Super Single-Ended Project

capohk

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Buoyed by my recent success with re-building my 5f1 using PTP and a small chassis, I am planning my next build. I have been listening to The Truth About Vintage Amps a bit too much and settled on the parallel SE 6V6 project that Steve Melkisethian first posted in the Angela catalogue in 1996. It's basically a 5F2 with a bigger OT and 2 6V6s in parallel single-ended configuration. I'm hoping there are some folk here who are familiar with the circuit as I'm right at the limit of my understanding with some of this stuff.

I am progressing my learning by trying to draw out the schematic for what I want to do, which is use a solid-state rectifier instead of the two 5Y3s, partly to reduce cost but also because I like the simplicity of it. I already have what I think is a good PT and OT in mind from Primary Windings UK. The specs are here:

PT
Primaries - 2 x 120V
Secondaries - 0 – 190V – 275V @ 160mA
Heater winding 6.3V at 3A centre tapped.

OT
Primary - 3000 Ohms, Inductance 2.5H at 1KHz, Leakage inductance 6mH
Secondaries - 4 Ohm, 8 Ohm and 16 Ohm
Power rating - 15W

Initial questions:

A choke is suggested for the first filter stage.
In the notes for the single 6V6 amp that Steve outlines, he suggests anything between 2- 10H with at least 120mA but no more that 100 ohms DC resistance.


Primary Windings also sell a couple of chokes and it would be good to get the iron all from the same place. However, the specs for the two that look reasonable are:

10H at 100mA, DC resistance 130 Ohms

Or

10H at 180mA, 200 Ohms DC resistance

Which specs are important and why?

Second, the 5F2A and 5F1 both have 22k negative feedback. The Angela schematic deletes the NFB. Why?

I'll follow up with my somewhat completed schematic and the Angela schematic for reference. Super appreciative of any feedback as always. The resistor and capacitor values for the SSE differ in several places from the 5F2 that it is based on. I'm leaving those blank on my schematic until I can work out why and what should be there.

Finally, Angela Super Single-Ended Guitar Amp Solid State Rectified has quite the catchy acronym. Maybe in future posts I'll just use that.

Matt
 
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capohk

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ZPKaWUXJU_TcfBECaNLYuO3chMatBi4mlZwGrQAuCjX_EtmhS-K8MWI0SzuEvsh04AnmXOPrEjnKGlu00wx4WplsW5ZmKNcr4lZQC_LamskmjDj6Bv9r2Yco-6Kv9IiHbZ-dNX13exT26VpbSA
 

SerpentRuss

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I'll be interested to hear this, ASSGASSR. I've never built an SE amp above 5 watts and I can tell you that you don't get that beautiful 2nd harmonic SE distortion until it's pretty darn loud. To drive a parallel 6V6 output section into that range may require ear plugs. In fact, one of my bucket-list items is to add a resistor network to a 12-inch cabinet I normally use so I have the option of running at 1/4 power. I have a bucket full of 6P1P tubes that I built the 5-watt head with, so running them hard is not a major concern.

Are you abandoning the power section, cathode-bypass switch and the 220k grid leak?
 
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FenderLover

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A parallel 6V6 is on my bucket list too.
I would think a SS rectifier would be a better plan than parallel glass rectifiers.
Remember that the 2-diode version is capable of sourcing more of the available B+ current than the bridge configuration. This is from the Hammond Design Guide:

Hammond_PS.png
 

bebopbrain

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use a solid-state rectifier instead of the two 5Y3s

Absolutely. The only thing dumber than a 5Y3 in a single ended amp (where the power supply voltage doesn't sag under load) is two of them.

