Power transformer clipping?!

[MWJB]

"I haven't re-measured the voltage at the speaker output yet. Can you confirm what the formula to convert speaker p-p voltage to Watts RMS is?" Easiest thing to do is to use a RMS meter. I'd halve the pk-pk volts then multiply by 0.7...comes out about the same as your method.

You could also consider a smaller dropping resistor in the B+ rail to raise preamp voltage generally, this will have happened to some degree already at the 2nd triode, thru rebiasing V1B, Another 30 or 40 volts on V1A might not hurt? Try another 2.2K cathode there too? Really, for more clean headroom you want to be looking at increasing the power tube plate voltage, possibly employ fixed bias, 6550 power tube?

If your amp is quiet wired with triode A as the input triode then there is no need to rewire, the majority of amps are wired this way, including all 5F1/5F2A. I'd eliminate hum from other areas, such as the heater circuit first.

[Steve Conner]

Steve, please inform me about the problems with non-air gapped core OT's for SE use. I've read that due to the constant DC current through them the core can saturate, rolling off the low frequency response. I've also read that you can help to improve their low freq. response by using one with bigger iron core

That's exactly right. Using an oversized one will help a bit, but it still won't be as good as a proper SE OT with a gapped core. At the end of the day though, if you like the way it sounds, does it matter?

[jimboyogi]

Hi All,

I've been trying various things, and learning much.

MJWB, I took your advice about increasing the cathode resistor value, and changed from 220 ohm to 270 ohm. On the CRO with sine input, this increased the asymmetry of distortion, with grid limiting beginning way before cut-off. So I tried increasing the value of screen stopper to compress the knee. It worked a bit, I was able to get a higher Wattage output before grid limiting began. But my plate dissipation was only 20.1 to 21.7 W depending on which screen stopper. So I put the 220 ohm back in, and found that a 1k screen stopper gave the most symmetry of distortion, as well as a plate dissipation of 24.2W.
Ip = 79.1mA, Vp = 325V, Vs = 309V, Vk = 19.4V.
With this set-up, and a clean 1KHz sine input, I am now getting max. 6.4 Watts RMS at the speaker before grid limiting/clipping, and 7.7 W at full output with a 0.1V sine input.
Not only that, it is sounding great! Lovely cleans, with a really smooth push into drive and a great fade back out of it.

I do not know how I can get any more clean headroom than this from this circuit, I was expecting up to 10 W clean. Plate dissipation is nearly at max. (max = 25W, I have 24.2W), onset of distortion is very close to symmetrical. Does anyone have any further ideas to tweak for more clean headroom with the current hardware (transformers and valves)?

Hum/noise levels are very low with no input and both volumes full, so I haven't bothered re-wiring V1 to swap V1a to pins 6-7-8.

Another thing that I noticed is a significant loss of treble when the guitar vol. pot is turned down a bit (1 meg pot, Fender Jazzmaster). In my layout i am running a (two conductor + shield) shielded cable from the control panel across the chassis to the socket of V1. This cable carries the input signal from the input grid stoppers to the socket, and from the cathode back to the panel where I am using a LED for bypassing and as a 'on' indicator. The shield is earthed to the input/preamp star at the panel end only. I was wondering if the extra capacitance between the grid and cathode signal wires could be causing any extra high freq. attenuation?
I measured the capacitance between the conductors as 10 pF, and between either conductor and shield as 20 pF.

Also, because this amp has two volume pots (between V1a and V1b, and between V1b and output valve), is there extra treble attenuation when the pots are not on full, and should I put a bright cap across one of the pots to make up for this?

Thanks again,

James

[MWJB]

"I do not know how I can get any more clean headroom than this from this circuit, I was expecting up to 10 W clean. Plate dissipation is nearly at max. (max = 25W, I have 24.2W), onset of distortion is very close to symmetrical. Does anyone have any further ideas to tweak for more clean headroom with the current hardware (transformers and valves)?"

10W is rather optimistic for such a low B+ amp, especially with cathode bias. Converting to fixed bias will allow you to exploit the full B+ available & will increase headroom a little. Start at around 50mA plate curent & increase if necesary. Also try a lower mu preamp valve like GE5751 or maybe even a 12AT7?

The pot between V1B and the power tube should be wired a rheostat (variable resistor, rather than a voltage divider) so that you are not introducing series resistance at settings other than fully up. For more headroom run this pot around <1/2 value.

