single ended output - hammond 125 SE?

[mike_mccue]

I think I want to build a simple Champ inspired amp out of some parts I have sitting here.

I'd like to use a Hammond 270BX that I already have. I'm looking at output transformers at vendors and I don't really understand what I can choose.

The 125 SE series has a range of wattage outputs. I guess a 5-8 watt rating is a good match for a Champ and a 6v6, but what are the possibilities that I might increase the output with a larger transformer? e.g. the 125ESE is rated at 15watts, is it also a good match for a 6V6?

Thanks,
mike

[Old Tele man]

...the tube is the major limiting factor for output power.

...using a higher-wattage rated transformer will NOT yield more output power, but it WILL enable you to "move up" later to a higher rated tube (such as 6L6 or 6550, etc.) assuming the new tubes' plate load resistance is similar to that used with the 6V6 (or you use a new speaker load).

...also, the beefier OT will have better low-end response, due to it's increased inductance (more iron core).

...for more power in the future, are you planning on: (a) adding another 6V6 in parallel with the existing tube? or, (b) going to a bigger tube?

...with option (a) the total effective plate resistance Ra1||Ra2 will become HALF of what it was originally, while option (b) will depend upon which tube you choose to use, but (luckily) the 6V6 has a rp-value that's about TWICE the other tubes, for example: 6L6 = 22.5K-to-33K depending upon voltage, while 6V6 is 50K-to-80K.

[mike_mccue]

I just want to build something :-) I'm in the mood.

I have a few Champs and so would really like to make some noticable variation. I thought about a very simple 6L6 se amp but after a quick search I think the 270BX I have is lacking in current capacity. It's also a bit low in voltage but I figured that would be OK.

When I saw the other output transformers I thought it might be something to consider.

I may just stick with 6v6 and a suitable 5-8 watt output transformer.

best,
mike

[mike_mccue]

I asked a related question about predicting current needs for a power transformer that may help me decide if I can use a 6L6 in place of the 6V6.

When you are speaking of the resistance of the tubes how does that relate to the impedance that the output transformer "sees"?

The 125ESE for example can be used in a 10kohm :: 8ohm configuration... how do you know what tubes that is suitable for?

The Champ replacements I see are something like 7kohm :: 3.2ohm 5+watts so it seems that 7kohm is a sweet spot for the 6V6.

How about a 6L6... how are you supposed to think thru that?

thanks,
mike

[Old Tele man]

...theoretically, the Dynamic Plate Impedance (rp) characteristic of a tube is truly an Impedance (Z), which is a vector sum of DC-resistance (R) and AC-reactance (XL ± XC).

...however, reactive resistance (XL and/or XC) can be viewed as simply impedance having a phase-angle, which is what's used these days and since the angle (usually) isn't worried about, it's dropped, leaving only the word resistance.

...but, the lowercase "r" in rp is the clue to what it REALLY is, because (by convention) lowercase letters denote AC-values and uppercase letters denote DC-values. Thus, Vbb is the plate-to-plate DC voltage while vbb is the plate-to-place AC-voltage.

[mike_mccue]

Ok :-)

I read, re-read and read your post again. I finally realized that you are responding to my sentence where I mentioned both resistance and impedance.


Thank you very much for the response but I really need a more basic "primer" level of understanding.

I was hoping to learn to understand the basic relationship between the the tube(s) and the out transformer with regards to how the OT's primary impedance relates to the circuit.

best,
mike

[Old Tele man]

...sorry for the pedantic "over-kill"

...tube rp is determined by the tube.

...push-pull output transformers (OT) are specified by their rated plate-to-plate impedance value (Zpp), often stated as 5.6K:4ohms, etc. How that value is determined is next:

...an OT 'steps-up & reflects' a HIGH load impedance (Zpri) back to the OT's primary windings (and the tubes) that is proportional to the OT's "Impedance Ratio" (Z-ratio) and the LOW load impedance (Zsec) that is connected to the OT's output terminals:

Zpri/Zsec = Z-ratio = (Npri/Nsec)^2

...if the OT turns-ration (Npri/Nsec) is 37.4:1 and a 4-ohm load is connected to the secondary winding, the OT will reflect a 5.6K ohm plate-to-plate load into the OT primary; connect an 8-ohm load and a 11.2K ohm plate-to-plate load will be reflected. Zpri is determined by Zsec.

