Trying to solve reason for low output (5F4 Super circuit)

[phydauex]

Just thought I'd throw this out there for any suggestions. I just finished my 3rd 5F4 build and while everything on the surface looks ok, it has a very low output. what I mean, is that the amp is subjectively about half as loud as the other two amps I built, and when I measure the AC across the speakers with a big chord with controls up full measure only 14 watts, (which confirms what I hear). Imeasure 8 ACVx8/ 4 ohms=16 watts . The others had about 12-14 ACV at the same settings and are much louder.
Ok, so I'm using the same OT's chokes and parts, layout ect. The only difference is that the others had a Hammond (270DX) and Classic Magnetics Tweed Bassman power transformers, which both had the same measured readings (voltages, bias, etc) and this new one I used a replacement Fender Tweed Bassman PT, which has the same specs, and measures close to the others (350AC/350 AC/ 412-415 dcv on plates and biased at 50 ma), but....

I swapped the power tubes, rectifier tube and all preamp/PI tubes and get the same result. I've retraced the layout with a meter to check continuity, measure resistances all good, a signal generator through it looks good on the ouput (no excessive distortion), both the output tranny and power tranny seem to check out (no shorts, out of whack readings), and the amp sounds real good, it just real quiet when the other two are much louder. Oh, and they all have the same speakers and when I swapped speaker hook-ups get the same result.
Any thoughts? things I should check/measure, etc. I suspect the power transformer not being up to snuff, but kind of scratching my head.

[Intergalactic Tourister]

the amp sounds real good, it just real quiet

I don't have a schematic... but first thing I would check is the input- double check the grid leak and/or grid stopper and double check they are the correct values (if it even has any of these).

measure resistances all good

Its easy to mix up resistor values too. I did this recently. It just sounds like maybe you could check to see that you didn't swap resistors by mistake and put them in the wrong spot.

I know you provided a lot of info and checked everything but... did you measure all the caps and resistors before you put them in your amp?

Sorry I couldn't help much but that would be my approach based on your info.

[phydauex]

Sis some more checking

Well, I did find that in my haste, I connected the bias leads to the wrong pin(4 instead of 5), so it was going directly to the grid and not through the 1.5k resistor. I swapped them and it didn't really make any difference. Although, I'm measuring about 10-11 ACv across the speakers for about 25-30 watts peak. but I must say, that it still doesn't have the clean headroom that the others had. I mean when you had the vol on 6, the clean sound was really loud, this one on 6 is clean, but very "polite", then gets louder and breaks up as you go up from there. I still think that the PT isn't able to supply enough current, or at least as much as the other PT's can. I know they all say 200ma, but there is a huge difference in volume and punch with the other amps, so maybe I'll swap the PT out.

I should also say, that I'm using the Hammond 194B (90ma) choke wired in after the plate supply, so this amp stays cleaner much louder than a stock 5F4 amp typically does.

[Enzo]

Instead of suspecting transformers (not that a transformer problem is impossible) try isolating the problem.

You have measured the output voltage and it is susbstantially lower. OK, How about the signal voltages through the rest of the amp? MAke a scratch copy of the schematic, or just take notes. Find a test signal. A generator is fine, but really even misuc will work. Whatever the signal is, don't change it. Now take a "good" amp and see what signal voltages there are at each stage. Now do that for the "bad" amp. At what stage does the difference start? Make sure both amps have the controls set the same. I suggest center the tone controls or max them, and turn the volume all the way up. Adjust the test signal level for reasonable loudness in the shop.

If you are concerned over a transformer, take B+ voltage readings to compare as well. Once you know WHERE the problem starts, we can determine WHY.

And as suggested above, do indeed check the resistor values. It is VERY easy to install a 470 OHM resistor in place of a 470k resistor.

[phydauex]

Thanks for the suggestions. Unfortunately I don't have the other amps nearby (another city), but after swapping the grid leads (pin5-6), that seemed to help a bit and then I put in a different rectifier (switched from a 5U4 to 5AR4) to raise the B+ up to 427v. That also helped, I'm now reading about 35 watts at break up point and about 45w at full on, which is what the other measured at. Everything else is as it should be. I wish I could put them side by side and go point to point, but I can't. The other transformers hit about 425v on the plates and this one was at 405v, and the player I'm building it for wants more clean volume, so raising the B+ makes sense.

It was really weird just how much quieter this amp was and I couldn't see anything obvious. Till I saw the grid /bias supply wire backwards. I didn't think that would make such a difference. I guess it's back to Tubes 101 for more learning.
matt

[Chuck H]

What rectifier tube are you using in the other builds?

