Fender Prosonic Combo Bias Sanity Check

[VinceM]

I just ordered some new tubes for my Prosonic Combo and I'm going to check the bias on it after replacing the tubes even though I've heard it's not necessary. I'm going to be looking for the Plate Dissipation on each of the 6l6GC tubes rather than simply measuring the current to the plates.

The Prosonic is the most intimidating amp I've ever worked on simply because of the rectifier switching system. For those not familiar with this amp, it has 3 possible settings controlled by a switch in the back that go from:

1. Class A 30 Watts (Tube Rectified)
2. Class A/B 45 Watts (Tube Rectified)
3. Class A/B 60 Watts (Diode Rectified)

I can't seem to find a pcb layout on the Prosonic but I'm fairly certain that this is the center tap of the output transformer:

If that's correct, I should simply be able to read the resistance from the Output Transformer center tap to Pin 3 of each output tube and the voltage drop. Then I can calculate the current (V/R), measure the Plate Current on each tube and then calculate the Plate Dissipation (Plate Voltage x Plate Current). I'm looking to get around 70% of the 6L6GC maximum power which should be 21 Watts.

Anyway, my question is, I should just be able to switch between the three different rectifier settings for each of these measurements right? I know there isn't much I can do on the first Class A setting since it's cathode biased but I could still measure everything that way anyway right? It should be literally as simple as flipping the switch to the next Class A/B 45 Watt setting to check the bias on that setting or am I missing something here?

(link removed) (it's the second schematic in the pdf) if anyone wants to take a look. I'm still not exactly clear on how all of the switching works with the rectification.

Thanks in advance.

 

[Wally]

VinceM, no, that is not where the OT's center comes to the B+ supply.
First of all, I was taught to look at that chassis with the back panel closer to me and with the control panel away from me. That is the way that the FEnder layout diagrams have alwys been presented, right?
In any case, the OT's red center tap comes to that small PCB board over which you have 'wrtieen' and drawn the arrow. That B+ contact is at the lower right hand corner of that PCB board if one has the chassis sitting with the back panel towards the camera or a working tech. IN the orientation in which you show that chassis, the B+ contact is in the upper left hand corner of that little PCB...underneath your 'writing' and those twisted red wires.
IF you trace the OT primarys that come through the chassis, you will see one blue, one brown and one red wire coming through one grommett. Follow that red wire, which is the B+ OT center tap wire. You will find it on thta particualar corner of the board.

One find the current draw in more than one way. I use the simple transformer shunt method. That is, with the negative probe of a multimeter on the power tube plate and with the positive(red) probe on that B+ contact for the center tap adn with the meter reading amps, one can read the curent draw directly. Danger, correct...live voltages. Then take the plate voltage reading..and figure you plate dissipation.
Ex: theoretical numbers...
450Pv x .035 amps(35ma) = 15.75 watts / 30 (max plate diss. 6L6) = 52.5% of max plate dissipation.

One could even figure the situation for the cathode-biased circuti if one wanted to do so...not much need, though. And....I don't believe that to be a true Class A operation...but that is another discussion. CAthode biased Push/pull(A/B), imho.....just as almost ALL amps that are cahtode-baiased and make the claim of Class A operation are.

I am a big fan of these ProSonics...most undervalued amp on the market, imho.

 

[VinceM]

Thanks Wally. I was actually going off someone else's picture since I haven't pulled the chassis on my Prosonic in a few weeks. Looking at this picture I see what you mean.

You know, now that I think about it, can't I just measure everything from pin 8 of the rectifier tube rather than messing around on the PCB with the Center Tap of the OT?

And yeah, I love my Prosonic. She's been acting up lately and I think it's time for new tubes since it's been almost a year. I opted for a 5751 to put in V1 because the Prosonic breaks up pretty quickly (volume at 4 on the normal channel). We'll see how it works out.

