis within that cabinet has a line filter anytime AC enters the chassis. Think about this... Ever see one of those IEC connectors with built in line filters? Guess what's inside those? If you guessed series coil and shunt caps from each line to ground, you'd get the prize.

OK, we got the amp properly grounded. But what if the house wiring is screwed up? You can't control the whole world. Just fix the part you can control and hope the other guy did his stuff properly. AND KNOW HOW TO TELL IF HE DID HIS PART RIGHT!

I can't think of anything else to say about two conductor power cords on guitar amps and line filters (death caps).
 

[John]

Thanks for that explanation and taking the time to type all that out. I knew why the 2-prong plug is a bad idea - from painful experience in fact - but didn't really know what the cap did.

I really have to start studying that big textbook again, but it's so much more fun burning solder. 

[Willabe]

OK, I have a question.

I had a BF SR for years that I giged with. When I bought it it already had a 3 wire power cord installed in it.

One night I was singing and my lip bumped the mic and whamo, I got zapped bad.                        I would always check the guitar and mic for zap problems before we'd start, but I must have for got that night. Shook me up pretty good, but I finished the song with the band.

When the song was over I fliped the ground switch on my amp and checked the mic/guitar pair to see if I would still get zaped, it was now fine.

It's been a long time since I sold that amp, but I am sure I remember seeing the 3'rd (ground wire) soldered to the chassis. I don't remember if the line filter cap was still hooked up.

So why did I still get zaped and why did the ground switch still work if there was a proper 3 wire power cord installed on the amp?


          Thanks,    Brad        
 

[sluckey]

So why did I still get zaped and why did the ground switch still work if there was a proper 3 wire power cord installed on the amp?

I can think of a few possible reasons. Maybe your amp was not grounded for some reason, defective power cord, poor solder joint (BTW, don't solder power cord green wire to chassis), etc. Maybe the mic and PA were not grounded properly. Maybe the house wiring or outlets involved were not grounded properly.

If all the convenience outlets in the house wiring were properly grounded, and your amp was properly grounded, and the mic/PA amp was properly grounded, you would not have been zapped. But since you did get zapped, there was a difference of potential between your amp/lips and the mic/PA. You could have measured the difference with a voltmeter.

For some reason, the PA amp chassis was not bonded to your guitar amp chassis. A properly grounded facility, guitar amp, and PA amp will bond all these things together and there can be no difference of potential to zap anything.

I know this doesn't totally answer the question why, but it's too late to get a definite answer now.

[The_Gaz]

I've had that problem with getting shocked by the mic before, but that was long before I started working on amps, and haven't thought much about it since. Similarly to you, I was able to fix the problem by switching the polarity of the ground switch, however it was on the PA, not the amp. I don't fully understand this issue either...

[Willabe]

Thanks sluckey, but your right, something had to be wrong. If I still had the amp and was still playing there I could probably figure it out, with help from you guys.

It's one thing to get zaped thru your hand, it's another when you get zaped thru your lip. That hurt!                

I got the amp from a guy named Henry Kern who owned/ran a music store and made a run at a guitar amp brand, KERN amps, 50 and 100w/4x12 cabs. They had 6417's (?) in them and he got a bad batch of them and that killed it, so the tale is told. He put the power cord in. He was very good at amp stuff.      

It was at a small bar called B.L.U.E.S. in Chgo. I knew the owners and the maintenance guy, David, pretty well. It was on the 1'st floor of an old gray stone 3 flat, and I know David had gone through the whole buildings wiring, including all the stage outlets. I worked there as a door man, so I was in the basement many times and David had showed me all the new (big) mains breaker boxes and new wiring/runs in conduit he had done. He was a very good all around handy man and his electrical work was as neat as any I'd seen on the jobs I'd worked on as a carpenter back then. But no ones perfect.      


So just to make shure I am understanding this right, once a 3 wire power cord is installed properly, fliping the ground switch with a line filter cap attached to the G Sw, it does nothing.

