Are 70w UL Fenders REALLY 70w?

[Dreams]

Got a Super Reverb on the bench.

http://el34world.com/charts/Schematics/files/fender/Fender_cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

Amp is in good shape, stock except for bias-balance to adjustable-bias conversion. Some resistors and caps changed, but stock values.

It's only putting out ~40 watts, which is something I would expect from a pair of 6l6gc.

These amps claim 70 watts @ 2ohms. So what gives? I wouldn't expect 70w out of a pair of 6l6 to last very long, and these amps seem to be biased fairly cold as it is...was Fender just lying, or is there something I'm missing?

Is this amp actually doing just what it's supposed to? Are 70w UL Fenders REALLY 70w?

[HotBluePlates]

Amp is in good shape, stock except for bias-balance to adjustable-bias conversion. Some resistors and caps changed, but stock values.

It's only putting out ~40 watts, which is something I would expect from a pair of 6l6gc. ...

You have the amp biased to about -58vdc on the 6L6 grids, as shown on the schematic?

If you warm up the bias, you're moving closer to the class A end of class AB (rather than idling cooler & moving towards the class B end). When you do this, you're trading away greater output swing for an output stage that is easier to drive to its max clean output power.

So to test this amp to see if it's capable of 70w output, you should idle at/near -58vdc as shown, then apply enough input test signal to net 56-57v peak input to one 6L6.

[shooter]

Like HBP said
my data sheet under operational characteristics show Plate 450, Z 6.5Kish, G2 350, G1 bias at -30 you just touch the 50W mark @ 12.5%THD

[HotBluePlates]

my data sheet under operational characteristics show Plate 450, Z 6.5Kish, G2 350, G1 bias at -30 you just touch the 50W mark @ 12.5%THD

Be careful... screen voltage (and ultralinear operation) matters in guessing output figures.

When screen voltage goes up, peak plate current capability increases. The OT in the 70w UL amp is fed from 500vdc, so plate & screen will both be high and near this figure.

And when you increase screen voltage, grid bias will have to get bigger to tame idle current. Hence the -58v on the schematic.

And we have a 4.4kΩ plate-to-plate load rather than 6.5kΩ...

[PRR]

> putting out ~40 watts

So it is sick. Find the problem and heal it.

> expect from a pair of 6l6gc

6L6GC will do 40 Watts/pair loafing at sane voltages. The "70W" runs the voltage way up; also the load Z way down. I won't argue 65W or 75W, but sure-golly more than 40W.

Check that V6 _is_ a 12AT7.

Check all voltages around driver and power stage and find what's way off.

Use fresh good 6L6GC-- this is not an amp for tired tubes.

[shooter]

Be careful...

AHH, I seen UL in the thread, but didn't for the data sheet specs
thanks

[Dreams]

Amp has new 6l6, PI is in fact 12at7

I had the bias running at ~ -52v when I posted before. Set it to -58v and applied 400hz ac to about 56v p-p at the grids. The output showed heavy clipping. Had to reduce signal on the grid to 40v p-p get clean output.

(I'm not completely confident in my scope's cal, though... I trust my volt meter, but then that's RMS...)

Getting very close to the same figures at -58v bias as I did with -52v. About 45w clean output.

My ac meter reads 9.7ac across 2.1 ohms giving me about 45w, yes?

If I plug in the number from my scope, 12v p-p, I can make the number look right, but we don't measure like that, do we?

Sorry if I'm being unclear, I'm not completely sure I'm grasping this UL thing, and i think I'm talking myself in circles, or at least missing the point. It looks to me that UL connection gives LESS power than the equivalent pentode connection...Everything looks more or less correct to me.

Am I chasing that 70w figure for nothing?

[HotBluePlates]

Assume we have a 4.4kΩ OT and want 70w RMS output. What will it take? Let's start by figuring out what voltage we need.

70w RMS = 140w peak, and we need peak voltage & current to know what the supply voltage and tube plate current will be. And since we know the stage will be class AB (required power output > 1/2 the total dissipation of the output stage), one side of the output stage will be cut off leaving 4.4kΩ / 4 = 1.1kΩ as the impedance presented to the side which is on.