...with Class-AB PP operation, each tube "sees" one-fourth of the reflected Zpp value, so the plate load (RL) seen by each tube is:

RL = Zpp/4

[mike_mccue]

I think I followed all that and I appreciate what you mean about the speaker load being the actual impedance which the transformer reflects.

I think the info I need to learn more about is about the plate load or RL. When you see a spec like that in the data sheet... what does it mean.

I understand by your demonstration that the real plate load is the result of the circuit.

How do you design for a good "match" or "bridge" between the tubes and OT? Is the listing on the data sheet an ideal circumstance? If so do you select te speaker and then work backwards with the OT design to hit the mark?

Thanks very much.

FWIW, I find that the unfamiliar coefficients and the abbreviations make it difficult to follow... so it takes me a while to catch up.

Also it occurs to me that I don't really know the relationship between plate resistance and plate load. Is there a direct relationship?

best,
mike

[Old Tele man]

...I don't really know the relationship between plate resistance and plate load.

...maybe this can answer some of your questions?

I. RULE-of-THUMB ‘rp’ VALUES

A) TRIODES: RL ≈ 2-to-4 times rp.
B) BEAM tetrodes and PENTODES: RL ≈ 1/6-to-1/10 of rp.
C) ‘Lower’ rp values for lower DISTORTION, %THD.
D) ‘Higher’ rp values for higher OUTPUT POWER, Po.

Excerpt from: John D. Ryder, ELECTRONICS FUNDAMENTALS AND APPLICATIONS, 3rd Ed., 1964, Prentice-Hall; Chapter 11 – Class A and B Amplifiers with Large Signals:

“11-5. Maximum power output-pentodes and triodes.

The distortion generated and the possible power output are both functions of the load assigned to the amplifier. Use of the condition RL = rp will furnish maximum possible power for a given input signal, but the amplitude distortion produced with this load will usually be greater than is considered allowable. Higher values of load, which permit greater driving voltages, may raise the power output and may increase or decrease the distortion. As used, loads for TRIODES are usually in the range of TWO-to-FOUR times the value of the plate resistance, and a small fraction (1/6-1/10) of the plate resistance for PENTODES and BEAM tubes.”

“The maximum percentage of distortion allowed is arbitrary, since it is dependent on subjective factors. Usually a maximum of 5-percent distortion is considered permissible, although only 1 or 2 percent is tolerated in high-fidelity service.”

** See 6V6 and 6L6 tube data sheet graphs for illustrations of Po and %THD versus RL curves; notice that “rated load” (usually 5KΩ with Vp = Vs = 250V) does NOT coincide with either minimum distortion nor maximum power!

II. THEORETICAL EQUATIONS (...deleted from this posting...)

III. NOMOGRAPHS for ‘rp’ VALUES
For information on nomographs used to ‘scale’ published tube values to new operating conditions, see the following:

A) “Valve Conversion Factor Chart,” Chapter 40, page 329, in RDH3 (1944).

B) Fig. 44 – Nomograph of tube conversion factors, page 33, in RCA Receiving Tube Manual, RC-30 (1975).

C) Fig. 8-10 and pages 255-256, in Chapter: Audio-Frequency Power Amplifiers, in J. D. Ryder, ENGINEERING ELECTRONICS, 1st Ed., 1957, McGraw-Hill.

...also, rp is traditionally tube value while RL is the combination of tube and it's reflected load, effectively but not exactly: rp||(Zpp/4)

...ra designates a plate load resistor, but with OT, there's no resistor, so RL is more commonly used.