[J M Fahey]

1) I think you have a big problem with your output transformer.
*Maybe* you can pull the lamination (you can heat the iron with a blowtorch until the varnish softens somewhat, yet not so much as to melt the plastic core. It can be done, al least I have done it, by sharpening a screwdriver tip into a "needle" and pulling as many Is and possible and then using one or two of them to push the Es. After the first 3 or 4 , it becomes easy ) and reassemble it with all Es pointing to the same side, and the pack of all Is closing the magnetic circuit.
You can insert a piece of printer paper to keep the gap open, and place just the first and the last E pointing to the other side, to make it mechanically stronger.
2) Asymmetrically grid clipping a 50V PP signal is to be expected, given that your output tube has a bias voltage (self-bias or fixed is the same here) of around 20V DC.
Whenever the positive side of your grid signal reaches thos 20V plus any forward voltage needed, the grid to cathode diode will turn on, as any diode, and clip the top of that semicycle.
When doing so, it will also place an extra negative voltage by charging the coupling capacitor to the peak rectified voltage.
3) I think that rather than a preamp problem, you have a power stage problem which you must solve first.
Good hunting.

[jimboyogi]

Thanks for the responses.

MWJB, thanks for the suggestions. I'm happy with the way the pre-amp gain is behaving atm, I was just wondering about headroom in the power stage. Fixed bias may be an option, am I correct in thinking that if my Vk (bias) on the 6CA7 is 19.4V, then converting to fixed bias could gain me this much extra B+ across the valve, and an extra ~ 38V voltage swing at the plate?

Also, with the second (master) volume pot, can you please clarify the best set-up? ATM this is a 500K pot that across it is the grid reference resistor for the 6CA7. Should I change this to a rheostat where the grid reference to ground is adjustable from 500K down to 0? Does this control the volume by changing the output load impedance for V1b, and Does this setup change the tone less?

J M Fahey, I think that you (and Steve earlier) are correct about my OT being a problem. I checked the OT low frequency response briefly last night and was really surprised by how much distortion and roll-off there was below 1KHz. At 100 Hz the OT was putting through effectively no un-distorted signal, the distortion began at such low voltages.
I am very tempted to try your advice re. modifying the core to introduce an air gap, but I'm wondering if it is then likely to have enough primary inductance to still be effective. Off the shelf SE 15W transformers have from 2 to 3 times the weight of my transformer, so I'm wondering if the iron/copper in mine is just not enough to do the job properly. I can borrow an inductance meter, and I'll probably try the mod. anyway, and measure inductance before and after. Is there any specific way I should measure L beforehand for reference, that I'll kick myself for not having done when it is too late?
Also, by changing the core to air gapped, and losing some L, will it change the Z ratios?

Thanks again,

James

[J M Fahey]

Hi James: on the brighter side of life: forget about audiophile-grade SE output transformers and think about this:
a Champ output transformer is minuscule, is SE and yet is part of a killer amp, so........ , considering yours would be a Champ on steroids, I dare guess that results will be good to excellent.
Anyway, measure inductance if you wish, but measuring power output at 1KHz and then sweeping down until you find the -6dB point will be all we need to know.
Anywhere below 200Hz is good enough for guitar use.
The Z ratio will not change, you will simply have rolloff below certain frequency.
You are not rewinding at all, so , as old computer geeks used to say: "WYGIWYG" (what you get is what you get), or something like that

[MWJB]

"am I correct in thinking that if my Vk (bias) on the 6CA7 is 19.4V, then converting to fixed bias could gain me this much extra B+ across the valve, and an extra ~ 38V voltage swing at the plate?" Converting to fixed bias will bump up the B+ by at least 19.4v...probably more, because you won't need to bias as hot, 50-60mA might well be satisfactory? Less the current draw, the higher the B+, obviously focus on having the current that you like the sound of, live with the voltage you end up with. At the moment you have a lot of current, but little voltage, this cuts headroom.

"Should I change this to a rheostat where the grid reference to ground is adjustable from 500K down to 0? Does this control the volume by changing the output load impedance for V1b, and Does this setup change the tone less?" Yes, you should. Voltages are higher at this stage in the circuit, therefore any series resistance has a greater effect in reducing fidelity, a rheostat will allow you to adjust the grid load without introducing any series resistance. It primarily changes the volume by altering V2's grid load, your coupling cap after V1B isolates the rheostat's load from V1B. A master volume changes the tone, that's its job, wiring as a rheostat changes one perameter, rather than 2.

