Not sure which output transformer to use.

[tpaairman]

I'm drawing out an amp I'd like to build, and the bulk of it is based on a Fender Pro 5E5, which I've linked below. However, the output transformer is not specified. When I look at the data sheet for the 6L6, it says that for 360 volts on the plate, it's 3800 Ω, and for 450 volts it's 5600 Ω. I'm sitting in between and not sure what impedance to use.

I've tried looking it up, and I can't find a model 5E5. On lists of Fender transformers, I can find a 5C5, 5D5 and 5E5A, which all show a part number of 1846, but I can't find any info on that part number.

EDIT - I accidentally attached the 5E5A schematic first, and it won't go away.

[Chuck H]

A few references I've seen indicate a pri z of 6.6k. That's outside the range you indicate. You could always just shoot in between with something like 4k, 4.2k. I think these are common values for BF era Fender and should be readily available. In fact I'd recommend that rather than 6.6k. That or add individual screen grid resistors to the design. My thinking is that if you're going to clip the power tubes much (and why wouldn't you want to ) the lower OT primary and/or the addition of individual screen grid resistors might mitigate overdissipation of the screens.?. But really, I didn't dig very hard to find a few references to 6.6k. You could do more. If authenticity is your goal then you should.

[Enzo]

You want to build the 5E5 power amp. You found the 5C5, 5D5, 5E5A all using the 1846, so I think we can safely assume the 5E5 uses it too, or something like it. The 125A6A or 125A7A do the same job. These are just guitar amps, don't obsess over exact impedances. The 125 type lists 4k in the AES catalog. Good enough for me. Good enough for fender. Should be good enough for anyone.

You want to build a Pro? Why not select a transformer specified as for the Pro?

For example:

https://www.tubesandmore.com/product...tput-35w-8-ohm

[tpaairman]

I'll go with the 4K transformer. I just didn't have any idea what impedance to even look for the start with.

Out of curiosity, where did you find the info for the 125's being an equivalent (or close enough) to the 1846? I couldn't find anything on the 1846 when I searched.

[Enzo]

I looked up the Pro, and found what transformers were used through the course of that model.

[Zozobra]

I've used a 4K primary with a pair of cathode biased EL34s at 380V. Works just fine. 6L6s will be much the same.
You can always draw the loadline as a sanity check. It'll be a good way away from max dissipation (and this can be exceeded pretty comfortably in AB push-pull anyway) with a sensible bias as folks use a 4K primary with much higher voltages with fixed bias.

[g1]

Not that I disagree with anyone's suggestions here, but 1846 subs are listed as 6K primary:
http://www.classictone.net/40-18088.html
https://www.tube-town.net/ttstore/Tr...ml?language=en

[Enzo]

4k? 6k? Neither is "wrong". Both of those and others have been used in successful amp designs for decades.



it is like Tater Tots. If a standard serving is 10 of them. Would 12 or 9 be "wrong"? Either way, they help me eat my burger.

[tpaairman]

I have a side question while were at it. Actually, I need someone to check my math. What would be the power output of this amp be? (Roughly) I found a formula on Valve Wizard to estimate the power output, which is P = 2 * (HT-50)^2 / Rload. With a 4K output primary, I'm coming up with almost 58 watts which seems a bit high.

[Dave H]

I have a cathode biased EL34 amp with 375V B+ voltage. It measures 30W clean (5% THD) into a 4k primary. I think it is nearer 25W with 5881s.

[loudthud]

Notice that both the 5E5 and the 5E5A schematics specify 6L6GB. That tube wants a higher primary impedance than what we normally think a 6L6GC would use because of it's lower rated plate dissipation. If you can find a vintage 5E5 with original transformers, it will probably clip at 25W or 30W. If you want the tone and feel of a vintage 5E5, go with the higher impedance.

[Chuck H]

I have a side question while were at it. Actually, I need someone to check my math. What would be the power output of this amp be? (Roughly) I found a formula on Valve Wizard to estimate the power output, which is P = 2 * (HT-50)^2 / Rload. With a 4K output primary, I'm coming up with almost 58 watts which seems a bit high.

The formula assumes a perfect world. That's probably even stipulated somewhere on the page that gave you that formula. And, of course, it's not a perfect world. Things that will reduce that wattage include:

The actual working voltage would be the plate voltage minus the cathode voltage. And that cathode voltage rises as current increases. This would be the "correct" voltage to insert in the formula.

Impedance varies with frequency. Typically being much higher than "rated" at one LF resonant point and then rising steadily as the mids move into the HF. This increased impedance will be very much less efficient than the ideal impedance at those frequencies. Of course wattage is typically measured into a resistive load and this only applies to using speakers as the load. But,..

OT's are also less than perfect in their power transfer. The formula assumes a perfect transformer. Which doesn't exist.

The formula also doesn't account for loading and sag in the power supply which further reduces the working voltage with current.

Most of these anomalies are variable enough that measuring an/the actual amp is really the only way to know how much power it has. And it will always be lower than the formula.

[Dave H]

The formula assumes a perfect world. That's probably even stipulated somewhere on the page that gave you that formula. And, of course, it's not a perfect world. Things that will reduce that wattage include:

It assumes that in a perfect world the amp can swing a peak voltage of HT-50 volts into half the primary which is 1k for a 4k p-p OT. As you say that's not the case in the real world. My amp only manages to swing down to 75V not 50V at the plate, the cathode resistor drops about 30V at full power, the power supply droops 10V and adding 10V for OT and other losses the peak voltage becomes 375-75-30-10-10 = 250V. Putting 250V for HT in the formula gives 31.25W. For the OP's amp with 390V B+ it works out at 35W.

[qweiczz]

I have a cathode biased EL34 amp with 375V B+ voltage. It measures 30W clean (5% THD) into a 4k primary. I think it is nearer 25W with 5881s.