Expected output power from SF Pro Reverb?

[Bruce / Mission Amps]

The Bassman 100 use that same phase inverter too... so it is actually fairly common to see the 1w 33K in the tail.

[Brian Swatton]

Well, I've had more time to spend on it today. I've found nothing to contradict the drawing suggested by JoeM, it still seems to be correct. I seem to be getting a touch more clean power today, reaching 9Vrms into 4 ohms, i.e. 20w.

According to MWJB's info, the OT looks ok at 35:1. For 300Vpp measured on each anode, I get 17Vpp at the load. Levels were chosen to be a) undistorted, and b) visible on this scope/probe setup which can only go to 400Vpp.

I am still suspicious of my phase splitter DC conditions, with the cathodes sitting at 150V instead of 105V. I've measured all the resistor values as ok in the cathode circuit. I've also checked all coupling and decoupling capacitors for capacitance and leakage, again ok. As a further check I lifted the caps to see if DC conditions changed, which they didn't. A replacement valve doesn't alter things much either.

I'm still trying to ascertain whether the distortion is occuring in the output pair or the PI. More careful measurement does suggest it's occuring in the output pair fractionally before it's seen at the PI outputs, but it's so close it's hard to be postive. I'm also mindful of PDF64's earlier comment about the input impedance of a saturated valve. I have run the PI without the output pair in circuit too, it provided 2 equal amplitude signals but was very close to clipping itself at the kind of levels I was using to observe onset of clipping earlier.

Could someone confirm what kind of mains ripple I could expect at the OT centre tap please? With no signal I see 445Vdc with 12Vpp ripple. When producing about 20w out it becomes 389Vdc with 30Vpp ripple. I think this may be sagging and rippling a bit too much, though I know I can expect some of both on an HT supply. As I mentioned before, HT caps do measure ok for capacitance and ESR, but really want changing anyway because of their age. I'll need to order them though, and wasn't going to do that until I'd worked out what else was wrong.

[MWJB]

With the 33K PI tail I would expect to see 150vdc at the PI cathode, because your amp uses 47K PI plate resistors rather than 100K/82K, so this does not result in lower than normal plate voltages at the PI, like it would if you used 33K PI tail with 100K/82K plate resistors.

So your OT turns ratio seems OK and it's doing it's job, you should have 150-160VAC RMS at each 6L6 plate at your 9VAC RMS clean, so now what do you get at the 6L6 grids, should be around a 5th or a 6th of the voltage at the plates?

Keep working back through the amp, until you find anything that is not amplifying in the correct proportion.

Do yourself a favour, change the filter caps now, you're bailing water out of a sinking boat before checking if it leaks in the first place.

[Brian Swatton]

With the 33K PI tail I would expect to see 150vdc at the PI cathode, because your amp uses 47K PI plate resistors rather than 100K/82K, so this does not result in lower than normal plate voltages at the PI, like it would if you used 33K PI tail with 100K/82K plate resistors.

I was going by what appears to be written on the circuit diagram, it shows the same values as I have. It is a bit smudged and not entirely clear though.

So your OT turns ratio seems OK and it's doing it's job, you should have 150-160VAC RMS at each 6L6 plate at your 9VAC RMS clean, so now what do you get at the 6L6 grids, should be around a 5th or a 6th of the voltage at the plates?

Grid signals are 1/6 of anodes.

Do yourself a favour, change the filter caps now, you're bailing water out of a sinking boat before checking if it leaks in the first place.

On the contrary, my query about ripple/sag was exactly about checking for leaks.

I am ordering caps later today anyway, as some want changing simply because of age, though they do measure ok. Some have been replaced already.

[Brian Swatton]

Update FYI.

Firstly, I have found the meter I was using to measure the rms output was reading low by about 10%, I've confirmed this with 2 other meters that have had their calibration errors checked and logged previously.

Having made comparative readings again, the actual powers I was getting clean into 4 ohms were :

Intial tests where I reported max clean at 8Vrms (16w) was really 9.1Vrms (20.7w)

Later tests, before filter capacitor replacement, where I reported higher clean of 9v (20.25w) was actually 10.1v (25.5w)

So, there looks to have been a change of about 5w at some point.

Now, after filter capacitor replacement, I am achieving 10.8V (29.2w) which is not looking not so bad. One of the 20uF I changed I am a little suspicious of as it's ESR has measured a little differently at different times - 1 ohm and 2 ohm, not much, but the other reads 0.5 ohm every time. It suggests something is changing in it.

I've had the unit on soak for several hours yesterday, the output has been very stable, I've also been watching HT and biases, which are too.

In all these tests i've adjusted mains input to be 240v with a variac. I've seen the mains input to the variac vary between 233v - 245v, so out of interest I checked what effect that would have on the max clean output power :

235v 10.3v 26.5w
240v 10.8v 29.2w
245v 11.3v 31.9w

Thanks again all for the input.

[pdf64]

I'm still confused about how it was putting more power into an 8 ohm load than 4 ohm; is that still the case?

[Brian Swatton]

I didn't repeat the measurements with 8 ohm again, I'd only done that in the early stages as I wasn't completely sure about the required load impedance at the time.

The amp is back with the customer now, and I've just had a very happy email from him after it's been put through a rehearsal, the changing headroom issue has gone, and it's sounding much less harsh and toppy now when driven hard.