Power = Volts²/Impedance, so rearranging:
Volts = √(Power * Impedance)
Peak Volts = √(140w Peak * 1.1kΩ) = 392v Peak.

We need to leave a good chunk of voltage from plate-to-cathode over the peak voltage for the tube to operate properly. According to the schematic, 500vdc is applied to the OT center-tap, so 500v - 392v peak = 108v plate-to-cathode at peak output. Check!

How much current will that be? 392v peak / 1.1kΩ = 356mA peak. Can the 6L6GC manage that?

Look at page 6 of the 6L6GC data sheet. This shows plate current when the G1 is driven to 0v for various screen voltages. 400v is the largest screen voltage given, and indicates a plate current of ~325mA near the knee of the curve. So that's pretty close, and additional screen voltage might put it over the top.

But that screen voltage varies with plate current, right (it is UL operation, after all)? TAD sells an OT for this amp which claims 20% taps for the screen. That's 20% of the total impedance from CT to end, or a tap at 220Ω.

220Ω * 356mA = ~78v, and
500v - 78v = 422v screen at peak output.

So it looks like that checks as well. Further, screen power (in the form of screen current times that 220Ω) will add to the total output power of the amp due to UL operation. This was not considered earlier, and actually reduces the peak current requirement from the 6L6's plate. As a reminder, RMS power (sine wave)= Peak power / 2, and (392v * 356mA)/2 = 69.8 watts

How much grid drive will be required? Well, we have to guess at the Gm for the output tubes. The data sheet cites ~5,000 micromhos, which is also stated as 5mA/V; we might guess as high as 6mA/V.

356mA / (6mA/V) = ~59v peak grid drive.

This assumes again that all of the required peak current to deliver the output power comes from the plate. But Look!! Fender had a bias indicated of -58v, and typically peak input equals the bias voltage or a bit less. Now isn't that interesting...

We don't know how much distortion is present at the claimed 70w RMS, and I haven't accounted for screen power in the above. However, it shows that 70w RMS is possible from 2x 6L6GC if the output stage is biased near what the schematic shows, and the tubes & driver are healthy. Hmmm... perhaps Fender's use of bias balance isn't only about neutralizing hum (I'm beginning to wonder if they didn't also want to make sure the output stage was set up as intended).

Getting very close to the same figures at -58v bias as I did with -52v. About 45w clean output.

My ac meter reads 9.7ac across 2.1 ohms giving me about 45w, yes? ...

Your calcs are correct. It would take 12.12v RMS across 2.1Ω to get 70w RMS output.

Amp has new 6l6, PI is in fact 12at7

I had the bias running at ~ -52v when I posted before. Set it to -58v and applied 400hz ac to about 56v p-p at the grids. The output showed heavy clipping. Had to reduce signal on the grid to 40v p-p get clean output.
...
Am I chasing that 70w figure for nothing?

I don't know if you're chasing 70w for nothing. However, something's not right with the phase inverter or output tube grid circuit. There shouldn't be anything clipping at a 40v peak input to an output tube biased at -58v. That makes me wonder if the phase inverter is clipping before the output tubes do, or if the output tubes (despite being new) can't manage the same peak current as true 6L6GC's.

The B+ feed to the OT center-tap is still the marked 500v (or close), right?

[Dreams]

Thanks HBP. Here are some voltages I took down.

All DC voltages at idle:

PI:
283v @ plates
113v @ cathodes
88v @ grids

Power tubes:
485v @ plates
485v @ screens
-58v @ grids
cathodes grounded via 1 ohm resistor  17-18mv

At onset of clipping I get:


PI:
275v @ plates
110v @ cathodes
87v @ grids

Power tubes:
470v @ plates
465v @ screens
-56.5v @ grids

cathode resistors read 65mv and 80mv

[HotBluePlates]

Note that what I showed above indicates that getting 70w RMS from a pair of 6L6GC's, while possible, is pretty much wringing everything out of them that can be gotten.

If you don't need (or want) the 70w output, then there's maybe no point in chasing it.

If B+ is lower than 500v, the amp probably won't make 70w RMS.