Since you have a big choke penciled in, I would be tempted to use maybe a 5H 150mA and put it after the initial reservoir capacitor and before the B+ to the output transformer (the A). A single ended amp will not cancel hum on the output transformer the way a push pull amp does. Then maybe add another resistor and cap to get to B (the screen voltage).
 

printer2

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A parallel 6V6 is on my bucket list too.
I would think a SS rectifier would be a better plan than parallel glass rectifiers.
Remember that the 2-diode version is capable of sourcing more of the available B+ current than the bridge configuration. This is from the Hammond Design Guide:

View attachment 981997
But you only use one winding at a time with the top transformer. And looking at the equations the top transformer has a multiplier of 0.71 as compared to the bottom 1.41. There is no free lunch between the two transformers as far as VA's are concerned. So the center-tapped version has half the voltage produced across the capacitor than the secondary of the bridged configuration. How do you get two windings in the same sized core to produce the same output voltage as the bottom transformer? You make the wire thinner and you end up with twice the number of turns. Along with the thinner wire you get a greater winding resistance and less current capacity. So while the equations say the top configuration delivers a higher current it also says the voltage is less.
 

FenderLover

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...So while the equations say the top configuration delivers a higher current it also says the voltage is less.
...And that's the good part. The recommendation is Deluxe PT which already has a high B+. It would be far more voltage friendly for SE 6V6 with less voltage, while being able to tap the reserve current. A design trade-off that makes sense in the application.
 

2L man

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It is good practice to use own cathode bias resistor / capacitor for each parallel SE power tubes to prevent more ”gainy” tube cooking.
 

printer2

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...And that's the good part. The recommendation is Deluxe PT which already has a high B+. It would be far more voltage friendly for SE 6V6 with less voltage, while being able to tap the reserve current. A design trade-off that makes sense in the application.
There is no good part... ...there is no magic.

The way the equation is set up for voltage on the center-tapped transformer is across the whole winding. But that is not how the circuit operates, it only gives the maximum voltage of one side of the winding, not the full winding. So while a bridged transformer would have an open circuit voltage of 200V the center-tapped transformer would have a voltage of 400V, or 200V - 200V referenced from the center tap.

The bridged transformer will have twice the current capacity as the center-tapped transformer, the center-tapped transformer has twice the voltage across the outside windings. Deciding on a transformer with the same VA between the two gets you to the same place. A 400V transformer where only one half the winding is used at a time gets you 200V. Selecting a transformer designed for bridge operation will have you looking for a spec of 200V.

400V x 0.71 = 284V
200V x 1.41 = 282V

So it is not a case of using a center-tapped transformer will give you a lower voltage than if you used a bridged transformer. If you need a lower voltage you pick a 380V transformer or a 190V transformer. There is no reserve current, the two transformers do not have the same current ratings. The center-tapped transformer needs two windings in the core to get to 400V. The way that is done by using the same number of turns for each winding as for the bridged transformer. But with the same volume in the core to put the windings in the 400V (200V + 200V) transformer the wire thickness will be reduced by (roughly) half. And this limits the current rating of the transformer.

Simply put, the transformer with the thicker wire will have more current available than the thinner wire transformer. So while the 400V transformer may have a multiplier of 1.0 x Rated Current, and the 200V a multiplier of 0.62 x Rated Current, the 400V transformer would have a secondary current rating of 100 mA while the bridged transformer would have a current rating of 160 mA. In the end you get to the same place.
 

FenderLover

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I'm going off the suggestion that the Deluxe transformer was recommended as a suitable choice. It does not know what rectifier will be connected to it. Increase voltage with a bridge for an already high B+ while neutering the current, or reduce voltage (often required when biasing the same tube type to SE) and provide the current are the choices. Power is the same in both cases. One choice is better for SE.
 

FenderLover

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In practice, I've noticed that too.
Point is, they are different, given different rectifiers (rectifier configurations) on the same transformer.
Edited to help Printer2 out. Doubt it matters.
 
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printer2

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I'm going off the suggestion that the Deluxe transformer was recommended as a suitable choice. It does not know what rectifier will be connected to it. Increase voltage with a bridge for an already high B+ while neutering the current, or reduce voltage (often required when biasing the same tube type to SE) and provide the current are the choices. Power is the same in both cases. One choice is better for SE.