        Thanks,     Brad      

      

[HotBluePlates]

Why do you think building inspectors walk around with a little outlet checker? There's at least 4-5 ways to wire the outlet wrong (and some that will still seem to work, when looking at only 1 thing plugged in THAT outlet).

Another item that Steve didn't mention:
In general, you think about resistors failing by burning, and becoming an open circuit. Since caps are already 2 conductive plates seperated by an insulator, you generally think about caps failing as a short-circuit (though they can fail open).

I forget the proper name (maybe class Y?), but there are caps specifically designed for line-filter duty, which are designed to fail safely (as an open-, not short-circuit). If you have a good ground wire, and good outlet wiring (and properly-sized breakers/fuses), you shouldn't need that because the breaker trips before you're shocked. But the fail-safe caps do make sure that the line filter won't be a potential cause for trouble.

[Willabe]

Why do you think building inspectors walk around with a little outlet checker? There's at least 4-5 ways to wire the outlet wrong (and some that will still seem to work, when looking at only 1 thing plugged in THAT outlet).

That's true and David could have made a wiring mistake.          If I were still playing out I would keep one of those outlet checkers in my guitar case. I was not aware of them back then. When I gutted my house inside and out down to the rough framing, I put in all the wiring and used one to check my work, they are handy.

I forget the proper name (maybe class Y?), but there are caps specifically designed for line-filter duty, which are designed to fail safely (as an open-, not short-circuit). If you have a good ground wire, and good outlet wiring (and properly-sized breakers/fuses), you shouldn't need that because the breaker trips before you're shocked. But the fail-safe caps do make sure that the line filter won't be a potential cause for trouble.

Yeah I posted over at KOC's a while back about cap ratings for cross line filters on PT secondary HT wires. He brought up what your talking about, type and class name. They are raited/listed with a DCv not an ACv on the tech sheets.
 
IIRC, he seems to favor a Poly Prop cap for this, altho he said you can use a ceramic disk if you get the right kind, something about different types of C disk caps are made for different frequencies/hash/noise?


           Brad        

  
  

[phsyconoodler]

Funny how no one seemed to mention that you could simply turn the
bias balance' pot to get those tubes drawing the same current.Maybe I missed that somewhere??
  I do that all the time when repairing those amps and simply adjust it so the bias voltage is the same at all the tube sockets(pin 5) and then check the tubes with my bias meter to know if I have to change the range resistor.
  I never use the current shunt method anymore.Too dangerous if you slip.

[HotBluePlates]

I don't like the arcing/sparking that happens with the current shunt method, don't like the danger if you slip, and often want to get the job done faster than shutting off and clipping the leads in for each tube.

But that's just me. If it was a customer's amp, and bias was a critical issue to them, I'd do whatever they want. I'd just charge for the time it took.

[Rev D]

OK, I have a question.

I had a BF SR for years that I giged with. When I bought it it already had a 3 wire power cord installed in it.

One night I was singing and my lip bumped the mic and whamo, I got zapped bad.                        I would always check the guitar and mic for zap problems before we'd start, but I must have for got that night. Shook me up pretty good, but I finished the song with the band.

When the song was over I fliped the ground switch on my amp and checked the mic/guitar pair to see if I would still get zaped, it was now fine.

It's been a long time since I sold that amp, but I am sure I remember seeing the 3'rd (ground wire) soldered to the chassis. I don't remember if the line filter cap was still hooked up.

So why did I still get zaped and why did the ground switch still work if there was a proper 3 wire power cord installed on the amp?


          Thanks,    Brad        
 

 Heh, been there done that... Wakes you up pretty good when you hit the mike huh? I had a old twin do that to me too, after that I always grabbed my strings and lightly tapped the stand and mike head to make sure it was all in synch, painful little jolt testing but quicker that carrying around a meter or something to check it.