 

[vanr]

Do what I did when I still had my Prosonic. Order some JJ 6L6GC's from Bob at eurotubes.com and tell them they are for a Prosonic. They will send you some tubes that will run just right in the 30-watt Class A cathode bias mode and also bias up OK in the 2 other settings.

PS. buy one of their $25 bias probes and a meter while you are ordering. You will still have to pull the chassis though because the bias wheel is on the PCB board where that giant cement cathode resistor is.

 

[Berndizzle440]

Thanks Wally. I was actually going off someone else's picture since I haven't pulled the chassis on my Prosonic in a few weeks. Looking at this picture I see what you mean.

You know, now that I think about it, can't I just measure everything from pin 8 of the rectifier tube rather than messing around on the PCB with the Center Tap of the OT?

And yeah, I love my Prosonic. She's been acting up lately and I think it's time for new tubes since it's been almost a year. I opted for a 5751 to put in V1 because the Prosonic breaks up pretty quickly (volume at 4 on the normal channel). We'll see how it works out.

I know this sounds kinda like neutering your prosonic but I thought the same thing with 12ax7s so in v1 and v2 I popped in some 12at7s worked like a charm I love my prosonic as well! Mine has been acting strange lately too... when I take it outta standby I hear slight crackling for a sec like static or something then I dont hear it anymore

 

[VinceM]

It's a great amp. I just wish it didn't weigh a ton! My poor back.

 

[Wally]

Vince, I work off of the center tap. Yes, the B+ voltage comes off of the rectifier, but I want to see what the first filter does to the electrons.
Re: lower gain in V1, ye a 5751 will change what goes on there. A
12AY7 is the tube that FEnder used in that 5F6A circuit. Take note of that schematic. THE triodes in soem of those tubes are shared. Ex: In V1, one triode functions as the first gain stage in the 'normal' channel while the other triode is the first gain stage for the gain channel.

vanr wrote: "They will send you some tubes that will run just right in the 30-watt Class A cathode bias mode and also bias up OK in the 2 other settings."

Since the cathode biased section is more 'forgiving', my first thought is that I want my fixed-biased circutis to bias exactly to what my ear wants to here...not just 'OK'. Best of all worlds....all of those 3 circuits bias perefectly for my ear. vanr, did you rebais that amp for those new tubes ..in the fixed-bias options?

 

[vanr]

vanr wrote: "They will send you some tubes that will run just right in the 30-watt Class A cathode bias mode and also bias up OK in the 2 other settings."

Since the cathode biased section is more 'forgiving', my first thought is that I want my fixed-biased circutis to bias exactly to what my ear wants to here...not just 'OK'. Best of all worlds....all of those 3 circuits bias perefectly for my ear. vanr, did you rebais that amp for those new tubes ..in the fixed-bias options?

Of course I re-biased. That is when I bought the bias tool from eurotubes. The tubes eurotubes sent me I biased up in AB mode (SS) and they didn't redplate when put in Class A 30-watt mode, unlike the tubes that were in the amp when I bought it used. My point is tell them you have a Prosonic and they will send you the lower current draw 6L6GC's you need for Class A setting and you can bias them to Class AB with a probe,

 

[Wally]

THat is interesting, vanr. I have never seen a set of tubes redplate in cathode-biased circuits...not to say that it can't happen. Did you happen to taek measurements and figure the current draw in that redplate state? Just curious...,..IF I saw that happen in a cathode-biased circuit, I would be curious about the resistor in that circuit. IF it was exhibiting the 'correct' resistance, then I suppose I would have to look for a cooler tube...or a bigger resistance in that resistor if I had a set of tubes that I wanted to use there.
To what plate dissipation did you bias those fixed-biasedcircuits? And....back to the 'hot' set of tubes you had in that amp....and just out of curiousity...did you measure the plate dissipation for those tubes in the fixed biased modes?
These questions are not meant as a 'cross examination' type of thing. I merely like to measure and learn as I work on these things. I think that is a good idea of that tube distributor to understand what that cathode baised circuit 'needs' to see in the way of a tube. Most people would need that help.
I have a PRoSonic on the bench right now....looking for an intermittent noise. I'll take some biasing measurements before I button it back up, I am sure....even though it sounds exactly like the other two I have here.

