Salvaging a 5c3 "Frankenstein" amp

[apwwest59]

Hi all,

First time posting on the forum - thanks in advance for the helpful replies!

I recent purchased an amp off craigslist with the intention of making it a "project". I've been interested in building a kit amp or project for a while, and it was pretty cheap so I figured I would give it a shot.

The circuit is supposed to be a 5c3 tweed deluxe, I had it looked over by an amp tech and there are 2 main problems: the transformers need to be replaced as they the amp is currently only putting out a couple of watts, and the internal wiring is VERY sloppy.

I was hoping someone could give any advice about if the amp is worth salvaging, and what the best way to do so would be. The amp tech suggested the easiest way would probably be to use terminal strips to clean up the wiring and put in some new transformers from somewhere like mojotone. Somehow it seems like using a board would be easier, but I'm not sure where I could fit it in the layout.

I have attached a few picture of the inside of the amp to give an idea of what it looks like. I like the tone of a 5c3, but if there are simpler options utilizing the components (5e3, etc) I would be willing to try that as well.

[Willabe]

Hi and welcome!   

The amp tech suggested the easiest way would probably be to use terminal strips to clean up the wiring

Yeah, I don't think that amp is safe with all the components floating in the air! I wouldn't play through it like that, but that's just me.

I had it looked over by an amp tech and there are 2 main problems: the transformers need to be replaced as they the amp is currently only putting out a couple of watts..........

What exactly did he say was wrong with the transformers?


                    Brad     

[apwwest59]

Yeah, after I saw the insides and the tech explained that it isn't safe (doesn't look safe either, incidentally) I'm definitely not going to be plugging up and playing.

He didn't say for sure what was wrong with them, presumably under-powered and also apparently wired wrong, obviously with no wiring instructions. He said it looked like they came of a tiny old PA or something of that nature.

I'm hoping that it will be possible to get all the components on a board and wire it up to where it isn't hazardous. There is about 6 inches of space between the PT and the side of the chassis (top right quadrant of first picture), but I don't know if thats going to be big enough to fit a whole board. I would plan to get the caps that are already on a small board reconfigured onto the same board with the other components.

[Willabe]

The iron set might be a little on the small size for a ~5E3 but I'm betting on wiring mistakes for the low power output.


                Brad     

[labb]

I have built a couple of 5B3 amps so maybe I can help a little. Go over to Schematic Heaven and you can find a layout for the 5B or 5C3 amps. The board in the ones I built is 8.5" long by 3" wide. That is with the power supply caps on the board. If you remove these big caps from the board then you could make the board 6" long. I would think that your problem is going to be the width of the board.

[apwwest59]

Is there any way to trouble-shoot that?

The tech mentioned another safety hazard pictured below - the wiring exposed on the side of the transformer.

The taps are as follows:

Blue wires out of #2 (bottom) and #5 (top middle) go to a 4 ohm output
Red wires out of #5 & #6 (top middle and right) go to an 8 ohm output
White wires out of #4 and #6 (top left and right) go to a 16 ohm output.

On the other side, there are 3 wires. 1 each to the 3rd pin of each of the power tubes, and 1 to a 450V/40uF cap (the purple one).

It would seem that these are the components of an amp that are quite dangerous to be messing around with, so without a wiring diagram for this OT, it seems pretty risky fooling around with it.

Would there be any point in trying a new OT, since I could ensure that it was wired up correctly? The tech said that he tried the amp out sending to a known-good OT and the wattage did come up, although not all the way to the full expected ~15 watts. Currently it is about 2-2.5.

Regarding the space: it looks like it could accommodate about 4-4.5 inches in width before you start running up on the tube sockets. The small board that has components wired onto it would be reconfigured onto the main board and would make some room (I think).

[apwwest59]

Also, I went over to Schematic Heaven, and I can see right off the bat that the caps are not the correct values. The 5c3 schematic calls for 3 450V/16uF caps, this one has a 450V/40uF cap in there. Not sure how much difference that makes, but I'm sure it isn't helping.

