Blues Junior output transformer specs

[sarkipinty]

Hi,
I am thinking about making a Blues Junior copy based on the plans found on this site.
I would like to know what is the impedance ratio that is recommended/acceptable for this amp

based on the EL84 datasheet a pair in push pull mode should match 8K impedance
but in the original amp specs I read 10K, also I found some cases where 4K is specified

what I have is a 1:25 ratio transformer, which translates to cca 5k to 8 ohm
any thoughts on this? thanks

[sluckey]

Hoffman used a 768 Heyboer output transformer which is 8K:4,8,16

[PRR]

EL84 happy-load tends to be near 8K. 10K and 6.6K are close-enough.

5K is a bit low but may be fine. Much depends on bias method, voltage, current, actual load (speakers are not exactly resistors), and how it sounds.

[tubenit]

From Kevin O'Connor  London Power FAQ

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and ridiculous that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

[sarkipinty]

thanks all for  the feedback,

actually I think I might have solved my own issue (sort of)
as you said the 5k ohm OT is not quite ideal for the two EL84s, however I have on hand a couple of 6L6 (russian equivalent) tubes
and when I checked the datasheet guess what it says the impedance should be? 5k :D (you should have seen my face...)

anyway, I often see posts about people modding their BJ with 6L6s, does that mean that the original components are suited to operate both EL84 and 6L6 tubes?
(of course the socket and the pinout will be different)

[sluckey]

6L6 will want to see 40 to 50 volts of bias so you will have to figure out how to do that with the original PT. Or you could just go cathode bias.

[sarkipinty]

unfortunately I am not very familiar with biasing... i found the wiring, now the question is the specific values


are the values of the cathode biasing resistor and capacitor given for specific tubes?
meaning: no matter the amp, a push-pull pair of EL84s will always need the same values for these components? or is it also depending on the rest of the power section?

I was thinking I could use the same cathode biasing as in the Orange Tiny Terror: a 220uF cap and 120 ohm resistor

[shooter]

depending on the rest of the power section?

here's the short version;
pick your tube
pick SE or PP
get tube data sheet, find all the cool values, typ B+, I, plate impedance, max power, max I
build a PS that gets you plate B+
calculate your bias point
plug in n play

[92Volts]

It depends on the amp.

Bias voltage (cathode greater than grid) limits the current flow through the tube. We usually do this to limit the power dissipation of the tube, which equals voltage across the tube times current through it. Imagine we want to limit each tube to 20W. For a 30W maximum tube like a 6L6GC, this is actually in the right ballpark.

If we have 300v across the tube, 67ma is our target current. 300v*0.67ma=20w

For this tube imagine we need 30v of bias to limit current to 67ma. Ohm's Law tells us V=IR, and we want the cathode resistor to produce 30v of voltage... we solve the equation to find 67ma through 450 ohms provides 30v... so 450 ohms will be our cathode resistor.

What if we have 400v across the tube? We must use lower current, 50ma, to stay within 20W limit. Ohm's law tells us we need a larger resistor to achieve 30v given the lower current, but we actually want to limit current to a greater extent than before, this varies by tube but let's assume we need 40v instead of 30v. Solve the equation and we find out we need an 800 ohm resistor! That's a huge difference!

To figure this out you need to know your plate voltage, ideally screen voltage too, and have your tube's datasheet. Fortunately there are calculators on the web (Weber's bias calculator for example) which can do a lot of this for you (specifically, they know the limits of tubes and the bias voltages required to control the current through them-- you need to provide your amp's voltages).

But you should start with the basics. It's actually not certain that your amp can handle 6L6s. What are you using for a power transformer? 6L6s will draw (very roughly) double the idle current as EL84s, with the full-tilt current being limited by the load impedance. Since you're using 5k instead of 8k... it's possible the overall power demands from the amp will be nearly double.

You can limit idle power consumption by biasing with your amp's power limitations in mind, instead of the limits of the 6L6GC tube in mind. During operation this won't quite reduce the demands of 6L6GCs all the way down to those of EL84s, though.

[sluckey]

anyway, I often see posts about people modding their BJ with 6L6s

Do you mean 6V6s???

are the values of the cathode biasing resistor and capacitor given for specific tubes?

250Ω for pp 6L6s and 6V6s, or 120-150Ω for pp EL84s are very common values.

[mozz]

I have 3 of these output transformers, i can measure them if you want.

[Ed_Chambley]

It is more about tone and current. If you like 6l6 loafing tone your tubes will last a long time. If you like el84 easy distortion 8 k is nice. Same with 6v6. 8k again is common. This sort of has some character of each.
Either case the trans Sluckey suggested is nice unless you have one. This will provide headroom for a 15 watt you may need. You can connect your 5k to a speaker double ohm and. It will reflect 10k which is ok too.

[DummyLoad]

Hi,
I am thinking about making a Blues Junior copy based on the plans found on this site.
I would like to know what is the impedance ratio that is recommended/acceptable for this amp

based on the EL84 datasheet a pair in push pull mode should match 8K impedance
but in the original amp specs I read 10K, also I found some cases where 4K is specified

what I have is a 1:25 ratio transformer, which translates to cca 5k to 8 ohm
any thoughts on this? thanks

blues jr. OT is 7K:8.

http://www.hammondmfg.com/pdf/EDB1750F.pdf


at least the hammond replacement is. i can't recall hammond winding a replacement that's not close to original specs.

just an idea: 20W deluxe reverb/tweed deluxe output x-former with the allen amps TP-24 power x-former coupled to a pair of 6L6GB/5881 in auto-bias. should make 25-ish watts. 


--pete