I would be very surprised indeed if the OT (if a suitable size, if a suitable Primary-Z) is your problem, air gap, or not. I would explore the more obvious & cost effective routes to increasing headroom first. After that, if you're still not happy, go for an air gapped OT.

The only commercially available SE guitar amps that I have seen, that run the B+ voltage you have, have been EL84 powered amps, even 6V6 amps typically run 50-80v more.

[Steve Conner]

If you take the core apart and reassemble it as a gapped one (I've done this before!) then it probably won't have enough inductance to make a good SE OT. You'd also need more turns.

[jimboyogi]

"am I correct in thinking that if my Vk (bias) on the 6CA7 is 19.4V, then converting to fixed bias could gain me this much extra B+ across the valve, and an extra ~ 38V voltage swing at the plate?" Converting to fixed bias will bump up the B+ by at least 19.4v...probably more, because you won't need to bias as hot, 50-60mA might well be satisfactory? Less the current draw, the higher the B+, obviously focus on having the current that you like the sound of, live with the voltage you end up with. At the moment you have a lot of current, but little voltage, this cuts headroom.


I would be very surprised indeed if the OT (if a suitable size, if a suitable Primary-Z) is your problem, air gap, or not. I would explore the more obvious & cost effective routes to increasing headroom first. After that, if you're still not happy, go for an air gapped OT.

The only commercially available SE guitar amps that I have seen, that run the B+ voltage you have have been EL84 powered amps, even 6V6 amps typically run 50-80v more.

I see what you are saying MWJB, but I am a bit confused. I was getting my expectation of 10W clean output at B+ of 300V from the single ended page on MerlinB's website. All of his other measurements/voltages etc match what I am getting fairly well, except for the WRMS at the OT secondary.
I have tried measuring, at 1 KHz,the Vpk-pk at the 6CA7 plate on full, but was WAY too much for my CRO with 1:10 probe, but my very rough estimation was ~600Vpk-pk, which is about what I should be getting. However on the OT secondary this was only converting to 7.7WRMS.
I just measured the plate voltage at max before clipping with my DMM and got 700Vac, but only 6.6Vac at the 8ohm OT tap? If I am really getting 600-700Vac of clean sine at the 6CA7 plate, then for a 4K OT primary at 100% efficiency I should be getting 27 - 31V pk-pk at the 8ohm secondary tap, = 11.6 - 15.3 WRMS.
So to explain my low wattage at the speaker, am I just losing a lot of power in the OT, ie. only an efficiency of 7.7/11.6 = 66%?

[jimboyogi]

Maybe it's time i bit the bullet and got a proper SE OT, if only to see what difference it makes.
I'm looking at the Weber WSE 15, 15W rating with a 5K primary, only problem is physically fitting it onto my chassis!
Has anyone used these, and are they any good? Are there any better or cheaper options for someone in Australia?

[MWJB]

"I see what you are saying MWJB, but I am a bit confused. I was getting my expectation of 10W clean output at B+ of 300V from the single ended page on MerlinB's website. All of his other measurements/voltages etc match what I am getting fairly well, except for the WRMS at the OT secondary." Have you noticed that manufacturers only make amps that go up in power in 5W jumps? If your amp makes 6-7W then it's called a "10W amp", no-one makes a 7W amp (well, actually Kendrick do). The W rating is nominal. Carr's 10W SE amps are fixed bias and take anything from a 5881 to a KT90 biased in the mid 50's mA wise, they run more B+ than your amp.

Even if it made 10W at the secondary, I wouldn't expect that to be clean W RMS. If you wanted clean 10W+ RMS you wouldn't really build a SE amp.

7.7W RMS sounds about right, perhaps even more than I would be expecting. Why all the references to pk-pk voltages, why not just measure the RMS voltages? Ideally, what you should do is start at the speaker, turn the pots fully up (perhaps the MV to 220K setting, rather than 470/500K). Apply your 1KHz at the input & see what you get at the speaker. Note the RMS at the speaker, then go back through the amp to the input, logging RMS voltages with an RMS meter. Note that you may end up applying 100mVAC, but with a 12AX7 preamp valve & a SE design, you may end up applying rather less than this. E.g. some schems show a 3-4mVAC (three to four millivolts) signal applied to the input at full RMS rating.

You seem to be being thrown off by reading the voltages from input to output, then trying to make the preamp voltages "fit" the RMS at the speaker...which seems counter-intuative to me.