[eleventeen]

Well unless my eyes are fooling me, I for one think your core problem could easily be that you have an amp built for 6V6 tubes into which 6L6 tubes have been stuffed. A 6L6 output section requires a fair amount more drive than a 6V6 section. Furthermore, the 6L6 tubes are going see (what is to them) lowish B+.


So just on an offhand basis, maybe thing #1 you want to do is to get 6V6 tubes in there and see if it works a lot better. Take note of your B+ both before and after you make the swap and try to get a read on the output tube current, which is easy measuring across the cathode resistor.


Then you have the issue of how you may want to proceed assuming you'd like to get the thing working. Looks to me like most of those resistors and caps have pretty long leads, and the jacks and pots are newish, so maybe you could do this pretty painlessly in terms of not having to buy lots of new parts. I'd throw some 6V6 in there, see how well (or not) it works with existing iron, and then make a decision as to whether you want new tranny/trannies.


The existing trannies don't look all that small to me, not to the point of being garbage. Of course one big issue is the actual B+ delivered to the output tubes. If that is low, that's a limitation. Maybe you can get some more ooomph by changing to a SS rectifier. That's pretty painless and you can pretty easily do that on a trial basis by soldering diodes to the 5Y3/5AR4 tube socket and see how it goes.


I think you have a pretty decent candidate but there is ALWAYS this thing about a squarish "used-to-be-a-hi-fi-amp" chassis, which is, you can't get a long skinny parts board in there without covering up terminals that you have to get to to wire the thing. But you CAN get a bunch of terminal strips in there, what I usually do is to put in more than I might need, which gives a fair amount of freedom when it comes to reconstructing the circuitry.


Keep us apprised as your progress!

[Willabe]

Also, I went over to Schematic Heaven,

If you scroll down to the very bottom of this page you'll see this, just click on it;

Check out our huge library of schematics here

Our host Doug has a very extensive library of tube amp schematics, some SS too.


                           Brad   

[apwwest59]

Hi, thanks for the reply!

My picture isn't that great, but they actually are 6v6 tubes.

In terms of using terminal strips - do you normally just kinda line them up in 2 parallel rows at a distance that can accommodate all the components? Maybe something like pictured?

[eleventeen]

That's the general idea. See if you can get a separate strip-pair for the filter caps, separated from the preamp circuitry, just for hum/noise purposes.

[apwwest59]

For the filter caps, is the main issue getting physical space between the pre-amp circuitry or being on a different strip or grounding, or what?

You could use a terminal with 3 lugs and position it all the way to the left towards the PT then try to squeeze the preamp stuff in towards the right. It looks like there would be enough space, but I guess you never know until you try.

[eleventeen]

"physical space between the pre-amp circuitry or being on a different strip or grounding, or what?"


We want distance between hum-generating parts (power supply) and hum sensitive (preamp) parts. And sometimes/often separate grounding helps. The grounding (and shielding of input wires if it becomes necessary) is something you want to pay attention to, but it can be reworked (in case of a problem) fairly easily as long as you don't solder in a bunch of parts each with their own individual grounds. You want to kind of gather grounds together in a PREAMP group and a POWER SUPPLY group. The idea here is that with a non-proven layout, you just want to use "best practices"  and avoid "worst practices" as much as possible. Giving yourself the best chance of success without having to rebuild the thing over and over. Nobody likes to do that.


I think it would be good for you (and us) to get a slightly larger picture going where the folks on the forum can get a complete gutshot view so we can best help you with possible layout changes. 

[apwwest59]

Here's a full photo of the chassis. The tube sockets from top to bottom are 5Y3, 6V6, 6V6, 6SC7, 6SC7. OT is in the top right, PT can been seen coming through the chassis body. Power switches top left, inputs bottom left (only 2, not 3 as in schematic), 3 outputs on the right side for 4, 8, and 16 ohms.

[eleventeen]

OK, that's great, thank you. All I can do is to ask you 20 questions, not nec in order and probably can't think of them all in one shot.


1: We would prefer to see the black (instead of the white, as is) wire of the line cord hit the fusepost...the inside terminal, then go from the side terminal to the switch....and have the WHITE wire go "full time" to the transformer. But don't mess with this yet, IMO.