If the output tubes have a Gm less than 5-6,000 micromhos, the amp probably won't get to 70w RMS. However, idle Gm may not tell for certain what happens at signal peaks.

If the 6L6GC's cathodes can't manage the full peak current indicated on the old data sheets, the amp likewise won't get to 70w RMS.

And if the phase inverter balks when trying to output ~58v peak (~41v RMS), the amp likewise won't get to 70w RMS output.

Personally, I'd rather have a blackface Super Reverb doing ~35-40w. Even that is too loud for my needs right now.

[HotBluePlates]

Thanks HBP. Here are some voltages I took down.

All DC voltages at idle:

PI:
283v @ plates
113v @ cathodes
88v @ grids

Power tubes:
485v @ plates
485v @ screens
-58v @ grids
cathodes grounded via 1 ohm resistor  17-18mv

At onset of clipping I get:
...
cathode resistors read 65mv and 80mv

Everything looks like it should. What a.c. millivolts do you get across those cathode resistors at clipping? That might tell the tale for the output tubes.

The phase inverter resistors values are stock? It should only take 1-1.4v RMS of input to the phase inverter to get ~58v peak (~41v RMS) output to drive the output tube grids.

[Dreams]

Thanks for walking through that.

This assumes again that all of the required peak current to deliver the output power comes from the plate. But Look!! Fender had a bias indicated of -58v, and typically peak input equals the bias voltage or a bit less. Now isn't that interesting...

;)

We don't know how much distortion is present at the claimed 70w RMS, and I haven't accounted for screen power in the above. However, it shows that 70w RMS is possible from 2x 6L6GC if the output stage is biased near what the schematic shows, and the tubes & driver are healthy. Hmmm... perhaps Fender's use of bias balance isn't only about neutralizing hum (I'm beginning to wonder if they didn't also want to make sure the output stage was set up as intended).

This is kinda where I'm at. I didn't do the bias circuit mod, but was suspecting something might be up with the change. I tried to figure out what Fender was doing with the stock circuit but then my eyes started hurting from trying to follow that schematic around...I'm tempted to put it back to see what happens, but I'm also tempted to just leave it alone....

I don't know if you're chasing 70w for nothing. However, something's not right with the phase inverter or output tube grid circuit. There shouldn't be anything clipping at a 40v peak input to an output tube biased at -58v. That makes me wonder if the phase inverter is clipping before the output tubes do, or if the output tubes (despite being new) can't manage the same peak current as true 6L6GC's.

The B+ feed to the OT center-tap is still the marked 500v (or close), right?

The PI is not clipping, I can get ~60v clean-ish on the 6l6 grid, but it does get a little "nipplly" above 55v or so. I have to reduce it to 40v to get clean output across the load. The B+ to OT CT is in fact the 500v node.

Everything looks like it should. What a.c. millivolts do you get across those cathode resistors at clipping? That might tell the tale for the output tubes.

Left Tube:  65mv DC, 80mv AC
Right Tube: 80mv DC, 102mv AC

[Ritchie200]

Not to argue with the great Fender gods, but that AT7 looks a little anemic in that UL role?  Makes me wonder what an AU7 would do in its place, since it is able to swing a lot more current?

Jim

[Ritchie200]

Ughhh, sorry, not a driver.  My bad.

Jim

[HotBluePlates]

Not to argue with the great Fender gods, but that AT7 looks a little anemic in that UL role?  Makes me wonder what an AU7 would do in its place, since it is able to swing a lot more current?

That amp doesn't need current in that position, but it does need voltage gain. It just so happens that with a 12AT7 and a gain a bit under 30 per side, 1v RMS into the phase inverter gives exactly the amount of drive needed to power the output section to full power.

[Ritchie200]

Yeah, I was thinking about another UL amp...

Jim

[HotBluePlates]

And a 12AU7 does a great job in that other UL amp!

As long at the OT doesn't short...    (and now I'll go play my Tele...)