[phydauex]

I was using a 5U4G. Which gave me about 415-425v depending on the wall voltage at the time. I didn't think about it too much, because the first two came out about the same. Namely loud and clean, with break up happening at about 7-12 on the volume dial. Plus I used all Hammond trannies on #1 (PT290DX/ OT270J); Classic Magnetics on #2 (18073PT/ the 35 watt OT), and both amps came out about the same as far as volume/voltages, etc. This one I used the cheaper Fender replacement PT for a tweed Bassman, which has roughly the same specs as the other two I used, but the amp was way quieter, which kind of caught me by surprise. With the 5U4 rectifier, my B+ was lower (405v) and even biasing up around 51ma was just about half the percieved volume at 6 on the dial than the other amps. I knew it right away when I got it wired up, but it wasn't till I traveled up north where the other two amps are that I could put them side by side. The difference was marked. The amp has the same basic tonal structure, it was just no where near as loud. The guy Im building it for wants and needs the level of clean volume that the others had. I would let him just take the first one, but I was also adding reverb to this one, which turned out sounding real good. I just need to get the volume to match the first two amps.

Too much info probably, but there you go. So at this point I have the reverb circuit disconnected and am trying to resolve the volume/power output before hooking it back up. I didn't mention the reverb part, because the amp was quieter before I ever hooked up the reverb circuit. Which is another reason Im going to change out the PT for the Classic PT, because it has more heater current available than the one Im using now, and a slightly higher voltage on the secondarys.
Matt

[big_teee]

I was using a 5U4G. Which gave me about 415-425v depending on the wall voltage at the time. I didn't think about it too much, because the first two came out about the same. Namely loud and clean, with break up happening at about 7-12 on the volume dial. Plus I used all Hammond trannies on #1 (PT290DX/ OT270J); Classic Magnetics on #2 (18073PT/ the 35 watt OT), and both amps came out about the same as far as volume/voltages, etc. This one I used the cheaper Fender replacement PT for a tweed Bassman, which has roughly the same specs as the other two I used, but the amp was way quieter, which kind of caught me by surprise. With the 5U4 rectifier, my B+ was lower (405v) and even biasing up around 51ma was just about half the percieved volume at 6 on the dial than the other amps. I knew it right away when I got it wired up, but it wasn't till I traveled up north where the other two amps are that I could put them side by side. The difference was marked. The amp has the same basic tonal structure, it was just no where near as loud. The guy Im building it for wants and needs the level of clean volume that the others had. I would let him just take the first one, but I was also adding reverb to this one, which turned out sounding real good. I just need to get the volume to match the first two amps.

Too much info probably, but there you go. So at this point I have the reverb circuit disconnected and am trying to resolve the volume/power output before hooking it back up. I didn't mention the reverb part, because the amp was quieter before I ever hooked up the reverb circuit. Which is another reason Im going to change out the PT for the Classic PT, because it has more heater current available than the one Im using now, and a slightly higher voltage on the secondarys.
Matt

Hi: good Thread.
I did a marshall build with the Classic Tone Transformers, good stuff.
You mentioned the 18073PT and a 35 Watt OT.
Do you know the OT Part number.
I'm wanting to do another build with the 18073 PT and maybe a 18006 40 Watt OT with the 4k Primary for 6l6s.
I guess this OT will handle the 6L6s ok, it was designed for the bandmaster?
I'm going to use the Marshall PreAmp Layout.
Later,
Terry

[Chuck H]

I would attribute the lower power to the lower B+. If the rectifier tube solved the problem then your good now. Lower B+ will lower power even with a hotter bias. It also reduces the dynamic volume, especially in the highs and lows. So not only do you have less watts but you also have less percieved volume due to the lower dynamics.

Don't put too much credence in the volume control setting as it relates to headroom. If the amp is producing 35 clean watts and your others are producing 35 clean watts they have the same headroom. Where this occures on the volume control has nothing to do with it. I read here ALL THE FREKIN' TIME where someone thinks their amp should be clean to a certain setting on the volume control and though the amp has the right amount of clean output they believe it has lower headroom than another amp that puts out the same amount of power at a higher volume control setting. I've even read advice here that says an audio taper pot gives more headroom than a linear taper pot. That's absolutely wrong. All that changes is the volume control settings. With an A10 log pot the amp is doing the exact same thing with the volume set to 5 as it would be with a linear pot and the volume set to 1. And both pots are seen the same by the circuit when set to 10. Headroom is a functional parameter. The volume knob setting is just a number. Save yourself a lot of time and don't get caught up in idea that a volume setting has anything to do with an amps headroom. Same goes for subing preamp tube types. A lower gain tube will make the amp cleaner to a higher volume knob setting but overall gain will be reduced and the actual number of clean watts the amp will make has not changed at all.