2: Not seeing (but I could be missing it) any center tap for the 6.3 volt heater winding....this is usually required, either an actual wire coming out of the PT, or, 2 qty 100 ohm resistors from the 6.3 outer wires to ground. The synthetic one is actually preferable but do NOT have BOTH. Not having such a CT, real or syn, can produce a lot of hum.


3: How much are you prepared to rip this apart?


4: This is going to sound ridiculous, but I would check to see that the prior builder chose the right OT wires---the CT of the primary to your highest B+ and the endpoints of the primary to the two 6V6 plates. Easy to check (power off, of course) with an ohmmeter.


5: I would like to see you swap the position of the choke and the OT, but that's pretty serious mechanical and wiring surgery. This would make it easier to move the filter caps to the other (top) end and that in turn would give you the room for the twin terminal strips that would sort of mimic the parts board---oriented north-south---at the bottom of the picture. That pretty much implies tearing out every R & C and starting fresh all the way back to a chassis with just the mechanicals---the tube sockets, pots & jacks---mounted.


6: Or, you could probably get away with adding a north-south terminal strip between the tube sockets and the front panel (can you squeeze it in there? it will be tight) to better support the caps that come off the tone controls while shortening their leads. Maybe 5-6 lugs (not incl grounds) and more is better---roughly centered between knob #1 and knob #3. If you did that, you may not need to install the twin-strips mimicing a parts board. And then you wouldn't need to swap the choke & OT.


Just throwing out some ideas. Lots depends upon how much work you want to do.

[apwwest59]

Thanks for all the info! I'll give my best shot at answering these.

1. Ok - will hold off on this for now.

2. There is a wire coming off the PT to ground, not sure if this is what you're talking about. Currently it has 1 to the power cord, 4 to the rectifier tube, 2 to the lamp, one to the AC/On switch, and 1 to ground (it is a light brown color that loops up and to the right and gets lost in the red/white/blue OT wires, but it goes to ground).

3. I do not mind a fair bit of moving things around. It seems more or less unavoidable to do so to some degree, so I may as well try to get it as "right" as I can since I'll be doing a fair bit of surgery on it anyway. This is my first time doing anything like this so I'm sure it will be slow and tedious either way, but it is interesting to me and I would like to learn.

4. Not exactly sure what this means... The OT does have a wire running to the 3rd pin of each 6v6. Is this what you mean? Again, I'm new to this so I may be asking questions about things that are pretty simple. The third wire coming out of that side of the OT patches into a red wire at the top of the photo then runs to the terminal above the purple filter cap. Also attached to this terminal are: the purple filter cap, 1 wire from the choke, 1 wire to the DC/Standby switch.

5. & 6. I'm not sure there is going to be enough room to squeeze a terminal strip between the tube sockets and the pots. My fingers are also not very nimble with a soldering iron yet, so the easier the better on that front. I should be able to switch the OT and choke without too much trouble - would that be a preferred layout in general, or out of necessity due to the layout of this chassis? I assume then you would essentially do the same thing with the filter caps and "float" them up at the top end, just how they are now at the bottom? That would free up some space and eliminate a couple of those longer red runs. From there, you would then just lay out all the pre-amp in the parallel row of terminal strips and should have lots of room.

In terms of separating the "preamp" elements and "power amp" elements physically, where does that distinction occur in the circuit? Obviously the filter caps, but would anything else move down to that end? The square ceramic resistor 250ohms/watts? I've tried to draw and outline over each section of the layout - poweramp, preamp, and input.

Also, I have been studying the schematic and have been puzzled by a couple of components in my amp - at the bottom of the photo, down along the bottom chassis wall, there are 2 resistors (presumably?), both labeled 7500 ohms, connecting from pins 7 & 8 of the first preamp tube, both connected to the same filter cap. I can't find anything like this on the schematic. Any idea whats going on there?

As an aside, I will need to order all new caps for the preamp section, as the white ones seen in the photos are the wrong values. At least whoever built this was better at building cabinets than they are at wiring up amps!

[eleventeen]

1: If you really decide to tear things apart, should do this. We want the fuse in the "hot" (blk) not the neutral (white) and we would like the fuse, if it blows, to remove power from as much of the innards as we can.


