Ouput Transformer Impedance Question

[rsmalling]

New Guy here,

I'm building a varent of an amp called the 2 stroke from a Dave Hunter book. Anyway, it's a pretty basic amp, class A, one power tube, 10 watt. You can use Different tubes as well. I'm going to use a Hammond 125ese OT Hammond Mfg. - Universal - Single Ended - Tube Output Transformers - (125 Series) ,but I dont know how to figure out what taps would be best for:

EL34 etc tubes @ 4, 8, and 16 ohm loads
6V6 etc Tubes @ 4, 8, and 16 ohm loads

I'm hoping I can get away with a 2 position impedance switch but don't know if this is possible.

Thanks for any help.
Rob S

[tubeswell]

2 x EL34 in parallel will want a 1k7-2k5 Load resistance (or 3k2-4k for 1 x EL34), and you need you keep sockets pins 1 and 8 wired together, because the suppressor grid (1) is not connected to the cathode (8) inside an EL34. You will also need to use chunky screen grid resistors (minimum 5W) for the EL34s. Wire each cathode pin with a separate resistor and bypass cap, so you can pull one of the output tubes at will.

2 x 6V6 in parallel will want a 2k5 - 5k load resistance (or 5k-8K for one 6V6).

Either way, its do-able with the 125ESE

If you build it with a tube rectifier that is switchable to SS, and with beefy filter caps, you will be able to run a range of B+ voltages to suit your different tubes.

[rsmalling]

Thank You tubewell,
This is my first build so I'm trying to really understand each phase of my project. Your detailed explaination helps me a great deal.
All the Best!
Rob

[Sleepthieves]

2 x EL34 in parallel will want a 1k7-2k5 Load resistance (or 3k2-4k for 1 x EL34), and you need you keep sockets pins 1 and 8 wired together, because the suppressor grid (1) is not connected to the cathode (8) inside an EL34. You will also need to use chunky screen grid resistors (minimum 5W) for the EL34s. Wire each cathode pin with a separate resistor and bypass cap, so you can pull one of the output tubes at will.

2 x 6V6 in parallel will want a 2k5 - 5k load resistance (or 5k-8K for one 6V6).

Either way, its do-able with the 125ESE

If you build it with a tube rectifier that is switchable to SS, and with beefy filter caps, you will be able to run a range of B+ voltages to suit your different tubes.

Is there some kind of reference for how much Load Resistance each tube or set of tubes wants? I should also ask is there another term or symbol for load resistance? I tried to look at some tube data sheets but i couldn't determine which value was Load resistance. On the duncanamps data sheets it seems like "Zout" is the only number that could be load resistance. Luckily for most tubes i can just look them up in my RCA tube manual as it just says load resistance. Other sites besides duncan seemed like they might have been using the Symbol "Rk" where the "k" is subbed to the "R".

Also how much wiggle room is there in matching the load resistance? I noticed in your answer that one El34 wants 3.2k or 4k, but that the 125ESE transformer only offers 2.5k, 5k, or 10k regardless of the Output impedance tap you choose. Even in Dave hunter's book when he talks about tube options for the Two-Stroke Amp he lists 1 EL34. The EL34 would either be .7 under(3.2-2.5) or 1k over (5k-4k) its preferred Load Resistance . Is that within the tolerable limits or is there something else going on there that allows the numbers not to match perfectly?

I hate to ask a million questions but I'm right at the point of building this amp and right before having actually soldered anything. I'm trying to leave myself the most amount of power tube options possible within reason while still being able to use 4 8 and 16 ohms out. You mention a solid state rectifier being needed to allow the B+ to be correct for different tubes. What kind of SS rect would you recommend? Also you said beefy filter caps, are the 40uf, 16uf, and 8uf in the design good enough or do you recommend more?

[lowell]

Usually the tube data sheet will have Zout or "ra" which means anode resistance or plate resistance. Tubes don't need exact loads. In fact you can tailor a load to the tone you're after... but as stated you probably wanna stay within the specified range. I think that the filter cap values in the design could be increased. Actually if it were me I'd do a pi filter of 100uf-100r-40uf and take the B+ for the OT from the 40uf. This has reduced hum very efficiently in my single-ended designs. You can put 2 1n4007 diodes in series with the tube rectifier plates... with the banded cathode facing the tube rectifier. Then use a switch to short the banded cathode side of those diodes to the top of the first filter cap-allowing you to switch your tube rectifier in and out.