 Platefire, I hear you on the sound of the twin, its a great gigging amp if your young enough to heft the 85+ pounds around, with 4 herniated disc it ain't happening for me anymore. I had a silverface Dual Showman reverb head that was pretty darn heavy too, sounded incredible and was probably one of the most powerful Fender's I'd every played, had HUGE bottom end and would seriously and literally shake the house playing it at home. With the bright switch activated using a telecaster you could really cut someone's head off with it (I always thought of Albert Collin's when I played it like that). Anyway congrats on the repair, these kinda deal's are cool as I always learn something when all the old hands here (and I mean old in the most respectful way) give input on the repairs.

Best regards,

Don

[Platefire]

Well I was wondering about what phsyco said!

On the balance bias pot when I was working on it I was wondering about what if I was to swing the hotter bias to the weak side that was reading 57mA(28.5mA per tube) and reduced the side that was 80mA(40mA per tube) to balance them out more even like 57+80=137 devided by 4=35.25mA each. If I understood correctly phsyco said I could have done that!!! If the balance pot it not for doing that, what is it for???

What I actually did was balanced the negative bias voltage to both sides so when & if the customer installs a matched quad set of new power tubes (like I recomended) it will already have the bias voltage equal. Did I do bad? Platefire

BTW-Thanks slucky for the detailed explination on the death cap. One thing I still don't understand about the .047 death cap. We use those type caps like that to block DC on the grid and let the low voltage AC that the guitar signal rides on pass, so why is it not letting the larger 120VAC pass? I don't know, just had to ask!

[HotBluePlates]

One thing I still don't understand about the .047 death cap. We use those type caps like that to block DC on the grid and let the low voltage AC that the guitar signal rides on pass, so why is it not letting the larger 120VAC pass? I don't know, just had to ask!

A few basics...

1. A .047µF cap looks like a 56KΩ resistor to the 60Hz line frequency. ...

Steve told ya already. 56k looks like a big resistance compared to the few ohms (or fraction of an ohm) for the PT primary. Use the formula for capacitive reactance to verify that.

120v/56k = ~2.1mA more current compared to the several amps drawn by the primary.

[Platefire]

Yes he did cover it, but I guess that flew right over my head, like a lot of other things! Thanks for pointing that out--and sorry sluckey. So now I fully understand the death cap thing.

That leads me to another question  on another thread I removed two .2 caps accross the two OT secondarys to ground on a Marshall JMP 50(see attached schematic). One cap failed and was blowing the fuse. Those caps were on some early Marshalls and not on others and apparently somone modded this amp to add them. Was the purpose to act as a secondary filter to line AC to hold down noise of if not what was the thinking on having those in that location? Platefire

[Platefire]

Bump! I'm bumping this up to hopefully get some attention to the questions in reply # 35 above in second paragraph. I really would like to know the purpose of those caps on the Marshall PT secondarys. Platefire

[LooseChange]

I believe those stop the pop noise you'll get when you flip the standby switch.
Bad design on that amp... The bias supply is on the wrong side of that switch.

[Platefire]

Thanks LooseChange

Well I pulled them out of that Marshall with customers permission before I gave it back to him. I never noticed any pop in the stanby switch operation. So your saying the tap to the bias supply should have been before the stanby switch so the bias voltage would be active the instant you turn the power on like the heater voltage. That way when the DC plate voltage hits the tube, it would be all ready to go?

I just got to wondering about it after the fact, what the purpose was. First amp I worked on that I seen these caps in that position. Platefire

[Willabe]

So your saying the tap to the bias supply should have been before the stanby switch so the bias voltage would be active the instant you turn the power on like the heater voltage. That way when the DC plate voltage hits the tube, it would be all ready to go?

Yep.

 Less stress on the power tubes. Bias won't come up to full -dcv untill the bias filter caps are fully charged, but the ss rectifer diodes will come on/up to their full +dcv right away. Power tubes run full tilt without -bias.       


             Brad