Your explanation using Hammond's drawing was wrong as you took the equations and assumed the transformers had similar specs. That is not the case, you are comparing a tall skinny jar to a wide squat one. They both hold the same water, they just do it differently.

Remember that the 2-diode version is capable of sourcing more of the available B+ current than the bridge configuration.

And how is the transformer suppose to work in full wave operation? The voltage from the windings is

Secondaries - 0 – 190V – 275V @ 160mA
For the transformer to be used in a full wave center tap circuit the two legs of the winding should be the same. The center tap of 190V is not half way in between 0V and 275V. (2 x 190V = 380V) In my books 275V does not equal 380V. The transformer is not suitable for full wave operation with a center tap.

But the transformer is suitable for a bridge rectifier. 190V x 1.4 = 266V. 260V x 1.4 = 385V Mind you that is theoretical. I generally start with 1.3 as a multiplier, it all depends on the current capacity of the transformer. So let us say 250V or 360V. Pick your poison. 250V if you want to get about 4W out of each tube (datasheet says 5k OT). Or 315V and 8.5k with output at 5.5W. But these are for center biased, Fender did not mind one side of the waveform clipping before the other as it got a little more power and did not sound too bad (to his ears).

So what to do?
 

printer2

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In practice, I've noticed that too.
Point is, they are different, given different rectifiers on the same transformer.
Why different rectifiers? The picture does not say they are different, it does not say one is tube and one SS. You could have a SS full wave circuit and a tube bridge circuit. You do need two tubes for that one though. Hammond rates their transformers at full output so the numbers they give end up lower than is normal, you end up getting higher than what they advertise.

Also there is no reason to have chokes with 100 mA or 180 mA ratings. The screen and preamp section will be under 50 mA.
 

capohk

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@capohk , don't miss what SerpentRuss pointed out. I think the grid to ground connection is mandatory to bias the power amp
View attachment 982028
Thank you, yes, that was an omission. Corrected in my own schematic now. There are a number of other options that I am considering, but I am a fan of KISS, at least to start with. I am thinking that the lack of NFB and the cathode bypass cap on V1b are there to increase gain, which I don't necessarily want.

As regards the choke position, are you saying that it is currently in the wrong place, or that I need to add extra filter cap?
 

andrewRneumann

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As regards the choke position, are you saying that it is currently in the wrong place, or that I need to add extra filter cap?

Not wrong, just ineffective in killing the ripple hum that a single-ended amplifier can’t cancel. Not a big deal in a Champ, but with twice the power tubes, your ripple hum is also going to be a lot more (I can’t remember if the math says it will be doubled too.) Extra capacitance won’t work as much as you’d think… law of diminishing returns due to source resistance of PT.

I am saying add another filter cap, then position the choke, then another filter cap for the plates, and on with the rest of the power supply. The current choke position can replaced with a small value / high wattage resistor so your plates and screens remain close in voltage.

Seriously, why are your diodes backwards? Trying to blow up some capacitors?
 

SerpentRuss

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Thank you, yes, that was an omission. Corrected in my own schematic now. There are a number of other options that I am considering, but I am a fan of KISS, at least to start with. I am thinking that the lack of NFB and the cathode bypass cap on V1b are there to increase gain, which I don't necessarily want.

As regards the choke position, are you saying that it is currently in the wrong place, or that I need to add extra filter cap?

The switch on the bypass capacitor acts like a negative feedback switch. If you look at the original schematic, both tube cathodes have a path to ground via the 250R resistor. With the cathode capacitor out of the circuit, there is a lot of gain lost and negative feedback takes place in the form of AC signal that can't go to ground. The output tubes are essentially constipated. Switch in the cap and boom, you have a lot more gain and grit.
 




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