Super Reverb voltage questions

[nosaj]

The Super Reverb I have I was checking voltages and noticed some weird ones as related to the schematic.

On v1 pin1 I get 238 (which I feel is ok compared to the 250 on the schematic) but on v1b pin6 I get 364(which is quite a ways from 250 on the schematic)
The same with v2q pin1 240 versus 250 I think is ok but v2b pin6 352 vs 250
V4a pin1 204 schematic lists 250 pin6 350 vs 250 on the schematic


V5a Pin1 is 384 vs the 250 on the schematic
Pin2 I have -41 I feel like I should not have this voltage here but not really sure.
v5b pin6 343 vs 330 think that ones ok but pin 8 is .01 so I feel that has to do with the 5uf cap or the 100k resistor.


At the standby switch off I get 505 on I get 487.

My over all feeling is the filter caps should really be changed old mallory's, but the B side of the preamp tubes voltages Should I be thinking worn tubes or drifted resistors?

Thanks,
nosaj

Here's the link to pics and schematic of the amp

[pdf64]

I suggest to check actual 'in circuit' resistances at relevant plates and cathodes, eg if cathode resistor drifts up in value then plate voltage will rise.

[Justin Thomas]

Agreed - cathode voltages will also help tell part of the story of high plate voltages. Could be bad grounds, or as pdf said, drifted cathode resistors. If the preamp plate is drawing low or no current (not "on"), then the plate voltages will rise. Also recommend changing out ALL the electrolytic caps - don't forget the bias supply! Slightly higher uF & voltage ratings won't hurt anything, since much of what's in there now may not be standard available values anymore.

If you have some, try new tubes, of course - they're in sockets!
Justin

[Enzo]

When you have plate voltage 100v higher than it ought to be, don't think resistors or caps, think the tube is not conducting. If you have a 100k plate resistor with 350 on the plate end, what is on the B+ supply end? If it is about the same, no current is flowing. You look at the cathode pins - 3 and 8 - are they at zero instead of a couple volts? That also means zero current. Look at the heaters, is that side glowing?

Never forget these are only guitar amps, not precision circuits, so 238 instead of 250 is nothing. Many Fender schematics even said right on them all readings +/-20%.

[nosaj]

When you have plate voltage 100v higher than it ought to be, don't think resistors or caps, think the tube is not conducting. If you have a 100k plate resistor with 350 on the plate end, what is on the B+ supply end? If it is about the same, no current is flowing. You look at the cathode pins - 3 and 8 - are they at zero instead of a couple volts? That also means zero current. Look at the heaters, is that side glowing?

Never forget these are only guitar amps, not precision circuits, so 238 instead of 250 is nothing. Many Fender schematics even said right on them all readings +/-20%.

Tired Preamp tubes. I swapped in some known good ones on the questionable readings I had and they were more inline with the schematic. So time to replace them and replace the caps in the doghouse.

Another question this amp has bias balance correct? I believe it is.

Also on this amp on the power tubes pin8 to ground there is a 2ohm resistor instead of the usual 1ohm. Any idea on the reasoning on that?

Thanks,
nosaj

[Tom Phillips]

...Also on this amp on the power tubes pin8 to ground there is a 2ohm resistor instead of the usual 1ohm. Any idea on the reasoning on that?

Fender didn't install bias current sense resistors in that model at all so I'd guess that someone installed the 2 Ohm resistors by mistake or because that is just what they had available. The 2 Ohm value will get the job done just fine as long as that value is used in the math equation to calculate the current after measuring the voltage drop across the resistor..
Tom

[nosaj]

Fender didn't install bias current sense resistors in that model at all so I'd guess that someone installed the 2 Ohm resistors by mistake or because that is just what they had available. The 2 Ohm value will get the job done just fine as long as that value is used in the math equation to calculate the current after measuring the voltage drop across the resistor..
Tom

Looks like I'm getting about 60watts clean out of a 70watt amp. New filter caps and Tad preamp tubes. Trying to find the last 10watts is that a fools errand or just what to expect when calculating wattage of amps. Close enough is good enough?

Thanks,
nosaj

[Tom Phillips]

Looks like I'm getting about 60watts clean out of a 70watt amp. New filter caps and Tad preamp tubes. Trying to find the last 10watts is that a fools errand or just what to expect when calculating wattage of amps. Close enough is good enough?

60W clean is as expected and normal for that amp. You should find that the amp will put out 70W if you push it into a moderate level of distortion. That is how Fender marketing came up with the "rated" 70W value.

[Leo_Gnardo]

Looks like I'm getting about 60watts clean out of a 70watt amp. New filter caps and Tad preamp tubes. Trying to find the last 10watts is that a fools errand or just what to expect when calculating wattage of amps. Close enough is good enough?