[2deaf]

I read through this a couple of times and I'm still not clear on the facts.  Isn't the UL Super Reverb 70W into 4 ohms?  Is this 2.1 ohm load a dummy load and if so, what kind?  The PI can put out 60 volts peak (60 up and 60 down) or 56 volts p-p (28 up and 28 down)?  The only reason to think the wattage is low is because of a meter reading (irrelevant to me, but significant to Jerry and his followers)?  There is some kind of problem calibrating the 'scope?

I would like to know what the peak positive and peak negative voltage across the load is on the 'scope when the signal starts to clip.  I would also like to know what it is with a 4 ohm load. 

[sluckey]

Isn't the UL Super Reverb 70W into 4 ohms?

Ah, I think everyone (including me) has overlooked this fact.

[PRR]

Wild muggy thoughts:

Yes, this seems to be a FOUR OHM output. If you are loading in 2 Ohms, there's the discrepancy.

Master Volume wide open? If not, the preamp may clip before the power stage (like it's supposed to do).

Signal frequency suitable to meter? I have seen some DMMs fall-off by 400Hz. (The early and cheap "True RMS" are prone to this.) I like peak-peak detection (old VTVMs) and average (needle meters). They don't lie, and usually good past 10KHz. You have to know how they read: the peak meter will stop rising when clipping starts, average will go up and up into gross clipping.

Fender probably speced at 10% THD. In a clean amplifier, this is significant clipping. I would expect 44W clean to be near 60W at 10% THD.

Reason it wants 12AT7: 47K+68K 6L6 G1 resistors. This would be a very heavy load on skinny 12AX7. 12AU7 would not be enough gain for this stage layout (killer with an added stage inside NFB). 12AT7 is the correct tube. And the low G1 resistors says that Fender had some concern about working 6L6GC this hard.

Pentode, UL, and triode modes may be different powers. But how different depends on a lot of things. Plate+G2 voltage is big, so is nominal load impedance. Also G2 current (6L6 has less of that than EL34). And whether you go for clipping or a reasonably low THD number at rated Watts. You can tune some amps so UL makes more pre-NFB Watts at 2% THD than the similar pentode mode. In this case I have suspected that Fender did UL (partly for fashion but also) for some measure of protection. As the plate pulls low, screen also goes low, tube current can not get huge.

[PRR]

Support for 6L6GC making 70 Watts, without even going as far as Fender did:

[Dreams]

Had to shelve this amp, then went on vacation; sorry for leaving it hanging, I appreciate all the input.

Isn't the UL Super Reverb 70W into 4 ohms?

That's what's shown on the schematic. The backplate on the amp states 2 ohm min, and there are four original 8 ohm speakers paralleled for 2 ohm total. So that's where I went with it.

Is this 2.1 ohm load a dummy load and if so, what kind?

Dummy load, yes. Resistors.

The PI can put out 60 volts peak (60 up and 60 down) or 56 volts p-p (28 up and 28 down)?

56 volts peak-to-peak (28 up and 28 down)

The only reason to think the wattage is low is because of a meter reading

Yes

There is some kind of problem calibrating the 'scope?

Not exactly. The scope is a Tek 214, sporting a whopping 2 1/2" screen. It was broken when it came to me, but I fixed it and haven't bothered to do any cal, since the readings I get are in the ballpark of what I should be getting. My concern regarding this scope isn't so much bad calibration, but resolution: It's too small to read the damned thing.

I would like to know what the peak positive and peak negative voltage across the load is on the 'scope when the signal starts to clip.  I would also like to know what it is with a 4 ohm load.

On the Scope

Across a 2 ohm load:
Negative clips at about -7.5v
Positive clips at about 7.5v
Volt meter gives me ~10v RMS across load =~50w


Across a 4 ohm load:
Negative clips at about -9v
Positive clips at about 9v
Volt meter gives me ~13v RMS across load =~43w

Once again, I take these readings with a grain of salt, due to a possible drifted calibration and the poor resolution of the scope screen.

Any thoughts?

Master Volume wide open? If not, the preamp may clip before the power stage (like it's supposed to do).