2: Not clear if you are saying there are TWO wires from the PT to ground. There MUST be one: The CT of the HV winding that produces the big B+ volts fed to the output tube plates. There COULD be another one: It's likely to be the CT of the 6.3 heater winding. It would be good to measure, ohmmeter w/amp power off, from either side of the fil winding (can grab this on pin 2 and/or 7 of either 6V6) to ground. You should get a low-ohm reading from either pin. If you DON'T get such a reading, then you should create a synthetic CT. If you DON'T get such a reading, rejoice, because installing the syn CT will generally reduce hum massively.


4: One of the things about old trannies is that yes, the leads are color coded, but the color may have shifted over many years of heat. We do not know who put together your amp as is. Some of his practices are sketchy. Some of those wires are covered with sleeving so we do not know the color of the wire, correct?


In terms of the output transformer, the CT of the primary winding needs to go to your highest B+. The "wings", the outer ends of that winding go to the 6V6 plates. Pin 3, pin 3. Just as a check against full-on stupidity, I would check to see that it really is the CT that is getting your B+ and it really is the wings that go to the plates. How? With power off (for a few minutes, make sure the wire that goes to the filter caps has roughly equal ohms to either 6V6 plate.


5: The square resistor is the cathode resistor, separating the joined cathodes of the 6V6 tubes from ground. Not power supply. It's not sensitive to noise. 


On a 6V6, it is pins 2 & 7 that are the heater pins. On your 6SC7, it is pins 7 & 8 that are the heaters. Those two resistors *could* be the prior builder's attempt to create the synthetic CT. You say they are going to "the filter" cap. The ground (-) end, I take it?


By the way, do you have at least a multimeter and have you messed with high, potentially lethal voltages before? Just checkin'.

[apwwest59]

Starting from the end:

Yes, I do have a multimeter. I have not used it extensively, but I can't imagine it being too difficult to operate.

Second, no, I have not messed around with anything like this before. I am going to keep the amp unplugged for now while I investigate what is going on, then double/triple/quadruple check everything I change before even thinking about connecting it to power. I have no assumption that I understand anything about this so I will exercise the upmost caution.


1. Makes sense - once the soldering iron comes out I will take care of this one.

2. Just 1 wire from PT to ground. Also - what do "CT" and "HV" refer to? (EDIT - "CT" = center tap, yes?)

4. I am not 100% clear on this still, but I think this is whats going on. See photo below. The top tap is labeled B+ (hard to see in picture) and runs to the purple filter cap. (EDIT - this terminal at the purple filter cap also connects to the Standby switch) The taps on the bottom are labeled "P" and run to pins 3 of each 6V6, respectively.

ALSO - I was able to determine that the OT is a Stancor A-3830 (output ~20 watts) so the incorrect wiring theory is looking more and more likely. I also found a partial instruction sheet at the following link, but I do not know how to read it:

http://plymouthcolony.net/starcity/radios/pages/instruct.html

Also, the original wires have obviously been removed (on the other side of the transformer) due to the presence of the red white and blue wires soldered onto the outside of the transformer. Is that something I could test with an ohmeter to determine which is which? I can still read the numbering, if that helps. I assume a standard multimeter also functions as an ohmeter, or is that typically a separate tool?

5. So those components can go where ever they fit best then I assume?

So would your recommendation on these resistors be to leave them out of the updated layout, if in fact the ground coming out of the PT is correctly wired?

Also, another simple question, but how can you tell the + end from the - end of a filter cap? From what I can determine reading online, the arrows will point toward the negative lead - if this is the case, then yes, these resistors are going to the negative/ground (-) lead

[apwwest59]

I've been doing some more digging to try to determine if the OT is wire correctly based on the manual I found. All of this is just guesswork/approximations, so please correct me if any of my assumptions are incorrect:

-A pair of 6v6 tubes looks for a load of ~8000 ohms (correct?)

The amp is wired as such:

16ohm out - taps 4 + 6
8ohm out - taps 5 + 6
4 ohm out - taps 2 + 5

-According to the manual listed above, the closest combination of OT taps at 8k ohms from the tubes to a 16 ohm load on the speaker would be 3 + 6 for about 14.8 ohms. As wired currently, the load at 8k ohm would be about 12.4 ohms.