At what distortion level are you measuring output wattage? Practically every instrument amp will yield disappointing results measured at clip or some other low distortion level, when compared to its maker's claims. Manufacturers seldom reveal the distortion factor when boasting about their amp's power level, it's typically anywhere from 5% to 20% and sometimes flat out square wave clipping. Not too many techs have a distortion % meter on the workbench, I sure don't. I could be wrong, but my "rule of thumb" is you can add 12% more power for every 5% of distortion. IOW the same amp could measure 100W or 150W, depending on whether it's just a smidge below clip, or at 20% distortion. If you're getting 60W at clip from a Super Reverb, that's fantastic & don't worry any more. Most I work on clip in the 40 to 50 watt range.

Another thing to consider is marketing. Lots of amps out there have some number in their name, this is intended to make the buyer/owner think they have an amp that can deliver that power level in watts. NOT SO! Sometimes the model's number increases as the years go by, presumably indicating that newer models deliver extra power. To some degree this was the case with Fender, but the tradeoff was newer higher power amps lost their fat mellow tones, instead sounding thin and piercing. So much for specsmanship. All too often "new and improved," isn't.

[nosaj]

60W clean is as expected and normal for that amp. You should find that the amp will put out 70W if you push it into a moderate level of distortion. That is how Fender marketing came up with the "rated" 70W value.

So to figure out what Watts you can expect I would think going to the datasheet would help. But I just looked in RC-20 for 5881 tube data.
It looks like it's saying 11.3 watts maximum signal output with 400v on the plates. So that in itself leaves me confused on how the amp derives 60watts(more voltage on the plates?). They're Sovteks 5881
http://www.thetubestore.com/lib/thet...ek-5881wxt.pdf
nosaj

[nosaj]

At what distortion level are you measuring output wattage? Practically every instrument amp will yield disappointing results measured at clip or some other low distortion level, when compared to its maker's claims. Manufacturers seldom reveal the distortion factor when boasting about their amp's power level, it's typically anywhere from 5% to 20% and sometimes flat out square wave clipping. Not too many techs have a distortion % meter on the workbench, I sure don't. I could be wrong, but my "rule of thumb" is you can add 12% more power for every 5% of distortion. IOW the same amp could measure 100W or 150W, depending on whether it's just a smidge below clip, or at 20% distortion. If you're getting 60W at clip from a Super Reverb, that's fantastic & don't worry any more. Most I work on clip in the 40 to 50 watt range.

Another thing to consider is marketing. Lots of amps out there have some number in their name, this is intended to make the buyer/owner think they have an amp that can deliver that power level in watts. NOT SO! Sometimes the model's number increases as the years go by, presumably indicating that newer models deliver extra power. To some degree this was the case with Fender, but the tradeoff was newer higher power amps lost their fat mellow tones, instead sounding thin and piercing. So much for specsmanship. All too often "new and improved," isn't.

On the scope I've got it to right before it starts the break the smooth curve.
I've got a Heatkit Distortion meter but have not put it to use yet, Recapped and calibrated but not used yet.

Thanks,
nosaj

[Justin Thomas]

It looks like it's saying 11.3 watts maximum signal output with 400v on the plates. So that in itself leaves me confused on how the amp derives 60watts(more voltage on the plates?)... They're Sovtek 5881s..

For the record, those Sovtek "5881s" are closer to a Super-6L6GC than anything, maybe a 7027 or what-have-you, even a 6550?. They may not put out the power, but they take lots of abuse. No use comparing them to an actual TS 5881 (NOT A "REISSUE!"). The original 5881 was a 19W tube, closer to the 6L6GB. Nobody really knows the specs on the Sovtek "5881," other than to say, they go in all the Ampegs w. 600+V on the plates and thrive.

Justin

[pdf64]

So to figure out what Watts you can expect I would think going to the datasheet would help. But I just looked in RC-20 for 5881 tube data.
It looks like it's saying 11.3 watts maximum signal output with 400v on the plates. So that in itself leaves me confused on how the amp derives 60watts(more voltage on the plates?)...

The conditions noted for 11.3 watts max signal power output is SE, 4k2 OT with 350V on the plate, 250V on the screen grid; those are rather different conditions to your SR!
Try working out you OT p-p primary and check the HT etc at full clean output, then see what nickb's calculator gives for those conditions Interactive Valve Data Sheets
Drag the sliders to the appropriate points.
I guess that the OT is ~5k, so as long as the HT doesn't drop below 460V, 70 watts seems feasible, but in the real world there are losses and 'sub-bogie' tubes to factor in.
60 watts seems very good for current production tubes.
If you have the 70 watt SR, I doubt you will get the rated output unless using the Sylvania STR387 power tubes that the amp was probably designed around.
http://bmamps.com/Schematics/fender/...ltralinear.pdf

[SoulFetish]

In the same amp, you won't hear a difference between 60W and 70W just before clipping anyway.

[nosaj]

In the same amp, you won't hear a difference between 60W and 70W just before clipping anyway.

Ok the amp shows fine on the scope. But here is another question. Will tired speakers not sound as loud as they once did? The only other cab I have to test with is a 15in 16ohm speaker. It just really should sound louder they are the original speakers in it and it looks road worn for sure.

How can you tell if a speaker is tired or worn out?

Thnaks,
nosaj