[jimboyogi]

"I see what you are saying MWJB, but I am a bit confused. I was getting my expectation of 10W clean output at B+ of 300V from the single ended page on MerlinB's website. All of his other measurements/voltages etc match what I am getting fairly well, except for the WRMS at the OT secondary." Have you noticed that manufacturers only make amps that go up in power in 5W jumps? If your amp makes 6-7W then it's called a "10W amp", no-one makes a 7W amp (well, actually Kendrick do). The W rating is nominal. Carr's 10W SE amps are fixed bias and take anything from a 5881 to a KT90 biased in the mid 50's mA wise, they run more B+ than your amp.

Even if it made 10W at the secondary, I wouldn't expect that to be clean W RMS. If you wanted clean 10W+ RMS you wouldn't really build a SE amp.

7.7W RMS sounds about right, perhaps even more than I would be expecting. Why all the references to pk-pk voltages, why not just measure the RMS voltages? Ideally, what you should do is start at the speaker, turn the pots fully up (perhaps the MV to 220K setting, rather than 470/500K). Apply your 1KHz at the input & see what you get at the speaker. Note the RMS at the speaker, then go back through the amp to the input, logging RMS voltages with an RMS meter. Note that you may end up applying 100mVAC, but with a 12AX7 preamp valve & a SE design, you may end up applying rather less than this. E.g. some schems show a 3-4mVAC (three to four millivolts) signal applied to the input at full RMS rating.

You seem to be being thrown off by reading the voltages from input to output, then trying to make the preamp voltages "fit" the RMS at the speaker...which seems counter-intuative to me.

Hi MWJB, I have been using pk-pk voltages because I don't have a RMS meter, just my pk-pk DMM (VAC setting) and mainly looking at the signal on the CRO and calculating.
I'm getting 7.7WRMS at the speaker with 100mV sine input at 1 KHz and both volume controls at full, tone at half (least attenuation), and NFB on. Plenty of distortion at this output though! With NFB off, same setup, I get 10WRMS out, with plenty more distortion.
Clean output, as determined by lack of clipping on signal on CRO - 6.5WRMS with NFB on, 5.8WRMS with NFB off.
Also, at 100mV input there is no clipping on either side of V1 (preamp), any clipping is in the 6CA7. As the master volume is turned up, the clipping as sighted on CRO at the speaker out begins quite symmetrically, ie. both top and bottom of waveform begin visibly distorting at the same signal amplitude.

I do have another PT with 300-300 HT, which should give me ~ 400v B+, but then I would need a SE OT with a Primary Z of ~ 6.5K (400^2/25W = 6.4K), with a 10W+ power rating.

Also, my PT's do not have a centre tap for the heater windings, and I have so far used the 100ohm resistor/artificial CT to power tube cathode (cathode bias) to ground and DC elevate the heater circuit to minimise hum. If I move to fixed bias, what techniques can I use to DC elevate the heater?

[Steve Conner]

Cheap DMMs don't measure peak-peak on the AC setting.

They rectify, average, and multiply by 1.1, to give a reading that's equal to the RMS value if the input is a sine wave. (And goodness knows what for other waveforms, but certainly not p-p.)

[MWJB]

"I'm getting 7.7WRMS at the speaker with 100mV sine input at 1 KHz and both volume controls at full, tone at half (least attenuation)," You mention "5F2A" in conjunction with this thread, tone pot full up is least attenuation (least lo freq bled to ground) with this circuit. Turn volume & tone fully up, set the master to 220K-ish. Measure RMS at the speaker & note clean output, use the signal generator as the level control, don't stick to 100mVAC necessarily.

But what you are generally describing sounds within the bounds of "normal". It would be better to actually measure the AC voltages.

"I do have another PT with 300-300 HT, which should give me ~ 400v B+, but then I would need a SE OT with a Primary Z of ~ 6.5K (400^2/25W = 6.4K), with a 10W+ power rating." No you wouldn't. You are already running 80mA plate current, you will be running less than that at 400vdc. Primary Z will be fine at 4.7K.

"Also, my PT's do not have a centre tap for the heater windings, and I have so far used the 100ohm resistor/artificial CT to power tube cathode (cathode bias) to ground and DC elevate the heater circuit to minimise hum. If I move to fixed bias, what techniques can I use to DC elevate the heater?" Remove the 2x100ohms, add one 220K 2W (or better) from main B+ node to one side of the heaters, from the same side of the heaters run a 22K-27K 1/2W resistor to ground, you want 35-40vdc (no more, no less) riding on the 6.3VAC.