-The outputs for the 8 and 4 ohm speaker outputs on the amp are similar. For the 8 ohm output the tap combination gives a load of 6.6 ohms, for 4 ohm output the tap combination gives an output of 2.7 ohms.

It seems like these would be "close enough" - or would they not? Could this explain why the amp is putting out only 2-2.5 watts? What other considerations could there be for this result? Blown transformer?

Any ideas welcome.

[sluckey]

This is how the primary of the OT should be connected (see pic).

Could this explain why the amp is putting out only 2-2.5 watts?

How do you know it's only putting out 2-2.5 watts? We need some voltage readings for the B+ rail and all tube pins. Otherwise, we're just shooting blind.

That amp is a trainwreck. And if you don't know what you're doing, maybe you should walk away. But until you actually plug it in and get some more symptoms and voltage readings, there's not much progress to be made.

[apwwest59]

The wires to pins 3 are correct. When you say "connect to B+" - where would that connection be? It is connected as show below.

The amp tech that looked it over said he measured the output at around 2-2.5 watts; I don't know if that is accurate or not, but that is the information I was working off of.

In terms of getting some readings off the amp - I can break out the multimeter this evening and check on things. (EDIT: newbie question here, but you meant every pin on all 5 tubes for voltage, being measured to ground, with the tubes in the amp? Just making sure...)

I realize this amp is in pretty bad shape, I'm hoping I can potentially get it working right and learn something in the process, otherwise pretty much my only option is selling it for parts/scrap, which I would like to avoid.

[sluckey]

(EDIT: newbie question here, but you meant every pin on all 5 tubes for voltage, being measured to ground, with the tubes in the amp? Just making sure...)

Yes.

I realize this amp is in pretty bad shape, I'm hoping I can potentially get it working right and learn something in the process, otherwise pretty much my only option is selling it for parts/scrap, which I would like to avoid.

You can always put it back on craigs list or ebay.

I'm not saying there's nothing to salvage. But I am saying you need to gut everything that's not connected to the chassis with a nut and bolt and start over.  As is, that amp is a TRAINWRECK. And I ain't talking about brands.

[apwwest59]

Ok, so I took the readings. I didn't feel comfortable reaching my hand in to try to get the reading off of preamp tube due to the web of caps, resistors, and exposed leads covering them, so I can't say about those at this point. There were several pins where the voltage seemed negligible - I'm guessing I just needed to change the range on the multimeter to get these accurately. I can re-do the testing if they turn out to be important, but for now I'll leave them blank.

The reading on the main B+ to the OT was ~2.5V

5Y3:

Pin 1 -
Pin 2 - 5.1V
Pin 3 -
Pin 4 - 254V
Pin 5 -
Pin 6 - 253V
Pin 7 -
Pin 8 - 2.5V

6V6 (closer to rectifier):

Pin 1 -
Pin 2 - 3.3V
Pin 3 - 2.5V
Pin 4 -
Pin 5 -
Pin 6 -
Pin 7 - 3.3V
Pin 8 -

Other 6V6:

Pin 1 -
Pin 2 - 3.3V
Pin 3 - 3.4V
Pin 4 -
Pin 5 -
Pin 6 -
Pin 7 - 3.3V
Pin 8 -

So... Trying to draw some conclusions here - would I be correct in assuming that:

1. My PT is ok, although slightly undersized, putting out ~250V to the rectifier.
2. Somewhere between the rectifier and the OT/power tubes, I am losing the voltage.

"It would be good to measure, ohmmeter w/amp power off, from either side of the fil winding (can grab this on pin 2 and/or 7 of either 6V6) to ground."


Per suggestion, I also measured this resistance and came up with 3.81k ohms.

Any ideas on what's going on?

I really don't see a safe way to get to the pins on the pre-amp sockets, so it's possible that the next step would be to re-wire that whole area...

[labb]

Do you have a spare 5Y3? If so give it a try and see what reading you get at pin 8

[apwwest59]

I don't have another 5Y3, but I do have both a GZ34 and a 5U4, both of which I know are good. Would those work for testing purposes, or is that a bad idea?