 

[vanr]

Well from what I understand the cathode resistor in the Prosonic is too low a value. I don't remember what the value is, but I remember reading on the internet that folks were bumping them up with higher value resistor. Also the tubes that red plated were Chinese 6l6GCR's which are really 6PC3's and probably only rated for 19 watts or so. This pair also drew quite a bit of current. I drew this conclusion from running them in another amp I had.

No I never bothered measuring the draw when the tubes redplated (actually I believe it was only one tube that redplated) I just pulled them out and replaced them with the JJ's I got from Bob.

 

[Wally]

vanr, that cathode biasing resistance value is low. (The Vox AC-30, IIRC, uses even less resistance there.) Liek I said, I am going to do soem investigation....but I am going to defer to Bruce Zinky, the designer of this amp, until I find to the contrary. I love these amps....and I particularly like the heat in the cathode-biased circuit. One thing I would not want is for the soncis while in that cathode-biased option to sound anything much 'cooler' than it sounds right now.
But....those Chinese tubes are not a good platform from which to draw conclusions about tube replacement, etc, imho. I don't run chinese tubes....YMMV. All 3 of these ProSonics I have here have different 6L6 power tubes in them, and none of them redplates in the cathode-biased circuit.

 

[VinceM]

vanr, that cathode biasing resistance value is low. (The Vox AC-30, IIRC, uses even less resistance there.) Liek I said, I am going to do soem investigation....but I am going to defer to Bruce Zinky, the designer of this amp, until I find to the contrary. I love these amps....and I particularly like the heat in the cathode-biased circuit. One thing I would not want is for the soncis while in that cathode-biased option to sound anything much 'cooler' than it sounds right now.
But....those Chinese tubes are not a good platform from which to draw conclusions about tube replacement, etc, imho. I don't run chinese tubes....YMMV. All 3 of these ProSonics I have here have different 6L6 power tubes in them, and none of them redplates in the cathode-biased circuit.

According to Bob at Eurotubes, Zinky has admitted the Class A bias isn't that great:

"These amps are a Zinky design and even Bruce admitted that the bias was a bit off the deep end in the class A mode, so it takes a cooler grade for the power tubes to come in right but with cooler tubes and a good GZ34 and preamp tubes these amps are really quite nice."

The cathode bias resistance value is a 220 ohm 15 watt resistor. I've read bumping it up to 240 ohms or 250 ohms does wonders. In fact, I'm pretty sure that resistor was replaced in mine. Last time I opened up my Prosonic it had one of these:

.

I'm pretty sure that's not the stock resistor.

 

[VinceM]

Just to update everyone, I pulled the chassis on my prosonic. First off, the 15 watt resistor for the cathode bias measures 222 ohms. It looks like it's been replaced but obviously the tech didn't up the resistor value since 220 ohms is the value on the schematic.

Secondly, I found the center tap (Thanks Wally). You can see in the picture below, it's the red wire that comes out of the grommet with the blue and brown wire and terminates on the board right above the diode. This is from looking at the chassis from the back where the knobs are furthest from you.

 

[Wally]

Good info, Vince...thanks. This one I have on the bench has the OEM sandcast 220ohm. I have the factory tech sheets for the changes for these circuits. One is to take some of the hum out of things with shielded cable. I will probably do these changes to this one of the '97's and see what goes on. I may change out that bias resistor depending on what I see there. I like the heat that is in the 'Class A' option...lots of thick midrange.
I have two March of '97's and one 1996 Lake Oswego CS amp here...they are all alike and stock. As I understand it, the tech changes were made on the production line maybe in late March or early April '97. I am thinking that these two Mar '97 amps were built just prior to those tech changes.