If you want to get a higher volume knob setting at max clean try a different taper pot or you must reduce gain. Niether is an increase in headroom but rather a perception by the user. I'm not poo-ing the desire to have an amp clean to a certain volume knob setting. It's user interface and that's part of what makes a good amp. But if you can wrap your head around the real issue you'll have a better chance of solving your problem.

[phydauex]

Yeah Chuck, I wasn't really hung up on control position other than it gave me a reference between the two amps. It's not an issue now that I think I have the output where it should be. Before I was only measuring about 14 watts on the output and at the same control positions there was a vast difference in volume. I probably shouldn't have put so much emphasis on it, again it gave me a point of reference in description.
I didn't think that 20 volts would make that much difference, plus the fact that I wasn't going thru the grid stoppers. I'm still not sure how that was affecting the circuit.

I always am refining my understanding of amps from all the comments and information shared here.
Thanks All

[Chuck H]

One interesting possibility about NOT going through the grid stoppers is that there could be an ultrasonic oscillation causing low volume. I would think that a measure of output voltage would have caught that though.

Plate voltage and bias are the whole game with respect to PA performance. Often (in fact usually with guitar amps) a hotter bias will DECREASE headroom. Since the ideal bias point has the amp right on the edge of crossover distortion, and we try to avoid this with guitar amps, biasing into the safe zone to avoid crossover distortion gets it closer to saturation on the positive swings as much as overlapping the crossover point, thereby reducing headroom. Biasing hotter to gain current in trade for voltage swing seems to make sense. In fact it's an unavoidable truth in my perception. But it just doesn't work in reality. Look at the tube charts and it clears up.

[phydauex]

That makes sense....

Actually, the first thing that happened when I fired it up for the first time, was a high pitched squeal. Kind of took me by surprise. Because I had wired up the OT the same way as my previous builds and didn't have a problem before with orientation of OT leads. So, I switched them and the squeal went away. But the high frequency oscillation your describing sounded like what I heard. Just for kicks, after I moved the grid leads, I swapped the OT leads back to where I thought they should be. The noise I heard when I took it off standby was a low freq motorboating, not the high pitched squeal.

Your also right about the loss of clean headroom with a hotter bias. I know that, but was just stumped when the volume was so much lower than the previous builds, that I was flailing. That of course is the way to get more saturation when the amp starts to distort, not the other way around. Without having the other amps right here, makes comparison damn near impossible. Other than my mental file that is. I think the first two were mutants, but w/o them here to measure has me blindfolded as to figuring out why this one doesn't have the same level of clean volume (it's better now, but still shy).

One thing I have noticed, is that the cathode of the second tube stage measures low. All my voltages are a bit higher: 183v on the plate(140v schematic), and only 1.3v on the cathode (2.2v on schematic). This is over the 1500 ohm resister, which means it's only drawing .0008 A (1.3v/1500 ohm = .0008A), which seems low. The current of the original schematic shows that stage biased at .0014 A, which is about 50% higher. I can't account for that.
Matt

[MWJB]

"One thing I have noticed, is that the cathode of the second tube stage measures low. All my voltages are a bit higher: 183v on the plate(140v schematic), and only 1.3v on the cathode (2.2v on schematic). This is over the 1500 ohm resister, which means it's only drawing .0008 A (1.3v/1500 ohm = .0008A), which seems low. The current of the original schematic shows that stage biased at .0014 A, which is about 50% higher. I can't account for that.
Matt"

Hi Matt, if you have a 12AX7 in V2 then you are probably fine, voltage & plate dissipation look to be ball park for that kind of tube. Looking at the 5E# schems the current draw & voltage of the 12AX7 triode A in V2 appears to be more in keeping with a 12AY7, rather than a 12AX7. I suspect that they prototyped the circuit with the 12AY in mind (as with the 5E6-A), took the voltages, replaced it with a 12AX7 and that's what is written on the schems...but figures are for a 12AY....if they even took the voltages at all - they may just have been pasted from another schem. There are lots of voltages in Fender schems that don't pan out, take them with a pinch of salt.