[stokes]

Mike,I also have always had a problem with those math formulas.I have gotten by for years just knowing what most circuits use for a pair of tubes in PP or SE and accepting the fact that a transformer only knows turns ratio.If you take any OT and put 1/2 vac on the secondary you measure across the primary and see 14vac you divide by 1/2 and get 28.That makes your turns ratio 28 to 1.to get the primary impedance you square the 28 and get 784 multiply this by your expected speaker load,say 8ohms and you get 6272 which,with an 8ohm load would be your primary impedance.That is close enough to what Fender runs their 6V6 PP at so you are in the ballpark.This is not so precise for lab use,but guitar amps are anything but lab grade,there is a lot of leeway in almost all the circuitry in a guitar amp.okay lets suppose you wanted to use the same OT for 6L6's.You merely adjust the speaker load,so for the same turns ratio 28 to 1 you get the same 784 times the new load of 4ohms you get 3136.Knowing that Fender ran a pair of 6L6's at about 4400 ohms primary you are close enough to use the same OT with a pair of 6L6's with a new speaker load of 4ohms.Assuming of course the OT can handle the increase in wattage.When you see that the actual speaker load varies with the frequency and that 4ohms is a nominal thing you dont have to be dead on with your primary impedance.For reference you can go by this.Fender ran a SE 6V6 with about a 7k primary into a 4 ohm load.A pair of PP 6V6's at about 6600 ohms into an 8 ohm load.They used a quad of 6L6's in the Twin with a 2000 ohm pri. into a 4ohm load.Now if you pull two 6L6's you change your spkr load to 8 ohms and it keeps things in balance.They ran their Dual 6L6's with about 4400 ohm pri.into an 8ohm load.Hope this helps.

[GibsonLover]

All of the load/impedance math is fun ... ;-) but I'd keep in mind two things. First, either 2.5K or 5K primary works with a 6V6 or a 6L6. It just depends if you like the sound or notice the difference. With the 125 series, you can try both. My point is the load is very forgiving.
Second - you mentioned the 270BX. It does not have the balls for a 6L6. Only 57mA for B+. The current rating for you B+ is a huge factor in how your amp will respond when you have your dial on 11. I built a champ with a 125ESE and 420V B+, pulling 64mA. (Can't remember the PT rating...)

[Old Tele man]

...from Leo Fenders era:

2 x 6V6 use 6.6K:8-ohm (early) or 8.0K:8-ohm (currently) OTs
2 x 6L6 use 4.0K:4-ohm (most all Fender amps w/dual speakers)
2 x 6L6 use 4.2K:2-ohm (4-10 Bass Man, 3-10 Concert, 4-10 Super Rev)
4 x 6L6 use 2.0K:4-ohm (Twin, Dual Showman, etc.)
4 x 6L6 use 2.0K:8-ohm (Single Showman)

[mike_mccue]

Honestly, that info you have all posted is very helpful for me to get some oversight.

Thanks very much.

I was wondering if there is some general guideline like the 1::10 ratio I often see as an example of a good impedance bridge when interconnecting audio devices with cables.

And I thought perhaps if there is some ratio like that then there might be a spec in the data charts to use while planning a circuit.

thanks again,
mike

[Old Tele man]

And I thought perhaps if there is some ratio like that then there might be a spec in the data charts to use while planning a circuit.

...which data charts are you referrig to: the OT or the tubes?

...typically, for PP-amp you start with the rp-value from the tube, scale that value by 1/6-to-1/10 (high/low Zpp range), then determine the speaker(s) load; and, lastly, select the PP-OT that has the input Zpp and output Zo specs to meet your needs.

...for SE-amp you start with the rp-value from the tube, scale that value by 2X-to-4X (low/high Zp range), then determine the speaker load; and, then select the SE-OT that has the input Zp and the output Zo specs to meet your needs.

...see if this article answers some of your questions: http://boozhoundlabs.com/howto/

[mike_mccue]

I think I'm following that.

To clarify rp = Dynamic Plate Impedance

does rp = "Plate Resistance" when I'm looking in a RCA tube manual?

The SE explanation seems simple enough... but I get an unexpected result when I compare it to a SE Fender Champ type circuit:

plate resistance of 6v6 = 60k

out transformer primary is 8kohm

how do a I get from 60k to 8k by multiplying by 2? I must need to learn more about something. :-)

Please help.

On the PP is it 1/6 - 1/10 of a single tubes rp? Or did I miss something in a previous post about PP?

Thanks very much.

mike