[Willabe]

I'm not saying there's nothing to salvage. But I am saying you need to gut everything that's not connected to the chassis with a nut and bolt and start over.  As is, that amp is a TRAINWRECK. And I ain't talking about brands.

I agree totally.


                Brad     

[sluckey]

So... Trying to draw some conclusions here - would I be correct in assuming that:

Looks like you took all voltage readings with your meter set to measure AC voltage. That ain't good.

[apwwest59]

Hah! Oops... 

Lets try this again...

[eleventeen]

With all candor, you asking to be instructed in this process which requires things like knowing how to use a meter and keeping your hands in your pocket when you are measuring volts and what "B+" means and any one of us who have done this 5, 10, or 50 times would probably start by staring at the thing for a while and thinking about whether we wanted to replace this part or keep that part and how much room this would take and where we would place ground. And by instructing you we are assuming you know enough not to zorch yourself to death yet at the same time you clearly don't understand using your multimeter.


If the project is taking you all the way from "learning how to use a multimeter" to "building a guitar amp from an old hi-fi amp using the parts that happened to come along with the thing and making 119 decisions as to what to replace and why"....I have to resign my teaching post.


Sorry.

[apwwest59]

No offense taken, thanks for the help thus far.

DC readings off the tubes:

5Y3:

Pin 1: 0
Pin 2: 261
Pin 3: 0
Pin 4: 4.5
Pin 5: 3.7
Pin 6:
Pin 7:
Pin 8: 263

6V6 closer to rectifier:

Pin 1: 0
Pin 2: 0
Pin 3: 260
Pin 4: 259
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 15.7

Other 6V6

Pin 1: 0
Pin 2: 0
Pin 3: 260
Pin 4: 258
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 15.7

[sluckey]

OK. Much better. Those voltages look lower than I was expecting. What about the other two tubes?

[apwwest59]

I can't find a safe way to get to the pins because there is too much stuff in the way (see below - wires, caps, resistors).

[PRR]

Is a good chassis. 250V B+ is fine (though not hot-rod), and you could hit 300V with Silicon in place of the 5Y3.

The wiring... it "might" be good, but looks shakey, which inspires ZERO confidence. Rip and re-do.

The OT might be OK, or not. I would really toss it and get a known-good OT.

Whether you are the person to do this, or it should be tossed back on CraigsList, I dunno.

> why the amp is putting out only 2-2.5 watts?

Honest accurate power testing is not a hasty or free job. I dunno that it is 2 Watts, you dunno, and even if true we don't know if it is a minor mistake or something serious.

[apwwest59]

Sounds reasonable. I guess the next course of action will be remove everything, re-wire the preamp/etc on to some terminal strips based on the Fender schematic, and re-test, likely a new OT.

Thanks for all the help, I'll check back in once the surgery/rewire is complete.

In the mean time, any suggestions for a possible replacement OT?

[PRR]

> suggestions for a possible replacement OT?

5C3 is a DeLuxe, right?

Right here (see "transformers" in the table at the bottom of this page).

http://hoffmanamps.com/MyStore/catalog/parts1.htm
Output for Tweed Deluxe and Deluxe Reverb
25 watts, 8 ohm secondary.
Two 6V6 power tubes.
Part number 041318.

There's also Hammond, various vendors, 1750H and 1760H: Deluxe, Deluxe Reverb 125A1A & 022640

[apwwest59]

Quick question on heaters for the preamp tubes...

On the Fender 5c3 schematic/layout, pins 1 and 8 are jumped together and grounded. Pin 1 is to the shell (assuming not connected for glass tubes vs. metal shell). Pin 8 is a heater.

Pin 7 is also a heater. In the schematic, pin 7-heater is wired back to the heaters on the 6v6 tubes, but pin 8-heater is jumped and grounded.

Does this mean that only 1 heater is enough for both triodes in the tube? Or am I missing something here?

[eleventeen]

The metal of the chassis constitutes the "other wire" in what we know to be the requirement of two "wires" to light up a heater. This was pretty common in the era this amp was built.