Signal frequency suitable to meter? I have seen some DMMs fall-off by 400Hz. (The early and cheap "True RMS" are prone to this.) I like peak-peak detection (old VTVMs) and average (needle meters). They don't lie, and usually good past 10KHz. You have to know how they read: the peak meter will stop rising when clipping starts, average will go up and up into gross clipping.

Master wide open. Meter goes to 10k, or so it claims, but I believe it.

[Dreams]

I should also add that the signal at the speaker output clips before the 12aT7 in the PI role does.

Everything looks (to me) just fine...until the OT secondary.

I believe I mentioned this before, but the only mod in this amp is the bias has been changed from the stock "balance" bias to the older style adjustable. The 47K and 68K bias feed resistors were changed to 220k, some wires moved around on the pot...very close to the AB763 bias circuit.

[super&plexi]

Hey I've read this whole Post and have been very interested... I've got some what the same in a pro Reverb 1976 to 78 l master volume pull switch sounds like exactly same thing, but with two twelves. It's a pretty long post I think I remember you saying you went into a dummy load, but I noticed you said 4 8 ohm stock speakers and on the schematic there's a couple models and I see there's 4/16 Ω also. And I know there's that weird second speaker output Jack I thought maybe somebody could have fudged with, I am not sure if you mentioned the caps in the power supply, and I assume of course that you check the numbers on the output Transformer and that the numbers match... did you mention what voltage was running into the amp, and more to the point I guess I was wondering how it sounds?. Mine I was always told that these were crap amps, and I haven't felt that way at all. My main thing is blackface super Reverb,, Stout Reverb( in 212,  green back about the size of an ac30, 72' Super Bass... basically fast-response amps, maybe except the Stout to some degree, but lately have been getting more into 5 and 16 watt amps and really kicking around the idea of ended,and the question I'm babbling on to is brought on by the fact that I got this Pro because I played my buddies black-faced Pro Reverb, actually he's got two or three of them, at some gigs and it's been some of the best tone I've ever had in a 200 and under size Club. I grabbed this  one up cheap and it had some water damage, and figured I've got input output tranny's and even an old NOS Fender board loaded with old orange drops,choke,  Reverb traany, the whole bit, and figured to build my own, but I'm not that turned off by the sound I get out of it. I've definitely not had this much compression out of any fenders but I also usually run them 0 + FB. Mine came with sovtek wxc orgy whatever... and yeah it runs them up in the 5's or very high 4's, but never seems to really be smoking. Maybe except for some of those large ceramic around the power supply but I've had much hotter in my super 60 that actually charred  the board, and that amplifier ran great. I've got this one apar, as I am working on the cabinet... I don't think it was completely submerged( I bought it a few years back when it quite possibly could have been from the floods), and just fooled with it at home a little bit because it's so heavy. I've come to believe more that maybe it was in someone's garage and maybe caught the sprinklers every now and then or something like that. That particle board or MDF whatever speaker baffle had sponged out at the bottom, but the Reverb pan wasn't all full of mud... the speakers seemed to be really weak and water damage at the bottoms, but I just ran some silicone around them and they sound ffine,  and really worn in and good actually. I'm not sure if these things lead to that compressed sound( the speakers mostly), but again I guess what I'm wondering because almost everything I see about the UL's. is always negative, is how it sounds and can you post anyclips, or is this just a repair thing and possibly you might not even play... good either way. Mine,  if I didn't get to it earlier is biased somewhere in the 56's, 58's, I think I remember one tube was a little stronger than the other. I do remember when I pulled the dog house pan I almost freaked when I saw the 220 μF's, until I saw they were in series, and then just ran 20 20 20 at 500V, and 100 100 I think at 100 each I haven't looked at the schematic lately and I'm guessing that was probably for the bias, as I seem to remember they were in series also and it seems like a good recap job although all the electrolytics inside the amp are original. Anyway I can see I'm completely running on and will close with saying that while my  numbers seem to add up, and it seems to be running as designed it really doesn't seem like I would expect something to be between A50 watter and 100 watter. It actually does sound something like 45 Watts. I'm playing around with the negative feedback on my super Reverb so I will do the same on this( well I'm really adding to the super Reverb, and planning on reducing NFB on this... but will try best to record some clips if anyone's interested.