 

[VinceM]

Mine is an "LO" as well and it does have a bit of the hum. I'm not sure if the previous tech did any of the suggested fixes but I suspect he did as I don't get the infamous pop when switching channels.

I'm hoping new tubes eliminate some of the hum so we'll see.

 

[Wally]

Vince, I have yet to experience this 'Pop' thing...and I do switch channels, but I am going to look at that aspect as well. The hum is there in these 3 amps.. It is similar to the hum of a single coil pickup in low gain situations...no stronger; so that might not bother some. I use humbuckers...even for 'single coil sounds'. So far, that light hum is all that bothers me about this amp....and it is very minor. In a playing istuation, it is of no concern. Everything else about the amp impresses me. One of FEnder's best considering build quality, versatility and....yes....I don't even mind the weight, Eugene!! LOL 55 lbs... I am thankful that my old 63 year old, bad back and all body can still deal with it. IN fact, the only FEnder amps of the post 1980 era that impress me as much are the Rivera'Jahns era SC, PRII and DRII. I was sold on a Super Champ the first time I heard one, and I am the same way about these PRoSonics. Bargain amp in themarket today, imho. The Rivera/Jahns amps are hand-wired, but the ProSonic is built like a
mid-'70's first generation Marshall PCB amp. The build quality is better than all but the handwired Reissues and Custom Shop amps....and that quality makes some of the other modern Fender tube amps pale in comparison, imho.
This is the o nly FEnder amp built in this manner that I know of. I like the biasing/rectification options...very neat and uncommon, right?
Yes...I like 'em.

 

[ThermionicScott]

Looking at the (link removed), I really like how they did the SS/fixed/cathode-bias switching. Having the CB option only with the tube rectifier helps keep things safer than if both options were independent!

I wonder if Zinky was using 7027(A) data when designing it since they're basically 6L6s with more advanced operational data -- the RCA book calls for a ~200-ohm cathode resistor when running them at 400/300, 380/380, or 425/425V, and with 6L6GCs that spec out in the ballpark of the old ones, that 220-ohm is perfect. A 250-ohm resistor is probably a good idea for more questionable tubes, though.

 

[Wally]

SCott, from a less technical point of view than yours, I like how and what Zinky did with the whole amp. It is something different with the Cathode biasing option and still maintains the ability to yield that 5F6A BAssman/Bluesbreaker type of thing along with the second generation of Marshall 5F6A 'clones'...the Md. 1987.
IT has struck me that those 3 options would/could allow one to tailor the amp to the venue to some extent...smaller venues?? go with the C-B, middle size...5F6A---#2 option with the tube rect/fixed bias. For the 'cleanest' sonics, the SS rec/fixed bias yields a tighter sound. There are a lot of possibilities with this rectification/biasing option switch......and we haven't gotten to the Gain channel. That Gain 2 control is a middle range control, right? Gain 1 control is a tube gain stage, but that mid-range Gain 2 knob is a nice touch, imho.
It is hard for an amp to 'do it all'...but these do a lot and do it all well, imho.....and are built to last.

 

[ThermionicScott]

Totally! I haven't tried one, but it seems like a really good design all around. Maybe I should keep my eyes peeled for one if I end up in a gigging situation.

 

[Wally]

TScott, back to that bias resistor, didin't the old Fender tweeds that used cathode-biasing with 6L6's use a 250ohm for that resistance? Leo probably did much of his component selection by the book, I would think.
That doesn't make 220ohm wrong...it just makes it hotter. IT will be interesting to see what the electrons say is going on with that 220 in the circuit. I amnot averse to changing a cathode-biasing resistor....but the sonics may be my guide and not some book. As I noted, I like the sonics with the 220ohm stock resistance there. As always, imho, if the tube is not redplating and it sounds good, I like what is happening. On the other hand, 30 more ohms of resistance won't make that much of a sonic difference, ime...but I may learn something new.