[It is less common today; the preference being running a separate wire instead of using the metal of the chassis. Some would suggest that changing from chassis to wire might reduce some hum in the output. And it *might* and it might not. It will NOT make the difference between the amp working or not]


Pin 1 wants to be grounded as a shielding issue wrt glass 6SC7s and metal 6SC7s, exactly as you said. Pin 8 wants to be grounded being one side of the heater string. The tube does not care if there are two separate ground connections to ground, separately, one from pin 1 to gnd and one from pin 8 to ground....OR, if pin 1 is jumped to pin 8 and THEN connected to ground on a single wire. The former would look like a "V". The latter would look like a "7" (maybe a backwards "7")  Doesn't matter.



If and when you light up the heater, all sections of a multisection tube will be "lit"; eg, a tube has only one heater. Note that the 6V6 tubes have their heaters pin 2 & pin 7. The preamp tubes are pins 7 & pin 8.

[apwwest59]

Thanks for the reply.

From how the wiring was previously configured, the 2nd heater pin on the preamp tubes is wired back to the other heater pin of the 6v6's. If I understand you correctly, either this configuration or the configuration in the schematic will accomplish the same thing?

Since 6sc7s are somewhat scarce, I opted to go for a 6sl7. After researching, it looks like the tubes have similar specs but different pin-outs. In order to sub this tube, it looks like the only wiring changes I will need to make are to jump pin 3 and 6 to create a common cathode, and to wire anything in the schematic going to pin 3 (grid 1 on 6sc7) to pin 1 (grid 1 on 6sl7). Will I be alright doing this or there any other considerations to keep in mind?

EDIT: grid t2 not t1 regarding the pinout

[eleventeen]

What you propose is electrically doable, but I would advise you to try harder and get the 6SC7(s). They are a little teeny bit scarcer than 6SL7 but not by much. Wherever you have to go that has 6SL7s will have 6SC7s---if not, go to the next potential vendor. I acquired a garage full of old tubes and had dozens and dozens of each, quite possibly more 6SC7s. All traded away. Those tubes "fly" together, where you find one, you'll find the other, in most cases. 


What you propose, again, is doable. To do it successfully, you are going to have to be very sure about what you are doing. Is it a big deal? No, not for those with experience. I am not sure whether to encourage you. You are going to have to create a marked-up schematic incorporating the mods required, and keep it with the amp, and have a changed tube diagram inside the cabinet...all doable...can you keep it straight?


It would be nothing for *me* to do, and it wouldn't be my preference, frankly.

[PRR]

> the preamp tubes...
>...pins 1 and 8 are jumped together and grounded. Pin 1 is to the shell (assuming not connected for glass tubes vs. metal shell). Pin 8 is a heater.
> Pin 7 is also a heater. In the schematic, pin 7-heater is wired back to the heaters on the 6v6 tubes, but pin 8-heater is jumped and grounded.
> Does this mean that only 1 heater is enough for both triodes in the tube? Or am I missing something here?

Last question *first*.
http://www.mif.pg.gda.pl/homepages/frank/vs.html
Look-up 6SC7. (The Tung-Sol sheets are often concise and clear.)

Each heater has two leads.

However both heaters are wired together.

This is same-as you have a two-lamp chandelier, or boat-light, or motorcycle dual headlight. There's two bulbs, two contacts on each bulb, but the chandelier maker wires them together, so you only have to deal with two leads not four leads.

Pin 1 on 6SC7 is Metal Shell. (6SC7G is glass and pin 1 is No/Connection, but Fender apparently anticipated metal shells.)

This older amp has one side of the heater *circuit* returned through chassis. Same as a car/cycle headlight often has one side returned through body metal. While this was OK on radios, in guitar amps it is marginal. Most later amps run two heater wires, don't flow heater current through chassis. An amp of this vintage, the slight hum is "authentic". A conversion to 2-wire heater power is historically incorrect (but might be done to use it in recording).

You may see 12AX7 in other amps. This also has two heaters, two leads each, but instead of bringing them out on 2 pins they do a 3-pin connection. This allows you to use the same tube with either 6V heat (most AC-power stuff) or with 12V heat (cars, some large systems). This is not an option for the 6SC7 (or 6SL7, though a 12SL7 is available).